Microstructure Heterogeneity In The Stir Zone Of Large Thickness 6082 Aluminum Alloy Joint Double-side Welded By FSW

6082 aluminum alloy has a good overall performance,is widely used in high-speed trains,subway and other rail transportation vehicles.China’s current aluminum alloy body is still the main MIG welding.However,using MIG welding method for large thickness aluminum alloy welding is more prone to stomata,welding deformation and welding hot cracking,joint softening serious.CoMPared with traditional fusion welding,friction stir welding(FSW)is a new kind of solid phase joining technology,which has many advantages in welding large thickness aluminum alloy,especially welding efficiency,welding heat input and joint,efficiency.In recent years,friction stir welding technology has been engineering applications used in railway vehicle aluminum alloy sheet welding,but in the aluminum alloy plate welding has not yet implemented batch engineering applications.In this study,40mm 6082-T6 aluminum alloy in coupler seat were welded by double-side friction stir welding.The evolution of the microstructure and properties of the welded joints in the thickness direction was studied systematically.Which provides the experimental basis for the application of friction stir welding in aluminum plate welding.In this study,the microstructural characterization and macroscopic and microscopic mechanical properties were used to study the inhomogeneity of microstructure and mechanical properties of FSW joints in 40mm thick 6082-T6 aluminum alloy.The results of microstructure study show that the a-Al matrix in the base material is fibrous,the texture type is mainly Brass texture {112}<111>,and there are a large number of fine needle-like β" phases in the intragranular.The width of the precipitates free zone is about 50nm.Stir zone is equiaxed grain organization,which due to the process of welding experienced a severe plastic deformation and temperature rise and then caused the dynamic recrystallization,the texture types of the grains are {111}<uvw>fiber texture.The grain size decreases from the top surface to the center because the weld metal,which along the thickness experienced high and low temperature gradient and the different degree of plastic deformation.The second phase distributed in the stirring zone also has obvious inhomogeneity along the thickness direction.From the top surface to the center layer,the number of β’ phase is increased and the number ofβ" phase is reduced.The size of β" and β’ phase are coarse.And the results of the micro hardness test along the thickness direction are in agreement with it.The thermal effect of the lower bead on the upper bead is the main cause of this phenomenon.Due to mechanical deformation and thermal cycling in the heat affected zone,the deformation of the grain along the plastic rheological direction of metal occurs and the dynamic recovery occurs,the texture types of the grains are {111}<uvw>fiber texture.The grain in heat affected zone became coarsen due to the role of welding heat cycle,the grains are mainly Cube texture ｛001}<110>.The influence of temperature is the main reason for the transformation from β"phase to β ’phase,and the softening zone is formed in the heat affected zone,which cause the mechanical properties deteriorate.6082 aluminum alloy is heat-treated aluminum alloy,the mechanical properties depend on the morphology and distribution of the enhanced particles.The micro hardness distribution shows that the hardness of the base metal region is about 120 HV,and the hardness decreases with the distance from the center of the weld.The softening zone appears in the heat affected zone near the TMAZ/HAZ interface,the hardness is about 60HV,the softening zone is "X" shape in the cross section of the joint,the hardness in the upper and lower welding overlap area for the entire joint is lowest.Unlike overlapping regions located in the center layer,the hardness in other areas of the stir zone increased to about 80HV,and the hardness of the lower stir zone is higher than that of the upper stir zone,this phenomenon is consistent with the results of particle morphology and distribution in the stir zone.Similar to the distribution of micro hardness,the tensile strength of longitudinal tensile test specimen in the stir zone is higher than that of the upper stir zone,The overlap area of the upper and lower welding has the lowest tensile strength.The transverse tensile tensile strength of the joint fluctuates in the range of 196-221MPa,the fracture sites were located in the softening zone of the heat affected zone on the advancing side of the joint,and is consistent to the location of the softening zone determined by the micro hardness distribution.After T6 heat treatment,the tensile strength of the transverse tensile test specimen is obviously increased from 277 to 322 MPa.From the upper surface of the welding to the center layer,layered transverse tensile strength tensile specimens gradually increased from the central layer to the lower surface of the weld.From the top surface to the center layer,the tensile strength of the transverse tensile test specimen gradually increased from the center layer to the lower welding surface,and the tensile strength of the transverse tensile test specimen gradually decreased,the tensile strength of the lower transverse tensile test specimen is obviously higher than that of the upper layer.In addition to the transverse delaminating tensile specimens taken from the center layer gauge overlay weld bead area,the remaining laterally delaminated specimens showed a tendency to fracture along the "Z" line.