When visiting a brazing job-shop recently, I was told that one of the shop’s clients insisted that some of their components be re-brazed because the braze fillets were not large enough, according to that client. When I examined the component myself, the braze fillet was actually perfectly OK, but the shop’s client thought that all fillets – welded or brazed – needed to be large.
“Isn’t that the purpose of a fillet?” the client asked when we talked to him on the phone. “Aren’t fillets supposed to add a lot of strength to the joint?” he then asked.
In the discussion that followed, I explained to him the purpose of a fillet in brazing, and the actual strength of a brazed joint did not come from the fillet but from the filled capillary space inside the joint. Let’s look at this further.
Braze fillets, such as those shown in Figures 1 and 2, are actually castings. They represent the amount of applied brazing filler metal (BFM) that remains on the outside of a joint after capillary action has drawn most of the molten BFM into the joint during the actual brazing process.
The amount of BFM that remains on the outside of the joint will solidify in a manner similar to any casting that is poured and then solidifies in place, but (it must be remembered) this external BFM fillet is not a significant factor in determining joint strength at all.
What does a fillet do?
An external fillet is, first of all, a natural outcome of any brazing process in which the BFM is applied along the outside of a joint to be brazed. The BFM may have been applied as a paste or as a solid metal (often in the form of a ring placed around a joint). After the BFM melts and much of it is drawn into the joint by capillary action, the small amount of BFM that remains along the edge of the joint is important evidence of the fact that the BFM has melted and flowed. If the quantity of BFM needed to fill the joint has been properly calculated prior to brazing, then – when the BFM melts and flows – just a tiny amount will remain on the outside of the joint to form a very small fillet, which is often called a “braze meniscus.”
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.
How should a braze fillet be inspected?
The best way to check the quality of a fillet is to simply look at it with your eyes. Is the fillet concave in shape? Does it go completely around the joint? Is it clean and smooth? These are the readily noticeable characteristics of any good brazed joint. Since the braze fillet (meniscus) is supposed to be very small, simply observing these few qualities of the meniscus should be totally adequate to verify the “goodness” of a brazed joint.
If there are any noticeable cracks in the fillet/meniscus, then that would be justification to reject any brazed assembly until further evaluation can be made to determine the nature and extent of the crack. If it is superficial and can be ground away, then perhaps that cracked area may be repaired by a localized braze (such as by a torch or induction coil).
NOTE: 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 may be fine for use in weld re-pairs, but it is not at all useful if you wish to repair a brazed joint for two primary reasons.
First, FPI merely shows that there may be surface imperfections on the outside of the fillet, but it tells absolutely nothing about the quality of the inside of the brazed joint where capillary action has caused much of the molten BFM to flow.
Second, FPI chemicals inside the crack in a brazed joint cannot be fully removed since nobody completely grinds out the inside capillary surfaces of a brazed joint and the external fillet in order to com-pletely eliminate the crack. Therefore, such cracks cannot be re-brazed. FPI merely presents a scrap option to brazers if cracks are found. It does not present any repair options.
Braze Fillets Should Never be Dimensioned on Drawings or in Specs
Fillets are not a dimensionally controlled part of any brazing process. The external braze fillet (meniscus) is merely the “after effect” of a brazing process. Because its size can never be completely con-trolled (especially in furnace brazing), the size of a braze fillet should never be specified on a drawing or in a spec.
Post-braze measurement of a fillet’s size becomes a meaningless waste of time in the shop, and it can be very costly to accurately perform. If a drawing specified that a fillet needed to be a minimum of 0.0625 inch in size, and it was found that the fillet was only about 0.050 inch upon inspection after brazing, what then? Theoretically, it is supposed to be rejected and re-brazed to add more BFM to the fillet. This is a time-consuming waste, especially since it has absolutely no bearing on the filled capillary space in the actual brazed joint itself, where all the “goodness” of a brazed joint resides.
Dan Kay operates his own brazing consulting practice in Connecticut (since 1996) and has been involved in brazing for almost 45 years. He received his BS in Metallurgical Engineering from Rensselaer Polytechnic Institute and his MBA from Michigan State University. Dan has been contributing brazing blogs for industrialheating.com for 13 years.
All figures/graphics provided by the author.