Study On The Tool Design,Joint Microstructure And Propertise Of Bobbin Tool Friction Stir Welded Aluminum Alloy

As the expansion of the field of friction stir welding technology,Bobbin Tool Friction Stir Welding(BTFSW)use another bottom shoulder instead of anvil plate,This technology not only significantly reduces the process loads in vertical direction,but also decrease the requirements of rigid clamping and the cost of fixture tools.moreover it eliminate the root defect problems.Therefore,it has high application value in engineering practice.4mm thick 6061-T6 Aluminum Alloy was bobbin tool friction stir welded by using different morphology of shoulder and pin with the aim of the effect of tool morphology and welding parameters on the formation and tensile strength of joint.The focus is placed on the characteristics of each region,summarizes the types of defects and the influence of the defects on the tensile strength of the joints.The main research results are as follows:Under the condition of the bobbin tool with a flat shoulder and a smooth cylindrical pin,with the plunge depth of up shoulder increase,the flash on the top plate is first decreased and then increased,and the flash on the bottom plate is increased.The scale-like ripple is appeared and become more uniform with the roating speed is increased.As the welding speed is increased,the flash on the retreating side of top plate is increase,and the scale-like ripple on both plate become more uniform.Based on the microstructual characteriration of the joint of bobbin tool friction stir welding,four distinct zone have been identified: base metal,heat affected zone,thermo-mechanical affected zone,weld nugget zone.The weld nugget zone is charactered by fine,equiaxed gains structure,plastic deformed gains is observd in thermo-mechanical affected zone and the gains in heat affected zone is coarsening.Through orthogonal test,we found that the order of the influence of each factor on the macro morphology of the joint was: the shape of the shoulder,the rotating speed,the piung depth of the shoulder,the shape of the pin and the welding speed.the order of the influence of each factor on the tensile strength of the joint was: the plung depth of the shoulder and the shape of the shoulder,the shape of the pin,the rotation speed and the welding speed.Defect types easy to appeared in joint was: void defects,wea-link,S line defect;the void defects often be found in the border of nugget zone and thermo-mechanically affected zone in the advancing side and the nugget zone near the retreating side.wea-link was appeared in the border of nugget zone and thermo-mechanically affected zone in the advancing side,S line defect was observed within the nugget zone near the retreating side.Compared with each joint which obtained S line defect showed that the S line defect region metallurgical bonding degree is different,if this regional metallurgical combination is less,it may affect the tensile strength of the joints,if this regional metallurgical bonding degree is good,it has no effect on tensile strength of joints.When the both shoulder were characterized by a combination of cavity and scroll,the pin surface with a thread and three flat,and the plung depth of the up shoulder is 0.05 mm,the plung depth of the bottom shoulder is 0.15 mm,the welding speed is 120mm·min-1,the rotation speed is 520r·min-1,the tensile strength of the joint can reach 230.36 MPa.