POWDER METALLURGY

fabrication of metal objects from a powder rather than casting from molten metal or forging at softening temperatures. In some cases the powder method is more economical, as in fashioning small metal parts such as gears for small machines, in which casting would involve considerable machining and scrap loss. In other cases melting is impractical because of the very high melting point of the metal-e.g., tungsten-or because an alloy is desired of mutually insoluble materials such as copper and graphite. Finally, powder metallurgy is used to produce a porous product that will allow a liquid or gas to permeate it. In the bonding process, powder particles are first compressed to the desired shape, then heated (sintered) at a temperature below the melting point of the metal or, in the case of an alloy, of the metal with the highest melting point. Metal powders are produced by either chemical or mechanical means. In chemical powdering, either a compound of the metal is reduced by a chemical agent or a liquid solution containing the metal is electrolyzed. In mechanical powdering, the metal is usually milled by power hammers or by balls in a rotating container. Ductile metals are usually combined in an alloy of two or more metals with a lubricant and then pressed or briquetted by a hard steel die. Refractory metals, those with high melting points, are compacted with an added binder, such as paraffin wax. Cemented carbides are formed by bonding the hard, heat-resistant particles together with a metal, usually cobalt. See also metallurgy.