The kinetics of melting: Liquid fraction versus time

The fraction liquid present during semi-solid processing has a critical effect. Conventionally the process window has been defined by inspecting the liquid fraction versus temperature curve (derived from thermodynamic prediction using a thermodynamic prediction software package for example, or derived from differential scanning calorimetry results). It has been assumed that a freezing range with temperature is required for semi-solid processing to be possible. However, recently a South African group (Curle, Moller and Wilkins) has shown that it is possible to rheo-process both high-purity aluminium and a binary Al-Si eutectic alloy i.e. materials with no freezing range. This behaviour highlights the fact that it takes time for liquid to form i.e. the kinetics of melting are important. Here the liquid fraction vs time for high purity aluminium is derived from experimental results to identify the process window in terms of time rather than temperature. The time sensitivity in thixoforming or rheocasting depends on the sample mass, the heat flux and the phase transformation temperature. It is also important in determining the vulnerability to defects such as hot tears, which tend to occur particularly with the alloys which are conventionally wrought rather than cast such as the 2000 series aluminium alloys.