Deformation analysis of hydro-bending of bi-layered metal tubes

Bending of an aluminum alloy thin-walled tube with larger diameter-to-thickness ratio is more difficult than that of a mild steel or stainless steel tube due to its low elastic modulus and yield strength. To solve this problem, experiments were conducted on hydro-bending of bi-layered tubes, which consist of an outer mild steel tube and an inner aluminum alloy thin-walled tube. The wrinkling behavior, springback, cross-sectional non-circularity, and wall thickness variation of the inner aluminum alloy tube were analyzed. It has been shown that the internal pressure plays a prominent role in preventing the wrinkling due to an additional axial tensile stress being introduced to the inner tube through welding the inner tube ends onto the surface of the outer tube. By increasing the internal pressure to a critical value, it is helpful to reduce the magnitude of compressive stress at the inner side of bent tubes, so the onset of winking is prevented. The springback and the cross-sectional non-circularity become smaller, while the thinning rate gets bigger with pressure increasing. Finally, a sound aluminum alloy thin-walled tube with diameter-to-thickness ratio of 63 and relative bending radius of 2.2 was successfully formed under the internal pressure of 25 MPa.