What is a typical cause of lack of wetting in brazing?

• A. Overheating
• B. Using inert gas during heating
• C. Oxide layer on base metal surface or insufficient flux or heating
• D. Too much flux flows away from the joint

Answer: (C)

Wetting in brazing is about the molten filler metal spreading smoothly over the base metal to form a continuous joint. If the surface isn’t properly prepared, the filler can’t wet the metal and will bead up instead of flowing into the joint. The most common reason is an oxide film on the base metal that blocks bonding. Flux is used to remove oxides and protect the surface from re-oxidizing as heat is applied, so insufficient or degraded flux leaves oxides in place and prevents wetting. Heating must reach the right temperature where the filler metal becomes liquid and can flow; too little heat stops melting and flow, while excessive heat can degrade flux or re-create oxides, also harming wetting. So a clean, oxide-free surface with adequate flux and correct heating is essential for good wetting, and a persistent oxide layer or inadequate flux or improper heating is the typical cause of lack of wetting.