Effect of Cooling Rate on Solidification Behavior and Microstructure Evolution of As-Cast Mg–5Al–2Ca–2Sm Alloy

Abstract

In the present work, the Mg–5Al–2Ca–2Sm alloy was fabricated in stepped type sand mold, the cooling rate varied from 0.3 to 3.5 °C/s. The solidification behavior and microstructure evolution of Mg–5Al–2Ca–2Sm alloy were carried out by computer-aided cooling curve thermal analysis method, optical microscope (OM), X-ray diffraction analysis, scanning electric microscope and transmission electron microscope. The experimental results showed that the nucleation temperature of α-Mg phase decreased with increasing cooling rate. In addition, the grain size of α-Mg phase in Mg–5Al–2Ca–2Sm alloy were 95.47 ± 1.2 μm, 88.65 ± 1.5 μm, 71.24 ± 1.7 μm and 42.35 ± 1.3 μm, which were responds to the cooling rates of 0.3 °C/s, 0.5 °C/s, 1.2 °C/s and 3.5 °C/s, respectively. There were both Al2Sm (particle structures) and (Mg, Al)2Ca (lamella structure) phases in the Mg–5Al–2Ca–2Sm alloy under different cooling rates. However, there were Mg2Ca (blocky structure) in the samples cooled in 0.5 °C/s and 0.3 °C/s. The solidification sequence of precipitated phase in Mg–5Al–2Ca–2Sm alloy could be obtained as: Al2Sm → α-Mg → (Mg, Al)2Ca → Mg2Ca. Furthermore, the volume fraction of precipitated phase increased with the cooling rate. The volume fraction of precipitated phase in Mg–5Al–2Ca–2Sm alloy were 11.86 ± 0.7%, 13.05 ± 1.2%, 16.19 ± 0.8% and 20.77 ± 0.9%, which were responds to the cooling rates of 0.3 °C/s, 0.5 °C/s, 1.2 °C/s and 3.5 °C/s, respectively.