Semi-solid metal casting (SSM) or slurry casting is an innovative variant of die casting, and is typically used for high-end castings. The process deals with semi-solid slurries, in which non-dendritic solid particles are dispersed in a liquid matrix. The process is based on the phenomenon of vigorously stirring the solidifying metal at the temperature that puts the metal between its liquidus and solidus temperature, and creating a slurry which retains a capacity for flow even when a substantial fraction of the total mass has solidified. In this intermediate condition, the material exhibits thixotropic behaviour, becoming less viscous with increasing intensity of stirring. The solidifying alloy is passed from a liquid reservoir through an annular gap between a cylindrical rotor and a concentric water cooled tube. This produces continuous shear of the growing crystals, preventing dendritic growth, so that the emerging slurry contains small globular solid particles, which can remain in relative motion during mould filling. Other forms of agitation, including electromagnetic stirring, are also feasible. There are a few different techniques using such semi-solid slurry; two most common ones are: rheocasting and thixocasting.
1 – liquid reservoir, 2 – molten alloy, 3 – rotor, 4 – induction heating,
5 – cooling, 6 – continuous alloy feed
Rheocasting
The semi-solid slurry is fed directly to the shot chamber of a high-pressure die casting machine and injected into a die in the conventional way. The residual shrinkage is reduced and the high viscosity promotes laminar flow, thus reducing porosity which is often associated with the turbulent flow conditions and shrinkage occurring in standard die casting.
Thixocasting
The semi-solid slurry is cast in the billet form of a non-dendritic microstructure and cut into solid slugs. The billets are re-heated to the semi-solid temperature range by induction heating, and then injected to the steel die by high-pressure casting machine. The material reverts to the thixotropic condition, again at the temperatures lower than those incurred in conventional casting. Thixocasting can produce extremely high-quality components exhibiting the product consistency resulting from using a pre-cast.