In contrast to this, if the shape of the fillet is convex instead of concave (as shown in Fig. 2), that would tend to indicate that there may be poor metallurgical compatibility between the BFM and the base metal, or that the base metal faying surfaces are not clean enough to allow proper BFM flow (faying surfaces contaminated with surface oxides or oils, etc.), or that the brazing atmosphere is poor, or 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. A braze fillet (meniscus) should be as small as possible, as shown in Fig. 3.
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 the number of imperfections that 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, from the base metal, 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 solidify, dendrites can form, and then as the remaining liquid continues to cool, it can pull away from the dendrites leaving a porous area. These fillet imperfections might act as stress-risers at the joint edge that could actually hurt the performance of a part in service.
1. Visual. The best way to check the quality of a fillet is simply to look at it, perhaps even using a 10X magnifier. Is the fillet concave in shape? Does it go completely around the joint? Is it clean and smooth or is it filled with porosity or cracks?
NOTE: Be very careful about specifying number of voids per linear inch (cm), or specifying size of each void etc. This practice can be a trap, and could result in the rejection of parts that might otherwise be perfectly fine. The fact that a fillet might have three (3) bubbles at its surface in a one inch length instead of only two (2) allowed bubbles, has nothing to do with the quality of the BFM that has flowed inside of the brazed joint. It also calls, once again, for a lot of extra inspection time to do these external fillet measurements, when what’s happening inside the brazed joint is actually more important.