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.

Braze Fillets in Order to “Spread the Stresses?”

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.