ASM Handbook, Volume 7: Powder Metal Technologies and Applications Copyright © 1998 ASM International® P.W. Lee, Y. Trudel, R. Iacocca, R.M. German, B.L. Ferguson, W.B. Eisen, K. Moyer, All rights reserved. D. Madan, and H. Sanderow, editors, p 834-839 www.asminternational.org DOI: 10.1361/asmhba0001576 Conventional Aluminum Powder Metallurgy Alloys

ALUMINUM P/M PARTS are used in an in- • The uniform introduction of strengthening methods are discussed in the article "Advanced creasing number of applications. The business features, that is, barriers to dislocation mo- Aluminum Powder Metallurgy Alloys and Com- machine market currently uses the greatest vari- tion, from the powder surfaces posites" in this Volume. ety of aluminum P/M parts. Other markets that This article focuses on more conventional alu- indicate growth potential include automotive The powder processes of rapid solidification minum P/M alloys and process methods. There components, aerospace components, power and mechanical attrition lead to microstructural are several steps in aluminum P/M technology tools, appliances, and structural parts. Due to grain refinement and, in general, better mechani- that can be combined in various ways, but they their mechanical and physical properties, alumi- cal properties of the alloy. In addition, RS can will be conveniently described in three general num P/M alloys provide engineers with flexibil- extend the alloying limits in aluminum by en- steps: ity in material selection and design. A variety of hancing super-saturation and thereby enabling pressed and sintered aluminum P/M parts are greater precipitation hardening without the • Powder production shown in Fig. 1. harmful segregation effects from overalloyed • Powder processing (optional) Conventionally pressed and sintered alumi- I/M alloys. Moreover, elements that are essen- • Degassing and consolidation num powder metal parts have been commer- tially insoluble in the solid state, but have sig- cially available for many years. Sintered alumi- Powder can be made by various RS processes num P/M parts are competitive with many nificant solubility in liquid aluminum, can be aluminum castings, extrusions, and screw ma- uniformly dispersed in the powder particles dur- including atomization, splat quenching to form chine products that require expensive and time- ing RS. This can lead to the formation of novel particulates, and melt spinning to form ribbon. consuming finishing operations. In addition, sin- strengthening phases that are not possible by Alternatively, powder can be made by non-RS tered aluminum P/M parts compete with other conventional I/M, while also suppressing the processes such as by chemical reactions includ- metal powder parts in applications where some formation of equilibrium phases that are delete- ing precipitation, or by machining bulk material. of the attractive physical and mechanical proper- rious to toughness and corrosion resistance. Powder-processing operations are optional and ties of aluminum can be used. The following These topics and the development of dispersion include mechanical attrition (for example, ball combination of properties make aluminum at- strengthened aluminum-base composites by P/M milling) to modify powder shape and size or to tractive for P/M parts:

• Light weight • Corrosion resistance • High strength • Good ductility • Nonmagnetic properties • Conductivity • Machinability • Variety of finishes

