Which condition most likely disrupts capillary flow in a brazed joint?

• A. Ambient air pressure.
• B. Excessive flux residue removal.
• C. Inadequate joint fit-up clearance and poor wettability.
• D. Using too much filler metal.

Answer: (C)

Capillary flow in brazing depends on the molten filler metal being drawn into a narrow, cleanly wetted joint by capillary forces. This works best when the clearance is just right and the surfaces are readily wetted by the filler metal. If the joint fit-up is inadequate, the gap can be too large or irregular, which reduces the capillary pressure driving the metal into the joint and can stop the flow. If wettability is poor—often from oxide films, contamination, or poor flux effectiveness—the molten metal beads up rather than spreading, so it cannot advance smoothly into the joint.

The other factors don’t disrupt capillary action as directly. Ambient pressure isn’t a primary control for capillary flow in typical brazing, and while flux management matters for oxidation protection, the key issue remains whether the surfaces can be wetted and the gap is properly fitted. Using too much filler metal can cause other defects, but it doesn’t inherently prevent the capillary action from functioning if the joint fit and wettability are appropriate.