Besides joint fit-up and wettability, what other factor strongly influences capillary flow in brazing?

• A. Surface cleanliness.
• B. Flux color.
• C. Ambient humidity.
• D. Base metal hardness.

Answer: (A)

Capillary flow in brazing depends on how well the molten filler wets the surfaces and how easily it can be drawn into the joint by capillary forces. Clean surfaces are essential because contaminants like oils, grease, oxides, or scale create barriers to wetting and raise the contact angle. When the surface is clean, oxides are minimized and the surface energy is favorable, so the molten filler spreads and is pulled into the joint more effectively. Flux helps during heating to remove oxides, but it can’t fully compensate for dirty surfaces beforehand, so cleanliness has the strongest influence on capillary flow. Flux color, ambient humidity, and base metal hardness don’t directly control the capillary action to the same extent: flux color isn’t a performance indicator, humidity mainly affects oxide formation rather than the wetting itself, and hardness doesn’t dictate how the liquid metal wets and moves into the joint.