When people look at a brazed joint, the only thing they can see is the external braze fillet. Judgments about the goodness of a joint are often made by merely looking at the fillet and, based on what the fillet looks like, deciding if the brazed joint is good or bad. This can often lead to erroneous decisions about joint quality since a braze fillet merely gives evidence as to whether or not the BFM has melted and flowed. Because of its small size and shape, a braze fillet is also commonly called a “braze meniscus.” Both terms are perfectly acceptable to use.
What are the desirable characteristics of braze 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 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, the base-metal surfaces are clean and the brazing “atmosphere” is good. This is very important! Due to surface-tension characteristics, the molten BFM wants to spread out over the metal surface, and it can do so only if the BFM is metallurgically compatible with the base metal (i.e., they are able to alloy with each other). When this happens, the molten BFM will diffuse into the base-metal surface, and the base-metal constituents will diffuse into the BFM. It doesn’t require a lot of diffusion, but some must occur to allow BFM-to-base-metal bonding. As an example, pure copper can only diffuse up to a maximum of approximately 5% into steel, but that small percentage is sufficient to produce good copper-BFM alloying with the steel and excellent capillary flow into a steel joint.
If, however, the shape of the fillet is convex instead of concave, then that would tend to indicate that there may be poor metallurgical compatibility between the BFM and the base metal. It could also mean that the base-metal faying surfaces are not clean enough to allow proper BFM flow (i.e., faying surfaces contaminated with surface oxides or oils), 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.
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. 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. As the remaining liquid continues to cool, it can pull away from the dendrites and leave a porous area. These fillet imperfections might act as stress risers at the joint edge and could actually hurt the performance of a part in service.