Influence of the multi-axial Bauschinger effet in cold forging

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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.