Development of a model to describe springback and residual stresses resulting from bending at elevated temperatures

为实现高强度轻量化材料在弯曲件上的稳定减重，采用快时钟级进模，既要保证零件的尺寸精度，又要解决常规成形工艺中成形性下降的问题，提出了新的挑战。在高温下通过感应加热在级进模具中弯曲代表了一种创新的解决方案，以生产高精度和复杂的弯曲零件，尽管由于更高的强度而限制了成形性。因此，该项目的目的是预测在高温下弯曲的回弹和残余应力，在此期间可能发生许多温度和时间相关的影响。在这种情况下，对回弹和应力状态有相关影响的温度和时间依赖的物理机制将在热拉伸试验和进一步的弯曲试验中得到表征。实验结果进一步通过解析或半解析的方法来模拟所产生的几何形状以及残余应力。重点将是蠕变应变在成形和加载保持，塑性流动期间淬火与主动载荷，和可变杨氏模量在冷和热状态。将通过热拉伸试验和参数分析来研究其机理。随后，模型将通过冷却和回火工具的弯曲测试来支持。该方法将通过数值模拟进一步验证。最后，推导出的相关关系给出了产品特性的解析理解，如弯曲角和残余应力与工艺温度、时间和应变率的关系。

For the steady weight reduction of bent parts by the application of lightweight metals with increasing strength, new challenges arise to keep dimensional accuracy and to deal with the decreased formability in conventional forming processes as the fast clocked progressive dies. Bending at elevated temperatures through inductive heating in progressive dies represents an innovative solution to produce high accuracy and complex bent parts in a large scale despite of the limited formability due to the higher strength. Hence, the aim of the project is the prediction of springback and residual stresses for bending at elevated temperatures, during which numerous temperature and time dependent influences can occur. In this context, temperature and time dependent, physical mechanisms with a relevant effect on the springback and the stress state will be characterized in hot tensile tests and further bending tests. The experimental results serve further to model the produced geometry as well as the residual stresses through an analytical or a semi-analytical approach. The focus will be on creep strains during forming and loaded holding, the plastic flow during quenching with an active load, and the variable Youngs modulus in a cold and a hot state. The mechanism will be investigated through warm tensile tests and a parameter analysis. Subsequently, the modeling will be supported by bending tests with cooled and also tempered tools. The approach will be verified further with numerical simulations. Finally, the deduced correlation gives an analytical comprehension of the product properties as the bending angle and the residual stresses in relation to process temperatures, times, and the strain rate.