Research On Mechanical Performance Of RFSSW Joints Of 2A14 Aluminum Alloy Strengthened By GNSs

2A14 aluminum alloy is a kind of hard aluminum alloy with good machinability,spot welding and seam welding performance.It is widely used in the manufacture of precision molds and important components of aerospace.In the area of aluminum alloy processing,welding is one of the most commonly used connection methods.Especially in aerospace aluminum components,fusion welding usually remains a great challenge because of solidification defects.Therefore,developing new spot welding connection technology for aluminum alloy has great significance for expanding the application of aluminum alloy,promoting the lightweight development of automobiles,and improving the aerospace and aerospace carrying capacity.In this context,friction stir spot welding is invented as a new welding method to replace resistance spot welding and riveting.This method uses the heat generated by the high-speed rotating welding tool to melt the material,thereby achieving an effective combination.As a new kind of friction stir spot welding,refilled friction stir spot welding can eliminate keyhole defects,which has attracted widespread attention.However,some studies have shown that the failure of refilled friction stir spot welding joint often results from hook defect in the joint.Therefore,research about reducing the impact of hook defect has important guiding significance for and improving the mechanical properties of spot welding joints.Hook defect is an unavoidable defect in lap joints.The formation of hook defect originates at the interface between the two welded sheets.During plunge process,the sleeve moves downwards while the pin moves upward to contain the material squeezed by the sleeve.Then the sleeve retracts upward while the pin moves downward to push the materials to refill the cavity created by the sleeve.In the retract process,materials around the sleeve will flow upwards when the sleeve retracts to form hook defect.So it is impossible to completely avoid the occurrence of hook defects.In this work,we proposed an innovative method to improve overall mechanical properties of RFSSW joint by adding GNSs to strengthen the tip of hook defect.The results show that the tensile/shear strength of RFSSW joints increases by 31% and the fracture toughness also gets improved by nearly 20% with addition of GNSs.Meanwhile,fatigue lifetime obtains significant extension on joint with graphene nanosheets.Strengthening and toughening mechanisms were discussed on the basis of microstructural observations,load-displacement curves,fracture characteristics and EBSD analysis.The synergetic effect including grain refinement,load transfer,dislocation strengthening and crack bridging,caused by high strength and surface-to-volume ratios of graphene nanosheets,leads to hardening behavior of graphene nanosheets pinned zone and a significant crack deflection during fracture process,which contribute greatly to improving strength and toughness of joint.