What property of a joint primarily governs the success of capillary flow in brazing?

• A. Joint fit-up clearance (gap) and wettability of the base metal by the filler metal.
• B. Color of the base metals.
• C. Ambient humidity.
• D. Base metal hardness.

Answer: (B)

Capillary flow in brazing is driven by capillary forces that pull the molten filler metal into the joint. The two main factors controlling this are how tightly the joint is fit-up (the gap) and how well the filler metal wets the base metal (how easily it spreads on the surfaces). A small, clean gap creates stronger capillary pressure that draws the molten filler into the joint, and good wettability (low contact angle) means the liquid spreads along the surfaces instead of beading up, filling the joint reliably. If the gap is too large or the surfaces aren’t wetted well, the capillary action is insufficient and the joint won’t fill properly, leaving voids or incomplete joints.

The other factors listed—color of the base metals, ambient humidity, and base metal hardness—don’t govern the capillary flow mechanism itself. They may affect appearance, surface oxides, or mechanical properties, but they don’t determine the ease with which molten filler metal is drawn into and spread across the joint.