The influence of Cu addition and pre-straining on the natural aging and bake hardening response of Al-Mg-Si alloys

The effect of pre-straining on the natural aging and bake hardening of Al-Mg-Si alloys with different additions of Cu were investigated by Vickers microhardness measurements, differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM) characterization. It is revealed that introducing pre-straining can suppress the natural aging and increase the bake hardening response (BHR) for both low and high Cu addition alloys. A pre-strain of 2% is found to be the optimum process. Addition of Cu can increase the effect of pre-straining in suppressing the natural aging and improving the BHR after paint baking. The slow diffusion rate of Cu atoms in the Al matrix is most probably the main reason for the significant suppression of natural aging in the high content Cu alloy after pre-straining. During paint baking, the dislocations introduced promote the preferential precipitation of the Q′ phase and produce a disordered structure within the precipitates. Thus designing new pre-straining processes should fully consider the influences of Cu addition to the alloys.