AC, DCEN and DCEP. When to use them. When not to use them. Part 4
AC is not really a polarity. But it is rather a combination of DCEP and DCEN, which creates a regular sine wave, instead of the straight line of DC current. The combination of rapidly changing DCEP and DCEN offers excellent benefits in TIG welding.
This back and forth action of the flow of electrons as the polarity rapidly changing provides cleaning for the aluminum, helping to scour away the oxide layer that is covering the surface of the aluminum. The DCEN portion offers penetration, while the DCEP offers cleaning. The bonus effect is that the tungsten size that is required is reduced, creating a more controllable and penetrating arc without loosing the cleaning action. Modern TIG inverters like the Everlast PowerTIG series have an AC adjustment which skews the balance of positive to negative flow of electons, allowing more negative to be used than positive which further extends penetration and reduces cleaning to a more desirable width, improving the aesthetics of the weld.
Though some welds may require up to 50% balance, an equal amount of cleaning and penetration, it is usually not necessary as amounts as low as 10% DCEP are effective at providing enough cleaning to weld well. For stick, AC is sometimes used to control Arc Blow. Arc blow is a condition where the magnetism created by the cables and the work surface destabilizes the arc, creating excess arc wander and spatter.
The even back and forth flow of electrons can help to reduce magnetism. But AC is not often used outside of that, except in cheap transformer welders and is the only option in some older, lower end engine driven machines. More expensive transformer Stick machines still do offer AC as an optional selection, but is not offered as an only process due to the undesirable roughness of the arc. For MIG, only the very cheapest lightweight Transformer MIG welders are AC.
The welds created by a cheap AC MIG transformer are typically not high in quality and is only used for light and non-structural applications as porosity can be an issue in an AC MIG weld. Though there are some experiments being done with AC MIG with the manufacturers to improve the quality of the weld and make it practical for commercial use, this is still not an accepted method of MIG welding except in the low end home-owner market.