Nickel brazing is unique in the brazing world in that each of the nickel-based brazing filler metals (BFMs) available for use today depends on the use of temperature-lowering ingredients (i.e., temperature-depressants) such as boron, phosphorus and/or silicon to enable the BFM’s usefulness for joining stainless steels and many other superalloys for critical aerospace applications.
Unfortunately, boron, phosphorus and silicon act as “hardeners” in the nickel-chromium alloy structure and can reduce the ductility of a nickel-brazed joint to zero if that joint is not brazed properly. By properly I mean that the gap clearance of a nickel-brazed joint should be kept very small – 0.000-0.002 inch (0.00-0.05 mm) thickness or less – if truly ductile behavior in end-use service is desired.
Fig. 1 shows an award-winning photomicrograph of a nickel-brazed component that was published in the December 1967 issue of Metal Progress magazine, a publication of the American Society for Metals (the organization’s name has since been changed to the American Society for Materials). The photomicrograph shows the microstructure of a variable-clearance brazed joint between a free-machining 18-8 stainless steel component at the top of the photo and a Vanadium-Permendur component on the bottom.
Fig. 1. Variations in centerline structures in nickel-brazed joint (Metal Progress magazine, December 1967)
The BFM used was the nickel-based AMS 4777 (also identified as AWS A5.8, Class BNi-2). It can be seen that the BFM fills three different gap-clearances: a large slot at the right side of the assembly, a wide joint gap in the center of the photo and a very thin joint at the far left of the photo.
Although there is a shrinkage-crack visible in the center of the BFM pool at the right side of the photo, the rest of the BFM material in the horizontal joint along the center of the photo is solid. It exhibits no cracks, voids or depressions. Instead, it shows a smoothly polished and etched centerline structure along the entire horizontal length of the joint. This centerline material is rich in zero-ductility chromium-borides. It is interesting to note that no such continuous centerline eutectic material is shown in the very thin vertical portion of the joint at the far left of the photo.
When a joint solidifies, solidification occurs from the outside edges of the joint to the center. The low-melting-temperature depressant ingredients (eutectic-forming elements) are the last to solidify in the center of the joint. In the nickel-based BFMs, therefore, these low-melting eutectic forming ingredients (boron, phosphorus or silicon) have their heaviest concentration along the center of the joint.
Look for more of this discussion next time.
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