Which factor is critical for ensuring good capillary action in brazing?

For effective capillary action during brazing, the proper joint fit-up is crucial. Capillary action refers to the ability of the liquid filler metal to flow into the gap between two closely fitted surfaces, and this flow is driven by the surface tension and viscosity of the filler metal. If the joint fit-up is too tight or too wide, it can hinder or completely prevent the movement of the molten filler metal into the joint, ultimately compromising the strength of the bond.

A good fit-up ensures that there is just enough space for the filler metal to enter while minimizing the potential for contaminants or air pockets that can interrupt the flow. It creates a suitable channel that allows for the necessary capillary attraction, enabling the filler metal to uniformly fill the joint and achieve a strong bond.

Other factors like temperature, filler metal composition, and type of flux can influence the brazing process, but without an appropriate joint fit-up, capillary action cannot be effectively established, making it the most critical factor to consider.