Second Phase Dissolution Influenced by Simultaneously Enhanced Mg, Cu Contents during Homogenization of As-cast Al-Zn-Mg-Cu Alloys

The dissolution of second phase with relatively high melting point in as-cast Al-Zn-Mg-Cu alloys was closely related to Mg and Cu contents. In present work, second phases in three Al-Zn-Mg-Cu alloys with simultaneously enhanced Mg and Cu contents (named by LMC alloy, MMC alloy and HMC alloy as Mg and Cu contents progressively enhanced) were analyzed and the correlated dissolution during homogenization was investigated. The results showed that both Mg(Zn,Cu,Al)2 phase and Cu-rich phase existed in as-cast alloys while HMC alloy possessed more eutectic phases. As homogenized by 470°C/24h, Mg(Zn,Cu,Al)2 phase had dissolved completely, LMC alloy contained little Al2CuMg phase and the amount of it for the three alloys was arranged as LMC alloy < MMC alloy < HMC alloy. As furtherly homogenized by a second stage at 480°C for 12h, no endothermic peak for Al2CuMg phase was observed for LMC alloy and only Fe-rich phase existed. Meanwhile, Al2CuMg phase still remained in MMC and HMC alloy. As the homogenization time prolonging to 36h, Al2CuMg phase in MMC alloy dissolved completely while that still existed in HMC alloy. Adding a third stage at 490°C for HMC alloy, no Al2CuMg phase could be observed for 24h. This gave rise to a method by incrementally grading homogenization temperature combined with prolonging soaking time to fulfill the dissolution of second phase for Al-Zn-Mg-Cu alloys with enhanced Mg and Cu contents