Extrusion: Second Edition Copyright © 2006 ASM International® M. Bauser, G. Sauer, K. Siegert, editors, p 195-321 All rights reserved. DOI:10.1361/exse2006p195 www.asminternational.org

CHAPTER 5

The Production of Extruded Semifinished Products from Metallic Materials*

THE HOT-WORKING PROCESS extrusion ered to be the most important of the hot-working is, in contrast to other compressive deformation processes. processes used to produce semifinished prod- ucts, a deformation process with pure compres- sive forces in all three force directions. These favorable deformation conditions do not exist in other production processes for semifinished products. Even in rolling, which is the most im- Extrusion of Materials with portant compressive working process for pro- ducing semifinished products, tensile forces oc- Working Temperatures cur in the acceleration zone of the roll gap as between 0 and 300 ЊC well as in the cross rolling process used to pierce blanks in the rolling of steel tubes. These Gu¨nther Sauer* tensile forces cause problems in the rolled prod- uct if the deformation conditions are not opti- mized. The benefits of this three-dimensional compression in terms of deformation technol- 5.1 Extrusion of Semifinished ogy, which have already been discussed in this Products in Tin Alloys book, can be clearly seen in Fig. 5.1 based on experimental results for face-centred cubic (fcc) Tin is a silver-white, very soft metal with a aluminum and zinc with its hexagonal lattice stable tetragonal lattice in the temperature range structure. 20 to 161 ЊC. The pure metal has a density of The extensive variations in the extrusion pro- 7.28 g/cm3 and a melting point of 232 ЊC. cess enable a wide spectrum of materials to be The material can be easily worked with its extruded. After rolling, extrusion can be consid- recrystallization temperature below room tem-

*Extrusion of Materials with Working Temperatures between 0 and 300 ЊC, Gu¬nther Sauer Extrusion of Semifinished Products in Magnesium Alloys, Gu¬nther Sauer Extrusion of Semifinished Products in Aluminum Alloys, Rudolf Akeret Materials, Gu¬nther Scharf Extrusion of Semifinished Products in Copper Alloys, Martin Bauser Extrusion of Semifinished Products in Titanium Alloys, Martin Bauser Extrusion of Semifinished Products in Ziroconium Alloys, Martin Bauser Extrusion of Iron-Alloy Semifinished Products, Martin Bauser Extrusion of Semifinished Products in Nickel Alloys (Including Superalloys), Martin Bauser Extrusion of Semifinished Products in Exotic Alloys, Martin Bauser Extrusion of Powder Metals, Martin Bauser Extrusion of Semifinished Products from Metallic Composite Materials, Klaus Mu¬ller 196 / Extrusion, Second Edition

perature. Consequently, the softest condition de- surface and the container inner wall because of velops rapidly in tin materials after cold work- these good bearing properties. Tin-base extruded ing. Work hardening by cold working is not alloys, therefore, tend to flow largely according possible. to flow pattern A (Fig. 3.11) in direct extrusion. Tin is very stable at room temperature and is The flow stress kf that has to be overcome for not poisonous. Unlike lead, it can therefore be the deformation of very soft pure tin is signifi- used in contact with food. The metal is used as cantly higher than that of pure lead, as can be the base material for soft solder, anode materials clearly seen in Fig. 5.2. It is, however, extremely as well as bearing materials and for tin plating low at 20 to 32 N/mm2 for logarithmic strains steel sheet (tin plate). ug up to 0.7 [Sac 34]. Therefore, a specific press Soft solders are the main application for tin. pressure of maximum 400 N/mm2 is sufficient Lead, silver, antimony, and copper are used as for the direct extrusion of tin alloys when the alloying elements. Special soft solders also con- low friction between the billet and the container tain cadmium and zinc. Aluminum is used as an liner surface is taken into account. alloying component for soft solders used to join Tin-base alloys for the production of soft sol- aluminum alloys. Bearing metals based on tin ders are generally extruded transversely at billet contain antimony, copper, and lead as the alloy- temperatures of 50 to 60 ЊC and a container tem- ing elements as well as additions of cadmium, perature of approximately 100 ЊC. It is often suf- arsenic, and nickel. ficient to heat the front third of the billet to this The production of semifinished products in tin temperature so that, on one hand, the billet up- alloys by extrusion is limited today to the pro- sets from the front to the back to prevent air duction of feedstock for the manufacture of soft entrapment and, on the other hand, the optimal and special soft solders on transverse extrusion welding conditions for the resultant transverse presses and to the direct extrusion of feedstock weld in the extruded product are obtained. This for the manufacture of anodes with solid and transverse weld in soft solders has to be capable hollow cross-sectional geometries for electro- of withstanding the drawing loads during the re- chemical plating. Tin is also used for the pro- duction in diameter of the extruded wires on duction of rolled strip, as the feedstock for the multispindle drawing machines. manufacture of high-capacity electrical con- Semifinished products in tin materials includ- densers, as well as for roll cladding of lead-base ing bar, wire, tubes, and sections are produced strips for organ pipes, wine bottle caps, and on direct extrusion presses. For productivity rea- Christmas ornaments (tinsel). sons, higher extrusion speeds are desired in this In general, tin-base materials have good bear- process than those used in the transverse extru- ing properties and are consequently used as the sion of soft solders, which are typically 4 m/min. base material for bearing metals. Very little fric- It is therefore advantageous to heat the billets to tion occurs in direct extrusion between the billet 100 to 150 ЊC. Again, it is beneficial to have a temperature profile with the temperature de- creasing toward the back of the billet. In auto- matic fully integrated extrusion plants complete

Fig. 5.1 Improving the workability of aluminum and zinc with a hydraulically produced increasing mean compressive stress p measured by the increase in the reduction Fig. 5.2 Flow stress kf as a function of the logarithmic strain in area at fracture u in tensile tests at room temperature of pure lead and pure tin (Source: Rathjen) Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 197

with melting furnaces and mold casting ma- Simple two-column horizontal oil-hydraulic chines, as shown in Fig. 5.3, the cast billets re- extrusion presses are used. Figure 5.3 shows a tain the heat from the casting process to assist typical two-column extrusion press for tin and the deformation process. Care has to be taken to tin alloys with a maximum press load of ensure that the heat from the casting process, 5000 kN. when combined with the deformation heat, does not result in excessive temperatures at high ex- trusion speeds in the deformation zone of the extrusion tooling. The solidus of the lead-tin eu- 5.2 Extrusion of Semifinished tectic at 183 ЊC has to be taken into account in Products in Lead Materials the tin alloys containing lead. If melting occurs in the solid tin matrix, transverse cracks will def- Lead is a soft metal, matte blue in appearance initely be produced in the extruded products. with a fcc lattice structure. The pure metal has Controlled cooling of the extrusion tooling im- a very high density of 11.34 g/cm3 and a melting proves the situation. In addition, the billet sur- point of 327 ЊC. Lead alloys can also be easily face is sometimes brushed with a lubricant to worked. Work hardening after cold working at improve the product surface. room temperature, for example, from 40 to 120 Tin alloys will weld during the extrusion pro- N/mm2 disappears within a few minutes because cess under suitable deformation conditions in- the recrystallization temperature of the base ma- cluding the extrusion load and temperature. terial is about 0 ЊC. Cold working of lead-base They can, therefore, be used with porthole and alloys can be approximately compared in its ef- bridge dies as well as for billet-on-billet extru- fect to hot working of other metals. Work hard- sion with feeder chamber dies. These extrusion ening by cold working is not possible. tools are, in principle, similar to those used in Pure lead is very soft and ductile. Soft lead aluminum extrusion. Their design is simpler in has a very low flow stress kf as shown in Fig. detail and the dies are nitrided. 5.2. It is, therefore, not possible to draw pure

Fig. 5.3 Modern integrated fully automatic 5000 kN extrusion plant for the production of bar, wire, tube, and sections in lead and tin alloys complete with casting oven, triple billet casting plant, hot-billet shear, and oil hydraulic long-stroke extrusion press (Source: Collin) 198 / Extrusion, Second Edition

lead to wire. However, lead alloys have higher the production of soft solders. Lead forms a eu- flow stresses. Additions of antimony and tin pro- tectic with 38.1% tin at 183 ЊC. These soft sol- duce solid-solution hardening of the lead, in- ders can be used to solder steel and copper al- creasing both the base strength and the work- loys. Organ pipes are produced from lead-tin hardening capability. This raises the alloy strips with similar tin contents. Tin con- recrystallization temperature. The loads needed tents from 45 to 75 wt% are used depending on in deformation processes also increase. Anti- the timbre. Lead-tin-antimony alloys with ad- mony and zinc are the main alloying elements ditives of arsenic, cadmium, and nickel are very for lead, although small amounts of arsenic and important as bearing metals. cadmium, as well as copper nickel and silver, Table 5.1 gives recommended lead alloys for are also used. the extrusion of rod, wire, tubes, sections, and Lead antimony alloys are referred to as hard cable sheathing. Lead alloys are extruded with lead because the antimony significantly hardens deformation temperatures in the range 100 to the soft lead. Alloys of this type with antimony 260 ЊC, depending on the composition, to obtain contents up to 3 wt% also age harden. For ex- economic extrusion speeds of typically 50 to 60 ample, the Brinell hardness of an alloy with 2 m/min. On the other hand, lead-base soft solders wt% antimony increases from 56 to 130 N/mm2 are extruded at significantly lower temperatures within 100 days after quenching from 240 ЊC. of approximately 55 ЊC and slower speeds of 2 The toughness and the fatigue strength, as well to 4 m/min. When determining the billet and as the corrosion resistance of these alloys, can container temperatures, care must be taken to also be improved by the addition of specific ensure that any eutectics on the grain boundaries amounts of tin. This is important for the me- do not melt as a result of excessive deformation chanically fatigue loaded sheathed cables on, for temperatures in combination with the deforma- example, bridges. Lead forms a eutectic with tion and friction heat during extrusion. A liquid 11.1% antimony at 253 ЊC. This has to be taken metal phase on the grain boundaries of a solid into account when determining the deformation metal matrix in extrusion will unavoidably pro- temperature for the extrusion of PbSb alloys duce transverse cracks in the extruded product. with eutectic, low-melting point phases on grain Lubricant is added to the billet surface with a boundaries. controlled brush stroke. Lead-tin alloys with their low solidus and li- Lead has even better bearing properties than quidus temperatures are particularly suitable for tin. Lead alloys, therefore, also flow mainly ac-

Table 5.1 Lead-base extruded materials

German Standardization Alloy constituents and Material Abbreviation Institute (DIN) permitted impurities, wt% Application Fine lead Pb 99.99 1719 Permitted impurities, max 0.01 Tube and wire for the chemical industry Pb99.985 Permitted impurities, max 0.015 Copper fine lead Pb 99.9 Cu 1719 Cu, 0.04Ð0.08 Pressure tube Permitted impurities, max 0.015 Pb remainder Primary lead Pb 99.94 17641 Sb 0.75Ð1.25.2006 Pressure tube Pb 99.9 As 0.02Ð0.05 Pb remainder Sb 0.2Ð0.3 Waste pipe Pb remainder Cable lead KbÐPb ... Standard cable sheath KbÐPb (Sb) KbÐPbSb0.5 17640 Sb 0.5Ð1.0 Cable mantle sheath resistant to fatigue KbÐPbSn2.5 failure resulting from severe vibration KbÐPbTe0.4 Sn Ն 2.5 Te Ն 0.035 Hollow anodes PbSn10 . . . Sn 8Ð12 Anodes for electrochemical coating of bores of bearing shells and bushes for bearing production Anodes for corrosion protection of bearing shells Source: Laue/Stenger Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 199

cording to flow type A in direct extrusion as shown in Table 5.1 (see the section on cable shown in Fig. 5.4 where the variation of the ex- sheathing in chapter 3). trusion load over the container cross-sectional area A0 is plotted as a function of the stem dis- placement and the extrusion ratio [Sac 34, Hof 62]. 5.3 Extrusion of Tin- and Lead alloys will weld during extrusion at suit- Lead-Base Soft Solders able temperatures and extrusion pressures simi- lar to tin alloys and certain aluminum alloys. The standard soft and special tin solders in Tubes and hollow sections can, therefore, be ex- DIN 1707, as well as nonstandardized special truded with porthole and bridge dies. Feeder soft solders, are extruded as solid bars, solder chamber dies have also proved successful for the threads, solid wire, and hollow wire (tube sol- extrusion of these materials and are required for der) filled with flux mainly on small hydraulic billet-on-billet extrusion. The tools used for the automatic extrusion presses. The maximum ex- extrusion of tin and lead alloys are in principle trusion loads are usually 2500 kN with a maxi- similar to those used for aluminum extrusion, mum specific pressure of 600 N/mm2, which is although the designs are simpler because of the required because of the extreme metal flow. The significantly lower thermomechanical stresses. billet-on-billet process is used on transverse All extrusion dies are nitrided. presses. Extrusion is usually followed by draw- Extrusion plants consist of integrated auto- ing to the finished diameter of 5 to 0.5 mm on matic complete plants with horizontal, hydrau- multispindle drawing machines. The majority of lically operated two-column presses and maxi- finished diameters fall in the range 1 to 2 mm. mum extrusion loads of 5000 to 10000 kN, as Special soft alloys with low cold-working ca- shown in Fig. 5.3. A specific extrusion pressure pacities sometimes have to be extruded as mul- of 400 N/mm2 is similar to tin-base extruded ma- tiple strands to the finished diameter using the terials, adequate for the extrusion of lead-base direct extrusion process. Economic wire solder alloys. presses operate completely automatically in the Large tubes with internal diameters up to 300 same way as large extrusion plants with auto- mm in lead alloys are sometimes extruded on matic billet feed called by the press during the old vertical indirect extrusion presses over man- extrusion cycle and automatic removal of the ex- drels and charged with liquid metal. These ex- truded semifinished product. Figure 5.5 shows a trusion presses can have loads between 5000 and fully automatic plant with an integrated melting 15,000 kN. Cable sheathing with lead alloys is furnace and a chill mold billet casting machine. carried out on special cable sheathing presses as Cored solders are hollow wire filled with flux by a special tool during extrusion as shown in Figures 5.6 and 5.7. The extrusion tool system needed for the extrusion of hollow wire with flux filling, which includes the die head with the hol- low mandrel, the extrusion die, and the welding chamber, and, in particular, their relative ar- rangement is similar to that for cable sheathing. Figure 5.6 depicts the construction of a die head. The application of “transverse extrusion” com- bined with “billet-on-billet” extrusion is an es- sential requirement for the production of contin- uous cored solder. Cored solder has, similar to the cable sheath, both longitudinal and trans- verse welds from this production method. The billets are usually produced by the chill casting process using casting machines and are normally processed with the cast skin forming the billet surface. Fig. 5.4 Specific extrusion pressure in N/mm2 as a function Typical widely used solders are the soft sol- of the stem displacement and the extrusion ratio V ders L-PbSn40, L-Sn50Pb, L-Sn60Pb and L- A0/AS in direct extrusion of soft lead (Source: Siebal/Fang- meier) Sn60PbCu2, the special soft solders L-SnCu3 200 / Extrusion, Second Edition

Fig. 5.5 Modern fully automatic extrusion plant for the production of soft solder with casting oven, chill mold billet casting plant, hot-billet shear, and oil hydraulic 2500 kN extrusion press. The extrusion press is fitted with a fixed dummy block on the stem. (Source: Collin)

and L-SnAg5 to DIN 1707 and, as an example of a nonstandard solder, the special soft solder L-SnCd25Zn5. The easy and moderately difficult to extrude soft solders are usually worked with a tempera- ture of 55 to 60 ЊC. Heating of the front third of the billet is sufficient. To avoid air entrapment the billet should upset under the influence of the extrusion load and the temperature from the front end to the back and readily weld to the previously extruded billet. The container tem- perature is set to 90 to 110 ЊC. With automati- cally operating integrated extrusion plants for solder wire that include a melting oven and chill mold casting machine, as shown in Fig. 5.3, the cast billet must be transferred with sufficient heat for deformation retained from the casting heat. Equally, an automatic extrusion plant for solder wire need not include a melting oven and chill mold casting machine. Plants with a large number of program changes and small produc- tion batches also operate economically without these facilities. The extrusion plant is then Fig. 5.6 Extrusion tooling for the extrusion of hollow solder. 1, connection to the flux container; 2, hollow man- equipped with a billet magazine and an induc- drel; 3, extrusion die; 4, extrusion die holder; 5, pressure nut; 6, tion furnace in line with a hot billet shear. This one-piece die head with the container; 7, front nut; 8, hollow mandrel screw adjustment; 9, extrusion stem direction during ex- is used to heat either the entire billet volume or trusion; 10, direction of extruded product (Source: Collin) the front third to the deformation temperature. Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 201

Special soft solders need to some extent lower plant specification. The billet end faces are the more difficult-to-extrude alloys’ higher de- sheared in a hot shear and thus cleaned of any formation temperatures, which can be as high as oxide [Lau 76]. This is necessary to ensure per- 400 ЊC. The container temperature has to be set fect transverse welds. After this operation the correspondingly high to avoid excessive heat billets collect in a magazine in front of the con- transfer between the billet and the container dur- tainer and fall one by one into the centerline of ing extrusion. the press when the stem is fully withdrawn. The wires are usually extruded to a diameter When the stem moves forward, the billet enters of approximately 15 mm and then draw on the container and is pushed against the previ- multispindle drawing machines to diameters be- ously extruded billet and the contacting material tween 5 and 0.5 mm. The transverse and longi- volumes weld together. The extrusion emerging tudinal welds formed in the welding chambers from the die is automatically fed into a multi- of the extrusion die are able to withstand the spindle drawing machine and heavily reduced in drawing deformation loads without any prob- diameter in one operation using a large number lem. Good extruded surfaces are achieved by of drawing dies. The extruded wire usually lightly lubricating the surface of statistically uni- passes through a water cooling system before formly selected billets with a special lubricant. reaching the drawing machine. The quantity applied of this lubricant must be The production plan given subsequently for controlled extremely accurately; otherwise, the the manufacture of solder wire with a flux core lubricant can enter the extrusion welds and pre- (hollow solder) explains the solder wire manu- vent perfect welding. The wires would then break at the welds during drawing and have to facturing process in more detail. be scrapped. Production of 1000 kg hollow solder with a The extrusion process for the production of diameter of 0.8 mm from the soft solder L- feedstock for solder wire commences with the Sn60Pb according to DIN 1707 filled with a flux billet call from the extrusion press. The extruded of type F SW32 according to DIN 8511 with wire passes continuously through a multispindle 2.5% by weight: drawing machine in line with the press corre- sponding to its capacity. The drawing machine 1. Casting in a chill mold casting machine or controls the extrusion press. The billet is trans- heating of 168 billets with dimensions: 72 ferred either directly from the chill molding ma- mm diam. 175 mm long (initial billet chine as shown in Fig. 5.8 or from a billet mag- weight: 6.05 Kg) using an induction rapid azine via an induction furnace, depending on the billet heating furnace to 50 ЊC.

Fig. 5.7 Die holder, left, and hollow mandrel, right, as well as the front nut of the container with the pressure nut for adjusting the hollow mandrel (See Fig. 5.6) (Source: Collin, Alchacht) 202 / Extrusion, Second Edition

2. Shearing of the billet end faces using the dou- 5. Drawing to: 1.35 mm diam. in one operation ble shear visible in Fig. 5.5 during the trans- with 40 drawing dies on an intermediate fer to the extrusion press. multispindle drawing machine 3. Extrusion into air with subsequent water 6. Drawing to: 0.8 mm diam. in one operation cooling after the press—one extruded bar per with 40 drawing dies on a fine multispindle extrusion with the dimensions 14.7 6.7 drawing machine. diam. using the billet-on-billet process and 7. Coiling the wire on bobbins. This operation transverse extrusion with simultaneous ad- is frequently combined with operation 6. dition of flux using an extrusion tool system Quality control of the wire surface is also car- shown in Fig. 5.6 from a container: 75 mm ried out. diam. on a 2500 kN extrusion press—con- 8. Control and labeling tainer temperature: 100 ЊC; exit speed: 3.02 m/min; extrusion ram speed: 5.5 mm/s; ex- Solder threads are produced by multiple trusion ratio V 32.86. drawing of extruded solid wire approximately 4. Drawing to: 6 mm diam. in one operation 15 mm diam., depending on the final dimensions with 14 drawing dies on a roughing multi- on the multispindle drawing machines and au- spindle drawing machine. This drawing ma- tomatically cut transversely after drawing. chine controls the extrusion press according The work hardening of the solder material to the wire requirement via a dancing roll. produced during the cold working is countered by the continuous softening from recrystalliza- tion during the multispindle drawing process. The process is helped by the heat of the drawing lubricant in the sump of the drawing machine, in which the entire drawing process takes place. Soft solder usually recrystallizes at room tem- perature following work hardening. Therefore, intermediate annealing is not needed to achieve the soft, i.e., recrystallized material, state re- quired for further cold working.

5.4 Extrusion of Zinc Alloy Semifinished Products

Pure zinc has a density of 7.13 g/cm3 and a melting point of 419.5 ЊC. The metal crystallizes with a hexagonal lattice structure, which, at room temperature only permits material slip on the (0001) basal plane. Consequently, polycrys- talline zinc alloys at room temperature and be- low, in particular, are brittle. Deformation of this material is possible only after heating to tem- peratures between 150 and 300 ЊC. Twin for- mation during the deformation improves the plasticity. Zinc alloys do not have any great work-hardening capacity because the recrystal- lization temperature of these alloys is at room temperature or just above. The main alloying Fig. 5.8 Oil-operated 2500 kN solder wire extrusion press as elements of zinc are aluminum and copper. Alu- shown in Fig. 5.5 with extruded hollow solder minum improves the mechanical properties of emerging transverse to the press longitudinal axis. The vessel on the left-hand side is filled with flux and linked by a tube to con- zinc alloys particularly when combined with hot nection 1 on the hollow mandrel 2 in the extrusion tool in Fig. working by extrusion. Copper also improves the 5.6. The container is located above the hollow mandrel, and the usually very liquid flux heated to approximately 60 to 120 ЊC mechanical properties of zinc-base alloys but flows under hydrostatic pressure to the hollow mandrel. not to the same extent as aluminum. However, Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 203

copper improves the fatigue strength and mach- modulus of elasticity of pure magnesium is ap- inability of zinc alloys. proximately 41,000 N/mm2 and that of magne- Typical zinc-base extrusion alloys are sium alloys between 45,000 and 47,000 N/mm2, ZnAl1Cu and ZnAl4Cu1. Bars, wires, tubes, and i.e., on average about 66% of that of aluminum sections are produced from ZnAl4Cu1. The ma- alloys. Pure magnesium has a liquidus tempera- terial can also be drawn. ZnCu is an alloy that ture of 650 ЊC and crystallizes with a hexagonal is used for the production of sections and hot- lattice structure. Metals with the hexagonal lat- stamping components. Car tire valves are pro- tice structure can only slip on the (0001) base duced from the alloy ZnAl15. plane at room temperature. Consequently, mag- After hot rolling, extrusion is the most im- nesium alloys are very brittle at room tempera- portant deformation process for zinc-base alloys, ture. However, temperatures above 200 ЊC per- although the main application of zinc alloys is mit the activation of other slip planes as well as pressure die castings. The extrusion of these al- the formation of deformation twins enabling loys has lost a lot of its importance. Zinc alloys these materials to be hot workable. The narrow are extruded only to a limited extent mainly be- temperature range between the brittle and the cause the limited workability associated with the plastic deformation behavior of magnesium al- hexagonal lattice structure also affects the hot loys is of interest and is illustrated in Fig. 5.9 workability. Zinc alloys are extruded at defor- for the alloy MgAl6Zn [Sac 34, Bec 39]. mation temperatures of approximately 150 to Whereas this material clearly has a limited 300 ЊC. Only very low extrusion speeds of ap- workability at 208 ЊC as can be seen by the shear proximately 3 to 6 m/min can be achieved be- stress cracks running at 45Њ, it can be readily hot cause of the limited extrudability of the extruded worked at 220 ЊC; i.e., a temperature increase of materials at high specific pressures [Sac 34, Lau only 12 ЊC produces significantly better defor- 76, Schi 77, Schu 69]. mation properties in this alloy. The formation of twin lamellae is associated with the improve- ment in the plastic workability. Only magnesium alloys are used as structural materials and in structural applications, the high notch sensitivity has to be taken into account. Extrusion of Materials with This also has to be allowed for in the cross-sec- tional geometry of extruded sections. In addi- Deformation Temperatures tion, magnesium alloys have a very elastic be- between 300 and 600 ЊC havior in compression because of their low elastic moduli between 45,000 and 47,000 N/ 2 Gu¨nther Sauer* mm , but this also increases the sensitivity to buckling. Specific alloy additions can raise the static and dynamic materials properties of magnesium 5.5 Extrusion of Semifinished by a factor of two or three. The principle alloy- Products in Magnesium Alloys ing elements of the wrought alloys are aluminum with contents up to 10 wt%, zinc up to 4 wt%, Magnesium is a metal with the low density of and manganese up to 2.3 wt%. Zirconium, ce- 1.74 g/cm3, which is approximately 40% below rium, and thorium are also added. More recently, that of aluminum. The densities of magnesium lithium has been increasingly used as an alloying alloys fall in the range 1.76 to 1.83 g/cm3. The element.

Fig. 5.9 Deformation behavior of the magnesium alloy MgAl6Zn in hot-compression tests in the temperature range between 200 and 220 ЊC (Source: Schmidt/Beck)

*Extrusion of Semifinished Products in Magnesium Alloys, Gu¬nther Sauer 204 / Extrusion, Second Edition

Aluminum is the most important alloying solutions with the phase MgZn. An addition of element for magnesium, which forms a solid up to 3 wt% increases the fracture strength and solution with aluminum at low contents and the fatigue strength of magnesium. Zinc contents the intermetallic phase Al2Mg3 at higher con- more than 3 wt%, however, result in a drastic tents. A eutectic occurs at 436 ЊC at an alu- reduction in the elongation to fracture. Zinc con- minum content of 32.2 wt%. Crystal segrega- taining magnesium alloys can be age hardened. tions can occur in wrought alloys at aluminum This can increase the fracture strength of mag- contents above 6 wt%. They can be dissolved nesium alloys by 30 to 40%. and thus removed by homogenization of the Manganese dissolves in magnesium by, de- cast in the temperature range of 400 to 450 ЊC pending on temperature, up to 3.4% at 645 ЊC. for correspondingly long times. The solid-so- At this temperature magnesium forms a eutectic lution formation at aluminum contents of 2 to with manganese. Manganese is dissolved by approximately 6 wt% increase the fracture magnesium with the formation of solid solutions toughness and hardness of magnesium alloys and the phase MgMn. It increases the strength as shown in Fig. 5.10. At higher aluminum of magnesium at contents Ͼ 1.5 wt%. Manga- contents, the fracture strength and, in particu- nese containing magnesium alloys have better lar, the hardness can be further increased by corrosion resistance than other magnesium-base the formation of the hard c phase, but with a alloys. Manganese contents of between 0 and reduction in the ductility of the material as can 0.5% are therefore added to all magnesium al- be clearly seen in Fig. 5.10 [Schu 69]. Mag- loys to improve the corrosion resistance. nesium-base wrought alloys have aluminum contents up to 9 wt%. Magnesium alloys can Additions of zirconium form finely dispersed be hot rolled at aluminum contents up to 7 zirconium oxides that act as the nuclei for a fine- wt% and hot worked by extrusion at aluminum grain structure and thus increases the tensile contents up to 9 wt%. The magnesium alloy strength of magnesium alloys with no reduction becomes increasingly more brittle at aluminum in the elongation. contents Ͼ 9 wt% as shown by the elongation Cerium also increases the fine-grain structure in Fig. 5.10. This also shows that magnesium and improves the hot strength. alloys with contents Ͼ 7 wt% can be age hard- Thorium improves the hot strength even more ened after solution heat treatment. At the same than does cerium. This has resulted in the de- time, the alloy suffers a drastic loss in ductility velopment of magnesium-thorium alloys with [Schu 69]. particularly high resistance to softening. Tho- Zinc readily dissolves, depending on the tem- rium also significantly improves the fatigue perature, in magnesium by the formation of solid strength and specifically the creep strength [Tech

Fig. 5.10 The influence of aluminum on the mechanical properties of extruded magnesium alloys (Source: Spitaler) Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 205

96/97/98]. However, in England, alloys with centive to improve the existing alloys and to de- more than 2 wt% of thorium are classified as velop new ones with the intention of: radioactive. In the United States, use of mag- ● nesium-thorium alloys is very restricted because Improving the static and dynamic materials thorium-free magnesium alloys have been sub- properties, including elongation and tough- ness stituted. ● Magnesium alloys are very susceptible to Improving the temperature resistance ● Further reducing the density corrosion compared with aluminum alloys be- ● cause they are electrochemically less noble. Ad- The development of alloys with self-healing ditions of nickel, iron, copper, and so forth pro- surface passive films mote the corrosion of the material. Therefore, The coarse cast structure is transformed into these additions are held in the range ppm by a fine grain elongated structure by hot working using purer starting materials as well as im- in the extrusion process. This structural trans- proved melt refining processes. This enables the formation is associated with a significant im- corrosion sensitivity to be significantly reduced. provement in the mechanical properties of the High-purity-based magnesium alloys today magnesium alloys in the same way as aluminum have corrosion resistances comparable to alu- alloys. The mechanical properties that can be minum alloys. achieved improve with increasing extrusion ra- This has been one of the factors that has in- tio, i.e., with increasing working during extru- creased the interest of the automobile industry sion. Figure 5.11 illustrates this process, which in magnesium-base alloys and provided the in- is of considerable importance for the supply of

Fig. 5.11 Tensile strength RM and elongation to fracture A as a function of the extrusion ratio determined on single cavity extruded round bars in the alloy MgAl10. Billets 175 mm diam were used for the experiments with a container diam of 180 mm (Source: Beck) 206 / Extrusion, Second Edition

extruded semifinished products [Bec 39]. The wrought magnesium-base alloys in addition to extrusion ratio V A0/AS should not fall below those listed, particularly abroad, for example al- 5, and it is better to specify the minimum at 7. loys of the MgAl family. The working tempera- A fine-grained extruded structure with good me- tures for extrusion are at 250 to 450 ЊC, depend- chanical properties can be achieved with extru- ing on the alloy and are higher than those used sion ratios of 25 [Har 47]. for hot rolling the same magnesium alloys. Ex- Magnesium alloys are mainly extruded using trusion ratios V A0/AS similar to those for alu- the direct extrusion process. Indirect extrusion minum alloys can be achieved. The extrudable is possible and is used. Magnesium alloys flow magnesium alloys belong to the category of dif- more uniformly in direct extrusion than do alu- ficult-to-extrude alloys mainly because of their minum alloys. The discards can be reduced in hexagonal lattice structure. The extrusion speeds thickness because of the lower distortion in the fall in the range of moderate- to difficult-to-ex- region of the billet surface and the resultant flat- trude aluminum alloys. According to [Har 47], ter dead metal zones in the deformation zone of the container. Magnesium alloys can be welded the extrudability of magnesium alloys decreases in the extrusion process. Therefore, porthole with increasing aluminum content as do the ex- dies and bridge dies can be used to produce trusion speeds. Only the alloys MgMn and tubes and hollow sections. Similar to aluminum MgMn2, as well as MgAl1Zn, can be extruded alloys, large-diameter tubes are extruded over at speeds up to a maximum of 30 m/min [Zol mandrels. It is also possible to use billet-on- 67]. The predominantly slow extrusion speeds billet extrusion with magnesium alloys. How- naturally result in long extrusion cycles. There- ever, the weldability is reduced by increasing fore, the extrusion container temperature has to aluminum contents. be set so that the billet does not lose any heat to Typical magnesium alloys for hot working by the container during the extrusion cycle; other- extrusion are given in Table 5.2. There are other wise, the billet will freeze in the container. In

Table 5.2 Extruded magnesium-base alloys

Properties Material designation Rp0.2 Rm AS/A10 Symbol No. (DIN standard) ASTM Composition, wt% Billet, ЊC Shape and condition N/mm2 N/mm2 % Mg 99.8H 3.5003 . . . Cu: 0.02 Si: 0.10 250Ð450 Bar, wire, tube, section ...... (DIN 17800) Mn: 0.10 Ni 0.002 Fe: 0.05 MgMn2 3.5200 M2 Mn: 1.20Ð2.00 250Ð350 Bar, tube, section 150Ð170 200Ð230 10Ð15 (DIN 1729) Mg: remainder Extruded and straightened Drawn and straightened MgAl2Zn ...... Al: 1.20Ð2.20 300Ð400 ...... Zn: 0.5 Mn: 0.1 Mg: remainder MgAl3Zn 3.5312 AZ31 Al: 2.50Ð3.50 300Ð400 Bar, tube, section 130Ð200 240Ð260 3Ð12 (DIN 1729) Zn: 0.50Ð1.50 Extruded and straightened Mn: 0.05Ð0.40 Mg: remainder MgAl6Zn 3.5612 AZ61 Al: 5.50Ð6.50 350Ð400 Tube and section 180Ð200 250Ð280 6Ð10 (DIN 1729) Zn: 0.50Ð1.50 Extruded and straightened Mn: 0.05Ð0.40 MG: remainder MgAl7Zn ...... Al: 6.50Ð8.00 300Ð350 Bar 200Ð230 280Ð320 3Ð10 (A5) Zn: 0.50Ð2.00 Extruded and straightened Mn: 0.05Ð0.40 Age hardened if required Mg: remainder MgAl7Zn 3.5812 AZ81 Al: 7.80Ð9.20 300Ð420 Bar and section 200Ð230 280Ð320 6Ð10 (A5) (DIN 1729) Zn: 0.20Ð0.80 Extruded and straightened Mn: Ͼ0.12 Age hardened if required Mg: remainder MgZn5Zn0.6 3.5161 . . . Zn: 4.80Ð6.20 300Ð400 Bar, tube, section 200Ð250 280Ð320 4Ð5 (A5) (DIN 1729) Zr: 0.45Ð0.80 Extruded and straightened Age hardened if required MgTh3Mn2(a) ...... Th: 2.50Ð3.50 . . . Bar, tube, section 180 270 4 (A5) Mn: Ͼ1.2 Extruded and straightened (a) Experimental material Fuchs Metallwerke. Source: Schimpke, Schropp, and Konig Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 207

Table 5.3 Dependence of the specific extrusion pressure for the extrusion of tubes 44 1.5 mm in the alloy MgAl3 on the homogenization treatment and the heating conditions of the billet 48 150 mm

Thermal treatment of the cast Heating before extrusion before heating prior to extrusion Temperature, ЊC Time, h Specific extrusion pressure, N/mm2 Not carried out 380Ð350 1 250Ð300 Not carried out 380Ð350 3 210Ð260 Not carried out 380Ð350 6 180Ð220 Not carried out 340Ð300 1 320Ð400 Not carried out 340Ð300 3 250Ð300 Not carried out 340Ð300 6 200Ð250 350 ЊCÐ12 h 340Ð300 1 190Ð250 350 ЊCÐ12 h 340Ð300 3 170Ð200 350 ЊCÐ12 h 340Ð300 6 150Ð180 350 ЊCÐ12 h 340Ð300 1 170Ð220 350 ЊCÐ12 h 340Ð300 3 160Ð200 350 ЊCÐ12 h 340Ð300 6 130Ð170 Source: Zubolob and Zwerev

addition, good soaking of the billet during heat- compared with aluminum alloys, even though ing is required. Magnesium alloy extrusion bil- the formation of a layer cannot be avoided with lets are normally extruded with the cast skin. magnesium alloys even when nitrided dies are Heat treatment such as homogenizing im- used. proves the hot workability of the magnesium- Cold working by drawing to improve the me- base extrusion alloys. The effect of homogeni- chanical properties is practically impossible for zation on the extrusion loads needed for hot wrought magnesium alloys because of their lim- working of the alloy MgAl3 is shown in Table ited cold workability associated with their hex- 5.3 [Zol 67, Lau 76]. agonal lattice structure. Extruded magnesium alloy semifinished Drawing in the form of a calibration draw is, products exhibit extreme anisotropy because of however, of interest, as a means of improving the hexagonal lattice structure of the metal. For tolerances. example, tensile tests on extruded semifinished magnesium alloys in the longitudinal direction, i.e., the extrusion direction, have significantly higher values for the 0.2% proof stress, tensile strength, and elongation than in the transverse direction. Extrusion of Semifinished If the ratio of the tensile strength transverse/ longitudinal is taken as a measure of the aniso- Products in Aluminum tropy of a material, magnesium alloys have val- Alloys ues in the range 0.6 to 0.7. The nitrided dies used for the hot working of Rudolf Akeret* magnesium extruded alloys have, in principle, the same design features as those used for the extrusion of aluminum alloys. Only the shape- forming regions of the die are matched to the 5.6 General specific properties of magnesium alloys by hav- ing longer bearings and larger radii. The typical Of all the materials processed by extrusion, hollow section die used for aluminum alloys can aluminum occupies the predominant role in in principle be used for these alloys because of terms of both production volume and value. their ability to weld during extrusion. In addi- Based on the annual volume of production of tion, in contrast to aluminum alloys, magnesium primary and secondary metal, aluminum is alloys have a significantly lower affinity for iron placed directly after iron. The majority are al- alloys [Tech 96/97/98]. This property results in a slower buildup of an intermediate layer of the *Extrusion of Semifinished Products in Aluminum Alloys, extruded material on the die bearing surfaces Rudolf Akeret 208 / Extrusion, Second Edition

loyed to produce wrought alloys and formed into loys. Cold and hot extrusion with lubrication of semifinished products of which 25% are ex- the container and the die is also used for bars truded. The majority of all extrusion plants, and tubes [Ake 73]. three quarters in Germany, for example, process aluminum alloys [Bau 82], 465,000 t in 1995. The physical metallurgical properties of alu- minum alloys make them particularly suitable 5.7 Extrusion Behavior of for the extrusion of products very close to the Aluminum Alloys finished shape and with attractive properties. The face-centered cubic (fcc) structure with 12 5.7.1 Flow Stress slip systems combined with a high stacking fault The extrusion temperature range, the flow energy is a requirement for good cold and hot stress variations, and the friction across the tool- workability. Corresponding to the melting point Њ ing determine the extrusion behavior of alumi- of 660 C, the hot-working temperature of alu- num alloys. Alloy and quality requirements de- minum alloys falls in the range of 350 to 550 Њ termine the necessary exit temperature for the C, which can be easily withstood by tools made extruded product and the temperature range for from suitable hot-working steels. In this tem- the deformation. In the range 350 to 550 ЊC, the perature range the flow stress is reduced to val- flow stress of aluminum alloys is very dependent ues that require relatively low specific press on the temperature and the composition [Ake 70, pressures for processing. The natural oxide skin DGM 78]. The increase in the flow stress with gives aluminum an attractive appearance and a increasing content of the most common alloying good corrosion resistance in the natural state. In- elements is shown in Fig. 5.12 [Ake 70]. creased surface protection is given by anodic ox- The dependence of the flow stress on tem- idation. Aluminum forms age-hardening alloys perature and speed is described in the context of with low-alloying additions that combine good the basics of metallurgy in Chapter 4. Figure hot workability with a high strength after a sim- 5.13 shows for some non-heat-treatable alloys ple heat treatment. that reducing the temperature by approximately Sections with an extensive range of function 100 ЊC results in an almost doubling of the flow specific cross sections can be extruded within stress providing the alloy additions stay in so- narrow tolerances from aluminum alloys. Hol- lution [Ake 70]. low sections in the form of rectangular tubes and With age-hardening alloys similar to Al- hollow engineering plates offer a high bending MgSi1 the variation of the flow stress is dis- and torsional stiffness. The extensive variety of aluminum sections used today in a wide range of technologies is described in detail in Chapter 2. The selection of the extrusion process is largely determined by the physical metallurgical properties of the aluminum alloys. The high af- finity to steel and thus the tendency to adhere to all extrusion tools has to be included in the ma- terial properties in addition to low extrusion temperature and the good extrusion weldability. Direct hot extrusion without lubrication and without a shell is used for the majority of ex- truded products, including solid and hollow sec- tions from the easily and moderately difficult al- loys. Direct extrusion can be used for practically the entire spectrum of products, from the simple round bar to complicated sections with a circum- scribing circle close to the container cross sec- tion. Flat dies are used for solid sections and porthole dies for hollow sections. Indirect extrusion comes into consideration Fig. 5.12 Increase in the flow stress of aluminum in hot for compact cross sections in hard-to-extrude al- working with different alloying additions [Ake 70] Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 209

Fig. 13 Flow stress of some non-age-hardening aluminum alloys as a function of the deformation temperature (maximum of the ˙ 1 flow curve in torsion tests with ug 0.655 s ) [Ake 70] placed by solution and precipitation processes at a maximum immediately below the surface according to the content of alloying elements in (Fig. 5.15) [Gat 54]. the matrix. A dead metal zone forms in front of the face of the flat die. The surface of the extruded sec- 5.7.2 Flow Process tion is not formed from the surface of the billet The aluminum alloys are almost always ex- truded in direct contact with the container and die manufactured in hot-working steel. How- ever, aluminum exhibits a significant chemical affinity and adhesion tendency with iron [Czi 72]. Even in the solid state it tends to adhere to tool surfaces. At face pressures above the flow stress, Coulomb’s laws of friction lose their va- lidity if the shear distortion of the peripheral layer requires less force than the slip along the surface of the harder frictional party [Wan 78]. The face pressure at the inner face of the con- tainer is of the order of 10 times the flow stress. Therefore, aluminum alloys flow in direct extru- sion according to flow type B1 as shown in Fig. 5.14 [Val 96].

