Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
6–20% cobalt. The WC particles are angular and equiaxed in shape, and their
size is usually in the range 1–10 µm.
Hot isostatic pressing (HIP) of metal powder/fibre composites is also
employed, with the materials contained in evacuated cans. Final densification
may be by hot extrusion.
When particles of sub-micron dimensions are mixed with metal powders
by normal powder metallurgical techniques, however, they tend to clump
together, with a corresponding loss in properties. Uniform dispersions of
fine particles are more readily achieved by the process of mechanical alloying
(MA). In this technique, elemental or alloyed metal powders, together with
the dispersoid are charged into a dry, high-energy, high speed ball mill.
During processing, the powder particles are repeatedly fractured and rewelded
to the ball surfaces until all the constituents are finely divided and uniformly
distributed through the interior of each granule of powder. The MA powder
is finally consolidated by extrusion under carefully controlled conditions of
temperature and strain rate.
Liquid metal techniques. There are a number of liquid metal techniques,
including squeeze casting (Fig. 6.1), whereby molten metal is introduced to
a die containing a fibre preform and then pressed to cause infiltration,
stircasting, in which the fibres or particulates are mixed with the molten
metal, followed by die-casting, and spray deposition, in which the metal in
Press
Heater
Molten metal
Fibre preform
Die
Base
Heater
6.1 Squeeze casting of a metal matrix composite.