A time-dependent power law viscosity model and its application in modelling semi-solid die casting of 319s alloy

The flow parameters in the conventional power law viscosity model are generally considered to be function of temperature without considering the time-dependent property of the thixotropic flow. Practically, however, semi-solid casting is a time-dependent process, especially where the solid fraction is relatively high. Therefore, the current study focusses on introducing the shear time effect into the power law model. This is achieved by considering the rate at which the structure can evolve during transitions in geometry and hence changes in shear rate. The temperature- and time-dependent flow parameters of 319s aluminium alloy are determined by isothermal compression experiments in the semi-solid state. In practice the shear strain during the geometric transition is proposed to identify the relevant time-dependent parameters for modelling the die filling process in a given geometry. Comparing with the practical thixocasting, the simulation employing this improved model works well in enabling visualization of the filling process of a complex product i.e. a turbocharger impeller. Furthermore, the non-filling defects can be successfully avoided with the aid of simulation analysis, showing the accuracy and potential improvement in properties by employing the improved viscosity model. The significance here is the development of the fundamental model, which will be applicable to other semi-solid alloys and using any software package which incorporates the power law model.

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