Effects of Mg contents on microstructures and second phases of as-cast Al–Zn–Mg–Cu alloys

The effects of Mg content on the microstructures and second phases of the as-cast Al–Zn–Mg–Cu alloys were investigated. The results show that the nonequilibrium eutectic structure consists of α(Al), Al7Cu2Fe, η(MgZn2) and T(AlCuMgZn) intermetallic compounds. The alloys with the highest Mg content generate nonequilibrium eutectic structures. The maximum value of nonequilibrium eutectic structures is 10.13 ± 0.62%. As the Mg content increases, the number of tertiary dendrites in the α(Al) matrix increases significantly, and more Mg can be dissolved into the Al- matrix. In addition, as the Mg content increases, the crystallization temperature range decreases from 169.8 K to 157.3 K. When the Mg content is higher than 2.6 wt%, the microstructure evolution of the Al–Zn–Mg–Cu aluminum alloy is as follows: Liq. → Liq. + α(Al) → Liq. + α(Al) + Al7Cu2Fe → α(Al) + Al7Cu2Fe + η(MgZn2) + T(AlCuMgZn). These results play a certain role in promoting the basic research of Al–Zn–Mg–Cu aluminum alloys with high Mg content