PREDICTION OF HOT TEAR FORMATION IN A MAGNESIUM ALLOY PERMANENT MOLD CASTING

A viscoplastic deformation model considering material damage is used to predict hot tear evolution in AZ91D magnesium alloy castings. The simulation model calculates the solid deformation and ductile damage. The viscoplastic constitutive theory accounts for temperature dependent properties and includes creep and isotropic hardening. The mechanical properties are estimated from data found in the literature. Ductile damage theory is used to find mechanically induced voiding, and hot tears are expected in regions of extensive damage. Simulations are performed for a test casting consisting of a 260 mm (10.2 in) long horizontal bar connected to a vertical sprue on one side and an anchoring flange on the other. The contraction of the horizontal bar is restrained during solidification and hot tears nucleate at the junction between the horizontal bar and the vertical sprue. The hot tearing severity is manipulated by adjusting the initial mold temperature from 140 degrees C (284 degrees F) to 380 degrees C (716 degrees F). For the simulation, a trial-and-error method is used to determine the mold-metal interfacial heat transfer coefficient from, experimental thermocouple results. The simulation results suggest that the predicted damage is in agreement with the hot tears observed in the experimental castings, both in terms of location and severity. The simulation results also confirm the observed decrease in hot tear susceptibility with increasing mold. temperature. These results suggest that the proposed viscoplastic model with damage shows promise for predicting hot tearing.