Investigations into a comprehensive tolerancing concept for incremental sheet forming through a local blank holder and stress relief annealing under partial constraints

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Incremental Sheet Metal Forming (ISF) has been developed for about 20 years as a solution for small series and individualized production of sheet metal parts. In ISF, forming is carried out using CNC- or robot-controlled, generic forming tools. To date, however, shape deviations in the order of millimeters are inevitable, which is regarded as the main reason for the very low industrial take-up of the process. ISF introduces a cyclic plastic bending deformation locally and furnishes the part with a complex residual stress state, which causes deviations from the desired geometry during trimming. Although the shape deviations led to the exploration of methods for increasing the accuracy of the process, only the geometry after forming is improved by existing approaches. In addition, only little research has been carried out with respect to avoiding secondary deformations resulting from the local bending near the primary forming zone. Shape deviations that occur during the removal of the clamps or during trimming cannot be controlled at all presently. With the current state of ISF, there are no strategies that enable compliance with workpiece tolerances. An annealing treatment under full constraint in dedicated tools could reduce the internal stresses before trimming, but this would have the disadvantage that in addition to the heat treatment high tooling costs would occur.The present project aims to develop scientific knowledge and technological solutions for a comprehensive tolerancing concept for ISF. Essential elements are an understanding of the residual stress formation, measures to increase the dimensional accuracy of the shaped component and to reduce the residual stresses by stress relief annealing under partial constraints.In this project application, two innovative approaches are being pursued: (i) a locally acting, elastic blank holder avoiding deviations in the vicinity of the forming zone produced by unbalanced bending moments and (ii) hybrid tooling concepts consisting of a steel frame with internal lamellae, which provide space for low-cost inserts made e.g. of Sika. The latter are removed before the subsequent stress relief annealing, so that the geometry is conserved by stress-relief annealing under partial constraints.On top of new technological approaches, new scientific knowledge about the generation of residual stress during cyclic plastic forming and the decrease of residual stresses during annealing of inhomogeneously deformed materials will be created.