Ta-10W难熔合金光纤激光焊接工艺研究

Process Research on Fiber Laser Welding of Ta-10W Refractory Alloy

采用6 kW多模光纤激光器进行了Ta-10W难熔合金的光纤激光焊接工艺研究。利用高速摄像机、光学显微镜、扫描电镜、显微硬度仪及拉伸试验机对焊缝成形、熔池形态、飞溅物形态、显微组织、显微硬度、接头强度及断口形貌进行分析。研究结果表明,在焦点位于板材表面且焦点直径为0.3 mm的聚焦参数下,当焊接速度为2~3 m/min,激光功率为1.5~2.0 kW时,焊接模式由热导焊转变为深熔焊。Ta-10W深熔焊时焊缝成形良好,焊接飞溅大且飞溅物不易附着于板材,焊缝呈钉头形。3 mm厚对接接头的焊缝组织主要为柱状晶,在焊缝靠上的位置,主要为垂直于板厚方向的细长晶粒;在焊缝中部与下部,形成的则是细等轴晶粒。拉伸试验结果显示,接头的室温拉伸强度为506.5 MPa,约为母材的92.1%;1500℃高温拉伸强度约为140.9 MPa,接头断裂在母材处。

Objective As a refractory alloy,Ta-10 W 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 hugh-quality Ta-10 W 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.Methods The welding equipment adopts a YLS-6000 fiber laser with focusing parameter of K=8 mm·mrad,wavelength of 1060-1070 nm,transmission fiber core diameter of 200μm,collimating lens focal length of 200 mm,focusing lens focal length of 300 mm,and focusing spot diameter of 0.3 mm.The Ta-10 W 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,an808 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.Results and Discussions Through the scanning welding of the Ta-10 W 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 is4 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 intergranular fracture at high temperature.Conclusions In the focused state,the critical power of deep penetration for multimode fiber laser welding of Ta-10 W alloy is in the range of 1.5-2.0 kW when the welding speed is 2-3 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-10 W 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.