Multi-step forming punch (MFP) for improving stretch-flangeability of advanced high-strength steel

Dissimilar materials, such as aluminum alloys, magnesium alloys, and advanced high-strength steel, are being increasingly used in vehicle structures while the flangeability of lightweight materials is of great challenges to the traditional process. In the presented study, the multi-step flanging in one press stroke accomplished by a novel punch with a notch was designed. The concept of incremental forming was implemented to improve the punch shape of stretch flanging and enhance the stretch-flangeability of advanced high-strength steel. Analytical solutions and numerical simulations were carried out to investigate the deformation mechanism and thickness distribution. Compared with the single-step forming punch (SFP), flanging process was divided into three steps in one press stroke by multi-step forming punch (MFP), which combined multi-stage characteristic of incremental sheet forming with high efficiency of traditional stamping. The results showed that the maximum tensile strain with MFP dropped from 0.406 to 0.28 at the edge of the deformed blank comparing with the single-step flanging by transferring large strain around the stretch corner edge to straight flanging region, which was in agreement with the theoretical analysis. In addition, excessive thinning was suppressed with MFP which decreased from 25.63 to 18.13% by sacrificing punch stroke that increased from 23 to 53 mm. Both the theoretical analysis and simulation results illustrated the validity of MFP for advanced high-strength steel.