Improving mechanical properties of welds through tailoring microstructure characteristics and fracture mechanism in multi-pulse resistance spot welding of Q&P980 steel

In the present research, the multi-pulse resistance spot welding involving three thermal cycles has been carried out on the Q&P980 steel sheet with the thickness of 1.8 mm. The influence of welding pulses on the microstructure evolution and mechanical properties of the welded specimens was analyzed through EPMA, SEM, TEM and tensile test, etc. It is found that there is a crack sensitive area in single and double-pulse samples, which is corresponding to the broadening of the partially melted zone with severe element segregation. Furthermore, the softened of nugget associated with microstructure coarsening on the outside of the re-melting zone can be observed during multi-pulse welding. As the nugget size increases, the enlarged connection/joint area between the sheets significantly enhances the ability of the nugget to resist tensile shear stress, leading to the increase of tensile-shear strength. Due to the crack propagating away from the partially melted zone into the softened nugget, where the tempered martensite provides a higher resistance to crack initiation and propagation, the cross-tension properties have been improved by applying the tempering pulse with low-level current. The multi-pulse welding samples show a better combination of tensile-shear and cross-tension properties, i.e. their peak load of cross tension (13.19 kN) is increased by 160% and 56% respectively, while the energy absorption (108.49 J) is raised by 390% and 107% respectively, compared with the single and double pulse samples.