Influence of punch shape on geometrical profile and quality of hole piercing-flanging under high pressure

Hole hydro-piercing-flanging is a new hybrid technology by using high internal pressure as the support medium instead of rigid die. By this process, holes can be punched and flanged successively on hydroformed tubular parts without secondary positioning and fabrication. However, punch shape as an important factor has not been concerned enough so far. In this study, the influence of punch shape on geometrical profile and quality of holes was investigated both by experiments and simulations with three different shapes of transition zone between the piercing punch and the flanging punch. Results show that the geometrical size of roll-over depth, straight-wall height, and thickness varied with punch shapes. Considering the quality of the hole’s edge, ellipsoidal punch is the best one with the largest smooth zone and the least damage, but the conical punch gives the worst results. Finally, the mechanism of hydro-piercing-flanging process affected by the punch shape was clarified by the stress and strain distribution during plastic deformation by finite element analysis.