A quantitative study of machining induced residual stress and its effect on subsequent creep age forming of aluminium alloy panels

A new method to investigate the effect of machining induced residual stresses (RS) on deformation and performance evolutions in subsequent creep age forming (CAF) of aluminium alloy panels has been developed in this study, based on which, upper limit diagrams quantitatively characterised the RS effect have been proposed and obtained for the first time. A hierarchical and multi-process finite element (FE) method is developed to efficiently simulate the whole procedures of the typical hybrid manufacturing process of aluminium alloy panels (milling + CAF). The hierarchical milling FE method could efficiently obtain both the through thickness and surface RS after milling and has been validated by the corresponding milling experiments. The CAF simulation and corresponding experimental results show that prediction accuracy in springback after CAF increases about 9%−31% and the prediction error of strength decreased within 2.5 MPa considering milling induced RS and deflections. In addition, a simplification strategy of RS has been proposed, based on which, RS effect upper limit diagrams, which quantified the effect of RS levels on springback of CAF panels with different shape conditions, have been proposed in this study. The diagrams indicate that effect of milling RS on springback of CAFed specimens are related to the ratio of maximum loading stress to milling surface RS. When the ratio changes from 0.26 to 0.18 for plates and 0.15 to 0.10 for stiffened panels within 5 mm stiffener height, 10% to 100% prediction errors in springback will be generated if RS effect are not considered in simulation.