As 3.6 pounds load on 75 mm diameter surface is small. It is still in the low wear rate regime where fine black powder debris are produced. these black powder is nothing more than fine particles of Al and steel and not much oxide.(see wear mecahnism maps for aluminium/steel). I could not understand how you get galling (gross material transfer, I suppose) at these low loads. It can happen if the intial apparent contact area is small and thus high contact stress. Check contact geometry whether the whole surface is in contact when you strat moving.
Alternatively, you could try Al-12 wt.%Si alloy instead of 7075-T6, if cast material is okay.
If you harden the steel or a hardened steel surface is used, generally Al alloy wear decreases (my Ph D thesis).
If you can try another combinations, try steel on steel with approriate hardness levels (i.e composition and heat treatment). Refer to any good book on wear or wear control handbook.
There are numerous possible combinations possible - metal/metal, metal/polymer, polymer/polymer, etc., apart from ceramic and composite materials, if you design allows. People who read these postings can give some guidance depending the info you supply.
Let me know if you need more info.
