A two-stage preforming process based on wrinkle formation is developed for the tube hydroforming process to accumulate material in the forming zone, thus reducing the thinning rate and improving the formability. In preforming stage one, the wrinkle onset is triggered with limited axial compression. In preforming stage two, the wrinkle grows stably and uniformly to a certain height. Then, the preformed wrinkles are flattened to conform to the die shape in the final tube hydroforming process. An analytical model based on bifurcation analysis and postbuckling analysis of the elastic-plastic circular cylinder under axial compression and internal pressure is used to study the wrinkle evolution characteristics in tube hydroforming. The analytical results offer valuable guidance to the process design of the two-stage preforming process. To validate this methodology, preform die sets for two axisymmetric parts were designed and tube hydroforming experiments were carried out on SS 304 tubing. Through this methodology, an expansion rate of 71% was achieved.

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