Investigation of As-Cast and Homogenized Microstructure in Mg–6 wt% Sn–1 wt% Al Alloy: An Experimental and Modelling Study

A wedge casting experiment was performed for ternary Mg–6 wt% Sn–1 wt% Al alloy to obtain solidified microstructure in a wide range of cooling rate (5–150 K/s). As-cast microstructural features such as secondary dendrite arm spacing and total second phase fraction were determined as a function of cooling rate. Furthermore, the as-cast samples were solution treated at 743 K and 793 K for 1, 6, 12 and 24 h to examine the variation of total secondary phases with respect to temperature and time. The experimental data revealed the dissolution of total secondary phases within 1 h and 12–24 h at 793 K and 743 K respectively. Solidification simulations incorporating solute back diffusion, secondary arm coarsening and dendrite tip undercooling were performed for accurate analysis of the experimental results. The study also presents a diffusion-based homogenization model to investigate the microstructural changes in the ternary alloy during solution treatment. In this model, simultaneous dissolution of Mg₂Sn and Mg₁₇Al₁₂ phases along with Sn and Al redistribution in the hcp Mg matrix was incorporated. The effect of temperature and time was ascertained on the solute distribution and secondary phases dissolution. The modelling results were compared with the homogenized microstructural features.