CHAPTER 13 PROCESSING OF MATERIALS
All metal parts and components go through a number of different processes in their manufacture. Most metals, after extraction from their ores, are melted and cast, either as ingots or as castings.
Ingots are usually made with a geometric cross section (round, square, rectangular, hexagonal etc. - sometimes with rounded, or mitred comers or with scalloped faces). The actual shape is determined by a combination of metallurgical and econontic factors. They are normally cast into metal moulds to provide rapid cooling and solidification of the metal. An ingot is simply a convenient shape and size for subsequent working of the metal by rolling, forging or extrusion. ---"'- , ------_.---- Castings are produced by pouring the liquid metal into a specially prepared mould where the metal may solidify and take up the shape of the mould. This shape is made to be as close as possible to the final shape of the part.
There are a large number of different casting processes, all of which have in common: • A suitable mould cavity must be prepared. • The size of the cavity must be larger than the fmished size of the part to allow for shrinkage and contraction of the molten mem] as it solidifies and cools. • There must be proviSion for air and gases to escape from the mould as the metal is poured in. • The mould must not unduly restrain the casting as it contracts during cooling. • It must be possible to remove the casting from the mould after solidification, and it must also be possible to remove any residual mould material from the cold casting. • The mould must be .strong enough to withstand the mass, pressure and heat from the molten metal.
Most moulds require the use of a pattern over which the mould is produced. This pattern is generally made from wood, aluminium or cast iron.
SAND CASTING
Sand, which is basically silica (SiO,), is intimately mixed with small quantities of clay and water and is packed tightly over a wood or metal pattern contllined within a moulding box (sometimes called a 'flask'). Most moulding boxes are made in two halves the top half called the 'cope' and the bottom half. called the 'drag'. The patterns are also split so that part of the pattern is moulded in the cope and the other part is moulded in the drag. The patterns are removed and the cope and drag placed together to form a closed mould.
There are two important considerations in preparing a sand mould: • The pattern must be shaped to allow its easy removal from the mould. Its side walls must be slightly tapered to prevent damage to the sand mould during removal of the pattern. • The sand must be packed or rammed in so that it is hard enough to withstand the pressure and erosion from the hot liquid metal, and yet loose enough to provide a porous mass through which air and gases may escape.
Sand moulds are always gravity fed by pouring metal into them from above. They also cont<'Iin a system of risers, which are reservoirs of molten metal contllined within the mould and located over those parts of the casting where met<1.i shrinkage may occur during solidification of the liquid metal.
Molten metals are characterised by atoms in a random arrangement. As the metal solidifies, these atoms take up an orderly arrangement which results in a particular crystal structure, and in which the space occupied by the atoms is reduced.
150 CHAPTER 13 PROCESSfNG OF MA TERfALS
Since the solid metal occupies less volume than the liquid it is necessary to feed in additional liquid metal as the casting solidifies to make up for this reduced volume. Failure to adequately "feed" a casting results in the development of large cavities known as 'shrinkage cavities'.
Another feature of sand castings is that holes and other cavities designed into the casting are produced by placing sand cores, made from sand containing about 2% linseed oil, into the mould at the proper locations.
Variations of sand moulding include:
• CO 2 sand moulds, sometimes called 'silicate' or 'sodium silicate' s,md moulds. These sands are mixed with a small amount of sodium silicate which can be hardened by gassing with carbon dioxide. Alternatively an acid hardener may be mixed with the sodium silicate sand which produces a self-setting mixture with a limited shelf life. • Cold setting sands, sometimes called furon or furane sands in which the sand is mixed with an organic compound such a furfuryl alcohol and an acid catalyst. These sands harden in a short time by the formation of a covalently bonded plastic binder similar to bakelite. These sands have good breakdown properties which allows easy removal after casting.
spruce riser
•
Fig 13.1 Making a sand casting
SHELL MOULDING
One of the limitations of the sand moulding process is the order of accuracy that can be achieved. Tolerances for the smaller sand castings are of the order of ±1.5 nun, and the tolerances increase as casting dimensions increase.
One of the systems devised to improve casting tolerances is the shell moulding technique, where tolerances of ±O.2 nun can be achieved. The sand is mixed with an artificial bonding material which is generally one of the formaldehyde group of plastics, a thermosetting resin.
The sand and resin mix is slung onto a heated metal pattern where the sand in contact with the pattern is partially cured by the heat. Excess sand is then removed by inverting the pattern and the partly cured shell is conveyed through a heating oven to fully cure it. The pattern is then removed and reused. The shell mould can be backed with sand or with metal shot to provide increased strength.
151 CHAPTER 13 PROCESSING OF MATERIALS
~ C
F
Fig 13.2 Making a sllell mould
INVESTMENT CASTING
Investment casting is another high precision process that can be used to cast complex shapes. In this process, the "pattern" is expendable, and is usually made of wax, although low melting point metals have also been used (as has frozen mercury). The process is sometimes referred to as the lost-wax process.
A master pattern is made from wood or metal, and from this, a set of master dies are made using a low melting temperarure metal. In some cases dies may be made by cutting the shape directly into a metal die such as steel. Whilst initially more expensive, a much greater service life is achieved.
These master dies are used to make the expendable wax patterns by pouring or injecting liquid wax into the die and allowing it to solidify. This wax pattern is then given a thin coating of refractory, placed into a moulding box and backed with a refractory plaster. The box is passed through an oven which hardens the refractory and melts out the wax. Because the 'pattern' is removed by melting, quite complex shapes are possible by this process. Molten metal can then be poured into the hot mould which is subsequently destroyed to remove the casting. This process is used extensively for the casting of the more expensive metals and alloys, including gold as well as those alloys that are difficult to machine. It provides high dimensional accuracy, and a very smooth surface finish. --
Fig 13.3 Tile Investment Casting Process
PERMANENT MOULD CASTING
A significant part of the cost of sand casting is the production of the sand moulds, each of which is used only once. Parts of relatively simple shape can be cast into metal moulds made from cast iron or copper, and coated with a refractory wash between each cast. The part solidifies quickly so that the system can be automated for high production.
152 CHAPTER 13 PROCESSING OF MA1ERIALS
A variation on pennanent mould casting is 'slush' casting, in which, once the metal in contnct with the mould has solidified, the mould is inverted and excess metnl poured out to produce a hollow casting with a roughly fmished intental surface, and smooth outer surface,
DIE CASTING
Die casting is a fonn of pennanent mould casting in which the liquid metal is forced into a metal die under pressure. The high cost of plant and dies dictates that the process requires relatively long production runs to be economical. It is used principally for casting the lower melting point metals such as zinc, aluminium, magnesium, and copper alloys.
There are two basic types of die-casting machines - cold chamber in which the molten metal is ladled into the injector from an external melting pot, and hot chamber in which the metal melting unit is contained in the die-casting machine and the injector is inunersed in the liquid metal. The fonner is used for the higher melting point metals such HS aluminium and copper alloys, and the latter for the lower melting point metals such as zinc and magnesium.
Metal ladle
Moving Fixed platen platen Fig 13.4 Die casting process
Many die-cHSting machines have facilities to integrally CHSt inserts such as threaded fittings and electric elements in the castings.
HOT WORKING OF METALS
Hot working involves .the plastic defonnation of metals at a temperature above its recrystallisation temperature. This means that the metal is not work hardened by the plastic defonnation, and that grain recrystallisation is occurring so that there is no distortion of the grains in the hot worked metal.
HOT ROLLING When a metal is rolled, its thickness and/or shape is altered by passing it through a set of rolls. These rolls are set a particular distance apart, and into which a certain shape may be machined. The rolls are driven so that they draw the metal into the gap between them. There is a limit to the amount of reduction that can be obtained in a single PHSS through the rolls, which is detennined by the "angle-of bite", HS shown in the figure below. The principal factor conttolling angle of bite is the roll diameter.
153 CHAPTER 13 PROCESSING OF MATERIALS
Angle of bite
Fig 13.5 Angle of Bite in Rolling
Rolling mills are classified according to the number of rolls in a stand, the main types being 2-high, 3-high, 4-high and cluster.
Upper roll Steel
Lower roll
2 high non-reversing 2 high reversing
3 high 4 high cluster
Fig 13.6 Types of rolling mills
Shapes and sections commonly produced by hot rolling include: • Blooms, which are really large section square bars having thicknesses 150 mm or greater • Billets, which are produced by further rolling of blooms down to round-edged square sections varying from 40 mm to 150 mm thick • Slabs, which are rewmgular sections which provide the raw material for sheet production • Plate, sheet and strip, are flat products produced in thicknesses up to about 40 mm • Bar product is rolled in round, square, rectangular, hexagonal and other special shapes • Special sections, include railway line, beams, angles and other special shapes.
Hot rolling is often used in the initial shaping or "breaking down" of an ingot because, at elevated temperatures, the as cast metal is malleable which allows relatively easy defonnation. Also, the hot
154 CHAPTER 13 PROCESSING OF MATERIALS work breaks up the as-cast grain structure and, by recrystallisation, new, smaller crystals develop within the metal.
I I as cast structure deformation recrystallization grain growth Fig 13.7 Effects of hot rolling on grain structure
Rolling does induce some directional properties into a metal due to the elongation in the rolling direction of macroscopic and microscopic inclusions. Mechanical tests taken patallel to the rolling' direction will generally yield slightly superior results to those taken normal to the rolling direction. Where transverse properties are important, steps can be taken to minimise these effects.
FORGINGS If a metal is hot worked by the application of localised comprehensive forces, it is said to have been forged. There are three basic forging processes:
I) The metal may be drawn out - its section is reduced and its length increased (sometimes called Smith Forging because it had its beginning in the days of the village blacksmith). The shape of the forging relies upon the skill of the operator. Forgings weighing from 0.5 kg to over 200 tonnes are made by this method.
pre-heated workpiece anvil
Fig 13.8 Smith Forging
155 CHAPTER 13 PROCESSING OF MATERIALS
2) It may be upset, in which case the section is increased and the length reduced. This process is used to forge the heads on bolts, as well as for forge discs, rings and gear blanks.
Fig 13.9 Upset Forging
3) It may be squeezed or hammered between specially shaped dies so that the metal takes up the shape of the cavity cut into the die. Automotive crankshafts and connecting rods are made by this method.
Fig 13.10 Closed die forging
The major advantage of forgings is in the "grainflow" that can be achieved by shaping the metal by plastic deformation. This grainflow provides significant improvements in strength in the critical areas of highly stressed components.
EXTRUSION Extrusion is a method of hot working of metals by which a heated billet is placed in a container and is forced out through a shaped hole in a die by a ram. Extrusion can be likened to squeezing toothpaste from a tube, or grease from a grease gun.
There are three principal methods of extrusion: 1) Direct extrusion in which the ram at the back of the billet forces the billet out through the die located at the front of the container.
I extruded shape
ram
Fig 13.11 Direct extrusion
2) Indirect extrusion in which the billet is placed in a container that is closed at one end, and the die is located on the end of a hollow ram - the ram forces the die into the billet so that it is back-
156 CHAPTER 13 PROCESSING OF MAlERlALS
extruded through the die and out through the ram. The main advantage of this method is that less energy is required because friction between the billet and the walls of the container is less significant as the billet is fixed in the container.
ram
shape Fig 13.12 Indirect extrusion
3) Impact extrusion uses a high kinetic energy punch to form a shape by a single blow onto a billet held in a die. It is used only on the softer metals such as lead, zinc, tin and aluminium. The punch strikes the billet located in a die designed such that the metal is extruded back over the punch. This method is used in the manufacture of battery cans and toothpaste tubes.
Fig 13.13 Impact extrusion processes.
Extrusion methods are used for the production of complex shapes and hollow sections, principally from aluminium and copper alloys.
Hollow sections are made by fitting a mandrel to the ram (direct extrusion) or locating a floating mandrel within the die.
billet
ram mandrel Fig 13.14 Extrusion o/Tube
COLD WORKING OF METALS
CoW:orking involves the controlled plastic deformation of a metal at a temperature well below the recrystallisation temperature of the metal. The terms 'cold' and 'hot' are related to the metals recrystallisation temperature, rather than to the actual temperature at which the process is undertaken. However, most cold working is done at temperatures between room temperarure and about 250°C.
