Effect of welding current on the microstructural evolution and lap-shear performance of resistance spot-welded 340BH steel

In this study, we used experimental characterizations and finite element analysis (FEA) techniques to investigate the effect that welding current exerts on the microstructural features and mechanical properties of bake-hardened (BH) steels. Resistance spot welding (RSW) experiments were conducted using 5.0 and 7.0 kA of welding current. The microstructure across the weld regions was analyzed using optical microscopy (OM) and electron backscatter diffraction (EBSD). FEA was employed to simulate the RSW process, predict nugget dimensions, and determine cooling rates. The results of the OM analysis showed that nugget size increased with an increase in the welding current, and EBSD characterization revealed similar microstructures under both levels of current, but significant variations were observed within weld zones. Lath martensite formed as a result of rapid cooling in the fusion zone (FZ). Whereas, fine and coarse grains were found in the heat-affected zone (HAZ). In addition, the formation of lath martensite in the FZ resulted in high hardness on a 2D map of microhardness. The welding current had no substantial effect on microhardness, but it did impact the mechanical properties under lap-shear test conditions, which resulted in significant changes in the fracture mode. The deformation and fracture behavior of spot-welded specimens was studied using a lap-shear test coupled with 3D digital image correlation (DIC). The experimental results indicated a fracture in the 5.0 kA current samples near the FZ/HAZ boundary, while 7.0 kA welding current resulted in base metal (BM) necking and tensile fracture. FEA was also employed to elucidate the local deformation properties and fracture modes during lap-shear testing. The results of this study show that nugget size has no effect on a failure site during lap-shear loading, but size does affect the nugget load-carrying capabilities. The failure behavior was influenced by the microstructural properties of the distinct weld areas. •RSWed 340BH steel was characterized using EBSD, lap shear test, and FEA.•Different weld zones revealed distinct microstructures influenced by local cooling rate.•Brittle fracture was observed near high stress concentration zone in the welded sample with 5 kA.•Enhanced weld strength resulted in ductile fracture via necking in the welded sample with 7 kA.•Nugget dimension and local microstructure play a crucial role in determining weld strength.