As you probably know, aluminium is TIG welded with AC current - the principle being that the current in one direction does the cleaning and the current in the other direction does the welding - the ratio of positive to negative being controlled from the welding set.
I am lucky enough to know an 'ace' welder, and when I questioned him about certain alloys being 'unweldable', his advice was to adjust the amperage low enough that it would heat the material but not actually weld it, and then do a couple of runs over the joint area. You can then see where the 'cleaning' phase of the arc has done it's job. Next, scrub the cleaned area with a stainless steel wire brush ( a plain steel brush will contaminate the area again) to remove any loose particles of dirt. Then adjust the amperage and weld as normal. As I understand it, pure aluminium rod is used on commercial pure aluminium sheet and sections - rod with 5 or 10 % silicon is the norm on alloy castings, etc.
Hope this is of some help,