Pure tungsten is used primarily in AC welding.  AC is recommended when welding Al and Mg, as it provides a good combination of cleaning and weld geometry (penetration & width).

additions such as thorium (up to 2wt%) and Zirconium change the thermionic emisivity of the electrode - which is essentially the amount of charge required to strip an electrode from the electrode surface. the addition of thorium allows for easier electrode stripping and thus reduces electrode heating. textbooks suggest that the 2%th electrode is great for straight current applications (dcen), where the anode end (the electrode) heats up more than the cathode end (the workpiece). they suggest that you use a 2%th electrode for steel, cause straight polarity welding concentrates the majority of arc heat at the electrode tip. (actually, 70% of electrode arc heat is located at the anode end, whether welding in straight or reverse polarity). I do a lot of aluminum 6xxx welding and use a 2%th electrode, not pure. aluminum welding, which operates in ac, does not heat the pointed electrode enough to create a ball tip, so a pointed tip remains thoughout the welding process. as a result, the weld arc is much more directional and allows for better root penetration. hope this helps.

Suggest you review ASME SecII PartC

The preferred electrodes are thoriated, ceriated or lanthanated tungsten as specified in AWS A5.12. Different Tungsten materials have different performance characteristics.  For quite some time, tungsten manufacturers have added an oxide to pure tungsten to improve the arc starting characteristics and the longevity of pure tungsten electrodes.  Below is a list of the major commercially sold tungsten types, their American Welding Society (AWS) and International Standards Organization (ISO) classifications, and the amount and type of oxide contained in the electrode.

Material    AWS Class    ISO Class    Oxide Content
2% Thoriated    EWTh-2    WT20    1.7-2.2%        ThO2
2% Ceriated    EWTh-2    WC20    1.8-2.2%        CeO2
1½% Lanthanated    EWLa-1.5N/A    1.3-1.7%        La2O3
1% Lanthanated    EWLa-1    WL10    0.8-1.2%        La2O3
Zirconiated    EWZr-1    WZ3    0.15-0.40%      ZrO2
Pure Tungsten    EWP    W    None

Mil-Std 248 permits the substitution of the electrode, as they are considered a non-essential element of the welding process.  

“No single material is the right choice for every application”.  Most welders used 2% Thoriated Tungsten for most GTAW welding applications.  “However, because this material is slightly radioactive and ingestion of the dust during grinding represents a health hazard, many welders have looked to other alternatives.  Non-radioactive 2% Ceriated Tungsten has proven itself to be very good in low amperage welding and is commonly used by orbital welders.  The new non-radioactive 1½% Lanthanated Tungsten electrode is an excellent substitute for 2% Thoriated Tungsten in almost every application.  In addition, independent, controlled tests have shown that it actually offers better arc starting and longer life.