Deformation behaviors of 21-6-9 stainless steel tube numerical control bending under different friction conditions

For contact dominated numerical control (NC) bending process of tube, the effect of friction on bending deformation behaviors should be focused on to achieve precision bending forming. A three dimensional (3D) elastic-plastic finite element (FE) model of NC bending process was established under ABAQUS/Explicit platform, and its reliability was validated by the experiment. Then, numerical study on bending deformation behaviors under different frictions between tube and various dies was explored from multiple aspects such as wrinkling, wall thickness change and cross section deformation. The results show that the large friction of wiper die-tube reduces the wrinkling wave ratio eta and cross section deformation degree Delta D and increases the wall thinning degree Delta t. The large friction of mandrel-tube causes large eta, Delta t and Delta D, and the onset of wrinkling near clamp die. The large friction of pressure die-tube reduces Delta t and Delta D, and the friction on this interface has little effect on eta. The large friction of bending die-tube reduces eta and Delta D, and the friction on this interface has little effect on Delta t. The reasonable friction coefficients on wiper die-tube, mandrel-tube, pressure die-tube and bending die-tube of 21-6-9 (0Cr21Ni6Mn9N) stainless steel tube in NC bending are 0.05-0.15, 0.05-0.15, 0.25-0.35 and 0.25-0.35, respectively. The results can provide a guideline for applying the friction conditions to establish the robust bending environment for stable and precise bending deformation of tube bending.