Effect of main elements (Zn、Mg and Cu) on the microstructure, castability and mechanical properties of 7xxx series aluminum alloys with Zr and Sc

In this study, a series of Al-Zn-Mg-Cu alloy with different compositions added with Zr and Sc refiners are designed. The effect of main alloying elements (Zn, Mg, and Cu) on the grain structure, hot tearing performance, and mechanical properties of the alloy were investigated. It was found that as the content of alloying elements increases, the grain size becomes finer, the grain morphology changes from equiaxed crystal to fine dendrite, and the eutectic phase increase. The maximum and minimum hot tearing susceptibilities of the alloy occur for Zn in 9 and 5%. The hot tearing sensitivity of the alloy is greatly affected by the latent heat of the alloy, the viscosity, and the surface tension of the alloy. The alloying feeding performance is good because of the high content of the non-equilibrium eutectic phase while the alloying content is high. It could be concluded that the mechanical property of the alloy increases while the elongation decreases significantly as the content of alloying element increases. When the Zn content exceeds about 9%, a lot of non-equilibrium eutectic phase remains after heat treatment that resulting in a decrease in the strength and elongation of the alloy.