Aluminum welding requirements

[performance]

Good but complex question to ask and answer.

But here is the short, abbreviated version.

1) AC is a combination of DC+ and DC-. DC+ cleans the oxidation from the aluminum while offering very shallow penetration. DC- gives the penetration but offers no cleaning action
2) TIG uses a non consumable (but really it is a consumable) tungsten that is able to withstand the high heat of DC+. Because when the polarity shifts to DC+ the majority of the heat is placed on the tungsten because the electrons are flowing from the workpiece to the singular point of the tungsten. You can weld Aluminum in DC+ with TIG, theoretically, but if you were welding at say 200 amps, you'd need a tungsten half as thick as your forearm because it would want to melt other wise. (not quite but in comparison to regular AC or DC-)
3) MIG is a different process and it is done in DC positive. The wire is constantly consumed, which is ok, and because it is in DC + cleaning action occurs. But because aluminum is welded in MIG in the spray mode only, the metal is transferred across the arc and into the molten weld puddle. Since the wire is the electrode, and is constantly consumed, the heat generated isn't an issue.

[blacksmith]

Ok, so the spool gun is just to sidestep the handling of the consumable and its tenancy to kink in the liner. Nothing magic about the gun, and the weld would look the same whether from the spool or the mig gun.