Effects of Zn on Microstructure Modification and Mechanical Properties Improvement of Al-Si-Cu-Mg Alloys

In this research, the influence of Zn on the microstructure and mechanical properties of Al-Si-Cu-Mg alloys under different solidification rates was investigated. Differential scanning calorimetry was used to detect the nucleation and growth of eutectic Si, while scanning electron microscopy was used to analyze Fe-rich phases. Two growth models were deduced to explain the modification of the eutectic Si and Fe-rich phases upon the addition of Zn. The results show that an addition of 0.6 wt pct Zn modified the eutectic Si from flake like and fibrous to micron- and submicron-granular structures, depending on the different secondary dendrite arm spacings of ~ 33 μm and ~ 16 μm, respectively. For the Fe-rich phases under steel mold casting conditions, β-Al₅FeSi was modified into π-Al₈FeMg₃Si₆ upon adding 0.6 wt pct Zn. Under sand mold casting conditions, the 0.6 wt pct Zn addition only refined the size of the β-Al₅FeSi. The Zn addition significantly improved the ultimate tensile strength and elongation. The excellent modification effect of Zn on the eutectic Si and Fe-rich phases was attributed to the fact that the reaction of the Zn-Si eutectic on the {111}_Si planes forced the Si atoms from these phases into the eutectic droplets.