Picture of the round brazement. Photo of Goodrich Aerospace (Delevan) 304-stainless steel assembly brazed with AMS 4777 Ni-based BFM. Note very tiny braze fillet.

Introduction

Brazing fillets can be a greatly misunderstood phenomenon in brazing. Some people insist that big fillets are needed, whereas others say that they are not. Let’s take a closer look at fillets in brazing, what they are, what they do and what characteristics about them are desirable.

A braze fillet is actually a casting along the outside of a braze joint that shows that the brazing filler metal (BFM) has melted and flowed along the edge of a braze joint. It doesn’t tell you if the BFM has adequately penetrated the joint, and caution is therefore strongly recommended to anyone attempting to use the many characteristics of a fillet as inspection criteria for judging the overall quality of a braze joint.

Fillets are not a significant factor in determining joint strength.

What Does a Fillet Do?

Fillets, first of all, are a natural outcome of the brazing process and merely give evidence that the BFM has melted and flowed. Fillets can also show whether or not there is good compatibility between the BFM and the base metal, and they may also be able to tell you about base-metal cleanliness. However, strong caution is advised against depending on fillets to be a distributor of stresses.

What Are the Desirable Characteristics of 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 tend to feather out at each edge and blend in nicely with the base metal. This 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! In contrast to this, if the shape of the fillet is convex instead of concave, that would tend to indicate the following:

  • Poor metallurgical compatibility between the BFM and the base metal
  • Base-metal faying surfaces are not clean enough to allow proper BFM flow (faying surfaces contaminated with surface oxides or oils, etc.)
  • Brazing atmosphere is poor
  • 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. 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 imperfections 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, base metal or flux (if used) 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 starts to “freeze,” dendrites form. Then, as the remaining liquid continues to cool, it pulls away from the dendrites, leaving a rough, porous area. These fillet imperfections may be of such a size that they become defects (causing part rejection) or might act as stress risers at the joint edge that could hurt service life and performance of the part.

Do Fillets Add to the Strength of Brazed Assemblies?

The strength of a brazed joint comes from a number of factors(there are other factors than these listed):

  • Design of the parts being brazed (and particularly the design of the outside edges of the brazed joint as far as stress concentration factors are concerned)
  • The cleanliness of the faying surfaces inside the joint
  • The gap clearance at brazing temperature
  • The differential expansion characteristics of the two materials being joined
  • The compatibility of the BFM and the base materials

Please note that “fillet size” and “fillet shape” are not in that list of factors. That’s intentional because fillets should never become a criterion for joint strength! Yes, there are exceptions to every rule, but the only reason that fillets would ever enter into strength-of-joint considerations is when there are major deficiencies in the other joint-strength factors I’ve just listed, which means the joint would then be questionable in any event.

This debated question about fillets and joint strength does not always have a perfectly clear answer. In a properly designed joint assembly, the answer is no – fillets do not add to joint strength. In such parts the presence of a fillet is not important other than to show that the BFM has indeed flowed completely around a joint. In a poorly designed part, however, it may be necessary to try to “cast” enough extra BFM around the joint to help “spread the stress” a little. However, this rarely works well in the long term and can put the part at risk in service when the brazed joint is trying to depend on the strength of a casting to survive the service conditions it will encounter. Therefore, depending on a fillet/casting is merely trying to compensate for the existence of other joint-design deficiencies in the part for a given end-use application.

It is shown above that brazing fillets can be a greatly misunderstood phenomenon. I mentioned that some people insist that big fillets are needed, whereas others say that they are not. It was shown that fillets are the natural external evidence that a brazed joint has been made, but a fillet cannot tell anyone about what actually happened between the faying surfaces inside the joint itself. Internal quality of a brazed joint must be determined by performance testing of the brazement and perhaps, if necessary, by such nondestructive techniques as ultrasonic testing. In some rare cases, such as with thin sheet-metal assemblies, this is done by radiography.

So then, how should someone inspect a brazed fillet to be able to get some clues about the “goodness” of the braze? Let’s examine that topic.

How Should Fillets Be Inspected?

