Mechanism of Improvement of Formability in Pulsating Free Bulging Hydroforming of Tubes

Abstract

The mechanism of improvement of formability by the oscillation of internal pressure in a pulsating hydroforming process of tubes was examined. Free bulging hydroforming experiments of steel tubes under oscillating and constant inner pressures were performed. For a high constant pressure, round bulging with local reduction in wall thickness was observed, whereas wrinkling occurred for a low constant pressure. The occurrence of these defects was prevented by oscillating the internal pressure in the pulsating hydroforming. In the pulsating hydroforming, uniform expansion without local reduction in wall thickness was obtained, and thus the formability is improved. It was found from an observation of deformation behavior using a video camera that the uniform expansion is attained by repeated small wrinkling and bulging. The cause of the uniform expansion for the pulsating hydroforming was also evaluated from the variation of stress components. In addition, a similar deference in deformation behavior between the oscillating and constant inner pressures was obtained from finite element simulation. The pulsating hydroforming is effective in improving the formability of tubes.