In addition, P/M technology can be used to re- fine microstructures compared with those made by conventional ingot metallurgy (I/M), which often results in improved mechanical and corro- sion properties. Microstructural refinement by P/M is made possible by two broad high- strength P/M technologies--rapid solidification (RS) and mechanical attrition (mechanical alloy- ing/dispersion strengthening). The advantages of P/M stem from the ability of small particles to be processed. This enables: Fig, 1 Typical pressed and sintered aluminum P/M parts made from alloy 601AB. Top: gear rack used on a disc drive. Bottom: link flexure used on a print tip for a typewriter. Right: header/cavity block used on a high-voltage vacuum • The realization of RS rates capacitor. Courtesy of D. Burton, Perry Tool & Research Company Conventional Aluminum Powder Metallurgy Alloys / 835 introduce strengthening features, or comminu- Table I Compositions of typical aluminum P/M alloy powders tion such as that used to cut melt-spun ribbon into powder flakes for subsequent handling. Cemlms~ ~t Grade Cu Mg Si AI Imbdcmm Aluminum has a high affinity for moisture, and aluminum powders readily adsorb water. 60lAB 0.25 1.0 0.6 bal 1.5 20lAB 4.4 0.5 0.8 bal 1.5 The elevated temperatures generally required to 602AB ._ 0.6 0.4 bal 1.5 consolidate aluminum powder cause the water of 2O2AB 40 b~ x5 hydration to react and form hydrogen, which can M~22 20 il; 63 b~ 15 result in porosity in the final product, or under MD-24 4.4 0.5 0.9 bal 1.5 confined conditions, can cause an explosion. MD-69 0.25 1.0 0.6 bal 1.5 Consequently, aluminum powder must be de- MD-76 1.6 2.5 ... hal 1.5 gassed prior to consolidation. This is often per- formed immediately prior to consolidation at essentially the same temperature as that for con- processed by conventional P/M press and sinter Compacting pressure, tsi solidation to reduce fabrication costs. Consolida- operations. The non-reducible aluminum oxide 1O 20 30 40 50 tion may involve forming a billet that can be coating hinders the development of strong inter- I Ijllll~l ~ I subsequently rolled, extruded, or forged conven- particle bonds. tionally, or the powder may be consolidated dur- The most common heat-treatable grades are 9O ~"601AB ;Iuminurn / ing hot working directly to finished-product comparable to the 2xxx and 6xxx series wrought form. aluminum alloys. Alloys 20lAB and MD-.24 are / most similar to wrought alloy 2014. They de- g so I/ velop high strength and offer moderate corrosion Powder Production resistance. Alloys 60lAB and MD-69 are similar o iron or - to wrought alloy 6061. These alloys offer high / reducedcopper Atomization is the most widely used process strength, good ductility, corrosion resistance, "o to produce aluminum powder. Aluminum is and can be specified for anodized parts. Alloy melted, alloyed, and sprayed through a nozzle to 601AC is the same as 60lAB, but does not i" form a stream of very fine particles that are rap- contain an admixed lubricant. It is used for idly cooled--most often by an expanding gas. isostatic and die-wall-lubricated compaction. / Atomization of aluminum is discussed in more When high conductivity is required, alloy detail in the article "Production of Aluminum 602AB often is used. Conductivity of 602AB 4O 0 200 400 600 "800 Powders" in this Volume. ranges from 24 x 106 to 28 x 106 S/m (42.0 to Compacting pressure, MPa Splat cooling is the process that enables cool- 49% IACS), depending on the type of heat treat- ment selected. ing rates even greater than those obtained in at- Fig. 2 Relationship of green density and compacting omization. Aluminum is melted and alloyed, and pressure liquid droplets are sprayed or dropped against a chilled surface of high thermal conductivity-- Aluminum P/M Part Processing for example, a copper wheel that is water cooled from 10 to 30 min. Nitrogen, dissociated ammo- internally. The resultant splat particulate is re- Basic design details for aluminum P/M parts nia, hydrogen, argon, and vacuum have been moved from the rotating wheel to allow sub- involve the same manufacturing operations, used for sintering aluminum; however, nitrogen sequent droplets to contact the bare, chilled sur- equipment, and tooling that are used for iron, is preferred because it results in high as-sintered face. Cooling rates of 105 IGs are typical, with copper, and other metal-powder compositions. mechanical properties (Table 2). It is also eco- rates up to 109 K/s reported. Compacting. Aluminum P/M parts are com- nomical in bulk quantities, If a protective atmos- Melt-spinning