5.7.2.1 The Dead Metal Zone Fig. 5.14 Flow types B, B1, and C and the flow inward of the billet peripheral layer along the dead metal The billet surface is stationary relative to the zone (path 1) and from the dummy block edge into the interior container inner wall, and the shear distortion is of the billet (path 2) [Val 96] 210 / Extrusion, Second Edition

but from the interior of the billet by shearing 5.7.2.2 In the Shape-Producing Aperture along the dead metal zone [Ake 88, Lef 92]. The outermost layer of the billet surface initially ad- The pressure conditions in the die aperture are heres to the inner wall of the container and is different. The axial pressure at the exit is zero, shaved off by the advancing dummy block. The or there can be a small tensile stress—if a puller material that is compressed together in front of is used. the dummy block and that contains oxide and The face pressure, which determines the fric- exudations from the billet surfaces ends up well tion, cannot be easily calculated because it strad- below the surface of the extrusion following the dles two boundary cases. If the section can slide path shown in Fig. 5.14, No. 2, and forms an as a solid body through the die aperture, the face incompletely bonded intermediate layer referred pressure, and thus the friction, is low. If the sec- as the piping defect. Material located farther be- tion undergoes a reduction in thickness through low the surface is compressed at the upper edge a narrowing die aperture, the face pressure is at of the dead metal zone and follows path 1 of the least equal to the flow stress. The large adhesion dead metal zone into the region of the surface affinity of aluminum increases the friction stress of the extruded product. Adhesion of the mate- to the same order of magnitude as the shear rial also occurs in feeder chambers and in the stress [Ake 88, Ake 85]. ports of porthole dies where there is a high pres- The friction in the die opening is responsible sure [Ake 88]. for the increase in the axial pressure against the flow direction. Slip with Coulomb friction is impossible in the extrusion of tubes over a mandrel because of the high radial pressure between the billet and the mandrel. If the extrusion is carried out over a stationary mandrel, relative movement with shear friction takes place between the billet and the mandrel from the start of extrusion over the entire billet length. The relative speed is there- fore of the order of magnitude of the stem speed. The billet material is only accelerated to the exit speed of the tube as it crosses the deformation zone and at the same time the pressure on the mandrel surface falls rapidly. The transition to slipping friction of the finish extruded tube over the mandrel tip takes place in the region of the die aperture. When a moving mandrel is used, the zone in which relative movement occurs between the billet and the moving mandrel is only the length of the deformation zone. The shear zone moves toward the back of the billet as the extrusion progresses [Rup 82].

5.7.3 Thermal Balance and Extrusion Speed The theoretical background to the thermal balance in extrusion is discussed in chapter 3 (see also Fig. 3.22). In the direct extrusion of aluminum alloys without lubrication, two to three times the me- chanical work is needed than would be required for an ideal loss-free deformation. The work car- ried out by the press on the material being ex- Fig. 5.15 Adhesion of the billet surface to the container wall and the shear deformation of the peripheral layer truded is practically completely transformed into [Gat 54] heat, which is partly transferred into the tooling Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 211

and partly removed in the emerging extrusion. mation zone, and the friction heats an even thin- In the adiabatic limiting case, each MPa of av- ner surface layer of the section. A peripheral erage extrusion pressure corresponds to an av- layer approximately one-third of the cross-sec- erage increase in temperature of 0.3 K. tional area is therefore heated to a significantly The magnitude involved is shown in Fig. 5.16 higher temperature than the core of the section for the case of an easily extruded material with [Ake 72]. This temperature difference is very a flow stress of 25 MPa in which a billet of short lived and is practically completely equal- length l0 4D0 is extruded as quickly as pos- ized on the way from the die to the press platen. sible with an extrusion ratio of 30 to avoid any Experiments with sheathed thermocouples in the heat losses. When a material volume corre- die land have shown that the maximum surface sponding approximately to the volume of the de- temperature can be 60 to 100 K higher than the formation zone is extruded, there is initially a mean section temperature [Joh 96]. steep increase in temperature. With loss-free de- With sections, the maximum temperature, formation, which approximately occurs in the which governs the onset of tearing, occurs dur- core of the extrusion, the material is initially ing the passage through the die opening usually heated by 25 K. at an edge where the longitudinal stress is the The shearing of the material flowing through highest and the temperature probably at the max- the deformation zone along the dead metal zone imum. requires approximately the same amount of The possible working range of the press, i.e., work as the pure deformation [Ake 72, Sah 96]. the “window” in the temperature-speed field, is An additional contribution, which cannot be ig- limited by the following thermal and mechanical nored, is the friction in the die aperture, the mag- conditions: nitude of which depends on the angle and the ● The surface temperature cannot exceed the length of the bearings [Ake 85, Moo 96, Mue maximum value that results in excessive 96]. The shear takes place in a surface layer ap- scoring or even transverse cracks at any proximately 10% of the diameter of the defor- point (limit curve 1 in Fig. 5.17). The tem- perature equalizes to some extent during the crossing of the deformation zone. ● The average section temperature has to be high enough to ensure adequate solution heat

Fig. 5.16 Heating from deformation and shear deformation of the peripheral layer in the adiabatic limiting case (example, AlMgSi0.5: flow stress 25 MPa, billet length: billet Ø 4, extrusion ratio 30). 1, temperature increase up to the entry into the deformation zone; On exit from the die: 2, tem- perature increase of the section core; 3, temperature increase of Fig. 5.17 Limit diagram for maximum surface and minimum the section surface; 4, mean temperature increase of the section mean section temperature 212 / Extrusion, Second Edition

treatment with AlMgSi alloys and to reduce nologies are influenced mainly by the need to the tendency of AlCuMg and AlZnMgCu al- avoid defects that would impair the mechanical loys to recrystallization (limit curve 2). properties or the decorative appearance [Bry 71, ● The press and its power source, on the other Fin 92, Par 96]. The following defects are par- hand, determine the maximum extrusion ticularly important: load and the ram speed. The latter decreases ● with increasing extrusion load (reduction in Uneven color tone (bands or flecks) ● Coarse recrystallization billet temperature) because of the increasing ● slip loss (limit curve 3). Excessive roughness (scoring or pickup) ● Imperfectly bonded brittle contact areas be- In the literature there are several different pro- tween material streams flowing together posals for depicting the process limits [Joh 96, (piping, defective welds) Hir 58, Ste 73, She 77]. ● Material separation in the form of transverse The range of extrusion speeds that can be cracks, blisters, and flaking practically obtained with aluminum alloys is ● Subsequent mechanical damage or corrosion shown in Fig. 5.18 [Ake 71] plotted against the flow stress at the extrusion temperatures used in The cause of these defects can be found in the practice. structure of the billets, the flow process, and the extrusion temperature, in a subsequent heat The exit speeds VA are scattered around a line with a steep negative gradient that can be ap- treatment or defects in the logistics (handling, proximated by the expression: transport, and storage of the finished sections). To determine and eliminate the causes of V ϳ k2 these defects, correct diagnosis and an accurate Af knowledge of the process technological relation- This empirical equation [Ake 68] can be ex- ships are necessary. The results obtained from plained by the laws of heat penetration [Lag 72]. more accurate analysis of the flow processes and the temperature field contribute significantly to 5.7.4 Section Surface and Surface Defects the understanding of these relationships. Men- tion should also be made of the extrusion defect In all stages in the production of aluminum catalog produced by the “extrusion” working extruded sections, the plant and process tech- party in conjunction with the Institute for Ma- terial Science of the RWTH Aachen University.

5.7.4.1 Section Surface and the Peripheral Layer Microgeometric surface defects and struc- tural-related tone variations (banding, flecks) are related to the method of formation of the section surface and the additional through working and heating of the peripheral layer compared with the section core. Using high resolution marking and evaluation processes, it is possible to local- ize the exact source of the peripheral layer in the billet [Val 88, Val 92, Val 92a]. Only a small material volume located between the dead metal zone and the deformation zone is involved in the formation of a 50 lm-thick peripheral layer, and this can be extended by a factor of more than 1000 [Val 92a]. This extreme working of the pe- ripheral layer deforms a cast grain to a strip that is thinner than the subgrain size corresponding to the deformation conditions. The subgrains then grow by recovery through the original grain boundaries, as shown in Chapter 4 in the section Fig. 5.18 Flow stress and extrudability [Ake 71] on metallurgical basics. Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 213

The peripheral layer therefore has a subgrain rium particles are nucleated on AlFeSi. Experi- structure without defined grain boundaries, pro- ments on the etching kinetics have shown that it viding the heat from extrusion or subsequent so- is these craters that have a strong effect on the lution heat treatment does not result in static re- matte finish [Sta 90].The number of particles per crystallization. This high deformation of the unit area depends on the average dendrite spac- peripheral layer does not produce any higher ing (cell size) in the cast structure and also on hardness in the as-extruded condition [Moe 75]. whether a region is normally elongated or highly An additional feature of the highly deformed deformed during extrusion. The impoverished peripheral layer is its ͗001͘ {211} shear texture, peripheral zones and the primary solidified re- which differs from the ͗111͘ ͗100͘ double gions appear bright after anodizing, whereas the fiber structure of the core zone [Moe 75, oxide skin and the segregation zones matte. The Auk 96]. controlling factors for the frequency of nuclea- Rows of precipitates are pulled apart by the tion of Mg2Si and AlFeSi are the temperature extreme deformation and moved closer together changes during billet heat treatment and during in the transverse direction so that the particles cooling or quenching after extrusion. appear to be irregularly arranged. The recrystal- Pits form during etching along the grain lization retarding effect of layers heavily deco- boundaries. The shape of the pits depends on the rated with particles is then lost. This results in formation of the age-hardening b-MgSi phase. undesired coarse grain recrystallization of the In general, different cooling conditions (influ- AlCuMg and AlZnMg alloys, particularly at the encing the pit width) as well as different grain end of extrusion where the peripheral layer size (influencing the pit length) can result in sig- forms up to 90% of the cross section. nificant variations in brightness. An orientation-dependent face etching occurs 5.7.4.2 Cast Structure and on the surface during etching and this consists Anodizing Capability of small cube faces with an edge length less than 0.5 lm. The deformation texture of the section Knowledge of the relationship between the lo- with the {011} ͗211͘ principle components can cation of the volume element in the billet and in form to different extents in different parts of the the section is the key to explaining and over- extrusion die. This results in an inhomogeneous coming linear and banded tone variations in an- distribution of the cube faces and thus in differ- odized sections [Ake 88, Bau 71, Lyn 71, ent reflection properties. This is how some of Sta 90]. the undesired leg marks can occur. The formation of the cast structure of the bil- Investigations on a hollow section [War 95] let is discussed in Chapter 4, in the section on have shown that different die designs for the metallurgical basics. From the point of view of same profile cross section can result in com- the uniformity of the anodized surface, the fol- pletely different decorative appearances. Feeder lowing regions of the billet surface are impor- chambers and splitting can affect the magnitude tant: and direction of the local strain of the surface ● The oxide skin (10Ð100nm) layer. The die design therefore has a strong in- ● The segregation zone (up to ϳ600 lm) fluence on the texture, precipitate distribution, ● The impoverished peripheral zone and grain size. ● The periodic cold shut (up to several mm) The three mechanisms just described for the ● The columnar solidified crust (up to 15 mm) roughening of the surface (depressions, pits, tex- ● The primary solidified regions scattered over ture) have approximately equal importance for the entire billet cross section the decorative appearance of the anodized sec- tion. In addition, precipitates of the base metal, Differences in the brightness of the oxide which can neither be dissolved nor removed dur- layer are primarily caused by differences in the ing etching, end up in the oxide layer, giving it number of depressions from the initial alkaline a cloudy or colored appearance. etching. The etch attack does not occur uni- The external oxide skin in single-cavity ex- formly but preferentially at all types of particles trusion follows path 2 (Fig. 5.14) and finishes in at grain boundaries. The brightness is affected, the interior of the sections forming a brittle layer in particular, by the depressions that form at all (piping defect). In multicavity extrusion, sec- AlFeSi particles. Particularly deep craters de- tions with reentrant angles, and hollow sections, velop at the locations where the Mg2Si equilib- the side adjacent to the center of the die consists 214 / Extrusion, Second Edition

of normally elongated material, whereas the ex- 5.7.4.3 Surface Roughness ternal side, particularly at the end of extrusion, can consist of highly deformed material [Ake In contrast to most deformed surfaces, ex- 88a] (Fig. 5.19). truded section surfaces of aluminum alloys in If the distance between two die openings is particular are not formed by the extension and large, a dead metal zone can form so that the dissipation of the surface of the starting material section surfaces extend into highly deformed [Kie 65, Mie 62]. Instead, the section surface is material. Within a short distance the central dead newly formed from the interior of the material metal zone disappears and the section surface by a cutting process between the deformation extends partly into the normally wrought fibrous zones and the dead metal zones. material [Ake 88, Val 96a]. In between the oxide Two shear zones meet during this cutting pro- and segregation layer of the billet appears on the cess. In one, the flowing material is sheared surface. along the dead metal zone; in the other, it is Cold shuts, which can extend several milli- moved into the exit direction. meters into the billet peripheral zone, can finish The extremely severe deformation of the out- up in the peripheral layer of the section follow- ermost surface layer in the region of the entry ing path 1 (Fig. 5.14). If the defects are deep, edge can result in cavitation defects in hetero- and if they occur in the front part of the billet, geneous materials. Hollow spaces form by the the entire section can be unusable [Bag 81]. separation of nonplastic particles from the ma- In addition, other variations in the material trix or by the fracture of these particles without flow and in the through working with subse- the matrix metal filling the space between the quent static recrystallization can result in varia- fracture pieces. If the hollow spaces bond to- tions in the size and orientation of the recrystal- gether in the direction of extrusion and the hard lized grains and thus to undesired contrasts in particles separate from the matrix, microgrooves brightness on anodized section surfaces. Known form on the surface [She 88, Clo 86]. examples include the side opposite to a branch The surface formed in the region of the entry in the profile cross section and the highly de- edge undergoes a change by adhesion wear as it formed material in the area of a weld (see also passes through the die aperture. The roughness section 5.7.6.5). In the extreme case variations of the section is therefore usually higher than in the billet structure and in the through working that of the die bearing. occur as regions with different tones on the same In the same way as the friction load as an flat polished surface of a section. integral value is strongly dependent on the

Fig. 5.19 Highly deformed peripheral zone material (light) and normally elongated core material (dark) at the end of extrusion [Ake 88a] Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 215

length and the angle of the bearing surface, the num nodules that adhere to the bearing surfaces locally effective mechanisms of friction and form grooves on the section surface. The for- wear of the surfaces of the section and die vary mation of layers of aluminum, which can be in- along the die bearing. terspersed with oxide, increases the number of The following possibilities have to be consid- adhesion points opposing the extrusion direction ered, as shown in Fig. 5.20: [The 93]. The nodules are finally torn off with ● Slip of the section as a solid body with a the section and appear as particles (“pickup”) on sudden speed increase at the steel surface the surface [Tok 88, She 88, The 93, Abt 96]. ● Formation of a coherent layer of aluminum In the majority of cases, die bearing lengths adhering to the bearing surface and slip of are less than the section thickness. If the bearing the solid section with a sudden speed in- surfaces are parallel, mixed friction occurs over crease between the section and the bonded the entire length. The surface roughness of the layer extruded product is governed by the adhered ● Adhesion of the section peripheral layer on layer, i.e., by the tendency for the material to the bearing surface, parabolic speed distri- adhere to the die [Mue 96, Mie 62, The 93]. This bution across the section thickness adhesion tendency cannot be completely sup- pressed by nitriding the bearing surfaces, but the Slip exactly corresponding to “the first point” is aluminum layer builds more slowly on nitrided not seen in hot extrusion of aluminum without die bearings. In the absence of lubrication, the lubrication. aim for a high surface quality is not the maxi- A friction “ab” with adhesion of discrete mum possible undistorted slip of aluminum over aluminum particles on the bearing surfaces cor- steel (slip process (a) in Fig. 5.20) but the most responds most closely to the experience of die uniform buildup of a thin adhered layer of alu- correctors and with the nitriding of bearing sur- minum on the die bearing surface. This aim is faces. Microscopic investigations on split die in- best fulfilled by die bearings ground perpendic- serts confirm that aluminum particles bond to the ular to the extrusion direction. Die surfaces bearing surfaces to an increasing extent and size ground or polished parallel to the extrusion di- in the extrusion direction and determine the mi- rection or polished favor an irregular buildup of crogeometry of the section surface. The alumi- the adhered layer in the form of long elongated and correspondingly wide and high ridges. Fig- ure 5.21 [Tok 88] shows the roughness variation over the length of a flat section on a small re- search press. The surface roughness of the extruded prod- uct decreases as the bearing length increases be- tween lk 0 and lk 1 to 3 mm because with increasing back pressure the adhesive layer re- gresses at the entry edge. As the bearing length increases, the roughness increases again as a consequence of higher temperatures [Wel 96] and possible mechanical activation processes [She 88] (Fig. 5.22). The optimum has been found to be bearing lengths of the order of half the thickness for flat sections [Mie 62, Tok 76, Tok 88, She 88, The 93]. As the number of extrusions increases, the fine aluminum nodules combine to form a layer that ultimately completely covers the die bearing surface. The relationship between the surface roughness of the extruded product and the buildup of the adhesive layer on the slip surface is shown in Fig. 5.23 [The 93]. In the initial region 1, no aluminum adheres to the bearing surfaces so that section roughness Fig. 5.20 Velocity distribution transverse across the die ap- erture is governed by the surface finish of the die. In 216 / Extrusion, Second Edition

Fig. 5.21 Influence of the finish of the bearing surface on the quality of the section surface [Tok 88] region 2, the aluminum adhesive layer builds up For a given billet temperature, the roughness, and the section roughness increases rapidly. the degree of pickup, and the scatter of the gray When the bearing has been completely covered, scale (as a measure of the degree of streaking) dynamic equilibrium develops in region 3 be- increase to a maximum with increasing extru- tween adhesion and tearing away of aluminum sion speed and then decrease at even faster particles so that the surface roughness remains speeds. This is explained by the hypothesis that almost constant. The section roughness is sig- at higher speeds the section surface slides over nificantly lower than the roughness of the ad- a thin layer of almost liquid metal [Par 96]. hered layer. A reduction in the section roughness Alloy type, alloy content, and differences in was periodically observed in trials involving 180 the thermal pretreatment of the billets can have extrusions, and this was attributed to a partial or a significant influence on the extrusion speed complete detachment of the coating. that can be achieved for given surface finish re- The angle of the die bearing has a significant influence on the friction force and results in a different formation of the coating [Mue 96]. However, it has only a small influence on the section roughness. Hand-filed bearing surfaces close up at the entry and open up at the exit. Mixed friction dominates on bearings that are not too long, yet the changes in the face pressure and the angle result in a different formation of the coating. In the opening exit area loose ad- hesions form and damage the section surface by die lines and pickup [Clo 90]. As the billet temperature increases, the ten- dency for adhesion between the section surface and the bearing surfaces increases [Mie 62, Tok Fig. 5.22 Influence of the bearing length on the roughness 88, Clo 90]. Ra of the section surface [She 88] Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 217

quirements. Precipitates of alloying constituents remains in compression, but a tensile stress pre- have the worst effect. vails in the peripheral zone. If this stress is too low to produce plastic strain, the section can 5.7.4.4 Transverse Cracks and Peeling flow as a solid body through the die aperture (Fig. 5.24a). Higher tensile stresses can result in The tensile stresses produced in the peripheral an elongation of the hotter and at the entry layer by the friction on the bearing combined slower flowing peripheral layer. This is seen par- with the considerably higher temperature [Moo ticularly on thin ribs and internal legs of hollow 96, Joh 96] and the correspondingly lower flow sections (Fig. 5.24b). The elongation of the pe- stress result in defects, which can penetrate deep ripheral layer after the removal of the dominat- into the section from the surface. Figure 5.24 (aÐ ing high mean compressive stress in front of the d) shows the qualitative variation in the velocity die results in various types of material defects, and the stress at the entry to the die aperture. depending on the structure and the peripheral The material being formed flows from the de- layer temperature. Lubricant residues that inad- formation zone into the die aperture. The profile vertently have reached the inner wall of the con- core flows faster than the surface section, which tainer can flow into the section following path 1 is retarded by the friction at the surface. The core in Fig. 5.14 and form an interface parallel to the surface. The surface layer can break away dur- ing extrusion (peeling) (Fig. 5.24c). In other cases, the material of the interface can break up during a subsequent heat treatment, and the sur- face layer bulges out in the form of a row of blisters. Peeling is also caused by the melting of isolated eutectic regions resulting from unfavor- able casting and homogenization conditions [Rei 84, Lef 96]. A classic defect process is the formation, growth, and the amalgamation of voids resulting in transverse cracks that penetrate deeply into the section (fir tree) (Fig. 5.24d), The propaga- tion and expansion of the cracks occur at the location of the plastic elongation of the periph- eral layer. At the same time the transfer of the tensile stresses into the billet is prevented so that less material flows into the peripheral layer. The volume deficit in the peripheral zone is therefore considerably larger than in the case of plastic elongation as shown in Fig. 5.24(b). In the common aluminum wrought alloys the melting of individual structural regions, e.g., along grain boundaries, is the defect initiating process (hot shortness). The transient exceeding of the solidus temperature in the peripheral layer is the most common cause of transverse cracks. The average section temperature measured at the exit is naturally well below the solidus tempera- ture. Another type of transverse crack that can oc- cur well below the solidus temperature occurs in materials with a high fraction (Ͼ15%) of non- plastic brittle structural components. 5.7.5 Procedures to Control the Thermal Balance As a general rule, the optimal productivity Fig. 5.23 Relationship between adhesive layer formation, friction stress, and surface roughness [The 93] and quality with aluminum alloys can only be 218 / Extrusion, Second Edition

achieved by taking the thermal balance of the perature below the desired average exit section extrusion process into consideration. The tem- temperature. The material reaches the desired perature control is optimized by measures that value only as it passes through the deformation either reduce the amount of heat that has to be zone. This is described in detail in Chapter 3. removed or accelerate the heat transfer [Ake 71, A uniform temperature gradient from the Ake 80]. These measures have been described front to the rear of the billet can compensate to some extent along with the extrusion process approximately for the increasing heat from the (Chapter 3). The technicalities of temperature shear deformation of the peripheral layer. Ad- control are described in more detail in Chapter 6. ditional heating of the front of the billet reduces the load needed at the start of the extrusion pro- 5.7.5.1 Material cess and ensures that there is sufficient solution heat treatment effect at the front of the section. The heat produced during the extrusion pro- This method of achieving “isothermal extru- cess from the transformation of the mechanical sion,” however, requires rapid localized heating work can be minimized by selecting the most or cooling because of the good thermal conduc- easily extruded alloy from those that meet the tivity of the aluminum. The billet then has to be product specification (see the section ”Materi- extruded with minimum delay. als”). In general, the effectiveness of the procedures The additional fine adjustment covers the op- in this first group increases as the billet cycle timization of the composition, the solidification time decreases, for example the extrudability of conditions, and the thermal treatment of the bil- the alloy improves. let. Small changes appear to result in consider- able differences in the extrusion speeds that can 5.7.5.3 Heat Removal be achieved [Ake 71, Sta 90, Rei 84, Sca 69, Lyn 71, Lan 82, Spe 84, Lan 84]. The opposite applies to the following proce- dures, which require heat flow into the tooling and which are most effective for slow extrusion 5.7.5.2 Billet Temperature speeds and long cycle times. The exit temperature is higher than the billet Container. The most important heat sink is preheat temperature because of the deformation the container, which plays an important role in induced heating of the material being deformed; the productivity in the extrusion of aluminum therefore, the billet is usually heated to a tem- alloys. The container is basically heated to a

Fig. 5.24 Influence of tensile stresses in the section peripheral layer. 1, appearance of the extrusion; 2, velocity variation across the section; 3, variation of the longitudinal stresses across the section Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 219

temperature below that of the specified billet depending on the thickness and the coolant. This preheat temperature. The temperature difference can be compared with the drive power of a press is usually limited to 50 K in order to avoid the of up to 400 kW. billet sticking in the container because of inade- The approximate calculations show that with quate press power (“sticker”). The thermal bal- the fast extrusion of alloys with a high extrud- ance in the system container-billet is described ability, only a small amount of deformation heat in Chapter 3. can be removed through a cooled die. With a The temperature set or recorded on the con- plate thickness of 50 mm, an aluminum flow of tainer heater controller applies only to the hottest 1 kg/s can at best be cooled by approximately 6 location, where, from experience, overheating of K with water and by only approximately 2.5 K the tool steel has to be avoided. The actual tem- with liquid nitrogen. perature field in the container is shown in the A significant reduction in the exit temperature example in Fig. 3.19. There are temperature dif- is achieved only with the slow extrusion of the ferences of the order of 100 K in the axial and difficult-to-work alloys. In trials it has been pos- radial direction. The temperature maximum sible to increase the exit speed of an AlCuMg moves during 5 to 10 extrusions from the heat- alloy by 30% using water cooling. Approxi- ing zone to the liner inner face [Ake 71, Ake 72, mately 15% of the deformation heat can be re- Sch 79]. moved by the water cooling [Bat 79]. The cooling, i.e., the heat flow from the con- The limited resistance of the die materials to tainer bore through the wall and into the atmo- temperature fluctuations makes water cooling of sphere, is determined largely by the thermal con- a die difficult. ductivity of the container wall. Cooling of the Liquid nitrogen is usually fed through chan- mantle surface by an intensive air blast has only nels machined either in the die itself [Sca 79] or an insignificant effect on the cooling. Blowing more commonly in the front face of the backer onto the container end face has more effect. [Wag, Sel 84, Yam 92]. If a separate feeder plate Today, combined heating and cooling systems is used, an additional network of cooling chan- are installed in containers for temperature con- nels can be machined in the rear face so that the trol (see the section on tooling in chapter 7). die is cooled from the front and the back [Ros]. Die. Removing excess deformation heat The aim is for the nitrogen fed in as a liquid through the die is helped by the short thermal to evaporate extensively as it flows through the path from the deformation zone to the location cooling channels. This gives an approximately of the cooling channels. Another advantage is constant cooling capability, the effect of which that the cooling takes place during the final stage is distributed over a more or less large aluminum of the deformation so that the load requirement material flow, depending on the extrusion speed. is increased only during the final deformation In experiments with nine different alloys, the stage. There is also the possibility of specifically extrusion speed of AlMgSi0.5 could be in- cooling the critical regions, e.g., at the edges creased by approximately 8% by cooling, but prone to tearing. with the more slowly flowing alloy AlZn- Because a significant amount of heat has to MgCu1.5, by a significant 80% [Sca 79]. flow through a relatively small die cross section, The dwell time of the deformation material in it is necessary from the cooling point of view to the die region is short so that the peripheral zone have an intense heat transfer to a coolant with of the section is cooled the most. This reduces sufficiently large thermal capacity. the temperature peaks, which are largely respon- This requirement is more easily fulfilled with sible for the tearing of the section surface at high liquid coolants than gas. On this basis, the fol- extrusion speeds (Fig. 5.25) [Sel 84]. lowing methods can be differentiated for die This is the reason why even with limited cool- cooling: ing effect the speed can often also be increased considerably with fast-flowing alloys. The evap- ● Water cooling orated nitrogen emerges from behind the die ap- ● Cooling with liquid-supplied evaporating ni- erture onto the surface of the newly extruded trogen section to utilize the protective atmosphere ef- ● Flushing with gaseous nitrogen fect (see below). Approximate calculations demonstrate that Gaseous nitrogen is fed through channels in the cooling capacity through a plate 200 mm in the gap between the die and the backer into the diameter is in the range between 2 and 12 kW, space behind the die exit. The mass flow of the 220 / Extrusion, Second Edition

nitrogen stream is usually not more than 1% of application to the optimisation of the extrusion that of the aluminum stream because of the low parameters will now be considered. Apart from density. The nitrogen flow is therefore not suf- other limiting additional parameters the extru- ficient to cause any serious cooling. sion parameters are at an optimum if the exit The regularly observed improvement in the temperature over the entire cross-section falls section surface must therefore be more attribut- within the permitted range for the alloy under able to an influence on the dynamic equilibrium consideration, the extrusion speed is as high as of adhesion and tearing off of the aluminum par- possible in the range limited by the equipment, ticles on the die bearing by the reaction of the and the extrusion load needed uses the maxi- newly formed aluminum surface with the at- mum press power of the press within a specific mosphere. safety factor [Rup 77, Rup 77a, Rup 83]. Mandrel. In contrast to heavy metal extrusion The parameters that can be set at the press are where cooling of the mandrel is unavoidable be- the ram speed, the initial billet temperature, and cause of the high thermal loading, in aluminum the container temperature. The billet tempera- extrusion, mandrel cooling is not required to compensate for high temperatures. In practice, ture and speed can be constant for a heating and mandrel cooling is therefore not used. However, extrusion cycle or be decreasing from the front temperature measurements on the extruded pro- to the back and then be adjusted in subsequent file have shown that the specific heat removed extrusion cycles. by cooled mandrels is at least equal to that as- Of all the set parameters, only the extrusion sociated with die cooling with liquid nitrogen so speed can be varied during a single extrusion that it appears to be possible to achieve higher cycle. The billet temperature and the tempera- maximum extrusion speeds by extruding over ture profile can, in contrast, be set for each cycle cooled mandrels [Rup 82]. before loading into the press. A change in the average temperature or the temperature profile 5.7.5.4 Optimization of the is possible in experimental work and under pro- Extrusion Parameters duction conditions in the next billet. The tem- Following the discussion of the different perature profile in the container with the usual means of influencing the thermal balance their arrangement of heating elements in the container mantle follows a change in the set temperature only after a long delay of several hours. A rapid change in the container temperature can be achieved with heating and cooling systems in- stalled in the liner [Har 94]. The parameter that has to be controlled is the section exit temperature; however, there are nu- merous difficulties in its continuous measure- ment, particularly in the case of aluminum alloys:

● The critical maximum temperature occurs in an unknown location in the die aperture that is practically impossible to access for mea- surements. ● The critical temperature occurs for only a very short time. ● The section surface is soft and easily dam- aged. ● The radiation is in the invisible infrared range. ● The emissivity of bright aluminum surfaces is very low and also is strongly dependent on the roughness and the wavelength (it is not a “gray” body). ● The measurement area is surrounded by Fig. 5.25 Temperature variation in the die with and without cooling with liquid nitrogen [Sel 84] strong sources of additional radiation. Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 221

● The display of all temperature sensors is as- Special applications of joining in extrusion sociated with definite delays and with sig- are cladding or locally reinforced sections of two nificant measurement scatter. different materials (two aluminum alloys) (alu- minum-copper, aluminum-steel), the compac- The original concept of isothermal extrusion tion and deformation of powder and granular assumed a continuous control of the speed based scrap, and the production of composite materials on the measured exit temperature [Lau 60]. This with a metal matrix. This is described later in concept is difficult to achieve because of the dif- this chapter. ficulties mentioned previously, as well as the de- layed response of the exit temperature to a 5.7.6.1 Extrusion Welding change in the speed associated with the heat con- tent of the material in the press. Changing the The general metallurgical principles of bond- speed with a rapid reacting control system would ing by extrusion are discussed in Chapter 4. The result in an unstable reaction in the control loop. following section describes how the extrusion The control systems used or proposed and process relevant properties of the aluminum al- tested today attempt to cope with these problems loys influence the technology of extrusion weld- associated with exit temperature measurement in ing and the quality characteristics of the ex- various ways. All these control systems are truded welds. based on optimization concepts with the aim of Aluminum alloys can be extruded in a tem- maintaining the exit temperature within the de- perature range at which the tooling is not over- sired range by suitable combinations of the heated so that the stability and service life of available parameters and simultaneously maxi- even complex hollow section dies are adequate. mizing the productivity of the press. One factor The high adhesion affinity for steel results in to take into account is that every change of one adhesive friction in the die at areas with high parameter changes all heat sources and heat surface pressures and to the formation of dead flows, i.e., the factors that combine to give the metal zones (Fig. 5.26). exit temperature [Ake 80]. This is described in After only a short time in air, newly formed more detail in Chapter 6. aluminum surfaces are coated with an oxide layer that cannot be reduced by any gas atmo- sphere or removed by diffusion of the oxygen 5.7.6 Joining by Extrusion into the parent metal. Aluminum therefore be- The production of complex sections with one longs to the group of metals that cannot be or more cavities plays a particularly important joined without macroscopic shape change, i.e., role in aluminum alloys. Approximately half the not by mere pressure and heat (diffusion weld- sections produced in the easy to moderately dif- ing) [Jel 79, Bry 75]. ficult to extrude AlMgSi alloys are hollow sec- tions such as box beams and hollow engineering plates. Compared with the functional corre- sponding solid sections (I-beams, plates with T- shaped reinforcements) the hollow sections have the advantage of a much higher torsional and bending stiffness. With the exception of tubes in the strictest sense, the hollow sections have lon- gitudinal welds where the metal streams split by the supporting legs of the mandrel reunite in a metallic bond. The joining of the metal from billets extruded one after the other by transverse welds is used for products in long lengths and large coil weights. In addition, having a material residue in the die entry or a feeder chamber provides exact positioning of the front of the section, which is essential for automation of the guiding of the front end. The next billet has to join to Fig. 5.26 Laminar material flow [Ake 92]. 1, adhesive fric- the residual material in the die. tion; 2, dead metal zone 222 / Extrusion, Second Edition

In order for two aluminum bodies to join in deviation processes (Fig. 5.27). In the first stage the solid state, these brittle separating oxide lay- (Fig. 5.27a), the billet is divided into two or ers require a significant combined deformation more streams that flow together under the legs to break up the oxide skin and to bring nonox- (Fig. 5.27b, c). In the deformation final stage idized metal to the joint. The generic term de- (Fig. 5.27d), the hollow section forms by the formation welding includes roll cladding, com- metallic bonding of the streams. posite impact extrusion, and powder extrusion, The processes at the contact areas are related as well as extrusion welding. It is possible from to the material flow (Fig. 5.26). The conditions observations of these processes using general on the exit side of the leg are critical where ei- metallurgical basic principles to draw conclu- ther a dead metal zone or a cavity can form. sions on the processes (previously studied only Normally, the pressure in the welding cham- to a limited extent) that occur during bonding by ber is so high that the space under the legs is extrusion. completely filled and a dead metal zone forms. The influence of the initial unevenness of the The conditions for the formation of a dead metal surfaces is also important [Ast 75]. The adhesion zone and for a good weld have been expressed process is the result of the plastic flow of the by several authors in the form of ratios that can metal in the contact zone, the deformation of the be deduced from the die geometry and provide rough tips, the formation of slip and shear bands a reference point for the magnitude of the total in the region of the contact surfaces, and the deformation. As well as the size of the welding movement of the dislocation fields of the indi- chamber, the ratio of the welding chamber cross vidual contacts. Uneven contact areas, however, section to the profile cross section is important rarely occur in longitudinal welds and then in [Gil 75]. conjunction with cavities under the support legs The pressure applied at the point of formation of the mandrel system. of the longitudinal weld is composed of com- ponents for the reduction in cross-sectional area 5.7.6.2 Longitudinal Welds in from the welding chamber to the section, for the deviation in the flow of the metal stream and for Hollow Sections overcoming the friction in the die aperture. When applying these thoughts on deforma- Oxide can only enter the welding chamber tion welding to extrusion welding, it is assumed with the first filling of the die when the free ox- that the latter consists of three sequential extru- ide coated front faces of the streams meet (Fig. sion processes and the associated transport and 5.28a) [Ake 92]. The fractured residues of these

Fig. 5.27 Extrusion of hollow sections. (a) Straight into the ports. (b) At an angle under the legs. (c) At an angle between two mandrel heads. (d) Emergence of the section [Ake 92] Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 223

oxide films are largely removed with the front forming on the exit side of the leg (gas pockets) part of the section (Fig. 5.28b). Further bonding [Val 95]. occurs between material regions that initially Two-dimensional numerical simulation and have been upset in front of the legs and mandrels experimental semiplanar extrusion both showed and have then flown along the flanks of the leg the formation of cavity with a section thickness with severe shear distortion to collect in the dead of 25 mm and a dead metal zone with a section metal zone under the legs. The streams that meet thickness of 12.5 mm for the same welding together have no free surface and, therefore, no chamber height (Fig. 5.29) [Wel 95]. roughness and no oxide film. The welding pro- The contact with the leg is lost under the con- cess then merely consists of the initially small ditions where a cavity is formed, and it is the contact area being extended over the total length free more or less roughened surfaces of the of the section and thus being enlarged by several streams that meet in the welding chamber. orders of magnitude, whereby the subgrains also The bonding is then restricted to the rough continuously reform over the original contact ar- crests [Val 95, Val 96], has little ductility, and is eas. This repolygonization is associated with the above all very brittle [Ake 92]. Voids in the joint movement of dislocations but does not require are recognizable in the fracture as bands without diffusion of atoms. fracture dimples and they are preferentially Die geometries where the cross section of the etched in the cross section by alkaline etchants. material flow path is too narrow should be Transverse cracks can also occur in the region avoided because these result in inadequate feed of the longitudinal welds. In principle these are into the weld chamber and in an inadequate no different to the surface and edge cracks that pressure buildup. This can result in cavities occur on solid sections when the peripheral layer

Fig. 5.28 Breakup of the surface layer in longitudinal welds. (a) Meeting under the leg. (b) Breaking apart. (c) Clean longitudinal weld [Ake 92]

Fig. 5.29 Formation of a hollow space (gas pocket) under the mandrel support leg at a low extrusion ratio (a) and a dead metal zone at a higher extrusion ratio (b). On the left, flow lines; right, lines of equal strain rate [Wel 95] 224 / Extrusion, Second Edition

reaches the hot-shortness temperature [Val 96a]. other than be pulled apart. In the most unfavor- The fact that the transverse cracks occur pref- able case, there is no bonding [Ake 72], particu- erentially next to the longitudinal welds can be larly with the current practice considered to be attributed to the severe heating of the peripheral unavoidable of lubricating the dummy block and layers of the metal streams, particularly under the discard shear [Joh 96a]. the legs [Ska 96]. With highly stressed sections, a length long enough to contain the tongue of the transverse 5.7.6.3 Transverse Welds weld is removed as scrap from the stop mark. This section usually includes one to two times Transverse welds are usually the severely the content of the entry zones and the welding curved surfaces where the material of the next chambers. billet bonds with the residue of the previously In most of the transverse weld the fractured extruded billet. This residue can be located in residues of the oxide layer are extensively pulled the container, in the feeder chamber of a die for apart. These profiles can be supplied with the a solid section, or in the entry ports of a hollow transverse weld for noncritical applications pro- die (Fig. 5.30aÐc). viding the material surrounding the tongues, The process where the residue is left as a thick which originates from the previously extruded discard in the container (Fig. 5.30a) is usually billet, can withstand the stresses from the used for the extrusion of cable sheaths and elec- stretching of the section. tric conductors in large coil weights. Special When the method of formation is considered, measures have to be taken to avoid smearing of it is easy to understand that there is a greater risk the contact surfaces. of transverse welds being impaired by surface The process shown in Fig. 5.30(b) and (c) in coatings than longitudinal welds. When material which the discard is removed in front of the separations apparently occur in longitudinal feeder chamber or in front of the entry ports is welds, usually the transverse welds running common for aluminum sections. Stripping of the along both sides are involved [Val 95]. discard (Fig. 5.30d) is also possible with alu- The most important sources of defects in ex- minum but increases in difficulty with more trusion welds are [Wei 92]: complex sections. The originally flat contact surface between the ● Lubricant or corrosion on the dies residue and the next billet is deformed to a ● Impurities on the billet front surface or dirt tongue because of the adhesion of the aluminum on the billet surface to the tool surface. The outline of the tongue ● Incorrect die design approximates to the contour of the correspond- ing material stream. In the region of the tips of Basically, all parts that come into contact with these tongues the contact surface can contract by the material being extruded must be clean and, as much as 50% so that the fractured residues of in particular, be free of oil, grease, or graphite. the oxide layer are more likely to slip over each The requirement for perfect separation of the

