Chapter 13 Processing of Materials
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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.