Questions for Helicomparator.
Why use welding at all to join gear wheels to shafts? That seems like an invitation for trouble.
Why not use a technique (I don't know the correct name; possibly shrink-fitting) whereby the two components are machined to a very tight interference fit, then the shaft gets chilled with liquid nitrogen, the gear wheel gets heated and after the two components are joined and once temperature normalises, the resulting joint is as good as machining the two components from one solid piece of metal?
From what I've been told, welding will always create some kind of stress raiser in metal, no matter what heat-stress-relieving process gets employed after welding.
If this technique is good enough for railway carriage wheels, it ought to be good enough for helicopter gearboxes.