Development of a reversible top-hat forming approach for reducing flange wrinkling in flexible roll forming

Abstract

Flexible roll forming (FRF) addresses the need for flexible manufacturing and light-weighting in electric vehicle production, but its widespread implementation is prevented by severe wrinkling in the flange. Wrinkling in sheet metal forming depends on the tool support and material deformation. In FRF the tool effects on wrinkling are complex, given that the sheet undergoes unsupported deformation when it enters and exits the forming roll and there is no fundamental understanding of material deformation in the critical forming regions where wrinkling occurs. This prevents an effective tool and process design. This work develops a new model to analyse the stability of the flange by combining plastic bifurcation analysis with thin-shell assumptions. The model defines a stress ratio parameter that can be used to determine when in the process, deformation becomes unstable considering the tool support conditions and material deformation. Based on this a new approach for wrinkling reduction is developed that includes the forming and bending back of a top-hat shape to provide additional flange stiffness when wrinkling is likely to initiate. The top-hat shape is optimised using a series of linear perturbation ‘buckle’ analyses to provide the highest possible stiffness increase in the flange. This is followed by experimental manufacturing trials to test and prove the new concept for wrinkling reduction and to validate the model. It is shown that the new top-hat forming approach can eliminate wrinkling in the flange even when flexible roll forming Ultra High Strength Steels. The analytical model results suggest that this is due to the stiffening effect of the top-hat and the reduction of the effective flange length which decreases the stress ratio. The new model in combination with the new forming approach to eliminate flange wrinkling represents a vital step towards a more effective process and tool design that will enable the widespread application of FRF in automotive manufacturing.