Sintering

Sintering involves metal in powder form, which is heated to approximately 70%-80% of its melting temperature and then squeezed to shape in a die.

The process is often used to form components made from materials with a very high melting temperature (e.g. tungsten). It also allows non-metallic materials, such as graphite and carbon, to be incorporated into the mixture.

The operation is usually conducted in a controlled atmosphere (typically argon or nitrogen) to prevent oxidation. Under high pressures, a metallurgical bond occurs (diffusion bonding) between the particles of powder. Typically, the sintered end-product is around 10%-20% porous and can then be impregnated with graphite (or high melting-point grease) to provide excellent, self-lubricating properties for plain bearings, bushes etc.

Sintering can be used, where the combined properties of materials are required, as when copper and graphite are used for electrical brushes (i.e. copper to carry the current and graphite to act as a low-friction contact).

Tungsten carbide cutting tools can also be produced in this way, by incorporating tungsten carbide particles within a cobalt matrix.

Hot, Isostatic-Pressing, uses a similar technique to sintering, but uses higher temperature and very much higher pressures to produce zero porosity. The technique is sometimes used to heal micro-porosity in super-critical castings.