First of all, let me state that I know the answer, as far as repairing a bike goes, is NO! The question is purely out of curiosity and I thought this the best place to ask (I wandered in from 50+) since I don't hang out on metallurgy forums.

And I am not now and never will be a frame builder. I will never try this at home. Promise.

So: If one desires to bend or otherwise re-form an item made of aluminum alloy while retaining its strength, how is it done? I assume there are highly skilled and possibly industrial processes used. Is there anything besides basically melting it and then casting/forging/machining anew?

I know hydroforming is one high tech fabrication method. Industrial for sure.

Like I said, just wondering. Hey, I think of crap like this cooped up inside.

Im just guessing, but maybe post-bending heat treating? I'm sure someone knows.

Aluminium and steel have very different cold hardening properties, so you can rebend a steel pipe and actually increase its strenght altought reducing its ductility: structural steel have roughly a ~300000psi ultimate strength, if you start with a ~100000psi steel, you can cold form it a couple times before it reach its ultimate stress. So, I won´t rebend a bike frame made of Reynolds high tensile steel tubing, because is near that limit.

Aluminum has 1/3 of steel Young's Modulus -70GPa (10million psi) vs 200GPa (29million psi), and also a lower ultimate strengh (aerospace aluminium alloys have an ultimate strength of ~75000 psi, so when you rebend an aluminium tube probably it is near that limit, and will break easily.

From an aircraft mechanics point of view, high strength aluminum is not easily bent without cracking. Depending on the alloy involved and the heat treatment.

6061-T6, a common sheetmetal alloy in aircraft use will not make a sharp 90 degree bend without cracking. If I use a gentle radius, I can bend it through 90 degrees. However, 6061-T0 will bend to nearly any shape required without much risk of cracking. It can then be brought to a harder condition after machining.

So: If one desires to bend or otherwise re-form an item made of aluminum alloy while retaining its strength, how is it done?

It would depend on what they actually use. If some of the strength comes from cold working, there's really not much you can do to reform while retaining strength. You could anneal and then form it, but strength would be gone. If it's a heat-treatable alloy, you could anneal it, form it, then re-heat treat it. For practical repairs, just re-form it and if it breaks, it breaks.