Characterization and optimization of the hydroforming process of AISI 316L steel hydraulic tubes

Hydroforming is a metal forming technology that enables the fabrication of complex parts in a low cycle time. The process is based on the plastic deformation of a blank sheet using a pressurized fluid. This paper focuses on the design of a tube hydroforming (THF) process to replace the current cut-and-weld practice for components produced by a company. Specifically, the study focuses on the characterization and optimization of the THF process for stainless steel T-joint parts produced in two sizes: small and large. The new production must improve the final components’ quality and maintain the technical requirements of the previous one, especially in terms of the parts’ geometry (in particular, the third branch minimum height and thickness) and material (AISI 316L), with competitive production costs. Accordingly, the process optimization is performed in three sequential steps. Initially, the process is characterized by the material flow stress and the friction between a tube and die. Subsequently, this information is used to develop a finite element method (FEM) model, which is validated based on experimental data. The FEM is used to optimize the process parameters (pressure, stroke, and trust force of the counterpunch) to improve the final component quality and guarantee the specific dimensional requirements. Finally, further improvements of the process are implemented (initial precrash of the tube, optimal length of the blank tube, and calibration pressure to avoid wrinkles in the final component). After the THF process optimization, emphasis is placed on the punch geometry. A study is conducted to avoid stress concentrations that may cause punch breakage. The results of this study allow the minimization of tube thinning during the hydroforming process, and guarantee the target value for the third branch height with minimal material consumption. Moreover, the evaluation of different geometrical alternatives allows the stresses acting on the punches to be reduced by 45%.