Investigation on deformation and strengthening mechanisms of Al-Zn-Mg-Cu alloy under a hybrid process of cryogenic incremental sheet forming and bake-hardening treatment

Incremental sheet forming process is featured with high flexibility and short manufacturing cycle, which has great potential in customized forming of complex components. However, the formability of high-strength aluminum alloy is still needs to be improved. In this paper, an efficient cryogenic incremental sheet forming (CISF) process combined with bake-hardening (BH) treatment is proposed to achieve excellent formability and strength for Al-Zn-Mg-Cu alloy. First, the effect of different cryogenic temperatures on the formability of Al-Zn-Mg-Cu alloy is investigated and the mechanism of plasticity enhancement during cryogenic incremental sheet forming was revealed. The fracture forming limit lines constructed from different components show a significant increase in the formability of as-quenched Al-Zn-Mg-Cu at −170°C. Specially, when forming cone component with variable wall angle at −170℃, the ultimate forming height is increased by 70.4 % compared with 25℃. The microstructure shows that dislocation cross-slip is suppressed at cryogenic temperatures, and the large number of subgrain structure and uniformly distributed dislocations reflect the improved deformation uniformity at cryogenic temperatures. Second, the effect of cryogenic incremental sheet forming on the strength of Al-Zn-Mg-Cu is explored. The vertical forming force can be increased by 37.4 % at −170°C compared to 25°C, and the components have higher post-forming strength. Moreover, the strengthening response of the components after the combination of cryogenic incremental sheet forming and bake-hardening treatments is investigated. The cryogenic specimens exhibited a rapid bake-hardening response compared to the room temperature specimens. The yield strength increased by 15.4 % to 387.2 MPa after 40 mins of bake-hardening treatment at 180°C. It has been shown that precipitation strengthening and dislocation strengthening are the main strengthening mechanisms during cryogenic incremental sheet forming and bake-hardening treatments. This study reveals the dual enhancement effect during the cryogenic incremental sheet forming process, and proposes an efficient approach which combines cryogenic incremental sheet forming with bake-hardening treatment to manufacture high-performance components.