Study of the microstructure, mechanical properties and forming behaviour of graphene-interlayered resistance spot-welded patch blanks of dissimilar ultra-low carbon cold-rolled and dual-phase steel sheets

The present study aims to investigate the role of graphene nanoplatelets (GNPs) coating as an interlayer during resistance spot welding (RSW) for the fabrication of patch blanks of dissimilar cold-rolled (CR-210) / dual phase (DP-780) steel combination. The desired nugget pull-out failure from the weaker CR-210 steel side of the weldments in lap shear tests was observed at a 9 kA welding current. Also, a joint strength improvement of similar to 36 % in the lap shear test was obtained at this welding current owing to the introduction of GNPs as an interlayer. The microstructural characterisation of the cross-sectioned sample revealed the grain refinement in fusion zone and existence of lath martensite. The dislocation pile-ups at grain boundaries accompanied by the formation of nano-precipitates were observed during transmission electron microscopy (TEM) because of the addition of GNPs. The interfacial shear load transfer from the matrix to the reinforcement was also noticed for the GNP-coated weldment during TEM analysis, which led to the increased joint strength during the lap shear test. Further, stretch forming of the patch blanks indicated that the maximum strain occurred at the thinner sheet away from the spot welds. However, the incorporation of the GNPs as an interlayer had a marginal effect on the formability of the patch blanks. Based on these findings, it was inferred that the patch blanks of dissimilar CR210 and DP-780 steel sheets with GNPs as an interlayer can be utilised for the auto body components for localised strengthening to avoid failure.