Abstract: Nozzle is a typical tube with variable wall thickness and diameters of the engine. Facing the challenges of low forming limits and susceptibility of wrinkles in traditional forming methods, the study proposed a tube upsetting-bulging forming process. The core technical approach is to achieve necking and thickening at the small end, while achieve bulging and thinning at the large end, with the action of an internal flexible mold and the restriction of an external rigid mold. A theoretical model for tube upseting-bulging method is established and simulations analysis of tube upseting-bulging method are conducted. The stress, strain, and thickness distribution are obtained. The defect evolutions and control methods are clarified, and experimental tooling is developed and validated. The results show that, the diameter of the small end is reduced and the wall thickness is increased, while the diameter of the large end is increased and the wall thickness is reduced. The tube with a wall thickness variation rate of ±20% and a diameter variation rate of 50% is successfully manufactured, which confirms the validity of the theoretical model and finite element simulation. The study reveals that tube upsetting-bulging method is an exceptionally technique for the precise manufacturing of tubes with variable thicknesses and diameters.

Key words: plastic forming, wrinkling, rocket nozzle, thickness distribution of tube