That depends on the metal and if it is post treated. Welded aluminum isn't as strong as the cast/extruded and heat treated pieces. Welding on 4130 steel can easily produce weaker joints if those joints are then treated. If they have an economical way to heat treat an entire cast piece, that might make it stronger than welded bits.
Heat treatment of modern, stabdardized alloys is a scirnce. Doing so with complex formed parts is still a bitch so. Especially big parts. Personally, I think induction hardening might work.
Well, welded structural parts, even produced in high vilumes, are a rather well understood problem so.