157 CHAPTER 13 PROCESSING OF MATERIALS
The major advantages of cold working, when compared with hot working, of a metal are: • Because the metal is not heated, fuel utilisation, and associated pollution, is reduced • Cold working provides a much improved dimensional control in the finished part - no allowance is needed for thermal contraction or for losses due to scaling • The lack of surface oxidation and scaling results in a much better surface fmish .' Cold working of a met,;'11 increases its strength • Parts made by cold forming generally require less machining, Hence there is less metal loss,
However cold working does have its limik~tions: • Because the metal is less malleable, much more powerful machines are required to deform it • The strain hardening induced by cold working may be undesirable so that the parts may require annealing after cold forming • The lower ductility of cold metal limits the amount of plastic deformation that can be achieved • The metal must be thoroughly clean and free of scale before cold forming. This often requires blasting with sand or shot or alternatively acid pickling • The materials must be in their softest condition so that maximum ductility is available.
Many of the processes by which metals are hot-worked are also carried out cold - these include rolling, forging, extrusion. Other cold working operations include bending, blanking, shearing, wire drawing, deep drawing and spinning.
Sheet, strip and bar products are cold rolled to produce materials with higher strength, better surface finish and better tolerances than can be obtained by hot rolling.
COLD DRAWING Wire and tube is produced by cold drawing (pulling) the metal through hardened steel, ceramic or diamond dies.
dies Fig 13.15 Wire drawing process
DEEP DRAWING Cup shaped components are cold formed by drawing and stretching metal over a mandrel and through a die. The stresses on the metal during deep drawing are very complex and special quality sheet materials are required for deep drawing purposes.
Products produced by deep drawing include ornament shell cases, stainless steel sink bowls and laundry tubs and aluminium milk chums.
One of the important factors in producing high quality products by the various cold working methods is lubrication - lubricants are required to prevent metal-to-metal conk~ct between tools and the job, and also to act as a coolant to dissipate the heat generated by the plastic deformation of the metal being drawn.
158 CHAPTER 13 PROCESSING OF MATERIALS
The complex nature of the strains imposed on the metal during drawing can result in tears, and wrinkles resulting from the thinning down and the thickening-up of the metal. 'Earing' is another problem which results from different amounts of defonnation due to directional effects in the sheet.
normal cup 'eared' cup
Fig 13.16 The effects of earing during drawing
The surface quality of deep drawn parts is very dependent upon the grain size in the sheet from which it is drawn - coarse grains result in an effect known as 'orange-peel', a surface roughness which resembles the surface of an orange in appearance.
rough 'orange peel' smooth surface surface at radius Fig 13.17 The orange peel effect
SHEARING AND BLANKING These are operations that are preliminary to many processes in that they are used to prepare blanks for subsequent forming. However shearing is also the basis of other processes such as piercing, punching, trinuning etc. used during the latter stages of a manufacturing cycle.
The mechanism of the shearing operation is shown in fig. 13.18. As pressure is applied to the workpiece by the punch, a maximum stress concentration occurs where the sharp edges of punch and die are in contact with the metal.
Cracks initiate at these points and, if the punch and die is correctly set, will meet at the centre of the section to produce a clean cut. If clearances are too small or too large, a ragged edge will result.
159 CHAPTER 13 PROCESSING OF MATERlALS
punch
die
clearance
insufficient clearance excessive clearance
Fig 13.18 Effects 0/ clearance in the shearing 0/ metals.
Clearances for blanking and shearing are dependent upon the thickness, hardness and type of metal being sheared, although clearances are generally in the range 5-10% of metal thickness, being towards the larger value for the more ductile metals.
NEWER FORMING TECHNIQUES MARFORMING Because of the high cost of deep drawing tools, a process employing low cost tooling has been developed for metal drawing. This uses a thick rubber pad as a "universal die". The metal is forced into the rubber die by a steel punch.
to cor.trolled hydraulic cushion unit Fig 13.19 Mara/orming Process
160 CHAPTER 13 PROCESSING OF MATERIALS
One particular advantage of this method is its ability to fonn sheet of different thickness to the same internal dimensions the rubber compensates for changes in outside dimensions.
HYDROFORMING In this process, the thick rubber pad is replaced by a thin rubber diaphragm backed up by hydraulic pressure. The operation is illustrated in fig. 13.20. Again, tooling is relatively cheap. For small quantities punches can be made from mild steel. It is capable of quite deep draws, in fact, deeper than with conventional deep drawing methods. It has also been used to concurrently produce pairs of mating parts by placing two blanks in the press together, one on top of the other.
1 1 I • 1
i r Fig 13.20 Hydr%rming Process
HIGH RATE FORMING It is known that deforming metals at a very high rate can partly overcome certain time dependent metallurgical changes that occur during nonnal fonning operations. Thus high rate forming can be used to plastically defonn metals to a greater extent than is possible under nonnal conditions.
One such method is explosive forming. This method is particularly useful for one-off or for small quantities because again the equipment required is cheap and simple to assemble. The one disadvantage is the fact that it does require the use of an explosive charge.
The sheet is placed over a female die and is held against the die by a pressure ring. This ring also acts as a reservoir for water. The explosive is submerged below the water where it is detonated to produce a high pressure wave which defonns the metal to the shape of the die.
Explosive fonning has been used to shape very high strength metals such a nimonics, stainless steels and titanium. There appears to be no limit to the size of parts that can be shaped by this process - dish shaped ends for very large vessels have been explosively fonned in lakes where the female die has been suspended from a large crane.
Explosive forming has little effect on the hardness or ductility of the defonned metal due to the high rate of plastic flow that overcomes the work hardening effects that occur during nonnal cold working processes.
161 CHAPTER 13 PROCESSING OF MATERIALS
explosive charge
explosive charge liquid workpiece
workpiece liquid
die die __.... _IIIr---' vacuum line
cylinder forming bulkhead forming Fig 13.21 Explosive forming of cylindrical and cup shaped parts
POWDER METALLURGY
If a metallic powder is subjected to a sufficiently high pressure a degree of bonding occurS between the metallic particles. If this compacted mass is then heated in a neutral atmosphere a further coherence develops between the particles by a diffusion process that is more active at elevated temperatures. The temperature used is usually higher than the recrystallisation temperature for the metal. but is well below the melting temperature.
Parts made from metal powders require the following processing:
1) Prepare the metal powder in a suitable degree of purity and Tmeness. This can be achieved by: • Mechanical methods such as grinding, rolling, hammering etc. - this is suitable only for brittle metals. • Atomising by passing a stream of molten metal through an orifice with a jet of air, gas, liquid, or steam. This method is used to prepare the more ductile and softer metals such as aluminium, copper, solders etc. • Chemical methods, which include chemical reduction, condensation and precipitation. Metals such as iron, cobalt, nickel and tungsten are chemically reduced from oxides, zinc is powdered by a condensation process, and silver and copper are prepared in very fme form by precipitation. • Electrolytic methods are used to prepare high purity powders of· copper and iron.
2) Blend the powders in the correct proportions, place into a mould and press with sufficient pressure to bond the particles together. This is usually done in steel or carbide dies at room temperature.
3) Sinter the compacted mass - this is done in a furnace at temperatures between the recrystallisation temperature and the melting temperature, although the temperature may exceed the melting point of one of the constituents present in the powder.
To prevent oxidation of the powder during sintering an inert or controlled atmosphere is maintained in the furnace.
If the part reqnires machining, it may be pre-sintered at a lower temperature, then machined and fully sintered. This is particularly useful for parts too hard to machine after fmal sintering.
4) Finishing and sizing - a further pressing or coining operation may be carried out if very high dimensional accuracy is required. This operation can also increase the strength of the sintered part.
162 CHAPTER 13 PROCESSING OF MATERIALS
The principal products of powder metallurgy methods include: • Porous products, such as porous bronze bearings that are impregnated with oil, and metal fIlters with very fme pore sizes. • Complex shapes such as gears, and camS that would require considerable machining by conventional methods, are made with little or no machining by powder methods. • Products difficult to machine, such as the tungsten carbide tips used for high speed machine tools. This material is a mixture of tungsten carbide powder with cobalt or nickel to provide a tough bond between the hard, wear resistant carbide. • Composites that are difficult to mix by conventional means. Copper and graphite are mixed as powders, shaped and sintered to produce brushes for electric motors, and copper or silver are combined with tungsten, nickel or molybdenum for other types of electrical contacts where high temperature oxidation resistance is required.
ADVANTAGES OF POWDER METALLURGY • Conventional methods of melting metals and mixing them to form alloys require that the meTals be mutually soluble it is not possible to mix metals if they are not mutually soluble in the liquid state. Mixed powders overcome this limitation because they can be blended, compacted and sintered. This allows for much wider variations in material compositions than is possible by the more conventional methods. • It is possible to incorporate non-meTallic substances into the part. For example the copper graphite brushes referred to earlier. • Because the powder can be pressed to its fmal shape, a great deal of machining is eliminated. This also means savings in material. • High production rates are possible because the process is simple to automate.
LIMITATIONS OF POWDER METALLURGY • Because metal powders cannot 'flow' like liquid metal, there are size and shape limitations. • The strength and toughness of parts produced by powder metallurgy are usually inferior to that of parts produced by casting or forging. • The initial cost of dies and tools are high, and require a high level of production to offset these costs. • The meTal powders themselves are expensive, although this cost is often offset by savings in production costs such as the elimination of machining. • Not all metals can be cold-pressed. Some tend to stick to dies causing die wear and faulty parts. • Some metal powders become reactive in air and so heat precautions are necessary to eliminate the risk of fire or explosion.
16"1 CHAPTER 13 PROCESSING OF MATERIALS
PROCESSING OF PLASTICS
Most plastic materials possess good "plastic" flow characteristics at slightly elevated temperatures, although in the case of the thennosetting plastics, the heating and fonning process is irreversible. Thus plastics lend themselves to moulding as a method of shaping. The principal moulding teclmiques are: • Injection moulding, in which granular plastic, usually thennoplastic, is fed from a hopper into a heated barrel where it is melted. A ram then forces the molten plastic into a cold metal die cut to the shape of the part. Pressure is maintained until the plastic has hardened, after which the mould is opened and the part removed. • Compression moulding, in which a weighed quantity of preheated powder is spread over a-heated mould cavity. The mould is then closed and heat and pressure are maintained until the plastic has fIlled the cavity and has cured. This method is widely used for the thennosetting plastics. The cure cycle means that the process is much slower than injection moulding. • Transfer moulding, is a modified compression moulding process the thennosetring material is put into an open cylinder where it is heated before transfer under pressure into a die cavity. • Blow moulding, is used to fonn hollow articles such as bottles and balls. Hot thennoplastic plastic is blown against the internal surfaces of a mould. Usually.~ a tube of- heated plastic is extruded into the mould and air is injected to expand the material onto the walls of the mould. • Extrusion. Just as with metals, plastics are extruded by forcing the heated material out through a cavity in a metal die. Tubes, pipes and continuous shapes are produced by extrusion. • Calendering is the process by which plastic sheet or film is produced. The material is passed through a series of heated rolls with reducing gaps to produce very thin plastic snip. • Casting, as the name implies, involves pouring molten plastic or a catalysed thennosetting resin, into an open mould where it sets or cures. There are other fonns of casting, for instance photographic film and cellophane are made by feeding a solution of cellulose acetate onto a moving polished metal band. • Vacuum fonning is used to fonn sheets of thennoplastic into three-dimensional shapes. The sheets are clamped around the edge, then heated and drawn down over a mould where it cools and hardens. • Foamed plastics are made from both thennoplastic and thennosetting plastics by expanding the plastic to a sponge-like structure during processing. This is accomplished by incorporating chemicals which decompose and liberate a gas at a critical point during the manufacturing cycle. It is also possible to dissolve certain gases in some plastics under conditions which will allow them to come out of solution at a later stage and so fonn bubbles in the solidified plastic.
