Microstructure and Mechanical Properties of Tantalum/Molybdenum Laser Welding for Electron Gun Bofeng Wang1,2, Xuhua Hu1, GuanLi Zhou1, Jianyong Zhou1, Xiaohua Li1, Yongqing Zhang1, Zhaochuan Zhang1 1.Key Labouratory of High Power Microwave Sources and Technologies, Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China E-mail:
[email protected] Welding Experiment The solid-state laser includes four major characteristics: a) the maximum output power is 600W ˗ b) the laser wavelength is 1.06Pm; c) the focus of laser beam diameter is 0.3~2mm; d) protective gas is argon gas. The solder for Ta and Mo is Pt of I0.3mm. The cross section is obtained after the process of cutting and polishing. Then the corrosion of HF and HNO3 (5:1) was applied for the cross section to be used for SEM and EDS experiments. X-ray diffraction was used to as a means of identifying the alloy structure. Random vibration experiments test the firmness of the welding joint.
Abstract: The laser welding test was carried out on the Tantalum(Ta) and Molybdenum(Mo) lap joints for electron gun. The weld surface morphology, microstructure, interface element distribution, fracture morphology, main phase, vibration and electrical property were studied by metallography microscope, scanning electron microscopy, hardness and mechanical properties of joints, X-ray diffraction, electron micro-hardness and tensile test, vibration table and vacuum degasing system. The results indicate that the morphology of welding surface can be improved through the optimised welding parameters and the electron guns survive in continuous high temperature working conditions. The vibration and electrical property test has examined the welding quality for the electron guns. Meanwhile, the reliability of laser welding process in the development, production, and application of microwave tubes has been verified.
Results and Discussion Figure 1 shows the welding sketch map of Mo and Ta with Pt solder. Ta and Mo are refractory metals, with obvious distinctions, such as melting point, density, heat conductivity and specific heat capacity. The melting point of Mo is lower than that of Ta, while the heat conductivity of Mo is higher than Ta. Ta is on the top of Mo and heat is transmitted to Mo. Ta and Mo are heated and melt at the same time, with good quality of welding joint.
Keywords: Tantalum and Molybdenum; laser welding; electron gun; mechanical properties; microstructure Introduction Electron gun is the heart of microwave tubes, which plays a decisive role in the mechanical and electrical property of electron tubes [1]. Therefore, the fabrication technology is the important process for the reliability of electron guns. Many refractory metals are widely used in the electron gun due to special working environment, such as Ta and Mo with high melting points, small thermal expansion coefficient and good thermal conductivity. It is difficult to firmly connect to these refractory metals by traditional resistance welding methods. The welding technology of electron gun includes resistance welding, argon-arc welding and laser welding. With the rapid development of laser technology, laser welding is widely used in microwave tubes. Laser welding belong to special welding, which plays an irreplaceable role. However, few studies have focused on the welding method and processing technology of the electron gun using pulsed laser welding for electron guns. In this work, laser lap welding of Ta/Mo dissimilar metal was comprehensively studied by adding Platinum(Pt) metal. The characteristic and regular of welding are researched from the welding process and method. The laser welding reliability is also verified by the production and application of the electron guns
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Figure.1 The welding sketch map of Mo and Ta
As shown in Figure 2, the results of the microstructure of Mo/Ta lap joint show that the welding surface can be improved at appropriate processing parameters when the welding power 1100W, pulse width 4ms, the defocus distance is 0 mm, and Ar gas acts as the protection gas with flow rate 10L/min. The fusion zone is evenly wide, mutual diffusion of elements has happened and thus excellent welded joint can be obtained. Random vibration for electron gun has been done with different test parameters. The vibrational frequency is 5 to 500 Hz and the acceleration is 20.3 m/s2. After vibration time gets to certain value such as 1 hour, the welding joint is firm welding spot to meet the demands of design and process engineering.
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protection gas with flow rate 10L/min. The welding joint is firm welding spot to meet the demands of design and process engineering, verified by the random vibration experiments. As is shown in the XRD pattern of weld joint, Ta, TaPt2, TaPt3, Mo and MoPt are formed in the welding interface, which is in accordance with the SEM morphologies and the EDS data.
Figure.2 Microstructure of Mo/Ta joint with Pt: (a) P=1100W, W=4ms; (b) P=1018W, W=4ms; (c) P=1100W, W=5ms; (d) P=1250W, W=4ms
Figure.4 Weld bead Ta/Pt (a), Mo/Pt(b) and element EDX line scaning Ta/Pt (c), Mo/Pt (d)
According to Chinese standard GB/2651-89, the shear measurement shows that the shear strength of the welding sample reaches 50.02Mpa. Microhardness distribution of Ta(a) and Mo(b) side in the joint is shown in Figure 3. The hardness in Tantalum and Molybdenum side is both higher than that of the based material, shear fracture is relatively flat and smooth, and parallel fatigue cracks appear [2]. WZ
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Figure.5 XRD pattern of weld joint
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Figure.3 Microhardness distribution of Ta(a) and Mo(b) side in the joint
Acknowledgements The present work was supported by the Major State Research Development Program of China under Grant2013CB328901.
Weld bead Ta/Pt, Mo/Pt and element EDX line scaning Ta/Pt, Mo/Pt are shown in Figure 4. Few small hole defects and cracks exist in the weld zone. EDX line scaning shows that the Ta/Pt weld fusion zone is between 8.5 and 12.5Pm with uniform width (see Figure 4a). Therefore, mutual diffusion or migration of Ta and Pt occur at the Ta/Pt weld interface in the welding process (see Figure 4c). Meanwhile, the Mo/Pt weld fusion zone is narrow and few Mo and Pt elements have mutual diffused [3] (see Figure 4b and Figure 4d ). XRD pattern of weld joint is shown in Figure 5. Ta, TaPt2, TaPt3, Mo and MoPt are formed in the welding interface, in accordance with the EDS data.
References 1. Ding YG. Design, manufacture and application of high power klystron. Beijing: National Defense Industry Press, 2010:328. 2. Zhou DW, Tian W, Xu SH et al. Microstructure and Mechanical Properties of Magnesium/Aluminum Alloy Laser Welding. Rare Metal Materials and Engineering, 2015,44(10), 2440-2444. 3. Fu J, Huang J, Yao CW et al. Laser Butt Welding f or Copper-Steel Joint. Chinese Journal of Lasers, 2009, 36(5), 1256-1260.
Conclusions The microstructure of Mo/Ta lap joint show that the welding surface can be improved at appropriate processing parameters when the welding power 1100W, pulse width 4ms, the defocus distance is 0 mm, and Ar gas acts as the
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