This is a very important topic that needs to be addressed. Design engineers and brazing-shop managers need to become “agents of change” to bring about a needed adjustment of thinking in industry about the need for external fillets in brazing.
The question of recessed braze fillets has already come up several times in 2023 at brazing shops in private industry and in the U.S. military, for which this writer has had to become involved. Too many brazing specifications still exist (written by both private and government agencies) that erroneously state that fillets in brazing cannot be recessed. Such statements are incorrect and can be greatly misleading. Numerous situations have been shared with me during the last year about existing specification requirements against recessed fillets, which then necessitate re-brazes of too many parts by a number of brazing shops. In a number of those cases, however, such re-brazing actually hurt the parts in question and ultimately resulted in scrapping those parts.
As can be seen in Fig. 1, the brazing filler metal (BFM) has filled the inside of the tubular joint but has a slight recess at the top edge of the joint. There is no large external fillet (or meniscus) of BFM on the outside of the brazed joint. Notice how the recessed material has a concave shape to it. The meniscus of any liquid is the curved shape of the surface of that liquid caused by surface tension. A meniscus can be either concave (very desirable in brazed joints) or convex. A convex-shaped fillet is actually poor for brazing since it tends to indicate poor wetting of the base metal by the BFM. It can be seen in Fig. 1 that the BFM’s recessed meniscus has a concave shape to it and has completely flowed around the circumference of the joint. It is a good, sound joint.
Many people erroneously believe that any fillet, whether it is for welding or for brazing, must extend beyond or outside the joint in order to be acceptable and that any joint that has a recessed meniscus (Fig. 1) has to be rejected as being incomplete. This is what I call “weld-think,” and it has resulted in many such joints being re-brazed (unnecessarily) in order to add more BFM to the joint until the resulting joint shows a large external fillet. This is erroneous thinking that can actually hurt the brazed assembly.
Fig. 2. Cross-section of a typical brazed joint, showing the filled capillary space (exaggerated thickness for clarity) between the two mating surfaces inside the joint. (Drawing courtesy of JWHarris, Div. of Lincoln Electric Co.)
Figure 2 illustrates a cross-section of a brazed joint. The joint thickness is greatly exaggerated to show the BFM flow in the joint. Notice that the BFM extends all the way through the joint between the two mating surfaces (the faying surfaces). All the so-called “goodness” of a brazed joint (i.e., its strength and its leak-tightness) comes from properly filling the inside of the joint between the faying surfaces. It does NOT come from any external fillets! Brazing designers and inspectors need to know this.
Interestingly, the drawing in Fig. 2 clearly shows that each end of the illustrated brazed joint has a slightly recessed meniscus. Yes, a recessed meniscus is perfectly fine. As mentioned earlier, large external fillets are not needed in brazing. Because external fillets actually solidify as castings, external fillets (especially if they are large) may actually hurt the performance of the parts in service rather than help. I have seen a number of external fillets develop cracks due to the external stresses acting at the edge of a joint, which results in premature failure of the parts.
It is very important to remember that brazing and welding are VERY different in their requirements, and large external fillets should be relegated ONLY to the world of welding. They are NEVER needed in brazing!
Fig. 3. Design joint edge with contours that will spread the stresses that would otherwise focus in a sharp corner right at the edge of a brazed joint.
Braze Fillets in Order to “Spread the Stresses?”
If someone thinks that large external braze fillets are needed to help “spread the stresses” at the edge of the brazed joint, they should think again! Spreading the stresses at the edge of a joint is NOT the job of BFMs. That can be dangerous.
If you are a mechanical engineer or a designer of joints for brazing, please know that the high stress concentration at sharp corners of assemblies should be removed by proper design/shaping of the base metals making up that joint. As shown in Fig. 3, the metal surfaces need to be properly shaped and contoured to spread the stresses at the joint edge rather than merely hope that a large BFM fillet will take care of that problem.
It is important to note that it is the job of the designer to design brazed assemblies in such a way that the smooth contours of the base metals at the edge of a brazed joint will naturally spread the stresses and keep them from concentrating at the edge of the joint. By so doing, the BFM can do its job, which is merely to bond the two surfaces together.
Figure 4 shows a close-up view of the “fillet” in the corner of a nickel-brazed joint, where any service stresses could cause a very high concentration of those stresses right at that corner. This could then lead to the initiation of a crack right through the BFM fillet into the 316L stainless steel base metal.
Fig. 4. Nickel-based BFM fillet in the corner of 316L stainless brazed joints.
External fillets on brazed joints are undesirable and unnecessary. They should never be added merely to help “spread the stress” in joint corners since external braze fillets are castings and cannot be relied on to be efficient spreaders of stress. In too many cases the external braze fillets actually crack due to the stresses they are trying to handle in service. The job of the BFM should only be to strongly bond two or more materials together, NOT to spread the service stresses that get focused at the outside edges of the joint.
The filled capillary space along the length of the two mating (faying) surfaces is the critical element in brazing. If the BFM does not extend to the outside of the joint, that should be allowed.
All figures/graphics provided by the author, except where noted.