MACHINING OF PLASTICS Most non-metallic materials are very poor conductors of heat, so that cutting and machining of these materials concentrates a large amount of heat at the cutting edge of the tool. For this reason, conventional tool materials tend to break down because of abnonnaltemperature rises. This heat also causes melting of the workpiece. Thus special tools are required for machining of plastics (e.g. drills with 60-90' tip angle and zero rake, and with large polished flutes).
In most machining processes on plastics, it is necessary to use a liberal supply of water as a coolant to prevent overheating of the cutting tools.
164 CHAPTER 13 PROCESSING OF MATERIALS
GLOSSARY Ingot Recrystallisation Casting Billet Shrinkage Slab Flask Forging Cope Extrusion Drag Cold work Binder Drawing Shell mould Maraforming Investment casting Powder metallurgy Die casting Calendering
QUESTIONS
1. Why is it necessary to maintain a reservoir of liquid metal to feed into a casting as it solidifies?
2. Describe briefly two different casting processes.
3. Compare the advantages and limitations of hot rolling and cold rolling of metals.
4. Describe two methods used for the extrusion of metals.
5. Describe three advantages of powder metallurgy methods.
6. Describe three plastic moulding processes and name two typical parts made by these processes.
HiS CHAPTER 14 JOINING OF MATERIALS
Most devices used by man are made from a number of parts, often involving the use of a variety of different materials, and all assembled together into a useful article, This assembly of parts requires that they be suitably joined or fastened.
There are a variety of means by which these materials may be fastened together, some of which may significantly disturb the structure of the material, while others provide only a minimum disturbance.
The principal methods by which materials are joined are: • Fasteners • Soft soldering • Brazing and silver soldering • Welding • Adhesives.
FASTENERS
The basic method of joining materials, particularly metals, together is called fastening - joining with bolts, rivets and screws.
In these operations, holes are drilled or punched through two or more pieces of material, and a fastener is fitted through the holes.
This method of joining has its limitations. Whilst it is simple to do, there is always the possibility that the fastener may work loose. However what is often more damaging is the possibility of corrosion in the crevice formed between the overlapping metals or between the fastener and the metal.
The design of these types of joints is often important because the distribution of forces can affect the stresses on the fasteners.
However fasteners such as bolts and screws are about the only way of joining materials when they must, at some time, be taken apart or opened up.
SOFT SOLDERING
In soft soldering, a thin fibn of molten alloy is introduced between the parts to be joined at a temperature below the melting point of the parts. The soft solder employed must fulfil the following requirements: • Its melting point must be lower than that of the metals to be joined, but higher than the maximum service temperature of the part • It must 'wet' and flow freely over the surfaces to be joined • It should solidify as a sound fiim of metal and adhere firmly to the parts • It must have adequate mechanical strength.
The most common soft solders are lead-tin alloys or lead-tin-antimony alloys, the latter being slightly cheaper because antimony replaces some of the more expensive tin in the solder.
The approximate strength of a soldered joint is about 50-60 MPa.
The main grades of solder are shown in Table 14.1.
166 ,
CHAPTER 14 JOINING OF MATERIALS
Table 14.1 Soft Solders
Composition Freezing Common Uses Sn Pb Sb temperature name 62 38 - 183°C Tinmans Solder Lowest melting point. Eutectic composition. Electrical & electronic work. 33 67 - 183-250°C Plumbers solder Wide freezing range produces a pasty stage which enables ioint to be wiped. 50 50 - 183-220°C General purpose As for plumbers solder. solder 43.5 55 1.5 188-220°C General purpose As for plumbers solder. * solder 12 80 8 243-250°C Soldering iron & steel. *
* These solders containing antimony should not be used on zinc or galvanised metal because of weak alloys formed with zinc.
For a solder to 'wet' the surfaces being joined, both the surfaces and the solder must be completely clean and solder must be completely clean and free from oxide films. It is the purpose of the flux to dissolve such oxide films and to prevent further oxidation during the soldering process.
Fluxes perform the following functions: • They chemically clean the surfaces to be joined • They prevent the formation of new oxide layers during the heating cycle for soldering • They assist the solder to run freely into the joint • They assist in the "wetting" process, by which the molten solder forms a thin surface alloy by diffusion with the metal being joined, thus creating a stronger joint
Table 14.2 lists the fluxes commonly used for soft soldering.
Table 14.2 Fluxes for soft soldering Flux Used on Action Killed Spirits (Zinc chloride Iron, tinplate, copper, brass, Chemically cleans surface and ZnCh bronze prevents re-oxidation. Will cause corrosion if not completely washed away Spirits of salts (Dilute Zinc galvanised steel Cleanses the zinc and forms hydrochloric acid - HCl) ZnC!,. Must be washed away. Sal Ammoniac (Ammonium Copper, Iron Cleans but not as good as ZnCh. chloride - NILCl) Must be washed away Dilute phosphoric acid - H,I'O. Stainless steel Cleanses, less damaging to austenitic stainless steel. Should be washed away Resin Most metals Non-corrosive. Used for electrical & electronic work Tallow Lead & lead rich alloys Has a protective action only. Does not cleanse the metal.
167 CHAPTER 14 JOINING OF MAlERIALS
BRAZING
Brazing is also known as 'hard soldering'. From a metallurgical viewpoint, brazing is similar to soft soldering except that the filler metal is a higher melting point alloy. Brazed joints are both stronger and capable of higher service temperatures than soldered joints. There are three primary groups of brazing alloys: • Brazing brasses • Silver solders • Self fluxing copper-phosphorous alloys.
BRAZING BRASSES Brazing brasses (or spelters) are copper-zinc alloys containing 50-60% copper. and sometimes up to 2% tin. with the balance being zinc. They melt in the range 860-890oC, and are used for general brazing of iron and steeL The joint strength is about 390-460 MPa
SILVER SOLDER Silver solders are commonly silver-capper-zinc alloys, with or without an addition of cadmium. They normally contain 50-60% Ag, 14-40% Cu, 10-20% Zn and sometimes 18-20% Cd. Their melting temperature is lower than that of the brazing brasses, being around 700-770oC for the Ag-Cu-Zn alloys and 620-640oC for the cadmium bearing alloys. Silver solders are very free-flowing and produce joint strengths in the range 340-450 MPa. They are used for the brazing of brass, copper, stainIess steels and nickel alloys.
As with soft solders, brazed joints require a clean, oxide free surface to allow the alloy to 'wet' the surface and provide a strong bond. There are two types of flux in use. For brazing alloys whose melting temperature is above 760°C, a borax-type flux is used. Below this temperature, borax is too viscous, so that a fluoride-type flux is required.
COPPER PHOSPHORUS BRAZING ALLOY Copper-Phosphorous brazing alloys are usually referred as self-fluxing brazing alloys. When melted in air, the phosphorous oxidises to form a fluid compound which acts as a very effective flux. However, they require an oxidising atmosphere (oxidising flame) to work effectively. Also these alloys should not be used on iron and steel or on nickel based alloys because they form brittle compounds which significantly weaken the joint
The self fluxing brazing alloys typically contain 4-7% Phosphorus, with or without 13-15% Silver and balance Copper. They melt in the range of 625-800°C,
ALUMINIUM BRAZING ALLOY Alumirtium is a metal which is very difficult to soft solder because of its hard, tenacious oxide film. Also, alumirtium has a relatively low melting point (600-6500C) so that it cannot be brazed with conventional alloys. However, special aluminium-silicon brazing alloys, with melting point below 600°C, are used for brazing of alumirtium.
WELDING
Welding processes may be broadly classified into two types: • Fusion Welding • Pressure Welding.
Welding is carried out at much higher temperatures than brazing, so that the metallurgical aspects are much more complex. The weld metal itself, is, effectively, a small casting in which the metal has been melted, cast and rapidly cooled. The nature of the welding process thus significantly controls the properties of this 'cast' weldment.
168 ,
CHAPTER 14 JOINING OF MATERIALS
The main welding processes are shown in figure 14.1.
Welding Processes i i FUsion }Nelding Pressu~ Welding i i J i i i i Arc Welding Gas Welding Thermit Welding Special Processes Forge Welding Resistance Welding I I I I Metallic Arc Oxy-acetylene Electron Beam Spot I I I I SUbme1ed Arc Oxy-propane Laser Beam Seam I Inert Gas Metal Butt Arc (MIG) I Inert Gas Tungsten Arc (TIG)
Fig 14.1 Welding processes.
METALLIC ARC WELDING All arc welding processes use the heat generated by an electric arc to melt the metal and to effect the weld.
In metallic arc welding, an arc is struck between a flux coated consumable metal electrode and the workpiece. The heat generated by the arc melts the consumable electrode, and the surface of the workpiece. The molten end of the electrode either falls, or is drawn, to the workpiece where it cools and solidifies .
. ~Oth alternating current (AC) and direct current (DC) systems are used. The latter is usually connected so that the electrode is negative and the job is positive.
The flux coating provides a gas that shields the molten metal to prevent oxidation, and a slag that solidifies over the weld bead to protect it as it cools. This slag is subsequently chipped away from the solidified weld.
Modifications to the arc welding process involve principally different methods of shielding the molten metal from the atmosphere.
electrode holder I --:.--...~ covered electrode
power
ground
Fig 14.2 Manual metallic arc welding.
These include:
Submerged Arc Welding Submerged Arc Welding, in which the electrode is a bare wire fed from a coil. The weld is shielded by a powdered flux fed from a hopper just ahead of the electrode. The flux in the area of the arc melts and forms a slag that protects the weldment from oxidation. Excess powder is drawn away by suction
169 CHAPTER 14 JOINING OF MATERIALS and returned to the hopper. This process is often automated. However it can only be used on a horizontal workpiece.
spool of filler wire
---c.. direction of travel __ wire drive & feed rolls
~.-.r+- power contact to wire granular flux hopper rr arc under flux flux blanket over ...... ,..."":'=;:,),;~--.lm--,I solidifying weld ground work
Fig 14.3 Submerged arc welding
Gas shielded arc welding Gas shielded arc welding, sometimes called "metal inert gas" or "MIG" welding uses argon (Ar) and/or carbon dioxide (CO,) [and occasionally helium (He) and nitrogen (N,)l as shielding gases for a bare metal wire electrode. The main advantages of this process are that it provides a faster deposition rate, and it can be adapted to automatic operation. If CO, gas is used, special electrode wire is required that is rich in deoxidants.
It is somewhat limited in the thickness that can be welded. However a modification of this process employs a flux cored electrode which uses a hollow wire with flux in the centre. This improves the quality of the weld but brings back the problem of flux removal.
filler wire shield gas
ground Fig 14.4 Gas shielded arc welding
Gas shielded tungsten arc welding Gas shielded tungsten arc welding, sometimes called "tungsten inert gas" or "T.I.G." welding differs from the other processes in that the arc is struck between the workpiece and a tungsten electrode. The welding electrode does not melt and so is non-consumable. On thin metals, the two edges of the workpiece are melted by the arc and fuse together without the addition of a filler metal. Otherwise a bare metal wire filler metal can be introduced into the arc to add filler metal. The shielding gas comes from an annular ring around the tungsten electrode, and is usually an inert gas such as argon or helium. It is a useful process for welding the more reactive metals such as aluminium and titanium, although it is somewhat limited to fairly thin materials such as sheet metals. Its advantages include a lack of slag, spatter free welds, the ability to weld in all positions, and the ability to be automated for production line welding.
170 CHAPTER 14 JOINING OF MATERIALS
torch tungsten electrode
power #'''----b~~refiller metal
shield
Fig 14.5 Gas shielded tungstell arc welding.
Plasma Arc Welding Plasma arc welding is a relatively new process in which a suitable gas such as argon is passed through a constricted electric arc. This ionises the gas by causing the atoms to split up into negatively charged eleClrons and positively charged ions, the mixture being called 'plasma'.
