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

金属板料渐进成形（ISF）作为一种小批量和个性化生产金属板料零件的解决方案，已经发展了大约20年。在ISF中，使用CNC或机器人控制的通用成形工具进行成形。然而，迄今为止，毫米级的形状偏差是不可避免的，这被认为是该工艺的工业采用率非常低的主要原因。ISF局部引入循环塑性弯曲变形，并使零件具有复杂的残余应力状态，这导致在修剪期间偏离所需的几何形状。虽然形状偏差导致探索提高工艺精度的方法，但现有方法仅改善了成形后的几何形状。此外，只有很少的研究已经进行了关于避免二次变形所造成的局部弯曲附近的主要成形区。在移除夹具期间或在修剪期间发生的形状偏差目前根本无法控制。在ISF的当前状态下，没有能够实现工件公差合规性的策略。在专用工具中进行完全约束下的退火处理可以减少切边前的内应力，但这也有缺点，除了热处理之外，还会产生高的工具成本。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。基本要素是对残余应力形成的理解，提高成型部件尺寸精度的措施，以及通过在部分约束条件下进行应力消除退火来降低残余应力的措施。在该项目应用中，采用了两种创新方法：（i）本地代理人，弹性坯料保持器，避免在成形区附近由不平衡弯矩产生的偏差，和（ii）混合工具概念由钢框架和内部薄片组成，为低成本插入件（例如西卡）提供空间。在随后的去应力退火之前，将其去除，从而在部分约束条件下通过去应力退火保持几何形状。除了新的技术方法之外，还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的降低的新科学知识。

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.