Can you provide a succinct explanation of braze erosion? What causes it, and where does the eroded material go?
The official definition of "erosion" in the brazing world (as shown in the AWS Brazing Handbook) is: "A condition caused by the dissolution of the base metal by the molten brazing filler metal (BFM), resulting in a reduction of the base-metal thickness."
Unlike water rushing through a river and wearing away (eroding the banks of the river), BFM erosion is more than just relative motion of liquid over solid. It also includes the strong effects of molten BFM interaction and diffusion with the base metals being joined. This latter point is very important.
Erosion also applies only to externally applied BFM, since erosion of base metals is only apparent outside a brazed joint. BFM inside the joint may diffuse into the base metals, but dimensional changes do not seem to occur. Consider the following:
Copper BFM can braze 304L stainless steel components very well, and, when applied outside a joint to be brazed, exhibits very little "erosion" of the base metals. This is because copper does not alloy much at all with stainless steels – less than 5% alloying with the steel. Nickel-based BFMs, however, alloy extensively with stainless steels (100% "miscibility" so to speak) and, thus, can cause extensive "erosion" under the following conditions:
1. Too much BFM applied – The joint being brazed can only be filled once by the molten BFM. Since excess BFM cannot flow into the joint, it may begin to extensively alloy with the base metal (BM) outside the joint. This inter-diffusion of BFM into BM and BM into BFM can change the dimension of that surface, sometimes resulting in reduced thickness of the base metal at the point of alloying. This change in dimension is not caused by run-off of the metals but by the alloying of the two materials together, resulting in a structure that can occupy less total volume. See Figure 1 of a nickel-brazed joint in which "erosion" of the outside surface can be seen.
2. Too high a brazing temperature – Most BFMs should be brazed at a temperature approximately 100°F/50°C above the published liquidus temperature for that BFM (there are always exceptions to this guideline). When brazing temperatures greatly exceed this guideline, such as when brazing at temperatures that exceed the liquidus by 300-400°F (150-200°C), erosion can become very pronounced!
3. Too long a time at brazing temperature when there is excess BFM applied – In a similar fashion as mentioned in 1 and 2, holding excessive quantities of BFM at brazing temperature for too long a time can also result in erosion of the brazed joint since the excess BFM sitting outside the joint has lots of time (and temperature) to diffuse into and alloy with the base metal. – This phenomenon of erosion can occur with any BFM family that can alloy extensively with the base metals being joined. Thus, BFM "erosion" can occur when aluminum brazing, with copper brazing of copper-based metals, with silver brazing of copper-based metals, etc. If the BFM can alloy extensively with the base metal being joined, then steps to prevent "BFM erosion" of the base metals should always be taken.