Introduction (200 words)
Glass/ceramic-to-metal or glass-to-metal sealing is performed in the temperature range 950-1000°C to produce hermetic electrical feedthroughs with a 304L stainless steel (SS) shell. The hermetic feedthroughs (headers) are then welded into stainless steel housings. To facilitate weight savings, Ti-6Al-4V component housings are being developed which will require dissimilar alloy joining of the header into the housing. Fusion welding of SS to Ti is problematic due to excessive brittle intermetallic compound formation and cracking. In this work, simultaneous glass sealing of the hermetic feedthrough and brazing of the SS shell into a Ti housing are being investigated. The dissimilar alloy brazing between 304L stainless steel and Ti-6Al-4V was performed using pure Ag and active braze Ag-1Cu-2Zr filler metals. The glass sealing temperature range is higher than desired for SS/Ti brazing, which is typically performed with AgPdGa (Gapasil-9) braze filler alloys at ~900°C or lower. The microstructure of the Ag and Ag-Cu-Zr joints will be described along with preliminary lap joint (AWS C3.2) mechanical testing and the implications of the constraints of brazing under the glass seal processing conditions will be discussed.
Experimental Procedure (300 words)
Brazing trials were performed in vacuum (Ag and Ag-Cu-Zr) and in nitrogen (Ag only) to simulate the typical glass sealing atmosphere. Brazing in N2 produced significant discoloration of the SS and Ti-6Al-4V but the braze joints were acceptable. Rectangular alloy specimens on an alumina base plate were brazed with 0.002 inch thick braze foil with a small weight on top of the assemblies. The hold time at 995°C was 10 minutes. Lap joints were also brazed in vacuum and N2 with the same foil thickness and overlap of 0.5 inches. Mechanical test specimens were electro-discharge machined (EDM) after brazing with dimensions per AWS C3.2. The lap joint specimens were tested at room temperature with quasi-static strain rate. Optical metallography, microhardness, scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS), and wavelength dispersive spectroscopy (WDS) electron microprobe are being used to characterize the braze joints and base metals. The microstructure characterization along with mechanical test results will enable selection of the best brazing conditions for the dissimilar alloy joints.
Results (300 words)
Preliminary microstructural characterization will only be described here. The braze joints showed good wetting and joint formation, with significantly more reaction on the Ti-6Al-4V side of the joint, especially for the Ag-Cu-Zr active braze alloy which experienced some runout onto the Ti-6Al-4V side. For pure Ag filler alloy brazed in vacuum, significant alloying occurred within the braze joint, although some regions of pure Ag remained. Ti, Al, and V diffused throughout the braze joint but less diffusion was evident on the 304L side of the joint. A Cr-rich, Ni/Fe-lean diffusion layer formed at the interface. Interestingly, Ni was rejected from this layer but was found at a greater diffusion depth into the braze joint and even into the Ti-6Al-4V. Some regions of crack/separation were found on the SS-side of the joint, possibly due to excessive Kirkendall void formation. For pure Ag brazed in N2, a similar morphology was observed but with less pure Ag remaining. Fine microcracks were found at the SS interface and within the Cr-rich diffusion layer suggesting a high hardness, brittle zone in that region.
In the Ag-Cu-Zr braze joints processed in vacuum, excessive runout on the Ti-6Al-4V side left behind void space, especially at the outside of the braze joint. The internal braze morphology was more uniform but complex. Fewer regions of separation or cracking were observed. In some areas, the SS-side of the joint exhibited a columnar morphology in the Cr-rich diffusion zone, with Fe, Ag, Zr, and oxygen also present. Further characterization and mechanical test results will be given in the presentation.
Fig. 1. Ti-6Al-4V brazed to 304L SS in vacuum with pure Ag filler (top) and Ag-1Cu-2Zr (bottom).
Conclusion (200 words)
This preliminary work has shown the feasibility of simultaneously brazing SS to Ti while also performing a hermetic glass sealing operation. The glass/ceramic-to-metal seal temperature range is somewhat high for dissimilar brazing with Ag braze filler and significant alloying throughout the braze joint is observed. Excessive wetting and runout are found on the Ti-6Al-4V, especially for the Ag-Cu-Zr active braze alloy. Although some interface separation and isolated microcracking are observed, the joints are expected to be hermetic. Limited reaction on the SS side suggests Ni-plating of the stainless steel prior to brazing may be warranted. No significant differences were found between vacuum and nitrogen brazing, with the exception of nitride discoloration of the base metals brazed in N2.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.