Retrogression and re-aging treatment of EN AW-7075 T6 to improve joining suitability for semi-tubular self-piercing riveting

In order to reduce the weight of car bodies in the automotive industry, the demand for new materials and innovative methods to achieve this goal is increasing. However, passive vehicle safety remains a central consideration in lightweight vehicle design and must be guaranteed. Therefore, aluminium alloys, particularly those of the 7000 series, have become highly attractive for lightweight manufacturing due to their high strength. This, however, presents a significant challenge for forming and mechanical joining technologies such as semi-tubular self-piercing riveting, which are well established in the automotive industry. To extend the joining suitability of EN AW-7075 in the T6 temper at room temperature, heat treatment can be applied. While heat treatment can improve formability by dissolving precipitates and reducing flow stress, the resulting mechanical properties deteriorate. In this study, different heat treatment strategies for aluminium alloy 7075-T6 are investigated to lay the foundation for crack-free joining using semi-tubular self-piercing riveting (SPR). Therefore, the temperature, quenching rate, and re-aging strategies were varied. Additionally, the short-term heat treatment has been conducted using a diode laser type LDM 3000–100 to identify the correlation between process parameters (e.g., temperature, quenching rate, cold aging, and warm aging) and their impact on the specimen's hardness. It has been shown that the applied short-term heat treatment enables crack-free joining of die-sided EN AW-7075 with punch-sided HCT590X but simultaneously reduces the hardness of EN AW-7075. However, the reduction in hardness after short-term retrogression can be compensated for, to a certain extent, by applying different re-aging strategies.