Driving the fracture mode transition in automotive dissimilar spot Welds: A study on DP590/HSLA440 joints

In this study, the mechanical properties of dissimilar resistance spot welds between DP590 and HSLA440 steel sheets were investigated, with a focus on heat-induced effects and geometric parameters. For this purpose, the steel sheets were prepared in accordance with the ISO 18278-2 standard. After welding at current intensities ranging from 6 to 11 kA, tensile shear tests were conducted at a strain rate of 1 mm/min. The results showed that increasing the welding current led to an increase in both the weld nugget diameter and the indentation depth. Specifically, when the welding current increased from 7 to 11 kA, the weld nugget diameter grew from 5.94 to 9.27 mm, and the electrode indentation depth increased from about 0.1 to 0.34 mm. Tensile test results indicated that the peak load increased from 16.67 to 23.83 kN, while the displacement at peak load rose from 3.44 to 8.86 mm as the welding current increased from 8 to 10 kA. The investigation of fracture mode variations in the welded samples showed that fractures at a welding current of 8 kA were of the interfacial fracture (IF), whereas those at 9 and 10 kA were of the pull-out fracture (PF). Furthermore, at both 9 and 10 kA welding currents, pull-out fractures occurred in the heat-affected zone (HAZ) of the HSLA440 steel, leading to an increase in the energy required for fracture.