Comment by petee

One reason certain metals can't be welded is how their crystalline structure is altered by the process, creating a weak or brittle weld/HAZ (heat affected zone: weaker metal surrounding the weld/filler metal altered by heat)

Some metals also have difficult or impossible cooling rates - welding stainless to carbon steel will make a weak weld that is almost guaranteed to crack while cooling because one metal cools and contracts much faster than the other.

The topic is fairly complex, but there is a good textbook/reference for welders put out by the Lincoln Arc Welding Foundation called "Metals and How To Weld Them" that goes deep on structure and metallurgy, in a readable way.