Study On High Cycle Fatigue Behavior Of Narrow Gap TIG Welded Joint Of TC4 Titanium Alloy

TC4 titanium alloy has the characteristics of strong corrosion resistance,high specific strength,good toughness,high fatigue strength and good weldability,and its comprehensive performance is excellent,which has been widely used in aerospace,automotive and other industries.At present,modern industrial equipment tends to be high performance,large scale and integration,and thus the welding demand for thick titanium alloy plate is increasing.Due to the performance differences at different positions of thick plate welded joints,the relevant research on the fatigue properties at the different positions of TC4 titanium alloy welded joint has certain guiding significance and reference value for the safe usage of TC4 titanium alloy welded components.In this dissertation,the magnetically controlled narrow gap TIG welded joint for TC4 titanium alloy was taken as the research object,and the high cycle fatigue(HCF)experiments were performed at room temperature for the samples at the upper,middle and lower layers of welded joint in the direction vertical to the plate thickness.And the difference in the high cycle fatigue properties of samples from three layers of titanium alloy welded joint was compared.The expression for the fatigue life of three layers of TC4 titanium alloy welded joint was determined.In addition,the microstructure analysis and fatigue fracture morphology characterization were performed to clarify the deformation and fracture mechanisms of three layers of titanium alloy welded joint under the HCF loading condition at room temperature.The results of high cycle fatigue test at room temperature show that at the same maximum cyclic stress,the fatigue lives for first layer of TC4 titanium alloy welded joint are the highest,the fatigue lives for third layer of welded joint are followed,and the fatigue lives for second layer of welded joint are the lowest.The fatigue limits for the first,second and third layers of TC4 titanium alloy welded joints are 420 MPa,390 MPa and 400 MPa,respectively.It means that the first layer of welded joint shows the highest fatigue limit,while the second layer of welded joint has the lowest fatigue limit.The observation and analysis results of fatigue fracture morphology show that the fatigue cracks initiate on the sample surface for the first layer of TC4 titanium alloy welded joint,while the fatigue cracks initiate on the sample surface and welding defects for the second and third layers of TC4 titanium alloy welded joint.In addition,the fatigue cracks propagate in a transgranular mode for three layers of TC4 titanium alloy welded joint.The transmission electron microscopy(TEM)observation results show that the dislocation configuration is mainly dislocation mesh and dislocation array in the first layer of TC4 titanium alloy welded joint,is mainly dislocation entanglement in the second layer of welded joint,and is mainly dislocation mesh and dislocation entanglement in the third layer of welded joint.For three layers of TC4 titanium alloy welded joint,the high cycle fatigue deformation mechanism at room temperature is mainly plane slip.The results of electron backscattered diffraction(EBSD)analysis show that the proportion of small angle grain boundary in α phase for the first layer of TC4 titanium alloy welded joint is the largest,while the proportion of large angle grain boundary in α phase for the second layer of TC4 titanium alloy welded joint is the largest.The strain in the fatigue deformation zone of three layers of welded joint mainly concentrates on the α/β phase boundary and its adjacent area.