Giant bake hardening response of multi-scale precipitation strengthened Al-Mg-Si-Cu-Zn alloy via pre-aging treatments

A high-performance Al-1.20Mg-1.60Si-0.20Cu-3.00Zn (wt%) alloy was designed based on the concept of high degree alloying with the main element content exceeding the range found for the 6xxx aluminum alloys. Using the optimized pre-aging (160 °C-6 min) treatment, compared to the 6016 and 6111 alloys, both the plasticity (T4P) and strength (BH) of the alloy were improved. In particular, the bake hardening response was as high as 180.2 MPa, which was more than 2 times higher than those of the 6016 and 6111 alloys. The enhanced performance of the alloy was attributed to micron/nanoscale multi-particle strengthening effects and optimized deformation heat treatments. The micro-scale phases were Mg2Si and Al(Fe,Mn)Si, and the nano-scale phases were GP zones and β"phases. Precipitation kinetics calculations, microstructure quantitative statistics, and strengthening contribution ratio analysis showed that the optimized pre-aging treatment effectively suppresses the negative natural aging effect, reduces the β "phase activation energy, and increases the mean diameter and number density of the GP zones (β"-nuclei) prior to paint baking and the β" phase number density after paint baking, thus improving the effectiveness of the paint baking treatment.