Analysis and suppression of flange wrinkling in AHSS chain-die forming channels with a curved axis

Abstract

Chain-die forming can be used to manufacture Advanced High Strength Steels (AHSS) channel components. Flange wrinkling is one of typical defects for chain-die formed products. The purpose of this paper is to investigate the cause of flange wrinkling in chain-die forming of AHSS and to propose a potential method to suppress it. A hat channel product with a curved axis is formed as a benchmark to illustrate the mechanism of flange wrinkling in chain-die forming. When forming such a profile, longitudinal compressive strain of the sheet metal is geometrically necessary, and the compression has to be achieved by plastic wrinkling due to the insufficient restricted boundary condition. Finite element models are established to better understand the complex interaction between the tools and plate. An initial imperfection calculated from the differential equilibrium equations for elastic plates with small deflections is implemented into the finite element model to predict occurrence of the flange wrinkling. In order to eliminate the wrinkling defect, an M-shaped die design is proposed, which intends to provide additional restriction to the sheet metal and increase longitudinal and transversal tension. A three-factor, three-level Design of Experiment (DOE) scheme for the M-shaped die design is conducted. The DOE results show that a benign wrinkling always corresponds to a larger compressive strain and the width of the ears for M-shaped dies is a decisive factor to suppress wrinkling. The verification experiments show that the plastic flange wrinkling is completely eliminated by employing the proposed method.