Just had a thought which I feel may be an eloquent non-adhesive solution for holding the aluminum pieces together for welding.
You mentioned the pieces you want to weld have flanges .... my suggestion is to get a handful of strong Alnico magnets and place one magnet on the flange of the part to be welded and a second magnet on the outer side of the aluminum it is to be welded onto. The force of the magnets pulling towards one another should keep the two pieces held snugly together while a couple of tack welds are laid down.
Sounds good in theory
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