Investigation of the microstructure and mechanical behaviour of resistance spot-welded CR210 steel joints using graphene as an interlayer

The present work investigated the resistance spot weldability of 0.85 mm thick galvanised CR210 steel sheets employing graphene nanoplatelets (GNPs) as an interlayer. The GNPs were drop-casted on the steel surface and resistance spot welding (RSW) was carried out at optimum welding current range of 6-9 kA with a constant weld time of 0.7 s. The lap-shear and cross-tensile tests were conducted to assess the mechanical behaviour of the weldments obtained with GNPs as an interlayer. An enhancement of -124% in the lap shear strength was observed in the specimen welded at 9 kA. However, the incorporation of GNPs led to the deterioration of the cross-tensile strength of the welded joints. The analysis of the different zones formed after welding was done using SEM and EBSD techniques which revealed the martensitic structure and finer grains obtained in the fusion zone. The presence of dislocation pileups, nano-precipitates, and interfacial shear stress transfer at the GNP-Fe interface was observed by TEM. Raman spectroscopy was carried out to get an insight into the stresses generated in the GNPs owing to RSW at different welding currents. Fracture surface analysis of lap shear specimens revealed the presence of shear dimples at the nugget zone welded at 7 kA and a mixed mode of fracture was observed in the specimen welded at 8 kA. However, a complete nugget pull-out was consistently found during the lap shear test of the specimen welded at 9 kA. Microhardness study revealed an increase in the fusion zone hardness at different welding parameters owing to the existence of GNPs and martensitic structure.