Microstructure, forming limit diagram, and strain distribution of pre-strained DP-IF steel tailor-welded blank for auto body application

In the present study, tailor-welded blanks (TWBs) of dissimilar material combination were fabricated by laser welding of interstitial-free (IF) and dual-phase (DP) steels using 2.4-kW power and 4 m/min scan speed. Subsequently, TWBs of as-received sheet materials and IF steels were pre-strained up to 20% major strain in the deformed specimens through an equi-biaxial pre-straining setup. It was found that highly non-uniform strain distribution with nearly plane strain deformation mode was induced in the pre-strained TWBs, whereas an equi-biaxial strain was recorded for IF monolithic blank. Microhardness profiles and the effect of weld zone on the microstructural and mechanical properties of the as-received and pre-strained TWBs were studied. Further, the forming limit diagrams (epsilon-FLDs) of as-received TWB and IF steel were experimentally evaluated. The epsilon-FLD of pre-strained TWBs was experimentally determined, and epsilon-FLD of the pre-strained IF material was estimated using the Yld89 anisotropy plasticity model with the Hollomon hardening law. Subsequently, all these respective epsilon-FLDs were implemented as damage models in the FE simulations for predicting the limiting dome height (LDH) of as-received and pre-strained TWBs. It was observed that the error in LDH prediction of pre-strained TWB domes was within 9.1% when the estimated epsilon-FLD of the pre-strained IF material was used as a damage model. The FE-predicted strain distributions and weld line movements of TWBs after the second stage of deformation were also successfully validated with the experimental data.