Microstructure And Hydrogen Embrittlement Sensitivity Of TC4 Titanium Narrow Gap Welded Joints

During the welding process of thick plates in titanium alloy,due to the effect of welding thermal cycles,there are significant differences in the microstructure of the weld zone compared to the base material.This not only affects the mechanical properties of the weld,but also has a serious impact on the susceptibility to hydrogen embrittlement of the welded joint.In this paper,the microstructure and mechanical properties of the narrow gap welded joint in thick plate of TC4 alloy were studied.The impact of hydrogen on the tensile properties of the TC4 alloy weld and base metal was studied while ensuring that the mechanical properties of the joint meet the requirements for use.The hydrogen embrittlement failure process and fracture mechanism of the joint were analyzed in depth,and the interaction mechanism between hydrogen and fatigue damage was studied through pre-fatigue experiments.The study on the microstructure and mechanical properties of narrow gap welding joints indicates that the distribution of phases in the weld area,including α and β phases,is different from that in the base metal and exhibits a basket-weave pattern.The tensile strength of the weld is 1 043 MPa,which is equivalent to that of the cast TC4 alloy and is 90% of the strength of the base metal.The mechanical properties of the weld joint exceed those of most TC4 alloy weld joints and meet the requirements for use.Observing the fracture morphology of different regions of the weld,it is found that all specimens experienced ductile fracture.The study on the hydrogen embrittlement properties of the weld and base metal shows that the continuity of the β phase in the base metal is poor,and the volume of the lamellar α phase is large,which hinders hydrogen diffusion.Therefore,the base metal has lower hydrogen embrittlement sensitivity.In comparison,the basket-weave phase distribution in the weld zone has high β phase continuity and phase boundary density,providing more convenient diffusion channels and absorption sites for hydrogen.Therefore,the weld has higher hydrogen embrittlement sensitivity.After 48 h of hydrogen charging,the amount of hydrogenated titanium in the TC4 alloy welded joint is much higher than that in the base metal,and the fracture mechanism of the base metal changes from ductile fracture to quasi-cleavage fracture.The fracture mode of the weld changes from a tough-brittle mixed fracture mode dominated by coarse feather-like tissue before hydrogen charging to a completely brittle fracture mode dominated by hydrogenation after hydrogen charging.The study on the hydrogen embrittlement failure behavior of the welded joint under fatigue damage shows that with the increase of pre-cycling times,the initial dislocation density of the specimen increases,w hich increases the severity of hydrogen embrittlement phenomenon.After 10 000 and 20 000 pre-cycles,the hydrogenenhanced local plasticity mechanism plays a dominant role in the tensile process of the welded joint fatigue damage hydrogen-charged specimen,manifesting as a intergranular ductile fracture.After 30 000 pre-cycles,the hydrogen-enhanced decohesion mechanism dominates the tensile process,manifesting as a transgranular cleavage fracture.