Effect of graphene nanoplatelets reinforcement on the microstructure and mechanical properties of friction stir spot welded AA6061–AA7075 dissimilar joints

Aluminum alloys such as AA6061 and AA7075 are widely used in transportation and structural applications due to their exceptional toughness and corrosion resistant. However, conventional resistance spot welding of these alloys often causes porosity, cracking, and interfacial defects that degrade joint quality. The preferred method for producing high-quality dissimilar joints with these alloys is friction stir spot welding (FSSW) as a solid-state welding process. This study evaluated the effects of volume of GNPs (5, 10, 20, and 30 mm 3 ) addition into the FSSW joint area and pin lengths (3.5 and 4 mm) on the mechanical characteristics and microstructure of FSSW AA6061–AA7075 dissimilar joints. Graphene nanoplatelets (GNPs) was selected for their outstanding mechanical strength and grain-refining ability, which can improve interfacial bonding and hardness. Results indicated that the GNPs greatly affected grain size and thus the mechanical properties of the AA6061–AA7075 joints. Increasing the volume of GNPs in the joints increased the grain refinement effect during the FSSW. Moreover, an increase in pin length leads to growth in grain size. The lap shear strength of the joints containing 10 mm 3 of GNPs and pin lengths of 3.5 mm and 4 mm showed remarkable improvements of 43.9% and 31.8%, respectively, compared with those without GNPs. Increasing the volume of GNPs beyond 10 mm 3 adversely affected the interfacial bonding within the weld nugget, leading to reduced lap shear strength due to GNP agglomeration. Furthermore, increasing the volume of GNPs (30 mm 3 ) led to an enhancement in the microhardness of the SZ of the joints. The maximum SZ hardness value of approximately 93 and 200 HV for the upper and bottom plates, respectively.