As a refractory alloy, Ta-10W has the advantages of high melting point, good high temperature strength, and low linear expansion coefficient. It has a good application prospect in the manufacturing of components under high thermal service conditions in the aerospace field. However, the traditional welding methods such as electron beam welding and argon arc welding have high costs and large heat input, so it is difficult to obtain high-quality Ta-10W joints with them. In contrast, this material has a high absorptivity and the fiber laser can be implemented in the atmospheric environment, but the relevant work has not been reported. In this paper, the effect of fiber laser welding parameters on weld morphology is studied, the butt-welding process test is carried out, and the structure and properties of the joint are analyzed.
The welding equipment adopts a YLS-6000 fiber laser with focusing parameter of Kf=8 mm·mrad, wavelength of 10601070 nm, transmission fiber core diameter of 200 μm, collimating lens focal length of 200 mm, focusing lens focal length of 300 mm, and focusing spot diameter of 0.3 mm. The Ta-10W refractory alloy sheet is selected as the test material. The size of the plate scanning sample is 50 mm×20 mm×4 mm, and the thickness of the butt-welding sample is 3 mm. In the welding process, argon gas is used as the shielding gas, and the back protecting tool and the protective nozzle are used as the two-way protection. When the plate is scanned, the welding speed and laser power are also changed. The welding parameter is 4 kW, the welding speed is 2 m/min, and the defocusing amount is 0 mm. The changes of the molten pool and the welding spatter are observed with a high-speed video camera. In the high-speed photography, an 808 nm band light source is used as the molten pool illumination light. The microstructure of the welded joint is analyzed, and the micro-hardness of the welded joint is tested. The tensile specimens are prepared to test the mechanical properties at room temperature and high temperature. The microstructure of the joint is observed by the metallographic microscope and the cross section of the tensile fracture specimen is observed by scanning electron microscope (SEM). The micro-hardness of the joint is tested by the hardness tester. The load is 1.96 N and the loading time is 15 s. The tensile testing machine is used to test the tensile properties of welds.
Through the scanning welding of the Ta-10W alloy plate, it is found that the weld depth is negatively correlated with the welding speed under the same power, while the weld penetration depth is positively correlated with the welding power under the same speed. With the increase of welding power, the weld shape gradually changes from a shallow arc shape in heat conduction welding to a nail head shape in deep penetration welding. The penetration depth is about 3 mm when defocusing amount is 0 mm, the welding speed is 2 m/min, and the laser power is 4 kW. There is no obvious oxidation on the surface of the butt joint, which is bright white and well formed. However, due to the high melting point of the base metal, the short weld pool length, the serious spatter phenomenon, and the little spatter on the plate surface, the spatter is solidified before falling back to the base metal. The grain shapes and sizes at different positions in the weld center are quite different. The grains at the upper position are fine equiaxed grains with a diameter of tens of microns, a length of hundreds of microns, and a width of tens of microns perpendicular to the thickness direction of the plate. The micro-hardness test of the joint shows that the micro-hardness of the weld zone fluctuates greatly, and the average hardness is about 294 HV, slightly higher than that of the base metal (280 HV). This is related to the heating condition and the grain size refinement near the fusion line. The results of the tensile test at room temperature and high temperature show that the average tensile strength of the joint is 506.5 MPa, 92.1% of that of the base metal, the average tensile strength of the upper part of the weld at 1500 ℃ is 140.1 MPa, and the average tensile strength of the lower part of the weld is 141.6 MPa. The fracture analysis shows that the fracture is an intergranular and cleavage mixed fracture at room temperature, and an inter-granular fracture at high temperature.
In the focused state, the critical power of deep penetration for multimode fiber laser welding of Ta-10W alloy is in the range of 1.52.0 kW when the welding speed is 23 m/min. In the deep penetration state, the weld shape is a nail head shape. There is large splash and it is difficult to adhere to the plate surface. When the power is 4 kW and the speed is 2 m/min, the 3 mm thick Ta-10W butt joint is well formed. Along the weld depth direction, there is a large gap in the grain morphology at the weld center, the upper part is slender grains, and the middle and lower parts are equiaxed grains. The micro-hardness of the butt joint is slightly higher than that of the base metal, and the butt joint has good mechanical properties at room temperature and high temperature. The fracture at room temperature is a mixed fracture, and the high temperature port has an inter-granular fracture.
