For your entertainment I present the crappiest aluminium welding I've ever seen (well, almost). I've put the centering ring on a couple of times and even mounted the motor and so far it hasn't fallen off.
While I don't claim to be the greatest welder, I feel I must point the finger at low argon flow for the very poor finish. I didn't want to attempt a continuous bead because the argon pressure is OK but the flow rate is very low, so you get a burst of gas when you pull the trigger and then very little. I'm not sure if I'm low on gas or the regulator is faulty (for argon I use very small disposable cylinders and my small regulator doesn't have pressure gauges).
The break profile allowed reasonably good relocation, so there is a chance I got the back into the right place. If the shaft runs smoothly after everything is assembled then I'll be very tempted to use this case instead of spending a few hours getting another one from the wrecker.
I'm using a 175A MIG welder, 1mm aluminium wire, argon shield gas, wire feed 5, power 6 and this is the second time I've tried to weld aluminium. I chose not to practice on scrap aluminium in case I ran out of gas completely -- I'm a little surprised that the weld is as good as it is.