Figure 1. A braze fillet that is slightly recessed down into the joint area (photo courtesy of Dan Kay)
This question comes up frequently and needs to be addressed again. As shown in Figure 1, the brazing filler metal (BFM) has filled the inside of the tubular joint but has a slight recess at the top edge of the joint. There is no large external fillet (or “meniscus”) of BFM on the outside of the brazed joint. Notice in the photo how the recessed material has a concave shape to it. The “meniscus” of any liquid is the curved shape of the surface of that liquid caused by surface tension. A meniscus can be either concave (desirable in brazed joints) or convex. In Fig. 1, there is a concave recessed meniscus to the BFM at the top edge of the joint. Is this okay?
Many people who see such a joint may incorrectly think that any fillet, whether it is for welding or for brazing, must extend beyond (outside) the joint in order to be acceptable and that any joint that has a recessed-meniscus, as shown in Fig. 1, has to be rejected as being “incomplete”. This is what I call “weld-think”, and has resulted in many such joints being re-brazed (unnecessarily) in order to add more BFM to the joint until the resulting joint shows a large external fillet. This is erroneous thinking that can actually hurt the brazed assembly.
Fig. 2 shows a cross-section of a brazed joint, and you can see that the BFM extends way all the way through the joint between the two mating surfaces (the faying surfaces). All the so-called “goodness” of a brazed joint, i.e., its strength, its leak-tightness, etc., comes from properly filling the gap (the inside of the joint) between the faying surfaces. It does NOT come from any external fillets!