Integrating reversion ageing and forming of high-strength Al alloys: Principles and theoretical basis
Chunhui Liu, Jun He, Zhuangzhuang Feng, Peipei Ma, Lihua Zhan
Abstract
Simultaneously improving the formability and post-formed strength of high-strength aluminum (Al) alloys, such as Al–Zn–Mg–Cu alloys, is essential in manufacturing complex-shaped panel components. The strict requirements on heat-treatment condition and high tooling costs limit the applications of current forming methods. A novel process called integrated reversion ageing and forming (IRAF) is proposed to form naturally aged (NA or T4 tempered) Al alloys. A principle-based concept analysis and systematic thermo-mechanical-metallurgical study of the IRAF process were performed. Additionally, tensile tests were conducted to evaluate the effects of parameters including heating rate , holding time, and forming temperature on formability and baked strength. The deformability of the AA7075-T4 alloy can be significantly enhanced through rapid heating to the reversion ageing temperature (150–300 °C), followed by short-term holding, as evidenced by the reduced yield strength of 200 MPa and increased uniform ductility. An instant strength increase to a value close to that of the T6 state was obtained after a short bake hardening (BH) treatment. Further, temperature-time-property (TTP) diagrams were established based on the correlation between the measured mechanical properties and through-process microstructure evolution to explain the mechanism underlying the optimised processing window of IRAF. The results indicate that fast-heating rate (>300 °C/min) promotes the reversion of NA clusters and inhibits re-precipitation of solutes, thereby improving the warm formability. Reversion ageing above 240 °C could induce the formation of coarse η'/η phases, leading to a considerably declined BH response. To accurately predict the strength evolution and deformation behavior during IRAF, a physical-based unified constitutive model was constructed by considering the reversion of NA clusters and solute re-precipitation. The bending and drawing tests on the AA7075-T4 alloy sheets verified that IRAF in the most-reverted state enabled optimum formability. The findings inspire promoting the reversion of pre-existing metastable particles to improve warm formability and post-formed age hardening.