THE moulded plastics industry has in the last few years, both in engineering and domestic directions, encroached upon the traditional fields of the metallurgist and it is now very noticeable even to the layman the extent to which all sorts of articles are made from plastics which were previously fashioned from metal castings or pressings. The metal‐lurgist is now, however, providing his reply by stealing the very essentials of the plastics technique and applying it to the moulding of metals. This technique of Powder Metallurgy broadly consists in taking a metal or alloy, or mixtures of metals or alloys, powdered to something similar to the consistency of flour and pressing it in a die under pressures varying from 5 to 50 tons sq. in. From the press is ejected a green compact which is then heat treated in a protective atmosphere at a temperature of approximately two‐thirds of the melting point of the alloy concerned. The product may be broadly classified into three types according to its qualities: (a) It may closely resemble normal cast or forged metals although generally possessing inferior, but not substantially inferior, physical qualities. In this field powder metallurgy becomes no more than another method of manufacturing metallic articles and finds its chief application where machining costs can be avoided. For example a start has been made in this country in the manufacture of small steel gears, almost equal in quality but appreciably cheaper than a machine‐cut gear. The powder metallurgy technique is able to handle a much wider range of alloys' than are customary in die casting (steels, heat‐resisting alloys, nickel, tungsten for example) but is much more limited in shape design. Broadly only those articles can be manufactured which are of a simple directly pressable shape without undercut surfaces. (b) It may be porous. The porosity can generally vary from 5 to 60 per cent by volume and the size of the pores from one millimetre in diameter to a porosity scarcely visible under the microscope. Porous bronzes saturated with oil have been in use for a number of years, particularly in circumstances where normal bearing lubrication is difficult, such as in refrigerators, vacuum cleaners, sewing machines and clocks, but the war has seen an enormous application of them in aircraft and tanks. Newer uses for porous metals are frequently being discovered and they have already extensive applications as oil, chemical and gas filters. (c) It may have qualities which are unobtainable in any other manner. Forged or cast metals are after all physically constructed with only those constituents which can be crystallized from a molten metal and this is a very severe limitation. By powder metallurgy this phase from this alloy and that from that alloy can be agglomerated into a mass having new physical properties; together, if desirable, with suitable non‐metallic materials. Powder metallurgy as a technique therefore has exten sive possibilities in providing new synthetic metallic materials. At the present time cemented carbide cutting tools, electrical contacts, impregnated diamond tools and friction surfaces are typical examples of this class and indicate something of what the future offers.
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