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

增量钣金成形（ISF）作为一种小批量和个性化钣金件生产的解决方案，已经发展了大约20年。在ISF中，成形是使用数控或机器人控制的通用成形工具进行的。然而，到目前为止，毫米级的形状偏差是不可避免的，这被认为是该工艺工业利用率极低的主要原因。ISF在局部引入了循环塑性弯曲变形，并使零件具有复杂的残余应力状态，这在切边时导致与期望几何形状的偏差。尽管形状偏差导致了对提高工艺精度的方法的探索，但现有方法仅改善了成形后的几何形状。此外，对于避免原生成形区附近局部弯曲引起的二次变形的研究很少。在拆卸夹具或修边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，没有能够符合工件公差的策略。在专用工具中进行完全约束下的退火处理可以在切边之前降低内应力，但这样做的缺点是除了热处理之外，还会产生高的工具成本。本项目旨在为ISF的综合容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，提高成形部件尺寸精度的措施，以及在局部约束下通过应力消除退火来减少残余应力。在该项目应用中，正在采用两种创新方法：(i)采用局部作用的弹性压边器，避免因不平衡弯矩在成形区附近产生偏差；（ii）混合模具概念，由带有内部薄片的钢框架组成，为西卡等低成本镶件提供空间。后者在随后的去应力退火之前被去除，从而使局部约束下的去应力退火保持了几何形状。在新技术途径的基础上，将产生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.