PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART D-JOURNAL OF AUTOMOBILE ENGINEERING, vol.229, no.5, pp.599-610, 2015 (SCI-Expanded)
A study was conducted to investigate the microstructure and the mechanical properties of dissimilar resistance spot welds between steel sheets of DP600 and DP1000 using different welding currents for automotive applications. Microstructural characterization, microhardness tests and tensile shear tests of the joints were conducted. The dissimilar joints had three main microstructural zones: the base metals; the heat-affected zones of both steels; the fusion zone. Additionally, the DP1000 side of the joints also had a transition zone between the heat-affected zone and the base metal, which exhibited significant softening. Such a soft transition zone was not observed on the DP600 side. The peak hardness values were observed in the centre heat-affected zones of both sides of the joints. The welding current significantly influenced the microstructure and the mechanical properties of the joints. With increasing welding current, the nugget size increased almost linearly and the widths of the heat-affected zones on both sides became smaller. The use of higher welding currents reduced the microhardness values in the outer heat-affected zones of both sides and the fusion zone of the joints; the hardness values in these zones decreased almost linearly with increasing welding current. For a welding current of up to 10 kA, the tensile shear loads of the joints increased linearly with increasing welding current. The peak values of the tensile shear load and tensile deformation were obtained for the joints welded with a current of 10 kA. All joints exhibited the full button pull-out failure mode under the tensile shear loading test. The failure location of all joints was in the centre heat-affected zone. The joints welded with relatively lower welding currents (8-9.5 kA) failed on the DP1000 side, while the failure of the other joints fabricated with relatively higher welding currents (10-11 kA) was located on the DP600 side of the weld.