Why does adding thorium to a TIG welding electrode improve the arc? What are the physics behind this and other tungsten alloys used for improving TIG electrodes?
A TIG electrode needs to have the following properties:
- initiate an arc easily
- live long
You use tungsten as the main material, you get the high melting point and longevity of the second point. However, the work function of tungsten is very high: 4.3 - 5.2 eV. By contrast, thorium has a work function of 3.4 eV, and has itself quite a high melting point (1755 C) so it won't affect the electrode life too much.
The lower work function means that electrons will be emitted at lower field (for a given temperature) and this in turn will make arc initiation easier.