Direct Current (DC) is the most used for TIG welding because it penetrates deeply.
For welding aluminum is one of the only that AC (alternating current)is preferred, AC current is used to weld aluminum
because its positive half cycle delivers a precise action and its negative half cycle deliver penetration.
As well it flips the polarity easily and quickly such as from 70 to 130 times each second. Therefore for welding aluminum AC current Use.