Does AC arc welding offer any advantage

There are a couple of advantages to AC arc welding.

In certain positions, like at the end of a long piece of metal, where the welding current has to travel a long distance across the piece, you can get enough of a magnetic field that it deflects your arc. This is called arc blow, and it’s really annoying, not to mention harmful, since an arc that doesn’t go where you want it to makes it hard to complete a good weld.

With my DC machine, I’d reposition the ground clamp till I found a position where the arc blow was minimal, and continue my work. Arc blow doesn’t affect an AC arc, because the current flow alternates and by the time the arc would deflect to one side, the current has reversed and it is now wandering towards the other side. If for some reason I couldn’t reposition the ground clamp, and I couldn’t handle the arc blow, I might consider switching to AC.

When welding aluminum, the aluminum oxide is an insulating layer and has a really high melting temperature. The arc doesn’t do a good job of blowing through the insulation, and it doesn’t do a good job of melting it away either. To get around this, it’s necessary to weld with alternating current.

Most of the heat in the work is generated during straight polarity, meaning the electrode is negative, the work is positive, and electrons spray from the electrode to the workpiece. This gives a deep, penetrating weld puddle, assuming it could get through the insulating, heat resisting oxide, which it can’t.

When the polarity is reversed, meaning the electrode is positive and the work is negative, electrons spray from the workpiece to the electrode, which doesn’t melt much of the work, but does tend to blow the oxide layer off. When these two polarities alternate quickly, the oxide layer is broken down and the straight polarity current can melt metal and complete the weld.

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