In brazing, which mechanism is primarily responsible for forming the joint?

• A. Fusing base metals together
• B. Evaporation of filler metal to form a bond
• C. Capillary flow of a filler metal that does not melt the base metals.
• D. Mechanical clamping without filler metal

Answer: (C)

In brazing, the joint forms mainly through capillary action of the molten filler metal. The base metals stay solid, while the filler metal, chosen to melt at a lower temperature, wets the surfaces and flows into the narrow gap by capillary forces. As it fills the joint and then cools, it bonds metallurgically to the base metals at the interfaces, creating the strong joint.

This is why melting the base metals or relying on evaporation isn’t how brazing works: the base metals are not melted, and bond formation comes from the flow and subsequent solidification of the filler metal. Mechanical clamping without filler metal wouldn’t create a joint either, since there’s no molten material to wet and fuse with the surfaces.

For effective brazing, surfaces must be clean and properly prepared, and the joint gap is kept small to maximize capillary flow of the filler metal.