| The 14 th Five-Year Plan has raised major deep-sea projects to the height of national strategy.Major marine engineering equipment plays an important role in maintaining marine sovereignty,coping with global climate change,and developing and utilizing marine resources.The manufacturing technology of major marine engineering equipment is crucial to the realization of the national blue ocean strategy,which reflects the comprehensive basic manufacturing level of the country.Deep-sea engineering equipment often needs to face extreme service conditions such as high pressure,shock and extreme cold.Titanium alloy is known as “marine metal” because of its excellent specific strength,corrosion resistance and fatigue resistance,and is widely used in marine engineering equipment manufacturing.At present,titanium alloy with large thickness and high strength and toughness is the preferred material for the core parts of the deep sea probe.The narrow gap welding of large thickness titanium alloy is an essential manufacturing process.At present,high-quality narrow-gap welding of large thickness titanium alloy is facing many challenges.Among them,how to overcome the deterioration of weld strength and toughness caused by the significant structural inhomogeneity of the joint is one of the most prominent technical problems.MagneticallyControlled Narrow-Gap Tungsten Insert Gas Welding(MCNG-TIG)and Electron Beam Welding(EBW)are widely used in the narrow-gap welding of large thickness titanium alloys.In this study,the MCNG-TIG joint and EBW joint of 100 mm thick dual-phase TC4(Ti6Al4V)titanium alloy are taken as the research object,and systematically compares and investigates the microstructure evolution of the joint and the mechanical response of the joint by the narrow gap welding forming process of two large-thickness titanium alloys The influence law of behavior,analysis and discussion of the reasons for the deterioration of joint weld strength and toughness,and establish an optimized post-weld heat treatment process to achieve the strengthening of large-thickness titanium alloy joints,in order to provide a theoretical reference for improving the manufacturing level of large-thickness titanium alloy components.First,the microstructures of two kinds of narrow gap welded joints in different area was systematically characterized.The research results show that the fusion zone structure of the multi-pass forming MCNG-TIG joint is dominated by needle-like ? phase,while the fusion zone structure of the EBW joint weld is dominated by the uniformly distributed ?/? mesh basket structure;According to the microstructure characteristics,the heat affected zone of the two joints can be divided into the near base metal zone and the near fusion zone.The heat affected zone microstructure is composed of primary ? phase and transformed ? phase.Based on the obtained results,the evolution process of the joint microstructure caused by different welding processes is further discussed.Then,compared the uniaxial tensile deformartion behaviors between the MGNG-TIG joint and EBW joint,the latter is better than the former in terms of strength and plasticity.The fracture positions of the two types of joints are also quite different.MCNG-TIG joints tend to break in the base metal area,while EBW joints tend to break in the middle of the weld.Based on the characteristics of different welding processes,the micro-mechanism of the difference in mechanical response between MCNG-TIG joints and EBW joints is discussed;Finally,the influence of different post-weld heat treatment process parameters on the microstructure evolution and mechanical properties of MCNG-TIG joints is systematically studied,the optimized post-weld heat treatment process is determined,and the microstructure optimization construction strategy of MCNG-TIG joints is put forward,and the joint strengthening mechanism is defined. |
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