Background

There are a number of reasons for making engineering components by powder metallurgy and these lead to the groupings below:

· Refractory Metals

· Composite Materials

· Porous Materials

· Structural (or Mechanical) Parts

· Special High-Duty Alloys
Refractory Metals

Certain metals, particularly those with very high melting points, i.e. the refractory metals, are very difficult to produce by melting and casting, and also are frequently very brittle in the cast state. Tungsten, molybdenum, tantalum and related metals come into this category. A sintered powder compact having a relative density of less than 90% can be mechanically deformed at a suitably elevated temperature, and gradually develops a microstructure with preferred orientation that gives the now dense material useful ductility even at ambient temperatures.
Composite Materials

These consist of two or more metals which are insoluble even in the liquid state, or of mixtures of metals with non-metallic substances such as oxides and other refractory materials. In this class appear:

· Electrical contact materials such as copper/tungsten, silver/cadmium oxide.

· Hardmetals, i.e. cemented carbides, used for cutting tools, wear parts such as, for example, wire-drawing dies, and tools for the hot forging of metals. Tungsten carbide bonded with cobalt was the first of this class of material and still has the lion’s share of the market, but other carbides and, more recently, nitrides, carbo-nitrides and borides are being used in increasing quantities, and substitutes for the relatively scarce and expensive cobalt are being tried. These include: Ni, Ni-Co, Ni-Cr, nickel-based superalloys, and complex steels.

· Friction materials for brake linings and clutch facings in which abrasive and other non-metallic materials are embedded in a copper or other metallic matrix.

· Diamond cutting tools especially grinding wheels in which small diamonds are uniformly dispersed in a metal matrix.

· In recent years several wrought products containing finely dispersed non-metallic phases have been developed and put into service. These dispersion-strengthened materials, referred to as ODS materials if the strengthening particles are oxide, have strength especially at elevated temperatures superior to that of case and wrought metal of similar basic composition. As in the case of refractory metals it is difficult if not impossible to make these composite products except by powder metallurgical processes.
Porous Materials

Most forms of metal are porous to some extent, sintered metals more so than most, but here we are concerned with the production of parts having a significant carefully controlled porosity designed to serve a useful purpose. The chief products in the group are filters and oil-retaining bearings often referred to as self-lubricating bearings. The latter is one of the major powder metallurgical products. Again the above products cannot readily or satisfactorily be produced by alternative processes.
Structural or Mechanical Parts

By any reckoning, this is by far the largest group. The bulk consists of iron-based parts, but significant tonnages of copper, brass, bronze and aluminium parts are produced, as well as some rarer metals such as beryllium and titanium. In general such parts do not have mechanical properties superior to those of equivalent parts made by forging or machined from wrought bar, often the reverse, but they are entirely suitable for the required duty. They often have advantage over forgings in dimensional accuracy, but in a large number of cases, the main justification for their use is economic - i.e. Powder metallurgy is a cheaper production process. Recently, however, developments have taken place that will require revision of the foregoing. It is now possible to produce sintered parts with properties equal to and even superior to those of parts made by more traditional routes.
Special High-Duty Alloys

An area that is growing very rapidly is the production from powder of high strength materials - high speed steels and so-called superalloys based on nickel and (or cobalt) to give a product having superior properties to those achieved by casting and forging. In general the powder is compacted into a blank or billet which is then subject to forging, or extrusion followed by forming in traditional ways. The advantages of the powder route are higher yield or usable material, and a finer, more uniform microstructure that confers improved mechanical properties, and, in the case of cutting tools and wear parts, longer life. The powder metallurgy process has also allowed the development of new types of materials based on powders having micro-crystalline or even amorphous (glass like) structures produced by cooling droplets of molten metal at very high rates. The final consolidated product is characterised by very high strength, ductility, and thermal stability.