Introduction and Objectives: In this study, the microstructural and mechanical properties of dissimilar resistance spot welds between DP590 and HSLA440 steels were investigated, with a focus on the effect of welding current.
Materials and Methods: For this purpose, steel sheets were prepared in accordance with AWS D1.1 standard. Welding was performed using currents ranging from 7 to 11 kA (in 1 kA increments), followed by mechanical testing and characterization. Tensile shear tests were conducted at a crosshead speed of 1 mm/min, and hardness tests were carried out for the 8 and 10 kA welds. Furthermore, fracture surface and weld microstructure analyses were performed using optical and scanning electron microscopy.
Results: Weld nugget was mainly consisted of lath martensite; its volume fraction increased with current and decreased toward base metal. In DP590, the supercritical and intercritical regions were martensitic and the subcritical region was tempered. In HSLA440, the supercritical regions showed martensite, the intercritical regions showed a combination of martensite and ferrite, and the subcritical region showed grain growth.Tensile strength enhanced from 10.84 kN (for 7 kA) to 24.34 kN (for 10 kA). Fracture mode shifted from interfacial to pull-out above 9 kA. Hardness increased with current, peaking at 430 HV (10 kA).
Conclusion: An increase in welding current caused to increase in nugget size and peak load. Also, softening was more pronounced on the HSLA440 side. The sample welded at a current of 10 kA with a tensile failure mode and maximum strength, hardness, and elongation was the optimal sample.