The state of residual stresses in the Cu/Steel bonded laminates after ISF deformation: An experimental analysis

Incremental Sheet Forming (ISF) is a novel sheet forming process. Being characterized by severe deformation, the process is likely to induce high residual stresses in the sheet metal components. These stresses in turn can affect the lifetime and integrity of the components. The present study, therefore, systematically analyzes the state of in-plane residual stresses as the function of technological parameters. The Cu/Steel bonded laminates are deformed and the hole drill tests are performed to determine the residual stresses in the laminates. The results show that the in-plane principal residual stresses are compressive in nature. Further, their magnitude decreases from one end to the other end of thickness section (i.e., from contact surface to non-contact surface). The in-plane maximum residual shear stress is found to be a way smaller than either of the in-plane principal residual stresses. The Analysis of Variance is performed to identify the role of technological parameters on the state of residual stresses. It is found that the tool diameter and wall angle are the most influential parameters while the rotation is the least influential parameter in this regard. The detailed analysis of response surfaces reveal that the combination of low diameter, high angle, middle step size, medium rotation, high feed rate and rolled condition of material (i.e., no annealing) promote the development of higher residual stresses. The relationship between the equivalent residual stress and the post-ISF yield stress shows that the former quantity is generally smaller than the latter one. Finally, empirical models are proposed to predict the state of in-plane principal residual stresses in the Cu/Steel components produced through the ISF process.