Microstructural Stability and Paint Bake Response of Pre-aged AA7075

High strength AA7xxx are attractive for use in the automotive industry, offering significant strength/weight benefits. These alloys are often used in tempers that require long ageing times of several hours. Pre-ageing has been proposed to be effective in suppressing natural ageing, and reducing the total ageing time by integrating the final ageing step into the paint bake cycle. However, the precipitate evolution during the pre-ageing and subsequent paint bake processes remains to be fully understood. In the present work, the pre-ageing process was studied for AA7075 over a wide range of temperatures and times. Small angle X-ray scattering (SAXS) was used as the main technique to investigate the precipitate evolution during the pre-ageing, natural ageing and paint bake, with the support from transmission electron microscopy (TEM) and isothermal calorimetry. For the first time, the results show that 8 hours pre-ageing at 80 ◦C can produce a microstructure consisting of GP zones with an average radius of approximately ∼0.9 nm, which remains stable up to at least 7 months. After a short paint bake process, 94% of the T6 hardness can be obtained by uniformly distributed precipitates with an average radius of approximately ∼2.6 nm. The final size and strengthening effect of the precipitates after 20 minutes baking is found to be insensitive to the heating rate, which has not been reported previously. The present results further suggest this process is robust for industrial application.