| With the urgent need for further development of the ocean,the requirements for the service performance of deep submersibles have been increasing.The deep submersible manned shell is one of the core components,which needs to pay more attention to safety.Thick plate TC4 titanium alloy was used in the manufacture of deep submersible shell because of its excellent corrosion resistance,high strength and lightweight characteristics,and welding is the main connection technology of titanium alloy.Narrow gap Gas Tungsten Arc Welding(GTAW)in thick plate TC4 titanium alloy connection has great potential for application.However,narrow gap GTAW of thick plate are generally welded in multiple layers with multiple passes,and the microstructural inhomogeneities caused by multiple thermal cycles can reduce the mechanical properties of the joint and further affect the fatigue crack growth behavior in each region of the joint.The fatigue performance of thick plate joints is dispersed and cannot be evaluated by a single region.Therefore,this paper analyzes the microstructural heterogeneities and mechanical properties of thick plate TC4 titanium alloy narrow gap GTAW joints caused by multi-pass thermal cycling,investigates the fatigue susceptibility of various regions of thick plate welded joints in simulated marine environments,and discusses the effects of tissue inhomogeneities and crystallographic features on crack growth behavior to reveal the relationship between welded joint organization and mechanical properties and fatigue crack growth mechanisms.The results of the study obtained in this paper are as follows:The thick plate joints generate coarse curved a prioriβ-columnar crystals during multiple heat cycles and show a competitive growth mechanism,and the columnar crystals grow across the layer channels with typical associative crystallization characteristics.The weld center is composed ofα′martensite with different orientations,and the martensite size increases with the increase of heat input.By analyzing the hardness and tensile properties of the joint,it was found that the distribution of hardness in the thickness direction was as follows:Bottom layer>Cover layer>Fill layer,while the general rule for the Cover layer and Fill layer was HAZ>WM>BM,except for the hardness of the WM in the Bottom layer which was higher than the HAZ.The tensile strength of each layer of the weld is much higher than that of TA17 filler wire,and its average strength coefficient is 93.4%of that of the BM,but the ductility is significantly lower than that of the BM,and the hardness and tensile strength of the WM are greatly enhanced by phase change strengthening and solid solution strengthening.The joint priming layer high-frequency fatigue specimens mostly fracture at the BM,while the Fill layer and Cover layer mostly fracture at the HAZ and WM,which is related to the change in microstructure gradient and heterogeneity.When the fatigue life is 10~7 cycles,the ultimate fatigue strength of the joint corresponding to the Bottom layer is 397 MPa,which is about 4.1%higher than that of the Cover layer and Fill layer.Through the analysis of the corrosion fatigue crack growth rate of the thick plate welded joints in the steady-state growth zone,it was found that the growth rates of the WM were the Bottom layer<Cover layer<Fill layer,while the growth rates of the joint along the thickness direction of the three layers of the region were WM<HAZ<BM,where the growth rate of the WM in the Bottom layer is the lowest.Fatigue cracks in the BM grow along theα/βphase boundary,while fatigue cracks in the weld grow mainly along the favorable orientation of theα′martensite,as grain-piercing cracks.When the BM is subjected to cyclic stress,the strain accumulation in theαphase crack nucleation is usually smaller than that in theβphase,and the fatigue crack grow along theα/βphase boundary due to the mismatch in strain accumulation.Theα′martensite of the WM has a high density of internal dislocations,which provides energy to impede dislocation movement,and crack growth becomes difficult,slowing down the fatigue cracking process,which makes the WM have better resistance to fatigue crack growth than the BM and HAZ.The rolling process makes the grains of the BM show a consistent orientation with a stronger{0001}<11-20>basal plane weave and a weaker{10-10}<11-20>columnar plane weave,and the grain boundary area is greatly reduced,which in turn shortens the distance of fatigue crack cracking along theα/βphase boundary.Due to the Burgers relationship,the basal and columnar slip systems of theαphase are compatible with those of theβphase,and the slip in theαphase is easily transferred to theβphase and causes a continuous slip state,so that the BM becomes a fatigue weak region.In addition,the weld thermal effect leads to enhanced weld anisotropy and increased weave strength,resulting in the accumulation of coarse slip bands at the martensite boundary and blocked slip deformation,which slows down the fatigue crack growth process.The fatigue crack tends to grow in the region of high Schmid factor,where the BM and WM of Fill layer have a large number of hard oriented grains,and the Schmidt factor range of 0.3 to 0.5 has a probability of occurrence of 87.34%and 83.68%,respectively,where the slip system is more likely to activate,ultimately leading to the fastest fatigue crack growth rate in the BM and WM of Fill layer.The proportion of large-angle grain boundaries and provides a source of dislocations.When the fatigue crack grows to the large-angle grain boundary,the dislocation movement is impeded and more energy is consumed,thus effectively preventing further crack growth.That is,the higher the content of large-angle grain boundaries,the stronger the resistance to fatigue crack growth. |
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