| Titanium and titanium alloys have been widely used as the manufacture parts of the nuclear submarine pressure shell and pipe system because of their advantages such as low density,high strength,excellent corrosion resistance and good weldability.Most of the structure parts of marine are large thickness welding parts,with the strategic planning of China's marine power and the demand for large thickness welding parts of marine increasing,which make the welding of thick titanium plates cause extensive attention.Electron beam welding(EBW)has a series of advantages including high power density,energy concentration,small deformation and wide range of welding thickness,which can avoid the defects of welding thickness titanium alloys caused by traditional welding methods and obtain good performance of joints.In this paper,pure titanium(TA1)with a thickness of 30mm and TC4 with a thickness of 100mm were jointed by electron beam welding(EBW),respectively.The microstructure,tensile and impact properties in various depth of joint cross section of EBW joints of 30mm TA1 and 100mm TC4 were analysed.Meanwhile the fracture appearance of the tensile and impact samples was observed by scanning electron microscope.The microstructure of electron beam weled joints of 30mm TAI from base metal to weld zone followed by equiaxed a?coarse ? columnar crystal + zigzag a +needle-like ?'.The microstructure of needle-like ?' in weld zone made the hardness and strength of joints higher than base metal,and the maximum growth of strength was up to 5.2%.The toughness and ductility were decreased and the reduction rate was between 10.7%and 19.3%;the strength of the upper joints and bottom joints were higher than middle joints.The number of ?' martensite increased after 750?×2h FC treatment and the ?' martensite resulted in a large number of disordered and short needle-like structures after 850?×2h FC in the weld zone.The overall hardness decreased after 650?×2h FC treatment,and the overall hardness increased and softened in the heat affected zone after 850?×2h FC treatment.The microstructure and properties became more uniform in various depths of TA1 joints after heat treatment.The microstructure of EBW joints of 100mm TC4 from base metal to weld zone followed by equiaxed(a+?)? needle-like ?' martensite.The hardness and strength were higher than base metal and elongation was lower than base metal in various depth of joint cross section of EBW joints of 100mm TC4.The maximum reduction of the elongation of joints was about 30.2%compared with base metal;the impact energy of weld zone was lower than base metal.a phase in weled zone got coarsening after 600?×2h FC treatment;the number of ?' rmartensite increased after 850?×2h FC treatment.The microstructure of weld zone was neele-like a' martensite + primary? grain boundaries after(920?×2h WC)+(500?×4h AC)treatment;the average tensile strength increased 141 MPa and the hardness increased about 60HV in the weld zone and 40HV in the base metal compared with welding parts without heatment.The microstructure and properties became more uniform in various depths of TC4 joints after heat treatment.The fracture mechanism of the tensile and impact of weld joints of 30 mm TA1 and the joints treated by 650?×2h FC and 750?×2h FC was dimple fracture.The fracture mechanism of tensile was dimple + quasi-cleavage fracture and impact was quasi-cleavage fracture of joints treated by 850?×2h FC.The fracture mechanism of tensile specimens of weld joints of 100mm TC4 and the joints treated by 600?×2h FC and 850?×2h FC was dimple fracture and joints treated by(920?×2h WC)+(500?×4h AC)was cleavage fracture.The fracture mechanism of impact of weld zone was quasi-cleavage fracture and joints treated by 600?×2h FC and 850?×2h FC was dimple fracture;joints treated by(920?×2h WC)+(500?×4h AC)was dimple + cleavage fracture and cleavage fracture was dominant. |
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