As the ions and eleClrons recombine, heat is released and an eXlremely hot 'electric flame' is produced. Temperatures of around 15,OOOoC can be produced by this technique.
Plasma is being used for cutting, drilling, and spraying of very refractory materials such as molybdenum, tungsten and ceramics, as well as for welding.
GAS WELDING Some welding processes make use of heat obtained by a chemical reaction. Probably the best known of these processes are the oxy-acetylene and oxy-propane processes.
These are torch welding processes in which the heat is generated by the combination of two gases. The gases are stored in pressurised bottles or tanks (acetylene cannot be 'pressurised' - it is stored by dissolving the gas in acetone which is contained in a porous packing within the cylinder).
Flame temperatures of around 3500'C are achieved from this process. A flux coated filler rod is also melted by the flame to supply the flller metal to the joint
The principal advantage of gas welding is its portability and its non-reliance on electric power.
bare filler metal
Fig 14.6 Gas Welding
THERMIT WELDING Thermit Welding is also a chemical process in which two powders are ignited so that they react and generate heat. Thermit powder consists of iron oxide and powdered aluminium, in calculated
171 CHAPTER 14 JOINING OF MAlERIALS proportions. By preheating the weld and igniting this powder, a chemical reaction occurs, which could be represented as: 3Fe,o4 + 8 AI --7 4 AlP, + 9Fe + Heat
The heat of this exothennic reaction is so intense that the iron produced is in a molten state. It runs down into a mould that has been prepared around the weld.
This process is used in the repair ofJarge steel castings and forgings, for joining railway track, and for fixing studs onto steel structures.
slag basin mould
riser -po>urirlg gate
gate for pre-heater nozzle
Fig 14.7 Tl!errnit welding process
ELECTROSLAG WELDING When welding thick workpieces, generally only the orthodox multirun processes, involving continuous removal of slag can be used.
Electroslag welding has been developed to join heavy sections in a single run by placing the sections to be welded in a vertical position, so that molten metal is delivered progressively to the vertical gap. The plates to be welded fonn two sides of a "mould", whilst water cooled copper shoes fonn the other two sides to fonn a dam that retains the molten metal until it freezes. _
This process is started much like the submerged arc process - an arc is struck between a consumable wire electrode and the workpiece. The heat from this arc melts the flux and fonns a slag over the molten metal. Once sufficient slag is fonned, further heat is generated by the electrical resist.1TIce offered by the slag which is sufficiently conductive to allow the passage of current from the electrode to the workpiece. As the electrode wire melts, the liquid metal falls into the weld pool beneath the slag, so that the level of weld pool rises. The heat from the slag also causes melting of the plate edges so that good fusion is obtained between the weld and the parent metal.
172 CHAPTER 14 JOINING OF MA1ERIALS
Fig 14.8 Electroslag Welding
As the weld metal solidifies, the copper shoes are moved up to maintain the dam around the rising weld.
The process is rmding considerable use in heavy engineering. It produces a sound weld free of porosity, and the slow rate of heating and cooling reduces the risk of cracking. The main disadvantage of the process is the relatively coarse grain size developed in the weld metal.
ELECTRON BEAM WELDING If a stream of high speed electrons strikes a target, the kinetic energy of the electron is converted to heat. Since the electron beam can be focused to converge on a point, intense heat can be generated there.
Electron beam welding is used industrially for specialised welding of 'difficult' metals such as the more refractory metals (e.g. tungsten and molybdenum), and the more reactive metals (e.g. beryllium and zirconium).
It is also used for the specialised welding of very thin sheet metals including nickel and nickel alloys, stainless steels and titanium alloys. Electron beams can also quickly pierce a very small hole through relatively thick sections of metal.
The principal limitation to electron beam welding is the need to set up the electron gun and the workpiece in an evacuated chamber. If performed in air, the collisions between electrons and molecules of gas in the air would make the process ineffective.
The process must be shielded because X-rays are also produced by the impingement of electrons onto the metal target.
173 CHAPTER 14 JOINING OF MATERIALS
Filament supply 3Va.c.
Field electrode Accelerating potential 30 kVd.c.
Anode-ib...... +
Focus -----;\< coil J,L_-- Magnetic lens supply ---+I< (focus coil) 12Vd.c.
To vacuum pump
Fig 14.9 Electron Beam Welding
LASER WELDING The tenn 'laser' is an acronym for 'Light Amplification by Stimulated Emission of Radiation'. A laser generator is a generator of monochromatic, in-phase, pulses or beams of light.
Light is also a fonn of energy which generates heat when it strikes a barrier. Very intense" laser beams can generate sufficient heat to melt, even to vaporise, metal in their paths. An advantage of tltis 'coherent' beam of light is that it can travel great distances (laser beams have been reflected back to earth by reflectors set up on the moon) with negligible loss of energy.
In the materials field, lasers are being used for cutting, drilling and welding, particularly spot welding and micro welding. Because it can travel great distances, laser cutting and welding does not require physical contact with the workpiece, nor does it require a vacuum.
PRESSURE WELDING PROCESSES Possibly the earliest method of welding two pieces of metal together was that used by the blacksmith, who heated the metal in a coke (non-oxidising) fife and then hammered them together on an anvil. This process is an example of pressure welding.
Modern pressure processes use electrical energy to provide the heat, and mechanical force to provide the pressure. A heavy electric current passes between copper electrodes held on opposite sides of a joint. The electric resistance at the interface of the two metals to be joined causes sufficient heat to pennit welding when pressure is applied.
174 CHAPTER 14 JOINING OFMATERlALS
The main pressure welding processes are:
Spot Welding In this process, the parts to be joined are overlapped and gripped between copper electrodes, When the electric current is passed, local heating of the workpiece occurs between the electrodes, Since the heated area is under pressure, a weld is produced,
Fig 14,10 Spot Welding
Seam Welding Seam welding resembles spot welding but produces a continuous weld by using wheels as electrodes,
I work pieces
Fig 14.11 Seam Welding
Butt Welding Butt welding is used to join bars, tubes and sections, The end faces of the two pieces are pressed together and electric current passed across the joint. Again, heat is generated at the interface, and the applied pressure is sufficient to cause welding.
175 CHAPTER 14 JOINING OF MA1ERIALS
• fixed clamp moving clamp
Fig 14.12 Butt Welding
Other pressure welding processes include induction welding. in which the heat is supplied by an induction coil external to the parts to be joined, and friction welding in which two pieces are rotated relative to each other whilst being forced together - the friction generates heat, and at the proper instant rotation is stopped and the ends are joined under pressure.
WELDING OF PLASTICS Thennoplastic materials can be welded together by methods which are basically similar to those used for welding metals; they depend upon the application of heat, and sometimes pressure.
Hot Gas Welding Hot gas welding, in which a jet of hot air from a welding torch is used. A thin filler-rod is used, just as in oxy-acetylene welding. This process is widely used in the fabrication of rigid PVC.
Seam and Spot Welding These methods are similar to their met.1.l counterparts except that, since plastics are non-conductors, the 'electrodes' are heated by high-frequency coils.
Friction Welding Friction welding, or spin welding operates on the same principle as the process for joining metals. One part is rotated at high revolutions whilst pressed against the other part. Heat is generated by friction. The motion is then stopped and the parts held fIrmly together until the joint hardens.
ADHESIVES
Adhesive bonding with vegetable and animal by-products is probably the oldest method used for joining materials. Adhesive materials were made from the hides and hooves of animals, or from parts of fIsh. .
Vegetable materials such as tree gum, starch and casein were also used.
None of these products provides a very strong bond, however, and they are all easily loosened by the action of moisture.
Nowadays, with the advent of thermoplastic and thennosetting resins, adhesives fmd extensive use in engineering and structural applications, including aircraft, automobiles ~md cans.
Many of these 'synthetic' resins have specifIc uses - some adhere well to soft porous surfaces like wood, while others work best with metallic or ceramic materials.
Because of their flexibility, structures bonded with adhesives often resist damage from vibration better than riveted or bolted assemblies.
176 CHAPTER 14 JOINING OF MATERIALS
Furthermore, a bond over a whole surface can result in lower stresses in an assembly. Connections such as bolts and rivets can also be a source of corrosion damage to a structure. Because corrosion involves a flow of electric current, and adhesives normally have a high electrical resistance, they can help to reduce corrosion. Thus they can be used to assemble unlike metals or even unlike materials.
The main limitations to adhesive bonding is their relatively low strength when compared to welded or riveted joints, and the fact that adhesively bonded joints carmot be used on parts subjected to elevated temperatures.
There are several theories used to explain the adhesion of adhesive materials: o The chemical bond theory in which electrostatic forces of attraction act between the adhesive and the bonding surface (adherend) similar to the ionic attraction that exists in ionically bonded solids. o Mechanical locking theory based on the observation that rougher surfaces bond better than smooth surfaces. The theory says that the adhesive 'wets' the surface and becomes mechanically locked into the hills and valleys of the surface. However, the 'super adhesives' can strongly bond very smooth surfaces. These adhesives will not cure in the presence of air, and so must be cured in vacuum or by reducing the gap between contacting surfaces so that all air is excluded. This generally requires a bond to be less than 0.05 mm thick. • The surface energy theory in which surface tension of the adherend is greater than that of the adhesive, thus causing the adhesive to 'wet' the surface and create a bond. o The polar theory which states that 'polarity' in the surface atoms or molecules of the adherend acts to attract molecules of adhesive. This theory is used to explain why certain 'non-polar' plastics such as polyethylene and polypropylene carmot be successfully joined with adhesives.
None of these theories adequately explain all of the characteristics or problems encountered in adhesive bonding. Perhaps bonding is achieved by some complex combination of all of these theories.
A very broad range of bond strengths is possible with adhesives. However, the most important factor in the strength of an adhesive joint is the design of the joint itself.
Most adhesives develop their maximum load bearing capability in the shear mode of loading, and most are weakest in peel or tear modes of loading, because these concentrate the stress at a very local area whereas, in shear, stress is spread over the entire joint.
shear
tension •
peel
tear
\0 Fig 14.13 Different types of adheSive joint design
177 CHAPTER 14 JOINING OF MA1ERlALS
The other factors that are important in designing an adhesive joint are the envirorunent and the strength level required. Just as with any other aspect of engineering design, it is important to calculate the area of bond required using bond strength data for the system.
There are numerous commercially available adhesives. However, most can be grouped into the types listed below in table 14.3
In order to obtain maximum bond strength, parts to be joined with adhesives should be thoroughly cleaned by blasting, degreasing, acid pickling or anodising (for aluminium). Water is almost always a contaminant so that surfaces should be thoroughly dried.
