AC cleaning action
The cathodic removal of the high-melting oxide layer on aluminum achieved during the positive half-cycle in AC TIG welding.
Aluminum always has a hard oxide layer in air. It only melts at a much higher temperature than the aluminum itself. That's why you weld aluminum with AC. In one half-wave, the arc blasts away the oxide layer; in the other, you melt the material. If you don't get rid of the oxide layer, it floats as dirt in the weld and you get porosity and an unstable arc. Set the balance on the machine so that the cleaning action is right – more cleaning share for thick oxide, less for normal material. Otherwise, the tungsten electrode overheats and you get tungsten inclusions in the weld. Before welding, clean the aluminum with a stainless steel brush and degrease. If the weld shows black spots or white edges, the cleaning wasn't sufficient.