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.
Fig. 3. A circular disc (right side of photo) being brazed to a flat substrate using a round BFM preform ring that completely encircled the disc.
An important way to prevent such gas-entrapment inside the joint would be to provide a vent hole (or holes) inside the joint. As the molten BFM pushes into the joint from each side, all gases can be pushed out through the vent holes. This will then be followed by the molten BFM sealing the hole (or holes) as shown in Fig. 4.
Fig. 4. If the brazed assembly shown in Fig. 2 had used vent holes in the inserted tubing, then the gases could have escaped and the joint could have been completely brazed.
Another way to prevent trapped voids inside a joint is to use a partial ring of BFM instead. See Fig. 5, where the BFM paste (or solid preform ring) is applied only a little more than halfway around the OD of the part being brazed to the substrate. As the BFM melts and flows into the joint, it will push any air, flux or other gaseous contaminants out of the joint ahead of it as it moves into the joint. The force of those expanding gases will be enough to prevent the BFM from encircling the joint and trap any of those items into the joint before the gaseous contaminants have escaped.
Fig. 5. Externally applied BFM paste (or solid preform ring) should only be placed halfway around the joint.
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