Niobium metal is a kind of heat-resistant alloy, mainly used as special steel, super alloy and alloy element of cutting tools. Niobium alloy has excellent high temperature strength, low temperature ductility and heat conduction properties, excellent cold processing formability and weldability, can be made into very complex workpieces, such as rocket thrusters and high temperature valves and other components, widely used in aerospace and nuclear energy industry structural materials. In this study, Ti-15Cu-15Ni(wt%) titanium base alloy or BNi-2 nickel base alloy were used as brazing filler to weld niobium hafnium alloy (C103) alloy. During the experiment, the microstructure of the joint interface was observed and the single stack compressive shear test was used to evaluate the influence of different brazing variables on the joint quality. Brazing variables include brazing temperature, temperature holding time, substrate surface roughness, lap distance, post-welding heat treatment, etc.
The experimental results show that the bonding strength of the two fillers is also affected by the surface roughness of the substrate and the overlap distance. The bonding strength of 0.59μm Ra is the best, and the bonding strength of 1T(2mm) is the highest, but the shear strength decreases with the increase of the overlap distance. TiCuNi filler has good wettability in Nb-10Hf-1Ti substrate. EDAX analysis shows that Cu and Ni remain in the bonding zone and form eutectic phase with Ti. Due to the high melting point temperature of the substrate, its component elements cannot be diffused into the joint zone to dilute Cu and Ni elements and eliminate the eutectic structure. The microstructure in the hard-welded joint zone is mainly composed of αTi-Nb alloy and Ti2Ni+ Ti2Cu eutectic phase, which decreases with increasing temperature and temperature holding time. The compressive shear strength of 1223 K for 5 minutes can reach 302MPa.
BNi-2 filler and Nb-10Hf-1Ti substrate were hardwelded at 1323-1423 K temperature for 5-20 minutes, and the microstructure also showed good wettability. The diffusion of B and Si from filler to substrate was the main control factor for the evolution of the microstructure of the bonding interface. The distribution of Nb, Ni, Cr, Fe, Si and other elements was obtained by EPMA analysis. Cr, Fe, Si gather in the center of the weld and form a hard and brittle eutectic phase. B has an abnormally high phenomenon near both sides of the joint surface. Due to the high melting point temperature of the substrate, the eutectic structure is not easy to be eliminated by the diffusion mechanism. The compressive shear strength increases with the increase of brazing temperature, for example, 1423K temperature for 10 minutes up to 337 MPa.
