Study On Microstructure And Fatigue Crack Propagation Behaviour Of Friction Stir Welding(FSW) Joint Of 6061-T6 Aluminum Alloy

The wide use of aluminum alloy is one of the effective measures to realize the lightweight of automobiles.Welding is an important manufacturing process of automobile,and fatigue failure is an important problem in joint service.Therefore,in this paper,the friction stir welding?FSW?joints of6061-T6 aluminum alloy was studied,and the microstructure and microhardness of different welding areas were characterized by means of optical microscope,scanning electron microscope,transmission electron microscope and vickers hardness tester.The distribution of welding residual stress was analyzed by X-ray diffractometer,the impact properties of different welding areas were studied by charpy impact test,the fatigue crack propagation behavior and failure mechanism of joints was studied by fatigue crack propagation rate test and fracture observation.The main conclusions are as follows:?1?The grain size of the weld nugget zone?¹0?m?is smaller than that of the heat-affected zone?²3?m?and the base metal?²7?m?,the nugget zone was of fine equiaxed crystal and the volume fraction of grain boundary was the largest,the“S”defect appears in the weld nugget zone due to low heat input.The average grain boundary misorientation angle was the highest and the low angle grain boundary fraction was the lowest in the weld nugget zone.The content of precipitate of base metal was the highest,and the size of precipitate was the smallest,and the dislocation density was the highest.?2?The weld nugget zone is the weakest position of the joint,the lowest hardness and tensile fracture are located in the center of the weld nugget zone.Both the base metal and FSW joints showed continuous yielding phenomenon during the process of tensile test,but the tensile properties after welding changed significantly,the joint coefficient reached 72.4%,indicated that the welding quality was still good,the delamination occurs in the tensile fracture due to the low heat input during welding.The impact absorption energy of the base metal?3.709 J?is slightly lower than that of the heat affected zone?3.973 J?,and the impact absorption energy of the weld nugget zone is the lowest?2.594 J?.There is obvious delamination of the impact fracture in the weld nugget zone also.The transverse residual stress in different region of the joint were smaller than the longitudinal,the transverse and longitudinal residual stresses in the weld zone were compressive stresses,the longitudinal residual stresses in the weld nugget zone were smaller than the base metal and heat affected zone.?3?When the crack length a=18 mm in the weld nugget zone,the crack closure occured,and the a-N curve of the weld nugget zone was deflected obviously at the same times,and the fracture shows two completely different morphology.The crack growth rate in the weld nugget zone was the lowest due to the residual compressive stress and grain boundary when?35?K was small,and the crack propagation rate in the weld nugget zone was the lowest,the crack propagation was hindered by precipitated particles and dislocation in the base metal when?35?K>15.69 MPa?m^1/2,in addition,the low hardness and“S”defect in the weld nugget zone were conducive to accelerating crack growth,so the crack growth rate in the weld nugget zone becomes the highest.The typical fatigue fracture morphology of the fatigue crack stable growth zone were fatigue striations and secondary cracks mainly,the rapid growth zone fracture distributed some dimples,and the unstable fracture zone fractures was dimpels dominate.