This question comes up frequently and needs to be addressed again. As shown in Figure 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 in the photo 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 (desirable in brazed joints) or convex. In Fig. 1, there is a concave recessed meniscus to the BFM at the top edge of the joint. Is this okay?
Many people who see such a joint may incorrectly think that any fillet, whether it is for welding or for brazing, must extend beyond (outside) the joint in order to be acceptable and that any joint that has a recessed-meniscus, as shown in Fig. 1, has to be rejected as being “incomplete”. This is what I call “weld-think”, and 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 shows a cross-section of a brazed joint, and you can see that the BFM extends way 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, its leak-tightness, etc., comes from properly filling the gap (the inside of the joint) between the faying surfaces. It does NOT come from any external fillets!
The drawing in Fig. 2 clearly shows that each end of the brazed joint has a slightly recessed meniscus, and this is perfectly okay! As mentioned earlier, large external fillets are not needed in brazing. And because external fillets actually solidify as castings, such external fillets may actually hurt the performance of the parts in service rather than help. I have seen a number of such external-fillets develop cracks in them due to the external stresses acting at the edge of a joint.
I strongly encourage you to remember that brazing and welding are VERY different in their requirements, and large external fillets should be relegated to the world of welding since they are NOT needed in brazing – EVER!
Braze fillets in order to “spread the stress”?
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 joint design — you should design the edge of the joint, as shown in Fig. 3, so that the metal surfaces are 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 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.
Fig. 4 shows a close-up view of the “fillet” in the corner of a nickel-brazed joint in which any high service stresses might cause very high concentration of those stresses right at that corner, which could then lead to the initiation of a crack right through the BFM fillet into the 316L-stainless steel base metal.
CONCLUSION. External fillets on brazed joints are not necessary at all. They should never be added merely to help “spread the stress” in joint corners since external braze fillets are castings and cannot be depended upon 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 brazing filler metal (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, and if the BFM does not extend to the outside of the joint, that should be perfectly okay. The BFM actually does not need to extend beyond the edges of the joint, and if there is a slight recess in the BFM at the edge of the joint, then that should be okay, as long as you can see the concave bottom of the entire recessed meniscus.
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Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 47-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Dan can be reached via e-mail at dan.kay@kaybrazing.com, and his website can be visited at http://www.kaybrazing.com/
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