If filler metal fails to flow, what does it indicate regarding the parts?

When the filler metal fails to flow during soldering or brazing, it typically indicates issues related to oxidation or insufficient heat. For the filler metal to melt and flow into the joint, the surfaces of the base materials must be clean and appropriately heated. Oxidation occurs when metal surfaces are exposed to oxygen, leading to a layer of oxide that can prevent proper adhesion and the wetting of the filler metal. This means that if the surfaces of the materials are not sufficiently prepared—clean and free from oxidation—then the filler metal will struggle to flow into the joint, resulting in an incomplete or failed joint.

Insufficient heat can also prevent the filler metal from reaching its melting point, which is essential for it to flow correctly. If the parts are not heated adequately, the filler metal may remain solid, hindering the entire process.

Understanding that both oxidation and insufficient heat can impede the flow of filler metal is crucial for successful soldering and brazing practices. This option captures the more common causes of failure in the flow of filler metal compared to factors like excessive heat, high pressure, or material incompatibility, which do not directly relate to the immediate problem of flow inhibition.