Study On Fatigue Fracture Behavior Of Al/Mg Dissimilar Metal Joint By Friction Stir Lap Welding

At present,friction stir lap welding?FSLW?joints are widely used in transportation,aerospace,military and other fields.It is very important to evaluate the fatigue performance of lap joints because they inevitably bear cyclic load in actual service,which affects the service life of the workpiece.In this paper,FSLW experiments were carried out on dissimilar metals of Al and Mg to obtain lap joints without defects on the macroscopic surface and internal structure.The microstructure,type and distribution of intermetallic compounds,micro-texture and the distribution of the average Schmid factor of aluminum-magnesium dissimilar metal lap joints were characterized by optical microscopy,scanning electron microscopy and electron backscattered electron diffraction.Meanwhile,the microhardness distribution,shear properties,fatigue properties and fatigue crack propagation process of lap joints were tested and evaluated.The main conclusions are as follows:During the FSLW process,the average grain size of AZ31 Mg alloy is refined from32?m to 8?m,and the average grain size of 7055 Al alloy is refined from 28?m to 2?m due to the mixing effect of the tool and the extrusion effect of the shaft shoulder.Distribution of intermetallic compounds,such as Mg₁₇Al₁₂,Al₃Mg₂ and Al₂Cu,can be observed at the lap interface.The micro-texture of the Mg alloy side produced significant anisotropy and increased density.There was no significant change in the micro-texture of the Al alloy side.As a result of grain refinement,the average hardness of AZ31 Mg alloy was increased from 61 HV to 68 HV.Due to the solid solution of the second phase in the matrix,the average hardness of 7055 Al alloy decreased from 145HV to 108 HV.The shear fracture showed that all joints were broken at the Al and Mg lap interface.Moreover,when the higher hook defect is located on the forward side of the Mg alloy,the shear strength of the joint is as high as 5.91 kN.In the high-cycle fatigue test conditions,the fatigue limit of Al and Mg lap joint was 1.45 kN,and all the lap joints were cracked,expanded,and broken from the thermo-mechanical affected zone on the advancing side of the magnesium sheet.During the observation of the structure at the fatigue crack initiation,it was found that due to the combined effect of slip and environment,oxygen atoms were adsorbed on the slip step in the thermal-mechanical affected zone of Mg alloy after the slip,forming an oxidation film,and the crack initiation took place at the oxidation film.In the initial region of fatigue crack propagation,the crack propagation is mainly dominated by slip;in the steady propagation stage of fatigue crack,the propagation path of crack is mainly controlled by slip and twinning.The grain types changed with the crack propagation.The initial sub-grains is gradually transformed into recrystallized grains and deformed grains,and the deformed grains dominated in the final transient fault zone.