When visiting a brazing job-shop recently, I was told that one of the shop’s clients insisted that some of their components be re-brazed because the braze fillets were not large enough, according to that client. When I examined the component myself, the braze fillet was actually perfectly OK, but the shop’s client thought that all fillets – welded or brazed – needed to be large.
“Isn’t that the purpose of a fillet?” the client asked when we talked to him on the phone. “Aren’t fillets supposed to add a lot of strength to the joint?” he then asked.
In the discussion that followed, I explained to him the purpose of a fillet in brazing, and that the actual strength of a brazed joint did not come from the fillet but from the filled capillary space inside the joint. Let’s look at this further.
Braze fillets, such as those shown in Figures 1 and 2, are actually castings. They represent the amount of applied brazing filler metal (BFM) that remains on the outside of a joint after capillary action has drawn most of the molten BFM into the joint during the actual brazing process.
The amount of BFM that remains on the outside of the joint will solidify in a manner similar to any casting that is poured and then solidifies in place, but (it must be remembered) this external BFM fillet is not a significant factor in determining joint strength at all.
What does a fillet do?
An external fillet is, first of all, a natural outcome of any brazing process in which the BFM is applied along the outside of a joint to be brazed. The BFM may have been applied as a paste or as a solid metal (often in the form of a ring placed around a joint). After the BFM melts and much of it is drawn into the joint by capillary action, the small amount of BFM that remains along the edge of the joint is important evidence of the fact that the BFM has melted and flowed. If the quantity of BFM needed to fill the joint has been properly calculated prior to brazing, then – when the BFM melts and flows – just a tiny amount will remain on the outside of the joint to form a very small fillet, which is often called a “braze meniscus.”