I have a client who fabricates structural aluminum weldments and nothing else. We always use the standard AWS welding symbols (AWS A2.4-98) for these structures.
The base metal has no relavance to the type of welding symbol drawn, so they are universal with regards to base metal type, whether it is steel, stainless, aluminum, titanium, bronze, bismuth, lead, copper, nickel, etc....
According to Aluminum Design Manual, 1994, Part III, Section 5.2.1, Page III-23, "The general recommendations and regulations for welding are provided in the American Welding Society D1.2-90 Structural Welding Code Aluminum. Acceptable weld profiles, standard welding symbols, inspections, and joint procedure qualification requirements are also provided in this code. Inspection methods are describrd in this code but are not required unless specified in contract documents."
I realize that this manual is not the latest edition, but it's the only one I currently have. The 2000 edition had been on sale for $175.00 plus S&H.