Brazing filler metal (BFM) is available in a variety of different forms, shapes and sizes, such as wire, rod, paste, sheet, foil, preforms and cladding. BFMs are often applied externally, but some of these BFM forms can be applied internally (i.e., inside the component assembly to be brazed like when foil, cladding and solid preform rings are used).

Please note, however, that whenever a BFM is applied externally, the BFM should only be fed into one side of the joint that is being brazed. This allows the BFM, when melted, to be pulled into and then completely flow through the joint by capillary action until it is visible at the other side of the joint, where it can be seen to produce a complete braze fillet (meniscus) around (or along) the opposite end of the joint.

As shown in Fig. 1, a BFM wire is being hand-fed into one end of a tubular joint being brazed. The joint-gap clearance shown is much thicker than desired only for illustration purposes. The BFM flows all the way to the other side of that joint, where it can be inspected for complete pull-through. This is an example of proper brazing in which you “feed from one side and inspect on the other.”


Fig. 1. BFM wire being fed into one side of the joint. (courtesy of J.W. Harris, Division of Lincoln Electric)

It is never wise to feed BFM from both sides of the joint. By so doing you can trap air, moisture bubbles and outgassing materials inside the joint between the two incoming walls of molten BFM, thus greatly increasing the void content in that joint. Figure 2 is a cross-section drawing of an actual tubular assembly in which BFM was applied from each side of the joint, thus trapping air inside it.


Fig. 2. Tubular joint in which BFM rings were applied on each end of the joint, trapping air between them and creating a large void inside the joint.