Deformation behaviors of thin-walled tube in rotary draw bending under push assistant loading conditions

In rotary draw bending (RDB) of thin-walled tube with small bending radii, by using pressure die or booster, the axial pushing force is applied to the tube to control the flow states of tube materials. While the roles of the push assistant loading (PAL) conditions are perplexing due to multiple defects inherent to tube bending. Based on an analytical description of the PAL functions, the deformation behaviors of thin-walled tube in RDB under three different PAL conditions are addressed with respect to wall thinning, cross-section deformation and wrinkling numerically combined with experiment considering PAL functions. The results show that: (1) besides the push assistant level fp and fb, the actual stretch amount Δl of tube, the relative slip distances L₁ between tube and pressure die and L₂ between tube and clamp die can be used to evaluate the PAL roles. (2) The suitable boosting method is that the tube is boosted directly at the trailing end of tube; The synchronous mode is the optimal one to reduce the wall thinning degree I_t and the cross-section deformation degree I_d among sole pressure die, independent and synchronous modes between pressure die and booster; the role of pressure die is limited under sole pressure die mode due to dynamic friction conditions, and the suitable range of f (fp & fb) for each matching mode is obtained. (3) the PAL function plays more significant effect on the extrados (reducing of I_t and I_d) than that on the intrados (enlarging of wrinkling tendency I_w); The PAL conditions under synchronous mode play more significant effect on bending behaviors than sole pressure die mode; With smaller R_d/D, the PAL can avoid L₂ and wrinkling near clamp die more effectively as well as reducing the I_t and I_d more effectively.