Manufacturing – Casting methods - European Aluminium Association

Metallurgical aspects of semi-solid processes
Thixocasting
The metal used has to show very fine and spherical grains to increase the shear thinning
effect during forming.
DC-casting is normally combined with strong electromagnetic stirring. This leads to
fine rosette like grains (fig. below),
Microstructure of AlSi7Mg0.3 before re-heating
Source: Alcan
During re-heating the metal, grains become spherical, as required for semi-solid
forming (fig. below),
Microstructure of AlSi7Mg0.3 after re-heating
Source: Alcan
During forming, the weak bridges between the grains break, leading to a strong reduction of
viscosity (shear thinning).
Rheocasting
During cooling the liquid to the semi-solid state, some specific conditions have to be fulfilled to
get the fine grains required.
A very high amount of grain seeds have to be built. This can be produced by using
chemical grain refiners, suitable thermal conditions or mechanical vibrations in the
melt,
During the process of partial solidification, the many grain seeds grow and, after
touching their neighbours, get spherical (fig. below).
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