Active Brazing Explosively Bonded Niobium-Copper Sheet to Alumina Ceramic

Similar to other refractory metals, commercially pure niobium is difficult attach using soldering processes without first plating with nickel-gold, nickel-tin or similar materials that are directly solderable. Currently used procedures require the aforementioned plating process or a step-brazing process in which copper substrates are brazed at a lower temperature onto the niobium surfaces eliminating the plating requirements. A solder-dipping process is then used to pre-tin the exposed copper surfaces, preparing them for next-assembly soldering steps.

As part of a product development effort to reduce or eliminate entire processes or processing steps, a project was initiated to replace commercially pure niobium sheet material with explosively bonded niobium-copper sheet. The exposed copper surfaces could then be subsequently coated using a solder dipping procedure. To simulate the component brazement geometry, explosively bonded niobium and copper metal sheets were actively brazed to 94% and 96% alumina ceramic test specimens. The thickness of the explosively bonded substrates was 0.5 mm and the thickness of the niobium metal approximately twice that of the copper. ASTM F 19 tensile buttons were fabricated using the explosively bonded niobium-copper material as the interlayers. The test samples were active brazed using a commercially available gold-based active brazing filler metal of the composition 35Au-62Cu-2Ti-1Ni (wt %). Brazing peak temperatures and soak times at peak temperatures were varied to assess the process robustness. Finite element analysis (FEA) simulations were performed to determine the theoretical residual stresses in the braze samples. Helium mass spectrometer leak detection data, brazed sample tensile strengths and scanning electron microscope image analysis of the niobium-copper, niobium-alumina and copper-alumina interfaces will be presented. This work was performed at Sandia National Laboratories*, Albuquerque, New Mexico, United States.

*Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin company, for the U.S. Department of Energy under contract DE-AC04-94AL85000.