Friction role in bending behaviors of thin-walled tube in rotary-draw-bending under small bending radii

For contact dominated rotary-draw-bending (RDB) of thin-walled tube, friction role should be focused to achieve precision bending under small bending radii ratio (R_d/D < 2.0, R_d-bending radius, D-tube diameter). By using explicit FE simulation combined with physical experiment, underlying effects of the friction on bending behaviors are explored from multiple aspects such as wrinkling, wall thickness variation and cross-section deformation. The results show that: (1) By using a simulative twist compression test (TCT), the dynamic contact conditions of RDB with large slipping are reproduced, and the coefficients of the friction (CoFs) under various tribological conditions in RDB are estimated, which provides physical basis for understanding friction role and boundary conditions for FE simulation. (2) Both positive and negative effects of friction role are observed since the friction on each interface affects the multi-defect with different or even contrary tendencies. The effect sensitivity on wrinkling is less obvious than that on wall thinning and cross-section deformation. Under smaller R_d/D, the bending becomes more sensitive to the friction conditions. (3) Considering the knowledge about friction role on individual interface of RDB, by changing two decisive parameters affecting the CoFs such as lubricant types and tube/tool materials, an optimal strategy is proposed to apply the tribological conditions and thus the stable and accurate bending conditions are established for precision forming of RDB under smaller R_d/D.