Dominant deformation mechanisms in single point incremental forming (SPIF) and their effect on geometrical accuracy

Abstract

The final geometrical accuracy of a part formed by incremental sheet forming depends on the deformation mechanisms and the residual stresses created in the part. In this regard, several studies have been reported in the literature, which investigate the forming mechanisms of the single point incremental forming (SPIF) process. Depending on the condition and experimental set-up, different research groups revealed that either membrane stretching, bending or shear deformation modes prevail. The current paper moves a step forward and quantifies the respective contribution of each forming mechanism, i.e. membrane stretching, bending, through-thickness shear, involved in the SPIF process and the dependence between geometrical accuracy and the dominant deformation mechanism. For this purpose, a validated numerical model of the SPIF process is used to record the respective deformation histories i.e. stress/strain components. Using an analytical approach, plastic energy dissipated during SPIF is split as a contribution of energies dissipated in membrane stretching, through-thickness shear and bending deformation modes. Further, a parametric study based on FE simulations is used to analyse the sensitivity of deformation mechanism to SPIF process variables, i.e. tool diameter, tool step-down, friction, sheet thickness and wall angle. The results of the numerical and analytical approach are validated by experiments. The results indicate that at any location on the geometry of a part formed with SPIF, the deformation is always a combination of these three modes. Dominance of one deformation mode over the other two depends on the process variables, for example, the bending mode of deformation dominates at larger tool diameters and shear dominates at increasing sheet thickness. The prevalence of each of the deformation modes as a function of the SPIF process variables is discussed. Further, the practical consequence of the dominant deformation modes on the geometrical accuracy is demonstrated by numerical simulations. A decrease of the energy dissipation in the bending deformation mode leads to lower residual moments. Hence, by increasing pitch and decreasing the tool diameter, an increase in the geometrical accuracy is achieved due to the decreased contribution of the bending deformation mode and lower residual moment.