It must be understood that this phenomenon of base-metal erosion (dissolution), further illustrated in Fig. 3 at the right, occurs for the following two reasons:
1. The brazing filler metal (BFM) being used is one that readily alloys with, and diffuses into, the base metal being joined. Such will be the case when aluminum BFMs are being used to join aluminum base metals, or when silver-based BFMs are used to join copper-based parent metals, or when nickel-based BFMs are used to join stainless steels or other super-alloys.
2. The brazing filler metal (BFM) is applied at the outside edge of the joint, is then rapidly heated, resulting in the BFM becoming liquid before the faying surfaces inside the brazing joint have fully reached brazing temperature. This liquid BFM may then aggressively attack (alloy with) the base metal on which it is sitting outside the joint, while it is waiting for the joint’s faying surfaces to come up to brazing temperature.
Erosion (dissolution) can also occur for the following reason as well:
3. Too much BFM is applied. The braze joint can only use enough BFM to fill the volume between the faying surfaces one time (obviously). Extra BFM that was applied, therefore, will just sit outside the joint, aggressively chewing up (dissolving) base metal, and thus “eroding” the base metal surfaces. The higher the brazing temperature used, and/or the longer the brazing cycle, the worse can be the results of this dissolution (erosion) of the base metals.
As mentioned in item#1 above, the primary reason for this erosion phenomenon is because the BFM chemistry readily alloys with the base metal on which it is placed. When the BFM chemistry is, however, such that the amount of alloying with the base metal is minimal (just enough to form a strong bond at the interface of the BFM and base metal), such as when using pure copper to braze steel components, or when using silver-based BFM to braze steel components, then the dissolution and erosion of base metals may not be readily apparent at all.
General Electric (GE) in their brazing manual entitled “Brazing for High Temperature Applications” (by R.R. Ruppender of GE’s Flight Propulsion Laboratory, 1956) indicates that the chemistry of some BFMs has a greater tendency to be erosive on some base-metals than does other BFMs, and that other outside factors also come into play. Here is their definition: Erosivity is the tendency of the molten filler alloys to dissolve the structural alloys being joined. While some solution is desirable, almost none can be tolerated when brazing very thin sheet metal. It must be remembered that erosion rate is a function, not only of alloy composition, but of brazing temperature and time as well. Except when brazing extremely thin sheet metal, erosivity should not be regarded as the first criterion for the choice of filler alloy.
For readers using Ni-based BFMs for high-temp vacuum brazing applications, please note that the most erosive elements in the chemical makeup of some of those Ni-based BFMs are boron (B) and silicon (Si), of which boron (B) is the more aggressive of the two. In light of this fact, it should be noted that there is something very important missing from the GE definition above, namely, any mention of the quantity of BFM being used. It is a very important variable in this discussion of base metal erosion.