Analytical prediction of forming pressure for three-layered tube hydroforming

Tube hydroforming is a special manufacturing process that uses high-pressure fluid to flow the material into a die cavity. This process has been used mostly in automobile and aerospace industries. Compared to the conventional metal forming processes, it has the advantages of weight reduction, less tooling cost, fewer secondary operations, and improved structural strength and stiffness. In special working environments, multi-layered tubes with combined material properties, high strength, and corrosion resistance are required to satisfy conflicting requirements. In this study, an analytical model for predicting the forming pressure range of a three-layered tube is developed and experimentally verified. Fundamental theories for calculation of the pressure for the inner, center, and outer tubes are proposed. Forming pressures are obtained from equilibrium equations for the action and reaction forces on the contacting walls of the tubes. In order to validate the proposed model, free bulge experiments are performed with stainless steel/aluminum alloy/copper alloy and stainless steel/carbon steel/copper alloy three-layered tubes. The measured forming pressures are in reasonable agreement with those obtained from the analytical model.