Effects of Cu or Li addition and multi-step aging conditions on the bake-hardenability of an Al-Mg-Si alloy

In recent years, automobiles with lower fuel consumption are required because the exhaust fume is severely regulated. The weight-saving is quite effective to realize such low fuel consumption, and therefore aluminum alloys become attractive as an alternative material of steels due to its high specific strength. 6XXX series Al-Mg-Si alloys exhibit good bake-hardenability during paint-bake treatment in the automobile manufacturing process, but to reduce further environmental impact, the paint-baking temperature is supposed to be lowered compared to the present temperature of about 443K. In this study, it was aimed to investigate the attained hardness after paint-bake treatment at various temperatures of 408-443K for an Al-0.55wt% Mg-0.90wt% Si alloy with/without microalloying elements of Cu and Li. The effects of multi-step aging conditions; e.g. pre-aging, natural aging and paint-bake treatments, were also investigated through Vickers hardness test, TEM observation and DSC analysis. From the obtained experimental results, it was clarified that the addition of Cu or Li to the Al-Mg-Si alloy increases the attained hardness even at a paint-baking temperature of 408K due to the increased volume fraction of precipitates. Furthermore, pre-aging treatment at 373K for 18ks was also effective in suppressing the increase in hardness during natural aging, resulting in the highest attained hardness among the investigated multi-step aging conditions; i.e. HV100 in the Li-added alloy paint-baked at 408K.