In recent years, refractory metals have been widely used in aerospace engineering, and they play an important role in the manufacture of liquid and solid rocket engine thrust chambers and nozzle extensions.
Niobium alloy is an active metal, the main difficulty of welding is that it is easy to be stained by nitrogen, hydrogen and oxygen, and the gap elements are more sensitive and harmful to niobium alloy than titanium alloy, so the welding process of niobium alloy is demanding. Vacuum electron beam welding, inert gas shielded welding and contact welding are used in niobium alloy welding abroad. Due to the hafnium alloy, C-103 niobium alloy has excellent welding performance and is not very sensitive to the changes of welding process factors. Engine C-103 niobium alloy nozzle extension section, using sealed box gas shielded gTAW arc welding. Considering that the box gas shielded welding has good accessibility and easy operation, and no expensive vacuum chamber is needed for large products, we studied the welding performance of C-103 alloy by this method, and discussed the possibility of welding Nb-752 alloy sensitive to impurity gas contamination by this method.
1. For tiG welding outside niobium alloy box, each procedure must be strictly controlled to effectively prevent the welding joint from being stained so as to obtain satisfactory joint performance.
2. The strength coefficient of C-103 joint is above 97%, the same test temperature as the base material at high temperature, the plastic brittle transition temperature is 191℃, after welding, the bending Angle of weld can reach 180° stably, the strength coefficient of Nb-752 joint at room temperature is above 98%, the bending Angle of weld can reach about 100° after vacuum annealing.
3. After adjusting the chemical composition of Nb-752 weld filler wire, the joint strength is not lower than the base material, and the weld plasticity is significantly improved. After annealing welding, the bending Angle of the other three filler wire welds can reach 180° except for 10Hf-Nb wire.However, the brittleness of the fusion line is relatively weak, especially the brittleness of the fusion line filled with TA-NB wire is more obvious, which remains to be discussed.
4. Vacuum annealing after welding of Niobium alloy C-103 and Nb-752 can cause overaging of weld microstructure, thus eliminating stress of weld microstructure and restoring plasticity.
5. C-103 nozzle has passed the thermal test and fully meets the design requirements.
