Effects Of Hydrogen On Microstructure And Properties Of Welding Joints Of TC4Titanium Alloy

In the welding methods of titanium alloys, electron beam welding (EBW) is one kind of advanced high energy density beam processing technology, which has many advantages that other traditional welding methods can’t compare with. Because there are hydrogen and cyclic stress in the work environment, the possibility exists that hydrogen-induced fracture will take place in the titanium alloy components welded by EBW. So it will be very important to study effects of hydrogen on fatigue properties of titanium alloy EBW joints. As a new solid-phase welding technology, linear friction welding (LFW) has many advantages such as high quality, high efficiency, material saving, pollution free, etc. However, LFW technology also has its disadvantages, i.e. the application of LFW is limited because of the large tonnage of LFW machine and complex technique for manufacturing equipment. The thermohydrogen processing (THP) is a technique in which hydrogen is used as a temporary alloying element in titanium alloys by controlling the microstructure and phase structure to improve the mechanical working properties. By using this technology, the workability of titanium alloy can be given to full play, and the requirement to equipment’s forming capability can be lowered. In this paper, TC4alloy is the research object. Firstly, the microstructure and sub-structure evolution of TC4alloy after hydrogenation were analyzed. Secondly, effects of hydrogen on fatigue properties of TC4alloy EBW joints were researched. Finally, linear friction welding performance of hydrogenated TC4alloy was researched, and the numerical simulation of hydrogenated TC4alloy during linear friction welding progress was carried out.Microstructure of titanium alloys at room temperature has much influence on their welding performance and the mechanical properties of welded joints. Thus optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction analysis (XRD) and Transmission electron microscope (TEM) were used to analyze the influence of hydrogen on the microstructures and substructures of titanium alloy. The precipitation mechanism of titanium hydride in hydrogenated TC4alloy was disclosed. The results show that original α phase fraction decreases gradually, while β phase fraction increases with the increase in hydrogen content. The amounts of twins also increase significantly in the hydrogenated alloy as hydrogen content increasing. When hydrogen content up to0.25wt%,δ titanium hydride having a fee structure can be observed in hydrogenated TC4alloy.The hydrogenated C(T) specimens were employed to measure the fatigue crack propagation rate (da/dN) of TC4alloy electron beam welded joints at room temperature. The results indicated that the da/dN of hydrogenated base metal specimens were higher than the non-hydrogenated ones in near-threshold stage and rapid rupture stage, but specimens with various hydrogen contents differed little from each other, and hydrogen showed little effects on da/dN in stable crack propagation stage (Paris stage). Thed da/dN of hydrogenated weld bead specimens were obviously enhanced during the whole crack propagation process and increased with the increment of hydrogen content. But the degree of da/dN increasing became small when hydrogen content increased from0.054wt%to0.101wt%. The analysis results of fracture surface morphologies showed that the weld bead specimens had similar surface morphologies with the base metal specimens. The amount of secondary crack increased with the increase of hydrogen content in the pre-crack region. In the stable crack growth region, the width of fatigue striation, amount and size of secondary crack increased when hydrogen content increasing, which indicated the increase of brittleness of titanium alloy. In the rapid cracking region, the surface morphologies of all specimens consisted of dimples, and the dimples of hydrogenated specimens were shallower and smaller.The effects of hydrogen on fatigue life values of TC4titanium alloy electron beam welded joints were evaluated by statistic approach. The fatigue fracture positions of joints as well as the fracture surface morphologies of specimens were observed and analyzed. It was found that the fatigue life was evidently reduced by hydrogen charging. The fatigue life of joint with0.028wt%hydrogen was just half of the hydrogen-free joint. When hydrogen content was0.120wt%, the fatigue life of hydrogen-charged joint decreased to one fifth of the hydrogen-free joint. The fracture occurred at heat affected zone (HAZ) of most fatigue specimens. This was due to the structure inhomogeneity and the higher hydrogen content of materials at HAZ. The fractographic observation results indicate that hydrogen accelerates the fatigue crack initiation and increases the speed of crack propagation, which leads to the obvious decrease of fatigues lives of TC4titanium alloy joints welded by electron beam welding.Systematic researches on linear friction welding performance of TC4alloy were carried out.The results show that the critical parameters for unhydrogenated TC4alloy to obtain a successful LFW joint having excellent mechanical properties are:f=30Hz, a=2mm, p=4T, t=2s. The minimum effective power density needed is8.0×106W/m2. Under the same welding parameters condition, the bonding rates of all hydrogenated TC4alloy LFW joints are higher than the unhydrogenated alloy joints, and the widths of weld bead of hydrogenated TC4alloy joints are narrower than the unhydrogenated joints. As hydrogen content increases, the axial shortening of hydrogenated TC4alloy first increases obviously and then decreases slightly. It can be concluded that TC4alloy containing0.3～0.4wt%hydrogen has better high temperature plasticity, which resultes in the optimum welding performance improvement of LFW. The THP process doesn’t reduce the mechanical properties of TC4alloy LFW joints. The minimum effective power density needed for hydrogenated TC4alloy to obtain a success LFW joint is calculated to be5.5×106W/m2. Comparing with the unhydrogenated TC4alloy, the reduce rate is up to30%. The high temperature plasticity of titanium alloy is enhanced by hydrogen mainly through mechanisms:changing phase proportion of α and β, promoting dislocation movement and dynamic recrystallization. As a result, the linear friction welding performance is improved by hydrogenation.By use of finite clement simulation (FHM) code COSMAP, the mathematical model synthetically concerning the coupled effects of temperature field, stress field and phase structure was established. Moreover, a three-dimensional elastic-plastic FHM model was established. Then the linear friction welding process of hydrogenated TC4alloy was well simulated, and the law of variation and distribution of temperature and stress fields as well as phase structure was obtained. Furthermore, the effects of hydrogen on the three fields were analyzed by comparative analysis method. By performing the measurement of temperature variation of a specific node and residual stress distribution at surface of the joint as well as observation of microstructure at weld bead zone, the simulation results were experimentally verified. By comparison, the calculation data obtained from this numerical simulation work was in good agreement with the experimental data. This numerical simulation result can be used to provide a reference for parameters optimization and joint quality control of titanium alloy in linear friction welding technology.