Integrating reversion ageing and forming of high-strength Al alloys: Principles and theoretical basis

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.