Fig. 5.30 Extrusion of aluminum “billet on billet.” (a) Extrusion of a billet on the discard R in the container. (b) With feeder die. (c) In the ports of a porthole die. (d) Pulling off the discard in a bridge die to avoid transverse welds [Ake 92] Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 225

dummy block and the discard, however, results rations and laminar enrichments of noncoherent in practice in compromise solutions in which particles to be detected. For example, the specific lubricants are considered to be indis- tongues of transverse welds can be localized. In pensable but have to be used as little as possible fatigue studies the quality of the extrusion welds and accurately applied. does not have a significant influence on the total number of cycles but does on the rate of growth of a crack in the extrusion weld at the end of the 5.7.6.4 Testing of Extrusion Welds endurance. The extrusion weld has to be considered as a Various Mechanical Tests. A critical me- surface of which a fraction fD is covered with chanical test for extrusion welds requires that the fragments of the original surface coating [Ake strain is reached at which failure will occur in 92]. The remaining fraction f1 consists of me- the parent material or in the weld. This critical tallic ligaments with the same flow stress as the strain is strongly material dependent because the parent material. These ligaments are very short energy released from the material separation in relationship to their width and can therefore (“the damage potential”) grows with the square withstand a high three-dimensional tensile stress of the elastic limit. Various technological testing under tensile loading. procedures [Gil 75] enable the controlled trans- In a tensile test perpendicular to an extrusion verse of strains that correspond in tensile tests weld the specimen can completely neck outside approximately to the region of uniform elonga- the weld. In the plastic region where the limit of tion up to fracture necking: elastic deformation is exceeded many times ● over, failure in the extrusion weld can still occur. Bending tests on samples taken transverse to The concentration of fragments of the surface the extrusion welds. The limiting bending coating on the contacting surfaces facilitates the angle is measured at a section dependent damage with increasing deformation, i.e., the specified bending radius. ● Folding tests along and between the welds formation, the growth, and the combination of Ϸ pores to form microcracks and, finally, failure. up to an internal radius 0 and elongation of the outer surface of Ϸ100% The usual parameters of 0.2% proof stress and ● the tensile strength from the tensile test have Axial compression of a tube section to the point of folding limited relevance to the quality testing of extru- ● sion welds. The increased sensitivity to damage Bending of tubes with a roll bending system in the region of the extrusion weld compared under longitudinal loading of the extrusion weld with the parent material is best expressed by a ● decrease in the reduction in area at fracture. The Expansion with a conical mandrel and mea- quicker the extrusion weld is damaged, the surement of the strain in the circumferential lower is the strain to fracture [Ake 92]. With direction decreasing extrusion weld quality, the elonga- These technological testing procedures are suit- tion and, in the extreme case, the tensile able for following quality variations along and strength, are also impaired as well as the reduc- between profiles with the same cross section and tion in area. the same material and for monitoring the main- Structural Investigation. Because the infor- tenance of tolerance limits that have been spec- mation from tensile tests is limited, numerous ified for the individual application. However, other testing procedures are used for testing the they do not supply any characteristic values that quality of extrusion welds. The location of the could be used for numerical validation of the longitudinal and transverse welds can be seen in safety of a design. The “calibration” of a tech- the macrostructure. If a section is cut from an nological testing process from the point of view extrusion at an externally visible interface be- of a fracture mechanics test value is still an un- tween two billets, it is possible to identify from resolved problem. the structure between neighboring transverse welds whether sufficient material has been cut 5.7.6.5 Influence of the Extrusion Weld out to achieve sound material. An extremely in- tensive etch attack will reveal the inclusion of a on the Surface Appearance lubricant or the peripheral layer. Although it is only a few atomic spacings Ultrasonic Testing, Fatigue Investigations. thick, an extrusion weld can affect the appear- Ultrasound enables both hidden material sepa- ance of the anodic film on a visible surface in 226 / Extrusion, Second Edition

the form of a band several times wider than the wall thickness [War 95]. Depending on the in- cidence angle of the light, the area of the same Materials extrusion weld can appear to be lighter or darker than the rest of the surface. The primary cause Gu¨nther Scharf* of this optical contrast between the region of the extrusion weld and the residual material is the The selection of the alloy for extrusion is usu- heavier working of the peripheral zones of the ally based on achieving a given property range split streams compared with the core zone. In with specific surface properties at the lowest the region of the weld the section consists of possible cost. These include the costs for the bil- heavily deformed material throughout its full let; the actual extrusion; the heat treatment; and thickness, which results in variations in the size the additional operations including stretching, and orientation of the grains and thus the reflec- detwisting, and cross-sectional correction. tion behavior. In contrast to rolling where the strength is in- creased significantly by cold working, extruded products are hot worked and, therefore, the ma- 5.7.6.6 Suitability of the Material for terial condition is practically soft. Cold working Hollow Section Extrusion can be considered for wires and tubes, which are drawn to their final dimensions. However, alu- The selection of the material and the extrusion minum alloys are usually extruded to the final temperature as well as the die geometry and the dimensions and the optimal mechanical proper- avoidance of impurities play an important role ties are produced by a suitable heat treatment. in determining the practical applicability of ex- With the exception of the isolated use of sil- trusion welding. As a basic rule, all wrought alu- ver, the usual alloying additions to aluminum minum alloys can be extrusion welded [Ake alloys have only a slight effect on the cost of the 72a]. The extrusion pressure, which is already billets. However, the influence on the cost of ex- increased by the flow through narrow angled trusion, heat treatment, and correction is much channels, increases with increasing flow stress higher. to values that overload the mandrel. However, The influence of the most common alloying hinge sections with a small core (the hole for the additions, magnesium, copper, silicon, and zinc, pivot) are extruded with bridge dies in the high- on the flow stress can be found in section 5.5 strength AlCuMg1.5 and AlZnMgCu alloys and in Fig. 5.12 in section 5.7. [Cre]. Engineering sections with large cores are Magnesium increases the flow stress the most usually produced in the alloys of the group followed by copper, whereas silicon only has a AlMgSi0.7, AlMgSi1, and AlZnMg. small influence and zinc, practically no increase Only the strength and toughness of the parent [Ake 70]. The influence of intermetallic phases, metal in the thickness direction is achieved in in particular of Mg2Si, has also been extensively the transverse direction across the weld because studied. This has shown that it is not only the of the fiber orientation in the region of the ex- amount of the alloying constituents that in- trusion weld. In alloys that have a high manga- creases the flow stress and thus reduces the ex- nese content to obtain the extrusion effect, trans- trusion speed, but also that the microstructure in verse sections frequently exhibit lamellar tears which the alloying constituents occur plays an along surfaces parallel to the extrusion weld, important role [Sca 69, Gru 66, Sca 69b]. Su- which are enriched with eutectically precipitated persaturated dissolved or fine dispersions of pre- particles. The mechanical properties in the re- cipitated Mg2Si increase the flow stress and thus gion of the weld can also be impaired by coarse- make deformation more difficult, whereas grain recrystallization of heavily worked mate- coarse precipitates of Mg2Si phases reduce the rial regions. There are alloys available for flow stress. hollow engineering sections that recrystallize The influence of the structure has a particu- completely and that include peritectically solid- larly significant effect on those alloying ele- ifying transition elements instead of manganese ments that tend to supersaturation during solid- [Scw 84]. ification and later precipitate during subsequent With the more easily extruded alloys, mainly AlMgSi0.5, a complete fine-grain recrystalliza- tion is desired to give the decorative appearance. *Materials, Gu¬nther Scharf Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 227

billet heat treatment as a reversible fine disper- treatment costs and correction processes also in- sion. The influence of manganese, in particular, crease with higher alloy contents. is strongly dependent on the precipitate state and The heat treatment of age-hardening alloys on the number and fineness of the intermetallic basically consists of the processes (see also particles [Gru 66]. Intermetallic phases includ- Chapter 4): ing Al Mn, MnFeSi, Al Zr, and FeAl reduce 6 3 3 ● the extrudability significantly [Sca 67, Sca 69a]. Dissolution of the alloying constituents ● Figure 5.31 shows the product strength of the Quenching to retain in metastable solution ● most common aluminum alloys as a function of Age hardening, usually hot, in one or more the flow stress at the extrusion temperature. temperature stages The majority of extruded products therefore An important simplification is possible if the are produced in heat treatable alloys with the product leaves the press with a temperature in lowest possible alloy content. The desire to re- the region of solution heat treatment and can be duce the magnesium content by a tenth or even immediately quenched to achieve the desired re- a hundredth of a percent is due to the strong sult. This requires that the interval between the influence of the flow stress on the cost of extru- solution temperature and the start of melting is sion as well as the downstream processes. The greater than the unavoidable temperature error extrusion load required is proportional to the and differences along the length of and across flow stress as is the deformation work converted the section. In other cases an optimum solution into heat. Another important consideration is heat treatment can be achieved only by a sepa- that the time needed for this heat to penetrate rate operation in a very accurately controlled into the tooling increases with the square of the oven. In this case, the material cannot be “press flow stress, and the extrusion speed decreases quenched.” accordingly. Apart from the dead cycle time, As the alloy content is reduced, the precipi- which is largely independent of the material, the tation pressure decreases. The section does not machine costs of the extrusion process alone in- have to be quenched so quickly but can be crease to the cube of the flow stress. The heat cooled using less drastic means. This has the ad- vantage that distortion resulting from excessive localized temperature differences is avoided. Thin AlMgSi0.5 sections can be adequately cooled in static air or under fans. A more inten- sive heat transfer is necessary for thick wall sec- tions and higher alloy contents (e.g., Al- MgSi0.7). This can be obtained with high air velocities (nozzles) or air water mixtures (mist nozzles) [Kra 93, Str 96]. The cooling require- ments provide an additional material-dependent cost factor partly related to the cost of the cool- ing system but mainly because of the correction costs that rapidly increase with increasing cool- ing rate. Because the extrusion load depends mainly on the flow stress kf, the wrought aluminum alloys are classified according to the flow stress into the groups easy to extrude, moderately difficult, and hard to extrude. Thus

Յ 2 Easy to extrude kf 30 N/mm 2 Moderately difficult kf 30 to 45 N/mm 2 Hard to extrude kf 45Ð57 N/mm

On top of this, there is the increasing investment Fig. 5.31 0.2% proof stress of extruded aluminum alloys in the standard as-supplied condition as a function in equipment and the cost of heat treatment and of the flow stress at the usual extrusion temperatures [Ake 71] correction as well as the quality control. 228 / Extrusion, Second Edition

The material behavior during plastic working curve is a material parameter for the ideal defor- can be derived from high-temperature torsion mation and depends on the working temperature flow curves. These show the deformation load kf and, in particular, the rate of deformation. as a function of the logarithmic principal strain Figure 5.32 shows the torsion flow curves for u. It should be emphasized here that the flow the extrusion alloy AlMgSi0.5 at 450 ЊC and a

Fig. 5.32 Torsion flow curves of AlMgSi0.5 [DGM 78] Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 229

logarithmic strain rate of du/dt 0.1 s1 as a and AlMgSi0.7, are considered to belong to the function of the logarithmic principal strain ug easily extruded alloys. Because AlMg1 and between 1 and 7.5. AlMgSi0.5 are frequently used for anodizing or The flow stress of the most important extru- bright finish applications, care must be taken to sion alloys is shown in Table 5.4 at a logarithmic ensure that the Mg and Mg2Si components are principal strain of ug 2.7. This corresponds dissolved in the solid solution in the as-extruded to an extrusion ratio of 15:1. The kf values listed state. Also, none or, if necessary, very small ad- illustrate the different deformation properties of ditions of insoluble or supersaturated dissolved the individual extruded alloys. These were ob- elements such as manganese, chromium, zirco- tained from torsion flow curves taken from both nium, or iron may be used. For this reason, the DGM Atlas of Hot-Working Properties, Vol- AlMg1 and AlMgSi0.5 are produced from a ume 1 [DGM 78], and internal investigations at base metal of a higher purity for bright finish VAW Aluminium, Bonn, Germany. qualities. In DIN EN 573-3 the individual alloys The aluminum alloys are designated below by are shown based on 99.85, 99.9 and 99.98 Al. If the chemical symbols, which was the custom un- this is not taken into account, secondary precip- til recently. Table 5.5 compares the material des- itates can form during billet heat treatment and ignation of the old DIN 1712 T and 1725 T with these can impair the surface quality of the ex- the new DIN EN 573-3. truded sections by streaking. The billet heat treatment—referred to as 5.8 Easily Extruded Alloys homogenization—of AlMg1 is mainly to reduce crystal segregation and dendritic residual melt- Յ 2 Alloys with low kf values (kf 30 N/mm ) are usually classified as easily extruded. To a first approximation, the lower the required flow stress the better is the workability. Normally the Table 5.5 Comparison of the aluminum alloys possible exit speed increases with the workabil- according to DIN and DIN EN ity [Ake 68]. DIN 1712 T3/DIN 1725 T1 DIN EN 573Ð3(a) 5.8.1 Aluminum Alloys Symbol Symbol No. Easy-to-extrude alloys In addition to aluminum, the naturally hard Al99.5 Al 99.5 1050A alloys AlMg1 and AlMn1, as well as AlMgSi0.5 Al99.8 Al 99.85 1085 Al99.98R Al 99.98 1098 E-Al E Al 99.5 1350 AlMg1 AlMg1 5005A Table 5.4 Flow stress of different aluminum AlMn1 AlMn1 3103 alloys AlMgSi0.5 AlMgSi 6060 AlMgSi0.5 AlMg0.7Si 6063 Experimental material: homogenized cast billets. Test: torsion AlMgSi0.7 AlSiMg 6005A flow curves; test temperature 450 ЊC. Logarithmic principal strain ug 2.7; deformation rate (log principal strain rate) Moderately difficult-to-extrude alloys u˙ 0.1 s1 g AlMg2.5 AlMg2.5 5052 DIN EN 573-3(a) AlMg3 AlMg3 5754 AlMg1Mn1 AlMn2Mg1 3004 Symbol No. Flow stress k , N/mm2 f AlMg2.7Mn AlMg3Mn 5454 Easy-to-extrude alloys AlMgSi1 AlSiMgMn 6082 Al99.5 1050A 17.9 AlMgSiCu AlMg1SiCu 6061 AlMgSi 6060 19.8 AlMgSiPb AlMgSiPb 6012 AlMn1 3103 23.6 AlZn4.5Mg1 AlZn4.5Mg1 7020 AMg1 5005A 28.2 AlCuLi(b) AlCu2LiMg1.5 2091 AlLiCuMg1(b) AlLi2.5Cu1.5Mg1 8090 Hard-to-extrude alloys Hard-to-extrude alloys AlSiMgMn 6082 41.5 AlMg5 Al Mg5 5056A AlMg3Mn 5454 43.8 AlMg4.5Mn AlMg4.5Mn0.7 5083 AlZn4.5Mg1 7020 44.2 AlCuMg2 AlCu4Mg1 2024 Difficult-to-extrude alloys AlCuSiMn AlCu4SiMg 2014 AlCuMgPb AlCu4PbMgMn 2007 Al4.5MgMn0.7 5083 46.7 AlCuMgFeNi(b) AlCu2Mg1.5Ni 2618 AlCuMg2 2024 48.2 AlZnMgCu0.5 AlZn5Mg3Cu 7022 AlCuMgFeNi 2618 51.5 AlZnMgCu1.5 AlZn5.5MgCu 7075 AlMg5 5056A 55.9 AlZn8MgCu(b) AlZn8MgCu 7049A AlZn5.5MgCu 7075 56.3 (a) The prefix “ENAW” has been omitted from the DIN EN 573-3 designations. (a) The prefix “ENAW” has been omitted from the DIN EN 573-3 designations. Al99.5 is ENAW-Al99.5; 1050A is ENAW-1050A. A199.5 is ENAW-A199.5; 1050A is ENAW-1050A. (b) Not standardized in DIN 230 / Extrusion, Second Edition

ing and to produce an easily worked cast struc- a smooth billet surface goes hand in hand with ture. In contrast, in AlMn1 the manganese exits the improvement of the internal structure. A in a supersaturated solid solution. This nonequi- fine-cellular dendritic cast structure with little librium state is activated by the thermal treat- variation in the cell size across the billet diam- ment and tries to attain equilibrium by the for- eter is desired [Sca 78] (see also the section mation of secondary precipitates. The size of the “Melting and Casting Processes” in Chapter 4). precipitated particles depends on the tempera- During the billet heat treatment (homogeniz- ture and the heat treatment time. The higher the ing), the plasticity of the cast structure can be temperature and the longer the heat treatment significantly improved. This occurs by the re- time, the coarser are the particles. It is known duction in grain segregation and the formation that the degree of dispersion influences the re- of the intermetallic AlFeSi cast phases during crystallized grain structure and the workability. the billet heat treatment just below the solidus It is therefore important with AlMn1 to heat treat line. These effects are the more marked the finer at a high temperature of 590Ð620 ЊC in order to the dendritic structure of the cast billet because produce a fine grain during extrusion. the initial conditions are then more favorable for The most common easily extruded alloy is the thermally activated processes. AlMgSi0.5. It differs from the materials de- The billet heat treatment cycle involves not scribed so far in that it achieves its properties by only the heat treatment temperature of 560Ð580 age hardening. AlMgSi0.5 is characterized by ЊC and the holding time of 6 to 8 hours, but also good mechanical properties as well as by out- the heating and cooling of the billets. The rate standing workability combined with an excellent of heating is not limited for any metallurgical surface quality. reasons. With AlMgSi0.5, rapid cooling from As already mentioned, the temperature range the heat treatment temperature is recommended for the hot-working process coincides with that because with slow cooling (Յ 50 K/h), the for solution heat treatment. In this case, separate Mg2Si is precipitated in a coarse format. With solution heat treatment and quenching of the ex- rapid billet heating to the extrusion tempera- truded section—as required for the high-strength ture—in an induction furnace—the heat treat- materials—is not required. ment time can be too short to redissolve the AlMgSi0.5 extruded sections only need to be coarse particles. This reduces the mechanical cooled with moving air because of the low properties after age hardening. In addition, quench sensitivity in order to retain the age- coarse precipitates reduce the surface quality, ␣ particularly in anodizing. hardening phase Mg2Si in -solid solution. A characteristic of this alloy is that Mg2Si can be retained in supersaturated solution with a cool- 5.8.2 Extruded Products ing rate of only 2 K/s. The desired increase in Extruded products are classified as bar, tube, strength can be obtained by subsequent cold or and extruded sections and are standardized in warm age hardening. DIN 1746, 1747, and 1748 (See The Aluminum The important requirements for AlMgSi0.5 Association, ANSI H35.1). and AlMgSi0.7 type alloys are small fractions The range of profile cross sections is almost of the principle alloying elements dissolved in inexhaustible for the easily extruded aluminum the ␣-solid solution at the press exit temperature alloys. Profiles with asymmetric cross sections and the avoidance of large additions of the re- and considerable wall thickness variations can crystallization retarding elements of manganese, be produced. Reference should be made to chromium, and zirconium because, in the form Chapter 2, which covers the wide range of ap- of fine secondary particles, these significantly plications. increase the flow stress and thus reduce the ex- Aluminum and the easily extruded AlMgSi trudability. In addition, they act as foreign nuclei types also have excellent extrusion welding properties. Hollow sections can therefore be pro- for the age-hardening phase. Mg2Si is deposited at the Al(Mn,Fe)Si containing crystals and in duced with extruded longitudinal welds using this form can no longer contribute to the age special dies (bridge, porthole, and spider dies). hardening [Sca 64]. The wall thickness that can be held with ex- The quality of the continuously cast billets has truded sections is determined by the following a not insignificant influence on meeting the high parameters: quality and productivity requirements of the ex- ● Material trusion plant. It is known that the formation of ● Specific pressure Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 231

● Dimensions and type of section Extruded products are standardized according ● Degree of difficulty to the product shape, material, and material con- dition, the technical delivery conditions, and the The problems and limits of hollow section dimensional tolerances. Table 5.6 summarizes production are covered in section 5.7.6.1. the standards for bar, tube, and extruded sec- Extruded AlMgSi0.5 sections are used for fa- tions. c¸ades in high-rise buildings because of their good surface quality, particularly after anodiz- ing. Particular attention has to be given to the 5.8.3 Extrusion and Materials Properties design of the die and the location of the extru- The interaction of a suitable alloy, the quality sion welds. Texture and grain variations in the of the cast billet and the correct billet heat treat- location of the welds can occur as a result of the ment, the optimal tool design, the extrusion con- material separation as it flows into the die and ditions and the cooling of the extruded section, the bonding of the metal streams in the welding and the subsequent age hardening is needed to chamber by pressure welding resulting in a dif- produce a high-value product that completely ferent appearance to the other visible surfaces. fulfills all the geometric, chemical, physical, and This is covered in detail in section 5.7.6.5. metallurgical requirements of the extruded sec- With high demands on the brightness of the tion. extruded sections, for example, for trim on The alloys AlMg1 and AlMn are naturally household goods, furniture, or automobiles, hard alloys in which the mechanical properties chemical or electrochemical brightening has to are obtained mainly from the solid-solution be carried out before anodizing. Because parti- hardening. Therefore, the extrusion conditions cles of AlFeSi phases impair the brightness alloy are determined by the material deformation pa- variations based on Al99.85, Al99.9 or Al99.99 rameters. The extrusion is selected to give the are used for both AlMg1 and AlMgSi0.5. optimal hot-working parameters. This, however, Large extruded sections are preferred for rail depends on the cast quality and the billet heat and road vehicles. It is possible to produce pro- treatment (homogenizing), which forms the ba- file cross sections with a circumscribing circle sis for improving the plasticity. The heating of of a maximum of 540 mm and a weight per me- the homogenized extrusion billets is usually car- ter up to 80 kg on presses in the range 72 to 100 ried out in induction or gas-fired continuous MN installed for this application. ovens to a temperature that can be found in Round and flat bars as well as tubes and sec- Table 5.7. tions are used as conductors in electrical engi- In general, the billet temperature should be as neering. The mechanical and electrical proper- low as possible in order to obtain a smooth sec- ties are standardized in DIN 40501 parts 1 to 4 tion surface at a high extrusion speed. It also for the product shapes mentioned. E-Al and E- depends on the degree of difficulty of the section AlMgSi0.5, which are mentioned in DIN EN and the extrusion ratio. With the easily extruded 573-3 are used (ASTM Internation Standards for alloys, the extrusion ratio should be between V aluminum bus conductors are B 236 and B 317). 20 and V 100, but, if possible, a value of A special heat treatment in the range of over- V 40 to V 60 is preferred. aging increases the conductivity of E- The container temperature is usually approx- AlMgSi0.5 without any loss in mechanical prop- imately 50 ЊC lower than the billet preheat tem- erties [Ach 69]. perature.

Table 5.6 Comparison of standards for extruded products

Material/material Technical delivery Dimensions, permitted Product format condition conditions deviations Other comments Round tube DIN 1746, part 1 DIN 1746, Part 2 DIN 9107 . . . Board 4.6 Round-, rectangular-, square-, DIN 1747, Part 1 DIN 1747, Part 2 DIN EN 755-3 ... and hexagonal bar Board 4.6 DIN EN 755-5 DIN EN 755-4 DIN EN 755-6 Extruded section DIN 1748, Part 1 DIN 1748, Part 2 DIN 1748, Part 4 Cross-sectional shape Rectangular tube Board 4.7 DIN17615, Part 1 DIN17615, Part 3 DIN 1748, Part 3 Board 4.1 Board 5.17 232 / Extrusion, Second Edition

With alloys that do not age harden, the exit nomic point of view because higher extrusion temperature and the rate of cooling of the ex- speeds are achieved with lower extrusion tem- truded section are not critical for the mechanical peratures. This, however, conflicts with the met- properties. Nevertheless, cooling with fans is allurgical requirement of the exit temperature recommended for economic reasons and to pre- Ն500 ЊC in order for the age-hardening phase— vent precipitation and grain growth. The guide- in this alloy family approximately 1% Mg2Si— lines for the extrusion parameters discussed for to dissolve in the ␣-solid solution. If this does the individual alloys are given in Table 5.7. not occur, the section cannot be age hardened to In the production of extruded sections in the the desired values. age-hardening alloys AlMgSi0.5 and Al- The exit temperature depends on the opti- MgSi0.7, the combination of special metallur- mized flow stress of the material produced by gical and process technological measures en- the billet heat treatment, the billet temperature ables not only the economic production of and the heating conditions before extrusion, the complicated profile cross sections but also si- extrusion ratio, and the rate of deformation. multaneously the attainment of favorable me- The billet heating in an induction oven re- chanical properties. This is possible because quires approximately 4 to 6 minutes. This is sig- with these alloys the temperature range for hot nificantly faster than a gas-heated continuous working and solution heat treatment largely co- oven where approximately 45 to 50 minutes are incide. A further positive characteristic of these needed. These differences influence the diffu- alloys is that the age-hardening phase Mg2Si can sion-controlled solution processes. It is easy to be retained supersaturated in solution even with understand that rapid heating of the billet before a cooling rate of only about 2 K/s and the me- extrusion is advantageous where rapid cooling chanical properties subsequently improved by from the billet heat treatment temperature pre- precipitation during age hardening. vents the formation of coarse Mg2Si particles. If It is important to set the process parameters preheating is carried out in a gas-fired continu- to meet both the metallurgical and production ous furnace, then the opposite applies because requirements. In many cases a compromise has the cooling from the billet heat treatment does to be found. The exit temperature is determined not play the decisive role. by two opposing criteria. The lowest possible It should be pointed out here that in the United exit temperature is needed to obtain a smooth States in particular, one method of operation is surface finish. This is also desired from an eco- to heat the billets to a temperature above the

Table 5.7 Typical values for billet high-temperature heat treatment and extrusion conditions of aluminum alloys

Alloy Homogenization, ЊC Holding time Preheat, ЊC Container temperature, ЊC Typical exit speeds, m/min Al99.8Ð99.99 580Ð600 6 380Ð450 330Ð350 50Ð100 AlMg1 530Ð550 6 400Ð460 350Ð380 30Ð70 AlMn 590Ð620 12 400Ð460 350Ð400 30Ð70 AlMgSi0.5 560Ð580 6 450Ð500 400Ð450 30Ð80 AlMgSi0.7 560Ð580 6 470Ð510 420Ð450 25Ð65 AlMg2.5 520Ð540 6 430Ð490 360Ð380 5Ð15 AlMg3 510Ð530 6 460Ð490 360Ð380 5Ð15 AlMg1Mn1 540Ð560 16 450Ð500 380Ð400 10Ð30 AlMg2.7Mn 510Ð530 16 480Ð510 380Ð400 10Ð20 AlMgSi1 480Ð570 12 500Ð540 400Ð420 15Ð35 AlMgSiCu 540Ð570 12 500Ð540 380Ð400 10Ð40 AlMgSiPb 470Ð480 12 450Ð530 400Ð420 10Ð30 AlZnMg1 460Ð480 12 420Ð480 370Ð430 10Ð40 AlCuLi 520Ð550 12 400Ð450 350Ð400 5Ð25 AlLiCuMg 520Ð550 12 400Ð450 350Ð400 5Ð20 AlMg5 500Ð520 12 440Ð480 360Ð410 1.5Ð3 AlMg4.5Mn 500Ð520 12 430Ð480 410Ð430 2Ð5 AlCuMg2 480Ð490 12 400Ð460 370Ð400 1.5Ð3 AlCuSiMn 475Ð485 12 400Ð460 380Ð400 1.5Ð3 AlCuMgPb 450Ð480 12 380Ð440 360Ð400 1.5Ð3 AlCuMgFeNi 520Ð530 24 400Ð440 360Ð380 1.5Ð3 AlZnMgCu0.5 475Ð485 12 420Ð480 360Ð410 1.5Ð3 AlZnMgCu1.5 475Ð485 12 420Ð480 360Ð410 0.8Ð2.5 AlZn8MgCu 475Ð485 12 420Ð480 360Ð410 0.8Ð2.5 Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 233

subsequent extrusion temperature to achieve The extruded sections are finally stretched by complete dissolution of the Mg2Si phase. The 0.5 to 1.5% to achieve the straightness specified. billet is cooled outside the oven to the desired The final operation after cutting to the fin- lower production temperature before extrusion. ished length is age hardening. With the AlMgSi Higher mechanical properties can then be alloys, the mechanical properties are usually achieved [Sca 64, Rei 88]. reached after 4 to 12 hours at 160Ð180 ЊC, de- As mentioned previously, a wide range of ex- pending on the selected temperature (Table 5.8). trusion ratios from V 20 to V 100 can be Preaging at room temperature has a favorable used for the easily extruded alloys. The selection influence on the properties that can be achieved usually depends on the container diameter. From with alloys with low contents of the main alloy- the point of view of the grain formation, it can ing elements [Dor 73]. be advantageous to change to higher extrusion ratios because the recrystallized grain size de- pends on the degree of deformation as well as 5.9 Moderately Difficult Alloys the temperature. Obviously, the deformation rate should be as Aluminum alloys with flow stresses Ͼ30 to high as possible for economic reasons. High de- 45 N/mm2 are classified as moderately difficult. formation rates are also desired for metallurgical The relative extrudability of these alloys in this reasons because the deformation temperature category is only 40 to 60% of AlMgSi0.5 if the has a significant influence on the exit tempera- extrudability of this easily extruded alloy is set ture, which in turn determines the hardening be- at 100% [Hon 68]. This reduction is due to the havior. Limiting factors are again the specific increase in the main alloying elements, which pressure (stem load over the container cross-sec- are added for alloy technical reasons to increase tional area) and the roughening or tearing of the the material properties as well as the addition of section surface. The effect of die cooling with manganese to produce the desired hot-working nitrogen can be found in section 5.7.5.3. structure. As a guideline, exit speeds of 30 to 100 m/ min can be expected with AlMgSi0.5, depend- ing on the degree of difficulty. With AlMgSi0.7, 5.9.1 Aluminum Alloys the values are approximately 15 to 20% lower The naturally hard materials include the because of the higher amounts of magnesium AlMg alloys with higher magnesium contents as and silicon, as well as small manganese addi- well as the AlMgMn alloys. The magnesium tions. contents vary between 1.8 and 3% and the man- Immediately after leaving the press, the sec- ganese content between 0.3 and 0.8%. The billet tions are cooled with air or an air water mist heat treatment temperature has to be lowered mixture to a temperature Յ200 ЊC. This proce- compared with the easily extruded alloys AlMg1 dure avoids deformation or twisting of the sec- and AlMn to avoid melting because the solidus tions. A cooling rate of 2 K/s is sufficient to line in the phase diagram has a strong tempera- retain the Mg2Si in solid solution. ture dependence (Table 5.7).

Table 5.8 Typical values for heat treatment of age-hardening aluminum alloys

Quenching at the press Separate heat Cooling Age-hardening Material Temper(a) exit temperature, ЊC treatment, ЊC method temperature/time AlMgSi0.5 T5 Ն 500 ... Moving air 160Ð180 ЊC/12Ð6 h AlMgSi0.7 T5 Ն 510 ... Moving air 160Ð180 ЊC/12Ð6 h AlMgSiCu T5 Ն 520 ... Moving air 160Ð180 ЊC/12Ð6 h AlMgSiPb T5 Ն 520 ... Moving air 160Ð180 ЊC/12Ð6 h AlMgSi1 T6 ... 530Ð540 In water ... AlCuMg2 T3 ... 495Ð502 In water ... AlCuMgPb T4 ... 480Ð485 In water ... AlCuSiMn T4 ... 500Ð505 In water ... AlCuMgFeNi T6 ... 525Ð535 In water ... AlZn4.5Mg1 T5 Ն 460 ... Air 185Ð195 ЊC/18Ð20 h AlZnMgCu0.5 T6 ... 475Ð480 In water 90 ЊC/10Ð12h120 ЊC/18Ð20 h AlZnMgCu1.5 T6 ... 475Ð480 In water 90 ЊC/10Ð12h120 ЊC/18Ð20 h AlZn8MgCu T6 ... 475Ð480 In water 120VC/20Ð24 h (a) Material temper condition according to DIN EN515 234 / Extrusion, Second Edition

The heat treatable materials that belong to the truded structure, particularly in the direction of moderately difficult alloys include the alloy extrusion [Die 66]. The formation of a coarse AlMgSi1 that is frequently used in Germany and grain recrystallized peripheral layer and an un- the alloy AlMgSiCu preferred in the United recrystallized core is well known in these alloys States, the free-cutting alloy AlMgSiPb and the after age hardening. readily weldable alloy AlZn4.5Mg1, as well as The same observations are made in the cop- the more recent lithium-containing alloys per-containing AlMgSi alloy where manganese AlCuLi and AlCuMgLi. is partly replaced by chromium, which metal- AlMgSi1 does not only differ from the lurgically functions in a similar way. The low- AlMgSi0.5 alloy in the higher Mg2Si content but copper addition favorably affects the grain for- also in the addition of about 0.4 to 1.0% man- mation and the mechanical properties as well as ganese. From the metallurgical point of view, the general corrosion resistance [Sca 65]. this results in the recrystallization temperature Both the alloys AlMgSi1 and AlMgSiCu are being increased to a significantly higher tem- significantly more quench sensitive than Al- perature and higher strain. It is important to note MgSi0.5 because of the manganese addition. that both the manganese dissolved in the ␣-solid Sections in these alloys have to be rapidly solution as well as the precipitated manganese- cooled immediately after extrusion with a water containing phases have a recrystallization re- mist or in a standing water wave. A separate tarding effect and increase the deformation re- solution heat treatment with water quenching is sistance. The size of the manganese-containing recommended with thick-walled tubes and bars phases can be controlled by the temperature of to maintain the strength and the toughness. The the billet heat treatment [Sca 69a] (see also the manganese-containing phases of the order of section “Extrudability of Metallic Materials” in 0.05 to 0.7 lm obtained from low-temperature Chapter 4). Coarse particles that form at a tem- billet heat treatment and which are uniformly perature of 560 to 580 ЊC have a lower recrys- and densely distributed in the matrix have a tallization retarding effect than fine particles that positive effect on the toughness behavior [Sca are produced at a heat treatment temperature of 82]. The particle arrangement described dis- 460 to 480 ЊC. places the fracture from the grain boundary into AlMgSi1 extruded sections do not have a re- the matrix by cavity formation. Manganese-con- crystallized grain structure like AlMgSi0.5 be- taining AlMgSi alloys therefore are less suscep- cause of the recrystallization retarding effect of tible to intercrystalline facture and thus have sig- the manganese additions but an elongated hot- nificantly higher toughness values. worked structure with deformation texture com- Another moderately difficult-to-extrude alloy ponents (Fig. 5.33). This formation of the grain that should be mentioned is AlZn4.5Mg1. Me- structure is referred to as the extrusion effect. It chanical properties even higher than AlMgSi1 is characterized by an increase in the mechanical can be achieved and a structure with the extru- properties over those of the recrystallized ex- sion effect is also needed for this alloy to achieve high mechanical properties. Figure 5.34 shows that only a small recrystallized fraction is to be expected if the billet heat treatment is carried out at 460 to 480 ЊC and the extrusion ratio is not high. With this alloy a higher billet heat treat- ment temperature should not be used as other- wise recrystallized grains occur, and there is a tendency toward stress-corrosion cracking (SCC), and the values in the standard are not reached [Sca 73]. AlZn4.5Mg1 is characterized by a signifi- cantly lower quench sensitivity than AlMgSi1. Maintaining a cooling rate of 0.5 K/s is not only recommended for process technology reasons but has to be held for metallurgical reasons to avoid SCC. Fig. 5.33 Extruded section (extrusion effect with thin recrys- tallized peripheral zone) of an AlMgSi1 bar (cross Since the middle of the 1980s, intensive at- section, Barker anodized, micrograph in polarized light) tempts have been made worldwide to develop Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 235

new aluminum alloys with improved materials tion. With extended age-hardening, this results properties. The specific aims for the aerospace in an increase in the elongation to fracture with industry are a higher component stiffness and a simultaneous increase in the flow stress. weight reduction. Lithium was considered as an Mechanical property variations across the alloying element because of its density of only cross section are found in lithium-containing al- 0.534 g/cm3 and because it is one of the few loys because of the nonuniform texture forma- elements that increase the modulus of elasticity tion [Tem 91]. of aluminum alloys. In addition, the Al-Li phase diagram shows a temperature-dependent solu- bility so that the requirement for an increase in 5.9.2 Extruded Products strength by age hardening is also fulfilled [Web With their outstanding corrosion resistance to 89]. seawater, the naturally hard alloys are particu- Age-hardening lithium-containing alloys larly suitable for industrial atmospheres and for based on AlCu and AlCuMg have been devel- pipe systems in the chemical industry. The oped with a 10% lower density and a 15% in- seamless tubes extruded over a mandrel are fur- creased modulus of elasticity. The lithium con- ther processed by drawing. tent has to be approximately 3%. Special The age-hardening materials with their favor- melting and casting technology is required be- able mechanical properties are mainly used for cause of the high reactivity of lithium-contain- superstructures in rolling stock and as hollow ing melts with oxygen and moisture and the ag- girders for bedplates as well as the manufacture gressive attack of the lining of the melting of processing plants. furnaces. The moderately difficult alloys can be pro- The age hardening of the AlLiCuMg alloys duced as either solid sections or hollow sections is, in contrast to the conventional aluminum al- whereby the wall thickness has to be set 20 to loys, characterized by the partly coherent 40% higher than the easily extruded alloys, de- Al2CuMg particles that occur in addition to the pending on the alloy, for the same circumscrib- coherent Al3Li precipitates during further hard- ing circle because of the higher deformation ening. These cannot be cut by dislocations and stress. The AlZn4.5Mg1 alloy is often used for there is an increase in cross slip, which has a highly stressed welded structures because of its positive effect on homogeneous slip distribu- good weldability and the rehardening in the heat-affected zone. In the “age-hardened” con- dition (2-stage aging), this alloy has a good resistance to general corrosion and SCC (Ta- ble 5.7). Lead-containing alloys such as AlMgSiPb are available for chip forming machining. They are particularly suitable for the machining on auto- matic drilling, milling machines, and lathes be- cause they enable fast cutting speeds to be used. The alloy AlMgSiPb has good mechanical properties, is corrosion resistant, and can be dec- oratively anodized. The extruded or extruded and drawn products are primarily supplied as round, hexagonal, and octagonal bars. The tol- erances are standardized in DIN1769Ð1799 and DIN 5000-59701 (Aluminum Association H35.2). Extruded sections in the lithium-containing aluminum alloys are used as stringer sections and floor plate supports in the aerospace industry because of their lower density and increased modulus of elasticity compared with the con- ventional high-strength alloys. Further applica- Fig. 5.34 Recrystallized structural fraction as a function of the billet heat treatment temperature and the ex- tions are limited by the present high billet pro- trusion ratio (extrusion temperature, 550 ЊC) [Sca 73] duction costs. 236 / Extrusion, Second Edition

5.9.3 Extrusion and Materials Properties trusion process, care should be taken to ensure that the exit temperature is at least 530 ЊC. The The flow stress of AlMg and AlMgMn in- preheat temperature should, accordingly, be 500 creases with increasing magnesium content, and to 520 ЊC because there is no significant tem- the melting point decreases. To compensate for perature increase from the heat of deformation. the lower extrudability of, for example, AlMg3, On the other hand, with a separate solution heat the preheat temperature can be set to 30 ЊC treatment a lower preheat temperature should be higher than for AlMg1. However, the exit tem- selected, and there is no demand on the exit tem- perature should not significantly exceed 500 ЊC perature. The exit speed that can be achieved to avoid the surface turning brown because of increased oxidation. As mentioned earlier with with AlMgSi and AlMgSiCu is approximately naturally hard alloys, the cooling from the ex- only half that of AlMgSi0.5. The extrusion ratio trusion temperature to room temperature is not on the other hand does not differ significantly that critical. The process parameters for the in- from that with AlMgSi0.5. dividual alloys are summarized in Table 5.7. The increase in the mechanical properties of the section is achieved by age hardening for 6 Higher mechanical properties are obtained Њ Њ with AlMgSi1 and AlMgSiCu by age hardening. to 8 hours at 160 C or 4 to 6 hours at 180 C. However, some compromises have to be made However, it should be pointed out that with for extrusion. The metallurgical procedures that AlMgSi1, the maximum achievable mechanical improve the extrudability (e.g., higher billet heat properties decrease with increasing room tem- treatment temperature) have an unfavorable ef- perature intermediate storage. The damaging in- fect on the grain formation and thus on the me- fluence of an intermediate room temperature ag- chanical properties [Sca 67]. ing can be largely overcome by a preventative In the production of AlMgSi1 sections, the heat treatment. The sections have to be heated Њ decision has to be taken in the selection of the to 180 C and held for 3 minutes, which elimi- metallurgical conditions whether the solution nates the cold aging that would otherwise take heat treatment and quenching are coupled with place by approximately 1 day [Koe 61]. the extrusion process or whether these should be The alloy AlMgSi1 is suitable for sections carried out as separate operations. For economic that have to be color anodized because the man- reasons it is better if additional operations can ganese content that is embedded in the metallic be avoided. From the metallurgical point of form in the oxide layer produces the brown tone. view, there are advantages in the formation of Attention must be paid to the grain structure be- the grain structure if no additional heat treatment cause different microstructures can result in in the shape of solution heat treatment has to be bands on the surface after anodizing. In this case carried out. The decisive factor is whether the it has proved advantageous to try and obtain a completely fine-grain recrystallized structure. age hardening producing Mg2Si is extensively dissolved at the exit temperature and can be re- This can be achieved when the recrystallization tained in solution during the cooling. AlMgSi1 retarding influence of the manganese is largely and AlMgSiCu sections have to be cooled excluded by the formation of coarse secondary quickly with water because they are quench sen- precipitates and thus the rate of nucleation ac- sitive due to the manganese content [Sca 64]. celerated and recrystallization promoted [Sca Whether this requirement can be fulfilled de- 69]. Billet heat treatment at 570 to 590 ЊC has pends on one hand on the section shape to be proved successful in this respect. The associated produced, in particular, the wall thickness, and simultaneous reduction in the mechanical prop- on the other hand, on the cooling system in- erties is not important for applications in build- stalled on the extrusion press. As a general rule, ings. the rate of cooling of AlMgSi and AlMgSiCu in The copper-free age-hardening AlZn4.5Mg1 the temperature range 530 to 150 ЊC has to be a alloys are very suitable for the production of hol- factor of 10 faster than with AlMgSi0.5; i.e., it low sections and flat sections because with this should be at least 10 to 20 K/s. The required material the working temperature in extrusion cooling rate can only be achieved with water. coincides with the solution temperature range, Quenching with water after the die can, how- and this alloy has a low quench sensitivity so ever, result in uncontrolled distortion of the sec- that air cooling suffices. tion that must be kept to a minimum by pullers. Also, the mechanical properties that can be If the solution heat treatment and quenching achieved are higher than AlMgSi1. However, are to be carried out simultaneously with the ex- with incorrect treatment, a tendency to SCC can- Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 237

not be completely excluded. Today it is gener- ular, the AlZnMgCu systems. Alloy technolog- ally recognized that the danger of SCC of ex- ical procedures have to be followed, however, truded sections can be excluded if the production that can be disadvantageous from other perspec- conditions are strictly maintained. These con- tives. The addition of up to a few percent copper ditions are [Sca 73]: results in the formation of low-melting-point in- termetallic phases. They also have an unfavor- ● The billet heat treatment must be carried out able effect on the general corrosion resistance. Њ at a low temperature (12 h at 460Ð480 C) Recrystallization retarding elements, including to suppress the recrystallization by fine man- manganese, chromium, and zirconium, also ganese, chromium, or zirconium phases. have to be added in order to obtain a hot-worked ● The extrusion temperature should be as low structure in the extruded sections that contains Њ as possible (450Ð490 C). elongated grains in the longitudinal direction. ● The cooling with air in the temperature range The mechanical properties in the longitudinal di- Њ 350Ð200 C should be in the range 0.5Ð1.5 rection are higher than in the transverse direc- K. Water quenching should be avoided at all tion. This is referred to as the extrusion effect costs. [Sca 69c]. The alloy technological measures de- ● The age hardening—after an intermediate scribed do fulfill the mechanical property re- room temperature aging of at least 3 days— quirements but have serious hot-working dis- Њ has to be carried out in stages (12 h at 90 C advantages. The additions of the main alloying Њ plus 12Ð24 h at 120Ð130 C) to produce an elements magnesium and copper and the addi- overaged structure. tions of manganese and chromium increase the The extrusion behavior of the alloy flow stress significantly so that AlCuMg2 and AlZn4.5Mg1 from the point of view of the ex- AlZnMgCu1.5 have to be classified among the trusion speed is comparable to the alloy Al- difficult-to-extrude alloys. The difficulty is in- MgSi1. The advantages of AlZn4.5Mg1 are the creased by the low-melting-point phases, which relatively low solution heat treatment tempera- severely limit the extrusion temperature range, ture and the use of air even with thick-wall sec- and if the latter is exceeded, hot cracking occurs. tions to achieve the necessary slow cooling rate. These alloys have a marked quench sensitivity The extrudability of the alloy AlLiCuMg is and a severely limited solution range so that in comparable with that of the moderately difficult most cases they have to be solution heat treated alloys AlMgSi1 and AlZn4.5Mg1. The flow and quenched in a separate operation. stress of AlCuMgSi is approximately 10 to 15% In addition to these alloys, the AlMg and lower than that of AlMgSi1. The maximum ex- AlMgMn alloys with 4 to 5% magnesium are included in the difficult-to-extrude alloys. The trusion speed that can be obtained is reduced to 2 a value of only 60% of that of AlMgSi1 because flow stress of 47 to 56 N/mm is attributable to of the low-melting-point copper phase. It is ben- the high magnesium content and the manganese eficial with the lithium-containing alloy as with secondary precipitates, particularly if these oc- the other high-strength materials to pass the sec- cur in a fine format and a high density in the tion through nitrogen as it leaves the die to avoid matrix. oxidation of the surface [Sca 79]. It should be pointed out here that new pro- The age hardening of the lithium containing duction processes are being used to develop new alloys is carried out by solution heat treatment aluminum alloys for extruded sections with im- at 530 to 540 ЊC and quenching in water. Cold proved materials properties. These include al- working (stretching) of 1.5 to 2% before aging loys produced by powder metallurgy and parti- at 185 ЊC has proved to be beneficial to both the cle reinforced conventional alloys [Web 89] (see magnitude of the strength and the elongation the section “Extrusion of Powder Metals” in this [Tem 91]. chapter).

