Here is part 2 of Dan Kay's blog about the purpose of a braze fillet. Part 1 can be found here.
 

What should a braze fillet look like?

• Fillets should be small, like that shown in Fig. 2 (the fillet is tiny and barely visible). Since a fillet is an external casting, the larger it is, the more casting imperfections may be present in that fillet. Strive to keep fillets as small as possible. The smaller the fillet, the fewer the number of imperfections that will be present in that fillet. Imperfections in large fillets may include voids, porosity, shrinkage cracks and open dendritic “fir-tree” structures (Fig. 1a).
• Typical causes of porosity and voids in fillets are outgassing of braze binders from an applied BFM paste, outgassing from the base metal, flux (if used) and surface contamination. Dendrites can begin to form in fillets as the liquid BFM starts to “freeze” (i.e., solidify) during cooling. More voids can form as the remaining liquid pulls away from the dendrites during cooling, potentially leaving a rough, porous area. These fillet imperfections may become defects (causing part rejection), or they might act as stress risers at the joint edge (which could hurt performance of the part in service).
• Fillets should be concave. When the fillet is concave (Fig. 3a), the edges of the fillets tend to feather-out at each edge and blend in nicely with the base metal. A concave meniscus (fillet) indicates three things:
  • There is good metallurgical compatibility between the BFM and the base metal. For good brazing to occur, the molten BFM must be able to alloy with (i.e., diffuse into) the two base metals being joined together. If the BFM is compatible with (can alloy with) each of the base metals being joined, then surface tension will draw (pull) the molten BFM along each base-metal surface, forming a con-cave-shaped fillet (or “meniscus” since it is so small).
  • The base-metal surfaces are clean. BFMs do not like to bond to or flow over oils, dirt, lubricants or oxides. Any of these contaminants on the metal surfaces will prevent the molten BFM from flowing into or along a brazed joint. Thus, the BFM may merely sit where it was applied, taking a convex (rounded) surface (Fig. 3b).
  • The brazing “atmosphere” is good.  As indicated previously, a poor atmosphere generally allows surface oxidation to occur, which will prevent the molten BFM from flowing into the joint by capillary action.

As described in (a) through (c), concavity of the meniscus is very important. Thus, since a concave meniscus indicates a good fillet on the outside of the joint, it can be safely assumed that the BFM has also flowed well into the joint. In contrast, if the shape of the fillet is convex instead of concave (Fig. 3b), that would tend to indicate that: there may be poor metallurgical compatibility between the BFM and the base metal; the base-metal faying surfaces are not clean enough to allow proper BFM flow (faying surfaces contaminated with surface oxides or oils, etc.); the brazing atmosphere is poor; or any combination of these three factors. This can then lead to the conclusion that the BFM did not flow well into the joint by capillary action, and the joint will display very poor braze properties.

Therefore, concavity of braze-joint fillets is an excellent quality-control feature of any brazed joint and should be used by inspectors as a general guide to the “goodness” of any brazement.

Does a fillet add to the strength of a brazed joint?

The fillet does not add to joint strength in a properly designed brazed joint. The fillet merely shows that the BFM has melted and flowed. If the corner of the assembly shown in Fig. 4a is bent or twisted, there will be a concentration of stress forces right at the sharp corner of the two pieces being brazed together. This focused stress concentration may cause that sharp corner to be the place where a crack can start, perhaps leading to the eventual service failure of the part that was brazed.    

Some people use this potential stress concentration as an excuse for building up a large braze fillet in that corner “to help spread that stress.” This is erroneous thinking since a large fillet is a large casting, and the casting imperfections inside the fillet may themselves become a source of premature failure of the joint. (Look at the poor quality of the cast fillet in Fig. 1a once again.)

To solve this problem, the joint designer should specify that the stress concentration be removed by smoothing the edge of the joint by removing some of the metal at that corner, perhaps by chamfering the metal or grinding down the corner (Fig. 4b).

NOTE: It is the designer’s responsibility to design the joint edge in such a way that a stress concentration is never present at the edge of a brazed joint. BFM is only supposed to bond two materials together. BFM is not supposed to also act as a “stress-spreader” at the edge of a joint to make up for design errors.

Fig. 4a and Fig. 4b showing effect of stress concentrations in corner of brazed components.  It is not the job of BFM to prevent that stress concentration. Instead, remove metal by shaping the component to spread the stress.