Deformation behavior of advanced high strength steel with different strain hardening exponents during tube hydroforging

Hydroforging is an innovative forming method used to form tubes with irregular cross-sections by using internal fluid pressure and axial force. Normally tube forming operations use solid mandrel to expand/bend the tube which does negative effect on tube or mandrel due to friction, buckling, and wrinkling. In this study, the tube material with various strain hardening exponents were hydroforged in a two-step numerical analysis with ramped fluid pressure and axial compression. The tube will be allowed to bulge with a ramped internal pressure and then shaped to a maximum expansion ratio by the axial compression in the shaping step. Some advanced materials exhibit the change in strain hardening exponent based on the plastic strain, the study focuses how the strain hardening exponent affects the deformation mechanics in tube hydroforging. It was found that the formability of tube during hydroforging is affected by the strain hardening exponent. The results show that tube can achieve a greater wall thickness reduction with increase in strain hardening exponent. Also, higher strain hardening exponent allowed tube to achieve a greater expansion ratio when axially compressed in the forging step. It was also observed that as the strain hardening exponent increases, the internal pressure requirements for the bulging and shaping steps decrease. The two model configurations of N01 and N02 failed from bursting and wrinkling, respectively, while every other model could form to the targeted disk-like shape. The forging step of the process showed that the die force for axial compression decreased as the strain hardening exponent increased for all successful models.