5.10.1 Aluminum Alloys 5.10 Difficult-to-Extrude Alloys There are various types of alloys with copper as the main alloying element. They differ in spe- The highest mechanical properties in the cial material properties. These alloys are usually aluminum alloys can be achieved with the age- cold age hardened to give the most favorable hardening alloys of the AlCuMg and, in partic- properties. AlCuMg2 has the highest mechani- 238 / Extrusion, Second Edition

cal properties and a good toughness. It is very The high-alloyed alloys AlMg5 and important in the aerospace industry. The alloy AlMg4.5Mn are particularly suitable for low AlCuMgFeNi has the best high-temperature me- temperature applications. The mechanical prop- chanical properties of all the conventional al- erties increase as the temperature falls; this also loys. The alloy AlCuSiMn is characterized by applies to the elongation to fracture. If the mag- high mechanical properties at room temperature nesium content exceeds 3%, some of the mag- and elevated temperatures. Because this alloy nesium exists preferentially as intermetallic also has excellent forging properties, preforms phases on the grain boundaries and some in the are extruded and then forged in dies. Bars for matrix. This applies in particular if the material drilling, milling, and turning are made from the is held for a long period in the temperature range alloys AlCuMgPb and AlCuBiPb. 250 to 150 ЊC. Intercrystalline corrosion suscep- The billet heat treatment of these alloys is tibility can occur under unfavorable conditions usually carried out at 480 5 ЊC. Only Al- if the application temperature is Ͼ80 ЊC. The CuMgFeNi is heat treated at 525 5 ЊC be- magnesium phase can be precipitated by suitable cause of the additions of nickel and iron. The thermal treatments during production so that no heating should not significantly exceed 50 ЊC/h coherent precipitates form and intercrystalline to avoid melting. After the heat treatment, slow corrosion can be practically eliminated. cooling to approximately 360 ЊC is recom- mended, preferably in the homogenizing fur- 5.10.2 Extruded Products nace. The cooling can then be carried out in air. The high-strength materials are mainly used These measures help avoid quenching stresses, in the aerospace industry, military applications, which can produce cracks in the billets in these and machinery. The products include solid sec- alloys. tions, bars, and seamless extruded tubes. The following AlZnMgCu alloys are the most AlCuMg2 and AlZnMgCu1.5 are normally not common: AlZnMgCu0.5 and AlZnMgCu1.5, suitable for the production of hollow sections which are both standardized, as well as alloys with extrusion welds because of the low-melt- with higher zinc contents in which manganese ing-point phases and the high extrusion pres- and chromium are replaced by zirconium. This sures required [Wei 78]. Simple, thick-wall tu- reduces the quench sensitivity of these alloys. bular components are, however, occasionally The highest mechanical properties measured on extruded with bridge or porthole dies. extruded sections are typically R 580Ð610 N/ p0.2 Whereas AlCuMg2 is usually used in the mm2 and R 610Ð640 N/mm2 and approxi- m room temperature age-hardened condition for mately 8Ð10% for A5. high-stress applications, AlCuSiMn and The billet heat treatment of these alloys is par- AlCuMgFeNi as well as AlZnMg alloys are ticularly important because of the complexity of mainly used after hot age hardening. AlCuSiMn the cast structure. Structural rearrangements oc- is used in the as-extruded or annealed state for cur during heating, holding at temperature, and subsequent processing to forged components. cooling, and these have a significant influence Only the alloy AlMg4.5Mn of the difficult-to- on both the mechanical properties and the ex- extrude naturally hard alloys can be extruded as trudability. Cast billets have a lower melting a hollow section with extruded welds. However, point than homogenized billets. The melting this is possible only with a large wall thickness point is increased by 70 to 90 ЊC by reducing because of the high deformation loads. The ap- the residual melt bands and segregations as well plications are primarily chemical equipment and as dissolution during the billet heat treatment. machinery. On the other hand, the supersaturated dissolved fractions of the peritectic phases are precipitated in a fine form in the matrix by thermal activa- 5.10.3 Extrusion and Materials Properties tion. In the billet heat treatment—usually 12 h Only extrusion ratios of V 10 to V 40 at 480 5 ЊC —the cooling of these alloys has are used for the difficult-to-extrude alloys be- a decisive influence on the extrusion speed [Fin cause of the high flow stress of over 45 to 57 N/ 96]. If the cooling is not fast enough, coarse par- mm2. The extrudability is less than 10% of the ticles can form during the cooling. These do not easily extruded alloys. dissolve completely during heating and then Indirect extrusion of high-strength alloys has melt in the extrusion process, resulting in sur- been used to an increasing extent in recent years. face cracks in the section. This has clearly confirmed the advantages of the Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 239

indirect extrusion process for the high-strength and the AlCuMgFeNi alloys to achieve the de- and difficult-to-extrude alloys AlCuMgPb, sired high mechanical properties. The age-hard- AlCuSiMn, and AlZnMgCu1.5 [Eul 75]. Indi- ening parameters are given in Table 5.8. rect extrusion has a higher productivity com- Finally, the difficult-to-extrude alloy AlMg pared with direct extrusion. Significant advan- 4.5Mn is a naturally hard alloy where the in- tages include the higher extrusion speed at lower crease in mechanical properties is not obtained temperatures (340Ð360 ЊC) and the use of longer by heat treatment but by solid solution and work billets (3Ð7 times the container diam). The dif- hardening. For this reason, the extrusion ratio ferent material flow in indirect extrusion pro- should not be too high; otherwise, the fraction duces a homogeneous structure and thus more of recrystallized structure increases to such an uniform properties. However, indirect extrusion extent that the specified values are not reached. places higher demands on the casting quality. This occurs, in particular, when a higher extru- Only defect-free turned or scalped billets give a sion temperature has to be used because of the good extruded surface finish. The distance be- limited specific pressure of the extrusion press. tween the die and the cooling section at the press exit is greater because of the design of indirect extrusion presses. This makes the quenching of quench-sensitive alloys at the press more diffi- cult. In addition, quenching at the press requires a sufficiently high exit temperature to achieve Extrusion of Materials with complete age hardening. In direct extrusion, the Deformation Temperatures extrusion speed of difficult-to-extrude alloys is increased by 20 to 40% by die cooling with ni- of 600 to 1300 ЊC trogen. This is demonstrated with the copper- containing alloys, which have a relatively low melting point. Die cooling enables some of the 5.11 Extrusion of Semifinished deformation heat to be removed so that the criti- Products in Copper Alloys cal melting temperature, which results in section cracking, is reached at a higher exit speed Martin Bauser* [Sca 79]. The solution heat treatment of the high- strength alloy AlZnMgCu1.5 is usually carried 5.11.1 General out separately after extrusion. The reason for 5.11.1.1 Copper, Bronze, and Brass— this has already been discussed. This is then fol- A Long History lowed by age hardening. Room temperature in- The knowledge and application of copper and termediate aging for 3 to 5 days has proved ben- some of its alloys extends back into prehistory eficial. The age hardening is carried out in one (Bronze Age). They were used up to the begin- or two stages (Table 5.8) and depends on the ning of our technical era mainly for jewelry and customer specifications. If the maximum me- household goods. The good workability of cop- chanical properties are required, age hardening Њ per and copper-zinc alloys combined with the is carried out at only 120 C to the maximum of attractive appearance is responsible today for the age-hardening curve. However, a slightly their use for metal wares, including containers, overaged condition is recommended because lamps, and trays as well as brass instruments. this significantly improves the stress-corrosion resistance for only a slight loss in mechanical properties. A two-stage age hardening is used to 5.11.1.2 Advantageous Physical and achieve the T73 designated properties. The sec- Chemical Properties tions are initially aged for 12 to 24 hours at 120 Copper is the commercial metal with the high- ЊC and then for 3 to 5 hours at 170 ЊC [Dah 93]. est electrical conductivity (58 m/X mm2 at 20 The alloy AlCuMg2 is usually aged at room ЊC). The additions to some low-alloy copper ma- temperature after quenching in water as hot age hardening has a negative influence on the cor- rosion properties; a tendency toward intercrys- talline corrosion can be detected. Hot age hard- *Extrusion of Semifinished Products in Copper Alloys, ening is the standard process for the AlCuSiMn Martin Bauser 240 / Extrusion, Second Edition

terials only reduce the conductivity slightly but 5.11.1.3 Importance of Extrusion significantly improve the mechanical properties. in the Processing The high thermal conductivity corresponds to Pure copper melts at 1083 ЊC. The melting the high electrical conductivity. Copper is lo- point is only increased by the addition of nickel cated close to the noble metals in the electrical (continuous solid solution). All other element chemical series and has a natural resistance to additions lower the melting point, sometimes to numerous corrosive effects. less than 900 ЊC (Table 5.9). The recrystalliza- The good corrosion resistance and the ease of tion temperature falls in the range 350 to 650 ЊC working make copper one of the most important depending on the composition. Copper alloys materials for water supply pipes. It is also par- are extruded at temperatures between 550 and ticularly suitable for heat exchangers because of 1000 ЊC corresponding to the melting tempera- its good thermal conductivity. The property of ture. Along with aluminum alloys, they belong copper used most is the excellent electrical con- to the group of materials where long semifini- ductivity, which secures its very wide applica- shed sections are mainly produced by extrusion. tion in electrical engineering and electronics— According to the statistics in Europe in 1988, naturally also in the form of rolled products. 1.4 million tonnes (metric tons) of extruded

Table 5.9 Extrusion data for copper alloys

Melting interval, Billet temperature, Maximum extrusion speed, Material ЊC ЊC Maximum extrusion ratio m/min Copper E-Cu 1080Ð1083 780Ð950 250 300 Low alloyed copper CuCrZr 1070 930Ð980 100 150 CuNi2Si 1040Ð1070 750Ð900 75 100 CuNi3Ni 1030Ð1050 850Ð950 50 75 CuZn (tombac and brass) CuZn10 1015Ð1035 825Ð875 150 100 CuZn20 950Ð990 750Ð850 60 100 CuZn30 910Ð935 720Ð800 150 150 CuZn37 900Ð920 710Ð790 200 150 CuZn38Pb1 880Ð900 650Ð750 250 250 CuZn40Pb2 875Ð885 650Ð750 300 300 Special brass CuZn28Sn2 890Ð930 750Ð780 75 100 CuZn31Si1 930Ð950 720Ð760 150 150 CuZn35Ni2 880Ð890 700Ð800 200 300 CuZn40Al2 880Ð890 600Ð700 250 250 CuZn40Mn2 880Ð890 650Ð700 250 250 CuSn (tin-bronze) CuSn2 1020Ð1070 800Ð900 100 150 CuSn6 910Ð1040 600Ð700 100 50 CuSn8 860Ð1015 650Ð720 80 30 CuAl (aluminum-bronze) CuAl5As 1050Ð1060 750Ð850 75 150 CuAl8 1030Ð1035 740Ð780 100 150 CuAl10Fe3Mn2 1030Ð1050 750Ð900 100 200 CuAl10Ni5Fe4 ϳ1050 750Ð900 50 100 CuNi (copper-nickel) CuNi10Fe1Mn 1100Ð1145 850Ð950 80 50 CuNi30Mn1Fe 1180Ð1240 900Ð1000 80 50 CuNi30Fe2Mn2 1180Ð1240 900Ð1000 CuNiZn (nickel-silver) CuNi12Zn24 ϳ1020 900Ð950 CuNi12Zn30Pb ϳ1010 80 50 4CuNi18Zn20 ϳ1055 850Ð920 CuNi18Zn19Pb ϳ1050 50 30 Source: Lau 76, Moe 80 Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 241

products were produced in copper alloys by 70 higher temperatures than with aluminum alloys. companies and approximately 120 presses [Zei However, as copper alloys can usually be ex- 93]. In Germany there are 14 companies with 37 truded at much higher speeds than aluminum, presses in the range 10 to 50 MN press power. the contact time with the tooling is so short that Sixty percent of all extruded products are bar, heating of the tools over the limit of 500 to 600 wire, and section of which the majority are pro- ЊC can be avoided. The tooling wear is, however, duced in copper-zinc alloys (brass). The remain- naturally much higher than with aluminum al- ing 40% are tube—usually in copper. loys (see the section “Tools for Copper Alloy Extrusion,” in Chapter 7).

5.12 The Groups of Extruded Copper 5.12.3 Structure Alloys—Their Important Copper and numerous copper alloys, e.g., Properties and Applications copper-tin (up to 8% Sn) and copperÐzinc (up to 37% Zn) have a pure face-centered cubic (fcc) 5.12.1 Alloy Groups ␣ structure up to the melting point and therefore have good cold workability but only moderately The DIN standards cover 81 wrought alloys good hot workability. On the other hand, the in seven groups that can be extruded under spe- body centered cubic (bcc) b phase, e.g., copper- cific conditions (Table 5.10). zinc over 40% zinc, has excellent hot workabil- The conversion to European standards had not ity but is difficult to cold work. been completed at the time of publication. Only product standards and not individual alloy groups, as is the case in DIN, are included in the 5.12.4 Typical Extruded Semifinished European standards. Each EN then usually ap- Products and Applications plies to all alloy groups (Table 5.11). ASTM In- The section “Copper Alloy Extruded Prod- ternational Standards cover alloys, product ucts” in Chapter 2 describes typical extruded shape, and specific applications. semifinished products and their applications. In addition to these alloys produced from cast Whereas bar and wire are produced over the billets there are a few composite materials and entire alloy range, tubes are mainly in SF-Cu for powder metals discussed in the sections “Extru- water supply, brass for plumbing fittings, and in sion of Powder Metals” and “Extrusion of Sem- special brasses and copper-nickel alloys for cor- ifinished Products from Metallic Composite Ma- rosive media. Large quantities of free-machining terials.” brass are machined to fittings and turned com- ponents, including bolts. 5.12.2 Tooling Temperatures in Extrusion In contrast to aluminum, the production of sections is no longer important. The higher ex- With extrusion temperatures between 500 and trusion temperature results in higher die tem- 1000 ЊC, the tooling is subjected to significantly peratures and thus greater wear and more severe tool defection than with aluminum alloys. The Table 5.10 Copper alloy groups and the associated composition DIN standards

No. of DIN standardized ASTM Table 5.11 Euro standards for extruded copper Alloy groups alloys standard alloy semifinished products DIN 1787: copper 6 B 133 Standard Designation DIN 17666: copper wrought 20 E 478 EN 1057 Seamless round copper tubes for water and gas alloys—low alloyed supplied for sanitary installations DIN 17660: copper-zinc alloys 31 B 371 EN 12735 Seamless round copper tubes for air conditioning (brass, special brass) EN 13348 Seamless round copper tubes for medicinal gases DIN 17662: copper-tin alloys (tin 4 B 505(a) EN 12449 Seamless round tubes for general applications bronzes) EN 12451 Seamless round tubes for heat exchangers DIN 17665: copper-aluminum 8 B 150, B 359 EN 12163 Bar for general application alloys (aluminum bronzes) EN 12164 Bar for machining DIN 17664: copper-nickel alloys 6 E 75 EN 12165 Feedstock for forged components DIN 17663: copper-nickel-zinc 6 B 151 EN 12166 Wire for general applications alloys (nickel-silver) EN 12167 Section and rectangular bar for general applications (a) For continuous casting EN 12168 Hollow bar for machining 242 / Extrusion, Second Edition

extrusion tolerances of copper alloys are wider authors have used various methods of measure- than with aluminum and it is also not possible ment these data are not suitable for comparison. to produce thin profile cross sections (see DIN Up to the extrusion temperature, copper and 17 674, page 3). With few exceptions the sec- low-alloy copper materials have the face cen- tions have to be subsequently drawn. Section tered ␣-structure, which does not have good hot- production is usually limited to copper and low- working properties. alloy copper materials as well as the easily ex- Brass in the ␣-b range (free-machining brass) truded ␣-b brasses. has very good hot workability. The extrusion of The most economic production of readily brass bars was the first large-scale application of cold-worked alloys to tubes, bar, and section de- this process (A. Dick, 1890, see the section “His- pends on the equipment available at the individ- toric Development of Extrusion” in Chapter 1). ual companies. Depending on the type of press, With difficult-to-extrude alloys, the flow the size, and the number and size of the drawing stress, depending on the press size and the ex- machines, the most suitable extruded dimen- trusion ratio, places a lower limit on the extru- sions vary for the same finished product. sion temperature and an upper limit on the sus- ceptibility to hot shortness [Lau 76]. Aluminum bronzes and lead-containing nickel silvers are particularly susceptible to hot shortness. The 5.13 Extrusion Properties of cracks can range from light surface cracks to fir Copper Alloys tree defects (Fig. 5.35). It is not only on eco-

Numerous authors have covered the extrusion of copper alloys [Tus 80]. Only the work rele- vant to practical applications is included here.

5.13.1 Extrudability of Different Materials Table 5.9 gives the extrusion temperatures, the maximum extrusion speeds, and the maxi- mum extrusion ratios. The data were obtained from practical experience in various extrusion companies. These can differ from plant to plant.

5.13.2 Temperature and Speed—Structure of the Extrusion The temperature dependence of the workabil- ity of different copper alloys is shown in the following section in the form of hot-strength curves. Data obtained from tensile tests only have limited application in calculating the forces needed for extrusion. However, if they have all been measured using the same method on soft specimens, they are suitable for comparing the materials (see also Chapter 4). Values of the flow stress kf obtained from torsion tests, which are often described in the literature, are more suited to calculating the load. Strains comparable to those in extrusion can be achieved. The Atlas of Hot-Working Properties of Non-Ferrous Metals, Volume 2, Copper Alloys [DGM 78] gives these kf values as a function of the logarithmic prin- Hot shortness cracking at excessive extrusion tem- cipal strain ug and the logarithmic principal Fig. 5.35 perature. (a) CuSn8 extruded tube with coarse, strain rateuú g . Unfortunately, not all the impor- moderate, and fine hot shortness cracks. (b) Extruded round bar tant alloys are included. In addition, as different in CuSn6 with gaping hot shortness cracks [Die 76] Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 243

nomic grounds that the extrusion speed should 5.13.4 Lubrication be as high as possible and the initial billet tem- Lubricants based on graphite oil mixtures are perature as low as possible. The output is higher well suited for the temperature range in which and the thermal stressing of the tooling during copper alloys are extruded. extrusion shorter. The material itself also cools There have been many attempts to introduce less during extrusion by the conduction of heat other lubricants for the extrusion of copper al- into the container, which is at a maximum of 500 loys. These have largely been disappointing. ЊC, which results in a more uniform section exit Usually, viscous oil blended with graphite flakes temperature and structure over the length of the is used. If automatic lubrication systems are extrusion. used, which is increasingly common on modern The age-hardening copper-chromium and equipment, the lubricant has to have a low vis- copper-chromium-zirconium alloys represent a cosity. “Lubricant sticks” of graphite containing special case where it is possible to quench small wax are available for die lubrication. The con- sections from the extrusion temperature, which tainer is usually unlubricated so that only the die is also the solution heat treatment temperature. and the mandrel in the case of tubes have to be In this case, extrusion is carried out at the high- Њ regularly lubricated. To reduce graphite inclu- est temperature possible (900 to 1000 C). sions and banding on the surface of the section, The maximum extrusion speed is frequently the minimum of lubricant should be accurately limited by the equipment, e.g., by the puller sys- applied. However, too little lubrication will re- tem for sections or by the speed of the down sult in wear between the tooling and the ex- coilers for wire. truded material. The extrusion temperature and the deforma- tion are normally so high that the extruded sec- 5.13.5 Extrusion with a Shell tion completely recrystallizes as it leaves the die, usually with a fine-grain structure. However, it Materials that are oxide free or have a limited is possible to have structural variations from the amount of oxide usually bond to the tooling in front of the extrusion to the back. At the start of the absence of lubrication. Brasses, aluminum extrusion the deformation is lower than in the bronzes, and copper-nickel alloys have to be ex- middle and at the end. Depending on the extru- truded with a stable shell. At the end of each sion ratio, the extrudability, and the extrusion extrusion the dummy block is pushed out and speed, the exit temperature can fall or even in- the shell removed with a cleaning disc. The shell crease. With multiphase structures the quantity thickness is usually about 1 mm. If it is too thick and distribution of the second phase can vary there is the risk that the material will flow back over the cross section and the length of the ex- between the dummy block and the container trusion. Details are given in the material-specific over the stem. This can occur in particular with sections 5.16 to 5.16.9. alloys that are easy to extrude. An incomplete shell can form if the shell is too thin. A fixed dummy block, which is used for the easily ex- 5.13.3 Extrusion to Finished or truded aluminum alloys, cannot be used for cop- Close-to-Finished Dimensions per alloys. Copper and low-alloy copper materials, as Because the ␣-b brasses are difficult to cold well as bronzes, tend to oxidize. The oxide layer work, brass bar and sections in these alloy adhering to the hot billet acts as a lubricant [Bla groups are extruded close to the final dimensions 48, Bei 76, Vat 70]. However, because the billet and then brought to the desired finished dimen- surface is generally not uniformly oxidized, par- sions and mechanical properties by a subsequent tial adhesion to the container can occur and ex- cold-working operation (usually by drawing). In trusion has to be carried out with a shell. contrast, the alloys that have good cold worka- The thin shell in the extrusion of copper and bility (copper, low-alloy copper materials, ␣- low-alloy copper materials can be easily re- brasses) are extruded well above the finished di- moved from the container and squashed so that mensions and then cold worked in several it is possible to operate with a combination stages. Cold working of sections can be difficult dummy block and cleaning pad (see Chapter 7). and requires considerable experience in deciding The front ring of this block has the diameter of the extruded dimensions, the tool design, and the the dummy block and the rear the diameter of drawing parameters. the cleaning block. In between there is a deep 244 / Extrusion, Second Edition

and wide circular recess into which the thin shell 5.13.6.3 Back End Defect folds. Extrusion with a shell is still carried out with the combination dummy block and clean- If the extrusion ratio is too low (heavy sec- ing pad but the additional cleaning operation is tions), there is the risk of the “back end defect” eliminated. forming particularly with material that flows ac- cording to type C in direct extrusion. The ac- celerating material in the center of the billet can 5.13.6 Different Material Flow Behavior form a funnel at the end of extrusion. Short billet lengths and large discard lengths can be used to In the direct extrusion of copper alloys with reduce this (see Fig. 5.47). a shell and unlubricated container, flat dies are Back end defect is seen less in indirect extru- used. Differences in the hot-working properties sion than in direct extrusion and only when the and the flow behaviors of different material extrusion ratio is too small and the discard too groups result in different types of material flow. thin. The material flow patterns can be found in Chap- ter 3. 5.13.7 Discard Length

5.13.6.1 Flow-Type C—Piping Defect Discard lengths between 20 and 40 mm are used for copper, ␣-brasses, and tin bronzes, Aluminum bronzes and ␣-b brasses adhere to which also flows according to flow pattern B in the container because of the minimal oxide for- direct extrusion. mation and flow according to type C because the For ␣-b brasses and special brasses, which layer close to the surface cools during extrusion flow following flow pattern C, discard lengths and does not flow as readily as the billet core. are between 30 and 50 mm. The danger of the piping defect (see Fig. 5.46) The lengths should be 40 to 70 mm with the is countered by restricting the billet length. hard-to-extrude aluminum bronzes—also flow However, the end of the extrusion has to be reg- pattern C. ularly tested using a fracture test. These are not always valid. Sometimes—in- It is advantageous with these alloys to change dependent of the material and the container—30 to the indirect extrusion process where there is to 50 mm discard lengths are used in direct ex- no friction between the billet and the container trusion and the possible defective section ends and the risk of the piping defect is removed. The removed by careful control of cross sections material then flows according to type B. Longer (fracture tests). billets can then be used, providing the other con- The discard length can be significantly ditions allow it, and the discard length is reduced smaller in indirect extrusion. [Zil 82].

5.13.8 Direct Extrusion with 5.13.6.2 Flow-Type B—Shell Defect Lubrication and without a Shell Copper and low-alloyed copper alloys similar Extrusion is rarely carried out with internal to CuCrZr or CuNiSi but also CuNi, CuNiZn, lubrication of the container, which reduces the ␣-brasses as well as CuSn are frequently ex- extrusion load because the friction between the truded at temperatures so high that, depending billet and the container is largely eliminated. on the material, a more or less thick oxide layer This process was adopted for the vertical extru- forms that in turn can result in extrusion defects sion presses that used to be used when high ex- including shell defects and thus blisters on the trusion ratios were required from relatively low- extruded section (see Fig. 5.39). The material powered presses. Even today thin-walled tubes flows according to type B. The dead metal zones and small hollow sections are produced on ver- when flat dies are used hold back the oxide. Nev- tical presses with lubricated containers. On hor- ertheless, the billet length has to be limited or izontal presses it can be necessary to process the discard thickness correspondingly increased difficult-to-extrude alloys, e.g., copper nickel, at to avoid shell and blister defects. This is covered higher extrusion ratios than would be possible in more detail in Fig. 5.38. taking into account the friction between the bil- The risk of shell defect is somewhat less in let and the container. Lubrication of the con- indirect extrusion than in direct extrusion. tainer can help in this case. Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 245

Blisters on the surface of the extrusion can be strands can be pulled with a single puller, with avoided on susceptible alloys by extruding with copper alloys a specific puller device must be a lubricated container. It is necessary to use a provided for each strand. The exit speeds of the conical entry in the die and to machine the billet individual sections are not exactly equal so that surface. The billet surface forms the surface of the slower section would be severely stretched the section because of the laminar flow and at the high exit temperature. every defect on the billet surface appears on the Indirect extrusion is often used for brass wire section. and rod because this avoids the piping defect and long billets can be used. 5.13.9 Extrusion into Water or Air A rod and section press—usually the direct process—has a section cross-transfer system as Alloys that tend to oxidation are often ex- well as the puller system and frequently a water truded into a water bath or through a water wave. trough in which the sections can be quickly This ensures that the extruded product does not brought to room temperature after crossing the have to be pickled before subsequent further critical temperature of approximately 300 ЊC. processing. It is also possible with copper tube Wire is also cooled in water after a specific cool- to restrict the secondary recrystallization ob- ing time in air. served when extruding into air and thus a coarse If a wide range of materials and dimensions grain on the exit from the die [Gre 71]. A fine have to be processed, a multipurpose press is grain is often required for further processing. still the correct choice. Water cooling should, as a general rule, be ␣ avoided for -b materials because otherwise, as 5.14.2 Tube Extrusion the material leaves the die, the structure con- sisting mainly of the b-phase is undercooled and Tube presses today obviously have a piercer it is possible for some of the ␣-phase to precip- system. Water-cooled mandrels are used that itate as needles. Both reduce the workability and usually move with the stem (moving mandrel). increase the strength of the extrusions [Bro 73]. The mandrel that is stationary in the die during If more rapid cooling than air cooling is required extrusion is subjected continuously to a high (e.g., with brass wire), the material can only temperature. The “stationary mandrel” is there- come into contact with water after air cooling fore used only for the extrusion of tubes with (to 500Ð300 ЊC, depending on the material) if it small internal diameters and suitable hollow sec- is to remain soft. Direct slower cooling with a tions. water-air mixture is possible. Copper sections are extruded under water (or occasionally in a protective atmosphere). In the 1950s and 1960s a large range of ver- tical tube presses were installed. They were built 5.14 Extrusion Processes and because the vertical axis simplified the align- Suitable Equipment ment of the mandrel to the container and the die giving a better tube concentricity. On today’s Whereas earlier extrusion plants were mainly horizontal presses the tools can be so easily ad- multipurpose plants capable of producing rod justed and guided that this advantage no longer and sections as well as wire and tube, today ex- applies. Horizontal presses need simpler foun- trusion plants are designed specifically for the dations and enable larger section weights to be production of large quantities of one product produced and have therefore almost completely [Ste 91]. Specific design details are given in displaced the vertical tube presses. Chapter 6. 5.14.3 Drive 5.14.1 Extrusion Presses for Whereas water and accumulator driven extru- Brass Wire and Sections sion presses were previously exclusively used There are some presses on which large quan- for copper alloys giving high ram speeds (up to tities of brass are produced where only wire is 150 m/s), these are used today only for copper extruded onto down coilers and others that are tube and thick sections. equipped with pullers (up to 4 sections) for the In other cases direct oil drives are used, al- production of rod and sections. In contrast to the though these allow only a maximum ram speed extrusion of aluminum sections where several of approximately 50 mm/s at an economically 246 / Extrusion, Second Edition

acceptable investment; this is usually adequate. with rod, sections, and tubes. With coiled wire The advantages of oil drive (good speed control this is obviously not necessary. In this case the and regulation over the billet length, simpler maximum billet length is selected. Alloys that maintenance, and smaller footprint) predomi- are susceptible to piping are usually limited to nate. Exact speed control is particularly advan- 2.5 to 3.5 times the diameter in direct extrusion. tageous for a linked puller and synchronously For extruded tubes the traditional rule of operating wire coilers. Variable oil drive also thumb is that the billet should not be longer than simplifies the automation of the extrusion pro- 5 times the mandrel diameter because of the in- cess. creasing risk of wall thickness eccentricity as the billet length increases. The billet length also has 5.14.4 Die Changing to be restricted because of the occurrence of shell defects (e.g., with low-extrusion-ratio cop- It must be possible to easily change dies and per tubes). to control them on copper alloy presses: the high thermomechanical stresses result in such severe wear and deformation of the shape forming dies 5.15.3 Billet Processing that dressing can be required after only a few In the case of lubricated extrusion or if the extrusions. Chapter 6 describes suitable die billet surface is defective (particularly with flow changing systems (rotating arm, slide). type B), the billet has to be skimmed. This ex- Quick container changing is more important pensive operation is, naturally, avoided as much than in aluminum extrusion because of the wear as possible by preferably extruding with an un- of the liner. lubricated container and a thicker shell that col- lects the casting defects. 5.14.5 Discard Separation In indirect extrusion without a shell and a con- The discard is normally removed with the ical die, the billet surface forms the section sur- saw, in contrast to aluminum extrusion where it face similar to direct extrusion with a lubricated is usually sheared. It is important that good container. All billets then have to be skimmed. swarf extraction prevents damage of the product To avoid this, extrusion with a shell and a flat from swarf. die is also carried out with indirect extrusion so In indirect extrusion and direct extrusion of that a dead metal zone forms. Both hinder the small solid sections a powerful shear is, how- flow of the billet surface into the surface of the ever, preferred [KM 77]. section. The risk of defects is, however, signifi- cantly greater than in direct extrusion and par- ticular emphasis has to be placed on a good, smooth billet surface. 5.15 Billet Production and Heating If large-format tubes are extruded from large- diameter billets, the piercing load of the press 5.15.1 Continuous Casting and may not be large enough and the billet has to be Homogenizing prebored. The general rule is that the bore di- Chill cast molds were still used up to the ameter should be approximately 5 mm larger 1960s. They were completely replaced by the than the mandrel diameter so that the lubricant continuous casting method developed circa 1930 is not wiped off as the mandrel enters the billet. (see the section “Systems for the Production of Copper Billets” in Chapter 6). Horizontal or ver- 5.15.4 Billet Quality Control tical casting is used depending on the billet cross section and alloy and usually on continuous The billet quality must in any case be care- plants. Homogenization of the billets is not nor- fully checked before extrusion. This includes mally required apart from tin bronze, which is porosity and crack monitoring of sensitive ma- susceptible to severe segregation, where the risk terial as well as visual inspection of the billet of cracking is reduced. surface.

5.15.2 Billet Length 5.15.5 Billet Heating The length of the extrusion billet is usually Billet heating is described in detail in the sec- determined by the extruded length of the section tion “Billet Heating Systems” in Chapter 6. Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 247

The most economic heating in a gas furnace nelectrical purposes. The STP and ETP desig- is adequate to heat materials with a relatively nations are used for electric bus. The most low extrusion temperature (e.g., free-machining expensive variation is OF-Cu that is oxygen free brasses), even though in terms of the final tem- without a deoxidation agent. perature it is less accurate and less reliable. A Oxygen-containing copper is sensitive to temperature profile can be applied, if needed, by heating in a hydrogen-containing atmosphere. subsequent heating in an inline induction fur- The oxygen forms water vapor in the pores of nace with several heating zones and, in any case, the annealed material with the diffused hydro- the billet temperature can be more accurately gen, and this can burst open the structure. If the controlled. There are also cases in which gas fur- material has to be suitable for welding, brazing, naces are used even with higher billet heating or annealing, the oxygen has to be bonded, temperatures (e.g., copper). In these cases in- which is carried out by phosphorus in SF-Cu duction furnaces are more common because the (0.015Ð0.04%). Water supply tubes are the main risk of overheating is reduced. High billet pre- application of this copper grade. heating temperatures are simultaneously close to the solidus temperature and thus there is a risk 5.16.1.2 Hot Workability, of the billet melting. Extrusion Temperature To reduce costs with high billet heating tem- peratures, the first heating step can be carried out The hot workability of copper is limited by in a gas furnace and the second in an induction the fcc structure up to the melting point (see the oven. hot-strength curve in Fig. 5.36). The extrusion In every case an inline electrically heated temperature is accordingly very high (800Ð950 equalization chamber can be used in which tem- ЊC), and a high extrusion ratio is possible only perature variations between the billet front and on powerful extrusion presses. back can be removed and in which billets can be maintained at temperature during press 5.16.2 Copper Tube downtime. 5.16.2.1 Application SF-Cu is used exclusively for domestic water 5.16 Copper Extrusion pipe, the most common use of copper tubes, be- cause these need to be welded or soldered. How- 5.16.1 General ever, SF-Cu is also used for underfloor heating Unalloyed copper is mainly extruded to tubes or for industrial applications (heat-exchanger and, to a small degree, to bar and sections. Cop- tubes in air-conditioning units, for chillers, etc). per wire is, in contrast, usually cast, hot rolled, and drawn.

5.16.1.1 The Different Grades of Copper, Their Properties and Applications The different copper grades are standardized in DIN 1787 (see Table 5.11 for the Euro stan- dards). Pure copper with a low oxygen content (less than 0.04%), which bonds the residual im- purities, giving the maximum electrical conduc- tivity, is used in electrical technology under the designation E-Cu. In North America a designa- tion beginning “OF” indicate oxygen free. Ex- truded and drawn sections as well as flat bar are used. Another high-conductivity copper grade that is oxygen free by deoxidation is referred to as SE-Cu in Europe. In North America deoxi- Fig. 5.36 Hot tensile strength of SF-Cu and low-alloy copper dized grades, DLP and DHP, are used for no- materials [Wie 86, Gra 89] 248 / Extrusion, Second Edition

Whereas water supply pipes are available as sively in the production of gas and water supply hard or half-hard pipes or annealed coiled tubes pipes. (DIN 1786/EN 1057), industrial tube is usually In the production of industrial tubes with wall supplied in annealed multilayered coils (coil thickness down to 0.35 mm and finned tubes, weight 60Ð120 kg). Internal and external finned the demands on the starting tube are very high tubes for heat exchangers are also produced in so that only the second and third processes with SF-Cu because of the good thermal conductivity extrusion are used. of copper and its excellent cold workability. 5.16.2.3 Extrusion 5.16.2.2 Production Methods The press and the tooling have to be accu- Numerous modern copper tube plants have rately aligned so that a low tube eccentricity been installed specifically for the production of (7% and under) can be achieved. In spite of SF-Cu tubes. The combination of the very good careful alignment of the centerline of the con- cold workability with the reduced hot workabil- tainer, stem, and mandrel as well as the die, ity enables hot working to be used to produce a movement of the mandrel during extrusion is al- tube larger than the finished dimensions fol- most impossible to avoid. Therefore, to achieve lowed by cold working to the finished size by a low eccentricity it is usual to limit the length cold pilgering and drawing machines without in- of the unmachined billet to 5 (up to 8) times the termediate annealing. Soft tubes are given a final mandrel diameter. anneal. Half-hard tubes are lightly drawn after The tube is usually extruded at high speed un- annealing. der water within a few seconds, which reduces Three processes compete for the production the risk of mechanical damage and produces the of copper tubes [Tus 70]: fine grain needed for extensive cold working. ● Production using a piercer, cold pilger ma- Only direct extrusion presses are used. chine (tube rolling), and drawing machines: To avoid water getting inside the tube, con- Heated billets are rolled with inclined rolls trolled mandrel movement is used at the start over a mandrel to tube blanks. A straight and the end of extrusion to give a tube closed at tube up to 150 m long is then produced on a both ends (Fig. 5.37). The extruded tube is then multistage cold pilger machine with a reduc- oxide free internally and externally and can be tion ratio of 10:1 followed by redrawing. passed to the cold pilger machine or drawing ● Production on an extrusion press, cold pilger machine without pickling. machine, and drawing machines: The usual dimensions of the extruded tube are 80 5.16.2.4 Oxide on the Billet Surface, 10 mm with a piece weight of more than 400 Shell and Blister Defect kg. As in the first process, the extruded tube is then further processed with cold pilger ma- As mentioned previously, there is the risk chines and drawing machines. Schumag ma- with copper flowing according to type B that the chines and spinner blocks are used. oxide produced on billet heating acts as a lubri- ● Production on an extrusion press and draw- cant between the billet surface and the container ing machines: A so-called thin tube, e.g., 73 and flows along the dead metal zone that forms 4 mm and a length of approximately 50 in front of the flat die: extrusion shell defects are m but a lower piece weight, is extruded. the consequence (Fig. 5.38). They produce blis- Without the need for a cold pilger operation ters when drawn tubes are annealed (Fig. 5.39). these tubes go directly to the drawing ma- Fast induction heating is recommended (at chines. least as the final stage) to ensure that the mini- mum of oxide forms on the billet surface. The In the first process (with no extrusion press) oxide on SF-Cu copper does not adhere strongly there is the risk of cracks and thus laps and oxide to the surface—in contrast to the oxide on oxy- inclusions, which impair the quality of the tubes gen-rich qualities. It can be largely removed by if the high demands of the casting process are a strong water spray at the furnace exit. At the not fulfilled. In the production of gas and water same time the billet hardly loses any heat. Hot supply pipe with a minimum wall thickness of scalping of the oxidized billet surface, which is 1 mm this does, however, play a minor role. This mentioned in the literature, is now rarely used very economic process is used almost exclu- [Vat 70]. Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 249

In conventional extrusion with a shell, the aim risk of lines of blisters is reduced by the selec- is for the oxide layer to be retained on the con- tion of a large extrusion ratio, which increases tainer liner. This does not occur completely. Be- the distance between the billet surface and the cause, as shown in Fig. 5.38, the oxide first ar- surface of the extrusion. If the billet is short rives at the die toward the end of extrusion, the enough, the flow of the oxide is stopped in time in front of the die. As already mentioned, the extruded shell is so thin and ductile that a combination dummy block and cleaning block can be used in extru- sion.

