Microstructure And Mechanical Behavior Of Electron Beam Welded Joints Of High Strength Metastable β Titanium Alloy

With the strategic focus of my country’s maritime power gradually developing towards the deep-sea and far-sea strategies,the service environment of marine equipment has become more and more severe.High-strength titanium alloy has excellent specific strength,fatigue resistance and corrosion resistance,and has the reputation of "marine metal".It is the best choice to promote the lightweight of marine equipment structure and ensure service reliability.Welding is a common process for manufacturing marine equipment,and the quality of welds has a crucial impact on structural reliability.In this paper,a new low-cost and high-strength Ti-3Al-5Mo-4Cr-2Zr-1Fe(Ti-35421)metastable beta titanium alloy vacuum electron beam welded joint is taken as the research object,the microstructure characteristics of each region of the joint are systematically characterized,and the room temperature plasticity of the joint is analyzed.The deformation behavior was studied,and the microstructure mechanism of the deterioration of the mechanical properties of the joint was explored.The optimization research on post-weld aging heat treatment process of joints was carried out,and the influence of different heat treatment process parameters on the microstructure evolution,hardness distribution and tensile properties of joints in various regions was explored,and the optimization and matching mechanism of microstructures in various regions of joints was revealed,and the mechanical properties of joints were proposed.Improved heat treatment process optimization strategies.The main findings of this study are as follows:The base metal of high-strength Ti-35421 metastable β-titanium alloy is mainly composed of rolled β-grains and two-state precipitates.lamellar secondary α phase(αs,average length258.7±148.4nm).Electron beam welding leads to rapid melting and cooling of the weld metal,and the temperature gradient of the molten pool leads to a large number of coarse columnar β grains in the fusion zone of the joint.nm).There are recrystallized β equiaxed grains and long β grains in the heat affected zone,and the content of α precipitates decreases.The average microhardness of the joint fusion zone was 302.7±5.4 HV,which was significantly lower than that of the base metal by 85.1 HV,and the joint fusion zone was significantly softened.The yield strength of the joint is 912.6±4.2 MPa,the tensile strength is930.2±2.5 MPa,the tensile strength coefficient of the joint is 0.83,and the elongation of the joint is only 2.9%,which only reaches 27% of the elongation of the base metal,and the plasticity of the joint deteriorates significantly.The results of the strain distribution during the tensile process show that the fusion zone yields first,and then the plastic deformation is further concentrated in the fusion zone,where significant strain localization occurs,and finally fracture occurs in the middle of the fusion zone.The fracture shows a mixed fracture mode with a large number of intergranular fractures and a small amount of transgranular fractures.The plastic deformation mechanism of the joint fusion zone is dominated by dislocation slip.With the increase of aging temperature,the hardness of fusion zone and heat-affected zone gradually increased,and the hardness of fusion zone reached the highest value(520 HV)after holding at 450 °C for 5 h.When the aging temperature is 520 °C and the holding time is 1-10 h,the joints can obtain the simultaneous improvement of strength and plasticity.When aged at 520 °C,the joint strength decreased gradually with the aging time,and the elongation increased gradually with the aging time.Aging and heat preservation at 520 °C for 10 h can increase the tensile strength of the joint to 1117.3 MPa and the elongation to 8.8%,obtaining a good strong-plastic matching.During the post-weld aging heat treatment process,the dispersed ω nanoparticles disappear in the fusion zone,and the fine lamellar α precipitates can play a significant role in precipitation strengthening.The tensile fracture position is transferred to the base metal zone,and the joint fusion zone is significantly strengthened.The overall elongation of the joint is significantly improved.