Study On The Crack Sensitivity And Structure Property Of Dissimilar Laser-Welded Joint Of Ta/Mo

The production of the main parts(such as the cathode support structure)of the traveling-wave tube(TWTs),which is the core part of the microwave device,often used Ta,Mo and other refractory rare metal.Laser welding technology commonly used to complete the joining of materials in the assembly process.The welding of dissimilar metals is an effective means of controlling the production cost of components and exerting the excellent performance characteristics of the materials.However,many factors affect the quality of dissimilar materials joint,especially in the poor weldability of refractory metal connection.Joint defects such as cracks is an important reason to limit the scope of application of the structure.The laser welding of 0.2 mm-thick Ta/Mo dissimilar metal sheet were studied in this paper,and the formation of Ta/Mo dissimilar materials was completed by the addition of filler materials respectively.The effects of the filling materials on the microstructure and mechanical properties of the joints were studied by means of metallographic microscope,scanning electron microscope,microstructure XRD,micro-hardness test and tensile test.The main results obtained in this paper are shown below:Transgranular fracture with small fraction of imtergranular was the characteristics of the fracture surface of the joint caused by the cracks in Ta/Mo joint is characterized by macroscopic transgranular cracks and imtergranular microcracks.Intergranular impurity element segregation is the is the root cause of the formation of crystal cracks in Ta/Mo joint.The solid solution formed by Ta and Mo significantly increases the micro-hardness of the weld but resulting in the decrease of the toughness of the weld.The crack is further extended under the action of welding thermal cycling because the weld grain is coarse and the mechanical properties of the material mismatch deteriorate the joint performance.The preheating before welding process can reduce the crack tendency of the joint to a certain extent,but it does not achieve the purpose of suppressing the weld crack.The addition of Ti and Ni fillers can suppress the generation of cracks in Ta/Mo laser welding joints,then the tensile strength of joints reach the maximum value respectively when the laser power is 20.8 W,the pulse width is 5 ms,the pulse frequency is 4.5 Hz,and the spot diameter is 0.3 mm.Wherein the tensile strength of the joint with Ti filler metal matched the ultimate tensile strength of Ta and the specimen fractured in the Ta base metal,and the tensile strength of the joint with Ni filler metal reached 81 % of that of the the weaker base metal,tantalum and the tensile specimen fractured along the interfacial zone on the Ni/Ta side.By adding the low melting point filling material,the heat input required for welding can be reduced,then reducing the amount of melting of the joint Mo and the cracks in the Ta/Mo joint has been suppressed.The obtained joint contains both the characteristics of the melting welding and the brazing characteristics.Interfacial zone between the weld metal and base metal constituted of Ni3 Ta,Ni2Ta and NiMo intermetallic compounds when the Ni filler wire was used but when Ti filler was used constituted of different solid solutions.The micro-hardness in the interfacial zone of the joint welded with Ni filler wire was found higher than that of solid solution area at Mo side when the Ti filler wire was used.The presence of the intermediate phases lead to a lower tensile strength of the Ta/Ni/Mo joint than the Ta/Ti/Mo joint.