Use of Calphad Thermodynamics to Simulate Phase Formation during Semi-Solid Processing

Abstract

In this work we will explore the use of thermochemical simulation methods (Calphad) to support alloy selection and processing in the semi-solid state. Semi-solid processing has been investigated extensively for aluminium alloys, in particular A356, but there is also an increasing interest in using semi-solid processing for steels, in particular high carbon steels. A key property for the semi-solid processing is the fraction of liquid phase as function of temperature. It is necessary to know the fraction of liquid phase in order to be able to control the process and in order to simulate the viscous flow during various forming operations. The approach used here is to calculate the fraction of liquid phase from thermodynamic (and diffusion) data, using equilibrium calculations, Scheil–Gulliver calculations and diffusion simulation. Normally only the solidification behaviour is considered, but during thixoforming also the melting behaviour is of importance. However, there is very little information on melting of alloys to be found in the literature. Here an attempt will be made to discuss also melting as it cannot in all cases be regarded as the reverse of solidification. In addition some further properties, such as enthalpy, heat capacity and density as function of temperature will be discussed. The materials treated are the aluminium alloy A356 and the tool steel X210CrW12. Interestingly they show fairly similar solidification behaviour.