Alex,
Not to be sarcastic, but I think you need some unobtanium. 

Really though, I don't think you are going to find any metal that has a high enough melting point to actaully turn red, and then not form any oxides, or nitrides, etc.

As far as fusing with another (assuming different) metal cast onto it.  That is generally called a dissimilar metal weld.  However, the welding heat source superheats the weld metal and puts additional heat into the base metal as well.  If you are wanting to pour barely metled metal onto another metal at room temperature and form a bond, it isn't going to happen.  You could always have some studs sticking out of the first metal that the 2nd metal could form around for a mechanical bond.

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?