QUESTION: We braze thousands of special cooling baffles for our customer each year, mainly thin sheet-metal AMS 5536 (Hast-X) parent material and BNi-2 braze powder with overall good results. But when we braze the same style of cooling-baffle insert and the parent material is AMS 5599 (INCO 625) brazing with the same BNi-2 filler metal, the filler metal doesn’t flow as well, especially in the lap-joint seams. Can Inconel 625 be more difficult to vacuum braze than the Hast-X when using a nickel filler metal such as BNi-2?
ANSWER: In my experience, Inconel 625 can sometimes be a bit more difficult to braze than Hastelloy X. Here are some chemistry sheets for both the Inconel 625 (Fig. 1) and the Hastelloy X (Fig. 2). You will notice that the Inco 625 chemistry shows special limits for titanium (Ti) and aluminum (Al) in its chemistry, but Ti and Al does NOT show up at all in the chemistry for the Hast X. The problem, in my experience, relates to WHEN the Inco 615 was actually cast and then rolled.
For many years, I used to manage a large casting and alloy-making shop, where we also melted/cast many lots of nickel-based brazing filler metals (BFMs). It was traditional in our casting shops to make the purest alloy first and then begin to add other metals to it to make more complex alloys on subsequent days. By the end of the week, we would make the most-complex alloys with the most metal additions to it. When that “worst-case” alloy was completed, we would thoroughly clean out the casting equipment and start over with the purest metal again and so forth.
This progressive alloying by subsequent additions is what I understand also happens with the Inconel family. They begin with their simplest, purest version, then move through the 600-series, then start adding titanium and aluminum to make their 700-series, etc. and then clean the system and start over. Unfortunately, not all “cleanings” are necessarily equal. Thus, it is quite possible (but rare) that a batch of Inco 625 might have some “carry-over” of just enough Ti or Al (or both) from earlier melts to slightly affect the flow of BFM on that particular lot/heat of Inco 625.
Notice in Fig. 1 the chemistry for Inco 625 does indeed allow for some of that Ti and Al carryover, allowing up to 0.4% of each of Ti and/or Al to take that carryover into account. That carryover of Ti and/or Al is not desirable from a brazing perspective! When the Ti and/or Al content is about 0.4% or greater, there would be enough Ti-oxide and/or Al-oxide on the surface to potentially affect BFM flow and bonding. This should not ever be the case with Hast-X, however, since Ti and Al are not even a slight “residual” concern with that alloy (Fig. 2).
Electrolytic nickel plating the Inco 625 surface would eliminate that problem completely. Since you don’t want to have to nickel plate the Inco 625 prior to brazing, however, it behooves you to get certs of finished-material analysis with each lot/heat of Inco 625 you receive so that you will know the actual amounts of Ti and Al that you have to deal with.