What condition must be true about the opening between base metals for effective capillary attraction?

For effective capillary attraction in the brazing process, the opening between the base metals must be quite small. This is because capillary action relies on the physical properties of the liquid filler metal, which moves into narrow spaces through cohesion and adhesion.

When the gap between the metals is small, the surface tension of the molten filler metal is sufficient to draw it into the joint, allowing for better wetting and spreading. A smaller gap helps to facilitate this movement, ensuring that the filler metal penetrates and fills the joint adequately. This ensures a strong bond due to the increased contact between the filler and the base metals.

In contrast, larger or wider openings would impede this capillary action, as there would not be enough surface tension to draw the liquid filler into the gap effectively. Additionally, if the opening were to be filled, that would imply that the necessary conditions for capillary action are not present, as the filler metal would not be able to flow into the joint. Therefore, maintaining a small gap is crucial for achieving effective brazing through capillary action.