Prediction of Misruns in ML5 (AZ91) Alloy Casting and Alloy Fluidity Using Numerical Simulation

Abstract

Predicting the misrun formation in thin-walled castings of magnesium alloys is a critical task for foundry. The computer simulation of casting processes can be used to solve this problem. Adequate results of simulation can be attained in the presence of the correct thermal properties of the alloy and a mold in a wide temperature range, interface heat-transfer coefficient between the casting and a mold, and the critical solid fraction (at which the melt flow in a mold is stopped). In this work, the interface heat-transfer coefficient between the ML5 (AZ91) magnesium alloy and a no-bake sand mold is found by comparing simulation spiral test lengths with experimental spiral test lengths under the same pouring conditions. Its values above the liquidus temperature are h_L = 1500 W/(m² K) at pouring temperatures of 670 and 740°C and h_L = 1800 W/(m² K) at 810°C. Below the solidus temperature, h_S = 600 W/(m² K). The critical solid fraction for the ML5 (AZ91) magnesium alloy was also determined for no-bake mold casting (with a cooling rate of ~2 K/s)—its value was 0.1–0.15. The critical solid fraction is refined by comparing the position of misruns by the results of simulation and in an actual “Protective cap” ML5 (AZ91) alloy casting poured into the no-bake mold. Castings are poured at temperatures of 630 and 670°C, and the critical solid fraction is 0.1 in both cases.