Fig. 1 A nice concave fillet (meniscus) at edge of brazed joint

Introduction

As mentioned in an earlier article, external brazing-fillets can be greatly misunderstood. Some people insist that big fillets are needed in brazing, whereas others say that they are not. Lets take another look at brazing-fillets, to show how size and shape of brazing fillets should be correctly interpreted.

Please note that a braze fillet is actually a casting along the outside of a braze joint that simply shows that the brazing filler metal (BFM) has melted and flowed along the edge of a braze joint. However, it doesn’t tell you if the BFM has adequately penetrated the joint. Caution is therefore strongly advised to anyone attempting to merely use the size of a braze-fillet as an inspection criteria for judging the overall quality of a braze joint.

What Does a Braze-Fillet (Meniscus) Do?

A braze-fillet, first of all, is just a natural outcome of the brazing process, and merely gives evidence that the BFM has melted and flowed. Because of its size and shape, a braze-fillet is also commonly called a “braze-meniscus”. Both terms are perfectly acceptable to use. A braze-fillet can also show whether or not there is good compatibility between the BFM and the base metal, and can also tell you about the base-metal cleanliness in the joint region, as well as about the furnace atmosphere quality.

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, as shown in Fig. 1.

A concave meniscus (fillet) 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! Due to surface-tension characteristics, the molten BFM wants to spread out over the metal surface, and can only do so 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-BaseMetal bonding. As an example, pure copper can only diffuse up to a maximum of approximately 5% into steel, but it is sufficient to allow good copper-BFM flow into a steel joint.

Convex meniscus (fillet)

Fig. 2 Convex meniscus (fillet). Why didn’t it flow?

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.

How Should Fillets Be Inspected?

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.

Braze fillet (meniscus) is very small

Fig. 3 Braze fillet (meniscus) is very small

2. Fluorescent Penetration Inspection (FPI) is NOT recommended. Many people still use FPI on braze-fillets to accept or reject parts. This can be a big mistake. FPI is fine for welds, but it is not really useful for brazed joints for two primary reasons: a) FPI merely shows that there may be surface imperfections on the outside of the fillet, but it tells absolutely nothing about the inside of the brazed joint itself; and b) FPI chemical removal requirements are very different in welding than in brazing. If FPI reveals cracks in a weld fillet, the entire fillet needs to be cut out or ground away (thereby completely removing all the FPI chemicals) and a new weld bead is then laid down in place.

However, in brazing, the BFM in the joint is not going to be cut away and replaced, and therefore any entrapped FPI chemicals have to be completely removed from the fillet itself either by ultrasonic cleaning or fluoride-ion cleaning (FIC) before a rebraze can be attempted. Do NOT think that FPI contamination in surface voids, cracks, or dendritic porosity will be effectively removed by soaking in a solvent, or by either hydrogen or vacuum furnace cleaning.

AWS C3.6 Specification for Furnace Brazing specifically discourages anyone from using FPI in brazing inspection procedures. It clearly states that penetrant inspection techniques “are not suitable for the inspection of braze fillets because they routinely give false results.”

Conclusion

A braze-fillet (meniscus) is a natural outcome of a brazing process. Visual inspection is easy and highly reliable when brazing is done properly. Merely look to see that BFM is indeed present all around the joint, that any filleting is concave in shape, and preferably as small as possible.

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