Most aluminum for fabrication is in a hardened condition, typically a heat treatment.
With an alloy designation say 6061 T6, the "T6" is the hardness. With a zero hardness meaning normal condition.
When you heat an aluminum part to a certain point (I'm not sure what it is but I think 400F is close) it brings the part back to normal condition, or zero.
That's why anodizing is a better way to coat aluminum and hard anodizing is the best anodizing, it's makes the surface harder.
Parts that have lost their hardness due to welding or inadvertent bearing installs <grin> can be re heat treated for a nominal fee, but warpage can occur, that's why parts are usually machined after being hardened.
There is also hardness testing to see if the hardness has been lost.
One of the "to win" series by Carrol Smith has a good chapter on aluminum alloys, there's also some good info on the web, I'm not at home right now or I'd post a link. .