178 CHAPTER 14 JOINING OF MATERIALS
Table 14.3 Adhesives
Name Advantages Disadvantages Uses Bond strength up to- Anaerobic High strength in a few High cost Production bonding 13.8 MPa l.yanoacrylates minutes Won't cure in air tight metal·to·metal joints such as screws and bushes Acrylic resins Clear high cost Bonding acrylics. 6.8MPa Good optical Sensitive to moisture styrene, polycarOOnale. oroperties ricidPVC Cellulose esters & Very soluble Moisture sensitive Bonding paper, wood, Very low ethers fabrics Epoxies Room temperature Cost. Some types have Structural, aircraft, 13.8 MJ>a cure. Long shelf life. low peel strength tools, abrasives, micro· Good optical electronic parts properties Phenolic Heat resistance Hard & brittle Abrasive wheels (Bakelites) Low cost electrical parts Polyimides Heat resistant to Cost Aircraft wheels, 315°(, High temperature cure electrical parts (370°C) Polyester Hardens without Cost Locking threaded presence of air limited adhesion ioints Plasticised vinyl Flexible. Low strength Vinyls O.6MPa cement Bonds to most materials Polyurethane Strong and flexible Moisture sensitive Flexible and rigid poor at high foams temperatures Butadiene High strength Low tensile strength Brake linings. Butyl neoprene nitrile Oil resistant Poor solvent resistance Structural metals. Pressure sensitive tapes Silicones Moisture resistant Low strength Contact cements 1.4 MPa Bonds variety of Vulcanised rubbers materials Urea formaldehyde Low cost Moisture sensitive. Wood glues for resins Room temperature plywood cure Vinyls High peel. Moisture sensitive Brake linings. 20MP. Polyvinyl butyroJ Vinyl plastisols can Household glue phenolic bond to oily surfaces Woodglue Vinyl plastisols PVC glue Polyvinyl acetate Polyvinyl chloride Solvent adhesives Special formulations 6.9MPa for bonding particular Methyl chloride Good for types of plastics polycarbonates 95% aqueous phenol Good for nylon
PVC dissolved in Rigid PVC & ABS tetrahydrofuran plastics
Styrene monomer Good for styrene (not foams) Acrylic esterin toluene Good fOfcellulosics & ri!ridPVC
17Q CHAPTER 14 JOINING OF MATERlALS
GLOSSARY Fastener Consumable Solder Submerged Arc Diffusion Plasma Brazing Thennit Flux Electroslag Weld Laser Fusion Adhesive
QUESTIONS
I. Name four properties of a soft solder.
2. Wbat type of soft solder is used for electrortic work? Wby?
3. Describe two different types of brazing alloy.
4. Describe two different arc welding processes, and name an application for each.
5. Describe the electroslag welding process.
6. Discuss the design of a joint for adhesive bonding.
180 CHAPTER 15 FINISHING AND COATING
The field of metal coating is a very broad one. Not many years ago, there were but two principal methods of coating metals - electroplating or painting. However, the technological developments in metal coating have resulted in a vast array of both coating types and methods of application. The main systems are summarised in Fig 15.1.
There are also many reasons for applying coatings. Some of the more important reasons are: • Aesthetic - to improve the appearance of the material • To improve corrosion resistance of a metal • To alter certain physical properties such as resistivity, arc resistance, reflectance etc • To alter the dimensions by building up a surface • To improve friction and wear characteristics.
Aesthetic and corrosion resist'll1ce improvements account for more than 75% of all met'll finishing work. However the ability to alter dimensions by selective metal buildup, and to reduce the friction between metal surfaces and so reduce wear are vitally important applications of metal coatings. Likewise in electrical and electronic applications metal coatings play an important role in controlling resistivities of metals and across the interfaces between two or more metals. The use of coatings for corrosion protection was discussed in Chapter 8.
Coatings I I I Metallic Nan-metallic I I I I Chemical Conversion Polymer Ceramic I I I I I I I Anodise Phosphate Chromate Spray Brush Extrude
Plating Hot Dip Spray Mechanical Vacuum Diffusion Plate Deposit I I I Electroplate Electroless Immersion Fig 15.1 Principal types of coating systems
Almost all types of coatings are used for aesthetic reasons, including electroplated metals, dyed anodising, grey phosphate coatings and paints and other polymers in a variety of colours and textures.
However, where corrosion is a factor, the limitations of the various systems must be considered. It is not good practice to use electroplated coatings of the more noble metals, or to use chemical conversion coatings such as anodise or phosphate treatments for immersion in water or chemicals. The presence of a pore or crack in these coatings would lead to rapid deterioration by galvanic attack. Galvanised coatings have excellent underwater resistance in cold water that is neither acid nor alkaline, so that galvanised plate is used extensively for construction of ship's hulls and underwater fittings.
Most water distribution authorities specify a bittuninous paint (not an enamel) for underwater protection on steel, and a galvanised coating for protection elsewhere. This is because large permanent steel strucrures are easier to fabricate from untreated steel - galvanised steel is difficult to weld - and are more easily protected and maintained by in-situ blasting and coating with a bituminous material.
Zinc and cadmium are sacrificial coatings when used on steel. Zinc is by far the most common form of protection. Cadmium has limited use because it can produce poisonous salts. Thus it is banned from use in the presence of food.
Chromium is the most familiar decorative protection because of its bright silvery shine. Most chromium plating is backed by copper and nickel to obtain adequate corrosion protection and
181 CHAPTER 15 FINISlllNG AND COATING adhesion. The clrromium itself is a very thin coating (about O.l3mm). This is because the chromium is very hard and difficult to polish. At higher thicknesses, it starts to go dull. For engineering applications such as a build-up on a bearing surface, thicknesses up to 0.5mm may be applied but its appearance is matt grey.
Anodising is undoubtedly the best coating for use on aluminium parts for resistance to atmospheric corrosion. However, in more corrosive environments, anodising is not recommended.
Phosphating is used as a preservation treatment on steels. However it will not stand up to handling or to even mild atmospheric corrosion, and so is never used alone. It should only be used in conjunction with a supplementary coaring such as oil, grease, wax, or paint.
If a coating is used to build up one or more dimensions of a part, the harder more durable coatings are generally used. These include hard rtickel and clrromium, in thicknesses up to 1.3mm. These built up, hard plated surfaces often require further firtishing by machining or grinding. Softer metals such liS tin or silver are often electroplated onto worn electric motor bearing housings to build up the hole so that a new bearing can be pressed in. These local plating jobs can often be done using portable equipment in which the plating electrolyte is contained in a special 'brush' and the metal is plated using a DC power supply.
One of the most important industrial applications for coatings is in the area of wear. Piston rings, cylinders, drive chains and cutting tools are just a few of the many industrial products that are coated for wear reduction. One of the interesting facts about wear is that it is not necessarily the hard coatings that provide best wear resistance. In tests of metal-to-metal wear, measured against hardened steel, silver plating produced both mirtimum coating wear and mirtimum wear in the steel block. The harder coatings caused severe wear on the mating steel, as did hard anodised aluminium. Electroplated tin, cadmium and rhodium did not cause wear to the steel, but did suffer extensive wear themselves.
ELECTROPLATING
In Chapter 8, it was shown that when a metal corrodes it goes into solution as metal ions, and furthermore that one of the causes of corrosion is the presence of stray electric currents. If these currents are harnessed, they can be used to cause both corrosion of an anode immersed in an electrolyte and reduction of those metal ions at a cathode immersed in the same electrolyte, i.e. electroplating.
182 CHAPTER 15 FINISHING AND COATING
Fig 15.2 A Basic Electroplating Cell
Referring to Fig 15.2, a basic electroplating cell contains two electrodes immersed in an electrolyte, and connected to a direct current power source. Provided that the combination of metal electrodes and electrolyte are favourable, the anode metal (A) will dissolve forming N> cations. These positively charged ions are atrracted to the negatively charged cathode (C) - the workpiece. At the cathode, the cations gain electrons and are discharged as metal on the surface of the cathode.
The laws governing electrode position were promulgated by Michael Faraday in the early nineteenth century. Faraday established that:
"The amount of metal deposited in an electrolytic reaction is a function of the amount of current flowing (amps) in the system, and the equivalent weight of the metal being deposited".
This is expressed mathematically as: M EW x Amps x Time(Seconds) 96500 where M = mass deposited in grams EW =Bquivalent weight
The equivalent weight of an element is equal to its atomic weight of that element divided by its valence in the electrolyte involved in the system. e.g. Copper in copper sulphate has an atomic weight of 63.6 and valence of 2. Thus its equivalent weight is 63.6+2 = 31.8
It takes 96,500 seconds (about 26.8 hours) to deposit one equivalent weight in grams of any metal using a current of one ampere. Asswning 100% efficiency, the mass of metal deposited in this time would be: Copper 31.8gm Silver 107.9 gm Nickel 29.3 gm Chromiwn (trivalent) 17.3 gm
lR1 CHAPTER 15 FINISHING AND COATING
The electrolytes used for electroplating are usually aqueous solutions of salts of the metal to be plated.
Typical electrolytes are: Copper - copper sulphate (CuSO.J Silver - Silver cyanide (AgCN) Nickel - Nickel sulphate (NiSO,) Chromium - Chromic acid (H,CrO,) Cadmium - Cadmium cyanide (Cd(CN),) Zinc - Zinc cyanide (Zn(CN),) Gold - Gold cyanide (AuCN)
The important requirements of a good plating electrolyte are: • The metal to be plated must dissolve in it • It must be a good conductor • It must be free of impurities that would contaminate the plating.
One of the most difficult problems in electroplating is obtaining a uniform current density over the surfaces to be plated so that a uniform coating thickness is obtained. Current density tends to be greater at corners and edges, and lower in holes and recesses. Thus plating tends to build up on corners and may not reach into holes. The actual thickness of this buildup is generally about twice the nominal plating thickness on the adjacent plain surface. If the nominal coating is thin, this build-up may not create any problems. However, for thicker coatings, it is often necessary to remove the buildup by grinding or machining. It can best be minimised by designing parts with radiused edges rather than sharp corners, or by preparing contoured anodes that will reduce current densities at the corners, and at the same time, will enable metal to plate in recesses.
