TIG (Tungsten Inert Gas) welding, can utilise both alternating current (AC) and direct current (DC), but the choice between the two depends on the material being welded.
For aluminum welding, AC current is typically used. The reason for this is twofold:
Cleaning Action: AC current has a cleaning effect on aluminum, which is essential because aluminum often has an oxide layer on its surface that has a higher melting point than the aluminum itself. The positive half of the AC cycle breaks up this oxide layer, allowing for a cleaner weld.
Heat Control: AC current helps in controlling the heat input. Aluminum conducts heat quickly, so controlling the heat input is vital to prevent burn-through. The alternating current assists in balancing the heat input by switching between penetration (negative cycle) and cleaning (positive cycle).
While AC is generally preferred for aluminum, DCEN (Direct Current Electrode Negative) or "straight" polarity can also be used for welding thin aluminum sheets or when a tighter arc and deeper penetration are required. However, it lacks the cleaning action that AC provides, which means the welder must take additional steps to remove the oxide layer, such as by using a chemical cleaner or mechanically removing it before welding.
For other metals, such as steel or stainless steel, DCEP (Direct Current Electrode Positive) or DCEN may be used depending on the desired penetration and bead profile. DCEP is often preferred for these materials as it provides deeper penetration into the workpiece.
DCEN - electrode negative is connecting the welding electrode (tungsten) to the negative of the welding machine and it does not provide cleaning action, suitable for deep and narrow welds where post welding cleaning can be achieved.
DCEP - electrode positive is connecting the welding electrode (tungsten) to the positive of the welding machine and provides cleaning action. Suitable for wide and shallow welds.
AC - provides cleaning action during the positive cycle and penetration action during the negative cycle.
Stainless steel and mild steel generally use DCEP and aluminium generally uses AC.