Bending mechanisms of the weld zone and coined-bead mechanisms applied for springback prevention in the tailor-welded blanks bending process

Many industrial and automotive parts are fabricated using sheet metal bending, and specific sheet metals, particularly tailor-welded blanks (TWBs), are widely used. TWBs often suffer from springback problem during bending. This study was aimed at elucidating the bending mechanisms of the weld zone for developing a springback prevention method. The effect of coined-bead mechanisms on springback prevention was also evaluated. The three-point bending model was applied to two types of workpieces (TWBs and non-TWBs), and their pre- and post-bending microstructural evolutions and microhardnesses were experimentally evaluated and compared. According to the experimental results, the weld zone in TWBs could not be deformed owing to a minimal weld zone with superior mechanical properties in terms of hardness and tensile strength. The bending and elastic band zones in TWB sheets were smaller than those of the non-TWB sheets, resulting in smaller springback characteristics of the TWB sheets. Next, the application of the coined-bead technique for springback prevention revealed its effectiveness in preventing springback only for the non-TWB sheets, in which an elastic band zone was formed. By contrast, for the TWB sheets, as the elastic band zone was not formed in the weld zone, placing the coined bead in the weld zone to reduce springback proved pointless. Based on these findings, it is very useful to recommend that the coined-bead application by coining the bead into the weld zone is useless for springback prevention in the TWB bending process.