techniques are somewhat pacted at low pressures and are adaptable to all phere is used, a dew point of-40 °C (--40 °F) or similar to splat cooling. The molten aluminum types of compacting equipment. The pressure below is recommended. This is equivalent to a alloy rapidly impinges a cooled, rotating wheel, density curve, which compares the compacting moisture content of 120 mL/m3 (120 ppm) maxi- producing rapidly solidified product that is often characteristics of aluminum with other metal mum. in ribbon form. The leading commercial melt- powders, indicates that aluminum is simpler to Aluminum preforms can be sintered in hatch spinning process is the planar flow casting compact. Figure 2 shows the relative difference furnaces or continuous radiant tube mesh or cast (PFC) process (Ref 1). The liquid stream con- in compacting characteristics for aluminum and belt furnaces. Optimum dimensional control is tacts a rotating wheel at a carefully controlled sponge iron or copper. distance to form a thin, rapidly solidified ribbon The lower compacting pressures required for best attained by maintaining furnace temperature and also to reduce oxidation. The ribbon could aluminum permit wider use of existing presses. at +_2.8 °C (~ °F). Typical heating cycles for be used for specialty applications in its PFC Depending on the press, a larger part often can aluminum parts sintered in various furnaces are form but is most often comminuted into flake be made by taking advantage of maximum illustrated in Fig. 3. powder for subsequent degassing and consolida- press force. For example, a part with a 130 cm2 Mechanical properties are directly affected by tion. (20 in. 2) surface area and 50 mm (2 in.) depth is thermal treatment. All compositions respond to formed readily on a 4450 kN (500 ton) press. solution heat treating, quenching, and aging in The same part in iron would require a 5340 kN the same manner as conventional hot-treatable Press and Sintered Aluminum Alloys (600 ton) press. In addition, because aluminum alloys. More detailed information on sintering of responds better to compacting and moves more aluminum can be found in the article "Produc- Commercially available aluminum powder al- readily in the die, more complex shapes having tion Sintedng Practices" in this Volume. loy compositions (Table 1) consist of blends of more precise and finer detail can be produced. Repressing. The density of sintered compacts atomized aluminum powders mixed with pow- Sintering. Aluminum P/M parts can be sin- may be increased by repressing. When repress- ders of various alloying elements such as zinc, tered in a controlled, inert atmosphere or in vac- ing is performed primarily to improve the di- copper, magnesium, and silicon. Press and sin- uum. Sintering temperatures are based on alloy mensional accuracy of a compact, it usually is tered aluminum alloys are based on elemental composition and generally range from 595 to termed "sizing"; when performed to improve blends. Prealloyed aluminum powders cannot be 625 °C (1100 to 1160 °F). Sintering time varies configuration, it is termed "coining." Repressing 836 / Materials Systems, Properties, and Applications

Table 2 Typical properties of nitrogen-sintered aluminum WM alloys Compacting Tensile Yield pt~sute Green density Green strength Siatered density mength(a) mength(a) l~bngatioa, Alloy MPa tsi % g]cm3 MPa psi % gJcm3 Temper MPa ksi MPa ksi % nartlaBes$ 60lAB % 7 85 2.29 3.1 450 91.1 2.45 T1 110 16 48 7 6 55-60HRH T4 141 20.5 96 14 5 80-85 HRH T6 183 26.5 176 25.5 1 70-75 HRE 165 12 90 2.42 6.55 950 93.7 2.52 T1 139 20.1 88 12,7 5 60-65 HRH T4 172 24.9 114 16,6 5 80-85HRH T6 232 33.6 224 32,5 2 75-80HRE 345 25 95 2.55 10.4 1500 96.0 2.58 T1 145 21 94 13.7 6 65-70HRI-I T4 176 25.6 117 17 6 85-90HRH T6 238 34.5 230 33.4 2 80-85 HRE 602AB 165 12 90 2.42 6.55 950 93.0 2.55 T1 121 17.5 59 8.5 9 55-60HRH T4 121 17.5 62 9 7 65-70HR}I T6 179 26 169 24.5 2 55-60HRE 345 25 95 2.55 10.4 1500 96.0 2.58 T1 131 19 62 9 9 55-60HRH T4 134 19.5 65 9.5 10 