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Aluminum Alloy can be used to produce Item Prototype at the Engineering Center.
In-game description

Aluminum Alloy is used to produce Weapon and Equipment Prototype Kits at the Engineering Center, and in the process gain more technology for the faction creating the prototype.

It is obtained by refining Aluminum at a 1:1 rate in a Refinery. Like other Resources it can be crated and uncrated at a Storage Depot or Seaport, with 20 Aluminum Alloy per crate.