The Importance of Recessed Braze Fillets (part 2)

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.

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.)

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.