70-75 HRH T6 186 27 172 25 3 65-70 HRE 20lAB 110 8 85 2.36 4.2 600 91.0 2.53 T1 169 24.5 145 24 2 60-65 HRE T4 210 30.5 179 26 3 70-75 HRE T6 248 36 248 36 0 80-85 HRE 180 13 90 2.50 8.3 1200 92.9 2.58 T1 201 29.2 170 24.6 3 70-75 HRE T4 245 35.6 205 29.8 3.5 75-80HRE T6 323 46.8 322 46.7 0.5 85-90HRE 413 30 95 2.64 13.8 2000 97.0 2.70 T1 209 30.3 181 26.2 3 70-75 HRE T4 262 38 214 31 5 80-85 HRE T6 332 48.1 327 47.5 2 90-95 HRE 202AB compacts 180 13 90 2.49 5.4 780 92.4 2.56 T1 160 23.2 75 10.9 10 55-60HRH T4 194 28.2 119 17.2 8 70-75 HRH T6 227 33 147 21.3 7.3 45-50 Cold-fomaed parts 180 13 90 2.49 5.4 780 92.4 2.56 T2 238 33.9 216 31.4 2.3 el}I-IRE (19% strain) T4 236 34.3 148 21.5 8 70HRE T6 274 39.8 173 25.1 8.7 85 HRE T8 280 40.6 250 36.2 3 87HRE

(a) Tensile properties determined using powder metal flat tension bar (MPIF standard 10-63), sintered 15 min at 620 °C (1150 °F) in nitrogen

IB0 , j 800 ~-- ~ 1200 800 ! Sinter I [ Lubricant Sinter - 1200 . Lubricant ~ Cool rate: Cool rate: 1200 burn Air ~" ~ ~0 110 *C/rain ~ ~" P 600 y- ~1 600 - Off f~ea~ COOl ~ ~ "bU~,,,~ (200 "F/min)" 9(]0 ~ ~14 '~/min • (25 °Flmin) 900 ~'~ Furna-- I ~ - 900 ~ 2 400 ~--~-~-~ ~ ~ 400 400 --,~i ~ , COOl , J ~ g ~, ~ ~ ~ ~- / Cold water quench I 600 E E E E 600 es 4.___.4 Cool rate: -I-- ~, t~ i.~ 200 E " ~/ I 40*C/min "t" 300 i.~ ~ 200 Vacuum 300 I- 0' I I 7~ °F/Tin) l 300 pumpAdown 0 0 0 20 40 60 BO 100 20 40 60 80 tO0 0 20 40 60 80 I00 Time, rain Time, rain Time, rain (a) (b) (c)

Fig. 3 Typical heating cycles for aluminum P/M parts sintered in (a) a batch fumace. (b) a continuous furnace. (c) a vacuum furnace may be followed by resintering, which relieves ing pressure usually does not exceed 345 MPa ing of alloy 601 AB-T4 at 425 °C (800 °F) fol- stress due to cold work in repressing and may (50 ksi). Forging normally is performed in a lowed by heat treatment gives ultimate tensile further consolidate the compact. By pressing and confined die so that no flash is produced and strengths of 221 to 262 MPa (32 to 38 ksi), and a sintering only, parts of >80% theoretical density only densification and lateral flow result from yield strength of 138 MPa (20 ksi), with 6 to can be produced. By repressing, with or without the forging step. Scrap loss is <10% compared to 16% elongation in 25 mm (1 in.). resintering, parts of >90% theoretical density conventional forging, which approaches 50%. Alloy 60lAB. Heat lreated to the T6 condition, can be produced. The density attainable is lim- Forged aluminum P/M parts have densities of 60lAB has tensile properties of: Red by the size and shape of the compact. >99.5% of theoretical density. Strengths are Forging of aluminum is a well-established higher than nonforged P/M parts, and in many • UTS, 303 to 345 MPa (44 to 50 ksi) technology. Aluminum also lends itself to the ways, are similar to conventional forging. Fa- • Yield strength, 303 to 317 MPa (44 to 46 ksi) forging of P/M preforms to produce structural tigue endurance limit is doubled over that of • Elongation, up to 8% parts. nonforged P/M parts. In forging of aluminum preforms, the sintered Alloys 60lAB, 602AB, 20lAB, and 202AB Forming pressure and percentage of reduction aluminum part is coated with a graphite lubri- are designed for forgings. Alloy 202AB is espe- during forging influence final properties. cant to permit proper metal flow during forging. cially well suited for cold forging. All of the Alloy 20lAB. In the T4 condition, alloy The part is either hot or cold forged; hot forging aluminum powder alloys respond to strain hard- 20lAB has tensile properties of: at 300 to 450 °C (575 to 850 °F) is recom- ening and precipitation hardening, providing a mended for parts requiring critical die fill. Forg- wide range of properties. For example, hot forg- • UTS, 358 to 400 MPa (52 to 58 ksi) Conventional Aluminum Powder Metallurgy Alloys / 837

400 SO i 300 \ 300 40 "6• wrought 4o ~,~ I 6061 -T6. wrought 200 2OO 30 g E E E x 2O :E IO0 100 I0 201 AB- I 601 AG-T1 ' - -@ 0 0 0 I '0 102 103. 104 105 106 107 108 166 102 ,0a to4 10s io6 ,o7 ,# I~ Cycles to failure Cycles to failure (a) (b)

