In sintering, a powder-based shaping process used in metallurgy and ceramics, the powder is pressed into a negative of the required final shape and heated to temperatures near to its melting point. The surfaces of the powder grains melt at their points of contact so that they join or weld together. Although the sintered workpiece contains pores (between the powder grains) and is thus lighter than a part of the same size produced by casting or machining, it still has sufficient shape stability for many applications.
Powder with grain sizes from 45 to 200 thousandths of a millimetre is used for sintering aluminium parts. Using a higher fraction of fine particles increases the strength but reduces the flowability. Lubricants (waxes or fatty acids) prevent welding together of the powder and the compacting tool (made of steel or carbide). Sintering is usually carried out at 625 °C (the melting point of aluminium is 660 °C) for about 30 minutes, in nitrogen to prevent a new oxide layer forming. In some cases, subsequent die forging is used to close the pores, thus reducing the dimensions, and allowing dimensional accuracy of up to ± 0.4 millimetres. In addition to unalloyed aluminium, mixtures with powders of other metals are also sintered; the latter diffuse into the aluminium and form the required alloy. Typical sintered parts made from aluminium are engine pistons and bearing bushes.
Sintering of alumina (aluminium oxide) produces very hard oxide ceramic, which is used, for example, for the balls in artificial hip joints or in ceramic foam (for filters in melt purification or for diesel soot filters in environmental protection).