Well, platinum iridium alloys won't oxidize in high heat (but other platinum alloys may), but they can react with the carbon in your flame. Best to work it with oxy-hydrogen if you need a flame.
Hafnium resists oxidation, even at the temperature of a plasma cutter electrode. Zirconium has very similar properties.
>"welds to form a single body when another metal is cast onto it"
Well, that's a tough one. Pouring liquid metal onto solid and assuming you'll get a weld there. Maybe, maybe not. Depends on a lot of things.
Back to reality, why does this have to be heated by a torch, and why does it have to be done in a reactive environment?