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 degrees 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 degrees C. This less pronounced reduction of mechanical properties at high annealing temperatures could be attributed to the dissolution of coarse overaged precipitates of eta phase (MgZn2) and S phase (Al2CuMg), while the more stable Al7Cu2Fe remained dispersed in the microstructure. Additionally, the alpha-Al matrix was also strengthened by the dissolution of alloying elements.