Fatigue Performance Of Friction Stir Welded TC4 Alloy Joints

Owing to advantages of great specific strength,high specific stiffness,excellent resistance corrosion and so on,titanium alloys have widely applied in the field of aerospace.Therein,the applied amount of double-phase TC4 titanium alloy approximately reaches 80% of the total amount in the aircraft structure.The welding methods of TC4 alloy have been attracting much attention at home and abroad.However,conventional fusion welding can produce some defects such as hot crack,large deformation,coarse microstructure and so on.Therefore,Friction stir welding(FSW),a solid-state joining technology,has some advantages.In this paper,the microstructure and mechanical properties of 2mm thick TC4 alloy FSW joint were investigated.As a main failure mode of welding structure,the fatigue failure was manly discussed.Firstly,in order to improve the effect of gas shielding and obtain the small microstructure of stir zone(SZ),low temperature welding method below the ?-transus temperature was adopted in this paper.The experimental results show that low temperature welding method is also beneficial to obtain uniform microstructure along the thickness.Under high rotational speed,the tearing defect in the welding joint is parallel to the joint surface,which is even throughout the whole bottom in SZ.The tearing defect decreases and even disappears with decreasing the rotational speed.The defect-free joint is obtained at 120rpm/30mm/min(simplified as 120/30)and 150/50.Secondly,the microstructure and microharness were investigated at 120/30 and 150/50.Compared with 120/30,the more uniform microstructure in SZ is attained at 150/50.The transformation of ? phase occurs in HAZ,heightening the grain size and the amount of ? phase.The hardness of SZ consisting of fine equiaxed grains is bigger than that of BM.The minimum hardness locates at HAZ.For the joint at 120/30,the hardness values at shoulder affected zone are bigger than that at pin affected zone.The distribution of hardness in SZ center at 150/50 is reverse.Thirdly,the tensile and fatigue tests were performed.The results imply that the average tensile strength of conventional FSW joint is superior to base material(BM).Group fatigue tests were performed using 75%-88% of the BM tensile strength as fatigue load.The fatigue limit of BM is about 708.3MPa by S-N curve-fitting method.Fatigue limits under 120/30 and 150/50 reach respectively 663.67 MPa and 650.90 MPa,which are about 93.70% and 91.90% of BM.Finally,intense cooling FSW experiments with liquid nitrogen were conducted at 150/50.Compared with air cooling process,intense cooling process can reduce peak temperature and accelerate cooling rate,leading to the smaller and more uniform microstructure of SZ center along the thickness.The maximum hardness of SZ increases from 355 HV to 360 HV.The tensile strength is heightened to 1084.14 MPa.The fatigue limit of 669.50 MPa reaches to 102.86% of conventional FSW joint.The fracture mechanism was discussed.The crack of defect-free joint initiates surface or subsurface with high stress and then it firstly propagates along the plane of maximum shearing stress.The sub-micro inclusion in intense cooling FSW joint reduces the rate of crack propagation,leading to increase of fatigue life.Under cycle loads,fatigue crack propagates first along maximum shear stress plane,next along maximum tensile stress plane until fracture occurs.