PREDICTION OF AZ91 CASTING MISRUNS AND ALLOY FLUIDITY USING NUMERICAL SIMULATION

Prediction of the misrun formation in thin-walled castings of magnesium alloys is a crucial task for foundry. The computer simulation of the casting processes can be used to solve this problem. A reasonable simulation results requires the correct thermal properties of the alloy and the mold over a wide range of temperatures and the value of interfacial heat transfer coefficient between the casting and the mold, and the critical solid fraction at which the alloy flow in the mold is choked off. In this paper we determine the interfacial heat transfer coefficient between the magnesium alloy ML5 (AZ91) and the sand mold with a furan binder. It was done by the comparing the simulated spiral test lengths with the experimental spiral test lengths obtained under the same conditions. Above the liquidus temperature the interfacial heat transfer coefficient IHTCL = 1500 W/(m2 ·K) at pouring temperatures 670 and 740 °С and IHTCL = 1800 W/(m2 ·K) at pouring temperature 810 °С. Below the solidus temperature the interfacial heat transfer coefficient IHTCS = 600 W/(m2 ·K). We also determined the critical solid fraction of ML5 (AZ91) magnesium alloy for the casting made in the furan bonded sand mold (at a cooling rate ~2 K/s) and it was 0.1–0.15. We compared the simulated misruns position and the experimental misrun position in the «Protective cup» casting produced from the ML5 (AZ91) alloy into the sand mold with furan binder. The value of the critical solid fraction was clarified. The castings were made at pouring temperatures 630 and 670 °C, and the critical solid fraction was 0.1 in both cases.