چكيده به لاتين
QP steels exhibit a remarkable combination of strength and flexibility, making them highly attractive for the automotive industry. However, the primary challenge associated with these steels is their relatively limited weldability. Therefore, further studies are necessary to enhance their weldability. In the present study, the effect of welding current intensity on the microstructure and tensile properties of resistance spot welded zinc-coated high-strength steel was investigated. For this purpose, a 1.5 mm thick QP1180 steel sheet was used. The sheets were cut to specified dimensions according to the AWS D8.9 standard. The cut sheets were then welded under 16 cycles and an electrode force of 4.5 bar at currents of 6, 6.5, 7.5, and 8.2 kA. To examine the microstructural and tensile properties, a scanning electron microscope (SEM), elemental analysis, and tensile testing at a rate of 1 mm/min were employed. The results of the study showed that with the increase in welding current from 6 to 6.5, 7.5, and 8.2 kA, the nugget diameter increased from 6.19 to 6.43, 6.67, and 7.26 mm, respectively. Liquid metal embrittlement (LME) cracking was observed at a current of 6 kA; however, it disappeared when the welding current was increased to 6.5 kA. Subsequently, with a further increase in current to 7.5 and 8.2 kA, the crack length increased. The increase in nugget diameter and the absence of LME cracks resulted in the welded sample at 6.5 kA exhibiting the highest strength (23.85 kN) compared to other samples. Following this, as the current increased to 7.5 kA, the strength decreased to 13.35 kN and then increased again to 18.12 kN at a current of 8.2 kA, while ductility was significantly reduced. Fracture surface analysis revealed that at low currents, brittle fracture was the dominant mechanism, whereas at 8.2 kA, it transitioned to ductile fracture. Additionally, the type of fracture was interfacial at currents of 6, 6.5, and 7.5 kA and changed to pull-out at 8.2 kA.
