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By Dan Kay
Erosion is defined in the AWS Brazing Handbook (published by the American Welding Society in 2007) as follows:
“Erosion is a condition caused by the dissolution of the base metal by the molten brazing filler metal, resulting in a reduction of the base metal thickness.”
Thus, the phrase “base-metal erosion by the brazing filler metal” is used to describe a process in which a molten brazing filler metal (BFM) which is highly soluble in a given base-metal (parent metal), is applied to the surface of that base metal, is heated to brazing temperature, and in so doing, actively diffuses into that base metal, alloying extensively with (dissolving) it.
Some people prefer to use the term “dissolution” of the base metal rather than “erosion” when referring to this phenomenon. That’s fine, and is actually a technically more correct descriptive term for what is happening (both terms, erosion and dissolution, have been used extensively for several decades, and the reader may use the term with which they feel more comfortable). “Erosion” is a word that quickly, and accurately, tells the reader or listener that something “less than desirable” is happening, and is a major reason, in my opinion, why it is so widely used in the industry.
Base-metal erosion (dissolution) is illustrated in Figures 1, 2, and 3 where it can be plainly seen that the brazing filler metal (BFM) has extensively alloyed with the base (parent) metal, resulting is a significant loss of base-metal thickness. This can have a very undesirable effect if the amount of base-metal dissolved by the molten BFM becomes too extensive.
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| Fig. 1 -- Nickel-based BFM eroding (dissolving, i.e., eating through) two stainless hi-temp alloy sheets, when held at brazing temp of 2200F (1200C) for 15-minutes. |
As can be clearly seen in Fig. 1, a large amount of nickel-based BFM was placed between the two sheets of stainless-type base metal above the spot-weld that joined the two sheet-metal pieces together (weld shown at the bottom of the photo). The molten BFM had no place to go, and thus, at the high temp of brazing began to rapidly dissolve (erode) the stainless base metal. Had it remained at brazing temperature much longer, the BFM would have completely eroded through the sheet metal. You can also clearly see the “casting” nature of braze fillets in that photo, since the dendritic structure and porous nature of the cast-fillet are clearly seen.
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| Fig. 2 -- BNi-2 BFM joining 316L stainless steel EGR cooler fins. |
Note in Fig. 2 that the nickel-based BFM has begun to dissolve the base metals above and below the brazed joint. Although the total thickness of the brazed assembly remains the same, the thickness of the base-metal gets less as the alloyed area between the BFM and base metal gets greater. Testing will be required to determine the effect (negative or positive) on the performance of the brazed part in service.
The erosive effects of a molten BFM may be readily observed when that liquid BFM finally gets pulled into the joint being brazed, and you can clearly see some undercutting, i.e., a reduced base-metal thickness, at the point where the BFM was sitting outside the joint while waiting for the faying surfaces inside the joint to come up to a temperature. This "under-cutting" (i.e., reduction in the thickness of the base metal) must be controlled.
The AWS C3.6 Specification for Furnace Brazing states in paragraph 5.5.1.3: “Any evidence of braze filler erosion of the exposed base metal surfaces is unacceptable if the erosion of either member exceeds 5 percent of the thickness for Class A and 15 percent for Class B or Class C of the thinnest component of the brazed joint.”
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Fig. 3 -- Aluminum mating surfaces showing significant erosion (dissolution) of base metal (Mating Surface A). Photo courtesy of Ed Patrick (E.P.Patrick Associates).
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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.
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| Fig. 4 -- The three important items to remember about BFM. |
Please note, however, that something very important is missing from the GE definition above, namely, any mention of the quantity of BFM being used.
Of the three shown in Fig. 4, the most important is, in my opinion and experience, the quantity of BFM applied. If just enough BFM is applied to fill the gap between the faying surfaces, then erosion (dissolution) of base-metals will be minimal. As I like to mention in each of my brazing seminars: “When the correct amount of BFM is applied to the joint, you can cook the parts until the cows come home…”, and you will not have any significant problems with either erosion or dissolution, since there physically is not enough BFM present to really cause any problems.
Additionally, if the BFM is pre-placed inside the braze-joint, then any concerns with erosion or dissolution are really minimized to the point of becoming basically non-existent.
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