Transient Behaviour of Semi-Solid Alloy Slurries

Semisolid metals, used in thixoforming, are thixotropic. The slurry viscosity is shear-rate and time dependent provided the semisolid microstructure consists of solid spheroids surrounded by liquid. Thixoforming takes less than 1 second, during which time the structure of the material breaks down. The transient behaviour of semisolid slurries is thus critical to die filling. For solid fractions less than 50%, this breakdown can be studied by viscometry, using rapid data-collection rates, by imposing rapid increases in shear rate on the slurry. For solid fractions greater than 50%, different techniques, such as rapid compression in the thixoformer itself, must be used. Here such experiments with Sn 15%Pb and with aluminium alloys will be described.