Learn how to correctly apply brazing filler metal in part 3 of Dan Kay's latest blog series.

Figure 3 shows a cross-section photo of a round electrical contact that was brazed onto a flat surface, where the applied BFM completely surrounded the disc-shaped part that was to be brazed to the substrate below it. When the BFM melted and tried to flow into the joint, it could not do so since the air and other contaminants inside the joint could not escape to the outside because the “wall of liquid BFM” around the outside of the joint was blocking it. Thus, a braze fillet was formed around the outside of the joint, but the BFM could not flow into the joint.

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).

Are you in the brazing business or in the heat-treating-of-fixtures business? Many brazing companies claim they are a brazing job shop doing lots of brazing for their clients, when in actuality they are only doing the brazing work for their clients in order to generate some income to pay for their real business (i.e., heat treating of furnace fixtures). Does that seem strange? Let’s take a deeper look at this.

Here is the conclusion of Dan Kay's blog series "Filler Metal Makes Braze Paste."

Notice in Table 2 that there are strict requirements for the amount of oversized and undersized powder particles in each mesh size group. For example, to be classified as a -140C powder (“C” stands for “course”), the screened powder can only have a maximum of 20% -325 mesh powder. Thus, 80% of powder classified as -140C-mesh must be -140 mesh/+325 mesh, meaning it should all go through the 140-mesh screen, but 80% or more must sit on top of, and not go through, a 325-mesh screen. 

Figure 3 shows a laboratory wire-mesh sieve screen used for 140-mesh powder. If atomized powder is not able to go through this screen because the openings in the screen are smaller than the size of those powder particles, then those large particles will remain on top of the 140-mesh screen and would be called “plus (+)” sized powders because they sit on top of the screen and cannot go through. Powder particles that can go through the screen are given a “minus (-)” particle-size designation, indicating that they can go through that screen.

A common way to create brazing filler metal (BFM) powder is by melting the raw metallic ingredients for the BFM in a large melting pot using induction heating and then pouring that alloyed liquid metal through a specialized atomization nozzle.

Dan Kay's blog series about tight joint clearances for nickel brazing continues with part 3.

Unfortunately, as mentioned earlier, all of these temperature-lowering, eutectic-forming elements in nickel-based BFMs are also hardeners; that is, the phase structures resulting from solidification of these elements have virtually zero ductility. Thus, the last phases to solidify (in the center of the joint) will be hard and non-ductile.