What signifies the right moment to start feeding the filler metal during brazing?

The moment to start feeding the filler metal during brazing is best signified by the melting of the flux. Flux serves multiple critical purposes in the brazing process, including preventing oxidation of the base metals and improving the flow of the filler metal. When the flux melts, it indicates that the joint area has reached a sufficient temperature to facilitate the bonding process.

At this point, the melted flux assists in the wetting and spreading of the filler material, allowing it to flow into the joint effectively. The proper interaction between the molten flux and the filler metal ensures that a quality joint can be achieved, enhancing the strength and durability of the braze. If one were to start adding filler metal before the flux has melted, it might not flow properly, leading to defects in the joint.

While changes in flame color, reaching specific joint temperatures, and visual steam production can all serve as indicators in the brazing process, they are not as definitive or reliable as the melting of the flux in signifying that the right conditions for introducing the filler metal have been achieved.