Deformation behavior in the hydroforming of overlapping tubular blanks

The hydroforming of overlapping blanks (HOB) is a novel method that uses overlapping tubular blanks rather than closed cross-sectional tubes to enhance the forming limit. However, the deformation and instability mechanism of HOB has not been elucidated yet. In this paper, theoretical analysis models were developed for the material flow field, the critical wrinkling stress, and the critical supporting pressure. On the basis of it, the HOB approach was performed experimentally and numerically to validate the proposed models, taking a variable-diameter part as an example. A new self-sealing loading tool made of highly elastic material was developed to realize the sealing. The deformation mechanism in HOB was revealed and the occurrence of material flow along the circumferential direction was demonstrated. In addition, the location and the morphology of potential wrinkles were well predicted. Wrinkling defects were prevented effectively by applying the normal load, which was achieved by a sub-plate covered upon the overlap at the outer layer. The comparison of the required internal pressure and the wall thickness distribution was analyzed in the HOB and the THF processes. It is feasible to reduce the forming pressure and the wall thinning using an overlapping blank.