The effect of combination of pre-ageing and regression heat treatment on the natural aging behavior in Al–Zn–Mg–Cu alloys correlated with precipitate dissolving ratio

Al–Zn–Mg–Cu alloys have remarkable plasticity and aging-hardening potential, while natural aging during storage and transportation dramatically increases the strength of the alloys and deteriorates the formability by generating massive high-density nano-scaled -clusters. In this study, a combination of pre-aging and regression heat treatment was employed to improve the negative effects of natural aging via regulating the distribution of precipitates and reducing the dissolving ratio. Atomic probe tomography and transmission electron microscope observations were mainly carried out to reveal the mechanism of microstructure evolution caused by heat treatments. The results reveal that the distributions of precipitate density and size vary with different pre-aging time, resulting in the differences in dissolving critical sizes and dissolving ratios of precipitates during regression heat treatments. For the limited pre-aging time, the dramatic dissolution of nano-scale clusters caused the surge of solute concentration in the matrix after regression heat treatments, resulting in considerable secondary precipitation of clusters during the subsequent natural aging process. With the increase of pre-aging time, the size and density of precipitates gradually increased, which reduced the dissolving ratio of precipitates during regression heat treatments. However, as the dissolving ratio declined from around 97.5 % (pre-aging for 0 h) to approx. 69.2 % (pre-aging for 18 h), the solute concentration in the matrix was lower than the secondary precipitation level of clusters, thus suppressing natural aging. Overall, this work provides a novel strategy to control the nature-aging behavior of Al–Zn–Mg–Cu alloys. Meanwhile, the correlation between microstructure evolution and mechanical properties under different heat treatments was systematically discussed.