Research On Microstructure And Mechanical Properties Of Friction Stir Welded Joints For 7075 High Strength Aluminum Alloy

7xxx series aluminum alloys have great application potential in the lightweight of automobiles because of their advantages of low density,high specific strength,good heat treatment strengthening effect.7xxx series high-strength aluminum alloys have poor welding performance and are called difficult-to-weld aluminum alloys.When welding using traditional fusion welding technology,the joints are prone to problems such as alumina slag inclusions,hydrogen holes,thermal cracks,alloy element burning,severe joint softening,and large workpiece deformation.Friction stir welding?FSW?is a solid-state welding technology that can avoid the occurrence of defects such as pores,cracks and slag inclusions,and provides an effective means for solving the welding problems of 7xxx series high-strength aluminum alloys.However,currently the strength of 7xxx series aluminum alloy FSW joints is generally not high,the joint strength coefficient is between 66%and 90%,and further improvement of the joint strength coefficient to more than 90%through process optimization still requires in-depth research.In this paper,the friction stir butt welding of 7075-T6 high-strength aluminum alloy sheet with a thickness of 3 mm was studied.The microstructure evolution and mechanical properties of the joint were studied.The relationship between the process and the microstructure characteristics and mechanical properties was established,and further improved the strength coefficient of FSW joints by post-weld heat treatment.Friction stir spot welding?FSSW?process was preliminarily explored,and the influence of friction stir spot welding process parameters on the microstructure and mechanical properties of the joints was analyzed.Friction stir welding test shows that through good pre-weld assembly and optimization of welding process parameters,welding defects can be eliminated and the FSW joint with smooth surface and excellent performance can be obtained.The recrystallized grain size in the weld nugget zone?WNZ?is much smaller than that of the base metal,and its grain size increases slightly with the increase of rotation speed and decreases gradually with the increase of welding speed.Compared with the base metal,the number of?'phase in the WNZ is reduced,and the insoluble?phase and other precipitated phases are stirred and broken.In the heat-affected zone?HAZ?,the??phase decreases,while the?phase increases,and there are a large number of coarse precipitated phases and a wide precipitate-free zone?PFZ?.The microhardness distribution of the cross section of the FSW joint shows a"W"shape,and the lowest value of hardness appears in the HAZ.The tensile strength of the FSW joint increases with the increase of the rotation speed,and increases first and then decreases with the increase of the welding speed.When the rotation speed is 1500 r/min and the welding speed is 120mm/min?1500-120?,the maximum tensile strength of the joint is obtained,with a value of441.2 MPa,and the strength coefficient is 80.0%.The post-weld heat treatment test shows that after direct artificial aging treatment of the FSW joint,no grain growth occurs in the WNZ,the width of the PFZ in the HAZ is reduced,and the small??phase dissolved in the original welding process re-precipitate,dispersed in the aluminum matrix.After T6 treatment,the grains in the WNZ grow abnormally,and the precipitate completely dissolves first,and then re-precipitates into a larger number of uniformly-sized disc-shaped precipitates and fine spot-like strengthening phases,distributed in the aluminum matrix.After direct artificial aging and T6 treatment,the tensile strength of the joint is improved,but the elongation is decreased.Among them,the tensile strength of the1500-60 joint treated by T6 is the best.The joint strength coefficient reaches 98.7%,and the elongation is 2.3%.Tensile fracture morphology analysis shows that the FSW joint after the artificial aging treatment is ductile-brittle mixed fracture,and the FSW joint treated with T6 is brittle fracture.The FSSW test of 7075 aluminum alloy shows that with the increase of the penetration depth,the distortion height?H?and the distortion angle???of the FSSW joint decrease first and then increase,and the width?L₂?of the deformation zone increases first and then decreases.When the depth is 5.1 mm,the width of effective bonding zone?L₁?and the width of deformation zone?L₂?are the largest,the distortion height?H?and the distortion angle???are the smallest.The metallurgical bonding area of the joint is the largest,and the tensile-shear force of the joint is also the highest,with a value of 12.44 KN.When the FSSW joint is subjected to shear force in the tensile test,a ductile-brittle mixed fracture occurs.