Effect of machining-induced residual stress on springback of creep age formed AA2050 plates with asymmetric creep-ageing behaviour

Effect of machining-induced residual stresses on springback of creep age formed (CAFed) AA2050 plates with asymmetric creep-ageing behaviour has been investigated experimentally and numerically in this study. A finite element (FE) model integrating unified asymmetric creep-ageing constitutive model for CAF of AA2050 has been developed through a commercial FE software-PAM-STAMP and a simplified machining-induced residual stress field for aluminium alloy plates has been proposed and implemented into the FE model. Different levels of residual stresses have been considered in the FE model and numerical results have been compared with corresponding CAF experiments and surface residual stress measurements. Results show that the springback prediction of the FE models has been significantly improved when the residual stress effect was considered. The best prediction can be achieved with a certain residual stress level for each plate thickness and the level increases with decreasing thickness of the plates produced by machining, which agrees well with the surface residual stress measurement results. For CAF of AA2050 with asymmetric creep-ageing behaviour, different springback behaviour has been observed with different residual stress levels: introduction of residual stress fields can either decrease or increase formed deflections of CAFed AA2050 plates, depending on the relative levels of residual stresses and the stresses caused by applied external forces. When the residual stress is large, subsurface of the plate may exceed the elastic limit after applying external force, leading to a significant decrease of springback in the CAFed plates.