Investigation of microstructure and mechanical properties of dissimilar electrical resistance spot welded TBF/DP600 steel sheets

Effects of welding parameters on microstructure and mechanical properties of spot-welded TBF/DP600 sheets were investigated. TBF was welded with and without electro-galvanizing. Microhardness and tensile tests were carried out, and microstructure characterized by optical microscopy. Nugget size was determined by image processing, indentation was measured by ultrasonic technique. Steels were not completely mixed with each other in FZ. Increasing in welding current and time expanded HAZ and increased mixing-ratio in FZ, nugget size, indentation and load capacity. TBF was more affected by thermal cycle than DP600. LME-based microcracks in HAZ of galvanized TBF at high heat input were observed, while corrosion started rapidly in weld zone of ungalvanized TBF. Highest hardness was observed in HAZ on TBF side. However, significant softening occurred in transition zone between HAZ and base metal on TBF side. The nugget size, indentation and load capacity were higher in galvanized TBF group. In dissimilar spot-welded TBF/DP600 sheets, DP600 has determined weld strength. Fractures are of fusion zone button type with higher weld strength in high heat input, while fractures are of partial fusion zone button type with lower weld strength in low heat input. In galvanized TBF, higher weld strength was obtained in lower heat input.