5.16.3 Copper Rod and Section

5.16.3.1 Dimensions and Shape, Further Processing The high extrusion temperature of copper pre- vents the production of thin-wall complex and sharp-edged sections. Extruded shapes therefore have to be brought to the finished dimensions by one or more cold deformation steps—on draw benches. The good cold workability of copper makes this relatively simple. This also produces the preferred hard state. Figure 5.40 shows ex- amples of extruded and drawn copper sections. As described in section 5.16.2.3 for copper tubes, rods and sections in copper are also ex- truded underwater to achieve oxide-free, dam- age-free products with a fine grain. Small cross- sectional areas can be extruded in long lengths through a water wave and then coiled.

5.16.3.2 Hollow Sections Hollow copper symmetrical sections for in- Fig. 5.37 Extrusion of copper tubes [Bau 93] ternally cooled bus bars in electrical engineering

Fig. 5.38 Extrusion shell formation in the extrusion of copper tubes [Bau 93] 250 / Extrusion, Second Edition

are, if possible, extruded from large billets oxide flowing into the weld seam. This can be pierced in the press and, for small openings, over avoided only by good technical knowledge and the mandrel tip (fixed mandrel). With difficult experience. The extrusion of copper through shapes, prebored billets can be necessary. The bridge dies is therefore rarely used. shape of the mandrel tip and the exact location in the die require considerable experience. 5.16.4 Extrusion of Low-Alloy Hollow sections have to be extruded through Copper Materials bridge dies if the openings are asymmetrical or if the section has several openings. This process 5.16.4.1 The Materials, Properties, is similar to that for aluminum alloys. However, and Applications with copper, deformation and cracking of the Low-alloy copper materials are covered by very expensive dies can occur along with the the standard DIN 17666 (UNS C-10100Ð

Fig. 5.39 Blisters on the tube surface of a SF-Cu-tube after annealing. (a) Transverse section. (b) External surface with line of blisters [Die 76]

Fig. 5.40 Example of extruded and drawn copper sections. (a) Solid section. (b) Hollow section (Source: Kabelmetal Osnabruck) Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 251

C15815). The composition, properties, and ap- are extremely high as is, as described previously, plications of the different alloys are described in the risk of flaking and blistering [Hes 82]. As a detail in the DKI information sheet 8 [DKIa]. general rule, only relatively short billets can be In the non-age-hardening alloys, additions of extruded (length:diameter 1 to 1.5:1). The silver, cadmium, magnesium, and iron increase billet heating time in the induction oven can the mechanical properties and, in particular, the sometimes be too short to completely dissolve softening temperature without reducing the con- the second phase and to achieve the maximum ductivity significantly. Tellurium and lead im- mechanical properties. prove the machinability. Additions of beryllium, The section has to be cooled in water to freeze nickel, silicon, chromium, and zirconium pro- the solid-solution state. Above a specific section duce age-hardening alloys that have high me- thickness (60Ð70 mm diam) water quenching at chanical properties and simultaneously high the press is no longer sufficient. In this case the electrical conductivity after solution heat treat- section has to be solution heat treated after ex- ment and age hardening. trusion in a special oven and then quenched. In Application examples of the low-alloy copper this case a lower extrusion temperature can be materials are found mainly in electrical technol- used because the extrusion process and the so- ogy and chemical plant construction, e.g., con- lution heat treatment are not combined. tacts and spring elements as well as (CuCr, The hot age hardening following solution heat CuCrZr) welding electrodes (see also the section treatment (e.g., 475 ЊC for CuCr) is carried out on copper alloy extruded products in Chapter 2). before or after the cold working, depending on Alloy development in recent years has re- the properties required. sulted in some nonstandard age-hardening ma- terials for elements of electrotechnology and electronics. The majority are used only in the 5.16.5 Extrusion of Copper-Zinc form of strip, and extruded and drawn products Alloys (Brass and Tombac) are rarely used. 5.16.5.1 Binary Copper-Zinc Alloys 5.16.4.2 Extrusion Properties, Structure. Brass alloys are the The low-alloy copper alloys have similar ex- most commonly used of the copper alloys and trusion properties to unalloyed copper. The risk long sections are almost completely produced by of flaking and blister formation by the inflow of extrusion. All copper-zinc alloys with a copper oxide into the funnel behind the dead metal zone fraction more than 50% are referred to as is usually even higher than with copper because brasses. More than 72% copper, the copper-zinc of the greater tendency to oxidation [Moi 89]. alloys are also referred to as “tombac.” These The oxide thickness also increases when a are alloys with a reddish color. As the copper higher extrusion temperature has to be used for content is reduced corresponding to an increase the same extrusion ratio because of the higher in the zinc content, the color changes more and hot strength. It is advisable in such cases to blow more to yellow, and at the same time the hard- off the oxide from the hot billet using a water ness increases. spray. The copper-zinc phase diagram is described Figure 5.36 shows some hot-strength curves. first because of the marked variations in me- Extrusion data are given in Table 5.9. chanical properties and structure with the zinc content. 5.16.4.3 Solution Treatment at the Press As shown in Fig. 5.41, three alloy groups can The mechanical and electrical properties of be clearly differentiated: the age-hardening alloys are obtained by solu- ● Single-phase ␣-alloys with a copper content tion heat treatment followed by hot age hard- above 61% ening. If the cross section is not too high, the ● Binary-phase ␣/b-alloys with a copper con- solution heat treatment of CuCr and CuCrZr can tent of 54 to 61% be carried out on the press. This process is well ● Single-phase b-alloys with a copper content known from the low-alloy aluminum materials. of 50 to 54% Because, however, in this case the solution heat treatment has to be carried out at approximately The single-phase ␣-alloys have a fcc lattice 1000 ЊC, the stresses in the container and the die similar to pure copper. They can correspond- 252 / Extrusion, Second Edition

Fig. 5.41 Copper-zinc phase diagram [Ray 49] ingly be easily cold worked (see structure in Chip forming additions—usually between 1 and Fig. 5.42). 3% lead—that are embedded as droplets in the The single-phase b-structure is body centered matrix further improve the machinability. cubic (bcc) and has very limited workability at Materials with only a low b-content (Cu room temperature. Between these limits (shown Zn36Pb1.5) can be readily machined and cold by the lines BE and CF on the phase diagram) worked. They are used where stamping or bend- there is a region in which the ␣-phase and the ing is required. The free-machining brasses b-phase can coexist. The greater the zinc con- (mainly CuZn39Pb3) are used in large quantities tent, the lower is the ␣-content. The cold work- as hard drawn rods for processing on automatic ability reduces in this region corresponding to lathes to turned components of all kinds (e.g., the increasing zinc content (see Fig. 5.43, ␣-b- bolts). They can be cold worked only to a limited structure). extent (see Table 5.11, EN 12164). Pure b-brass The Materials, Properties, and Applica- tions. DIN 17660 covers the commercially used copper zinc alloys (See also ASTM B455 for Copper-Zinc-Lead [leaded brass] extruded shapes). The DKI information sheets i.5 and i.15 [DKIb, DKIc] cover these CuZn alloys in detail (See also Copper Development Association web site www.copper.org). The brasses with a pure ␣-structure as well as the tombac (low-alloy CuZn) alloys are used predominantly in the form of sheets and strip, less as tubes, and rarely in the form of bar and section. The producers of jewelery and metal ware, brass instruments, lamp bodies, and light- bulbs use ␣-brasses. Tubes are used for air brake pipes in goods vehicles and for manometers. The alloys in the ␣-b field with a b-fraction between 20 and 40% can be readily machined Fig. 5.42 Structure of ␣-brass, etched. Image width is and are therefore widely used as rod material. 0.5mm [Wie 86] Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 253

(e.g., CuZ44Pb2) is almost brittle at room tem- significantly lower flow stress than the ␣-struc- perature but is used for extruded sections that do ture. The ␣-b brasses and especially pure b- not have to be subsequently cold worked be- brasses therefore have a high extrudability. Fig- cause of its excellent hot workability. Sections ure 5.45 illustrates this by hot flow stress curves. in ␣-b brass can, on the other hand, be cold The workability of the CuZn alloys decreases drawn to a limited extent and therefore be sup- initially in the normal extrusion temperature plied to tight finished tolerances. Figure 5.44 range of 600 to 800 ЊC with increasing zinc con- shows some examples of extruded and drawn tent and then increases again from approxi- brass sections. They are covered by the DIN mately 30% zinc when b-phases occur during 17674 standard (EN 12167). extrusion. Extrusion. At the normal extrusion tempera- Њ The low flow stress of the b-phase at the ex- ture of more than 600 C, the b-structure has a trusion temperature enables high extrusion speeds and extrusion ratios to be attained. With free-machining brass extrusion ratios up to V 900 and exit speeds up to 8 m/s can be reached. Table 5.9 gives extrusion data on copper-zinc alloys. Because the phase boundaries ␣/␣-b and ␣-b/ b are displaced to higher zinc contents on cool- ing, the ␣-phase fraction extends into the ␣-b field on cooling (see Fig. 5.41). The ␣-precipi- tation can be partially suppressed by quenching (frozen) so that the b-fraction remains higher than would be the case in the equilibrium state. If quenching is carried out after cooling to 500 to 300 ЊC, depending on the composition, the ␣- precipitation has stabilized and the structure no longer changes. By quenching from the deformation heat and aging at temperatures approximately 200 ЊC, ␣- b-brasses can be given a substantial hardness in- Fig. 5.43 Brass with acicular ␣-b-structure, etched. Image width is approximately 0.275mm [Wie 86] crease by the fine precipitation of the second phase [Bro 73]. This is rarely used in practice because it is difficult to maintain the extrusion

Fig. 5.44 Extruded and drawn brass sections (Source: Wie- Fig. 5.45 Some hot tensile strength curves for copper-zinc land-Werke, Ulm catalog) alloys and SF-Cu [Wie 86] 254 / Extrusion, Second Edition

conditions constant from billet to billet, and at Indirect extrusion is recommended for ␣-b- the same time the cold workability is restricted. brasses where there is the risk of the extrusion Possible Extrusion Defects and their Pre- piping defect forming in direct extrusion be- vention. The high deformation resistance of cause the different material flow excludes this low-alloy ␣-brasses requires a high extrusion defect. Significantly longer billet lengths can temperature for a given extrusion press power. then be used [Sie 78]. This increases the danger of coarse grain for- As mentioned elsewhere, in direct extrusion, mation and in the extreme case, hot shortness. if the extrusion ratio is too low, particularly with Low initial billet temperatures as well as low brass, the acceleration of the center of the billet extrusion speed reduce the risk of coarse grain can be so severe that cavities can occur toward formation and hot shortness, as with all materi- the end of the section (Fig. 5.47). Their forma- als. However, at the lower extrusion temperature tion in the section can be prevented only by lim- the deformation resistance is higher and the pos- iting the billet length and leaving a sufficiently sible minimum extruded cross section for a long discard. given press power larger. A further defect, particularly with ␣-brasses With copper-zinc alloys containing more than with a low zinc content, is the formation of zinc 80% copper, copper oxide forms on the billet flakes on the surface of the extruded product. surface during heating in air. With alloys with Zinc vaporisation from the newly formed surface less than 80% copper, zinc oxide mainly forms immediately behind the die condenses onto and this has no lubrication properties in contrast cooler parts of the tooling in the form of fine to copper oxide. Whereas the CuZn alloys with drops, which can be picked up by the passing more than 80% copper can still have shell and section. The droplets form zones of high zinc blister defects, this risk does not occur with content, containing b phase, on the section sur- higher-zinc-containing materials because the ad- face. These are brittle and can result in defects. hesion between the billet and the container is so These zinc flakes can also be found on the inter- large that a closed shell is formed when extrud- ing with a shell. On the other hand, the combi- nation dummy block and cleaning disc that can be used in copper extrusion is not suitable be- cause of the large shell thickness. A separate cleaning cycle has to take place after extrusion. As the zinc content increases, the thermal conductivity of brass decreases and the periph- eral cooling of the billet in the container is no longer equalized. Because the b-rich hotter billet core flows more easily than the b-impoverished peripheral zone, the typical flow pattern type C forms (see Fig. 3.11 in Chapter 3) with the risk of the piping defect. Figure 5.46 shows an ex- ample. The billets should have a perfect surface qual- ity to avoid extrusion defects. The difference be- tween the container diameter and the billet di- ameter should also not be too large to avoid folds forming as the billet is upset in the container. With a good quality liner surface, the shell from the previous extrusion can be completely re- moved with the cleaning disc [Lot 71]. In direct extrusion, the length of the billet is limited to reduce the risk of the piping defect. As a general rule the billet should not be longer than 2.5 to 3 times the diameter. Routine fracture testing of the end of the sections can remove any sections containing the extrusion defect and en- sure that the remaining length is defect free. Fig. 5.46 Extrusion defect (piping) in ␣-b-brass [Die 76] Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 255

nal surface of tubes. In order to prevent them terial emerging from the die is very soft and it forming it is necessary to ensure that the hot sec- is necessary to use a defined low puller force to tion does not contact the tooling behind the die. avoid undesired stretching of the sections. Handling the Section. Because the ␣-b- If the cross-transfer conveyor is not large brasses and the b-brasses are very soft at the exit enough, water cooling has to be used at the end temperature and sensitive to mechanical contact, before the sections can be cut to length. Figure good guiding out of the die (possibly lined with 5.48 shows the principle of the extrusion of graphite) is needed to avoid damage of the sec- brass sections. tion surface combined with careful handling on Cu-Zn wires, usually in free-machining brass, the runout table. are extruded from one or two cavity dies into Whereas thick bars from single-cavity dies are down coilers, the speed of which are synchro- extruded into air on graphite or steel-lined nized with the ram speed. Periodic oscillations plates, or onto a freely rotating roller conveyor of the rotation speed (wobble) ensure that the and then cross transferred until they can be individual layers on the drum are not directly cooled—possibly in a water trough—and cut to above each other. To avoid damage, the strands length, pullers are usually used for sections and are usually extruded into pans, which at the end thinner bar. Up to four sections can be simulta- of extrusion are removed from the coilers and neously pulled using independent jaws. The ma- transported on a roller conveyor until the brass

Fig. 5.47 End cavities in a brass bar [Die 76]

Fig. 5.48 Schematic of a section and wire brass extrusion plant (SMS Hasenclever catalog) 256 / Extrusion, Second Edition

wire has cooled sufficiently and can be lifted out. Again, as with brass there are single-phase Further processing is carried out after pickling materials with a fcc ␣-structure and two-phase by a combination of drawing, straightening, and materials with an ␣-b structure. In both groups cutting to length. additional intermetallic phases can occur de- Straightness—b phase distribution—Fur- pending on the type and amount of the additions. ther Processing of Bar. The straightness of the The b-phase fraction in the ␣-b-mixed struc- bar is very important for machining on auto- ture, which has a strong influence on the extrud- matic lathes with high cutting speeds. Bars that ability and the cold workability, can be con- are not clean or straight chatter in the bar feed. trolled by these additions of alloying elements If the b-phase fraction and its distribution in the (see Table 5.12, Fig. 5.49). cross section vary over the length of the extru- Semifinished Products and Applications. sion, the rectification effect in the combination The alloys CuZn20Al2 and CuZn28Sn1 are re- drawing, straightening, and cutting to length ma- sistant to seawater and corrosion. They are used chine changes from the first bar to the last of an in heat exchangers and condensers. Because extruded coil. they have a pure ␣-structure and consequently The indirect extrusion process again offers ad- are difficult to extrude but have good cold work- vantages over the direct process. The variation ability, the tubes are extruded with large cross in the size and distribution of the b-phase in the sections and brought to the finished dimensions ␣-b mixed structure between the start of extru- on cold pilger machines and draw benches with sion to the end in indirect extrusion is less than intermediate annealing. in direct because of the uniform material flow CuZn31Si1 is a material with an ␣-b-mixed over the length of the extrusion. structure. Tubes for bearing bushes are produced In multicavity extrusion the billet no longer flows symmetrically in one strand because the die apertures are arranged asymmetrically. One Table 5.12 Influence of element additions on side of the section surface stems from the outer the b-phase fraction and the extrudability of region of the billet and the other side from the special brasses inner region. It is therefore particularly impor- Alloying Zinc tant to ensure a uniform cast structure and good element b-portion equivalent(a) Extrudability through heating; there are then no disadvantages Silicon Is improved 10 Is improved for the quality of the section and, in particular, Aluminum Is improved 6 Is improved Tin Is improved 2 Is improved for the straightness of the finished bars. Manganese Is improved 0.5 Is improved In order to obtain uniform mechanical prop- Nickel Is improved 0.9Ð1.5 Is improved erties and good straightness, care must be taken (a) 1% the alloying element corresponds to one in the effect on the structure to ensure that the degree of deformation in the condition one as “zinc equivalent” of indicated multiples of the effect of zinc. Source: Lau 76 individual drawing operations is held within nar- row limits requiring that the dimensional toler- ances of the extruded bar are within tight limits. Suitable selection of the hot-working materials for the die and its design are discussed in detail in the section on tooling for the extrusion of cop- per alloys in Chapter 7.

5.16.5.2 Copper-Zinc Alloys with Alloy Additions (Special Brasses) The Different Materials and Their Prop- erties. If additional elements, including alumi- num and tin, are added to copper-zinc alloys, properties such as the b-phase fraction, the cor- rosion resistance, and the strength change sig- nificantly. These materials are referred to as spe- cial brasses. They are also covered by DIN 17660 copper-zinc-aluminum alloys are known Fig. 5.49 Displacement of the phase limits in the copper- as aluminum bronzes and copper-zinc-tin alloys zinc system using aluminum additions as an ex- are tin brasses. ample [Bar 32] Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 257

in this alloy. Other nonstandardized special DIN 17662 describes the composition of the brasses (with additions of Mn, Ni, Si, and Pb) common alloys suitable for extrusion with a have proved themselves as wear-resistant bear- maximum tin content of 8%. The DKI infor- ing materials in special cases. mation sheet i.15 [DKId] describes the copper- High-zinc-containing special brasses are con- tin alloys in detail (See also Copper Develop- struction materials with moderate and high me- ment Center. These tin bronzes are also called chanical properties used in the manufacture of phosphor bronzes). chemical plants. Tin-bronzes belong to the oldest known cop- Casting and Extrusion. Casting defects, in- per alloys. They are still today very important in cluding porosity and cracks, have to be assumed the chemical industry and ship construction be- in special brasses, particularly with aluminum cause of their good electrical and thermal con- and manganese additions. Careful testing of the ductivity with simultaneous high-strength and billet quality (e.g., using a penetrant such as the favorable corrosion properties. In electrical tech- Met-L check testing on the cut surface) is nec- nology, significantly more tin-bronze strip is essary. Billets with defects on the mantle surface used than wire and sections. have to be machined. CuSn4 and CuSn6 tubes are used for manom- In the same way as with pure brasses, the ex- eters or suction pipes. Highly stressed compo- trudability depends on the amount of the b- nents, including gear wheels, bolts, and bear- phase present at the extrusion temperature. This ings, are made from CuSn8. is shown by the hot tensile test curves in Fig. The phase diagram in Fig. 5.51 shows that the 5.50. The possible defects and their avoidance wrought materials in the equilibrium state have follow those described in section 5.16.5.1. a pure fcc ␣-structure and therefore behave like copper in hot and cold working. 5.16.6 Extrusion of Copper-Tin Alloys (Tin Bronzes)

5.16.6.1 The Different Alloys, Their Structures, Properties, and Applications The tin content of tin-bronzes can extend to 20%. However, the higher-alloyed materials (more than 10% tin) are extremely difficult to deform and are, therefore, usually used only as cast materials. Complex tin-bronzes can contain additions of zinc and lead.

Fig. 5.50 Hot tensile strength curves of special brasses and SF-Cu [Wie 86] Fig. 5.51 Copper-tin phase diagram [Ray 49] 258 / Extrusion, Second Edition

5.16.6.2 Casting of Copper-Tin Alloys cause of the low speed and the large difference between the initial billet temperature and the Because the cast structure—particularly at the container temperature. Only short billets can higher tin contents—tends toward segregation therefore be used. because of the wide solidification interval and With the good cold workability of the fcc lat- the billets can exhibit tin sweating as a result of tice the section can be extruded above the final inverse billet segregation, turning of the billets dimensions and then worked to the final size in is advised. Homogenization of the billets (630 several stages—including where necessary in- to 700 ЊC for several hours) reduces the risk of termediate annealing—by section rolling and extrusion defects in the form of longitudinal and drawing. This also produces the frequently re- transverse cracks in the higher-alloyed materi- quired high mechanical properties. als. With more than 6% tin, the brittle ␣-b eu- tectoid that forms on solidification is partly re- tained in the cooled billet; it can be dissolved 5.16.6.4 Flow Type and Shell Defects only by this homogenization. Some phospho- Because tin-bronzes tend to oxidize and this rous is added to the 8% tin-bronze to suppress oxide exhibits good lubrication properties, flow the tin-oxide formation in melts that have not type A occurs in the direct extrusion of these been completely deoxidized. These tin-bronzes alloys, giving the risk of shell and blister are referred to as phosphor-bronzes. defects—particularly with low extrusion ratios, i.e., thick bars [Vat 70]. With large extrusion ra- 5.16.6.3 Extrusion of Copper-Tin Alloys tios—thin bars—this risk is reduced. Extrusion is normally carried out without container lubri- The flow stress of the tin-bronzes is higher cation and with a thin shell. Care has to be taken than that of copper as shown in the hot tensile in the cleaning cycle to ensure that no residues strength curves in Fig. 5.52 and increases with of the shell from the previous extrusion remain. increasing tin content. Tin-bronzes are best worked between 700 and 750 ЊC. The extrusion data can be found in Table 5.9. 5.16.6.5 Competition from Wire Casting Tin-bronzes containing up to 6% tin are mod- As mentioned previously, only short billets erately difficult and those with 8% difficult to can be used in wire extrusion giving low coil extrude. High-alloyed tin-bronzes are very sus- weight. The extrusion process and subsequent ceptible to cracking and can be extruded only downstream processing is correspondingly ex- with slow speeds [Moe 80]. pensive. Direct continuous casting of wire in Bars smaller than 25 mm diameter are ex- large coils has grown in competition to the ex- truded as several strands and coiled. There is the trusion of thin bar from which tin-bronze wire risk of the billet “freezing” in the container be- is produced by rolling and drawing. The production of tin-bronze wire today is mainly by this continuous casting process [Bau 76].

5.16.7 Extrusion of Copper-Aluminum Alloys (Aluminum Bronzes)

5.16.7.1 The Different Alloys, Their Properties, and Applications The composition of copper-aluminum alloys, usually referred to as aluminum-bronzes, is cov- ered by DIN 17665 (see Table 5.11 for the Euro standard). The DKI information sheet i.6 [DKIe] describes aluminum-bronzes in detail (see Cop- per Development Association). Binary aluminum-bronzes up to approxi- mately 9% aluminum have a homogeneous Fig. 5.52 Hot tensile strength curves of copper-tin alloys and SF-Cu [Wie 86] structure with the fcc ␣-phase, as can be seen Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 259

from the phase diagram in Fig. 5.53. In contrast, from larger extruded cross sections by cold complex aluminum-bronzes with aluminum working, then only the 5% aluminum bronzes contents of approximately 10% and additions of can be considered. iron, nickel, manganese, and silicon individually Extrusion and Extrusion Defects of Cop- or together usually have a heterogeneous struc- per-Aluminum Alloys. The oxidized billet sur- ture. face, mainly with Al2O3, does not act as a lu- Aluminum-bronzes are corrosion- and oxi- bricant in the container in contrast to copper dation-resistant alloys because of the formation oxide. The friction resistance is so high that it is of Al2O3-containing surface layers. They are possible to extrude with a stable shell. Because also very resistant against erosion-corrosion and the cast billets frequently have surface defects, cavitation and are particularly suitable for sea- machining is recommended. Testing the cross water applications. Aluminum-bronzes are also section for cracks and cavities is also advisable. found in the chemical industry and in highly If the billet heated to more than 750 ЊC cools stressed machine components. during extrusion—it can only be extruded rela- tively slowly—variations in the ␣-b-ratio occur 5.16.7.2 Binary Copper-Aluminum Alloys over the length of the extrusion along with vari- ous forms of the ␣-phase, resulting in differ- Materials and Extrudability. The two stan- ences in the mechanical properties. dardized alloys CuAl5 and CuAl8 have a fcc ␣- As with all difficult-to-extrude materials, a phase structure. They can be readily cold higher extrusion ratio for the same press power worked. Their hot workability depends on the can be attained with the aluminum-bronzes by aluminum content. Figure 5.54 shows the tem- extruding with a lubricated container without a perature dependence of the hot tensile strength shell through a conical die. This process is used of the aluminum-bronzes. only in exceptional cases. The poor extrudability is revealed by the ex- If the aluminum content is 8% or more, then trusion data in Table 5.9. the structure at the high extrusion temperature The phase diagram in Fig. 5.53 shows that the above 900 ЊC consists of an ␣-b mixed structure, boundary line ␣/␣-b is displaced to higher alu- the flow stress of which is less than the pure ␣- minum contents with decreasing temperature. structure. However, in the direct extrusion of The b-content thus decreases during cooling these alloys, there is the risk of flow type C oc- from the extrusion temperature. Even with nor- curring and the formation of the piping defect at mal cooling to room temperature from the ex- the end of extrusion in the same way as with trusion temperature, residues of the bcc b-phase brass. The countermeasures are the same as are retained with the 8% aluminum-bronze so those mentioned previously; the piping defect that only limited cold working is possible. If can be controlled by a suitable short billet and smaller final cross sections are to be produced

Fig. 5.54 Hot tensile strength curves of copper-aluminum Fig. 5.53 Copper-aluminum phase diagram [Ray 49] alloys and SF-Cu [Wie 86] 260 / Extrusion, Second Edition

long discard. Fracture tests of the ends of the type C similar to CuAl8, so that allowance has extrusion are recommended in any case. to be made for the piping defect. Further Processing. The cold workability of 5.16.7.3 Copper-Aluminum Alloys the complex aluminum-bronzes is very re- stricted compared with the binary alloys because with Additions of the high b-phase component. They are, there- (Complex Aluminum-Bronzes) fore, normally supplied as extruded. Materials, Structure, and Properties. The Similar to special brasses, the complex alu- properties of alloys CuAl8Si, CuAl9Mn, minum-bronzes can be heat treated [Ben 93]. CuAl10Fe, CuAl10Ni, and CuAl10Fe3Mn2 are Higher mechanical properties can be obtained given in the standards. with CuAl10Fe and CuAl10Ni by solution heat The mechanical, physical, and chemical prop- treatment in the ␣-b-region followed by quench- erties of the complex aluminum-bronzes depend ing and age hardening at 500 to 650 ЊC. On on the composition. The aluminum-bronzes with quenching a b-martensite forms that partly iron, manganese, and silicon additions are usu- breaks down on aging. The quenching can take ally extruded in the b-phase where they have a place at the press, but better values are obtained lower flow stress than CuAl8 (see Fig. 5.54). On by a separate heat treatment after extrusion. the other hand, CuAl10Ni has an extremely high flow stress (about 70% higher than CuAl8 at 700 5.16.8 Extrusion of Copper-Nickel Alloys ЊC). This alloy is extremely difficult to extrude. Extrusion of Complex Aluminum-Bronzes. Complex aluminum-bronzes have a strong ten- 5.16.8.1 Materials, Structure, dency to stick to the container wall. In contrast Properties, and Applications to brass, the shear stress needed to shear away The copper-nickel alloys contain between 4 the shell is very high at the extrusion tempera- and 50% nickel, usually approximately 5 to ture of 600 to 700 ЊC. The related high stem load 30%. Nickel forms a continuous solid solution needed to shear the shell and the high flow stress with copper (see Fig. 5.55) so that all copper- result in the complex aluminum-bronzes being nickel alloys are single phase and have a fcc ␣- very difficult to extrude in spite of the high b- structure. phase component. Often, if small cross sections The most important copper-nickel alloys are have to be extruded there is no alternative to covered by DIN 17664 (ASTM E 75). They are working with a lubricated container without a CuNi10Fe, CuNi20Fe, CuNi30Fe, and CuNi30 shell and with a conical die entry. Mn1Fe. A special case in the complex aluminum- The most important alloy technically is bronzes is the copper-aluminum-nickel-shape CuNi30Fe, which is used for tubes in ship heat memory alloy (about 13% aluminum, 4% exchangers, seawater desalination plants, and air nickel) in which the composition, billet pretreat- ment, and the extrusion conditions have to be exactly maintained to obtain the desired shape memory properties [Don 92]. The sticking of complex aluminum-bronzes to the extrusion tooling can sometimes cause prob- lems in tube extrusion if the mandrel lubrication or the mandrel taper is insufficient. The internal surface of the tube can then exhibit cracks or flaking. Another almost typical defect with the com- plex copper-aluminum alloys is the “wood grain fracture” attributable to severe gas porosity of the billet or the inclusion of aluminum oxide films. The billets therefore have to be carefully checked (e.g., by penetrant check) for sound- ness. The complex aluminum-bronzes in direct un- lubricated extrusion usually flow according to Fig. 5.55 Copper-nickel phase diagram [Han 58] Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 261

and oil coolers. However, CuNi10Fe alloys also be taken into account when casting the billets find applications here. Whereas pure binary al- into water-cooled molds or by continuous cast- loys are used primarily for the production of ing. coins, the extruded alloys used for other appli- A homogenization heat treatment of the bil- cations, e.g., condenser tubes, have additions of lets is not considered to be necessary in spite of iron and manganese to improve the properties. the nickel concentration variations resulting to These elements increase the corrosion and ero- some degree from the casting process. The bil- sion resistance and raise the strength as well as lets often have to be machined to remove the the recrystallization temperature. The DKI in- cast skin. formation sheet i.14 [DKIf] describes the cop- per-nickel alloys in detail. (See UNS C/70100 to C/72950 and ASTM B 151.) 5.16.9 Extrusion of Copper-Nickel-Zinc Alloys (Nickel-Silver) 5.16.8.2 Extrusion of Copper-Nickel Alloys 5.16.9.1 Material, Structure, Properties, and Applications The melting point and recrystallization tem- perature increase with increasing nickel content, Alloys of copper, nickel, and zinc are referred and the cold and hot workability decrease. As to as nickel-silver because of their silver color. can be seen in Table 5.9, the extrusion tempera- The copper content of the technically most com- ture increases with increasing nickel content. mon alloys can range from 45 to 62% and the The range is between 850 and 1000 ЊC and is nickel content 7 to 26%, with zinc forming the the highest of all the copper extruded alloys. remainder. Alloys suitable for turning and drill- Figure 5.56 shows the hot strength of the copper- ing with an ␣-b-phase contain up to 2.5% lead nickel alloys as a function of the temperature. It as a chip breaker similar to brass. Small addi- does fall within reasonable limits at the high ex- tions of manganese are usual to reduce the risk trusion temperatures mentioned, but when de- of cracking at high temperatures (“annealing termining the extrudability, the service life of the cracking”). tooling plays the decisive role. The copper-nickel-zinc alloys are covered by Copper-nickel alloys should and can be ex- DIN 17663 (see ASTM B 151). truded as quickly as possible because of the high Figure 5.57 shows the copper corner of the extrusion temperature and the associated ther- copper-nickel-zinc system. The solid lines in the momechanical stressing of the tooling. Usually, phase diagram show the room temperature state; relatively thick-walled tubes are extruded and the dashed lines apply at 850 ЊC. then further processed on cold pilger machines In the majority of the commercial nickel-sil- and by drawing with intermediate annealing be- ver alloys, the fractions of nickel and zinc are cause the copper-nickel alloys have good cold- working properties as a result of their fcc lattice. Extrusion is usually carried out with a shell and without container lubrication through flat dies. This is not possible if small cross sections, i.e., a high extrusion ratio, have to be extruded in these difficult-to-extrude alloys. Container lu- brication is then recommended with extrusion through conical dies and without a shell. The resultant significantly lower friction between the billet and the container enables higher extrusion ratios to be produced or lower extrusion tem- peratures to be used. However, attention has to be paid to having a high-quality container bore and a good billet surface to avoid defects on the surface of the extrusion. The increase in hydrogen solubility with in- creasing nickel content and thus the risk of gas Fig. 5.56 Hot tensile strength curves of copper-nickel alloys porosity with inadequate melt treatment has to and SF-Cu [Wie 86] 262 / Extrusion, Second Edition

Fig. 5.57 Copper corner of the copper-nickel-zinc system [Sch 35] completely dissolved in the copper so that only ␣-solid solution exists. As with brass, only the high-zinc-containing alloys are heterogeneous and consist of ␣-b mixed crystals. Tableware and spring elements are produced from lead-free nickel silver. It is also used for frames of glasses and zippers. The ␣-b-nickel silvers, also with lead addi- tions, are used for all kinds of turned compo- nents and for fine mechanical fittings. The DKI information sheet Nr i.13 [DKIg] describes the nickel-silver alloys in detail (Also refer to ASTM B 151).

5.16.9.2 Extrusion of Nickel-Silver Alloys Fig. 5.58 Hot tensile strength curves of copper-nickel-zinc The extrusion data for the nickel-silver alloys alloys and SF-Cu [Wie 86] is also given in Table 5.9. As with the brasses the b-component determines the hot and cold cracking, and a structure with an excessive grain workability of the different alloys. A lead addi- size. tion hardly reduces the hot workability of the b- The billet surface of the nickel-silver alloys is containing nickel-silvers but reduces it signifi- usually turned off and extrusion is carried out cantly in the ␣-alloys. The hot tensile strength with a thin shell. Because a defect-free section can be obtained from Fig. 5.58 for some Cu-Ni surface requires the lowest possible extrusion alloys. These alloys can be readily extruded in temperature, it is frequently possible to extrude the temperature range between 600 and 700 ЊC. only with a large cross section, i.e., a low-extru- In principle, the lowest possible extrusion tem- sion ratio. Further processing is then carried out perature should be used to avoid oxidation, hot by rolling and drawing. Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 263

5.16.9.3 Competition from Wire Casting With extrusion temperatures of 850 to 1150 ЊC, the extrusion process largely corresponds to Lead-free nickel-silver is—similar to tin- that of iron and nickel alloys. Titanium alloys bronze—occasionally cast in wire format and are, therefore, usually extruded on steel extru- then cold worked without hot working. This pro- sion presses. An analogous technology is used. cess is economically superior to the manufacture Similar to steels and nickel alloys forging, hot of wire by extrusion but frequently produces de- and cold rolling are preferred when possible to fects in the strand, which reduce the quality the more expensive extrusion process for the [Bau 76]. production of bar material of titanium. Thin-wall tube in unalloyed titanium is also usually made from strip with longitudinal welding. Extrusion is, however, the most suitable method of pro- duction for thick-wall titanium tubes, titanium- Extrusion of alloy tubes, and for sections. Semifinished Products in 5.17 Materials, Their Properties, Titanium Alloys and Applications

Martin Bauser* 5.17.1 The Structure and Its Њ Influence on the Properties Titanium with a melting point (Ts) of 1668 C is one of the high-melting-point metals. Al- The example of the phase diagram for tita- though it is the fourth most abundant metal in nium with aluminum (Fig. 5.59) shows that pure the earth’s crust after aluminum, iron, and mag- titanium has a close-packed hexagonal lattice up nesium, the high cost, in particular, of the re- to 882 ЊC, the so-called ␣-phase. Above this duction to the pure metal but also of casting and there is the bcc b-phase. In alloys there is a more further processing, makes titanium products ex- or less wide field within which the ␣- and b- pensive [Sib 92]. Its low density of 4.5 g/cm3 phases can coexist. compared with iron and, simultaneously, the Titanium alloys are classified into three very high strength of some of its alloys makes groups according to the structure at room tem- it particularly useful where a favorable ratio of perature after the most common deformation strength to density is required, i.e., in the aero- and heat treatment processes: ␣-alloys, ␣-b- space industry. The very good corrosion resis- alloys, and b-alloys. tance against numerous media is attributable to b-alloys in general have a lower strength and the strongly adhering oxide film that forms even elongation as well as an inferior fatigue perfor- at room temperature. It therefore finds numerous mance compared with the ␣-b-alloys. However, applications in the chemical and petrochemical the b-alloys are superior to the ␣-b-alloys in industries. Titanium alloys can be classified as terms of creep strength and fracture toughness one of the exotic extruded metals because of the [IMIa 88]. high cost. Because its industrial application only The ␣-alloys are relatively difficult to work at started in the 1950s, all production and appli- room temperature [Mec 80], whereas b-alloys cation possibilities have certainly not been ex- can still be readily deformed. hausted. The alloying constituents of the technically Special mention should be made of supercon- most important materials are aluminum, which ductors of titanium-niobium alloys, shape-mem- increases the strength and stabilizes the ␣-phase, ory alloys with titanium and nickel as the main as well as chromium, manganese, molybdenum, constituents, as well as intermetallic phases of vanadium, copper, tin, and zirconium. Most of titanium and aluminum for high-temperature ap- these elements stabilize the b-phase and reduce plications. The book Titanium and Titanium Al- the ␣/b transformation temperature and are con- loys by U. Zwicker gives an overview [Zwi 74]. tained in the precipitates after age hardening. Tin as an alloying element, which slightly stabilizes the ␣-phase, and zirconium have a high solubil- *Extrusion of Semifinished Products in Titanium Alloys, ity in the ␣-phase and harden it [Mec 80]. Zir- Martin Bauser conium increases the hot and creep strengths. 264 / Extrusion, Second Edition

Fig. 5.59 Titanium-aluminum phase diagram [Kum 92]

