Examination and Simulation of Silicon Macrosegregation in A356 Wheel Casting
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abstract
Macrosegregation is commonly seen and has been extensively studied in large ingots in a variety of alloy systems. If severe, the occurrence of this defect can lead to rejection of the ingot. In comparison, this defect is rarely investigated in small aluminum shape castings. To address this shortcoming, a numerical model has been developed to investigate silicon macrosegregation during the Low Pressure Die Casting of aluminum alloy (A356) automotive wheels. The model results are compared with silicon distribution maps measured using an optical, phase area-based technique. The model of the wheel casting process has been implemented within FLUENT, a commercial CFD software package. In the formulation adopted, liquid metal flow is driven solely by solidification shrinkage due to the variation in density between the liquid and solid phases. Buoyancy and die filling have been ignored. Additionally, the model includes Darcy flow in the two-phase mushy zone, the release of latent heat, and solute redistribution at the micro-scale using the Scheil approximation. The model has been validated against temperature and segregation data taken from a commercially cast wheel and shown to be qualitatively correct in predicting trends in temperature histories and segregation. A closer inspection of the data shows the model is quantitatively accurate to within 10 to 30%, depending on the location. Several areas within the wheel were shown to have significant silicon enrichment. The implications of accounting for segregation include an improved ability to predict shrinkage porosity, hydrogen porosity, and, the mechanical properties of the component.
