From what I undertsand brake disc are usually cast iron.
Welding to cast iron is not a simple process, usually involves pre-heating. Preventing warping and cracking could be an issue. Obviously will need to be machined afterwards.
Probably easier to just get new ones cast from a pattern. If you make or have the pattern made and keep it, getting a batch done every now and again, I'm sure there would be market for Pinz replacement discs.