As aluminum is welded with AC rather than DC mode, the electrical characteristics are different compared to welding mild steel and the amount of energy put into the tungsten electrode is higher when AC welding. For these reasons, pure tungsten, Zirconiated or 2% Ceriated tungsten are recommended for aluminum welding.
Although 2% Thoriated tungsten can be used, this electrode contains thorium, a radioactive material that can pose health and environmental risks at elevated exposure levels. Thorium is a low-level radioactive material that primarily emits alpha particles as well as some beta and gamma radiation. It is for this reason that is falling out of favor in the welding industry.
In addition, the electrode diameter for AC welding has to be significantly larger than when using DC. It is recommended to start with an electrode that is 1/8" and adjust as needed. Zirconiated tungsten can carry more current than pure tungsten electrodes. Another helpful hint for AC welding is to use a blunt tip- the arc tends to wander around a pointed tip.
Tungsten Electrode in TIG welding for aluminum