The Effects of Interfacial Heat Transfer Coefficient on the Microstructure of High-Pressure Die-Cast Magnesium Alloy AM60B

This paper describes the details of a quantitative experimental and numerical study on the influence of solidification conditions, including the apparent interfacial heat transfer coefficient (IHTC) between the die and solidifying metal, on the resulting local microstructure. Multiple runs of the commercial casting simulation package, ProCASTTM, are used to model the mold filling and solidification events employing a range of IHTC values. The simulation results are used to estimate the centreline cooling curve at various locations through the casting. The centreline cooling curve, together with the die temperature and the thermodynamic properties of the alloy are then used as inputs to compute the solution to the Stefan problem of a moving phase boundary, thereby providing the through-thickness cooling curves at each chosen location of the casting. Finally, the local cooling rate is used to calculate the resulting grain size and skin thickness via previously established relationships. A comparison of the predicted and experimentally determined grain size profiles enables the determination of the apparent IHTC, which, in this study, was approximately 12000 W/m2·K. Additional useful observations from the numerical study suggest that the IHTC has a significant influence on the skin thickness and grain size in both the skin and core regions of the casting, while the effect of die temperature is limited to influencing the skin grain size only.