The Importance of Recessed Braze Fillets (part 1)

This is a very important topic that needs to be addressed. Design engineers and brazing-shop managers need to become “agents of change” to bring about a needed adjustment of thinking in industry about the need for external fillets in brazing.

The question of recessed braze fillets has already come up several times in 2023 at brazing shops in private industry and in the U.S. military, for which this writer has had to become involved. Too many brazing specifications still exist (written by both private and government agencies) that erroneously state that fillets in brazing cannot be recessed. Such statements are incorrect and can be greatly misleading. Numerous situations have been shared with me during the last year about existing specification requirements against recessed fillets, which then necessitate re-brazes of too many parts by a number of brazing shops. In a number of those cases, however, such re-brazing actually hurt the parts in question and ultimately resulted in scrapping those parts.

As can be seen in Fig. 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 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 (very desirable in brazed joints) or convex. A convex-shaped fillet is actually poor for brazing since it tends to indicate poor wetting of the base metal by the BFM. It can be seen in Fig. 1 that the BFM’s recessed meniscus has a concave shape to it and has completely flowed around the circumference of the joint. It is a good, sound joint.

Fig. 1. A braze fillet that is slightly recessed down into the joint area.

Many people erroneously believe that any fillet, whether it is for welding or for brazing, must extend beyond or outside the joint in order to be acceptable and that any joint that has a recessed meniscus (Fig. 1) has to be rejected as being incomplete. This is what I call “weld-think,” and it 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.