The effects of Cu, Zn and Zr on the solution temperature and quenching sensitivity of Al–Zn–Mg–Cu alloys

This study is aimed at exploring the effects of the addition of different amounts of Cu, Zn and Zr on the solution temperature and quenching sensitivity of Al–Zn–Mg–Cu alloys. The main concerns are that low solution temperature and high quenching sensitivity will bring about disturbances during the manufacture of AA7075 aluminum alloys. Therefore, overcoming these difficulties is the goal. It was observed in the experiments that when a high copper alloy was subjected to heat treatment over 530 °C, there was an obvious melting phenomenon at the grain boundaries. After reducing some of the copper, an homogenizing heat treatment led the eutectic T phase (Mg₃Zn₃Al₂) and η phase (MgZn₂) of the Al–Zn–Mg–Cu alloy to dissolve fully into the aluminum matrix rather than precipitating the low melting point's (~495 °C) S phase (Al₂CuMg). This enhanced the alloy solution temperature and improved the solution treatment efficiency. Besides, by adding a certain amount of zinc to the alloy, the precipitation amount of strengthening phase η′(MgZn₂) was increased. This compensated for the reduction in the mechanical strength of the alloy containing less copper. The results showed that the addition of the zinc atoms not only elevated the strength of the alloy, but also caused no damage to the ductility. Furthermore, the addition of zirconium atoms improved the quenching sensitivity of the alloy.