Fig. 4 Fatigue cu ryes for (a) P/M 60lAB (b) P/M 201 A8

• Yield strength, 255 to 262 MPa (37 to 38 ksi) somewhat less ductile than similar wrought com- the conductivities of sintered aluminum alloys • Elongation, 8 to 18% positions. Annealed specimens develop the high- with wrought aluminum, brass, bronze, and iron. est impact strength, whereas fully heat-treated Machinability. Secondary finishing opera- When heat treated to the T6 condition, the parts have the lowest impact values. Alloy tions such as drilling, milling, turning, or grind- tensile strength of 20lAB increases from 393 to 20lAB generally exhibits higher impact resis- ing can be performed easily on aluminum P/M 434 MPa (57 to 63 ksi). Yield strength for this tance than alloy 60lAB at the same percent den- parts. Aluminum P/M alloys provide excellent condition is 386 to 414 MPa (56 to 60 ksi), and sity, and impact strength of 20lAB increases chip characteristics; compared to wrought alu- elongation ranges from 0.5 to 8%. with increasing density. A desirable combination minum alloys, P/M chips are much smaller and Properties of cold-formed aluminum P/M al- of strength and impact resistance is attained in are broken more easily with little or no stringer loys are increased by a combination of strain- the T4 temper for both alloys. In the T4 temper, buildup. This results in improved tool service hardened deusification and improved interparti- 95% density 20lAB develops strength and im- life and higher machinability ratings. cle bonding. Alloy 60lAB achieves 257 MPa pact properties exceeding those for as-sintered (37.3 ksi) tensile strength and 241 MPa (34.9 99Fe-lC alloy, a P/M material frequently em- ksi) yield strength after forming to 28% upset. Properties for the T4 and T6 conditions do not ployed in applications requiring tensile strengths Powder Degassing change notably between 3 and 28% upset. Alloy under 345 MPa (50 ksi). and Consolidation 602AB has moderate properties with good elon- Fatigue is an important design consideration gation. Strain hardening (28% upse0 results in for P/M parts subject to dynamic stresses. Fa- The water of hydration that forms on alumi- 221 MPa (32 ksi) tensile and 203 MPa (29.4 ksi) tigue strengths of pressed and sintered P/M parts num powder surfaces must be removed to pre- yield strength. The T6 temper parts achieve 255 may be expected to be about half those of the vent porosity in the consolidated product. Al- MPa (37 ksi) tensile strength and 227 MPa (33 wrought alloys of corresponding compositions though solid-state degassing has been used to ksi) yield strength. Highest cold-formed proper- (see comparisons of two P/M alloys with two reduce the hydrogen content of aluminum P/M ties are achieved by 20lAB. In the as-formed wrought alloys in Fig. 4). These fatigue-strength wrought products, it is far easier and more effec- condition, yield strength increases from 209 levels are suitable for many applications. tive to remove the moisture from the powder. MPa (30.3 ksi) for 92.5% density to 281 MPa Electrical and Thermal Conductivity. Alu- Degassing is often performed in conjunction (40.7 ksi) for 96.8% density. minum has higher electrical and thermal conduc- with consolidation, and the most commonly Alloy 202AB is best suited for cold forming. tiviiies than most other metals. Table 4 compares used techniques are described below. The vari- Treating to the T2 condition, or as-cold formed, increases the yield strength significantly. In the Table 3 Typical heat-treated properties of nitrogen-sintered aluminum P/M alloys T8 condition, 202AB develops 280 MPa (40.6 ksi) tensile strength and 250 MPa (36.2 ksi) Grades yield strength, with 3% elongation at the 19% Heat-treatedvark~les and properties MD-22 MD-24 MD-69 MD-76 upset level. Solution treatment Temperature, °C (°F) 520 (970) 500 (930) 520 (970) 475 (890) T'mle, min 30 60 30 60 Atmosphere Air Air Air Air Properties of Sintered