| 6082-T6 aluminum alloy has the advantages of low density,high specific strength,good weldability,corrosion resistance and so on.It is an ideal material for lightening rail transit vehicles.Using friction stir welding technology to connect aluminum alloy can reduce the defect rate of welding structure,reduce the residual stress and improve the strength.Post-weld heat treatment will change the microstructure of welded joint,and then affect the mechanical properties and corrosion resistance of the joint.Therefore,it is of great theoretical significance and engineering application value to study the effect of post-weld solution and aging treatment on the structure and properties of joints.In this paper,friction stir welding of 6082-T6 aluminum alloy with 5mm thickness has been carried out with welding speed of 1000mm/min and spindle speed of 1400r/min.The welded joint was heat treated（550℃×2h solid solution+170℃×10h aging）,and the microstructure,mechanical properties and corrosion resistance of welded joint and heat treated joint（welding zone,thermomechanical affected zone,heat-affected zone,base metal）were compared and analyzed.The microstructure of the joint was analyzed by optical microscope,scanning electron microscope and energy spectrometer.The microstructure of the weld nugget is fine equiaxed grain,the grain of the HAZ is partly grown,and the deformation degree of the grain in the TMAZ is weakened.Heat treatment changed the number and distribution of precipitates.The precipitates were mainly Mg2Si particles.The mechanical properties of welded joints were studied by means of microhardness,tensile and impact tests.The tensile and impact fracture morphology was observed by SEM.The hardness of FSW joint increases by about 5～10HV after heat treatment,and the hardness of TMAZ and HAZ increases slightly compared with that of the welding state,but the hardness of base metal has little change.The tensile strength of FSW joint is 250MPa,reaching 77%of the base metal and the elongation is 8%.After heat treatment,the tensile strength increases by 20MPa and the elongation decreases by 2.2%.The impact energy of the joint is 4.7J,2.35 times of that of the base metal.After heat treatment,the impact energy of the joint increases2.4J.The tensile fracture and impact fracture of the welded and heat-treated joints show ductile fracture characteristics.After aging treatment,the number of precipitates in the matrix increases and the distribution of precipitates is fine and dispersive,which improves the mechanical properties of the joint.The pitting corrosion and intergranular corrosion of the joint were studied by immersion test and electrochemical test.The corrosion morphology was observed by SEM,and the corrosion products were analyzed by EDS.Pitting corrosion resistance of each region of the joint:weld nugget zone>retreating side heat mechanical affected zone>advincing side heat mechanica affected zone>heat affected zone>base metal;intergranular corrosion resistance of each region of the joint:base metal>heat affected zone>retreating side heat mechanical affected zone>advincing side heat mechanica affected zone>weld nugget zone.After heat treatment,the pitting corrosion and intergranular corrosion degree of each region are the same as that of the welding state,and the corrosion resistance is improved.Solid-solution aging makes the precipitates present homogenization and fine dispersion distribution,which reduces the tendency to form corrosion microcells. |
