D finite element simulation of pulsating free bulge hydroforming of tubes

Application of pulsating pressure is a new and effective method to improve the formability of the tube hydroforming process. However, the factors that cause this improvement are still unclear. In this paper, the forming mechanism of pulsating free bulge hydroforming of tubes is studied using both finite element simulation and experiment. The effects of oscillating pressure on deformation behavior, thickness distribution, strain path and friction force are examined. It is shown that for a constant pressure path, the wall thickness decreases quickly up to bursting; whereas for the pulsating pressure, the thickness decreases gradually, and thus, local thinning is prevented by oscillating internal pressure. Formability is improved due to an increase of the longitudinal compressive strain and better wrinkling behavior. Small harmonic wrinkles appear and are removed during the pulsating process, hence, by this mechanism bursting and wrinkling are prevented, causing improvement of formability.