Which property of filler metal is essential for effective brazing at lower temperatures?

For effective brazing at lower temperatures, high fluidity of the filler metal is essential. High fluidity ensures that the filler metal can easily flow into the joint and fill any gaps between the base metals. This is particularly important at lower temperatures, where the filler metal may not wick into the joint as readily as it would at higher temperatures due to increased viscosity.

When the fluidity is high, the filler metal can be drawn into the joint through capillary action, leading to a strong and reliable bond. This fluidity allows for better wetting of the surfaces, which is crucial for achieving a good metallurgical bond between the filler and the parent materials. Therefore, the ability of a filler metal to flow well at lower temperatures directly influences the quality of the brazed joint.

In contrast, while a low melting point can facilitate brazing, it does not specifically address the fluidity required for joint filling at lower temperatures. High viscosity, on the other hand, would hinder the flow of the filler metal, making it more difficult to achieve an effective bond. Low thermal expansion is important for stress management, but it doesn't influence the initial filling capability during the brazing process as fluidity does.