| In this paper, the vacuum electron beam welding was used to weld the 20 mm thick TC4 titanium alloy by three ways, which are no scanning, scanning and local heat treatment. The scanning waveform includes elliptical, circular, triangular, sawtooth, etc.. The surface focus and positive focus of electron beam were used for local heat treatment. The microstructure evolution of the weld and grain size gradient in the direction of penetration were analyzed by optical microscope. The influence of different process parameters on the microstructure was studied. The mechanical properties of the joint were analyzed by the layer microhardness test and tensile test, and the tensile fracture was studied by SEM. The results show that:In the case of 720mm/min, the grain size of the weld decreases with the increase of the penetration depth with non scanning welding. With the electron beam current increases, the grain gradient decreases, and the grain gradient is minimum when the electron beam current is 105 mA. When the electron beam current increases from 95 mA to 105 mA, The size of the martensite ?? and intergranular ? phase and ?? lath in the upper of the seam is larger than that in the middle and lower of the seam. With the electron beam current increasing, intergranular ? phase becomes wider and ?? lath becomes thicker. When electron beam current is reduced to 85 mA, there is no intergranular ? phase and ?? lath in the lower of the seam.The microhardness curves of the non scanning welded joints showed "M" shape. The microhardness of the weld is higher than that of the solid solution base metal. When the electron beam current is 105 mA, the microhardness is highest and uniform. The electron beam current reduces to 85 mA, the microhardness gradient becomes larger. The strength of the seam acquires a maximum value of 1265 MPa when the intensity of the electron beam is 95 mA. which is higher than that of the base metal. The upper of the joint strength is the lowest. The fracture mode of solid solution base metal is ductile fracture, and the joint with no scanning is mixed fracture with ductile fracture and quasi cleavage fracture.The grain gradient of the welded seam is minimum by using circular wave scanning welding, which is effective to the improvement of the grain gradient. There is no obvious difference between the microstructure of the joint with scanning welding and non scanning welding. The microhardness of the joint is higher than that of the as cast base metal. The average microhardness of the weld is between 343~353HV under different scanning waveforms. The microhardness of the scanning welding becomes more uniform. The strength of the welded joints were higher than that of the as cast base metal. The intensity gradient of the circular wave is the largest, and the intensity gradient of the sawtooth wave is the smallest. The fracture mode of the cast base metal is brittle fracture, the welding joint for scanning is mixed fracture with cleavage fracture and ductile fracture.When the joint made by local heat treatment and the electron beam was focused at the surface, the weld center have coarse structure. As the defocused degree increased, the structure at the middle area are fine and the bottom part of the weld consist of fine ?? grains. When the surface focus mode adopted, the average microhardness at the weld seam is 349 HV. When the positive focus mode adopted, the microhardness distribution at the up part of the weld varied from 344 HV to 353 HV. The microhardness of the weld was as the same as that of as-cast base metal. The weld tensile strength with local heat treatment was higher than that of the as-cast base metal. The influences of surface focus on the weld tensile strength were relatively low. As the defocused degree increasing, the tensile strength of the upper part of the weld decrease from 1038 MPa to 984 MPa, the middle part decrease from 1078 MPa to 1000 MPa, the bottom part of the weld was The joint made by local heat treatment failed with brittle and ductile hybrid fracture mode. |
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