| TC4 alloy possesses many advantages such as high specific strength,strong corrosion resistance and good mechanical properties at low and high temperatures,and the TC4 alloy thick plate has been widely used in the ship-building,marine engineering,aerospace,nuclear power engineering,mechanical manufacturing and other fields.The conventional welding methods for the titanium alloy thick plate have such disadvantages as the low welding efficiency and poor joint quality.The narrow gap TIG welding method adopts the deep and narrow I-type or small angle V-groove,and shows such advantages as the less filling materials,high welding efficiency,small welding deformation and high quality of joints.In this dissertation,the narrow gap TIG welding method was adopted to perform the single channel multilayer-filling welding test for the TC4 alloy with a thickness of 30mm,and the influence of welding technology on the forming performance of weld seam as well as the microstructure and mechanical properties of welded joints,in order to solve such problems as the non-fusion of side wall and interlayer in the welding process of titanium alloy thick plate and to provide the guidance for the practical application of the present welding technology.Through optimizing such welding technological parameters as the welding current,welding voltage,welding speed,shaking speed and shaking angle and performing the measurement of weld forming parameters and the observations on microstructure of welding joints,the optimum welding technological parameters under the I-Type groove of 10mm are as follow.The welding current is 210A to 250A,the welding voltage is 11V,the welding speed is85mm·min-1 to 120 mm·min-1,and the shaking speed is 35rmp to 55rmp.The welding of TC4 alloy thick plate with a thickness of 30mm was performed with the optimized welding technological parameters.The macroscopic topography,microstructure,microhardness,tensile properties and tensile fracture surface of the welded joints were characterized,The experimental results show that the welded joints with the good fusion of side wall and interlayer as well as the perfect forming and perlage can be obtained with the optimized welding technological parameters.At the weld seam,the grains are coarse,and the microstructure is mainly composed of?'martensite.The heat affected zone is narrow where the width is about 1mm to 2mm,and the microstructure mainly contains the residual?phase and a few?'martensite.The microhardness of weld seam and heat affected zone is higher than that of base material,where the microhardness of weld seam of cover weld is lower and has a value of 328HV and the microhardness of weld seam in the middle region of joint is higher and has a value of 355HV.It indicates that the weld seam possesses the higher hardness,and the obvious softening area does not appear in the joint.The average ultimate tensile strength of welded joint can reach to 973MPa and is about 95%of ultimate tensile strength of base material.The fracture position is all at the weld seam.It suggests that the welded joint with the good mechanical properties can be obtained after the narrow gap TIG welding technology is optimized.The tensile fracture surface of welded joint had a cup cone shape,the equiaxed dimples with different depth and size can be observed,and the corresponding fracturing mode is the ductile fracture. |
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