激光焊接钛合金接头的组织演变和力学行为

Titanium alloy had the advantages of high specific strength, good corrosion resistance and high temperature performance, so it was often used as a structural part in aerospace and military fields. Laser welding had the advantages of fast welding speed, small heat affected zone and no vacuum environment, and it was mainly applied to TC4 plate in titanium alloy, which mostly applied to thin plate welding. TC6 titanium alloy had higher strength and better corrosion resistance than TC4, as well as good formability. The research on welding of TC6 could provide theoretical basis and experimental basis for its application in industry. Therefore, laser welding of medium thick plate TC6 was carried out in this paper and the microstructure and properties of its joint after welding were studied. TC6 titanium alloy plate with thickness of 6.5 mm was successfully welded by high power disk laser under the parameters of welded power of 6.0 kW, welding speed of 2.4 m·min^-1, defocusing distance of -2 mm and protective gas of 15 L·min^-1, and the surface of the welded joint was silvery white with good formability and no obvious defects. The microstructure evolution and mechanical properties of the welded joint were studied under these welding parameters. The observation by optical microscope (OM) showed that the welded joint had a typical "I" type shape, the top and bottom surfaces were wider and the middle was narrower. The inside of the welded joint was the coarse β grain similar to the casting state, in which the fine β grain was in the fusion line of the welded joint, and the columnar β grain was in the center of the welded joint. Scanning electron microscopy (SEM) showed that the coarse β grains of the weld were mainly composed of fine and acicular structures, and a small amount of equiaxed α phases were also contained in the columnar β grains. Through transmission electron microscope (TEM) observation and diffraction spot calibration, it could be confirmed that the fine and acicular structure inside of the welded joint was α' phase. X-ray diffraction (XRD) analysis also showed that the inside of the welded joint mainly contained α' phase. SEM images showed that the α phase in the heat-affected zone near the fusion line grew up and became spheroidized, and a large number of acicular α' phases were formed inside it. In the heat affected zone near the base metal, only part of the lamellar α+β were transformed into equiaxed α phase. The microhardness test results showed that the hardness of the base metal was about HV 361 and the hardness of the welded joint was about HV 487. The microhardness of the welded joint increased about 35% compared with that of the base metal. The static tensile test results showed that the tensile strength of the welded joint was about 1056 MPa, slightly higher than that of the base metal (1039 MPa); and the weld elongation of welded joint was about 9.6%, slightly lower than that of the base metal (11.7%). And the slight increase in the strength of the welded joint compared with the base metal was caused by the strengthening of the acicular α' phase in it. Then the welded joints were made into smooth tensile specimens and notched tensile specimens respectively, and static tensile tests were carried out; the fracture of the smooth tensile specimens were the base metal, and the fracture of the tensile specimens with notch were the welded joint, and the notch sensitivity coefficient (q) was 1.04. SEM images showed that there were a lot of fine and uniform dimples in the fracture of the smooth tensile specimen, which indicated that the fracture was ductile fracture. The fracture of the tensile specimen with notch contained a large number of tearing edges and shallow dimples, which indicated that the fracture was quasi cleavage fracture. TC6 titanium alloy could work stably at 400 ℃ for a long time, and was an ideal aerospace structural material. At the same time, it also had a certain application prospect in the field of armor. In those application scenarios, TC6 would be mostly used in the form of thicker plates. So, using efficient welding method to weld TC6 plate to obtain reliable welded joint would be one of the key research projects in the field of titanium alloy welding in the future.