Rather than weld the key just Loctite the entire thing to the shaft. I used it all of the time to hold the wheel wedges in older IH tractors. Had a wheel come off the IH 1466 and it made a real mess. So I loctited the wedges. That stopped the problem.
Plus if you loctite it if you need it apart you just have to heat it up and it will come back apart. Much better than welding it.