A new method for compression bending of thin-walled tubes in hydro-bending process

Tube bending is widely used in aerospace, automotive, and several other industries. The main problems of this operation include wrinkling, cross-section deformation, and variations in wall thickness of thin-walled tubes during process. If bending radius to tube diameter ratio (R/D) is less than 1.5, the bending process will fail using conventional methods. Proposing a new technique capable of eliminating wrinkles from the tube cross section which results in minimal changes in tube wall thickness would be very important. One of these bending methods is hydroforming, in which compressive force and internal pressure can be simultaneously used to better control the bending process. In this study, in order to uniformly apply the pressure of fluid to the inner surface of the tube, thin-walled tubes with closed ends were first fabricated. Since this fabrication is difficult, initially, seamless tubes with closed-ends were fabricated using multi-stage deep drawing and ironing processes. Then, the tube bending process with R/D ratio of 1, using a new hydro-bending process, was investigated through experiments and simulations. After examining different pressure paths, the path by which the tube completely took the shape of the die without wrinkles was obtained. In addition, the effect of pressure variations on the distribution of wall thickness along the inner and outer radii of the bend area was studied. The formation of tubes with fixed R/D ratios through changing bending radii and tube diameters simultaneously was also compared with that of tubes with variable R/D ratios of less than 1. Finally, the effects of friction coefficient between the die and tube and the tube length were evaluated.