If you are gonna tig it, just build up with steel or some other strong material that will not wear like bronze. Tigging with bronze will leave islands of steel anyway that will not clean up the same as bronze, and may catch your cutter and chip it.
If you are dead set on braze, I would Oxy/Acet silver braze a layer of Easy-Flow 50 or some other high strength silver braze if the shaft is not sensitive to annealing.