normal anode - normal anode - contoured anode - uneven deposit improved contour of even deposit part even deposit Fig 15.3 Edge Build-up ill Electroplating
Most metals in the periodic table can be electroplated. However only about 30 of these can be plated from aqueous solutions and of these ten are important industrially - copper, nickel, chromium, silver, gold, tin, zinc, cadmium, rhodium and platinum. Some alloys of these metals can also be plated. However, to successfully electroplate an alloy, the equivalent weights of the components must be about the same. Thus we can electroplate brass (a copper/zinc alloy) and bronze (a copper/tin alloy), because the approximate equivalent weights are: Cu-64J2=32 Zn-6Yz =32.5 Sn-l1%=29.8 The quality of a plated coating is dependent upon the properties and cleanliness of the substrate - the metal under the plating, and on the conditions under which the electroplating is carried out. The adherence of an electroplated metal requires absolute cleanliness of the substrate, because the bonding between substrate and electroplate is atomic. The presence of even a thin oxide fibn can prevent such bonding. For this reason, metals that form inert corrosion resistant oxides such as til<'Ulium, Sl 184 CHAPTER 15 FIN1SHING AND COATING The electroplater can control, to some extent, the soundness and the hardness of an electroplated deposit. By plating chromium from a hot electrolyte, its hardness can be reduced. Additives (such as the anificial sweetener, saccharin) are made to nickel electrolytes to reduce the hardness of nickel plate. Generally the softer coatings contain less stress and so are more likely to be free of cracks and pores, and are less likely to produce distortion in plated pans. IMMERSION PLATING We noted in Chapter 8 that immersing a metal in an electrolyte containing ions of a metal lower in the electrochemical series will result in corrosion (oxidation) of the metal and plating out (reduction) of the ions. If we immerse steel in copper sulphate, steel will corrode and copper will plate onto its surface, without the aid of an external emf. Such deposits have poor adhesion and so have little industrial importance. However immersion plating without the aid of an external electric potential is sometimes used for temporary masking steel pans with copper, and also as a first step in preparing some of the hard-to-plate metals for electroplating. In this case, copper is deposited from a copper cyanide bath (not copper sulphate) because better adhesion can be achieved. Once a base coating is deposited, copper sulphate may be used to achieve a more rapid build-up of copper. ELECTRO LESS PLATING (AUTOCAT AL VTIC PLATING) This is also an immersion process. However the mechanism for plating of a rather hard, adherent coating is a chentically induced reduction of metal ions by a reducing agent dissolved in the electrolyte. The pan to be plated simply acts as a catalyst. The process is carried out at about 100°C. It continues to deposit nickel whilst ever the part is immersed. The deposit is not pure nickel, it is a nickel/phosphorous alloy of relatively high hardness. A panicular advantage of the process is that all wetted surfaces, including holes and recesses, are coated. VACUUM DEPOSITION In this process, the metal to be deposited and the part to be coated are contained within an evacuated chamber. The deposit metal is electrically heated till it vaporises - the vapour condenses on the cooler workpiece. Its main advantage is that it is a technique for depositing a metallic coating onto a non metallic substrate such as plastic and ceramic materials. OXIDE COATINGS Oxide coatings are produced by controlled corrosion or oxidation in either attnospheric conditions or by immersion in chemical baths. One or the oldest 'oxide' coatings is the 'blueing' produced on steel pans by heating them at around 370°C in an atmosphere of steam. A nticroscopically thin coating is formed which is porous and so absorbs oils to provide an improved resistance to rusting in normal attnospheric conditions. Black oxide coatings can be produced, generally for decorative purposes, on stainless steels, copper and steel by immersion into chemical baths, or by applying proprietary pastes to the surfaces requiring treattnent. 185 CHAPTER 15 FlmSHlNG AND COATING PHOSPHATE COATINGS Phosphate coatings are produced on steel surfaces by inunersing the part into a solution containing dilute phosphoric acid and additives. The metal reacts to dissolve some surface atoms and precipimtes an insoluble metal phosphate at the surface. Phosphate coatings are weak and porous, and can absorb oils or waxes to produce a degree of atmospheric corrosion resismnce, as well as some temporary wear iesismnce which is often useful on new machine parts to reduce wear during the breaking-in period. Phosphate coatings are often painted as they greatly enhance the protective effect of the paint. CHROMATE COATINGS Chromate coatings are used extensively on zinc and aluminium parts to prevent the fonnalion of powdery white oxide that usually fonns on these metals when exposed to mildly corrosive conditions. The yellow-green coating is produced by dipping the parts into an aqueous salt solution such as acidified pomssium dichromate. The solution atmcks the metal causing reduction of chromium ions to fonn a film on the metal surface. The treatment time is very short, usually only a few seconds, because the film will redissolve if the part is left inunersed in the bath, and the coating is quite fragile until it has thoroughly dried. As well as zinc and aluminium, magnesium and cadmium parts may also be treated by chromate coating. It provides excellent corrosion resismnce in situations where there is no wetting of the part. A typical application is carburettor bodies. ANODISED COATINGS Anodising, or anodic oxidation, is an electrolytic process which thickens the natttral oxide film on aluminium parts. Anodising can also be carried out on timnium, zirconium, and beryllium. Anodising is done in a sulphuric acid bath by making the metal part the anode in an electrolytic cell. A chemical conversion of the surface of the metal from metal to metal oxide occurs. Chromic acid or oxalic acid/sulphuric acid mixtttre may also be used as the electrolyte for anodising. However sulphuric acid is preferred because it produces a pearly white translucent coating that is pleasing in appearance and is easily dyed. The chromic acid electrolyte is used for anodising aircraft parts because any electrolyte that may become trapped in crevices will not redissolve the coating. The oxalic acid/sulphuric acid mixture is used at low temperatttres (O-4°C) to produce thick hard coatings on parts subject to wear such as gears. Aluminium oxide is hard and porous and must be sealed by inunersion in boiling water. Coloured anodised coatings are produced by dipping the part into a dye bath before sealing. Some dye processes ~ require a double dip into two baths to produce the desired colour. Not all aluminium alloys are suimble for anodising - those containing greater than about 3% copper or 5% silicon discolour and form a dark smut on the surface. However, there are some high silicon grades that can be anodised to a unifonn grey colour. NON-METALLIC COATINGS Many components and structttres are designed so that they incorporate the high strength of metal allied with the corrosion resismnce of non-metals by coating the metal with a suimble non-metallic material. 186 CHAPTER 15 FINISHING AND COATING PAINTS AND POLYMER COATINGS Painting is a general tenn for the application of a thin organic coating to the surface of a material for either decorative or protective purposes. Painting offers many advantages: • The material and labour costs are relatively low • Paint is available in a wide range of pigments and types • Paints have been developed that are capable of withstanding severe corrosive conditions • The insulating properties of paints inhibits the galvanic action between dissimilar metals • Paint can be applied by a range of different methods dipping, spraying, brushing, electrostatic etc. The different types of paint include: Lacquers These are basically polymers dissolved in a volatile solvent. When the solvent evaporates, the polymer forms the desired protective coating. They include rtitrocellulose, vinyls and acrylics. Nitrocellulose lacquers are used in automotive fIrtishes, the vinyl lacquers on chemical plant, and the acrylics for transparent fIlms over copper, brass, and aluminium to prevent tarnish. Alkyds These are liquid resins derived from drying oils which dry by polymerisation on contact with oxygen in the air. Varnishes Varrtishes are transparent coatings based essentially on drying oils, resins and solvents. Chlorinated Rubber Chlorinated rubbers are elaslOmeric resins dissolved in hydrocarbon solvents. They have good chemical resistance but are soft and are affected by sunlight. They are the most durable of external futishes and are frequently used as coatings on merchant ships, and as sealers to prevent water entering concrete and masonry materials. Oil Paints Oil paints are suspensions of pigment in linseed oil with thinners and dryers added. On application, the oil polymerises and forms the binder for the pigment and creates a protective fIlm. Oil paints are fairly permeable to moisture and generally require a priming coat to produce the required passivity when used on metals. Polyurethanes Polyurethanes are two-component mixtures that polymerise on mixing to form the desired coating. They are hard and impervious coatings with good corrosion resistance. Epoxies Epoxies are also two-component mixtures with the added benefIt of excellent adhesion to the metal. They form hard tough coatings that are resistant to most chemicals and are well suited to severe indusnial service. Fluorocarbons These are applied as dispersions of tetrafluorethylene in a water spray and are fused at about 370'C to form a TFE enamel with outstanding resistance to chemical attack. They are primarily used for their dry fIlm lubricating properties. The drying, which drives the water off in the early stages, can result in pinholes in the coating which limits the use of these coatings for corrosion applications. There are several other fluorocarbon systems that do not require the high temperature bake, although porosity can still be a problem. Vitreous Enamel These provide a coating which, provided it is free of cracks and porosity, is entirely impervious to the passage of electrolyte. There use is however lirrtited because of the low shock resistance of these coatings. IR7 CHAPTER 15 FINISHING AND COATING PAINT SYSTEMS A paint system is a multiple coat system in which each coat fulfIls a specific function. A basic paint system consists of tluee coats: 1) Primer. The main purpose of the primer is to ensure maximum adhesion to the surface of the job and, in the case of mel<'lls, it may also provide some protection'against corrosion. Typical primers include zinc rich paint and zinc chromate for metals, 'pink' pigmented primer for wood, and alkali resiSl<1Ilt primers for concrete and brickwork. 2) Undercoat is applied after priming to hide the background colours. They may be coloured to lead up to the colour of the finishing coat. They should be compatible with the primer and fInishing coats, and are generally low gloss and highly pigmented, usually with titanium dioxide (TiO,). 3) Finishing coat. This is used to provide colour, gloss or texture, as well as to provide some protection from weathering. In industrial situations, special formulations may be used to provide improved resiSl<1Ilce to weathering or corrosion using chemically inert pigments and resins. Special paints include: • lndustrial maintenance paints containing zinc, titanium dioxide or red oxide pigments. • Fungus resisting paints containing copper or cobalt compounds. • Anti-fouling paints, some of which contain toxic additives such as cupric oxide or complex tin compounds. • Heat resisting paints which resist discolouration and cracking up to about 600°C, and are based on silicone alkyd resins, or butyl titanate paints that are stable up to about 900°e. • Fire retardant paints used on timber and chipboard, are based on chlorinated rubber compounds or synthetic resin emulsions containing frreproofing ingredients. The most important step in the painting of a surface is surface preparation. Surfaces to be painted must be clean and free of grease, scale, loose particles and corrosion products. Also since paints do not 'fill' a surface, a smooth fmish requires a smooth base. General surface preparation procedures include: • Iron and Steel i) Solvent clean. ii) Shot blast or wire brush to remove scale or corrosion products. iii) Pretreatment with phosphate solution to provide a passive surface. • Aluminium i) Solvent clean. ii) Pretreat with a chromate/fluoride or chromate/fluoride/phosphate solution. • Zinc (and galvanised steel) i) Solvent clean. ii) Pretreat with acidifred zinc phosphate or acidifred dichromate solution to remove the zinc carbonate frlm. • Copper i) Abrade and solvent clean. ii) Pretreat with etch primer (based on alcoholic phosphoric acid and basic zinc chromate pigment). • Timber i) Rub smooth with sand paper and dust off. ii) Apply primer. • Plaster i) Allow plaster to dry, brush to remove loose particles, wash with water and again dry. ii) Apply alk.'lli-resistant primer or emulsion paint. Paint may be applied by dip, brush, roller, air-spray, airless spray (no air, pressure is applied directly to paint), electrosl<~tic spray (paint is atomised and electrostatically charged, which causes it to be attracted to the part which is earthed). 188 CHAPTER 15 FINISHING AND COATING COATING SELECTION Now that we have discussed the properties of various surface coatings, we must face the task of selecting an appropriate finish for a particular application, A good place to start is to detennine the environmental conditions - is the coating to be mainly decorative or must it provide a degree of protection to a surface? Is the coating to be exposed to sunlight and the weather? Will it be immersed in or subject to splashing from a liquid? What temperatures will the coating be required to withstand? It is difficult to generalise about the properties of specific coatings since formulations may vary between vendors, Once conditions are detennined, the best action is to seek data from manufacturers and suppliers on specific properties of their coating systems, 189 CHAPTER 15 FINlSmNG AND COATING GLOSSARY Electroplate Autocatalytic plating Anodise Vacuum deposition Phosphate Chromate. Equivalent weight Paint QUESTIONS 1. Sketch and describe a typical electroplating cell. 2. Why are phosphate coatings used on steels? 