Thanks to the two of you for the info about annealing and hardening.

I can remember doing that as a kid when I used to make model aircraft
undercarts, but had long forgotten the details.

But the aluminium I used to use was an alloy that's much stiffer than
the aluminium I usually find in shops.

Is there an easy ( and cheap ) source of an alloy that's more suitable
than bog-standard soft ali ?