Parts Quench medimn H20 H20 H20 H20 Mechanical Properties. Sintered aluminum Aging P/M parts can be produced with slrength that Temperature, °C (°F) 150 (300) 150 (300) 150 (300) 125 (257) equals or exceeds that of iron or copper P/M Time, h 18 18 18 18 Atmosphere Air Air Air Air parts. Tensile strengths range from 110 to 345 MPa (16 to 50 ksi), depending on composition, Heat-treated ('1"6)properties(a) density, sintering practice, heat treatment, and Transverse-rupaLrestrength, MPa (ksi) 550 (80) 495 (72) 435 (63) 435 (63) Yield strength, MPa (ksi) 200 (29) 195 (28) 195 (28) 275 (40) repressing procedures. Table 2 lists typical prop- Tensile strength, MPa (ksi) 260 (38) 240 (35) 205 (30) 310 (45) erties of four nitrogen-sintered P/M alloys. Prop- Elongation, % 3 3 2 2 erties of heat-treated, pressed, and sintered Rockwellhardness, HRE 74 72 71 80 grades are provided in Table 3. Electrical conductivity, %IACS 36 32 39 25 Impact tests are used to provide a measure of (a) T6, solution heat treated, quenched, and artificiallyage hardened toughness of powder metal materials, which are 838 / Materials Systems, Properties, and Applications

Table 4 Electrical and thermal conductivity of sintered aluminum alloys, wrought chips I I precipitationC,em., II Atomization II coatingS0,., II spinning"" II depositionSpr.y aluminum, brass, bronze, and iron I Eledric~ Tne~m] eooducavity(a) conductivity(b) at~*c (~*~3, at ~*c (~0~3, Material Temper %IACS callcm.s.°C 60lAB T4 38 0.36 T6 41 0.38 I Powder I T61 44 0AI I I 20lAB T4 32 0.30 i I T6 35 0.32 I I I I T61 38 0.36 Can Open tray Cold Degas 602AB T4 44 0.41 degas press degas T6 47 0.44 II II II II O.n. 1"61 49 0.45 I I I I 6061 wrought alu- T4 40 0.37 minum v.cuudegas II hotv. pressu_ II v.oo.degas T6 43 0.40 I1 o. o,.I Brass (35% Zn) Hard 27 0.28 I I I I Angled 27 0.28 Bronze (5% Slt) Hard 15 O. 17 Hot press press Annealed 15 0.17 I I il Iron (wrought plate) Hot rolled 16 0.18 I (a) Determined with FM- 103 Magnamster. Ca) Converted from Decan I electrical conductivity values I ,,. I I Form: I - ...... ous aluminum fabrication schemes are summa- I Extrude I rized in Fig.5. I Fo~a I Can Vacuum Degassing. This is perhaps the I Ro, I most widely used technique for aluminum de- gassing because it is relatively non-capital inten- Fig, 5 Aluminum P/M fabrication schemes sive. Powder is encapsulated in a can, usually aluminum alloys 3003 or 6061, as shown sche- matically in Fig. 6. A spacer is often useful to increase packing and to avoid safety problems The ultimate degassing temperature should be Evacuation tube, ~ I when the can is welded shut. It has been found selected based on powder composition, consid- that packing densities are typically 60% of theo- ering tradeoffs between resulting hydrogen con- tent and microstructural coarsening. For exam- w.,0.0 II retical density when utilizing this method on me- disc ' 0ssp:::;es chanically alloyed powders. Care must be used ple, an RS-P/M precipitation-hardenable alloy to allow a clear path for evolved gases through that is to be welded would likely be degassed at the spacer to prevent pressure buildup and ex- a relatively high temperature to minimize hydro- plosion. gen content (hotter is not always better). Coars- To increase packing density, the powder is ening would not significantly decrease the w' li often cold isostatically pressed (CIP) in a reus- strength of the resulting product because solu- able polymeric container before insertion into tion treatment and aging would be subsequently the can. Powder densities in the CIPed compact performed and provide most of the strengthen- of 75 to 80% theoretical density are preferred ing. On the other hand, a mechanically attrited Welded II II because they have increased packing density P/M alloy that relies on substructural strengthen- with respect to loose-packed powder, yet allow ing and will serve in a mechanically fastened °'"°--1-k II sufficient interconnected porosity for gas re- application might be degassed at a lower tem- I[ II