Friction stir lap welding of AA6061 aluminium alloy with a graphene interlayer

The present study aims to enhance the strength of friction stir lap welded aluminum alloys by using an interlayer of graphene nanoplatelets (GNPs) at the weld interface. With GNP interlayer, the weld strength and percentage elongation increased by 121 and 53%, respectively, as compared to the weld without GNP interlayer. The interlayer also changes the mode of fracture from brittle in the weld without GNP to ductile mode. Grain size in the weld with interlayer decreased by similar to 38% as equated to the weld without GNP. The height of the hook defect (HD) and cold lap defect (CLD) decreased by 26% and 41%, respectively, in the weld with GNP interlayer as compared to the weld without interlayer. In weld with interlayer, the bottom of the top plate on the retreating side acts as the potential site for fracture due to the presence of an interfacial defect, and undeformed GNP layer. The strengthening of the weld is attributed to various primary strengthening mechanisms like thermal mismatch, grain refinement, Orowan looping, and load transfer. Moreover, GNP interlayer also prevents the formation of the Al2O3 layer at the lap interface and thus contribute significantly in the weld strengthening.