Solidification microstructures obtained by a novel twin-screw liquidus casting method

Abstract

Thixo processing, where an alloy billet is reheated to the semisolid state and shaped into a component, has so far been the most common semisolid processing route. Feedstock preparation is important for thixo processing, as the billet should exhibit thixotropic behavior when reheated to the processing temperature. In the present investigation it has been shown that casting near the liquidus temperature has tremendous potential for such feedstock production. A modification of the normal liquidus casting involving shearing the melt in a twin-screw machine prior to casting at the liquidus temperature is employed. Using both a model Sn-15 wt.% Pb alloy as well as an industrially important AZ91D alloy it has been shown that the resulting liquidus cast structure is refined, uniform and globular throughout the billet. The twin-screw liquidus casting addresses the shortcomings of normal liquidus casting and, therefore, is promising as an independent processing technique or for preparing thixotropic feedstock that requires minimal or no holding at the processing temperature.