Effect of Subcritical Annealing on the Microstructure and Mechanical Properties of a Precipitation-Hardened Al-Zn-Mg-Cu Alloy

Commercial precipitation-hardened Al alloys are widely employed in structural components and might be subject to moderate temperatures during application. Consequently, overaging process could take place leading to a significant decrease in strength and toughness. To study the change in mechanical properties during reheating at subcritical temperatures, the effect of annealing treatments was investigated in a commercial Al-Zn-Mg-Cu alloy in a solution-treated and artificially aged condition (7075 T651). For annealing treatments at temperatures between 150 and 350 °C, a decrease of 50% in tensile strength was identified. However, there was only a 20% decrease for annealing close to the solution temperature of 460 °C. This less pronounced reduction of mechanical properties at high annealing temperatures could be attributed to the dissolution of coarse overaged precipitates of η phase (MgZn₂) and S phase (Al₂CuMg), while the more stable Al₇Cu₂Fe remained dispersed in the microstructure. Additionally, the α-Al matrix was also strengthened by the dissolution of alloying elements.