Influence of the multi-axial Bauschinger effet in cold forging

在冷锻过程模拟中，通常不考虑包辛格效应。因此，初始屈服应力的显着降低，以及由作用应力的符号的变化引起的冷锻材料的永久软化效应未被考虑。忽略这些影响会导致局部产品性能、残余应力以及工艺力的数值预测有缺陷。初步研究表明，冷锻材料的循环塑性行为首次可以在冷锻条件下系统地捕获。这些条件包括大应变，这是传统材料测试方法无法达到的。通过对预应变试件进行拉伸和压缩试验，可以覆盖相关应变的整个区域。对材料行为的新见解允许使用迄今为止仅用于金属板材成形模拟领域的本构模型。在这一领域，模型导致的回弹预测的改善。在体积成形操作中，材料的大区域经历循环变形，由此包辛格效应变得活跃。当模拟多阶段成形操作时，包辛格效应的考虑变得更加重要。通过使用组合硬化模型，可以大幅改善模拟结果，这一点已经在选定的材料中得到了验证。最终，改进的局部产品性能、残余应力和工艺力的数值预测可以减少时间和成本消耗的部件测试，可以更彻底地开发轻量化设计的潜力，并延长刀具寿命。

Usually the Bauschinger effect is not considered in the simulation of cold forging processes. Consequently, the significant reduction of the initial yield stress, as well as the permanent softening effect of cold forged materials caused by a change of sign of the acting stresses are not taken into account. Neglecting these effects leads to a flawed numerical prediction of local product properties, residual stresses as well as process forces. Preliminary studies show, that – for the first time – the cyclic plastic behaviour of cold forged materials can be captured systematically under cold forging conditions. Such conditions include large strains, which cannot be reached by means of conventional material testing methods. By the application of tensile and compressive tests on prestrained specimens, the complete region of relevant strains can be covered. The new insights into the material behaviour allow for a use of constitutive models that are so far only used in the field of sheet metal forming simulation. In this field, the models lead to an improvement of spring-back prediction. In bulk forming operations large regions of material undergo cyclic deformation, by which the Bauschinger effect becomes active. The consideration of the Bauschinger effect becomes even more important, when multi-staged forming operations are simulated. By use of combined hardening models a drastic improvement of simulation results can be expected, which could already be proven for selected materials.Ultimately, the improved numerical prediction of local product properties, residual stresses and process forces lead to a cut of time and cost-consuming component tests, potentials for lightweight design can be exploited more thoroughly and tool life can be prolonged.