Experimental and numerical analysis of forming-induced residual stresses for fatigue strength increase of highly cyclically loaded high-purity copper components

随着交通和能源政策的转变以及由此导致的电力牵引传动和发电机的使用增加，高纯铜部件的疲劳行为问题正在成为当前研究的重点。高纯铜部件是高循环负载的电池电动汽车动力总成中的动力传输部件。构件的多阶段成形过程对其疲劳寿命的影响是复杂的、相互影响的。特别是，成形引起的残余应力的影响到目前为止还没有被考虑，类似于在钢部件中的使用，可能有很大的潜力改进部件。该项目的目标是应用和转让在战略规划中为钢部件开发的方法知识，即使用成形引起的残余应力来提高高纯铜部件的疲劳寿命。重点是在考虑残馀应力的情况下建立广义的工艺-结构-性能联系。以弯曲法生产的导线钢轨为例进行了研究。通过引入残余压应力来提高其使用寿命。为此，计划在制造过程中进行额外的工艺步骤。除了对残余应力的影响外，整个过程还会导致局部材料状态的变化。这就需要考虑残余应力、局部硬化和与工艺有关的显微组织对疲劳寿命的影响。通过实施有针对性的循环材料表征，对残余应力影响下的定义材料状态进行了研究。参考部件的成形借助适当的工艺建模被虚拟地映射。利用该工艺模型，以失效临界点的残余压应力分布为目标，对工艺进行了优化。利用随同的残余应力测量结果对模型进行了验证。随后，对选定的工艺变量进行了实验成分表征，以确定工艺变量对使用寿命的影响。基于编制的虚拟数据和实验数据，建立了考虑与工艺有关的残余应力和局部材料状态的疲劳寿命计算方法。在知识转移项目结束时，可以将详细的工艺-结构-性能联系作为为提高高纯度铜部件寿命而进行的工艺设计的基础。通过对不同材料状态下残余应力影响的大量实验，建立了可传递的知识，扩展了铜材料的数据库。

With respect to the transformation of transportation and energy policy and the resulting increased usage of electric traction drives and generators, the question of the fatigue behavior of high-purity copper components, which serve as parts for power transmission in the highly cyclically loaded powertrain of battery electric vehicles, for example, is becoming the focus of current research. The multi-stage forming process of the components results in complex and interacting influences on their fatigue life. In particular, the influence of forming induced residual stresses has not been considered to date and, analogous to the utilization in steel components, could have high potential for component improvements. The objective of the project is the application and the transfer of methodological knowledge developed for steel components in the SPP on the use of forming-induced residual stresses for the improvement of fatigue life to components made of high-purity copper. The focus is on the establishment of generalized process-structure-property linkages taking residual stresses into account. A conductor rail produced by bending is used as an example for the investigation. Its service life is to be improved by inducing compressive residual stresses. For this purpose, an additional process step during manufacturing is planned. In addition to the influence on residual stresses, the overall process causes a change in the local material state. This makes it necessary to consider the influence of residual stresses, local hardening and the process-related microstructure on fatigue life. By implementing a targeted, cyclic material characterization, the investigation of defined material states under the influence of residual stresses is carried out. The forming of the reference component is mapped virtually with the aid of suitable process modeling. Using the process model, the process is optimized with a residual compressive stress distribution at the failure-critical point as the target. Accompanying residual stress measurements are used to validate the model. Subsequently, experimental component characterization of selected process variants is carried out to determine the influence of the process variants on the service life. Based on the compiled virtual and experimental data, a computational fatigue life evaluation is developed considering process-related residual stresses and local material state. At the end of the knowledge transfer project, elaborated process-structure-property linkages are available as a basis of the performed process design for lifetime improvement of high-purity copper components. Through the extensive experiments on residual stress influence for different material states, transferable knowledge is developed, which expands the data base for copper materials.