Effects of Cu additions on the precipitation activation energy and mechanical properties of prestrained Al–Mg–Si alloys

Abstract

We investigated the effect of different Cu additions (0.1 wt.% and 0.3 wt.%) on the precipitation activation energy and mechanical properties of Al–Mg–Si alloys under different prestraining levels (0%, 3%, and 5%). Tensile tests were performed before and after paint baking (PB). Differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) analyses were conducted to explore the clustering and precipitation behavior. Our results show that the precipitation activation energy was reduced by higher Cu addition but increased with prestraining level. Before PB, higher Cu addition significantly improved the elongation and work-hardening behavior, and the improvement was more pronounced in the prestrained alloys than in the nonprestrained alloys. After PB, higher Cu addition slightly increased the bake hardening response (BHR) of the alloy, and this improvement was enhanced in the prestrained alloys. However, only by introducing an appropriate amount of prestraining (3%) could the high-Cu alloy increase the BHR more significantly than the low-Cu alloy. When 5% prestraining was introduced, the high-Cu alloy had no advantage in increasing the BHR. After the introduction of 3% prestraining, a small amount of pre-Q′ phases was additionally formed near the dislocation lines, but the β″ phase was still the main strengthening phase. Higher Cu addition significantly refined the β″ phase and increased its number density, thereby improving the BHR. The formation of pre-Q′ phases was more pronounced in the high-Cu alloy under 5% prestraining, which suppressed the formation of the β″ phase and limited the BHR.