Study on Microstructure Evolution and Heat Treatment Process of Al-Si-Cu-Mg Cast Alloys Under Two-Step Solution Treatment

The heat treatment process plays a decisive role in the microstructure and mechanical properties of alloys. However, it is difficult to optimize the heat treatment process of Al-Si-Cu-Mg cast alloys because of the different solubilities of Al₂Cu and Q-Al₅Cu₂Mg₈Si₆. In this study, the parameters of the heat treatment process were optimized by using thermal simulation and experimental, and the microstructure evolution during the solid solution process was studied. The results show that the dissolution temperatures of low melting point eutectic phase Al₂Cu and Q-Al₅Cu₂Mg₈Si₆ are 502°C and 539°C by using DSC. Spherical Al₂Cu phase is easily dissolved, and irregular Al₂Cu phase and Q-Al₅Cu₂Mg₈Si₆ phase are not easily dissolved under the one-step solution treatment. However, the two-step solution treatment makes the Q-Al₅Cu₂Mg₈Si₆ phase dissolve, which improves the solid solution degree of the alloy. The spheroidization degree of eutectic Si is the highest under the optimum one- and two-step solution process. The microhardness was the highest (76.4 ± 1.2 HV, 95.6 ± 2.0 HV). The optimal heat treatment processes are 500°C + 9 h + 170°C + 3 h and 500°C + 9 h + 535°C + 7 h + 170°C + 3 h, and the peak aging hardness is 132.9 ± 1.9 HV and 139.9 ± 2.1 HV, respectively.