I fully acknowledge that AC TIG, DCEN TIG with helium, and spool gun MIG are all superior processes for most aluminum welding situations. I am not a professional weldor, I do not currently have access to one of the preferred processes, and I am not willing to invest in another welder or another gas cylinder at the moment. I have a Lincoln Invertec V275-S DC stick/TIG welder, I have an argon cylinder, and I'm curious. DCEP TIG with argon can weld aluminum, but it provides shallow penetration, it requires oversize tungsten, and the arc is difficult to control. Flux makes oxy-fuel welding aluminum and stick welding aluminum possible. I ran some aluminum stick welding electrodes DCEP with this welder, but I'd like to see if I can achieve more control and a cleaner weld. This page focuses on welding aluminum with a DC TIG welder, electrode negative (DCEN), argon, and flux. The argon cools the TIG torch, shields the tungsten, and stabilizes the arc. DCEN should provide good penetration. The flux cleans oxides from the aluminum and it helps shield the molten puddle. For my testing, I used TM Technologies Premium Aluminum Gas Welding Flux. It's designed primarily for oxy-fuel welding aluminum, but TIG is also mentioned on the product page. I haven't had a chance to try the Forney 37025 Alum-A-Flux yet. You can use this process with scratch start, touch start / lift arc start, or high-frequency start DC TIG.
This page, A short history of welding aluminum, has an excellent overview of the various aluminum welding processes. It does a good job of explaining some of the difficulties and potential problems with flux in OFW and SMAW aluminum welding. The flux discussion is also relevant here.
I mixed up a 2:1 flux:water very thin paste in a glass jar. The flux is very corrosive, so use glass or plastic to store it. Metal containers are not recommended. Before this, I tried DC TIG with a stick welding aluminum electrode. That worked, but the large electrode size and huge amount of flux made things difficult. You have a lot more control and visibility with a conventional TIG rod and thinly applied flux. I used an old toothbrush to brush the flux on the area to be welded.