Microstructure and mechanical properties of dissimilar resistance spot welds of tbf and hsla300 steel sheets

Abstract

The welding behavior of the resistance spot welded dissimilar TBF-HSLA300 steel joints under different welding parameters was investigated. TBF steels were used as galvanized or ungalvanized. Microstructural characterization and mechanical tests were performed. Higher heat input resulted in the higher possibility of shrinkage cavities at the weld nugget and also increased the risk of liquid metal embrittlement (LEM) crack in the heat affected zone (HAZ) on galvanized TBF steel sheets. A partial melting zone was observed on HSLA300 side. The welds with ungalvanized TBF sheets had the higher nugget size and indentation. The highest hardness values (over 500 HV0.1) were observed in HAZ on TBF side. The lowest hardness values (below 300 HV0.1) on TBF side were observed in a very narrow softening zone between tempered zone and base metal. The fractures of all spot welds in the tensile shear test occurred on the TBF steel side with the thinner thickness. Low-strength steel sheet have to be sufficiently thicker in order to achieve higher welding strength. Higher welding time led to higher weld strength in the ungalvanized TBF combination. In the galvanized TBF combination, higher heat input led to the drastic decrease in weld strength due to LEM cracks.