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By heating it, you're annealing the metal - it will then work harden as you beat it to shape, and may need re-annealing part way through the process to reduce the possibility of cracks forming.
Many aluminium alloys melt before they reach red heat - old school / low tech metalworking would have you polish the surface and smear it with household (bar) soap before heating, and remove from the heat as soon as the soap turns black. Polish again before shaping - it's easier to clean if you haven't beaten burned soap into the surface...
MrE-
Would re heat the alloy after beating make it less likely to crack? Can't be bothered now it's re sprayed...
It's only preventing the rack from pivoting backwards, most effort is in the legs so of be surprised if it ends up breaking... If it does, the rack comes with two struts so I'll have to start again...
Thanks for the tip!(Note: it looks red on the picture, but that's only the discoloration of the epoxy paint, the alloy didn't get that hot)
veLLo
I bought a Tortec rear rack, as usual, the provided struts didn't accommodate for short(ish) wheelbase (I suppose) frame and unless I was ready to have a non level rack on my bike (don't mind on the front but rear never feels right to me).
It turns out that by heating the alloy on a cooker it flattens quite easily (with the help of a hammer too). I could then file to an acceptable shape, repaint (powder coat would have been better but...) and have my perfectly adjusted strut... I thought it was a useful enough trick to share... (Sorry if repost)