3. Name two metals that can be fInished by anodising. 4. Discuss the advantages and limitations of non-metallic coatings. 190 l CHAPTER 16 SPECIAL MANUFACTURING TECHNIQUES PRINTED CIRCUIT BOARDS Printed circuit boards consist of two basic materials, the base component being an insulator, and the circuit component being a conductor. INSULATING MATERIALS Insulating materials used in printed circuit board manufacture fall into two broad categories, orgmlic and inorgmlic. Each has a significantly different chemistry, mode of fabrication and properties. Within the orgmlic group there is also a wide range of analyses. They are often classified as Imninar or non-lmninar. Lmninar orgmlic base materials are produced from thermosetting plastics such as phenolics or epoxies in the form of resin impregnated paper reinforced sheets that are bonded together at pressures in the range 7000 to 14000 KPa and temperatures of around 150°C. The copper cladding is treated to produce a suitable surface for bonding and is adhesively bonded to the Imninate using a modified phenolic adhesive to provide compatible insulation and adequate mechmlical properties. Non-lmninar orgmlic base materials include a broad range of thermosetting and thermoplastic plastics and are produced by casting, moulding and extrusion processes. The main criteria for suitability as a base material, other than the dielectric requirement, is resistance to softening during soldering operations. Other properties that must be considered in selecting a non-lmninated plastic material include arc resistance, dimensional stability, and degradation. Some castable plastics, such as the phenolics, are high in carbon and tend to form carbonaceous deposits which act as a conducting path within the insulator. Dimensional stability relates mainly to long term shrinkage effects prevalent in borne plastics such as the antinoplasts, which results in deformation of the copper circuitry, and some types are susceptible to long term degradation in high humidity or high temperature environments. Perhaps one of the most outstanding non-lmninated moulding plastic is diallyl phthalate, or as it is better mown, DAB. These materials are often reinforced or filled with mineral glass, or synthetic fillers, and exhibit excellent resistance to arcing, shrinkage and degradation. Inorgmlic or ceramic materials form the other basic material used in the manufacture of the base component of printed circuit boards. Cermnics are solid materials composed mainly of inorgmlic non metallic materials in a polycrystalline state. The crystals are ground to a fme powder, mixed with a binder, moulded to the desired shape and sintered by heating to a very high temperature. Their major advantage over the orgmlic materials is the greatly superior dimensional stability, temperature resistance and resistance to chemical attack and degradation. They also exhibit excellent dielectric and mechmlical properties. Cermnics for electronic applications are generally divided into four classes: 1) Those with dielectric constants below 12, used as insulators. 2) Those with dielectric constants above 1£, used as capacitor dielectrics (e.g. some titanates). 3) Those exhibiting piezo-electric or ferro-electric properties, used for transducers and resonators (e.g. barium titanate). 4) Those exhibiting ferromagnetic properties, used in switching applications (e.g. ferrites). 191 CHAPTER 16 SPECIAL MANUFACTURING TECHNIQUES It is the type 1 cermnics that are used as P .C.B. base materials. The oldest such material is porcelain. However this has been superseded by cermnics such as alumina, zircon and beryllia. These materials are fmding increasing use in P.C.B. manufacture, particularly in areas of microminiature circuitry. CONDUCTING MATERIALS The material used as the conductor in almost all P.C.B.'s is high purity copper. There are two methods by which the foil is manufactured - electrolytic and cold rolling. Electrodeposited foil is made by electrolytically depositing copper from either copper cyanide or copper sulphate onto a large rotating lead or stainless steel drum, from which it is continuously stripped. Electrodeposited foil is very smooth on the drum side, although great care is required to avoid drawing inclusions from the drum, particularly if a lead drum is used. The outer surface exhibits a more matte fInish. The smooth side is ideal for resist printing and as a base for electrical contacts. The matte surface provides a good surface for bonding to the base material. Rolled foil is manufactured by conventional rolling techniques. However there is great diffIculty in achieving the necessary flatness and uniformity of thiclmess by this method because of the foil itself is only about 0.02 - O.OSmm thick. Rolled foil has a shiny surface on both sides and is generally hllfder than the electrodeposited foil. However it is also freer of inclusions and pin-hole defects than the electrodeposited material. Most P.C.B.'s employing copper foil are manufactured by an etching process, in which the conductive circuits are produced by chemically etching away all unwanted foil. An alternative method of manufacture involves plating the conducting circuitry directly onto a lmninate. A microscopically thin silver deposit is sprayed onto the lmnirtate, the non-conducting areas are then coated with a photo-resist ink, and the board placed into a copper cyanide or copper sulphate electroplating bath. ' The ink and silver film are then removed chemically from the non conducting areas of the board. A recent improvement in this technique involves electro less deposition of copper on the base. This method provides a coating that extends into drilled holes and so has bycome a popular method for P.C.B. work. It depends upon an inert catalyst included within the resin of the plastic which promotes the deposition of copper, particularly along the internal edges of a drilled hole. A thicker deposit can be achieved by electroplating over the electroless deposit. ENCAPSULATION MATERIALS The manufacture of electrical and electronic components involves the use of a wide range of metallic, metalloid, and non metallic materials. The successful operation of these components requires a predictable and reliable performance over a range of operating conditions such as temperature and humidity. Metallic materials, whilst normally quite stable, are severely affected by hostile environments. Likewise the performance of dielectric (non-metallic) materials can be affected by the environment. Hence many electronic components must be suitably sealed, or encapsulated, to maintain their reliability. The ultimate form of encapsulation is hermetic sealing; the exclusion of all air and moisture by enclosing the components in glass which is fused to seal around the various leads or connections. 192 CHAPTER 16 SPECIAL MANUFACTURING TECHNIQUES However tltis type of encapsulation is impractical for a number of electronic components because hermetic sealing involves heating the glass to around 400°C at which temperature the components themselves may be permanently damaged. Alternative materials that have been in use for many years include oils, waxes and bitumen. However these are often messy and tend to fail due to softening and flow at moderate temperatures. The materials mostly favoured for encapsulation today are the thermosetting and thermoplastic plastic materials. Thermosetting mixtures can be used cold in the liquid form and hardened by a very moderate heating. Some thermosetting mixtures such as epoxies harden without the aid of heat. Thermoplastic materials are generally used in a molten state at a relatively low temperature of around IOO-200°C. In tltis condition, they may be moulded around the part using sufficient pressure to ensure a homogeneous encapsulant. Unfortunately most plastic materials are not completely impervious to moisture. Hence in an electronic component, it is important that the plastic is fully bonded to the component, and that there are no voids or gaps between the plastic and the component. The presence of voids allows an ingress of moisture which can build up in the void and cause breakdown of the component. The effect is often made worse because the moisture permeating through the plastic can become 'polluted' by the chemicals in the plastic, with the result that ionic materials which act as excellent conductors, and are often corrosive, are carried through to the component. One area where moisture may easily gain access to a component is along the leads and terminals that connect to the component. As a result a highly efficient bond between the encapsulant and the metal is most important in these areas. Glass can form a chemical bond with the metals, such as tinned copper, that are used as leads in many electronic components. However the plastics do not form such a strong bond, and it is often necessary to apply some type of primer to the metal to ensure a satisfactory bond to the encapsulant when using plastic encapsulating materials. The materials used to encapsulate modern resistors, capacitors and other components are mostly epoxy resin types of thermosetting plastics. These materials may be drawn into a simple mould under vacuum to maximise bonding and to minimise the formation of voids. However because continued exposure to these materials can be hazardous to health, precautions are necessary to prevent such exposure. Alternatively they may be cast, dipped or sprayed. The advantage of the epoxy systems is the wide variety of formulations available that can be adapted to a variety of methods of application. Another material used as an encapsulating material is rubber, due mainly to its extreme deformability, and its ability to quickly recover its original shape. This makes rubber particularly useful as a seal material because it can be deformed during fitting or assembly, and will recover and perform its duty as a seal to prevent the ingress of contaminating materials. There are a range of different types of rubber, including: • Butyl rubber - which is used because of its good resistance to permeation of gases. • Natural rubber - has a relatively high strength and wear resistlUlce. • Neoprene - used mainly because of its good weather resistance. • Nitrile rubber - which have outstanding resistance to degradation by oils and hence comprise the majority of all seal materials. • Acrylic rubbers - have a superior heat resistance, but are of lower strength when compared with the nitrile rubbers. • Polyurethane rubbers - are the highest strength rubbers available, and show excellent resistance to tearing. • Silicone rubbers - have good thermal resistance. • Fluorinated rubbers - which have excellent resistance to attack from a wide variety of chemicals. 193 CHAPTER 16 SPECIAL MANUFACTURING TECHNIQUES SEMICONDUCTOR MATERIALS In Chapters 1 and 2 we learnt that certain of the 105 elements in the periodic table exhibited dual characteristics - they could behave as metals and conduct electric current, or they could behave as non-metals and so act as non-conductors. These elements form the backbone of the semiconductor industry. . The elements concerned are the metalloids silicon (Si), germanium (Ge), arsenic (As), Selenium (Se), antimony (Sb), gallium (Ga) and carbon (C). By far the most common elements used in semi conductor manufacture is silicon and, to a lesser extent, germanium, both of which are characterised by the diamond crystal structure, a tetrahedral configuration of four covalent bonds. Silicon is the second most abundant element in the earth's crust. In the ultra pure state, it is a very poor electronic conductor. However the addition of just a trace of impurity can Significantly change the conductivity of silicon. The addition of trace elements such as arsenic, antimony and phosphorous, each of which has five electrons in its outer shell, causes the atoms of these elements to 'substitute for', or take the place of some of the silicon atoms in the tetrahedral structure. Because this structure is covalently bonded, and at each atom site, four covalent bonds are available which rigidly tie up four of the five free electrons in the impurity atom, there is one 'spare' electron. This electron is free to move through the silicon crystal under the influence of an applied electric charge. Thus the impure silicon can act as a conductor in which the carrier of the electric charge is a free negatively charged electron. This type of conductivity in a semiconductor is called negative or n-type. The addition of trace elements such as aluminium and boron which have only three outer electrons leaves a vacant hole in the structure because no electron is available to engage the fourth covalent bond. By means of an internal exchange of electrons, this "hole" can move through the lattice under the influence of an electric charge, again resulting in conduction in which the hole becomes the free carrier of the electric charge. This type of conductivity is referred to as positive or p-type. ~ By bonding n-type and p-type silicon crystals, a p-n junction is formed at the interface. It is these junctions that are responsible for the unique properties of most semiconducting devices. MANUFACTURE OF SILICON CHIPS The manufacture of silicon semiconductors begins with a molten bath of high purity silicon. The high purity is achieved by a process known as 'zone refining' in which the silicon, contained within a vertical quartz tube, is heated to fusion temperature as it passes upward through several heating coils. Impurities will normally tend to congregate in the molten silicon. Thus by progressively melting a zone of a silicon "bar", and moving that zone down along the bar, impurities are extracted from the metalloid. The silicon is then melted in a crucible, and a 'seed' is introduced at the surface of the melt Pure silicon begins to crystallise on to the seed, a large single crystal of silicon is produced. In some processes, the required impurity atoms are introduced during this stage of growing of the silicon crystals which are typically 75-100mm diameter. 