The undercooling capability of the b-phase The strength is not a decisive criterion for and the reducing solubility with the decreasing thin-wall tube for liquid transport in the chemi- temperature of numerous alloying elements re- cal or petrochemical industry and, for example, sult in interesting hardening and tempering pos- in water desalination plants and power station sibilities. Solution heat treatment is usually car- cooling towers. The requirement is far more for ried out in the b-region or just below followed the outstanding corrosion resistance of titanium by rapid cooling so that the ␣-phase or other against a range of media and its oxidation resis- precipitates can form only in a finely distributed tance at normal temperatures. In these applica- manner after subsequent age hardening. Age tions different grades of pure titanium are used hardening cannot be carried out on ␣-materials that differ from each other only in their oxygen [Lam 90]. content (Table 5.13). The often complicated transformation kinetics As with iron alloys the move is away from the of the ␣-b alloys are represented in time-tem- production of soft tubes by extrusion, cold pilger perature-transformation diagrams similar to mills, and tube drawing. This area has had to be steel [Zwi 74]. conceded to the more economic longitudinal The lowering of the transformation tempera- welding of cold-rolled strip. ture by b-stabilizing additions is important for Bar and section in commercially pure tita- hot working because the body-centered cubic b- nium can be extruded and is produced by this phase has better forging and extrusion properties route but the demand is small. than the hexagonal ␣-phase. The aerospace industry is the main user of The niobium-titanium alloy with 52% Nb and semifinished products in titanium alloys apart 48% Ti used for superconductors has a relatively from special products, including racing wheels low transformation temperature and can there- and sports cars. In aircraft, they are used for en- fore be extruded at approximately 900 ЊC. gine components, fuselage, and wing elements [Pet 92] (Table 5.13). Wherever the strength and 5.17.2 The Most Important mechanical properties of aluminum alloys are Titanium Alloys and Applications insufficient, titanium alloys are used for light- Table 5.13 shows the most important titanium weight fabrications. alloys with their properties and application Extruded sections, tubes, and bars are pro- areas. duced from the higher-strength materials as well Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 265 Application Application turbines, jet engines, implants Aircraft components Jet engines Aircraft components Most common alloy, Gas turbines Aircraft components Aerospace Gas turbines Aerospace Application C C, soft Њ Њ petrochemical industries Properties Properties C C C C C C, still suitable for 500 Њ 400 Њ 300 Њ readily worked Њ Њ Њ hot working Creep resistant up to Suitable for hot-working ,% ,% 5 5 A 8 Creep resistant up to 9 Creep resistant up to 9 Creep resistant up to 940 8 Creep resistant up to 450 . 10 Creep resistant up to 500 12 Used up to 500 30 Used up to 350 Ͼ Ͼ Ͼ Ͼ Ͼ Ͼ Ͼ Properties Elongation, Elongation, A 2 2 , N/mm , N/mm m m 900 R 1100 Ͼ Ͼ 630Ð790 950Ð1080 1000Ð1300 1000Ð1250 Titanium High Temperature Alloys ,% 5 A Elongation, 2 . (d) IMI brochure: , N/mm with m Strength R b structure b ␣ ␣ in ␣ -mixed structure -mixed structure used -mixed structure -mixed structure with heat-treated and tempered precipitates; high- temperature heat treatment; precipitates b b b b - - - -Phase - ␣ ␣ ␣ Quenched and tempered Fine ␣ Age-hardening ␣ -phase 250Ð590 30Ð8 Very corrosion resistant The same areas -phase 290Ð590 30Ð8 Corrosion resistant, readily worked Tubes and components for the chemical and ␣ ␣ Structure at room temperature Titanium Medium Temperature Alloys Composition Composition Composition 6 2 4 2 ...... Almost 2 11 5 1 0.2 . . . Al Sn Zr Mo Si Nb Structure at room temperature Strength Cu Al V Sn Mo Si Structure at room temperature Strength R 5.5 3.5 3 0.25 0.3 1 Almost ... 6 6 2 ...... Materials for High-temperature alloys TiAl6V6Sn2(a) TiAl4Mo4Sn2 (IMI 550)(c) . . . 4 . . . 2 4 0.5 TiAl6V4(b) (IMI 318)(c) . . . 6 4 ...... IMI 829(d) 6242(d) (USA) Intermediate temperature alloys Ti1PdÐTi3Pd(a) Addition of 0.12Ð0.25% Pd Table 5.13 Some titaniumLow-temperature alloys alloys suitable for extrusion, properties, and applicationsMaterial (IMIa 88, IMIb 88,Ti1Ð and Ti3(a) personal communication) Unalloyed titanium with min 99Ð99.5% Ti Material TiCu2.5 (IMI 230)(c) 2.5 ...... Material IMI . 679(d) . . (a) DIN 17850/17861/17862. (b) DIN 17851/17961/17862. (c) IMI brochure: 266 / Extrusion, Second Edition

as sheet and strip. The alloy TiAl6V4 is the most 700 ЊC and can become brittle in hydrogen-con- common not only as a construction material be- taining atmospheres. Very fast heating is op- cause of its high strength, but also in surgery for posed by the poor thermal conductivity of tita- implants because of its good biocompatibility. nium. Heating in a salt bath (e.g., with barium chloride as a deoxidant) is less environmentally 5.17.3 Titanium Alloys Produced by friendly and is therefore rarely used. Powder Metallurgy Titanium alloys produced by powder metal- 5.18.2 Extrusion, Lubrication lurgy (P/M) are being offered to an increasing Table 5.14 shows the important data for the extent. This occurs on one hand when high-alloy extrusion of titanium alloys produced by melting materials, e.g., with intermetallic titanium alu- metallurgy. minide, cannot be produced by melting metal- Titanium alloys are preferentially extruded in lurgy or cannot be or only to a limited extent be the b-phase region where the flow stress is sig- hot worked and, on the other hand, if it is dis- nificantly lower than in the ␣-b mixed region (or persion-hardened material in which the disper- in the pure ␣-phase). The uniform structure, soids can only be added to the metal using P/M. which is obtained by the recrystallization that These materials are described in Chapter 4. occurs during extrusion in the b-region, is also advantageous. If the temperature in the b-region is too high, there is the risk of coarse grain for- 5.18 Billet Production, Extrusion mation, which has to be avoided at all costs. With some alloys it is possible to obtain a mar- tensitic structure if the section is not cooled 5.18.1 Casting, Billet Preparation, slowly enough from the b-phase. Subsequent Billet Heating annealing is then required. The billet pre-material is usually obtained Because titanium becomes brittle not only in from vacuum arc furnaces with melting elec- hydrogen but also nitrogen and oxygen atmo- trodes of titanium foam and recycled material spheres, the extrusion temperature should be as [Kra 82]. If the ingot diameter is too large, it has low as possible without crossing the ␣/b tem- to be forged to the final billet diameter. This has perature limit. the advantage that an originally coarse grain If, on the other hand, deformation is carried structure with low hot workability can be re- out just below the ␣/b phase boundary, the var- crystallized with a fine-grain structure by careful iations in heating during extrusion over the cross selection of the forging temperature intervals. section and along the length of the section result Mechanical surface processing is the final in a changing mixed structure that produce var- stage: a smooth billet surface is the requirement iations in materials properties over the cross sec- for a good product surface in the same way as tion and along the length. with steel extrusion because in direct extrusion Titanium alloys are extruded in the tempera- without a shell, with lubrication and a conical ture range 850 to 1150 ЊC. Above 1000 ЊC, die entry, the billet surface becomes the product which is in the b-region, it is necessary to use surface. glass lubrication similar to steel and nickel The billets have to be rapidly heated in an alloys. induction furnace or a protective atmosphere The glass film hinders the contact between the (argon) because titanium easily oxidizes above titanium material and the extrusion tooling and

Table 5.14 Data for the extrusion of some titanium alloys (Lau 76, Zwi 74, IMI personal communication)

Alloy b-phase boundary Extrusion temperature Structure at extrusion temperature Deformation at extrusion temperature Ti 1ÐTi 882 ЊC 700Ð900 ЊC ␣-phase TiAl6V4 995 ЊC 900Ð950 ЊC ␣-b-phase ϳ120 N/mm2 1050Ð1100 ЊC b-phase ϳ80 N/mm2 TiAl6VSn2 945 ЊC 800Ð1040 ЊC ␣-b or b-phase TiAl4Mo4Sn2 975 ЊC ϳ900 ЊC ␣-b-phase ϳ160 N/mm2 ϳ1100 ЊC b-phase 100 N/mm2 TiAl6Sn3Zr3MoNbSi 1015 ЊC 1050Ð1150 ЊC b-phase ϳ100 N/mm2 Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 267

thus the destruction of the bearing surfaces by The pickling away of the copper film in a mix- corrosion, to which titanium tends in contact ture of nitric and hydrofluoric acid is unpleasant with steel. The molten glass also acts as a ther- and is the reason why cladding in copper is mal insulation barrier and hinders the heat flow avoided as much as possible. It has been re- from the hot material into the colder tooling. The ported that an extrusion in the ␣-b-mixed region glasses 4 and 5 for the lower temperature range is possible without a copper cladding in spite of are preferred from the recommended glass the severe adhesion tendency between titanium mixtures for stainless steel tubes (see Table 5.18 and the tooling surface if a special graphite and [Mar 74]). grease lubrication is used [Boy 89]. The extrusion technology with titanium alloys differs slightly from that for iron-base alloys: ti- 5.18.3 Flow Stress, Extrusion Defects tanium alloys are, if possible, coated with a glass powder suspension at 80 to 150 ЊC. During the Compared with many stainless steels, the ti- subsequent heating to the extrusion temperature, tanium alloys have a high flow stress at the ex- the glass coating can act as a barrier against the trusion temperatures mentioned, and they are reaction of the billet surface with air constituents therefore difficult to extrude (see Fig. 5.60). This [Mar 74]. means that on a press usually used for extruded However, the method described for stainless semifinished products in steel, titanium can be steels, rolling the billet heated to the extrusion extruded only with a low deformation. Extrusion temperature over a table in glass powder is com- ratios of V 10 to V 100 are used. mon, particularly with tube extrusion and when The flow stress is—particularly in the ␣-b- induction furnaces are available for heating. mixed region—very temperature dependent: It Extrusion is usually carried out quickly, di- increases rapidly with falling temperature as rectly through a die with a conical entry with a shown in Fig. 5.60. Care must therefore be taken glass disc in front to minimize the cooling of the to ensure that the billet is not given time in the material during the extrusion process and to en- container to cool, producing a large radial tem- sure that the tooling is subjected to the high tem- perature gradient. Otherwise, the material from peratures for only a short time. At a high speed the temperature of the emerging section can even increase from the front to the back. If the speed is too high, in particular with sec- tions, there is the risk that the glass film will tear. The instantaneous failure of the tool working surfaces is the result [Mar 74]. The extrusion speed is in the range 0.5 to 5 m/s. There are also alloys that have to be extruded in the ␣-b-mixed region with temperatures be- tween 700 and 950 ЊC, especially if subsequent cold working is not carried out and a fine-grain structure is needed in the as-extruded condition [Zwi 74]. The turned billet is then wrapped in a thin copper sheet and extruded with a lubricated container, in this case, a graphite grease mixture, through conical dies. At the upper temperature range of 850 to 950 ЊC, an iron foil is recom- mended as an intermediate layer to avoid a re- action between the copper and the titanium. With adequate lubrication the friction be- tween the billet and the container is, similar to glass lubrication, so low that a quasi-stationary deformation with laminar material flow occurs, and the surface copper layer at the start and end of the section has an approximately constant thickness. This would be even better with indi- rect extrusion, but usually only direct extrusion Fig. 5.60 Flow stress kf as a function of the temperature presses are available. [Zwi 74] 268 / Extrusion, Second Edition

the billet interior will flow first producing a pip- bium (ZrNb2.5) are used as construction mate- ing defect at the end of the section. If the extru- rials in nuclear reactors, especially for the casing sion is too slow there is also the risk of the bil- material for the fuel rods because of the low ab- let’s “sticking.” sorption of thermal neutrons, good heat transfer, The dependence of the flow stress on the rate good corrosion resistance, and high hot strength. of deformation is higher for titanium than for The chemical industry also uses zirconium-alloy steel. Nevertheless, extrusion has to be carried semifinished products for specific critical cor- out quickly for the reasons just mentioned. rosion conditions. The production of the very pure metal nec- essary for processing is costly, and zirconium 5.19 Tooling, Further Processing alloys are therefore correspondingly expensive. Zirconium melts at 1852 ЊC and has a density The tooling is discussed in Chapter 7 in the of 6.5 g/cm3. Pure zirconium has a hexagonal ␣- section on tooling for the extrusion of titanium lattice up to 862 ЊC. Above this transformation alloys. temperature, there is a bcc b-phase. With al- The tooling loading is similar to the extrusion loys—also with small quantities of element ad- of stainless steels. When extruding in the b- ditions—there occurs a more or less wide tem- region with glass lubrication—particularly in perature interval with the simultaneous presence section extrusion—only one or two billets can of the ␣- and the b-phases [Web 90]. The struc- be extruded before the die has to be reworked. ture is similar to that described for titanium al- The die service life is significantly longer with loys, and the properties during hot and cold copper sheathed billets and lower extrusion tem- working are comparable. peratures. Chapter 7 describes coated section dies that are much more expensive but have a significantly longer service life. 5.21 Billet Production, Extrusion Normally, the lowest limit for the wall thick- ness of sections is 2 mm. Sections used in air- 5.21.1 Casting, Billet Preparation craft construction often have to have thinner walls (down to 1 mm wall thickness). Because Similar to titanium production, the starting cold working of the common alloys is possible material is melted from zirconium sponge in only to a limited extent, drawing and stretching vacuum or protected atmosphere furnaces with of the extruded sections at a higher temperature self-consuming electrodes. After forging or roll- is utilized [Mar 74]. However, 350 ЊC should not ing the large-diameter cast ingots, the billets for be exceeded to avoid undesired structural the extrusion of tubes have to be turned on the changes. outside to remove surface defects and also be prebored. Because zirconium has the same prop- erty as titanium of readily welding to the tooling during deformation and also reacts with oxygen, hydrogen, and nitrogen at high temperature, the Extrusion of billets are normally externally and internally clad. Copper sheet material is usually used Semifinished Products in sometimes with an intermediate steel layer when Zirconium Alloys the temperature is high to prevent diffusion of the copper into the zirconium base material. The copper cladding can, however, be applied by Martin Bauser* plasma spraying. To avoid an expensive zirco- nium discard, a copper disc can be attached to 5.20 Materials, Properties, the back of the billet to form the discard, at the and Applications end of extrusion. The cladding of the billet can be avoided by using special lubricants. Pure zirconium and, in particular, zirconium alloys with tin (ZrSn1.5 zircaloy) and nio- 5.21.2 Extrusion, Influence of the Structure

*Extrusion of Semifinished Products in Ziroconium Alloys, If zirconium, for which there is no great de- Martin Bauser mand, is processed on presses installed for steel Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 269

or copper alloys, then the technology used fol- If extrusion is carried out with copper clad- lows that for the corresponding main material. ding, the cladding material has to be removed The flow stress of most zirconium alloys is from the extruded product either mechanically not very high at the extrusion temperature, and or with nitric acid. Zirconium is not attacked by the dependence on the temperature is tolerable nitric acid. Coarse grain billet material leaves an so that the extrusion temperature and speed can orange peel effect on the pickled surface as an be varied within relatively wide boundaries with image of the grain structure. A fine-grain start- no risk. ing material is therefore preferred. Because the b-phase has the best hot work- The subsequent processing of zirconium alloy ability, it makes sense from the deformation tubes is carried out on precision cold pilger ma- point of view to extrude in the temperature range chines [Jun 93]. Drawing requires careful prep- 800 to 1100 ЊC, where the flow stress is rela- aration by bonderizing and lubrication because tively low. This, however, rarely occurs because the material has a tendency to weld to the tool- the billets would then have to be clad in steel. ing. More recently, drawing has been carried out In this case, lubrication is carried out with a using an ultrasonic vibrating mandrel, which im- glass powder mixture similar to steel and tita- proves the internal surface. Zircalloy cladding nium. for fuel rods can be produced from extruded In practice zirconium alloys are extruded tubes without drawing by multiple cold pilger preferentially in the temperature range 675 to operations with intermediate annealing (70% re- 800 ЊC to reduce the risk of gas absorption, duction per cycle can be achieved). which severely impairs the toughness of the ex- truded semifinished product. Just below the ␣/b transformation temperature and (in the case of alloys) in the ␣-b mixed phase region the work- ability is very good [Sch 93]. The best method, but also the most expensive, Extrusion of Iron-Alloy is cladding the billets in copper. Clad billets can be heated in an induction furnace and lubrication Semifinished Products with oil-graphite suffices similar to standard copper alloys. Dies with conical entries are nec- Martin Bauser* essary to obtain a predominantly laminar flow and thus to obtain a uniform cladding thickness over the length of the extrusion. 5.23 General If extrusion is carried out without cladding, it is important that the billets are brought rapidly 5.23.1 Process Basics to temperature and protected as far as possible from atmospheric influences during heating. Salt The high melting point of iron alloys (pure Њ 3 bath heating is, therefore, preferred over other iron: 1535 C, density 7.9 g/cm ) corresponds to methods (75% BaCl, 25% NaCl is referred to in a high recrystallization and hot-working tem- [Lus 55]). perature. Hot working usually is carried out in Special glass mixtures are used as lubricants the fcc austenite region, depending on the alloy, Њ when extruding without cladding at approxi- between 1000 and 1300 C. This is associated mately 800 ЊC. They have a low viscosity even with high tool wear because of the high thermal at this low temperature and substantially protect and mechanical stresses and, depending on the the billets from gas absorption. composition, a more or less severe oxide for- mation. As a result, it was relatively late before these alloys could be successfully extruded. 5.22 Tooling, Further Processing The oil graphite lubricants known from cop- per alloys, and which were used for steel extru- When selecting the tooling materials and the sion around 1930, were not really suitable. They tooling design, the experience gained from the were initially replaced with mixtures of oil, extrusion of copper alloys and—when extruding graphite, and cooking salt, which could be ef- at high temperatures—of steels can be utilized. As a general rule, conical dies (2␣ 140 to *Extrusion of Iron-Alloy Semifinished Products, Martin 90 ЊC) are used to produce tubes and round bars. Bauser 270 / Extrusion, Second Edition

fective only with very short contact times be- uneconomic. The tooling costs can also be tween the hot material and the shape-forming very high. In contrast, extrusion can be via- tooling. This method of lubrication is used only ble for quantities as low as three billets. rarely today for the production of mild steel ● Experimental or pilot production of tubes tubes on vertical mechanical presses (see section and sections that will later be produced in 5.24). large quantities more economically by A significant extension to the application of rolling the extrusion process followed the development of the Ugine-Se«journet process in 1950 in which Approximately only 30% of all steel tubes are glass of a specific composition was used for lu- produced as seamless and of these, less than brication [Se«j 56]. The molten glass not only 10% are produced by extrusion [Bil 79]. protects the heated billet from oxidation and acts Three product groups are described in detail as a lubricant between the material and the tool- subsequently: ing, but also acts as thermal insulation so that ● Mild steel tubes the die and container heat more slowly than with ● Alloy steel tubes the lubricants previously used. It was now pos- ● Steel sections sible to produce alloy steel tubes and steel sec- tions on horizontal extrusion presses with a lu- bricated container. Steels are extruded using the direct extrusion 5.24 Mild Steel Tubes process with lubricated containers without a shell. With this process it is possible to achieve short contact times with fast extrusion speeds— 5.24.1 Use of Mechanical Presses important for the necessary high extrusion tem- The numerous vertical mechanical presses peratures. The use of conical dies result in a ma- previously used in the technically highly devel- terial flow in which the billet surface forms the oped countries are no longer in operation. Car- surface of the extruded product (see Chapter 3, bon steel tubes are usually produced on contin- the section on material flow in direct extrusion, uous production lines that have a significant Fig. 3.31). higher productivity. However, these mechanical presses are still used in other countries. 5.23.2 Importance of Carbon steels, free cutting steels, low alloy Steel Extrusion Today hot and cold working steels and high speed steels can be processed on mechanical presses. The growth in the extrusion of steel in the 1950s and 1960s was followed by a continuous decline. The production of seamless tubes in mild steels and low-alloy structural steels on 5.24.2 Application of Mechanical Presses vertical presses has largely been replaced by Mechanical extrusion presses are similar to more economic continuous rolling processes. the machines used in the drop-forging industry. Seamless tubes in these steel grades are now re- The billets are pierced and extruded in one placed whenever possible by the less-expensive operation on vertical mechanical presses. The longitudinally welded tubes. billets are first heated to 1100 to 1300 ЊC and Today, the extrusion of stainless steel tubes loaded into the vertical lubricated container. and steel sections is used only when the material, Then, at the upper top dead center the crankshaft the section shape, or the low volume required is connected to a continuously operating electri- cannot be produced by other processes or only cally driven flywheel with a horizontal axis. In with significant expense. one revolution the stem and piercing mandrel The reasons for extruding steel tubes are: fall and the billet is extruded down into a curved ● Crack-free production of long products even channel and the stem and mandrel retracted. The in materials that are difficult to hot work and discard is sheared with a shearing tool and re- that tend to crack during rolling moved from above. ● Production of small volumes. If unusual di- The high deformation temperature and the ex- mensions or materials are involved, then fre- trusion time, which lasts for only a few seconds quently the setting and operation of rolling on the mechanical press, enables extruded tubes processes designed for mass production is in mild and free-cutting steels to be hot reduced Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 271

directly after extrusion on a stretch reducing mill Table 5.15 Production ranges for the different without reheating. methods for producing seamless steel tubes The glass lubrication used on horizontal (Man 86) presses to protect the tooling (see section Process Tube diameter, mm 5.25.4.2) is not really applicable for vertical Extrusion 35Ð250 presses because the glass powder does not ad- Tube continuous rolling 20Ð180 Diagonal rolling and Pilger process 160Ð660 here securely to the billet surface in the vertical Plug rolling procedure 180Ð400 position. The older method of lubrication with a vis- cous oil-graphite salt mixture has to be used. This partly evaporates and can even burn during for extrusion can be produced relatively inex- pensively but wear much more rapidly than the extrusion. Today’s environmental requirements tooling in the rolling process. are difficult to fulfill even with careful extrac- The most important method of producing tion. seamless alloy steel tubes in the same dimen- sional range as the extrusion press is the contin- 5.24.3 Dimensional Range and uous tube-rolling process in which a forged Throughput round ingot formed to a hollow billet in a pierc- ing mill is hot rolled over a mandrel through Mechanical tube presses are restricted in their numerous profiled roll pairs and then brought to press loads because there is a limit to the load the finished size on a stretch reducing mill after that can be economically transferred mechani- reheating. This process has at least 4 times the cally. They are, therefore, used only for extru- throughput of an extrusion press [Bil 79]. sion loads up to a maximum of 17 MN. Because The extrusion process today is restricted to: the upper limit of the billet weight naturally de- ● High-alloy stainless ferritic and austenitic pends on the extrusion load that can be devel- steels oped, the maximum billet weight that can be de- ● Heat-resistant high-chromium ferritic and formed is 120 kg with a diameter of 200 mm. austenitic alloy steels The entire extrusion process lasts no longer ● High-temperature austenitic alloy steels than 3 s so that it is possible to have up to 200 working cycles in one hour (average 120 to The dimensional range of these tubes is be- tween 35 and 250 mm external diameter and a 130). The mandrel length is restricted to 5 to 7 wall thickness of 5 to 50 mm with a minimum times the mandrel diameter because longer man- internal diameter of 30 to 40 mm [Ric 93]. The drels can deflect sideways within the press dur- extruded tubes are usually processed further by ing piercing. rolling and/or drawing. The dimensional range of the extruded tubes To produce the infrequently required dimen- extends from 40 to 120 mm external diameter sions over 200 mm external diameter, an alloy and 2.5 to 5 mm wall thickness [Sar 75]. steel tube extrusion press would have to be so massive that the high cost would generally pro- hibit the use of the extrusion process [Bil 79]. A 5.25 Seamless Alloy Steel Tubes 55 MN press designed for this purpose was sup- plied to Russia several years ago. Horizontal water-driven tube presses and only 5.25.1 Extrusion in Competition with the direct process without a shell are used. Fig- Other Hot-Working Processes ure 5.61 shows the extrusion principle. There has also been a large reduction in the Alloy steel tubes produced by extrusion are extrusion of seamless alloy steel tubes over the sold as hot finished or further cold worked. Ap- plications are in the chemical industry, plant last few decades and numerous presses have had manufacture, nuclear technology, and the pet- to be closed down. The extrusion of seamless rochemical industry. alloy steel tubes has to compete with a range of more economic rolling processes, the production 5.25.2 Alloys and Extrusion Properties capacity of which can be seen in Table 5.15. Economic analysis has shown that extrusion 5.25.2.1 Alloy Groups, Structure, is inferior in output to the continuously operat- Properties, and Applications ing rolling processes because of the low billet Table 5.16 shows a selection of the alloy weight and the long dead cycle times. The dies steels currently extruded. 272 / Extrusion, Second Edition

Fig. 5.61 Extrusion of alloy steel tubes on a horizontal press [Sar 75]

Table 5.16 Examples of stainless heat-resistant and high hot strength steels that are processed by extrusion (Ric 93)

Chemical composition, % Material No. C Cr Ni Mo Other Similar UNS No./Common Name Stainless steel Ferritic ...... 1.4016 Ͻ0.08 16.5 ...... S43000/430 1.4510 Ͻ0.05 17.0 ...... TiϾ4 (CN) Ͻ 0.8 S43036/430Ti Austenitic 1.4301 Ͻ0.07 18.0 9.5 ...... S30400/304 1.4306 Ͻ0.03 19.0 11.0 ...... S30403/304L 1.4401 Ͻ0.07 17.5 12.0 2.2 ... S31600/316 1.4404 Ͻ0.03 17.5 12.5 2.2 ... S31603/316L 1.4571 Ͻ0.08 17.5 12.0 2.2 TiϾ5 %C Ͻ 0.7 S31635/316Ti Heat-resistant steels (austenite) 1.4845 Ͻ0.15 25.0 20.5 ...... S31000/310 1.4841 Ͻ0.20 25.0 20.5 . . . Si 2.0 S31400/314 1.4876 Ͻ0.12 21.0 32.0 . . . Al Ti High-temperature steels (austenite) 1.4910 Ͻ0.04 17.0 13.0 2.5 N 0.14 S31653/316LN 1.4961 Ͻ0.10 16.0 13.0 . . . Nb 10 %C ϳ 1.20 S34700/347 1.4959 0.07 20.0 32.0 . . . Al Ti, V 0.07 ...

The heat-resistant materials are covered by notch impact values. Suitable billets for ex- DIN EN 10095, the other materials by DIN EN trusion are therefore difficult to produce free 10216-5 and DIN 17456 (the European standard from cracks in the high-chromium-contain- [EN] has replaced the previous DIN standards). ing materials. The following material groups are classified Hot-brittle lead and sulfur-containing by the hot-working temperature range [Ric 93]: free-machining steels can only be hot rolled with difficulty. Only piercing and rolling ● ␣ Ferritic steels with bcc -iron structure. All over a mandrel are suitable for processing ferritic chromium steels over 12% Cr content these alloys apart from extrusion, which is as well as the steels alloyed with molybde- still the most common process used for this num and/or titanium belong to this group and material group. have a bcc ␣-iron structure. They are char- ● Austenitic steels have chromium contents acterized by a flow stress that decreases rap- over 16% and nickel contents exceeding 8%. idly with increasing temperature so that they At the extrusion temperature the structure of can be readily hot worked. these alloys is the fcc c-iron lattice. The aus- However, this alloy group tends to brittle- tenite is, therefore, usually characterized by ness due to precipitated phases and to grain very good hot workability as well as a low coarsening during the hot working of the tendency to embrittlement. The undesirable starting material by rolling or forging before coarse grain formation found with the ferritic extrusion. The embrittlement reduces the steels does not occur with the austenitic ma- Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 273

Fig. 5.62 Phase diagram according to Schaeffler-DeLong [Ric 93]

terials so that they can also be readily mium and nickel content. The effect of other al- welded. loying elements is determined from their chro- The good workability and the good cor- mium or nickel equivalent. rosion resistance against many media pro- In austenitic steels, d-ferrite is particularly vide the austenitic materials with a wide dangerous because its occurrence limits the range of applications in chemical plant man- extrusion temperature; otherwise, transverse ufacture and in energy production. cracking occurs in the extruded section. The higher the nickel content, the higher is the hot strength of austenitic materials and 5.25.2.2 Extrusion Properties, Defects the more difficult they are to extrude. High- nickel-containing materials can conse- The extrudability and extrusion temperature quently be extruded only at low extrusion range of the different materials are given in Ta- ratios. ble 5.17. ● Austenitic–ferritic alloys with chromium The fewer the precipitates, the better the ex- contents of 18Ð25% and nickel contents of trudability of ferrite and austenite. Single-phase 4Ð7% are used for a range of applications. materials—particularly in the c-range with a fcc They combine to some extent the advantages structure—are particularly easy to extrude [Bur of both the alloy groups described previ- 70, Ben 73]. At the normal extrusion temperatures of 1100 ously. They have a lower susceptibility to Њ embrittlement than do ferritic materials and to 1350 C, the flow stress is in the range 150 are more easily worked than the austenitic MPa (materials with high extrudability) to 400 materials and also offer advantages in spe- MPa (materials with low extrudability) for aus- cific types of corrosion attack. The most tenite and ferrite. The extrusion ratio (V) is usu- well-known representative of this group is ally 8 to 40. the material 1.4462 with 22% Cr, 5.5% Ni, Examples of the relationship of the flow stress and 3% Mo, which is an alloy steel with and the deformation capability on the composi- many applications in the petrochemical in- tion and the temperature are shown by the curves dustry. for different steels in Fig. 5.63. Whereas the workability of homogeneous An overview of the structure of the three ma- structures always increases with increasing tem- terial groups is given by the phase diagram ac- perature, a marked decrease can occur with Cr- cording to Schaeffler-DeLong (Fig. 5.62), which Ni steels as a result of the formation of d-ferrite shows the structure as a function of the chro- above approximately 1200 ЊC. This results in a 274 / Extrusion, Second Edition

risk of transverse cracking during extrusion. The stituents that can form precipitates, the extruded limit is considered to be 2 to 3% d-ferrite at section usually has a fully recrystallized struc- room temperature (corresponding to approxi- ture. mately 8% at the extrusion temperature). At low extrusion ratios, i.e., large tube cross- The calculation of the extrusion load is dis- sectional areas, the core region tends to accel- cussed in detail in Chapter 3. erate. The resultant internal longitudinal stresses The high extrusion temperature produces a can result in lamination such as internal cracking large temperature difference between the mate- within the extruded section that cannot be de- rial and the container and a rapid loss of heat. It tected externally. These defects can only be dis- is necessary to extrude quickly to prevent the covered using ultrasonic testing. billet freezing during the extrusion process. This In contrast, in thin-wall tubes, these stresses also aids the die life. result in cracks originating at the surface. In the deformation zone immediately in front of the die, an adiabatic temperature increase of 5.25.3 Billet Production up to 150 ЊC occurs during extrusion. The higher the flow stress of the material and the faster the 5.25.3.1 Melting, Casting, Hot Working extrusion speed, then the higher the temperature Alloy steels are melted today by the following increase will be, i.e., the closer the process ap- processes [Ric 93]: proaches adiabatic conditions. It is therefore necessary to carefully match the temperature and ● Electric arc melting process with down- speed for materials with low-melting-point con- stream argon-oxygen-decarburization (AOD) stituents to avoid melting and thus cracks in the or vacuum-oxygen-decarburization [VOD] extruded section. ● Induction melting in air or in a vacuum Because the extrusion temperature is above ● Remelting using the electroslag refining pro- the solution heat treatment temperature of con- cess (ESR)

Table 5.17 Extrudability of different material groups (examples) (Ric 93)

Group Characterization Nominal flow stress kf MPa Example, alloy (DIN No.) Billet preheat temperature, ЊC 1 Easily extruded 150 410 (1.4006) 430 (1.4016) 439 (1.4510) 1000Ð1200 2 Good extrudability 200 304 (1.4301) 304L (1.4306) 316 (1.4401) 316L (1.4404) 316Ti (1.4571) 1050Ð1250 3 Difficult to extrude 250 314 (1.4841) 310 (1.4845) . . . (1.4876) 1100Ð1300

Fig. 5.63 Flow stress and workability of steels measured from the number of turns to failure in torsion tests as a function of temperature. (a) Flow stress. (b) d. (c) Deformation capacity [Ben 73] Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 275

The latter guarantees a high cast purity and an 5.25.4 Billet Heating, Lubrication extensively homogeneous distribution of the ele- ments that tend to segregation such as niobium 5.25.4.1 Billet Heating and molybdenum. Continuous casting is carried out if the charge Apart from the uniform through heating of the size permits; otherwise, mold casting is used. billet, it is important during heating to avoid ox- Only pure and low-alloy carbon steels can be idation and surface decarburization of the heat directly extruded without previous hot working. treatable steels. Higher-alloyed materials have to first be forged Billet heating to the required temperature of or hot rolled to destroy the columnar cast struc- 1100 to 1300 ЊC is carried out in rotary hearth ture and produce a homogeneous structure [Man furnaces with protective atmospheres, in induc- 91, Bil 79, Bur 70]. tion furnaces, or in a combination of both with a gas furnace for preheating to approximately 700 ЊC and final heating in an induction furnace. 5.25.3.2 Homogenizing, Billet The slower heating in the rotary hearth fur- nace with its more uniform through heating can Preparation have a homogenizing effect on the structure of Homogenizing of the rolled or forged billets some steels. is rarely carried out and only when the homo- The cost advantage of gas heating also plays geneity or grain distribution is inadequate for a role, particularly with infrequent alloy defect-free extruded bars [Ric 93]. The need for changes. The possibility of a rapid temperature homogenization increases with increasing con- change with small batch sizes of different steels tent of the elements molybdenum, niobium, and is an argument for induction heating. titanium, which result in the formation of seg- Induction furnaces are more expensive to op- regations. erate but heat more quickly, and the temperature Because the billet surface in direct extrusion can be more accurately controlled, which is the with lubrication without a shell and using coni- reason why induction furnaces are used in par- cal dies becomes the surface of the extrusion, ticular for high-alloy steels with exactly speci- the 8- to 12-m-long forged or hot-rolled round fied preheat temperatures. Complex alloy steels bars have to be cross sectioned and carefully ma- often have only a narrow extrusion temperature chined by turning or peeling. Coarse turning range. The upper limit is determined by the sol- marks result in fish-bone patterns on the ex- idus temperature and the lower limit by the press truded tube surface. A chamfer on the front face power. of the billet simplifies the uniform flow of the The temperature of the billet core lags behind during the rapid induction heating, which can glass lubricant during extrusion (see below). result in core cracking in crack-susceptible Billet diameters from 200 to 300 mm and bil- steels, e.g., heat-resistant ferrites with high chro- let lengths up to 700 mm are standard. mium contents. In this case the preheating in ro- Whereas in the extrusion of copper alloy tubes tary hearth furnaces mentioned previously is large billets are pierced in the press, this process recommended. is not possible with alloy steels, which are also Although induction furnaces with the shorter processed on hydraulic horizontal presses. The heating times of 5 to 15 minutes largely heat free mandrels cannot withstand the severe thermo- of oxidation, a protective atmosphere is often mechanical stresses and would rapidly wear be- passed through the induction coil with the oxide- cause of the absence of the lubricating film. The susceptible steels. This can be necessary, in par- billets are therefore bored, although for diame- ticular, with furnaces with a vertical axis in ters up to 60 mm, a deep hole without expansion which the billets oxidize more severely because is sufficient. The billet length to bore ratio of the larger air throughput from the chimney should not exceed the value of 7:1; otherwise, effect. the bore will deviate. With larger bore diameters, Salt bath ovens for billet heating are rarely the predrilled billet is expanded in a separate used. Their advantage is the capability of exact piercing press [Bil 79]. The hole diameter is al- temperature control and the avoidance of oxi- ways larger than the mandrel diameter so that dation. They are, however, unwieldy and expen- the lubricant spread in the bore is not stripped sive and the salt carryover presents a large en- off by the mandrel. vironmental risk [Sar 75, Lau 76, Kur 62]. The 276 / Extrusion, Second Edition

diffusion of nitrogen damages the surface zones not break up even when being extruded of austenite and, as a result, the extruded tubes through the die aperture. It should prevent sometimes have to be ground over. contact with the tool working surfaces as If the turned billet has to be pierced before well as reduce the load. extrusion, this process is carried out in vertical ● It should form an insulating layer between piercing presses linked to a vertical single-billet the billet and the extrusion tooling and thus furnace in order to balance out the heat losses increase the ability of the tooling to with- and to exactly control the extrusion temperature. stand the thermomechanical stressing. A low In the piercing press, for example, the existing thermal conductivity is desirable. bore diameter is expanded from 35 mm to a ● The lubricant applied to the billet surface im- range of 53 to 74 mm, increasing the billet mediately after heating should protect this length by up to 20%. and the emerging extrusion from oxidation Carbon steels and low-alloy steels can oxidize with a thick film. Thin oxide coatings that so severely that removing the thick oxide layer have already formed should as far as possible with a strong jet of high pressure water after be removed or absorbed by the lubricant. billet heating is recommended. This process is ● Glassy lubricants have a larger volume con- so quick that hardly any heat is lost. traction on solidification and cooling than The high billet temperature of high-alloy steel, and the lubricant can readily break off, steels necessitates a fast transport from the oven particularly with rapidly cooled extrusions. to the press. Otherwise, the material would cool too quickly, particularly with small billet diam- Glasses that are optimally suited for all steels eters where there is a large surface to volume with their different extrusion temperatures and ratio. flow resistances do not exist. Therefore, glasses The use of radiation protective sheaths has with different compositions and thus a different been reported in the United States. These have viscosity temperature dependence are recom- to be removed just before extrusion when the mended for the various material groups (Table billet is loaded into the container [Lau 76]. How- 5.18). ever, a thick glass layer, which surrounds the hot The criterion for the upper limit of the tem- billet as a lubricant, also acts as insulation. perature application of glasses in extrusion is the so-called “hemispherical” temperature (see foot- 5.25.4.2 Lubrication note in Table 5.18). The viscosity is then ap- proximately 2 104 poise. The lower limit is Similar to other difficult-to-extrude materials, characterized by the compressive softening tem- e.g., the nickel alloys, the development of lubri- perature (CST) where the glass is extremely cation technology was an important requirement tough (approximately 1010 poise) but no longer for the economic extrusion of alloy steel tubes. breaks in a brittle manner. The extrusion tem- The graphite and salt containing oil suspensions perature should be at least 200 to 300Њ above the initially used as lubricants would carburize the CST. surface of low-carbon steels, among other dis- Standardized glasses are available from vari- advantages. Glass lubrication technology was ous manufacturers. Today only a few types are rapidly adopted and had to meet numerous re- used in extrusion with SiO as the main constit- quirements [Ben 73]: 2 uent. They contain oxides of sodium, potassium, ● The lubricant with its good slip properties calcium, magnesium, aluminum, and boron. has to form a closed lubricant film that does Barium oxide is also sometimes added. These

Table 5.18 Glasses used successfully as lubricants for extrusion (Deg 74)

Composition Extrusion temperature

No. Na2OK2O MgO CaO BaO Al2O3 B2O3 SiO2 Maximum HK(a), ЊC Minimum DET(b), ЊC 1 ...... 1300 765 2 ... ... 1220 690 3 ... 990 645 4 710 510 (a) Hemispherical temperature (HK) is the temperature at which a specific quantity of the glass powder melted in a heated microscope forms a hemisphere. Viscosity is approximately 2 104 poise, i.e., highly fluidic. (b) Compressive softening temperature (DET) is the temperature at which the glass has a viscosity of approximately 1010 poise and is extremely tough but no longer brittle. Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 277

additions lower the melting point and stabilize The stem loading is, as usual, restricted to a the glasses. maximum of 1200 N/mm2. In order to achieve The billet from the preheating furnace is this, there are presses with several hydraulic cyl- rolled over a sloping table covered in the glass inders that can be individually switched off to powder and also has powder spread in the bore avoid overloading with small billet diameters. using a spoon. The powder immediately melts The press capacity of these presses is 35 to a and protects the billet from further oxidation and maximum of 50 MN. Extrusion ratios (V)of10 can even dissolve any oxide that has formed. If to 40 are standard. the billet is not pierced before it is loaded into Because extrusion is carried out without a the press, the procedure for applying the glass shell the dummy blocks used have to be matched powder has to be repeated (after the reheating). as closely as possible to the container bore; a Different glasses are sometimes used for internal cleaning pad is not required. and external application. The grain size of the A laminar quasi-stationary material flow is glass also plays a role because fine grain-size achieved by using dies with a conical entry inlet powder melts more quickly than coarse grain. angle of 120 to 150Њ or dies with a curved inlet. In front of the die there is a pressed disc of A fast die-change system is needed to avoid the same glass powder or fiber glass (with water overheating of the dies and to enable die re- glass as the binding agent) that slowly melts dur- working after every extrusion. If a die has to be ing the extrusion process and encloses the front reused, it is cooled to approximately 250 ЊCin of the extrusion. The thickness of the glass layer a water bath. is of the order of 10 lm. The container is not heated during extrusion Careful matching of the type of glass, the ex- because the heat transferred from the hot billet trusion temperature, and the extrusion speed, as is sufficient to keep it warm. The container only well as the quantity of glass is important to pro- has to be maintained at approximately 500 ЊC duce a perfect glass, coating on the extrusion. If before starting extrusion and during production too little glass flows through the die, grooves interruptions. There are extrusion presses with form; if the quantity is too high, the surface of rotating container devices in which two contain- the section exhibits bulges or a so-called “orange ers are used in turn: while one container is being peel surface” corresponding to the individual used for extrusion, the other can be cooled and grains [Bur 70]. cleaned. It should be mentioned that the removal of the The extrusion mandrel moves with the stem glass film from extruded tubes is expensive and during extrusion so that no mandrel cross section did not have to be carried out with the previous has to have prolonged contact with the hot de- graphite-containing lubricants. formation zone in the die. The mandrel is cooled internally as with copper materials or sprayed 5.25.5 Extrusion Process externally with water between extrusions. A high extrusion speed is possible for the ma- In order to remove the glass lubricant from jority of iron alloys. Depending on the material the tooling, the following operations have to be and the extrusion ratio, ram speeds in the range carried out between extrusions: of 20 to 300 mm/s are attained so that the billets are extruded within a few seconds. High extru- ● The mandrel is sprayed externally with wa- sion speeds are necessary to keep the stresses on ter. the tooling to a minimum. The tooling tempera- ● The container is cleaned with a steel brush. ture should not exceed 500 ЊC. The melting ● The die is quenched in a water bath. properties of the glass used for lubrication pre- vents the use of the maximum extrusion speeds The discard and tube are separated with a hot so that in practice, the minimum ram speed is 50 saw or a shear between the opened container and mm/s and the upper limit is 200 mm/s. the die. The extruded length is restricted to a High extrusion speeds can be achieved only maximum of 20 m for steel tube extrusion to with hydraulic accumulator drives. In the steel ensure that the contact time between the material industry, direct oil operating systems are not and the die is not too long. Runout tables up to used. 50 m, which are used for aluminum and copper The construction of the horizontal hydraulic alloys, are not possible with steel. The runout tube extrusion presses are basically similar to trough for steel can also be fitted with powered those used for copper tube extrusion. rollers. 278 / Extrusion, Second Edition

In the majority of cases, quenching with water Extruded and metallic clean semifinished is carried out directly behind the die because products can be directly used as hot-finished most of the alloys produced have to be cooled semifinished products after finishing (straight- as quickly as possible to avoid precipitates of ening and cutting to the finished length). If nec- carbides and intermetallic phases such as the r- essary, the surface has to be ground. and v-phases. Precipitated phases result in em- In other cases, the extruded tubes are further brittlement and in numerous cases, unfavorable processed by cold pilgering and/or cold draw- corrosion behavior. Water quenching also has ing. the advantage that the glass film breaks up so that it can be easily removed. Quenching should 5.25.7.2 Testing be avoided only with materials where there is a In order to prevent d-ferrite occurring in aus- tendency for cracking with rapid cooling and tenitic steels, the billets are tested randomly by with ferrites where martensite can form [Ric 93]. a magnetic balance. The permitted upper limit is 3%. Crack testing of the billet surface (penetrant 5.25.6 Tooling testing) is required only occasionally. As well as the standard visual inspection of Tooling design and other details are covered the tube surface, ultrasonic testing is required, in Chapter 7. particularly with high-alloy materials and for The shape-forming tools—die and mandrel— some critical applications. This is carried out are usually made in steel 1.2343 (H11, UNS with either fixed test heads and rotating tubes or T20811). The die with the conical entry is usu- with static tubes and rotating test heads. Water ally located in a die holder with a truncated is used as the coupling agent. shaped external face, which mates with the con- The danger of internal cracks occurs only with ical container seat. Container liners are also fre- the highest-alloyed materials (molybdenum- quently manufactured in the steel 1.2343. containing). Internal inspection with an endo- Dies that are removed and cooled can last be- scope is specified only for these. tween 20 and 40 extrusions and can then be opened up to another dimension. The mandrel life is around 400 extrusions and that of the con- 5.26 Steel Sections tainer about 4000 extrusions, with the possibility of reworking. 5.26.1 General The use of high-alloy materials for dies and liners in the extrusion of alloy steels has been 5.26.1.1 Extrusion Process, Materials tried but for economic reasons is not used As with alloy steel tubes, only horizontal hy- [Ben 73]. draulic extrusion presses are used for steel sec- tions. The laminar extrusion takes place with 5.25.7 Further Processing, Testing glass lubrication and without a shell. In this case the competing processes, which are economi- 5.25.7.1 Further Processing cally more competitive for suitable quantities and specific sections, again have a higher market Steel, ball shot blasting (VacuBlast) is used, share than the extrusion process. followed by pickling in nitric acid with hydro- In principle, all steel grades that can be hot fluoric acid additions to remove residual glass worked can be extruded to sections. Neverthe- lubricant from the internal and external surfaces less, the tool wear increases drastically with in- of the tube. Hot molten sodium and calcium salts creasing extrusion temperature. The higher the are used to some extent for surface cleaning, but flow stress at the extrusion temperature, the they are expensive and environmentally un- lower the possible extrusion ratio will be and friendly because salt carryover is difficult too also, as a rule, the length of the extrusion. avoid. Heat treatment after extrusion is necessary if 5.26.1.2 Competition with Other it is not possible to arrange the cooling of the Deformation Processes hot extrusion so that a precipitation-free struc- ture is obtained. The heat treatment after extru- The processes used to produce sections are: sion then has the nature of a homogenization or ● Hot rolling: The dimensional range falls a solution heat treatment. within a circumscribing circle of 250 mm di- Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 279

ameter. The possible weight per meter is 1 to 7 kg with minimum wall thickness of 3 mm. The minimum wall thickness tolerance is 0.3 mm. In the hot-rolling process, cross-sectional undercuts are impossible and hollow sections cannot be produced. A rela- tively large tonnage is required to justify the cost of the manufacture of the profiled roll pairs needed for the sequential tools. ● Machining from solid material: The high material loss and the expensive process en- sure that this process usually follows a non- machining deformation and only when other methods of producing the final shape have to be excluded. ● Cold profile forming from steel strip: This process requires material that can be cold bent and the section must have a uniform wall thickness. The final shape is produced from the flat sheet using several roll sets in Fig. 5.64 Extruded steel profiles [Hoe 90] the bending machine. The form-shaping tool pairs are expensive, so the process is eco- nomic only for large quantities. Extrusion is also preferred for sections that ● Joining of part sections by longitudinal can be more economically produced by welding welding, riveting, or bolting: Extruded sec- but which cannot have any weld for safety tions are frequently used to produce more reasons. complex sections or larger cross-sectional The average lot size is 10 billets per order and areas. is therefore generally small. If larger quantities ● Extrusion: The possible dimensional range are required, an alternative method of produc- falls within a circumscribing circle of ap- tion is usually sought for cost reasons, e.g., hot proximately 250 mm diameter with a weight rolling, even if the shapes have to be slightly per meter of 1.5 to 100 kg/m and a wall modified and simplified. thickness of at least 3.5 mm. The thickness Of the steel sections produced in a section tolerance that can be achieved is 0.5 mm. mill, only 8 to 10% are extruded. Approximately Complicated sections and also hollow sec- 65% of sections are hot rolled and the others are tions can be produced (Fig. 5.64). produced using other processes.

