Strengthening and toughening mechanisms in refilled friction stir spot welding of AA2014 aluminum alloy reinforced by graphene nanosheets

Refilled friction stir spot welding is proposed to replace friction stir spot welding to solve the problem of keyhole in joining of light metals. However, it is rather challenging to ensure the mechanical property of refilled friction stir spot welding joints because of hook defect. In this study, a novel method was proposed to use graphene nanosheets to strengthen the tip of hook defect and improve overall mechanical properties of joint. The joints of AA2014 aluminum alloy with 0.6%wt graphene nanosheets were fabricated by refilled friction stir spot welding. The results show that the tensile/shear strength of joint increases by 31% and the fracture toughness also is improved by nearly 20% with graphene nanosheets. Extension in fatigue life was achieved on joint with graphene nanosheets. Microstructural observations, fracture characteristics and electron back scattered diffraction analysis were systematically investigated to clarify the mechanism for improvement of strength and toughness. The synergetic effect including grain refinement, load transfer, dislocation strengthening and crack bridging, caused by graphene nanosheets, leads to hardening behavior of graphene nanosheets pinned zone and a significant crack deflection during fracture process, which contributes greatly to improving strength and toughness of joint. Keywords: Refilled friction stir spot welding, Graphene nanosheets, Strengthening and toughening mechanism, Hook defect, AA2014 aluminum alloy