A review on flexibility of free bending forming technology for manufacturing thin-walled complex-shaped metallic tubes

Free bending forming (FBF) is a flexible forming technology that gained significant attention recently because of its broad applications in critical industries such as the automotive, aerospace, and nuclear industries. Thus, this paper aims to present a comprehensive review of FBF technology and its flexibility in manufacturing thin-walled complex-shaped metallic tubes with variable curvatures. First, the historical development and state of the art of FBF technology were reviewed and discussed its importance in tube forming recently. Then, the fundamental principles, theoretical modelling, deformation mechanisms during complex bending, critical process parameters, and forming defects with a discussion of the recent research and technological developments of FBF technology under three-, five- and six-axis processes were explained. After that, the system dies and tools design of the FBF machines and the trajectory control of the bending die were presented. Afterwards, the mechanisms of manufacturing spiral and involute hollow metallic tubes and outstanding solutions for critical issues during tube manufacturing, such as springback, cross-section deformation, over-thinning (even cracking), and wrinkling instability were introduced. Finally, the paper is concluded by providing the discussions on the system, die and tool design methodology, and outlook of the FBF technology.