perature to reduce the annealing out of disloca- moval. At packing densities of about 84% and Fig. 6 Degassingcan used for aluminum P/M process- higher, effective degassing is not possible for tions and coarsening of substructure. several atomized aluminum-alloy powders (Ref When a suitable vacuum is achieved (for ex- 2,3). Furthermore, one must control CIP pa- ample, <5 millitorr), the evacuation tube is rameters to avoid inhomogeneous load transfer sealed by crimping. The degassed powder com- through the powder, which can lead to excessive pact can then be immediately consolidated to and then re-evacuated. Several backfills and density in the outer regions of the cylindrical avoid the costs of additional heating. An extru- evacuations can be performed resulting in lower compact and much lower densities in the center. sion press using a blind die (that is, no orifice) is hydrogen content. In addition, the degassing can Such CIP parameters often must be developed often a cost-effective means of consolidation. often be performed at lower temperatures to re- for a specific powder and compact diameter, Dipurative Degassing. Roberts and cowork- duce microstructural coarsening. The canned powder is sealed by welding a cap ers at Kaiser Aluminum & Chemical Corpora- Vacuum Degassing in a Reusable Cham- that contains an evacuation tube as shown in Fig. tion have developed an improved degassing ber. The cost of canning and decanning ad- 6. After ensuring that the can contains no leaks, method called "dipurative" degassing (Ref 4). In versely affects the competitiveness of aluminum the powder is vacuum degassed while heating to this technique, the vacuum-degassed powder, P/M alloys, This cost can be alleviated some- elevated temperatures. The rate in gas evolution which is often canned, is hackfilled with a dipu- what by using a reusable chamber for vacuum as a function of degassing temperature depends rative gas (that is, one that effectively removes hot pressing. The powder or CIPed compact can on powder size, distribution, and composition. water of hydration) such as extra-dry nitrogen, be placed in the chamber and vacuum degassed Conventional Aluminum Powder Metallurgy Alloys / 839 immediately prior to compaction in the same pressure, and which enables the transfer of hy- ACKNOWLEDGMENT chamber. Alternatively, the powder can be "open drostatic stress to the powder. Early fluid dies tray" degassed, that is, degassed in an uncon- were made of mild steels or a Cu-10Ni alloy, ASM International gratefully acknowledges fined fashion, prior to loading into the chamber. with subsequent dies made from ceramics, glass, J.R. Pickens, Martin Marietta Laboratories. Sig- The processing time to achieve degassing in an or composites. nificant portions of this article were adapted open tray can be much less than that required in The preheated die that contains powder can be from his article, "High Strength Aluminum P/M a chamber or can, thereby increasing productiv- consolidated in <1 s in a forging press, thereby Alloys" Properties and Selection: Nonferrous ity. Care must be exercised to load the powder reducing thermal exposure that can coarsen RS Alloys and Special-Purpose Materials, Vol 2, into the chamber using suitable protection from microstrnctures. In addition, productivity is ASMHandbook, 1990, p 201-215. ambient air and moisture. The compacted billet greatly increased by minimizing press time. De- can then be formed by conventional hot-working pending upon the type of fluid die material being REFERENCES operations. used, the die can be machined off, chemically Dined Powder Forming. One of the most leached off, melted off, or designed to "pop off" 1. M.C. Narisimhan, U.S. Patent 4,142,571, 1979 cost-effective means of powder consolidation is the net-shape component while cooling from the 2. J.R. Pickens, J.S. Ahearn, R.O. England, and direct powder forming. Degassed powder, or consolidation temperature. For aluminum-alloy D.C. Cooke, High-Strength Weldable Alumi- powder that has