Kimmo 

You know how ally gets old and starts to crack (I had to take 6mm off the top of my headtube to save the frame)... well, I just found myself wondering, shouldn't heat-treating make it like new again?

And if so, and granted an affordable heat-treating service or access to such facilities, wouldn't that mean junked frames are a source of tubes?

Falanx

Fatigue is a microplastic yielding phenomenon. It affects all materials (yes, even those ones that are claimed to have a fatigue life limit, like steel and titanium. It only visibily exhibits in them under ultraherzian loadings, though). At grain triple points and other such extremely high energy internal defects - ones that magnify applied stress locally so significantly that the point loadings are above the macroscopic yield strength of the material, work is built up by repeated sub-macroscopic yield loadings that at tiny points, the local work causes miniscule fractures. Once those fractures have occurred, the only way to heal them is by incipient melting.

So no, unfortunately, while solution treating and aluminium alloy frame and then re-aging it may remove almost all the plastic work done to the frame by a recrystallisation process, it won't weld the microcracks up, unless you take it high enough for them to melt together. You'll get a degree of sintering, but not proper melting.

Kimmo 

Thanks for the hardcore jargon and the translation

And that's a bummer... oh well.

I don't suppose ultrasound or microwaves or something might help...?

Falanx

No, not even slightly.