For capillary attraction to occur, what must be done to the base metals?

For capillary attraction to occur, the base metals must be cleaned and fluxed to create an effective joint for brazing. Cleaning is critical because any contaminants, such as dirt, grease, or oxidation, can prevent the molten filler metal from flowing properly into the joint. Clean surfaces allow the filler metal to make proper contact with the base metals, ensuring good wetting and spreading, which are essential for strong joint formation.

Fluxing further aids this process by preventing oxidation during heating and promotes the flow of the molten filler metal into the joint by lowering its surface tension. It essentially prepares the surfaces and supports the capillary action necessary to draw the filler metal into the space between the closely fitted metal parts. This combination of cleaning and fluxing is what facilitates effective capillary attraction, leading to the desired strength and integrity of the brazed joint.