thats cast iron - I’ve never welded cast before and not really sure what that would entail, but I’ve always heard it’s a challenge.
Cast is notoriously difficult to weld due to needing to be preheated, and the possible alloys that it could be as well. If done improperly, it can crack. I'd let a professional do it since I would consider that a pretty big safety item. Or just find a new knuckle.