Effect of Zn/Mg ratio on the microstructure and mechanical properties of as-cast Al–Zn–Mg–Cu alloys and the phase transformation during homogenization

Abstract

The content of Zn and Mg directly influences the microstructure characteristics of Al–Zn–Mg–Cu alloys. In the present study, the solidification phases of three as-cast Al–Zn–Mg–Cu alloys with different Zn/Mg ratio (1.5, 1.8 and 2.1 at.%, successively) were thoroughly investigated, and their evolutions during homogenizations were analyzed with the aid of optical microscopy (OM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The results showed that serious dendritic segregation existed in the as-cast microstructures. In the as-cast alloys with three different Zn/Mg ratio, the secondary phases are represented by non-equilibrium intermetallic phases (Mg(Zn,Cu,Al)2 and Al2Cu) and precipitate MgZn2 phase. Thereinto, the Mg(Zn,Cu,Al)2 phase possessed a majority, which was determined through computational analysis of solidification processes. With the increase of Zn/Mg ratio, the amount of Mg(Zn,Cu,Al)2 phase continuously decreased, while the content of Al2Cu phase showed only minor variation. As the Zn/Mg ratio increased, the strength and hardness of the as-cast alloy gradually improved. The Zn/Mg ratio has a significant impact on the dissolution and transformation of the secondary phases during homogenization process. As the Zn/Mg ratio increased, the melting point of the as-cast eutectic gradually decreased. Additionally, due to the higher diffusion rate of Zn compared to Mg and Cu elements, the dissolution rate of the secondary phase increased accordingly. At relatively low temperatures (380, 410, and 440 °C), phase transformation from the Mg(Zn,Cu,Al)2 phase to the Al2CuMg phase occurred in all three alloys. However, phase transformation was only present in Low/Mid-Zn/Mg ratio alloys at 470 °C. These differences are attributed to the varying diffusion rates of Zn, Mg, and Cu elements during the homogenization process. The implementation of a two-stage homogenization process effectively addresses the issue of residual Al2CuMg phase in Low/Mid-Zn/Mg ratio alloys.