been manufactured with great powders, unconfined degassing is critical to op- num Alloys Made from Rapidly Solidified care to avoid contact with ambient air, can be timize the cost effectiveness of the ROC process. Powder, Proc. of the AIME-TMS Syrup. on consolidated directly during the hot-forming op- An electrodynamic degasser was developed for Powder Metallurgy (Toronto), GJ. Hildeman eration. Direct powder extrusion and rolling this purpose. and M.J. Koczak, Ed., 15-16 Oct 1985, p 105- have been successfully demonstrated numerous Just as in the case of HIP, the stress state dur- 133 times over the past two decades (Ref 5, 6). It still ing ROC is largely hydrostatic. Consequently, 3. J.R. Pickens, J.S. Abeam, R.O. England, and remains an attractive means for decreasing the there may not be sufficient shear stresses to D.C. Cooke, "Welding of Rapidly Solidified cost of aluminum P/M. break the oxide layer and disperse the oxides, Powder Metallurgy Aluminum Alloys" End- Hot Isostatic Pressing (HIP). In HIP, de- which can lead to prior powder-particle bound- of-year report on contract N00167-84-C-0099, gassed and encapsulated powder is subjected to ary (PPB) failure. Consequently, a subsequent Martin Marietta Laboratories and DARPA, hydrostatic pressure in a HIP apparatus. Can hot-working step of the ROC billet is often nec- Dec 1985 vacuum degassing is often used as the precursor essary for demanding applications. More de- 4. S.G. Roberts, U.S. Patent 4,104,061 step to HIP. Furthermore, net-shape encapsula- tailed information on this technique can be 5. G. Naeser, Modern Developments in PM, Vol tion of degassed powder can be used to produce found in Ref 8. 3, Hausner, Ed., 1965 certain near net-shape parts. Relatively high HIP Dynamic Compaction. Various ultrahigh 6. D.H. Ro, "Direct Rolling of Aluminum Pow- pressures (~200 MPa, or 30 ksi) are often pre- strain-rate consolidation techniques, that is, dy- der Metal Stdp" Report for conWact F33615- ferred. Unfortunately, the oxide layer on the namic compaction, have been developed and 80-C-5161, 1 Sept 1980 to March 1981 powder particle surfaces is not sufficiently bro- utilized for aluminum alloys (Ref 9). In dynamic 7. J.R. Lizenby, "Rapid Onmidirectional Com- ken up for optimum mechanical properties. A compaction, a high-velocity projectile impacts paction" Kelsey-Hayes Company, Powder subsequent hot-working operation that intro- the degassed powders that are consolidated by Technology Center, 1982 duces shear-stress components is often neces- propagation of the resultant shock wave through 8. Rapid Omnidirectional Compaction, Powder sary to improve ductility and toughness. the powder. The bonding between the powder Metallurgy, Vol 7, Metals Handbook, 9th ed., Rapid Omnidirectional Consolidation. En- particles is believed to occur by melting of a American Society for Metals, 1984 gineers at Kelsey-Hayes Company have devel- very thin layer on the powder surfaces, which is 9. D. Raybould, Dynamic Powder Compaction, oped a technique to use existing commercial caused by the heat resulting from friction be- Proc. of the Eighth Int. HERF Conference forging equipment to consolidate powders in tween the powder particles that occurs during (New York), American Society of Mechanical several alloy systems (Ref 7). Called rapid omni- impact. The melted region is highly localized Engineers, I. Berman and J.W. Schroeder, Ed., directional consolidation (ROC), it is a lower- and self-quenched by the powder interiors 1984; also, in Carbide Tool J., March/April cost alternative to HIP. In ROC processing, de- shortly after impact. Thus, dynamic compaction 1984 gassed powder is loaded into a thick-walled has the advantage of minimizing thermal expo- 10. D. Raybould and T.Z. Blazynski, Dynamic "fluid die" that is made of a material that plasti- sure and microstructural coarsening, while Compaction of Powders, in Materials at High cally flows at the consolidation temperature and breaking up the PPBs. Strain Rates, Elsevier, 1987, p 71-130