Hope you are well.
This is the kind of question that should be directed at Kath Lillicrap in future, as she is your main point of contact for Technical queries on Purchase Orders.
We don’t actually have an abbreviation for Aluminium, so your suggestion of AL is just fine (This is what Plymouth would use also)
Example:
2” 150 ASME B16.20 AL SPGH 316L IR + OR