Improvement of Formability by Wrinkling Controlin Hydroforming of Tubes

The formability of hydroforming of tubes was improved by controlling an internal pressure path for the wrinkling. In the former stage of hydroforming, wrinkles occur at a relatively low pressure, and then the wrinkles are eliminated by increasing the pressure in the latter stage. Local thinning caused by bulging is prevented by the occurrence of wrinkling in the former stage; thus, the tube does not burst. The formability for the wrinkling control is also improved by delaying contact with the die, because local thinning occurs owing to the restriction of material flow induced by such contact. For the wrinkling control, a hydroforming process of steel tubes with a box die was performed in both finite element simulation and experiment, and the range of the pressure path without defects was determined. Moreover, the forming range was extended by eliminating large wrinkles with impulsive pressure. It was found that the wrinkling control for the tube hydroforming is effective in improving the formability.