What state must the filler metal be in for capillary attraction to occur?

Capillary attraction occurs when the filler metal is in a liquid state. This state allows the filler metal to flow into the joint or space between two closely fitting surfaces, which is essential in brazing processes. When heated to its melting point, the filler metal liquefies and can be drawn into the joint by capillary action due to surface tension.

While in a liquid state, the filler metal can easily penetrate small gaps and irregularities between the surfaces being joined. This is a vital aspect of brazing, as it ensures a strong bond once the filler metal cools and solidifies. The ability of the liquid filler metal to flow and completely fill the joint significantly contributes to the overall strength and integrity of the finished assembly. In contrast, solid, gaseous, or frozen states of the filler metal would not allow for this necessary movement and bonding within the joint.