Rheological modelling on semi-solid metal alloys and simulation of thixocasting processes

Semi-solid metal alloys, as used in thixoforming, have a special microstruc ture of globular grains suspended in a liquid metal matrix. The material un der investigation is a tin-lead alloy (Sn-15% Pb) which exhibits a similar microstructure as aluminum alloys. The experiments were performed with conc entric cylinder rheometers. initially, the liquid alloy is cooled down to t he semi-solid range under constant shearing and then kept under isothermal conditions for further experimentation. The microstructure is characterized in dependence of the shearing time. The rheological techniques consisted o f step change of shear rate and shear stress ramp experiments fur different solid fractions (40-50%). Based on the experimental data a single phase mo del has been derived: where the semi-solid alloy is regarded as a homogeneo us material with thixotropic properties and the microstructure is character ized by a structural parameter. The model consists of two parts: the equati on of state, including a finite yield stress, and a rate equation for the s tructural parameter. The model equations are employed into numerical softwa re and used for the simulation of a characteristic thixocasting process. Th e results are compared to real experiments. (C) 2001 Elsevier Science B.V. All rights reserved.