It can't be a weld obviously since that can only
be between identical metal.

There are plenty of heterogeneous welding processes around. They're
somewhat obscure, but they're common enough when you need them. All you
need is a filler material that is easily dissolved in the parent
material - nickel rods for arc welding cast iron are one, some of the
gas-torch bronze welding processes are another. Then there are the
non-filler process like explosive welding to laminate plates together,
some friction weld process for spin-welding fastening studs onto sheet,
or even an 18th(?) century technique like making Sheffield Plate by
diffusion welding silver foil onto copper sheet (heat, time and
pressure).