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

板料渐进成形技术作为一种解决小批量、个性化生产板材零件的方法，已经发展了近20年。在ISF中，成型是使用数控或机器人控制的通用成型工具进行的。然而，到目前为止，形状偏差在毫米量级是不可避免的，这被认为是该工艺工业接受率很低的主要原因。ISF在局部引入循环塑性弯曲变形，并为零件提供复杂的残余应力状态，这会导致在修剪过程中偏离所需的几何形状。虽然形状偏差导致了提高加工精度的方法的探索，但现有的方法只改善了成形后的几何形状。此外，对于避免一次成形区附近的局部弯曲引起二次变形的研究也很少。在拆卸夹具或修剪过程中发生的形状偏差目前根本无法控制。在ISF的当前状态下，没有任何策略能够实现对工件公差的合规。在专用工具中进行完全约束下的退火处理可以减少修整前的内应力，但这样做的缺点是除了热处理外，还会产生较高的模具成本。本项目旨在为ISF的全面公差概念开发科学知识和技术解决方案。在本项目的应用中，两种创新的方法正在寻求：(I)局部作用的弹性压边机，以避免因不平衡弯矩而在成形区附近产生偏差；(Ii)由带有内部片层的钢框架组成的混合模具概念，这为低成本插件提供了空间，例如西卡。在随后的应力消除退火法之前，去除了后者，从而通过部分约束下的应力解除退火法保留了几何形状。在新的技术方法的基础上，将创造关于循环塑性成形过程中残余应力的产生和非均匀变形材料退火过程中残余应力降低的新的科学知识。

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.