Obviously, given the severe thermal stressing 5.26.2 Materials, Starting of the form-producing tooling, the degree of Material, Process complexity and the range of sections that can be Extrusion produces the same mechanical produced cannot be compared with aluminum properties as hot rolling. In both processes the sections. Sharp edges are impossible because of hot-working temperature is higher than the re- the risk of tooling failure and the thermome- crystallization temperature, and a fine-grain re- chanical localized stresses. External edges crystallized structure is produced. should, therefore, have a minimum radius of at Structural steels are processed along with heat least 1.5 mm and internal edges on hollow sec- treatable steels and alloy steels (stainless, heat tions a minimum of 4 mm (Lin 82). Tolerances resistant as well as tool grades), as well as that are too wide for the application can often nickel-base alloys and, more rarely, cobalt-base be reduced by subsequent cold drawing. alloys and titanium alloys. Extrusion is used even for sections that can The starting material for carbon steels is con- be produced by hot rolling when it is not eco- tinuously cast and supplied in approximately 10 nomical to produce the roll sets needed for the m lengths from the foundry cleaned by pickling multistand mills because of the small volume or shot blasting. required. Extrusion can then be used for the pro- The starting material for alloy steels has to be duction of prototypes and first series. hot rolled or forged in the same way as for tube 280 / Extrusion, Second Edition

production for homogenizing and to achieve a the container has been opened, the discard is cut fine-grain, crack-free structure. from the section with a hot saw. The section is The extrusion process with a lubricated con- pulled back through the die and then removed tainer and without a shell resembles that de- on a powered roller conveyor. After pushing out scribed for tube production. The large billets the discard together with the dummy block, the used for heavy sections and, in the case of hol- die is changed for a new or reworked die using low sections, prebored billets also have to be a rotating arm or a slide. The die has to be chamfered on the end surfaces to achieve the checked for the dimensional tolerances after uniform flow of the glass lubricant. each extrusion and, if necessary, reworked be- cause of the high thermomechanical stresses. 5.26.3 Billet Production, The length of the section that is extruded as Extrusion, Further Processing quickly as possible has a maximum length of 20 m so that the die that is deformed by the tem- perature effect during the extrusion does not ex- 5.26.3.1 Billet Preparation, ceed the extrusion tolerances toward the end of Heating, Lubrication the extrusion. As soon as an order is passed to production, The required high ram speeds of up to 300 the bars delivered to billet production are cross- mm/s can only be achieved by a water hydraulic cut, peeled, turned or ground and chamfered. system. The billets for hollow sections have also to be The sections cool in free air but bend and bored. The hole is larger than the circumscribing twist significantly in the longitudinal direction circle of the mandrel cross section. The billets and deform in the transverse direction because are heated to 1000 to 1300 ЊC in a rotary hearth of the different flow behaviors of different cross- furnace with a reducing protective gas burner or sectional areas in the die and the faster cooling in an induction furnace similar to steel tubes. of thin legs after extrusion. To avoid accidents Salt bath heating is also used. from moving sections, the runout table is occa- After the billets have been removed from the sionally covered to form a tunnel. furnace, they are rolled down a table with glass In multihole extrusion, the extruded sections powder. The glass film formed from the molten usually have varying lengths because of the dif- powder prevents high thermal losses and oxi- ferent amounts of lubricant in the die openings. dation of the billet surface and can even dissolve the oxide that has formed. A 4 mm-thick glass powder disc bonded with water glass is placed 5.26.3.2 Further Processing in front of the conical die. The type of glass used The extruded sections have to be straightened is the same as for the extrusion of steel tubes and detwisted by stretching 2 to 3% on stretchers (e.g., type 3 in Table 5.18 for carbon steels). with rotating stretcher heads and capacities of Extrusion. The tube and solid extrusion up to 3000 kN. Hot stretching is also used for presses used have a capacity of 15 to 25 MN. the higher-strength alloys (alloy steels, Ni, and The diameter of the billets varies from 150 to Ti alloys). This stretching process is sufficient 250 mm, depending on the section cross-sec- with carbon steels to break off the 10 to 20 lm- tional area, and the length can extend to 900 thick glass film from the lubrication. Alloy mm. Extrusion ratios up to 100 are possible but steels, however, have to be pickled and some- rarely used. times first shot blasted. Hollow sections are extruded over round or After stretching, it may be necessary to carry profiled mandrels that are not internally cooled out further straightening on a roller correction and that move with the stem during the extrusion machine or even on a straightening press with process. The thickness should be at least 20 mm profiled tools. This straightening process adds to be able to withstand the large thermal stresses. significantly to the costs of producing steel ex- It is not possible to use bridge dies as with alu- truded sections [Lin 82]. minum and copper because of the high extrusion If a coarse grain structure or, in the case of temperature, the relatively high flow stress, and alloy steels, excessive mechanical properties are the resultant thermomechanical stresses on the detected, a subsequent annealing treatment is re- tooling. quired. The billet loaded into the container is ex- Tight tolerances are obtained by bright draw- truded to a discard length of 10 to 20 mm. After ing. If necessary, sections can also be ground. Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 281

5.26.4 Tooling Copper forms a continuous solid solution with nickel. Molybdenum increases the strength by Information on the tooling for steel sections the formation of a solid solution. Intermetallic can be found in Chapter 7 in the section on tool- phases (mainly Ni Al) increase the strength as ing for the glass-lubricated extrusion of tita- 3 do carbides and carbon-nitrides in conjunction nium, nickel, and iron alloys. with titanium, niobium, molybdenum, and chro- mium.

Extrusion of Semifinished 5.28 Materials, Properties, Products in Nickel Alloys and Applications (Including Superalloys) Table 5.19 refers to the nickel alloy DIN stan- dards. Table 5.20 shows some important and typical nickel alloys that are processed by extru- Martin Bauser* sion. Pure nickel and low-alloy nickel materials have properties that are particularly suited to 5.27 General chemical processes and electronic applications. Nickel alloys are corrosion resistant to many re- Nickel alloys are used for many applications ducing chemicals and cannot be bettered for re- in machinery, chemical engineering, industrial sistance to strong alkalis. The food industry is furnaces, electrical engineering, electronics, and an important application. Nickel also has a high in power stations. Extrusion is used to produce electrical conductivity, a high Curie tempera- tubes and wire as well as bars for feedstock for ture, and good magnetostrictive properties. Bat- the manufacture of turned, forged, and impact- tery components and spark electrodes are appli- forged components. cation examples. High-alloy nickel materials (in particular with Low-alloy nickel materials are often used in iron, cobalt, and molybdenum) are referred to as heat exchangers because of the good thermal superalloys and are suitable for applications at conductivity. Good workability and good weld- temperatures of more than 1000 ЊC. They are ability mean they can be readily worked. used in gas turbines and jet engines. These mul- Monel, i.e., nickel-copper alloys, are the most tiphase alloys can often hardly be referred to as widely used high-nickel-containing alloys and nickel alloys, but they do not belong to any spe- have been used for over a hundred years. Their cific alloy group. Extrusion is important for the strength is higher than pure nickel, but in spite production of bars and rods in these high- of this, they can be easily worked and possess strength alloys because they are almost impos- good corrosion resistance against many environ- sible to forge [Vol 70]. mental factors. Their good resistance to acids is The extrusion process is largely identical to utilized in the chemical industry and the good that described for alloy steel tubes. thermal conductivity relative to other nickel al- Pure nickel melts at 1453 ЊC and has a density loys in heat exchangers. The seawater resistance comparable to iron and copper of 8.9 g/cm3. of Monel is useful in ship construction. Nickel and many technically important nickel Certain nickel-iron alloys have a special co- alloys have a fcc lattice up to the melting point efficient of expansion property, which makes and therefore have good hot and cold workabil- them suitable for use with glasses. The soft mag- ity. The flow stress varies considerably depend- netic behavior of nickel-iron alloys is also util- ing on the alloy. ized. The high-alloy superalloys are very difficult Nickel-chromium alloys and nickel-iron- to extrude because of both the high extrusion chromium alloys are characterized by their good temperature (up to 1300 ЊC) as well as the high corrosion resistance, good mechanical proper- extrusion loads needed. ties, and excellent oxidation resistance at high temperatures. Combined with their good work- *Extrusion of Semifinished Products in Nickel Alloys (In- ability, these alloys have a wide range of appli- cluding Superalloys), Martin Bauser cations in heat treatment furnaces, incineration 282 / Extrusion, Second Edition

Table 5.19 German Standardization Institute (DIN) standards for nickel alloys

Similar Comments ASTM Standard Designation to DIN standard Title DIN 17740 Nickel in semifinished Composition B39 Standard Specification for Nickel products DIN 17741 Low-alloyed nickel Composition ...... wrought alloys DIN 17742 Nickel wrought alloys Composition B167 Standard Specification for Nickel Chromium-Iron with chromium Alloys (UNS N06600, N06690, N06025) Seamless Pipe and Tube DIN 17743 Nickel wrought alloys Composition B164 Standard Specification for Nickel-Copper Alloy with copper Rod, Bar, and Wire DIN 17744 Nickel wrought alloys Composition B335 Standard Specification for Nickel Molybdenum- with molybdenum and Alloy Rod chromium DIN 17745 Wrought alloys of nickel Composition B407 Standard Specification for Nickel-Iron-Chromium and iron Alloy Seamless Pipe and Tube B408 Standard Specification for Nickel-Iron-Chromium Alloy Rod and Bar DIN 17751 Tubes in nickel and Dimensions, B161 Standard Specification for Nickel Seamless Pipe nickel wrought alloys mechanical and Tube properties DIN 17752 Bar in nickel and nickel Dimensions, B160 Standard Specification for Nickel Rod and Bar wrought alloys mechanical properties DIN 17753 Wire in nickel and nickel Dimensions, B473 Standard Specification for UNS N08020, UNS wrought alloys mechanical N08024, and UNS N08026 Nickel Alloy Bar properties and Wire B475 Standard Specification for UNS N08020, UNS N08024, and UNS N08026 Nickel Round Weaving Wire

plants, steam generators, and resistance heating Nickel alloys can be continuously cast. If the elements. charge size is insufficient for continuous casting, The superalloys contain cobalt, molybdenum, chill mold casting with all its disadvantages iron, titanium, and aluminum, in addition to (coarse, radial columnar, and frequently crack- chromium and nickel. A wide range of appli- susceptible structure) is preferred. For this rea- cations are possible at high and very high tem- son, as well as reducing the large chill mold cast- peratures. These include gas turbines, jet en- ing to the billet dimensions, they are usually gines, and nuclear power stations. Hot-working broken down by forging or rolling. This aligns tooling is also produced from these alloys. the grains axially and frequently produces a fine- There are high-strength dispersion-hardened grained recrystallized structure. It is then nec- nickel alloys that have to be produced by powder essary with the high-alloy materials to follow metallurgy. These alloys are discussed in the this with a homogenization heat treatment to section “Extrusion of Powder Metals.” minimize the segregation. The feedstock material supplied to the press as long bars is sawn to length, turned, and cham- fered. If tubes are being produced, the billets 5.29 Billet Production usually have to be bored. A good billet surface is necessary as is the case with alloy steels be- Depending on the composition and the ma- cause it will form the surface of the extrusion as terial requirements, various routes are used to a result of the glass lubrication (see below) and produce the starting material and these have the resultant laminar material flow. been described in section 5.41.5 [Ric 93]. High-alloy materials are melted in an electric arc furnace and, if a specific purity is required, subjected to the Vacuum-oxygen-decarburiza- 5.30 Billet Heating tion (VOD) process in an evacuated ladle. This removes the carbon and nitrogen and reduces the The billets have to be heated in a low-sulfur sulfur content drastically. furnace atmosphere because of the sensitivity of Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 283

nickel and its alloys to intercrystalline attack by oil for low extrusion ratios and thus low extru- sulfur. It should be weakly reducing in gas-fired sion temperatures, higher-alloyed material bil- furnaces because nickel and nickel-copper al- lets are only lubricated with glass using the Se«- loys in particular tend to intercrystalline corro- journet process similar to alloy steels [Pol 75]. sion and thus embrittlement in an oxidizing at- Suitable glasses for the corresponding tempera- mosphere even without sulfur [Vol 70]. ture are selected from Table 5.18. The glass with Because the oxide layer of severely oxidizing the highest melting point is used for the high- nickel-iron and nickel-copper alloys also re- alloy materials with extrusion temperatures of duces the effect of the lubricant in extrusion and almost 1300 ЊC. The glass powder is applied produces a poor surface quality, these materials even with two-stage heating (gas furnace-induc- should be heated as quickly as possible—at best, tion furnace) only after the billet has left the in- in an induction furnace. duction furnace. With high-alloy, crack-sensitive alloys on the other hand, and with coarse grain cast billets, a slow heating rate is important because if the 5.31.2 Deformation Behavior heating is too rapid the thermal stresses can re- and Extrusion Data sult in grain-boundary cracking. Some nickel- Figure 5.65 shows the tensile strengths of chromium-cobalt alloys cannot be directly various nickel alloys as a function of tempera- charged into the hot-gas-fired furnace if they ex- ture. hibit coarse grain or severe segregation but have The flow stress of pure nickel is low enough to be slowly heated from a low temperature to for extrusion even below 1000 ЊC. Similarly, the the extrusion temperature so that the temperature soft alloys of nickel with copper or with up to gradient remains low in all phases of heating 20% Cr have a relatively wide temperature range [Pol 75]. Reducing continuous, chamber or ro- for hot working. The complex alloys on the tary furnaces are used for this. other hand have narrow deformation tempera- High-alloy materials that are not crack sensi- ture ranges, in particular, the molybdenum-con- tive and those that have been well homogenized taining and the high-strength nickel-chromium and have a fine-grain structure can, however, be alloys, and these must be accurately maintained heated in an induction furnace that guarantees to avoid cracking. the exact final temperature. With complex al- The addition of strength-increasing elements loys, the exact setting of the final temperature is to improve the mechanical properties at high very important if a low-melting-point eutectic has formed (e.g., with niobium). To save energy and to ensure the slow heating of crack-sensitive materials mentioned above, a gas furnace can be used for the base heating (up to 1000 ЊC) and an induction furnace for the final stage [Ric 93]. Salt bath heating cannot be used because of the risk of the diffusion of embrittling elements.

5.31 Extrusion

5.31.1 Billet Preparation, Lubrication As with alloy steels, prebored or pierced bil- lets are used for the production of tubes to avoid high mandrel wear and to obtain a low eccen- tricity [Eng 74]. With large internal diameters the billets are again expanded on a separate piercing press and then reheated in an induction furnace. Whereas nickel, low-alloy nickel materials, Fig. 5.65 Tensile strength of nickel alloys as a function of NiCu, and NiFe can be lubricated with graphite the temperature [Inc 88] 284 / Extrusion, Second Edition Application Application Application lightbulbs, electrode tubes plugs units, ship construction, chemical industry industry integrated circuits Corrosion-resistant components, Lightbulbs, electrotubes, spark Corrosion resistant construction Chemical industry, petrochemical Relay components, transformers ... Properties Properties thermal conductivity, corrosion resistant corrosion resistance Properties slightly magnetic ...... Low temperature expansion Best coefficient of expansion, Glass/metal ceramic bonds, e.g., for product Strength product Strength Semifinished Semifinished product Strength Semifinished 2.0 Mn R, S, D 450Ð700 Good strength, best high- Other Other Ͻ Other Al Fe 0.35 ...... R, S, D 370Ð590 High electrical and Ͻ 0.4 Ͻ Ni 63 28Ð3463 1.5Ð2.5 27Ð34 . . 0.5Ð2.0 . 2.2Ð3.5 0.3Ð1.0 Ti R, S, D 600Ð800 Age hardening 97.0 . . . 1.5Ð2.5 ...... S, D 99.2 Ͼ Ͼ Ͼ Ͼ Alloy 42 40.0Ð43.0 Remainder . . . R, D Alloy 48 46.0Ð49.0 Remainder . . . R, S, D ... NA 11 Alloy 200 NA 13 Alloy 400 Na 18 Alloy K500 Standard British/U.S. Ni Cu Fe 2.4110 2.4066 2.4360 2.4375 1.3917 1.3922/26/27 NiMn2(b) Nickel-copper alloys DIN (Monel) 17741 Table 5.20 Some important(a) Nickel typical and nickel low-alloyed alloys nickel materials that are extruded MaterialNi 99.2 Standard DIN 17740 British/U.S. NiNiCu30Fe Fe DIN 17743 NiCu30Al DIN 17743 MnMaterial Al Ni 42 Standard DIN 17745 British/U.S. Ni 48 DIN 17745 (c) Nickel iron alloys Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 285 Application tools industry industry aircraft petrochemical industry, chemical industry Jet engine Gas turbine, hot-working Chemical industry, paper Chemical industry, paper Chemical industry, Gas turbine, Application gas turbines C Њ Properties C 1010 properties properties Њ age hardening, corrosion resistant temperature, resistant to oxidation 700 Stable up to high 1200 Creep resistant up to 1100 Creep resistant up to 900 ϳ ϳ ϳ Properties product Strength Semifinished product Strength S, P, R, D 1100 Age hardening, high hot strength Components subjected to creep loading Semifinished 2 14Ð172 15Ð17 . . . 3.0Ð4.5W S, R, D S, R, D 690Ð850 690Ð850 Excellent corrosion Excellent corrosion Ͻ Ͻ 1.9Ti 3 1.5 18 . . . 2.5 Ti1.5Al S, P, R Ͻ Ͻ Cr Fe Co Mo Other 5 0.2Ð0.5 . . . S, P, R, D 650 High oxide and corrosion resistance Furnace components, heating conductor, Ͻ 58 20 72 14Ð1765 6Ð10 18Ð21 ...... 2.3 . . . 1.0Ð1.5Al S, R, D 550 Heat resistant, corrosion resistant Furnaces, chemical industry, spark plugs 72 18Ð21 Ͼ Ͼ Ͼ Alloy C4 Remainder 14Ð18 Alloy 90 Alloy C276 RemainderAlloy 625 14.5Ð16.5 Remainder 4.0Ð7.0 20Ð23 ...... 8Ð10 3.15Ð4.15Nb S, R, D 690Ð830 High strength without Alloy 617 Remainder 20Ð23 . . . 10Ð15 8Ð10 Al, Ti S, R, D NA 14 Alloy 600 Alloy 80A Alloy 75 Standard British/U.S. Ni 2.4610 2.4819 2.4856 2.4663 2.4816 2.4952 2.4951 NiCr15FeNiCr20TiAl DIN 17742 DIN 17742 (e) Nickel alloys with chromium, molybdenum, and/or cobalt super alloys NiCr15Co14MoTiAl 2.4636 Alloy 115 51 15 . . . 14 4 4Ti5Al S, P (d) Nickel-chromium (iron) alloys MaterialNiCr20Ti Standard DIN 17742 British/U.S. NiMaterial CrNiMo16Cr16Ti Fe Din 17744 Ti Other NiMo16Cr15WNiCr22Mo9Nb DIN 17744 DIN 17744 NiCr23Co12Mo DIN 17744 NiCr20Co18Ti 2.4632 NA19 286 / Extrusion, Second Edition

temperatures results, on one hand, in a reduction mains between the two limits. In other words, if in the liquidus temperature and, on the other the plant is capable, “isothermal extrusion” is hand, an increase in the flow stress. required. Extrusion presses built especially for These difficult-to-extrude alloys require a nickel have a suitable speed control system. Oil high degree of homogeneity in the starting ma- hydraulic presses are preferred to water hydrau- terial (see section 5.29), uniform billet heating lic ones [Lau 76]. A high press power (up to 60 (see section 5.30), correct lubrication condi- MN) simplifies the extrusion of the high-alloy tions, and exact maintenance of the extrusion nickel materials with high flow stresses [Eng conditions. Economic production is, therefore, 74]. Because the billet rapidly cools at this slow not possible without an extrusion plant with a speed with the risk of freezing, only short billets correspondingly high extrusion power [Inc 86]. can be used. Table 5.21 gives data for the extrusion of Ultrasonic testing of extruded products is rec- some nickel alloys. ommended for the crack-sensitive materials. In the high-strength nickel alloys, there are some with a low-melting-point eutectic (e.g., with niobium) that can only be extruded with a maximum of 1150 ЊC and thus with a low ex- 5.32 Tooling, Further Processing trusion ratio (e.g., Inconel 625). Some superalloys, for example, the high- Information on the tooling used and the tool- molybdenum-containing alloys, have to be ex- ing design is given in Chapter 7. truded above 1400 ЊC to obtain low flow The dies are, as described for alloy steel tubes, stresses. Preferably, extrusion is carried out be- conical to obtain a laminar flow with the glass low 1300 ЊC because of the high tool wear and lubrication. the low viscosity of the glasses used, and the The further processing is also similar to that resultant low extrusion ratio is accepted. This described for alloy steel. The glass lubricant film naturally means more expensive further pro- is removed by shot blasting with steel grit fol- cessing. lowed by pickling. If the glass skin is broken up by quenching at the press (to suppress precipi- tation), shot blasting and pickling may not be 5.31.3 Defects and Their Prevention required. Cold pilgering or drawing is then car- Narrow limits are placed on the extrusion ried out, possibly with intermediate annealing. speed, particularly for the complex molybde- num-containing nickel alloys. If the extrusion speed is too slow, the lubricant film breaks up, resulting in a rough surface on the extruded product and/or the extrusion process “freezes.” If the extrusion speed is too high, the exit tem- Extrusion of Semifinished perature can increase to such an extent that transverse hot cracking occurs or—particularly Products in Exotic Alloys with thick bars—lap defects occur. Conse- quently, especially for the slowly extruded com- Martin Bauser* plex high-alloy steels with extrusion speeds be- low 10 mm/s, the extrusion speed has to be All the materials described in this section are controlled to ensure that the exit temperature re- rarely used in long lengths and are therefore not

Table 5.21 Data for the extrusion of some nickel alloys (see Table 5.20 for examples indicated by (b), (c), (d), and (e) (Lau 7, Vol 70, Ric 93)

2 Group Characterization Approximate Kf value, N/mm Billet preheat temperature Examples 1 Easily extruded 150 900Ð1100 ЊC All nickel and the low-alloyed nickel materials 2 Good extrudability 200 1050Ð1150 ЊC Monel(b) Nickel iron(c) 3 Difficult to extrude 250 1100Ð1250 ЊC Ni-Cr materials(d) 4 Very difficult to extrude 320 1150Ð1280 ЊC Ni-Cr-Mo materials Super alloys(e)

*Extrusion of Semifinished Products in Exotic Alloys, Martin Bauser Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 287

extruded in Germany. However, they are in- cluded for completeness.

5.33 Beryllium

5.33.1 Properties and Applications There are special areas of application for be- ryllium in optical components, precision instru- ments, and space travel because of its unusual combination of physical and mechanical prop- erties. Selection criteria are the low weight (den- Fig. 5.66 Elongation to failure and tensile strength of com- sity 1.85 g/cm3), a high E-modulus, and low ra- pacted beryllium as a function of the temperature diation absorption. [Sto 90, Kau 56, Lau 76] Beryllium melts at 1283 ЊC and has a hex- agonal lattice. It is characterized by a high ther- extrusion and a texture favorable for the me- mal capacity and heat resistance combined with chanical properties is formed. In “warm” extru- good corrosion resistance and high strength and sion, the compacted and possibly sintered billet is therefore used in reactor technology. The very can be lubricated with graphite or molybdenum poisonous metal can only be melted and pro- sulfide. Warm extruded bars and tubes can be cessed under the strictest conditions [Sto 90]. drawn to the finished sizes. The “hot” extrusion process is selected for 5.33.2 Billet Production large extrusion ratios because of the lower ex- Cast beryllium has a coarse grain, is brittle, trusion loads. The billets sheathed in steel are and tends to porosity. It can consequently be fur- extruded like alloy steel with glass lubrication. ther processed only with difficulty. For this rea- The sections leave the press fully recrystallized son, plus the importance of a fine grain for the and largely texture free. properties, beryllium is usually prepared by powder metallurgy. This also applies to extrusion (see the section 5.34 Uranium “Extrusion of Powder Metals”). Usually, beryl- lium powder with the minimum possible oxygen 5.34.1 Properties, Applications content is consolidated to billets by hot-isostatic compaction in vacuum [Sto 90]. The best-known application is the extensive If extrusion has to be carried out at a high use of uranium in the form of oxide powder as temperature (approximately 1000 ЊC), a billet nuclear fuel in nuclear power stations. Uranium clad in a steel jacket has to be used. The com- is also sometimes used in other areas as a pure metal and dilute alloys because of the very high paction of the beryllium powder directly into the 3 jacket with a ram, the subsequent welding of the density (19.1 g/cm , which is 68% higher than jacket and the evacuation (to prevent oxidation) lead) and the good radiation absorption. Typical has been described [Kur 70]. nonnuclear applications are radiation protective shields and counterweights [Eck 90]. Natural uranium contains up to 99.3% of the 5.33.3 Deformation Behavior, Extrusion weakly radioactive isotope U238 and only up to According to Fig. 5.66, beryllium exhibits 0.7% of the nuclear fuel U235. Whereas this low two maxima in the elongation to fracture, one at radioactivity has little effect on the workability, 400 ЊC and another one at approximately 800 the poisonous nature and the ease of oxidation ЊC. From experience, the elongation to fracture of uranium necessitates special measures. can be taken to be a measure of the workability. Uranium melts at 1689 ЊC. It has a rhombic Consequently, it is possible to differentiate be- lattice up to 665 ЊC(␣-phase) and a tetragonal tween “warm” extrusion with billet temperatures lattice (b-phase) above this temperature. At 771 from 400 to 500 ЊC and “hot” extrusion with ЊC this lattice transforms into the body-centered billet temperatures from 900 to 1065 ЊC. In the cubic (bcc) c-phase. Because the tetragonal b- first case, no recrystallization takes place during phase is difficult to deform, extrusion cannot be 288 / Extrusion, Second Edition

carried out between 650 and 790 ЊC because of be melted in vacuum induction melting furnaces the risk of cracking. or in vacuum arc furnaces. Because the structure is similar, the extrusion is the same as for pure 5.34.2 Deformation Behavior, Extrusion uranium [Eck 90]. Because the b/c phase bound- ary is displaced to lower temperatures by these The billets are melted in induction furnaces alloying additions and because in the c-region under a vacuum and cast in molds. secondary phases are held in solution, the extru- The deformation behavior of uranium is sion of alloys is easier. shown in Fig. 5.67. The extrusion in the lower ␣-region (between 550 and 650 ЊC) avoids the difficulties of high-temperature extrusion de- 5.35 Molybdenum, Tungsten scribed subsequently but does require a high press power because of the higher flow stress. 5.35.1 Molybdenum The extruded sections are fine-grain recrystalli- zed. It is possible to work in this temperature Molybdenum is usually used as an alloying range without cladding with the usual graphite- element in steels and high-alloyed materials. grease lubrication. Preheating the billets in a salt However, it is also important as a pure metal and bath prevents oxidation. They should, however, a low-alloy material. Tools in TZM (Mo-0.5 Ti- be subjected to the atmosphere for only a short 0.17Zr), for example, can withstand tempera- time at the extrusion temperature. tures well over 1000 ЊC. Further application ar- In the c-region (between 800 and 1000 ЊC), eas are cathodes, electric resistance elements (up where significantly higher extrusion ratios can to 2200 ЊC), and high-temperature components be used, uranium reacts very rapidly with iron, in space travel and for rockets. The highest ap- nickel, and other metals and is very susceptible plication temperature is 1650 ЊC. Molybde- to oxidation. The billets for this hot extrusion num’s good resistance to hydrochloric acid is of are usually clad in copper, evacuated, and then interest to the chemical industry. The density of extruded using graphite grease as a lubricant. molybdenum is 10.2 g/cm3 [Joh 90]. Only a few low-alloy-content uranium alloys The high melting point of 2622 ЊC permits with higher mechanical properties and better only powder metallurgical processing. The pow- corrosion resistance are known (with titanium, der is obtained by hydrogen reduction of mo- zirconium, molybdenum, and niobium) that can lybdenum oxide and then cold compacted and sintered. These sintered billets can then be either directly extruded or melted in a vacuum arc fur- nace. The bcc lattice of molybdenum is the reason behind the excellent deformation characteristics. This can be deduced from the hot tensile strength curves in Fig. 5.68. Pure molybdenum is extruded in the range 1065 to 1090 ЊC and the most common alloy TZM (Mo-0.5Ti-0.1Zr) at 1120 to 1150 ЊC. The billets are heated in conventional gas or oil-fired furnaces or by induction. Above 650 ЊC, molyb- denum oxidizes as a gas with a weight loss of 1 to 5% without an oxide layer forming. Rapid heating obviously reduces the loss from oxida- tion. Similar to steel, glass has to be used for extrusion. Bar, tube, and simple shapes can be produced by extrusion [Joh 90]. The glass ap- plied as powder after the billet has left the fur- nace melts and protects it from oxidation.

5.35.2 Tungsten Tungsten melts at the extremely high tem- Fig. 5.67 Hot tensile strength and elongation of uranium as a function of the temperature [Eck 90] perature of 3380 ЊC. The structure and properties Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 289

bium oxidizes severely above 425 ЊC so that the billets have to be protected from oxidation by cladding and evacuating. Niobium forms a continuous solid solution with titanium. Niobium with 46.5% titanium is the alloy most commonly used as a super con- ductor [Kre 90]. Fine wires of this niobium alloy are embedded in a copper matrix (see the section “Extrusion of Semifinished Products from Me- tallic Composite Materials”). The niobium-tita- nium starting material is extruded to bars using glass as a lubricant in the same way as steel and titanium. These are then clad with copper and further processed in several stages.

5.36.1 Tantalum Fig. 5.68 Hot tensile strength of some high-melting-point Tantalum has similar structure and properties metals and alloys as a function of temperature [Kie 71] to niobium. It also has a bcc structure but first melts at 3030 ЊC and is twice as heavy with 16.6 g/cm3. are similar to molybdenum. With its high density The widest application of tantalum—apart of 19.3 g/cm3, it dominates where high weight from an alloying element in steel like niobium— is a main requirement. The high melting point is as an anode material in electrolytic cells. In combined with its high electrical resistance the chemical industry it is known for its resis- makes it the most common material for heating tance to nitric acid, hydrochloric acid, and sul- conductors and filaments in lamps [Joh 90a]. furic acid. The high melting point ensures ap- In contrast to molybdenum, a significantly plications in the high temperature region higher hot-working temperature of 1500 to 1800 [Pok 90]. ЊC is required, as shown in Fig. 5.68, which Processing usually follows a powder metal- makes tungsten unsuitable for extrusion. Hot lurgical route. Electron beam cast tantalum is forging of sintered powder billets is preferred usually too coarse grained for further processing [Par 91]. and has to be forged before it can be extruded. The extrusion of compacted and sintered powders has been mentioned but does not have any great importance because the recrystalliza- 5.36 Niobium and Tantalum tion temperature, and thus the extrusion tem- perature, is very high [Pok 90]. Niobium melts at 2468 ЊC and at room tem- perature has a bcc structure with a density of 8.4 g/cm3. It can be readily worked at room tem- perature. Niobium alloys have a wide range of applications in space travel because of their rela- tively low weight and high hot strength. Nio- Extrusion of bium and its alloys are also in demand in the Powder Metals chemical industry because of the resistance to specific corrosive media [Ger 90]. Similar to other high-melting-point metals, Martin Bauser* the extrusion billets are made by a powder met- allurgical route. Figure 5.68 shows for niobium a relatively 5.37 General low hot-working temperature. Extrusion tem- peratures between 1050 and 1200 ЊC with extru- Oxide-containing aluminum powder was ex- sion ratios of V 4toV 10 have been de- truded in the 1940s under the name sintered alu- scribed for niobium alloys with zirconium, hafnium, and tungsten [Ger 90]. However, nio- *Extrusion of Powder Metals, Martin Bauser 290 / Extrusion, Second Edition

minum powder (SAP). In the 1950s the advan- tallic particles as dispersions by extrusion is also tages of the extrusion of powder was known for discussed in this chapter (metal matrix compos- reactor materials and beryllium. This process ites, or MMCs). Chapter 4 discusses the physical has only recently found a wide application, par- properties, in particular those at high tempera- ticularly with aluminum alloys but also high- tures, and the processes in the deformation of alloyed and dispersion-hardened materials. dispersion-hardened materials. Frequently, me- A good overview of powder metallurgy (P/ tallic or nonmetallic fibers are mixed with the M) can be obtained from the textbook on powder metal powder to achieve certain properties and metallurgy by W. Schatt [Sch 86]. The work of then extruded [Boe 89]. The resultant so-called Roberts and Ferguson gives detailed information fiber composite materials are described in the on the extrusion of powder metals [Rob 91]. section “Extrusion of Semifinished Products from Metallic Composite Materials.” 5.37.1 Main Application Areas of Powder Metallurgy 5.37.3 Powder Production Processing by extrusion plays an important Atomization of the melt using water or gas role compared with other processes in P/M. The streams is the most common of all the different most important is the processing of metal pow- casting, mechanical, and chemical processes ders to near final shape and thus the economic used to produce metal powders. Water gives the production of molded components with a piece fastest cooling rate, but the powder particles are weight below 2 kg, and mainly in iron alloys. coarser (approximately 500 lm) and in a spat- The powder is compacted on vertical compac- tered format. If, however, atomization is carried tion presses in dies by a stem and then sintered out in an air or protective gas jet, more rounded at a high temperature, during which the particles shapes and smaller particles are produced (down form a solid bond. For large pieces with simple to a few lm, depending on the process param- geometries (e.g., forging billets), the compaction eters) (Fig. 5.69). The output per hour of atom- is carried out by fluid pressure applied on all ization plants is relatively low. Because a re- sides (cold isostatic pressing, or CIP) before stricted particle size is usually required and the they are sintered. Compaction and sintering car- powder usually falls in a wide particle spectrum, ried out in a single operation by hot isostatic the particles of different sizes have to be sepa- pressing (HIP) is also used occasionally. rated by sieves. The powder costs are corre- spondingly high. A desired material composi- 5.37.2 Advantages of Metal tion can be obtained by mixing powders if it Powder Extrusion does not exist in the atomized melt. “Reaction milling” in ball mills is a new pro- Where long semifinished products can be con- cess that enables the mechanical alloying of ventionally produced by casting and extrusion, metal components. Nonmetallic particles can powder extrusion is usually less favorable be- also be finely dispersed by this method. The cause the production of suitably mixed and powder particles are repeatedly broken down sieved powders is usually too expensive. and welded between steel balls. A fine structure The production of extruded semifinished is obtained after a certain milling time [Inc 86]. products by the powder route is worthwhile when: 5.37.4 Spray Compaction ● The material cannot be processed conven- In the new process of spray compaction de- tionally by melting and casting. veloped by the company Osprey [Cra 88], the ● An extremely fine grain size and finely dis- gas-atomized material from the melt is sprayed tributed precipitates have to be achieved. onto a rotating block with a vertical axis ([Man Rapidly quenched powder can have a signifi- 88] (Fig. 5.70). The slowly sinking block grows cantly extended solid-solution range layer by layer. This process saves precompac- [Tus 82]. tion, encapsulation, and evacuation. ● A uniform distribution of very small inclu- Hard particles can be blown into the spray by sions to achieve specific properties is re- an injector and form a dispersion in the finished quired (dispersion hardening). billet. This last-mentioned processing of metal pow- The disadvantage of spray compaction is that ders with mixed or reaction produced nonme- a considerable part of the spray droplets miss the Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 291

Fig. 5.69 Example of a gas atomization plant for metal powder [Wei 86]

block and fall to the bottom as powder “over spray,” which can be remelted or used as a powder. The powder particles cool on the solidified block so slowly in spray compaction that a struc- ture similar to casting forms, but with signifi- cantly better homogeneity and with finely dis- tributed particles and dispersed particles. Because the billet diameter varies slightly, it has to be turned to the extrusion dimension. The spray-compacted billets can be extruded like cast billets—also to tubes with piercing man- drels. Plants are in operation producing alumi- num, copper, and steel.

5.38 Powder Extrusion Processes

5.38.1 General The individual powder particles are plasti- cally deformed in the extrusion direction during the extrusion of metal powders, usually at the same temperature as cast billets, and the surface area increases. Oxides and other films on the Fig. 5.70 Schematic of spray compaction [Arn 92] particle surface break up and release new reac- 292 / Extrusion, Second Edition

tive metallic surfaces. The powder is compacted during extrusion and the newly formed surfaces bonded by pressure welding. Even with low- density powders, a complete compaction and porous-free material is obtained by extrusion if the particles can be sheared sufficiently. This as- sumes that no gas porosity can form, e.g., from moisture. Several types of powder extrusion are known, including (a) the rarely used shaking of the loosed powder into the vertical container of an extrusion press, (b) the precompaction outside the press, and (c) the encapsulating of the pow- der before extrusion (Fig. 5.71). The new pro- cess of spray compaction is described above.

5.38.2 Shaking of Loose Powder into a Fig. 5.71 The classic processes for powder extrusion. (a) Ad- Vertical, Heated Container and dition of loose powder. (b) Precompaction outside Direct Extrusion (Version a) the press. (c) Encapsulation before extrusion [Rob 91] The loosely filled powder has a low bulk den- sity (maximum 50% of the theoretical density), 5.38.4 Encapsulation of the Powder because the large number of interstitial space. Therefore, a relatively long container is neces- before Extrusion (Version c) sary. The stem first compacts the powder before Usually the metal powder is compacted in the the actual extrusion process commences. This metal sleeve before sealing. After this it is often apparently economic process has a low through- evacuated (possibly at an elevated temperature) put if the powder first has to be heated in the and then vacuum sealed (Fig. 5.72). container. It can be used only rarely. An example is the extrusion of magnesium-alloy pellets with grain sizes from 70 to 450 lm to rods [Rob 91].

