When connecting copper to steel, the recommended brazing filler alloy is a...

• A. Copper/phosphorus alloy
• B. Silver-copper alloy
• C. Aluminum-silicon alloy
• D. Nickel-based alloy

Answer: (A)

When joining copper to steel, you want a brazing filler that will wet and flow across both metals and fill the joint cleanly. A copper–phosphorus alloy does this effectively because phosphorus helps break and penetrate oxide layers on both copper and steel, lowering surface tension and promoting capillary action. This active wetting lets the filler flow into the joint at a practical brazing temperature, creating a strong bond between the dissimilar metals without requiring extensive flux or harsh conditions.

Silver–copper alloys can also braze copper, but they’re more expensive and may require fluxing and higher temperatures in some cases. Aluminum–silicon alloys are designed for aluminum, not copper or steel. Nickel-based alloys are useful for high-temperature or highly corrosion-resistant joints, but they don’t provide the same easy wetting of a copper-to-steel joint at typical brazing temperatures. So the copper–phosphorus filler is the most suitable and economical choice for copper-to-steel brazing.