The Microstructure Evolution Of New Mg-Si-Sn Alloys During Isothermal Heat Treatment

The as-cast Mg-Si-Sn alloys were obtained by means of permanent mold casting method, and the effect of different Sn contents and isothermal heat treatment with relatively higher heat treatment temperatures and shorter holding times on the growth morphology of Mg₂Si in Mg-Si-Sn alloys were investigated and their mechanisms were also discussed. Furthermore, the tensile properties for the as-cast and heat-treated Mg-Si-Sn alloys with different Sn levels were also carried out. Afterwards, the Mg-Si-Sn alloy semi-solid billets were fabricated by semi-solid isothermal heat-treatment process and strain-induced melt activation (SIMA) process, and the effect of different heat-treatment processing parameters (heat-treatment temperature, heat-treatment time and predeformation) and Sn contents on the semisolid microstructures of Mg-Si-Sn alloys were studied. Moreover, the semi-solid microstructure evolution mechanism for Mg-Si-Sn alloys was also researched.The results show that the coarse primary Mg₂Si phases in the as-cast Mg-Si-Sn alloys can transform into fine polyhedral shape with appropriate Sn content. After isothermal heat-treated of 2 hours at 550℃, the morphology of the eutectic Mg₂Si in Mg-3Si-1.70Sn alloy transforms completely from the Chinese script type and fibriform shape into fine, spherical shape and the spheroidization mechanism of the eutectic Mg₂Si phases seems to be attributed to the "Rayleigh shape instability". It has been indicated that the tensile strength and elongation for the Mg-Si-Sn alloys at ambient and elevated temperatures were improved to some extent after isothermal heat-treatment.The optimized heat-treatment processing parameters for the fabrication of the Mg-Si-Sn alloy semi-solid billets are obtained based on the analyzing results of the semi-solid microstructures. The results show that the preferred heat-treatment processing is suggested at 640℃for 15min and the degree of predeformation is chosen around 20% for the predeformed alloys. The microstructure will coarsen when the temperature is higher or the time is longer. The degree of microstructure spherdization will worse when the temperature is lower or the time is shorter. The addition of Sn results in the decreasing of the eutectic transformation temperature, so the semi-solid temperatures should be changed accordingly for the Mg-Si-Sn alloys with different Sn additions.The semisolid microstructure evolution mechanism for Mg-Si-Sn alloys was got at last. The results indicate that the semisolid microstructure spheroidization mechanism produced by semi-solid isothermal heat-treatment process is the dendrites melting. However, the penetration of liquid phase along the sub-grain boundary is the main spheroidization mechanism for the Mg-Si-Sn alloys produced by SIMA process.