Due to their high strength, Al-Mg-Zn(-Cu) wrought alloys, also known as 7xxx series alloys, are currently attracting interest of automakers as possible materials for car body applications. However, their low formability at room temperature and limited joinability has so far prevented their widespread use in industrial automaking.
Low-temperature heat treatments can stabilize the hardness of a modified AA7021, making sheet material suitable for forming operations and self-pierce riveting with constant quality during that time. The stabilized hardness is lower than the hardness of the unstabilized and naturally aged material which means that the natural ageing process is inhibited. Here we present long-term hardness data (>3 years) for stabilized sheet along with microstructural investigation by atom probe tomography.
The kinetics of the natural ageing process is remarkably different from usual natural ageing kinetics of pre-aged 7xxx series alloys. By calculating kinetic parameters, the transition time from the stabilized state to a conventional natural ageing behaviour can be predicted.