Tim as you say, heat treating aluminum is kinda tricky.
from what I recall of my time in the aero repair industry , fixing broke crop dusters,,
different alloys require different methods,, some radically so.
I cant recall which alloy it was, but it would harden with age, so you would anneal it, work it, then just let it sit around for a period of time and magically
it would go back to the harness you desired,,
others ,, ( my memory says 5051 but that could be wrong) we would get it in an "0" state, and it was used for holding windscreens in place and things like that, it was wonderfully mallable, dead soft,, it was like working with a peice of leather or something it was that workable,,but if you looked at the strenght, not so much,,
To weld or work a crankcase of aluminum, I would think it extremely critical to know what alloy was used, otherwise you could process it wrong and destroy it,,
as was mentioned in another post, some things are meant to be replaced not repaired,,