Stabilization of 7xxx aluminium alloys

High strength Al-Zn-Mg(-Cu) alloys are rarely used in the automotive industry due to pronounced natural ageing after quenching and resulting poor formability at room temperature. Furthermore, the paint bake response is often suboptimal. To overcome these challenges, we performed various low temperature pre-ageing treatments followed by storage at room temperature and a short high temperature heat treatment simulating industrial paint baking. The intermediate and final mechanical properties were assessed and the nanostructure, precipitation, and local chemistries of the samples were studied by atom probe tomography and differential scanning calorimetry. It was found that a properly designed stabilization treatment can inhibit the natural ageing process in AA7021 resulting in a stable condition with a lower hardness than samples naturally aged for longer than approximately one week. Atom probe tomography revealed that the stabilization leads to co-precipitation of Mg and Zn (indicating GP-zones) hindering the further increase in hardness over the course of three weeks. Furthermore, the paint bake response of AA7075 was improved resulting in yield strength only 2% short of a peak aged sample.