Abstract:The effects of Mg content and cooling rate on the solidification behaviour of Al-7%Si-Mg(mass fraction) casting alloys have been investigated using differential scanning calorimetry, differential thermal analysis and microscopy. The Mg contents were selected as respectively 0.00%, 0.35% and 0.70%(mass fraction). DTA curves of Al-7%Si-0.55%Mg(mass fraction) alloy at various cooling rates were accomplished and the alloy melt was cast in different cooling rates. The results indicate that increasing Mg content can lower the liquidus and binary Al-Si eutectic transformation temperatures. Large Fe-rich π-phases (Al₈FeMg₃Si₆) are found in the 0.70%Mg alloys together with some small β-phases (Al₅FeSi); in contrast, only β-phases are observed in the 0.35%Mg alloys. The test results of the Al-7%Si-0.55%Mg alloys identify that the liquidus and binary Al-Si eutectic transformation temperatures decrease, and the quantity of ternary Al-Si-Mg₂Si eutectic phase decreases as the cooling rate increases.

Key words: Al-Si-Mg casting alloys; solidification; cooling rate; eutectic transformation