Study On Microstructures And Properties Of Bobbin Tool Friction Stir Welding Joints Of 6082-T6 Aluminum Alloy

With the rapid development of national economy and rail transit manufacturing industry,lightweight materials such as aluminum alloy are more and more widely used in rail transit vehicle manufacturing.Friction stir welding?FSW?is a new solid phase bonding technology,which is helpful to prevent metallurgical weldability problems in aluminum alloy fusion welding,so it is widely used in the manufacturing of rail vehicles.However,due to applying the pressure on the upper surface of the workpiece and the necessary rigid support on the workpiece back during FSW,the joining of special structures such as hollow structure and closed profile cannot be realized,which limits the application scope of FSW.The bobbin tool friction stir welding?BT-FSW?is an extension technology of FSW.It uses the tool with top and bottom shoulders,and the bottom shoulder replaces the rigid support on the back of FSW workpiece,so it is beneficial to solve the above problems for FSW.But so far,there are relatively few reports about BT-FSW.Therefore,it is of great scientific significance and engineering application value to study the microstructures and properties of BT-FSW joints of 6082-T6 aluminum alloy for rail transit vehicles.In this paper,the characteristics of thermal cycle,microstructures and mechanical properties of BT-FSW joints of 6082-T6 aluminum alloy,the influences of welding parameters and postweld aging treatment on microstructures and mechanical properties of the joints,the microstructures and mechanical properties of the underwater BT-FSW joints,and the finite element analysis of the thermal process of BT-FSW were systematically studied.The results showed that different zones of the BT-FSW joints experienced different thermal cycles.As the distance from the weld center increased,the peak temperature and the cooling rate of the thermal cycle decreased,and the peak temperature of the retreating side?RS?in the joints was higher than that of the advancing side?AS?.It is mainly attributed to the transfer of plastic metal from AS to RS accompanying the transfer of heat and the large deformation amount and strain rate of RS metal generating more plastic deformation heat.BT-FSW joints can be divided into base metal?BM?,stir zone?SZ?,thermo mechanical affected zone?TMAZ?and heat affected zone?HAZ?.Obvious"S-line"and gray white strip structure can be seen in SZ.BM is mainly composed of striated?-Al grains and nano needle-like?''phase??-Al+?''?.SZ underwent a strong friction thermo-mechanical stir?thermo-mechanical?coupling and dynamic recrystallization,which significantly refined the?-Al grain and formed a small amount of GP zones in the grain??-Al+GP zones?.Compared with SZ,the thermal-mechanical coupling effect of TMAZ was reduced,which makes the striated?-Al grains bend in a certain direction and form a small amount of GP zones??-Al+GP zones?.Under the effect of thermal cycle,the?-Al grains of HAZ tended to coarsen,and the phase transformation of?''??'+Q'occurred,therefore HAZ microstructures with?-Al+?''+?'+Q'were formed.As the distance from the weld center increased,the peak temperature of HAZ decreased,the?'and Q'phases decreased,and the?''phase increased.The formation of"S-line"and gray white banded structure in SZ is mainly related to the interface segregation of?-Al?Fe Mn?Si and Al₂O₃ particles flowing along the plastic metal.Based on above results,the formation mechanism and microstructure evolution mechanism of BT-FSW joints of 6082-T6 aluminum alloy were proposed in this paper.The micro-hardness distribution of BT-FSW joints was W-shape.The hardness of BM was significantly increased due to the precipitation strengthening of?'';the hardness of SZ was higher than that of TMAZ due to the obvious refinement of?-Al grain;the minimum hardness of joints appeared in the HAZ near to TMAZ,which was mainly related to the precipitation of?'and Q'and their coarsening in this area.Under the action of tensile stress,the joint was mainly fractured in the softening zone of HAZ,so it was the weakest zone in BT-FSW joints.The welding parameters?rotating speed and welding speed?and postweld aging treatment have significant effects on microstructures and mechanical properties of BT-FSW joints.As the rotating speed?welding speed?of the tool increased,the peak temperature of the thermal cycle tended to increase?decrease?.With the increase of the rotation speed from 600 r/min to 1200r/min,the grain size of SZ decreased?9.1?m-7.1?m?,the hardness of SZ increased?80 HV-94HV?,and the minimum hardness of HAZ decreased?71 HV-68 HV?.At the rotation speed of 600r/min,the bonding performance between SZ and TMAZ was affected due to weakening of thermal-mechanical coupling,which seriously deteriorated the tensile strength of BT-FSW joints?198 MPa?.When the rotation speed was increased to 800 r/min,the joint strength was increased.A further increase for the rotation speed?1000 r/min,1200 r/min?resulted in decreasing joint strength,which is mainly associated with the increased welding heat input promoting the growth and coarsening of?'and Q'in HAZ.With the increase of welding speed from 300 mm/min to700 mm/min,the grain size of SZ decreased?9.9?m-7.1?m?,the hardness of SZ increased?71HV-84 HV?,the minimum hardness of HAZ increased?66 HV-70 HV?,and the tensile strength of joints increased firstly and then decreased,its maximum value appearing at the welding speed of 500 mm/min.The mechanical properties of the joints can be improved by the appropriate combination of welding parameters?the rotating speed of the tool of 800 r/min and the welding speed of 500 mm/min?.The tensile strength of the joint is 263 MPa,which is 81%of BM strength?323 MPa?.On this basis,the effects of postweld aging treatment on BT-FSW joints were studied.The results showed that after natural aging treatment?PWNA 60 d?,the GP zones of SZ increased,and the microhardness and tensile strength of the joints tended to increase.Compared with PWNA,the effects of postweld artificial aging?PWAA?on microstructures and properties of the joints were more obvious.After PWAA 180?/6 h,the phase transformation of SS?GP??"took place in SZ,the microhardness?110 HV-120 HV?of SZ and tensile strength?280 MPa?of joints increased significantly,and the joint strength reached 87%of the BM strength.The research of underwater BT-FSW of 6082-t6 aluminum alloy showed that compared with BT-FSW in atmospheric environment,the peak temperature of thermal cycle of underwater BT-FSW was significantly reduced and the cooling rate was significantly increased due to the strong cooling effect of water medium.Its grain size of SZ decreased?5.5?m?,the hardness increased?94 HV–103 HV?;the width of HAZ softening zone decreased,the microhardness increased,and the?'and Q'phases had refined tendency;the tensile strength of the joints was increased obviously?308 MPa?,which can reached 95.5%of the BM.Therefore,underwater BT-FSW process is more favorable for improving the mechanical properties of 6082-T6aluminum alloy joint.The heat production of BT-FSW process consists mainly of the friction heat between the shoulder and the contact surface of the workpiece,the friction heat between the surface of the pin and the workpiece,and the plastic deformation heat of the metal near the tool.During the welding,both shoulders can contribute 90.44%of the friction heat,which plays a leading role.According to the heat production model and heat conduction model,the temperature fields of the BT-FSW joint were simulated by using COMSOL finite element software.The simulation results show that the temperature field of BT-FSW is elliptical,and the temperature distribution in the middle of the weld is hourglass-shape.The rotating speed of the tool has a greater impact on the peak temperature,while the welding speed mainly affects the isotherm distribution of the temperature field.Compared with the atmospheric environment,the high temperature range of the underwater BT-FSW temperature field is significantly reduced.The temperature field characteristics of BT-FSW joints obtained by numerical simulation are in good agreement with the experimental results,which provides the important thermal basis for improving the microstructures and properties of the BT-FSW joints of 6082-T6 aluminum alloy.