Influences of graphene nanoplatelet addition and pin lengths on the microstructure and mechanical properties of 7075 aluminum alloy under friction stir spot welding

Abstract

This study investigated the impact of graphene nanoplatelets (GNPs) and pin lengths on the mechanical properties and microstructure of AA7075 Al alloy in friction stir spot welded (FSSW) joints. FSSW was conducted using varying volumes of GNPs (5, 10, 20, and 30 mm3) and different pin lengths (3.5 and 4 mm) with 1250 rpm of rotational speed and 10 s of dwell time. The results of this study indicated that the addition of GNPs led to significant grain refinement in the stir zone (SZ). An increase in GNPs volume from 5 to 30 mm3 resulted in substantial grain refinement during FSSW. This outcome was obtained from the pinning effect generated by nano-sized GNPs. This effect successfully hindered grain growth, followed by recrystallization during FSSW. Furthermore, the grain size grows as the pin length increases. The results of tensile lap shear tests revealed that 3.5 mm pin length and 10 mm3 volume of GNPs are the best welding conditions for the fabrication of the best-quality welds. The increase in the volume of GNPs of more than 10 mm3 induced a negative impact on the tensile shear strength due to the agglomeration of the GNPs. The tensile shear strength of the joints with 10 mm3 GNPs and pin lengths of 3.5 and 4 mm exhibited remarkable increases of 52.6% and 41.2%, respectively, compared with that of those without GNPs. Moreover, the microhardness of the SZ of the joints improved by increasing the addition of GNPs volume.

Keywords

• FSSW
• GNPs
• Graphene
• Pin lengths
• AA7075
• Mechanical properties