Brazing fillets can be a greatly misunderstood phenomenon in brazing. Some people insist that big fillets are needed, whereas others say that they are not. Let’s take a closer look at fillets in brazing, what they are, what they do and what characteristics about them are desirable.

A braze fillet is actually a casting along the outside of a braze joint that shows that the brazing filler metal (BFM) has melted and flowed along the edge of a braze joint. It doesn’t tell you if the BFM has adequately penetrated the joint, and caution is therefore strongly recommended to anyone attempting to use the many characteristics of a fillet as inspection criteria for judging the overall quality of a braze joint.

Fillets are not a significant factor in determining joint strength.

What does a fillet do?

Fillets, first of all, are a natural outcome of the brazing process and merely give evidence that the BFM has melted and flowed. Fillets can also show whether or not there is good compatibility between the BFM and the base metal, and they may also be able to tell you about base-metal cleanliness. However, strong caution is advised against depending on fillets to be a distributor of stresses (these items will all be covered in the two parts of this blog – this week and next).

What are the desirable characteristics of fillets?

1. Fillets should be concave. The shape of a fillet is very important, and concave is the desired shape. When the fillet is concave, the edges tend to feather out at each edge and blend in nicely with the base metal. This indicates three things: (a) there is good metallurgical compatibility between the BFM and the base metal, (b) the base-metal surfaces are clean and (c) the brazing “atmosphere” is good. This is very important! In contrast to this, if the shape of the fillet is convex instead of concave, that would tend to indicate the following:
  • Poor metallurgical compatibility between the BFM and the base metal
  • Base-metal faying surfaces are not clean enough to allow proper BFM flow (faying surfaces contaminated with surface oxides or oils, etc.)
  • Brazing atmosphere is poor
  • Any combination of these three factors
2. Fillets should be small. This is where people often get themselves in trouble. Some people erroneously believe that the larger the fillet, the better the braze joint. In actuality, just the opposite is true. Since a fillet is an external casting, the larger it is, the more casting imperfections will be present. Conversely, the smaller the fillet, the fewer imperfections will be present. These imperfections include voids, porosity, shrinkage cracks, open dendritic “fir-tree” structures, etc.

Typical causes of porosity and voids in joints are outgassing from the filler metal, base metal or flux (if used) and surface contamination. Cracks and dendritic structures generally become more pronounced as fillets get larger. When the liquid BFM in the fillet begins to cool and starts to “freeze,” dendrites form. Then, as the remaining liquid continues to cool, it pulls away from the dendrites, leaving a rough, porous area. These fillet imperfections may be of such a size that they become defects (causing part rejection) or might act as stress risers at the joint edge that could hurt service life and performance of the part.

Do fillets add to the strength of brazed assemblies?

The strength of a brazed joint comes from a number of factors(there are other factors than these listed):
  • Design of the parts being brazed (and particularly the design of the outside edges of the brazed joint as far as stress concentration factors are concerned)
  • The cleanliness of the faying surfaces inside the joint
  • The gap clearance at brazing temperature
  • The differential expansion characteristics of the two materials being joined
  • The compatibility of the BFM and the base materials
Please note that "fillet size" and "fillet shape" are not in that list of factors. That's intentional because fillets should never become a criterion for joint strength! Yes, there are exceptions to every rule, but the only reason that fillets would ever enter into strength-of-joint considerations is when there are major deficiencies in the other joint-strength factors I’ve just listed, which means the joint would then be questionable in any event.

This debated question about fillets and joint strength does not always have a perfectly clear answer. In a properly designed joint assembly, the answer is no – fillets do not add to joint strength. In such parts the presence of a fillet is not important other than to show that the BFM has indeed flowed completely around a joint. In a poorly designed part, however, it may be necessary to try to “cast” enough extra BFM around the joint to help “spread the stress” a little. However, this rarely works well in the long term and can put the part at risk in service when the brazed joint is trying to depend on the strength of a casting to survive the service conditions it will encounter. Therefore, depending on a fillet/casting is merely trying to compensate for the existence of other joint-design deficiencies in the part for a given end-use application.