Copper-nickel alloy

Sometimes called Monel? I've used cupronickel brazing rod, but that'd be welding rod in this case; it must be possible to weld, I've seen Monel sinks that were obviously weld-ups.

I've seen a number of marine heat exchangers, said to be cupronickel, that had the end plates soldered on but I am not sure whether it was lead-tin solder or silver solder, but I did visit a shop that was making them and the assembly was being done with an acetylene torch.

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Go look for Keith Fenner on You Tube or Facebook. He can likely tell you all you would need to know.