NUMERICAL MODELLING OF FLUID FLOW AND HEAT TRANSFER IN THIXOMOLDING

摘要

Thixomolding is an innovative moulding process for producing metal alloy net shape parts. The main feature of the process is to apply the techniques developed for the injection moulding of plastics to the casting of metal alloys & composites. The process consists of the following steps: 1) the chips of the feed stock are loaded into a heated barrel and through the shearing action of a high torque screw the metal alloy is converted from a solid state to a semi-solid or thixotropic state; 2) the semi-solid alloys are injected into a preheated mould cavity to form the net shape part. Compared to other metal casting processes, Thixomolding is capable of producing more complex parts with reduced porosity. The reason for this is that the alloys at the semi-solid state have a much larger viscosity and therefore the flow is laminar. In this work, a numerical model representing the Thixomolding process has been developed. The flow is modeled using the Hele-Shaw approach coupled with the temperature calculation across the part thickness. A hybrid FEM/Control Volume model is implemented to solve the flow and heat transfer equations and track the material flow fronts. The viscosity of the semi-solid material is evaluated using a Power law-type model. This model takes into account the viscosity dependence on shear rate and fraction of solid. To demonstrate the capabilities of the model, an industrial part produced by Thixomolding has been simulated. Numerical results are compared with experimental "short shots".