Aluminium

Aluminium was a grade 1 premium scavenged metal crafting material.

Source
Aluminium could be gathered from an Aluminium Salvaged Heap node, which required 1 skill in scavenging. It was most commonly found on Coruscant and  Dromund Kaas.

The following materials could also be found in Aluminium Salvaged Heap nodes:

It could also be obtained from grade 1 scavenging missions. Additionally, there was a chance to obtain Aluminium from reverse engineering the appropriate level and type of gear.