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.