Influence of Cu and Mg Content on Recrystallization Behavior in Al-Zn-Mg-Cu-Based Alloy Sheets

The recrystallization behavior of cold-rolled Al-Zn-Mg-Cu-based alloy sheets was systematically investigated by varying the Cu and Mg contents. With increasing Cu and Mg contents, the recrystallized grain size was reduced from 54.9 to 23.3 mu m owing to the reduced ratio of the size to volume fraction of microscale particles. Interestingly, the sensitivity of different particles to particle-stimulated nucleation (PSN), which is reflected by the linear experimental coefficient (k), was dependent on their morphologies and relative fractions. The k value was considerably small (0.07) in the Cu-free alloys, implying that the Zn and Mg containing particles have an insignificant effect on the PSN. However, the k value increased significantly to 0.61 with the increase in Cu content to 1.3 wt%, because the rod-shaped Al7Cu2Fe particles formed a large plastic deformation zone around them. The addition of 2.0 wt% Mg led to the increase in the relative fraction of round-shaped Al2CuMg particles, increasing the k value from 0.46 to 0.86. The Al7Cu2Fe particles were a stronger source of PSN than the Al2CuMg particles, forming more recrystallized grains. Although Mg addition could increase the yield strength of the aged sheets, a strength-ductility trade-off arose. However, the addition of Cu simultaneously increased the yield strength and elongation because the uniform distribution of grain boundary precipitates caused by grain refinement could reduce the stress concentration during plastic deformation.