1 QLl CHAPTER 16 SPECIAL MA>'1UFACTIJRING TECHNIQUES A slice is then cut from the crystal into which the complex three-dimensional distribution of p-type and n-type impurity atoms are introduced. The quantity and distribution of these atoms is critical to the success or otherwise of the manufacturing process, and there are a number of processes used for this purpose. However the most common processes are: • Diffusion process, in which a gaseous impurity compound is passed over a specially prepared surface of the cut crystal for a carefully controlled time. The surface is masked by a photographic process that causes the fonnation of silicon dioxide on the surface of the crystal. • Epitaxial growth in which a mixture of hydrogen and a suitable silicon compound are passed over clean silicon slices at temperatures of around 10500C to 1200oC. Chemical reactions produce silicon which is deposited onto the surface of the original silicon. At the same time suitable impurity compounds are introduced in controlled quantities which lead to the controlled p- or n type doping of the growing epitaxial layers. These processes produce, within the silicon slice, an array of components which must be interconnected to fonn a circuit. This is achieved by a photolithographic process in which holes are etched into the silicon dioxide at places where the electrical connections are required. The whole surface can then be coated with a film of metal, generally aluminium after which the unwanted metal is removed by a further photOlithographic process. Up to this stage, a number of silicon chips are produced within a single silicon slice. Each 'chip' is then tested, and the slice cut up. Those chips that pass the electronic and visual inspections have wires soldered on, they are sealed, further tested and built into electronic components. The processes used in the manufacture of gennanium devices are basically similar to those used for silicon - zone refining, crystal growth, junction fonnation, and testing. 195 CHAPTER 16 SPECIAL MANUFACTURING TECHNIQUES GLOSSARY Insulator Encapsulation Conductor Semiconductor Lmninar organic base Zone refIning No~-laminar organic base Diffusion Dielectric Exptaxial growth Electrodeposited QUESTIONS 1. Discuss the use of laminar and non-laminar organic base materials in printed circuit boards. 2. Describe the methods used to for printed circuit boards. 3. Compare the use of oils and waxes with polymer materials for encapsulation. 4. Describe the manufacture of silicon transistors. 196 l CHAPTER 17 THE MATERIAL SELECTION PROCESS Earlier in this book, we referred to the fact that the best material for a particular application is the one that gives the optimum combination of desired properties at the minimum cost. All materials exhibit individual physical, mechanical and chemical properties. Unfortunately we carmot isolate anyone property, without giving consideration to all of the other properties. Further no single book can be considered a reference manual to the infInite number of materials available today to the designer. Hence the selection process is often a very diffIcult one. A general procedure for the selection of a material can be summarised in the following steps: ANALYSIS A careful analysis of the requirements of the material. TIris includes: • Functional requirements - what you require from the material e.g. strength, toughness, lightness, corrosion resistance, heat resistance, light transmission etc. • Processability requirements - the material must be able to be shaped into the desired shape, fabricated into the desired assembly, machined etc. • Availability - the material must be available when it is required, and in the required quantities. If the project is a continuing one, then the continued availability in uniform qualities is important. • Cost is usually the controlling factor in evaluating materials because in the majority of applications, the ultimate cost of the project signifIcantly affects its viability. However material cost carmot be divorced from other costs associated with a project such as processing costs and replacement and warranty costs. The cheapest material does not necessarily produce the cheapest product. • Reliability of a material can sometimes be estimated from previous history with a material. However subtle changes in service conditions can SignifIcantly affect reliability, so that service testing may be necessary. TIris requires a detailed study of the performance of the material under actual or simulated service conditions. ALTERNATIVE SOLUTIONS Once the functional requirements have been determined, a list of suitable materials can be prepared. TIris list will include a number of a1temate materials. It is important that considerable creative thought be given at this stage to seek out possible new materials, and not be restricted to the more traditional materials. This phase creates alternatives, often without complete regard to their ultimate feasibility. Once the list is complete, those materials that are obviously unsuited to the application can be deleted to leave a short list of possible alternate materials for a more detailed analysis and assessment. EVALUATION Having reduced the list of possible materials to a more manageable number which includes those materials that do not violate any of the rigid material requirements the detailed valuation to fmd the best material begins. Unfortunately there is no single formal procedure for this stage, nor is it likely that one single correct answer will emerge from this evaluation. Much depends upon the nature of the materials problem, the manufacturing facilities of the company and even the preferences of the company's technical staff. 197 CHAPTER 17 THE MATERIAL SELECTION PROCESS However it is important that the evaluation phase begin with the most critical property required from the material, and then proceed to assess the lesser important properties. It is useful to incorporate some type of rating system in this assessment so that the best material may show up as the material with the highest score. More complex ratings involve determination of such factors as cost per unit strength, cost per unit mass, and strength to weight ratios. One factor that must be considered here is the cost, not only of the material and its manufacture, but cost of maintenance, downtime, lost production, repair and warranty. Thus it is necessary to predict the life expectHncy of the product when manufactured from each of the alternative materials on the short list. THE FINAL SELECTION The second last step in the process involves making a decision on the optimum material. Sometimes the outcome of the evaluation process may result in a reasonably clear cut answer. However, in other cases, a decision will be based upon knowledge and experience. If none of the materials meet the design parameters, consideration must be given to either a relaxation of the specification or a change in the design. THE LAST STEP Yes, there is one final phase in the selection process follow up the manufacturing and service history of The part. The information gained can be extremely valuable next time you are required to select material. 198 l brazing, 168 brightray series, 142 INDEX brinell hardness, 56 brinell test, 57 brittle coatings, 79 A busbars, 137 ABS plastics, 107 butt welding, 175 absorptance, 148 acrylics, 107 c acrylonitrile-butadiene-styrene, 107 calendering, 164 addition polymerisation, 144 calorimeter, 37 adhesives, 176 carbon steels, 129 aeration, 97 carburizing, 134 aliphatic compounds, 120 carryover, 117 alkalis, 97 cast iron, 126 alkyds, 187 casting, ISO, 164 alloy steels, 129 cathode, 84, 95 alpha brass, 138 cathode ray oscilloscope, 73 aluminium brazing alloy, 168 cathode reaction, 92 aluminosilicate glass, 147 cathodic inhibitors, 102, 118 amines,108 cathodic protection, 105 anion, 17 cation, 17 annealed, 140 caustic embrittlement, 117 annealing, 130 cavitation corrosion, 101 anode, 84, 85, 95 cellulosics, 107 anode reaction, 92 cementite, 123, 125 anodic inhibitors, 102, 118 ceramic, 7, 145, 191 anodising, 182, 186 ceramics, 83 apparent viscosity, 121 charpy test, 44, 62, 63 aqueous corrosion, 102 chemical change, 4 arc resistance, 29 chemical conversion coatings. 104 arc welding, 169 chlorinated rubber, 187 artesian wells, 114 chlorination, 116 AS2008,121 circular magnetisation, 71, 72 AS2341,121 cladding, 104 asphalt, 120 clark's process, 115 atom, 13 composite, 9 atomic number, 14 compound, 3 atomic weight, 15 compression moulding, 164 austempering, 132 concentration cell, 90 austenite, 124, 125 concrete, 9 austenitic stainless steels, 96 condensation polymerisation, 144 autocatalytic plating, 185 conductivity, 28 copper cladding, 191 B copper phosphorus brazing alloy, 168 bar magnet, 71 cor-ten, 103 bar product, 154 corrosion, 83, 95, 97, 117 bases, 97 corrosion cell, 92 beta brass, 138 corrosion fatigue, 99 beta-ray, 80 corrosion prevention, 117 billets, 137, 154 covalent, 18,21 birefringent, 79 creep, 46 bitumen, 120 crevice corrosion, 91, 98 blackheart, 127 crystal probe, 77 blanking, 159 cupolas, 123 blooms, 154 cupronickel, 138 blow moulding, 164 curie, 32, 33, 40, 70 borosilicate glass, 147 199 INDEX o F de-alloying, 100 faraday, 95, 183 deaemtion, 118 fasteners, 166 dealuminification, 100 fatigue, 46 degradation of materials, 83 ferrite, 125, 126 demagnetisation, 73 ferromagnetism, 29, 70 density, 33, 40 ferrous metals, 123 depolarising agents, 102 flashpoint, 121 dezincification, 100 fluorocarbons, 107, 187 diamagnetic materials, 30 foaming, 117 die casting, 153 force, 42, 51 dielectric, 146 fretting corrosion, 10 1 dielectric materials, 39 friction welding, 176 dielectric strength, 29 furane sands, 151 differential absorption, 74 furon, 151 differential aeration, 90, 91 fused silica, 147 dilatometer, 37 fusion welding, 168 dipping, 104 direct extrusion, 156 G dissociation of water, 93 galvanic cells, III distillation, 116 galvanic corrosion, 86, 88, 98, 102 doubly refractive, 79 galvanic couples, 96 ductility, 45, 121 galvanic series, 87, 88 durability, 121 galvanising, 105 dynamic viscosity, 121 gamma phase, 124 gamma rays, 75,76 E gas carburizing, 134 e.m.f.• 85 gas shielded arc welding, 170 eddy current, 38, 73, 80 gaseous, 2 elasticity, 44 gauge length, 49, 51 electric arc furnaces, 123 glass, 145 electrical conductivity, 38 grainflow, 156 electrical resistivity, 38 graphite, 126 electrochemical corrosion, 84 grey cast iron, 126 electrochemical series, 87 gunmetal, 138 electrodeposited foil, 192 e1ectrodeposition, 104 H electroless plating, 185 half life, 75 electrolyte, 84, 184 hard water, 116 electrolytic tough pitch, 137 hardness, 43 electromagnet, 72 hardness of water, 114 electromagnetic radiation, 74 hardness test, 55 electromotive force, 85 heat capacity, 26, 37 electron, 13 heat resistance, 27, 38 electron beam welding, 173 hermetic sealing, 192 electroplate, 182, 185 high rate forming, 161 electroslag welding, 172 homogeneous encapsulant, 193 element, 2 hot gas welding, 176 elongation, 50 hydrofluoric acid, 83 emulsion, 76 hydroforming, 161 encapsulation, 192 hypoeutectoid steel, 129 epitaxial growth, 195 hysteresis, 31 epoxy, 108, 187 etp, 137 I explosive forming, 161 immersion plating, 185 extrusion, 156, 164 impact extrusion, 157 impingement corrosion, 101 200 INDEX l indirect extrusion, 156 mixture, 3 ingots, 150 molecular bond, 20 inhibitors, 102 molecule, 17 injection moulding, 164 mould, 150 insoluble hydroxides, 118 mnntz metal, 138 insulating materials, 191 intergranular attack, 100 N intergranular corrosion, 99, 100 neutron, 13 ion-exchange, 116 nitriding, 135 ionic bond, 16 nodular iron, 128 ionising electromagnetic radiation, 74 non-destructive testing, 69, 74, 78 ions, 17 non-ferrous metals, 137 iron oxide, 123 non-laminar organic base materials, 191 isothermal treatments, 132 non-metals, 7,16 isotope, 15 normalising, 132 izod, 44, 62 nylon, 107 K o kinematic viscosity, 121 oil paints, 187 open hearth furnaces, 123 L oscilloscope, 73 lacquers, 187 oxidation, 83, 92, 106, 185 lamellae, 125 oxygen blown furnaces, 123 laser, 174 lead glass, 147 p light amplification by stimulated emission of pack carburizing, 134 radiation, 174 paper radiographs, 76 liquid, 2 paramagnetic materials, 30 liquid carburizing, 134 pascal, 42 liquid penetrant inspection, 69 pearlite, 125, 126 longitudinal magnetisation, 71 penetrant, 70 penetration, 121 M percentage elongation, 52 magnetic field, 30, 71 permanent hardness, 115 magnetic flux density, 30 phenolics, 108 magnetic induction, 30 phosphate glasses, 147 magnetic particle, 70 phosphating, 182 magnetic yoke, 72 photo-elastic strain gauging, 79 malleability, 45 photographic fihn, 76 malleable iron, 127 physical change, 4 marforming, 160 piezo electric, 77 martempering, 132 pig iron, 123 martensite, 132 pitch, 120 mass number, 15 pitting corrosion, 98 mastic, 120 plasma, 171 maximum use temperature, 26, 38 plasticity, 45 mean free path, 28 plastics, 7,144 melting temperature, 26, 38 plate, 154 metal, 5 plating electrolyte, 184 metal spraying, 104 polar theory, 177 metallic arc welding, 169 polarising agents, 102 metallic bond, 20, 21 polycarbonates, 107 metallic corrosion, 83 polyesters, 108 metalloids, II, 16 polymer, 7, 19,83 metals, 16 polymerisation, 7,19,144 Michael Faraday, 183 pol yolefmes, 107 micro hardness tester, 61 pol ystyrene, 107 201 INDEX polyurethane, 108, 187 spot welding, 175 powder metallurgy, 162 stimulators, 102 pressure welding, 168, 174 strain, 42, 50,78 primary coil, 73 strain hardened, 140 primer, 188 strength, 42 priming, 117 stress, 42 proof stress, 52 stress corrosion, 99 proton, 13 stress relief, 132 pyrolysis, 106 stress-rupture, 46 submerged arc welding, 169 R super-cooled liquids, 147 radiograph, 74, 76 superficial rockwell test, 60 radiography, 74, 76 radioisotopes, 75 T redox potentials, 87 tar, 120 reduction, 84, 92 temper brittleness, 65 reduction of area, 51 temperature, 36 refractive index, 33,34,40 tensile strength, 52 refractory metals, 143 tensile test, 48 resistance strain gauges, 79 thermal conductance, 24, 25,36 resistivity, 27 thermal expansion, 25, 37 rockwell, 59 thermit welding, 171 rolled foil, 192 thermocouple, 36 rowland ring, 39 thermometer, 36 rubber, 8, 83 thermoplastic, 8, 193 rust, 84 thermosetting, 8, 193 thermosoftening, 8 S toughness, 44 sand, 150 transfer moulding, 164 scale deposits, 117 transition temperature, 44, 65 scale prevention, 117 trisodium phosphate, 116 scission, 106 seam welding, 175 U secondary sensing coil, 73 ultrasonic, 77, 78 segregation, 96 , undercoat, 188 semiconductor, 194 uniform corrosion, 97 semiconductors, 147, 194 shadow graph, 76 v shearing, 159 vacuum deposition, 185 shore durometer, 61 vacuum forming, 164 shore sclerescope, 61 valence, 14, 15, 16 silica, 147, 150 van-de-waals bOnds, 106 silicon chips, 194 varnishes, 187 silver halide emulsion, 76 vibrations, 77 silver solder, 168 vickers test, 58 single phase alloys, 96 vinyls, 108 slabs, 154 vitreous enamel, 187 slaked lime, 116 soda lime silica glass, 147 w sodium hexameta-phosphate, 116 water, 114 sodium tripolyphosphate, 116 water absorption, 29 soft solder, 166 waves, 77 soft water, 114 welding, 168 softening point, 121 whisker, 10 solid, 2 white cast iron, 127 special sections, 154 whiteheart, 127 spheroidal graphite, 128 wood, 9,145 spheroidise, 131 INDEX x x-ray tube, 74 x-rays, 76 y yield stress 52 YOUflC1'~ s M'-odulus, 52 Z zeolite, 116