Investigation of the Microstructure and Mechanical Properties of Brazing Joints between Niobium and 316L Stainless Steel using Silver-Copper-Palladium Filler

Abstract

This paper introduces an approach for vacuum brazing of niobium-316L stainless steel transition joints for application in superconducting radiofrequency cavity helium jackets. The study takes advantage of good wettability of ag-Cu-Pd brazing alloy to suppress brittle Fe-nb intermetallic formation, hence improve the joints’ mechanical performance. The wettability of ag-Cu-Pd filler metal on niobium, the interface microstructure and mechanical properties of the transition joints were investigated. Two kinds of ag-Cu-Pd filler metals had been studied and wet well on the niobium, and the wettability of ag-31.5Cu-10Pd filler metal on niobium was better than ag-28Cu-20Pd filler metal. Microstructure characterization demonstrated the absence of brittle intermetallic layers in all of the joint interfaces. Mechanical properties of samples prepared with ag-31.5Cu-10Pd filler metal were also better than their peers made with ag-28Cu-20Pd filler metal both room temperature (300 K) and liquid nitrogen temperature (77 K). The transition joints displayed shear strengths of 356-375 MPa at 300 K and 440-457 MPa at 77 K, respectively. after undergoing ten thermal cycles between the room temperature and the liquid nitrogen temperature, the transition joints’ leak rates were all lower than 1.1×10 –11 mbar·L/s. Therefore, ag-Cu-Pd filler metal is applicable to high vacuum vessels used at cryogenic temperatures.