5.38.3 Precompaction of the Powder outside the Press “Green” compacts can be produced by cold isostatic pressing (CIP in autoclaves), particu- larly from angular particles or flakes. These are so stable that they do not break up during han- dling before extrusion. The powder filled into a plastic container is subjected by a pressurizing liquid to a uniformly applied high pressure (2000 to 5000 bar max), which increases the original powder density of 35 to 50% of the den- sity of a cast billet to 70 to 85%. The risk of the fracture of the precompacted billet during han- dling can be reduced further if necessary (e.g., with round particles) by sintering before extru- sion by heating. An alternative to this two-stage process is to carry out the compaction itself at an elevated temperature (HIP). In this case, the powder has to be filled into a thin-walled metal Fig. 5.72 (a) Encapsulation of powder. Cladding sealed at can that does not melt at the pressing tempera- the back and with evacuation tube. (b) Evacuation ture (special case of version c). [Rob 91] Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 293

The reasons for encapsulation include: made for an equalization time before loading the ● billet into the furnace (possibly in an equaliza- Excluding a reactive powder material from tion furnace). air and extrusion lubricants ● Encapsulated powders (version c) are usually Protection from poisonous materials during extruded with lubricated containers and conical handling (e.g., Be and U) Њ ● dies (90Ð120 ) or indirectly. This gives a laminar Risk of breaking up of green compacts of material flow and a uniform cladding material round powders or other particles that are dif- thickness on the extrusion. The selection of the ficult to compact in a billet shape ● lubricant depends on the cladding material (e.g., Improved lubrication and friction behavior aluminum: unlubricated; copper: graphite-oil; and better flow through the die by the correct iron: glass). Nevertheless, a uniform laminar selection of the container material ● material flow can be achieved only with round Keeping the base material away from the die bar or tubes and simple section shapes. Section and the zone of severe shear deformation. extrusion is successful only when a suitable die This is important only for materials with low inlet has been developed. ductility [Rob 91]. A billet produced from powder usually has a With high-purity materials, processing has to lower density than a cast billet. A longer billet be carried out in a clean room with careful clean- is therefore needed for the same section weight. ing of the can and evacuation at an elevated tem- In extrusion the stem initially pushes together perature. the still not highly compacted powder in the con- A large disadvantage of this process is that tainer before the particles are subjected to a the can material remains on the surface of the shear deformation and friction between each extruded product and can be difficult to remove other in the entry (shear) zone to the die and in by machining or by pickling. There has been the die itself. The bonding of the grains on the reference to a degassed and hot-compacted alu- newly formed surface occurs by friction (or minum powder in an aluminum capsule, which pressure) welding under high pressure and takes is removed from the billet before extrusion by place more quickly than the more conventional machining [Sha 87]. sintering process used in powder metallurgy. The extrusion process can produce a better bond between individual particles than sintering. This applies in particular to aluminum alloys, 5.39 Mechanism and the powder particles of which always exhibit Flow Behavior in the surface films of aluminum oxide because of the Extrusion of Metal Powders high reactivity of the element aluminum. These films are broken up to such an extent in extru- If loose powder is extruded (version a), the sion that the higher the extrusion ratio the container first has to be sealed on the die side to greater the number of newly formed surfaces ensure good compaction by the stem. The die is available for particle bonding. An extrusion ratio then opened. of at least 20 is often stated to be certain of With precompacted green blanks (version b), achieving a perfect bond between the powder a “nose” or a disc of the cast material corre- particles [Sha 87a]. The resultant structure of the sponding to the powder is often placed in front extruded powder corresponds to the structure of the die. This compensates for the risk of the formed during the extrusion of solid billets. surface of the extrusion tearing due to insuffi- Encapsulated powder has to be well com- cient initial pressure. Indirect extrusion is pre- pacted and degassed. If the strength of the pow- ferred because of the more uniform material der material and the encapsulating material dif- flow. Extrusion with a lubricated container is fer too severely, or if the can is too thin and the avoided because of the risk of lubricant penetra- powder inadequately compacted, there is the risk tion of the green blank. of folds forming during extrusion (Fig. 5.73). The thermal conductivity in the green blank Diffusion between the can material and the pow- is relatively low. Too-rapid billet heating there- der material should be prevented as far as pos- fore results in localized melting with the risk of sible, and it should not be possible for a eutectic cracking. Slow heating in a chamber furnace is to form between them. When selecting the can the most suitable. If heating has to be carried out material, consideration should always be given in an induction furnace, allowance has to be to the fact that it later has to be removed. 294 / Extrusion, Second Edition

Fig. 5.74 Load variation in the extrusion of powder and spray compacted material compared with cast (Al18SiCuMgNi), indirectly extruded. 1, cast; 2, spray com- Fig. 5.73 (a) Fold formation during extrusion of insufficiently pacted; 3, compacted powder [Mue 93] precompacted metal powder. (b) Avoiding fold formation by advance of the capsule back wall [Rob 91] ever, it is rare for exactly the same material to be extruded as a cast billet or as a powder, which An interesting variation of the extrusion of prevents exact comparisons. metal powders is the hydrostatic extrusion of sil- The use of indirect extrusion has proved ben- ver alloy powders. Presintered billets can be ex- eficial in the extrusion of powders in order to truded without a can because the pores close reduce the extrusion load. during the pressure buildup, preventing the pen- etration of the pressure medium and the produc- tion of defects. As mentioned previously, spray-compacted 5.41 Examples of Powder Extrusion billets behave similarly to cast billets—the pow- der compaction measures then no longer apply. In almost every powder metallurgically pro- duced material group there are materials with solid insoluble inclusions (dispersoids). These dispersion-hardened materials can be produced 5.40 Load Variation only by powder metallurgical processes. The variation in the load in the extrusion of powders differs considerably from that with cast 5.41.1 Aluminum Alloys billets because there is a less severe increase in It is practically impossible to produce alumi- the load during compaction in the first part of num powder without an oxide skin because of the stem movement. The gradient of the load- the high affinity of aluminum for oxygen. Nor- displacement curve varies with the degree of mal sintering is therefore hindered or prevented compaction. with aluminum alloys. Extrusion, therefore, re- The high load peak often seen with aluminum mains as the only practical possibility of pro- powders during upsetting can cause problems, ducing defect-free dense materials by P/M of and there is still no definite explanation for it aluminum alloys because the oxide skins on the (Fig. 5.74). powder particles are torn away to leave oxide- Additional work also has to be carried out free, easily weldable surfaces. Pure aluminum during extrusion to overcome the friction be- powder can be bonded to a metallic dense ma- tween the grains and for their bonding. It is terial with at least 80% cold deformation therefore often assumed that the extrusion load [Gro 73]. in extruding powders is somewhat higher than The starting material is usually quenched in the extrusion of cast billets. In a few cases a powder from a gas stream atomization plant that lower load has been mentioned [Nae 69]. How- is first compacted in a cold isostatic press or, for Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 295

small dimensions, in a cylindrical compression because the dispersed particles are finer and bet- die to a green blank with 75 to 80% of the theo- ter distributed. retical density. In the extrusion press the green Milling aluminum powder with electrogra- blank is almost 100% compressed with an ini- phite followed by heat treatment for complete tially closed die and then extruded using the di- transformation of the carbon into Al4C3 pro- rect or the indirect process at 450 to 500 ЊC [Sha duces a dispersion-hardened aluminum material 87]. Placing a “nose” or a disc of aluminum in with embedded Al4C3 and Al2O3 particles mar- front of the powder billet can promote sufficient keted under the trade name Dispal [Arn 85]. In compaction during upsetting and also prevent variations of the material solid-solution hard- the start of the extrusion from breaking up. The ening AlMg5 or AlSi12 is used as the base ma- abrasive effect of hard particles in a powder mix- terial instead of pure aluminum to achieve better ture on the shape-forming tools is also reduced. mechanical properties even at lower tempera- Encapsulation with degassing at 500 ЊC and hot tures. Whereas the solid-solution hardening and compaction is required only if absolute freedom precipitation hardening are lost at higher tem- from hydrogen is specified. The cladding can be peratures, the dispersion hardening is retained. removed by machining prior to extrusion. Dispal materials with high silicon content are In practice, the extrusion of aluminum powder preferably processed by spray compaction to ex- materials is worthwhile only if it produces a ma- trusion billets. As well as saving the cost of cold terial that cannot be produced by casting tech- isostatic pressing of powders, the main advan- nology. Most attention is paid to the high-alloy tage is the fine distribution of the embedded par- materials with high room temperature strength ticles. After extrusion, rapidly solidified alumi- as well as dispersion-hardened materials that num powder with iron and nickel additions have a much higher strength and better mechan- contain finely distributed particles of interme- ical properties at higher temperatures than nat- tallic phases of the types Al3Fe and Al3Ni (me- urally hard and precipitation-hardened alumin- tallic dispersoids) [Sha 87]. Alloys based on ium alloys (Fig. 5.75). AlFeCe, AlFeMo, AlCrMnZr, and AlNiFeMn In the 1940s, a process was developed for pro- with additions of copper, magnesium, and tita- ducing and working an oxide-containing powder nium, which also form intermetallic dispersoids, from aluminum powder by a milling process. are used to achieve high hot strengths. This re- This product was referred to as sintered alumi- sults in alloys that have an approximately 100% num powder (SAP) [Irm 52, Jan 75]. Originally, higher hot strength in the temperature range 250 the extruded aluminum material contained 12 to to 350 ЊC, compared with conventional materi- 15% Al2O3; today less than 5% Al2O3 is used als. The fatigue strength of such materials can also be significantly improved. The E-modulus is also increased by 20 to 30%. These improve- ments in the mechanical properties are, however, obtained with an increase in density to 2.8 to 3.0 g/cm3 because of the increased content of heavy elements [Sha 87a]. In powder metallurgically produced aluminum alloys with silicon contents of 10 to 30%, the eutectic and the primary phases are very finely formed in the structure (Ͻ25 lm). This structure has an advantageous effect on the hot and fatigue strengths [Sha 87a]. The alloys can be readily mechanically worked and are characterized by a low coefficient of thermal expansion. For bearing elements, the ex- trusion of powder- and spray-compacted mate- rials produces an uneven finer distribution of hard particles than that achieved by casting. The wear properties are significantly improved. Ma- terials that contain other elements in addition to Fig. 5.75 Temperature dependence of the tensile strength of silicon are being tested in engine construction different aluminum alloys. 1, aluminum 99% soft; 2, AlZnMgCu1.5; 3, sintered aluminum powder (SAP) with 10% [Arn 92a], where, however, the high manufac- Al2O3; 4, Al with 4% C (as Al4C3) [Sch 86] turing cost is a major barrier in spite of the sig- 296 / Extrusion, Second Edition

nificantly better properties compared with con- cipitates. However, with spray compaction, the ventionally produced components. A particles are so finely distributed that the billets breakthrough is high-silicon-containing alumi- can be readily extruded without difficulty to rod num tubes for cylinder sleeves that have recently and tube. Graphite particles, which improve the been extruded from spray-compacted billets slip properties, can be included by an injector [Hum 97]. during the spray compaction (see Fig. 5.70). A Other aluminum alloys produced by mechan- further application of spray-compacted-pro- ical alloying with oxides and carbides as disper- duced tubes of high-alloy bronze is as super con- soids and that contain 1 to 4% copper and mag- ductors [Mur 96]. nesium are commercially available. They are hot Dispersion-hardened silver materials (e.g., compacted and then extruded. However, only with AgNi2 or with cadmium oxide) have better simple cross sections are available. After extru- erosion properties, a lower contact resistance, sion they can be further processed by forging, and less tendency to welding than homogeneous hammering, rolling, or drawing [Rob 91]. materials. Silver and nickel are almost com- pletely insoluble in each other at room tempera- ture. AgNi alloys, therefore, cannot be produced 5.41.2 Copper and Noble by melting metallurgy but only by powder met- Metal-Base Alloys allurgy. A uniform distribution of the embedded If powders of electrolytic copper and alumi- nickel is obtained by currentless nickel plating num oxide are cold isostatically compacted, sin- of silver powder followed by extrusion tered, and extruded to rods, the very fine Al2O3 [Mue 87]. particles (approximately 0.3 lm), which should As described previously for copper, finely dis- be finely distributed, prevent recrystallization so tributed particles of cadmium oxide can be that a high strength is retained up to 1000 ЊC formed as a dispersoid in silver by internal ox- [Zwi 57]. idation of alloy powders of the noble metal sil- Copper alloys with 1.1% aluminum oxide are ver, which practically cannot be oxidized, and supplied as wire and rods and processed to spot the base metal additive cadmium. The fraction welding tips. These tips have a very good elec- of cadmium oxide is very high, up to 25%. trical conductivity and a high flow stress at the Platinum and its alloys, which are used for welding temperature. At room temperature the special heating wires and other high-temperature material can be cold worked like copper without components, are dispersion hardened whereby intermediate annealing. they are extruded as powder mixtures with tho- The strength-increasing effect up to very high rium oxide, yttrium oxide, or zircon oxide as temperatures of the aluminum oxide particles as dispersoids [Rob 91]. dispersoids requires extremely fine particles (3Ð 12 nm) and a uniform distance between them of 5.41.3 Titanium Alloys less than 0.1 lm. This is achieved by a so-called A dispersion-hardened titanium alloy is, for internal oxidation of copper-aluminum powders, example, a modification of Ti-6242 with 6% Al, i.e., an oxidizing annealing in which the oxygen 2% Sn, 4% Zr, 2% Mo, 0.1% Si, and 2% Er, transforms the aluminum to oxide, whereas the which is produced by internal oxidation during more noble copper is retained as a metal. the annealing of the powder Er O , with ex- Dispersion-hardened copper with 0.1 to 0.5 2 3 tremely small particles. Of interest is that the lm large TiB particles (3 vol %) is produced 2 structure obtained by very rapid solidification is by the following original process. not changed by extrusion [Rob 91]. Two different copper melts are reacted to- gether in a reaction vessel whereby the dispersed phase is produced by a precipitation reaction in 5.41.4 Iron Alloys situ in the melt. This melt is then atomized to The production of chromium-nickel-steels powder, which is filled into a copper can and and chromium-aluminum-steels by extrusion of evacuated [Sut 90]. This material can also be steel powder produced by water atomization was extruded and processed to spot welding elec- reported in 1969. The process did not go into trodes. production [Nae 69] because, normally, even CuSn and CuSnNi alloys with high tin con- with iron alloys the production, or steels using tents are known as bearing materials but cannot powder, and compaction is more expensive than be extruded because of the coarse d-phase pre- by melting and casting. Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 297

However, because fewer operating steps are strength is usually obtained by precipitation needed than between casting and the production hardening. of the bar and a better homogeneity is obtained, The molten metallurgical production route is powder extrusion is again being promoted for the most economic solution when possible. It is, some iron alloys. A Swedish manufacturer of- however, accompanied with coarse precipitates fers powder metallurgically produced tubes in and reduced hot workability and is not possible stainless chromium-nickel-steels as well as at all with wide melting intervals. nickel and cobalt alloys [Asl 81, Rob 91]. The Oxide dispersion strengthened (ODS) alloys melt is atomized under a protective gas and the are materials in which fine particles, in contrast powder then cold isostatically compacted in an to precipitates, are also resistant to high tem- iron can before being extruded over a mandrel. peratures, act as strength increasing dispersions. The hollow billet also has to be internally en- There is no melting metallurgical alternative to capsulated to produce tubes. The lubricant is powder metallurgical production. Oxides of yt- glass, the same as with conventional extrusion, trium are mainly used [Rob 91]. and extrusion is carried out at approximately Table 5.22 summarizes the superalloys pro- 1200 ЊC. The process is supposed to have eco- duced by P/M. nomic advantages over the conventional one In the ODS process, the rapidly quenched [Tus 82]. (more than 103 K/s) powder produced by at- Some high-speed and tool steels that are se- omization in an inert gas stream after sieving verely segregated when produced by the normal and mixing is encapsulated in a steel can, which process are extruded as bar from powder to elim- has to be evacuated at a high temperature to inate segregations and to retain elements in so- eliminate micropores. If a particularly uniform lution above the equilibrium value. This is not precipitation-free structure is required, cooling possible with molten metallurgical production. has to be carried out extremely quickly. Special Because atomization is usually carried out under rapid solidification rate (RSR) processes have a protective atmosphere, the round particles that been developed (up to 106 K/s). Extrusion has do not readily bond have to be encapsulated for proved to be the most suitable compaction compaction. Water-atomized material can be method with these materials because the large cold compacted without a can and then sintered deformation breaks up oxide films as described before extrusion [Rob 91]. for aluminum, disperses impurities present, and The dispersion hardened iron based alloys provides very good compaction. The encapsu- MA956 and PM2000 with chromium and alu- lated powder is precompacted by HIP or forging minium as the main alloying elements and yt- to approximately 85% before it is extruded. The trium oxide as the dispersoid are described in bars extruded at an extrusion ratio of 5Ð7 with the next section. glass lubrication at approximately 1200 ЊC are further processed by forging and rolling after re- 5.41.5 Nickel and Cobalt-Base moval of the can. It is possible with extrusion to significantly reduce the residual porosity and High-Temperature Alloys thus the notch impact sensitivity. The extrusion Alloys with high contents of nickel, chro- temperature with these alloys has to be very mium, and cobalt are necessary for high-tem- carefully controlled to prevent the risk of coarse perature applications, particularly in gas turbines precipitates forming at high temperature (e.g., and in the aerospace industries. The high carbide in Rene«95) [Rob 91].

Table 5.22 Composition of some iron and nickel superalloys produced by powder metallurgy (addition in wt%)

Designation Fe Ni Cr Co Mo W Ti Al Nb Ta Zr B C Y2O3 MA 956(a) 74.0 . . . 20.0 ...... 0.5 4.5 ...... 0.5 PM 2000(b) 73.0 . . . 20.0 ...... 0.5 5.5 ...... 0.5 MA 754(a) 1.0 77.5 20.0 ...... 0.5 0.3 ...... 0.05 0.6 MA 6000(a) . . . 68.5 15.0 . . . 2.0 4.0 2.5 4.5 . . . 2.0 0.15 0.01 0.05 1.1 PM 3000(b) . . . 67.0 20.0 . . . 2.0 3.5 . . . 6.0 ...... 0.15 0.01 0.05 1.1 Rene`95(c) . . . 62.0 13.0 8.0 3.5 3.5 2.5 3.5 3.5 . . . 0.15 0.01 0.07 . . . Nimonic AP1(a) . . . 55.5 15.0 17.0 5.0 . . . 3.5 4.0 ...... 0.025 0.02 . . . (a) Inco Alloys International. (b) High temperature metal GmbH. (c) Nuclear Metals Inc. Source: Sch 86, Rue 92 298 / Extrusion, Second Edition

The alloy Rene«95 and Nimonic AP1 are pre- Because the production of alloy powder would cipitation-hardening alloys where solid-solution be very expensive, element powders are mixed hardening is also effective at moderate tempera- and cold compacted. The green blanks are en- tures because of the addition of molybdenum. cased in aluminum and extruded at approxi- They are used for gas turbine discs. mately 500 ЊC. This prevents the extreme exo- In the ODS alloys Ma 965 and PM2000 (Fe- thermic reaction from the formation of the Cr-basis), as well as Ma754, Ma 6000, and PM intermetallic phases occurring during the extru- 3000 (Ni-Cr-basis), the Y2O3 is worked into the sion. It takes place—possibly after further de- metal powder by mechanical alloying in high- formation steps—in high-vacuum furnaces or in energy ball mills and finely dispersed. The fiber an HIP unit. Another method is “reaction extru- texture obtained by extrusion is utilized by suit- sion” at such a high temperature that the phase able heat treatment (coarse-fiber crystals from formation actually occurs during extrusion. zone recrystallization) to achieve a very good fatigue and creep strength. MA956 and PM2000 with high oxidation and hot corrosion strength up to 1100 ЊC are used in combustion chambers, in gas turbines, and other cases demanding maximum resistance at high Extrusion of Semifinished temperatures. MA6000 and PM3000 are used in the first and Products from Metallic second stages of jet engines and can be hot and Composite Materials cold rolled after extrusion.

5.41.6 Exotic Materials Klaus Mu¨ller* Beryllium tubes are the ideal construction ma- Metallic composite materials are microscopi- terial in communication satellites because of 3 cally heterogeneous, macroscopically homoge- their low density (1.85 g/cm ), the high E-mod- neous-appearing materials that consist of two or ulus, and their considerable elongation. In order more components intimately connected with to achieve a fine structure, P/M processing is each other and in which at least the component preferred to melting and casting. The material with the highest volume is a metal or an alloy being processed is encased in carbon steel be- [Rau 78]. Its structure can be matched to the cause of the poisonous nature of beryllium and stresses of the component. the susceptibility to oxidation. This can be re- Pure metals and alloys have a defined prop- moved by pickling with hydrochloric acid after erty spectrum so that the determination of a extrusion with glass lubrication. The workability property for a given material also determines all in the extrusion direction is increased by the tex- other properties. In composite materials, in con- ture formed in extrusion of this hexagonal crys- trast, the properties of different components are tallizing material [Rob 91]. combined, resulting in a new extended property Ceramic uranium oxide powder has to be Њ spectrum. The spatial arrangement of the com- heated to 1750 to 2000 C for plastic flow. Be- ponents in the composite gives rise to typical cause no deformation tooling can withstand this composite properties, the so-called structural high temperature, experiments have been re- properties. Composite materials can also exhibit ported in which the hot powder is filled into rela- properties resulting from interactions between tively cold steel cans at approximately 700 ЊC the components, the so-called product proper- and then extruded. ties. Always present are the cumulative proper- This two-temperature process has also been ties, i.e., the resultant properties from the addi- investigated for the production of chromium tion of the component properties. This is shown tube and bar whereby hot chromium powder is in Fig. 5.76 [Sto 86]. filled into a colder steel can and immediately The application of metallic composite mate- extruded. The aim is to ensure that the very dif- rials in place of conventional alloys produces in ferent deformation behaviors of the two mate- most cases economic as well as technical advan- rials at the same temperature are matched to each other. Intermetallic phases such as Ni3Al4 or Ti3Al4 *Extrusion of Semifinished Products from Metallic Com- can only be deformed at approximately 1100 ЊC. posite Materials, Klaus Mu¬ller Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 299

Fig. 5.76 Properties of metallic materials tages. Composite materials are produced today, ● The capability of influencing the deforma- the metallic components of which form solid so- tion zone by die design lutions or intermetallic phases corresponding to the phase diagram or are insoluble in each other. Depending on the materials combination, the The application of the composite materials is de- materials structure, the application, and the ex- termined by the adhesion at the boundary faces trusion temperature, metallic composite materi- of the individual components. The general con- als can be produced by direct, indirect, or hy- ditions for a satisfactory boundary surface ad- drostatic extrusion. hesion with common deformation of heteroge- neous materials include: ● Adequate face pressure (compressive stress 5.42 Terminology and Examples in the deformation zone) ● Adequate increase in the surface area during The metalic composites can be classified ac- the deformation cording to the spatial arrangement of the com- ● A limited inhomogeneity of the flow in the ponents: deformation zone that is still sufficient for the structures to conform to each other but, ● Fiber composite materials on the other hand, is not so high that the ● Particle composite materials composite material is subjected to unaccept- ● Penetration composite materials able internal stresses. The composite com- ● Laminated composite materials ponents have to be capable of flowing to- gether under the process-specific stress Figure 5.77 [Lan 93] shows schematically the conditions. geometric structure. Composite materials in which fibers of the For longitudinally orientated semifinished other components aligned or randomly oriented products (section, bar, wire, and tubes), the cri- are embedded, aligned, or randomly orientated teria described previously for stresses, surface in the matrix of the predominating component increase, and flow field formation are fulfilled by volume are referred to as fiber composite ma- by the deformation processes rolling, drawing, terials extrusion, and (to a limited extent) by forging. . Examples are: However, extrusion provides the most favor- ● Directionally solidified eutectics able conditions with reference to the three basic ● Copper-sheathed aluminum conductors for requirements: electrotechnology ● Compressive deformation ● Superconducting materials, including NbTi ● Large strains in one operation multifilaments in a copper matrix 300 / Extrusion, Second Edition

Fig. 5.77 Spatial arrangement of the components in composite materials [Lan 93]

● Electrical contact material, including Ag/C, ● All the components involved are deformed Ag/Cu/C, and Cu/Pd together whereby different strains can occur ● SiC or B fiber-reinforced light metal mate- within the components. rials Because the composite materials produced by Composite materials in which the other com- extrusion are predominantly metallic fiber com- ponents are embedded without a marked pre- posite materials and the production of dispersion ferred orientation in the matrix of the predomi- composite materials are described in the section nating component by volume are referred to as “Extrusion of Powder Metals,” the basic prin- ciples are described using this type of material. particle composite materials. They are discussed The range of possible material combinations, in the section “Extrusion of Powder Metals.” which are not fully utilized today, are illustrated Composite materials in which the various by way of an example in Fig. 5.78 [Mue 91]. components form an intermingled structure are Single or multicore wires can be produced as referred to as penetration composite materials. well as wires with solid or powder cores. In Composite materials that have a laminated some cases, viscous wire fillers (glasses during structure of the composite materials are referred hot working) can be used. If thermal material to as laminated composite materials. influences and diffusion are taken into account as well as the structure along with the associated metallurgical possibilities, the wide range of ma- terial technical possibilities is clear [Mue 82]. 5.43 Flow Behavior in the Extrusion Metallic fiber composite materials, corre- of Fiber Composite Materials sponding to the structure described (outer tube and one or more wire cores), can only be pro- The following cases can be differentiated duced, depending on the application, by indirect from the point of view of the deformation: or hydrostatic extrusion because the outer tube cannot have any distortion relative to the core ● Only one or not all of the components are material resulting from adhesion or friction with deformed (aluminum-steel bus bars, metal the container liner. powder combinations with ceramic addi- Both in indirect extrusion and hydrostatic ex- tives). trusion there is no friction between the billet and Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 301

Fig. 5.78 Schematic structure of metallic fiber composite materials the container. Under comparable stress condi- ● Die opening angle ● tions in the actual deformation zone both pro- Flow stress ratio kf1/kf2/...... /kfm cesses can be differentiated by the billet upset- ● Extrusion speed ting. This is important because the billet used to ● Lubricant (influences the friction conditions produce a core filled sheathed tube contains an in the die in particular) empty volume of between 15 and 23%. In hy- ● Temperature control of the extrusion process drostatic extrusion, this empty volume is re- ● Bonding quality in the initial billet moved by the hydrostatic pressure conditions; in indirect extrusion this occurs by the reduction of These relationships are shown in Fig. 5.79 the billet length and the increase in the billet for the two-component-composite aluminum- diameter. This can result in twisting and buck- copper. ling processes that can produce defects in the Figure 5.80 represents a homogeneous defor- composite material. mation in which the individual components are The risk of twisting and buckling increases subjected to the same reduction for the three-part with an increasing length/diameter ratio of the composite CU/Ni/Cu with the relevant HV 0.1 core wire in the tube sheath. The structure can values in the billet and the extruded section. be so severely distorted that further extrusion results in elongated folds and doublings that can initiate cracks and thus render the composite ma- terials unusable [Mue 80]. 5.43.1 Criterion for Homogeneous Deformation In the combined extrusion of metallic mate- rials with different flow stresses kf, the material flow can take place in different ways. The aim of composite material production by extrusion must be, in addition to pure cladding, to trans- form the individual components into a compact undamaged material. This requires that the in- dividual components have the same flow stress kf in the deformation zone. This condition can be achieved by the selection of the following parameters: Fig. 5.79 Influence of the extrusion ratio and core fraction ● on the type of deformation. Material: sheath Al, Extrusion ratio core Cu, flow stress ratio 2.7, die opening angle 2 alpha (m ● Volume distribution or volume fraction 45Њ). Area for achieving homogeneous deformation [Osa 73] 302 / Extrusion, Second Edition

Fig. 5.80 Homogeneous deformation of the composite Cu/ Ni/Cu

A nonhomogeneous deformation in which the individual components are extruded with differ- ent reductions can result in the failure of the composite with the pure cladding material not being deformed with the core material. 5.43.2 Deformation with Failure of the Composite Material The most common failures are core and sheath fracture, if defects from undesired reac- tions during the deformation are excluded. Ex- Fig. 5.81 Example of core fracture. The core material used amples of core failure are shown in Fig. 5.81 for is periodically broken. both a two- and three-component composite. However, there is no external visible defect on the extrusion. This type of failure occurs with casing cracking because of the absence of the combination of hard core material and soft friction between the billet and the container. sheath material. Sheath failure can be seen ex- ● The bonding of the composite components ternally at least for the case of the two-part com- to one another is strongly promoted by the posite with the material composition hard sheath surrounding compressive stress state during material and soft core material. In a three-com- the deformation. ponent composite, the damage need not be visi- ● ble at the surface. The innermost core can have Because the process can be carried out over core oscillations, i.e., periodic irregular defor- a wide temperature range, boundary surface mation. Figure 5.82 shows examples. The works reactions can be suppressed by suitable tem- of [Ahm 78, Rup 80, Hol 78] give various so- perature control where they reduce the prop- lutions to avoid these failures for the composite erties and promoted where they improve the Cu/Al. composite material. Depending on the geometry of the billet used, 5.44 Production of Metallic composite materials with a fibrous, particle and Composite Materials laminated structure can be produced. Figure 5.83 illustrates this for the composite copper- Indirect and hydrostatic extrusion offer the palladium [Sto 87]. following favorable conditions for production: The production of a metallic fiber composite ● It is possible to extrude composite billets material corresponding to the schematic struc- with thin casing tubes without the risk of the ture shown in Fig. 5.78 and consisting of a cas- Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 303

acceptable handling. Thus, in simple extrusion, the number of fibers in the end product is deter- mined by the maximum possible internal diam- eter of the casing tube. The total production is largely determined by the cost of the production of the casing tube. These include casings in the form of closed cups (produced by drawing and redrawing); casings machined with a solid or hollow tip, the angle of which is matched to the angle of the die; and casings that consist of flanged tube sections. The simplest case from the production point of view is shown in Fig. 5.84. A tube is used for the casing material, the short inlet of which is conically upset outside the press. The upper and lower closure of the core wire packet is made from embedded stamped blanks (indirect extrusion) or from a stamped blank and an embedded radial plug that simultaneously can act as the guide for the billet in the container (hydrostatic extrusion). With suitable materials selection, this component can be reused. In the application of composite billets that consist of only two components and thus have almost no volume gaps (e.g., copper-clad alu- minum bus bar conductor), there are no addi- tional influences resulting from the geometry of the initial material apart from those described. This is not the case when multicore composite billets are used (simplest billet preparation, no precompaction of the billets). In the case of hy- drostatic extrusion, the billet is radially com- pacted at the start of extrusion because upsetting cannot occur. Twisting and bending of the wire bundle does not occur from experience; how- ever, with thick wires their contour can be seen in the form of corrugations in the encasing tube. With thin wires, the compaction results in the Fig. 5.82 Examples of sheath failure geometry of the billet deviating from a circular shape. In both cases there is nonuniform flow in the ing tube and one or more core wires is possible die and explosive lubricant breakdowns in the by extruding a number of thin core wires (the die aperture, resulting in the extrusion shatter- number of wires corresponding to the desired ing. In the case of hydrostatic extrusion of non- number of fibers in the finished product, simple compacted multicore billets, with simple billet extrusion) or by extruding a less number of thick preparation, these problems have to be expected core wires in a casing to a relatively thick sec- if very small core wires (1 mm diam.) are not tion, a small number of sections of which are used [Mue 81]. then bundled into a common casing and reex- Under the stress conditions of indirect extru- truded (repetitive extrusion). sion, the composite billet is not radially com- Handling of thin wires can be difficult. It can pacted as in hydrostatic extrusion but upsets therefore be assumed that a core wire thickness with a reduction in length and increase in cross of approximately 1 mm is the lower limit for section to the diameter of the container. The re- 304 / Extrusion, Second Edition

Fig. 5.83 Structure of extruded copper/palladium-composite material. Micrograph image width is approximately 3.6 mm duction in length of the core wire bundle can ● Buckling of the individual wires or the entire take place in three ways: bundle ● Twisting of the core wire bundle ● Compression of the core wires

Fig. 5.84 Multicore billet preparation for (a) hydrostatic extrusion and (b) indirect extrusion Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 305

Fig. 5.85 Schematic of indirect repetitive extrusion

The first two mechanisms do not initially con- in the rod bundle, the support of the casing, and tain any cross-section increase of the individual the mechanical properties of the material. wires. There is the risk of the casing buckling or [Mue 81] shows that twisting and buckling bending giving rise to extrusion defects. The processes can be identified at an L/d ratio of 20 twisting represents energetically the lowest state for a fiber composite material of AlMgSi0.5 and can therefore always be expected. Whether wires; however, no material defect occurs. At an compression or buckling occurs after twisting L/d ratio of 100, the casing material exhibits depends on the length-diameter (L/d) ratio of the folding and a defect-free product cannot be ob- individual rods as well as their mutual support tained. 306 / Extrusion, Second Edition

Fig. 5.86 Different structures in Ag/C (billet diam, 30 mm; bar diam, 12 mm). ZF, number of fibers in sheath tube

Fig. 5.87 Cross section after progressive extrusion stages (bar diam, 12 mm). ZF, number of fibers in the sheath tube

Therefore, the so-called indirect repetitive ex- Ag/Cu/C contact materials is described in more trusion is a particularly suitable variation of the detail in this chapter [Mue 85]. Silver or copper indirect extrusion for the production of compos- billets were drilled and filled with graphite pow- ite materials with complex structures. A billet der for the production of silver/graphite and cop- made from a specific number of core wires per/graphite composite materials. The diameter (mantle wires) is extruded to a rod, which is then of the bores was determined by the graphite con- sectioned and the cross sections reextruded. This tent that had to be achieved. The billets were process can be repeated as often as required to sealed, preheated to extrusion temperature, and produce the desired structure. The process se- indirect extruded to bars. After cutting the sec- quence is shown schematically in Fig. 5.85. tions to length, a specific number of the cross The process for the production of Ag/C and sections was arranged in a thin-walled silver or Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 307

Fig. 5.88 Geometric arrangements in indirect repetitive extrusion

Fig. 5.89 Process principle for the production of Al-alloy steel composite bus bar copper casing. The number of bar sections de- ber of fibers in the composite is shown in pends on the extrusion ratio. Fig. 5.88. Depending on the extrusion ratio and the number of extrusion stages, different structures 5.45 Application Examples can be obtained Fig. 5.86. The casings were also sealed at both ends and The applications described below subse- extruded like the powder filled billets. The ex- quently for metallic composite materials pro- truded multicore bars were again cut to length duced by extrusion can illustrate only a few ar- and reprocessed as described previously. The se- eas of the comprehensive range. There has been quential structures can be seen in Fig. 5.87. no attempt to provide a full coverage of all pos- The relationship between the casing used, the sible and, to some extent, still-experimental ma- wire diameter, the extrusion ratio, and the num- terials. 308 / Extrusion, Second Edition

5.45.1 Simple Structures and Coatings movement between the steel and the aluminum. In this way, two-mirror image, composite bus Lead sheathing of electrical cables can be bars conductor rails are extruded with a metallic considered as the oldest and also the most well- bond between the steel and the aluminum. The known example of the production of a metallic reason for simultaneously extruding two com- composite material. In the mid-1960s, the com- posite sections is to avoid friction between the posite extrusion of bar, tube, and section became steel strip and the die. In this process the steel technically important [Ric 69], particularly for band does not contact the die surface but the processing of reactive metals including be- emerges with no wear. This is achieved as the ryllium, titanium, zirconium, hafnium, vana- steel strips are completely surrounded by the dium, niobium, and tantalum. The problem of aluminum and never come into contact with the attack of the extrusion tooling is a major prob- die bearings. Because the steel strips do not stick lem in the processing of these metals and their to each other, the two sections can be easily alloys. Technically, this means that the standard separated from each other. Figure 5.89 shows the lubricants used in extrusion are not able to guar- process principles. antee reliable separation between the metals be- Simple composite structures are also found in ing extruded and the extrusion tooling so that the area of steel- and iron-alloy composite tubes localized welding occurs. Billets of reactive and sections [Deg 89, Gut 91, Hug 82, Lat 89, metals have to be clad with a metal that provides Vil 92], as well as aluminum-clad steel wire [Hir the lubrication as a “lost shell.” Ti/Cu ignition 84] that has excellent corrosion resistance even electrodes are produced this way. in marine atmospheres and is used as wire cloth The development of suitable lubricants, and and wire netting. of low-melting-point glasses in particular, en- ables conventional extrusion without a cladding material to be carried out today in many cases. 5.45.2 Metallic Fiber Composite Materials Cladding with, for example, 18/8 chromium- with Complex Structures nickel steel, is used today for the extrusion of Outstanding examples in this area are the in- cast intermetallic materials including NiAl, dustrial metallic superconductors NbTi and Ni Al, and TiAl to counteract embrittlement by 3 Nb3Sn [Web 82, Bre 79, Hil 84]. Very different oxygen absorption at the high extrusion tem- complex structures of usually three or more in- perature of approximately 1250 ЊC. dividual components are produced depending on Since 1963, laminated composite materials of the desired current-carrying capacity. The ex- copper-clad aluminum for conductors have been amples in Fig. 5.90 shows examples of niobium developed as a substitute for copper [Hor 70, wires embedded in a copper-tin matrix. Nyl 78, Fri 67]. The aluminum core consists of Copper forms the core of the composite ma- EC aluminum, the copper cladding of electro- terial and is separated from the CuSn of the su- lytic copper. For technical and economic rea- perconductor by a tantalum barrier. Structures sons, the copper cladding is 15% of the cross- with up to 10,000 individual filaments and a fil- sectional area. This material is produced only to ament diameter between 3 and 5 lm in wire di- a limited extent today because of the changes in ameter of 0.4 to 2.0 mm are obtained by multiple metal prices. combination and followed by multiple extru- In contrast, the process developed by the com- sion, comparable to the repeated indirect extru- pany Alusingen for the production of composite sion already described but with a packing den- electrical bus bars conductors in aluminum-steel sity of 95 to 98%, followed by drawing. for high-speed and underground trains is com- After the final operation to produce the re- mercially important [Mie 87]. The basic process quired dimensions, the material is heat treated is that two alloy steel strips are fed into an ex- to produce the superconducting compound trusion die. The steel strips are fed from decoil- Nb3Sn by diffusion of the tin from the copper- ers. They pass through stations for chemical and tin matrix and reaction with the niobium wires. mechanical pretreatment to apply corrosion pro- The copper core remains unaltered (protected by tection. They are then fed into the extrusion die the tantalum barrier). Because the supercon- Њ and turned through 90 . Both metals bond to- ducting intermetallic phase Nb3Sn cannot be de- gether in the welding chamber under the influ- formed, this production route has to be used. ence of the high extrusion pressure and the in- The possibility of influencing the material creased temperature as well as the relative and properties by changing the structure and by Fig. 5.90 Cross section of a filament super conductor [Web 82]

Fig. 5.91 Property comparison between Cu/Pd composite materials (VW) and corresponding Pd/Cu alloys (electrical conductivity and hardness after cold working) 310 / Extrusion, Second Edition

Fig. 5.92 Schematic process for reaction powder metallurgy. (a) Longitudinal and transverse structure of Ti48.5Al, unreacted. (b) Longitudinal and transverse sections in the extruded state specific heat treatments can be applied to other Pd composite alloys a high ductility (Fig. 5.91). materials. The work by [Sto 87] and [Mue 86] This provides excellent further processing pos- describes the contact material copper palla- sibilities, providing savings in the noble metals dium. Palladium-copper alloys with 15 or 40 and economic production processes for com- wt% Cu are used extensively for contact ma- plex components. terials for switching direct current because of their high resistance to material movement. Pal- ladium and copper form a continuous solid-so- 5.45.3 Metallic Fiber Composite lution series with superstructure phases. This governs the electrical conductivity of the alloys. Materials with P/M Structures There are significant technical and economic This application area includes all those fiber advantages in using Cu/Pd composite materials composite materials that can be produced from instead of conventional PdCu alloys. Care powder starting materials. These are mainly alu- should be taken to avoid the formation of solid- minum-base fiber-reinforced materials [Sch 87, solution zones at the boundary surface between Ros 92, Bei 90, Moo 85], metal matrix compos- copper and palladium in the manufacture of the ites (MMC) [Sta 88], and materials for electrical composite materials. It is possible, by control- contacts [The 90]. The production of fibers [Boe ling the structure and the temperature in the de- 88] also provides a means of materials strength- formation process, to obtain the microscopic ening. heterogeneous composite structure. The indirect Alloys based on the intermetallic phase TiAl extrusion of coils of copper and palladium strip have a large potential for use as high-tempera- is particularly suited for this. This produces ture, light structural components. The deforma- contact materials that exhibit a completely dif- tion potential of this phase, which is usually brit- ferent property spectrum compared with con- tle under a conventional stress state, is very ventional alloys of the same composition. The limited. Semifinished and structural components electrical and thermal conductivities of Cu/Pd can be produced economically by reaction pow- composite materials are up to 10 times higher der metallurgy [Mue 90, Wan 92, Mue 93] (see than the corresponding Cu-Pd alloys. The ab- also the section “Extrusion of Powder Metals”). sence of solid-solution hardening gives the Cu/ The production of titanium aluminides by reac- Chapter 5: The Production of Extruded Semifinished Products from Metallic Materials / 311

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