Preparation And Microstructure Control Of New Mg-Al-Sn-Si Alloy

The as-cast Mg-3Al-3Sn-1Si (ATS331) and Mg-6Al-3Sn-1Si (ATS631) alloys were fabricated by means of permanent mold casting method. The phase compositions, microstructures and mechanical properties of the as-cast composites were investigated by means of X-ray diffraction (XRD) analysis, optical microscopy (OM) analysis and tensile test. The results show that the coarse primaryα-Mg phases in the as-cast Mg-Al-Si-Sn alloys can transform into fine shape after the alloys were added with Sn. The as-cast alloys have improved tensile strength but lower elongation with increasing Al contents. The formation of Mg2Si and Mg2Sn particles improved mechanical performance of the experimental alloys at elevated temperatures, and the higher Al content the effect more obvious. However, the Mg2Si phases existed in the form of coarse Chinese script in the matrix, which would greatly deteriorate the strengthening effect of Mg2Si.The processes of solution (T4) and solution + artificial aging(T6) heat treatment were used to strengthen the as-cast alloys. It is obvious that the tensile stress and the tensile strain increase dramaticly after dealing with the processes of T4 and T6 heat treatment. However, the Mg2Si phases change slightly, the morphology of the eutectic Mg2Si still keep its Chinese or fibre script in the matrix.In this paper, ATS331 and ATS631 alloy billets were obtained through semi-solid isothermal heat treatment. The effects of processing parameters on nondendritic microstructural evolution during isothermal heat treatment were investigated. The results indicate that the relative optimum holding temperatures and times should be chosen:575℃and 10-20 min for ATS631,610℃and 10-20 min for ATS331, respectively, thus fine and globular or spherical grains were attained, as well as appropriate liquid fraction in the semi-solid microstructure.To study the mechanism for formation of non-dendrite microstructure of semi-solid Mg-Al-Sn-Si alloy billets, including the mechanism of a-Mg spheroidization, the spheroidization mechanism of intragranular Mg2Si phase and the evolutional mechanism of Mg2Si phase at grain boundary. The first mechanism could be illuminated by dendrite melting, The second one seems to be attributed to the "Rayleigh shape instability". The final mechanism is associated with the resolution-precipitation.