This study investigates the resistance spot welding behavior of dissimilar low-carbon CR3 steel and coated transformation-induced plasticity-aided bainitic ferrite (TBF) steel sheets widely used in automotive body applications. The effects of welding current (6-7-8-9 kA) and welding time (240–400 ms) on weld microstructure, nugget size, indentation depth, and mechanical performance were systematically examined. Metallographic analysis revealed no macroscopic weld defects; however, limited mixing was observed in the fusion zone, particularly at lower heat inputs. Increasing heat input enhanced material mixing and widened the heat-affected zone (HAZ), especially on the TBF side, but also promoted liquid metal embrittlement (LME)-induced surface microcracks associated with Zn-coating penetration along grain boundaries. The maximum nugget size reached 8.34 mm at 9 kA–400 ms, while the indentation depth remained within automotive acceptance limits, with a maximum value of 21.65%. Tensile-shear tests showed that all fractures occurred in the HAZ of the thinner CR3 sheet in a tearing mode, with a maximum failure load of 4.25 kN at 7 kA–240 ms. Despite the presence of microcracks in the TBF HAZ, the joint strength was governed by the lower-strength CR3 sheet. The results highlight the importance of optimized welding parameters to balance sufficient nugget growth and minimize LME susceptibility in dissimilar advanced high-strength steel (AHSS)–mild steel combinations.
İmren Öztürk Yılmaz (Sun,) studied this question.