Effect of Zn/Mg Ratio on Second Phase Dissolution during Solution Treatment of Al-Zn-Mg-Cu Alloys

The main alloying elements have a decisive influence on the type and quantity of the second phase of the Al-Zn-Mg-Cu alloy, and even on the dissolution of the second phase during solution treatment. The effect of Zn/Mg ratio on second phase dissolution during solution treatment of Al-Zn-Mg-Cu alloys was investigated by means of differential scanning calorimetry (DSC), scanning electron microscope (SEM) and electrical conductivity testing. The results showed that Mg (Zn,Cu,Al)2 phase and Fe-rich phase existed in the as-deformed alloys. In addition, a small amount of Al2CuMg phase was found in the low and medium Zn/Mg ratio alloy. The number of Mg (Zn,Cu,Al)2 phases as the major second phase in the alloys was inversely proportional to the Zn/Mg ratio. Mg (Zn,Cu,Al)2 phase essentially dissolved into the matrix after solution treatment at 465°C/2h. Increasing the solution temperature and time were both beneficial to the dissolution of the Al2CuMg phase. With the increase of the solution temperature from 465°C to 475°C, the conductivity of the alloy showed a decreasing trend initially and then increased. As the solution time increased at 470°C, the electrical conductivity of the low-Zn/Mg ratio alloy decreased and then increased due to the more secondary phase. After the second phase was fully dissolved in the alloy, the electrical conductivity gradually increased with the increase of the solution time.