1. Visual – The best way to check the quality of a fillet is simply to look at it. Maybe that sounds corny, but simply look at the fillet closely, 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?

Picture of T-joint. Photo by Dan Kay of 304-stainless steel joint using AMS 4777 BFM. Note tiny fillet at joint.

Picture of T-joint. Photo by Dan Kay of 304-stainless steel joint using AMS 4777 BFM. Note tiny fillet at joint.

Lack of fill – “Lack of fill” is not a fillet characteristic that alone is cause to reject a part. It all depends on the design of the braze joint and the end-use service conditions to which the part will be exposed. In a properly designed joint that sees low service stresses, it may be perfectly fine to allow a certain amount of lack of fill to occur. Should that happen, look inside the slight “depression” represented by the lack-of-fill area to see the shape of the BFM surface inside that area. If it is shallow and shows a concave surface down inside, then it may be perfectly OK to allow this lack-of-fill area. If, however, the inner surface is rough or convex in shape, it may indicate poor wetting inside the joint itself and may be cause for concern. In parts that see high stress concentrations at the edge of a joint due to thermal cycling or mechanical cycling, it may be desirable to disallow any lack of fill in a fillet, since it might serve as a stress riser.

It is also wise in such cases to keep the fillet size small to ensure that no “casting defects” (usually evident in larger fillets) could interfere with service performance. Likewise, if the joint is to be used in food service, medical applications or jewelry, any lack of fill may represent an unacceptable surface condition. Additionally, any lack of fill areas might cause disturbance in fluid flow or airflow and might be cause for concern.

Size and quantity of surface voids – Be very, very careful about specifying number of voids per linear inch (cm) or specifying size of each voids, 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 bubbles at its surface in a 1-inch length instead of only two 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.

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: FPI merely shows that there may be surface imperfections on the outside of the fillet but tells absolutely nothing about the inside of the brazed joint itself; and FPI chemical removal requirements are very different in welding than in brazing. If FPI reveals defects 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, fillets are not going to be cut away and replaced. 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 effectively done.

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. Although some types of oils used in the FPI might theoretically be removable by such furnace cleaning, those furnace processes may actually turn out to be ineffective at completely removing FPI oils and probably will not even touch any of the solids in the FPI chemicals (such as developers, etc.) even though the FPI chemicals are claimed to be volatile or water soluble.

AWS C3.6 Specification for Furnace Brazing specifically discourages anyone from using FPI in brazing inspection procedures. It clearly states in paragraph 5.4.3.1 of that spec that penetrant-inspection techniques “are not suitable for the inspection of braze fillets because they routinely give false results.”

How should fillets be specified on drawings or in specs?

1. Fillets should not be dimensioned on drawings – Be very careful about specifying on a drawing the size of a braze fillet. Once specified, it means they must then be measured, and this becomes a meaningless waste of time in the shop and can be very costly to a manufacturer. If a drawing specified that a fillet needed to be a minimum of let’s say 0.0625” in size, and upon inspection after brazing it was found that the fillet was only about 0.050” in size, what then? Theoretically, it is supposed to be rejected and rebrazed to add more BFM to the fillet, a time-consuming waste of time. Therefore, I strongly recommend against anyone specifying fillet size on drawings unless they can come up with a truly magnificent reason for so requiring. I can think of none.

2. Preferred “visual” wording – Instead of specifying that fillets are needed and that all lack-of-fill is forbidden, or that only a certain number of voids are allowed, it would often be better to merely indicate the following: “There shall be evidence of brazing filler metal at all edges of the joint.”

Conclusions

Fillets are a natural outcome of most brazing processes and should not be dimensioned on drawings nor used as critical accept/reject criteria for brazed joints. Fillets exist, but they aren’t truly relevant when it comes to joint integrity or strength. Merely look to see that BFM is indeed present all around the joint and that any filleting is concave in shape and preferably as small as possible. That can often be achieved by following the brazing rule of thumb: “If a little BFM is good, then less is probably even better!” That’s because the volume of a brazed joint can be filled only once. Any extra BFM added beyond that amount will result in large external fillets (without adding to joint strength) or run down the outside of the parts by gravity, causing a lot of extra work to clean up the parts.

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