The Processing of Metals as Semi-Solid Slurries.

Aluminium - 10% magnesium alloy slurries with a range of non-dendritic globular microstructures have been produced by powder processing routes. These processes have also been used to include various fractions of ceramic fibre. Billets of these materials have been reheated into the semi-solid range and pressure die-cast using a cold-chamber machine adapted for this purpose. The quality of the castings has been shown to depend on the fraction of solid present on die injection and the influence of the processing conditions on the solute distribution, and hence the fraction of solid, has been considered. The microstructure of the castings indicates that pockets of liquid may form inside the solid particles during the reheating stage and it has been shown that this phenomenon arises from the development of a solute supersaturation combined with the availability of intragranular nucleation sites. The transformation strain associated with this is high, but the strain energy is reduced to levels low compared with the driving force available by plastic relaxation of the matrix around the pocket. Intragranular liquation may therefore affect the effective fraction-solid vs temperature characteristics of the system