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Powder Metallurgy

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Details

Powder metallurgy, abbreviated by PM, is a metal processing technology in which parts are produced from metallic powders. In the usual powder metallurgy production sequence, the powders are compressed into the desired shape and then heated to cause bonding of the particles into a hard, rigid mass. Compression, called pressing, is accomplished in a press-type machine using tools designed specifically for the part to be manufactured. The tooling, which typically consists of a die and one or more punches, can be expensive, and powder metallurgy is therefore most appropriate for medium and high production. The heating treatment, called sintering, is performed at a temperature below the melting point of the metal.

Although parts as large as 22 kg (50 lb) can be produced, most powder metallurgy components are less than 2.2 kg (5 lb). The largest tonnage of metals for powder metallurgy are alloys of iron, steel, and aluminum. Other metals include copper, nickel, and refractory metals such as molybdenum and tungsten. Metallic carbides such as tungsten carbide are often included within the scope of powder metallurgy.

Pros

Cons

Parts can be mass produced to net shape or near net shape, eliminating or reducing the need for subsequent processing. The process itself involves very little waste of material. About 97% of the starting powders are converted to product. This compares favorable to casting processes in which sprues, runners, and risers are wasted material in the production cycle. Owing to the nature of the starting material, parts having a specified level of porosity can be made. This feature lends itself to the production of porous metal parts, such as filters, and oil-impregnated bearings and gears. Certain metals that are difficult to fabricate by other methods can be shaped by powder metallurgy. Tungsten is an example; tungsten filaments used in incandescent lamp bulbs are made using powder metallurgy technology. Certain metal alloy combinations and cermets can be formed by powder metallurgy that cannot be produced by other methods. Powder metallurgy compares favorably to most casting processes in terms of dimensional control of the product. Tolerances of ±0.13 mm (±0.005 in) are held routinely. Powder metallurgy production methods can be automated for economical production.

Tooling and equipment costs are high. Metallic powders are expensive. There are difficulties with storing and handling metal powders (such as degradation of the metal over time, and fire hazards with particular metals). There are limitations on part geometry because metal powders do not readily flow laterally in the die during pressing, and allowances must be provided for ejection of the part from the die after pressing. Variations in material density throughout the part may be a problem, especially for complex part geometries.