Question: I recently read your blog about isothermal solidification (ITS). I want to be sure I understand how it relates to ordinary furnace brazing with the BNi-2 nickel filler metal we use, where the time at temperature is limited to only 10 minutes. How will our short time at brazing temperature affect the remelt temperature of the BNi-2 brazing alloy?
Answer: Brazing aerospace components with certain nickel-based brazing filler metals (BFMs) can actually be done in a manner that will significantly increase the remelt temperature of the BFM in the joint. This can be important if, for example, that nickel-brazed component were subjected to sudden, accidental high-temperature extremes (much higher than the original brazing temperature) during end-use service.
As I described in a previous blog here (relating to isothermal solidification), the AWS A5.8, Class BNi-2 (also known in the industry as ASM 4777) is a good example of a Ni-based BFM whose remelt temperature can be significantly increased if held at brazing temperature for a long enough time. The amount of this increase in the remelt temperature (which can typically range from 200-700°F/100-400°C above the actual brazing temperature used) depends directly on how much boron (B) is in the chemistry of the BFM, how high a brazing temperature is used and how long the BFM is held at that temperature.
Please note that the BNi-2 alloy chemistry has about 3.5% of boron added to it as a temperature-depressant (i.e., an element added to the BFM to significantly lower its initial melting temperature during brazing). It needs to be emphasized once again that boron is a very small atom compared to the much larger atoms of nickel, chromium, iron and silicon that make up the rest of the BNi-2 chemistry (Fig. 1). As an “interstitial” type of atom, it is loosely bonded in the metal structure.
At brazing temperature, the atoms in the hot metal have opened up their distances from neighboring atoms in the metal matrix, and the tiny atoms of boron can very quickly move out of the matrix entirely. Remember, the boron was added into the BFM chemistry because it is very effective at lowering the BFM’s melting temperature. Therefore, does it not seem logical that the melting point of the BFM should go back up when the boron leaves (i.e., diffuses away from) the BFM? In fact, that does happen quite rapidly.
We will finish this discussion in part 2..