Study On Ta/Mo Dissimilar Metal Laser Welding Process And Numerical Simulation

In the microwave and millimeter wave power amplifier, many refractory metals arewidely used in the Cathode Assembly of TWT (traveling wave tube) due to special workingenvironment, such as tantalum and molybdenum. It is difficult to firmly connect to theserefractory metals by traditional resistance welding methods. In recent years, with the rapiddevelopment of laser technology, laser welding is widely used in the microwave andmillimeter wave power amplifier. Laser heat source has some advantages such as narrow heataffected zone, energy concentration, precisely controlling the heating time and weldingheat-input. It becomes one of the most preferred welding methods for refractory metals.In this dissertation, laser lap welding of Ta/Mo dissimilar metals was comprehensivelystudied by adding different interlayer metals Ni and Pt. Three kinds of lap joints wererespectively Ta/Mo, Ta/Ni/Mo and Ta/Pt/Mo. A feasible welding process of dissimilar metalslaser lap welding of Tantalum and Molybdenum was proposed. Firstly, the morphology,microstructure，metallurgical behavior and welding defects of three different kinds of lapjoints were analyzed. Secondly, the mechanical properties of three different kinds of lap jointswere tested and compared, and further discussed fracture morphology and fracture mode ofthe three different kinds of joint. The experimental results show that the joints are seriouslyoxidized if unprotected by the inert gas during laser welding. There are different defects ofdifferent kinds of welded joints. For the joint of Ta/Mo, there are thick grains of Molybdenumin heat affected zone and cracks near the fusion line; for the joints of Ta/Ni/Mo and Ta/Pt/Mo,there are mainly cracks and pores. Finally, for the mechanical properties of three kinds ofjoints, tensile shear strength of the joint of Ta/Pt/Mo is the largest in three kinds of joints, andit is80.88Mpa. Fracture analysis shows that fracture characteristics of Ta/Mo joint andTa/Ni/Mo joint are the intergranular fracture, and Ta/Pt/Mo joint is mixed type ofintergranular fracture and transgranular fracture.Meanwhile, a moving Gaussian heat source model was established in this dissertationbased on ANSYS Workbench software. The temperature field and residual stress field werecalculated by numerical simulation methods during the laser welding process. The residualstress of Ta/Pt/Mo joint was measured by X-ray diffraction method. Simulation results show that the isotherm of temperature field is close to circular, and cross-section of the weld issimilar to the shape of a bowl which is in good agreement with the shape of welding pool oflaser conducted welding. With the increase of laser moving speed, temperature range of thehigh temperature zone narrows, and the cross-section of the weld pool changes from the "cup"to "bowl". In terms of residual stress, there is a larger longitudinal residual tensile stress onthe area of the weld, and the maximum residual tensile stress is about54Mpa. There is aresidual compressive tensile stress away from the weld zone, and the maximum residualcompressive stress is about20Mpa. The simulation result of weld is in good agreement withthe experimental results.