I have thought about an Al frame, but as NewRider mentions, the welding is the tricky part. Unlike steel (Chromoly), the welds will be the weakest point, and even a good looking weld may break at the wrong time.
Technically, welding any metal introduces a "heat affected zone", which would benefit from post work heat treating, even steel. But this is really only important if you are pushing the envelope of materials.
Russ Barrow
Russell ATV