Fig. 1. Variations in centerline structures in Ni-brazed joint, from extreme (right side of photo) to almost-none at far left (ASM Metal Progress magazine, Dec. 1967 The famous photomicrograph in Fig. 1 clearly shows how the hard centerline matrix forms in the center of the joint (since solidification moves from the edge of the base-metal/brazed-joint interface to the center of the joint).
1. Nickel-based brazing filler metals (BFM) can leave a hard, non-ductile eutectic phase in the middle of a brazed joint.
The hard, non-ductile metallurgical phase-structures that form upon solidification of Ni-brazed joints must be carefully controlled, or else they can, and will, result in cracks inside the joint in stressful mechanical or thermal-cycling service.
The last phases to solidify when brazing with nickel-based brazing filler metals (BFMs) will be those phases that are the lowest-melting, i.e., those phases rich in the temperature-lowering, eutectic-forming, elements (meaning those that are rich in boron, silicon, or phosphorus). Remember, “eutectic” refers to the composition of an alloy that is the lowest melting point portion of the BFM. Thus, eutectic phases will not only be the first composition to start melting during heating of the BFM but also will be the last to solidify during cooling. Thus, during cooling these eutectic-phases will “migrate” towards the center of the joint as the “solidification-front” of the BFM moves from the base-metal/BFM interface toward the center of the joint, and will be forced to solidify right at the center of the joint.
Unfortunately, all of these temperature-lowering, eutectic-forming, elements in nickel-based BFMs are also hardeners, that is, the phase-structures resulting from solidification of these elements have virtually zero ductility! Thus, the last phases to solidify (in the center of the joint) will be hard, and non-ductile. If the joint is thicker than only about 0.004” (0.10mm) max., these hard centerline eutectics can actually form a continuous line down the center of the joint, and cause the joint to become very prone to cracking under any kind of thermal or mechanical stress or strain in service.
Notice, however, the portion of the brazement shown on the far left side of the photo in Fig. 1, which is at a right angle to the larger horizontal joint structure in the photo. Because that joint on the far left side is very thin (less than 0.003”/0.075mm), the amount of eutectic “hardener” available in that joint is minimal, so much so, that there is physically not enough of that temperature-lowering additive present to enable the formation of any kind of continuous centerline eutectic. Therefore, this thin joint would behave in a very ductile fashion in service.
All nickel-brazed joints, therefore, should be designed with this in mind, if they are to perform adequately in service.
2. These centerline-eutectics can become initiators of cracks in components when subjected to thermal or mechanical shock and/or fatigue.
To further illustrate this concept, look at the photo in Fig. 2, which shows a polished cross-section of a round-pin that was nickel-brazed into a much larger diameter round hole. Note how the solid-pin drifted to one side of the hole, causing the joint on the right side of the photo to be thin, with just a few spots of what is called a “non-continuous centerline eutectic”. Those darks spots appearing in the joint on the right side of the photo are not voids, but are actually dark-etching centerline-eutectic phases, surrounded by ductile nickel-chrome solid solution material. By contrast, the left side of the joint in that photo is quite wide, resulting in the formation of a continuous centerline-eutectic structure, which then cracked in service, as clearly shown in that photo.