Springback and Longitudinal Bow in Chain-Die Forming U and Hat Channels

Springback and longitudinal bow are two major shape defects in gradual forming processes like chain-die forming and roll forming. In this study, the springback and longitudinal bow of AHSS in chain-die forming of hat and U profiles are investigated through experiment and finite element simulation. The disparity of springback along the longitudinal direction and longitudinal bow signifies complex deformation during chain-die forming. Based on finite element simulation of the chain-die forming, the gradual forming process realized by sequential die blocks causes bending and reverse-bending in the web area and redundant deformation of the sheet metal, which leads to nonuniform bending moment and accumulated stresses along the longitudinal direction. At the same time, the redundant deformation will also result in the longitudinal strain on the edge, while the downhill characteristic of chain-die forming reduces the maximum longitudinal strain on the edge and introduces longitudinal strain on the web. The nonuniform longitudinal strain distribution along the transversal direction causes disparate downward and upward bowing longitudinal bow for chain-die formed AHSS hat and U profiles. By the established model, the disparate springback and longitudinal bow behavior can be quantitatively evaluated, which is helpful to chain-die forming process design.