Structure-property characterization of an age hardenable Al-Mg-Si alloy after straining and flash annealing

The objective of this research is to characterize the structure-property relationship of a heat-treatable Al-Mg-Si aluminum alloy that is subjected to various straining and heat treatments. The effects of paint bake cycle (PBC), pre-straining and annealing on mechanical tensile properties were investigated. Pre-straining combined with rapid, high temperature annealing can noticeably improve ductility and increase formability. While pre-straining by itself does not change the hardening behavior of the as-received material, combining pre-straining with rapid, high temperature annealing does alter the hardening behavior and decreases the final post-PBC yield strength. It is well documented that PBC increases the yield stress in Al-Mg-Si alloys by precipitation of the β″ phase. The reduction in precipitation hardening after pre-straining and annealing is caused by the high temperature annealing treatment that promotes over-aging and conversion of the β″ phase to β′ and β precipitates, even with a short annealing time. The yield stress after PBC can be increased if the pre-strained and annealed material is strained a second time after annealing, although this increase may not reach the strength levels of the alloy without the annealing step. Hence, the intermediate annealing and forming technology is most useful for age hardenable alloys if applied locally to complex areas of a panel where increased formability is more important than high strength.