Thickness dividing circle in hydroforming of variable diameter tubes

To investigate the effect of process parameters and geometry of the tube on thickness distributions in hydroforming of variable diameter tubes, the analytic formula of the thickness dividing circle was derived by using the mechanical analysis and the total strain theory. The effect of friction coefficient, expansion coefficient, ratio of axial stress to internal pressure, relative length of feeding zone, relative thickness of tube, and transition cone angle of workpiece on the relative position of thickness dividing circle can be given by this formula quantitatively, and the analytic results are in good agreement with the FEM. The results show that with the increasing of friction coefficient, the relative length of feeding zone, the ratio of axial stress to internal pressure, the distance between thickness dividing circle without thinning and tube end decreases, that means the increase of the thinning zone; and with the increasing of relative thickness of tube, the distance between thickness dividing circle without thickness thinning and tube end increases, that means the decrease of the thinning zone.