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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?

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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.

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  • $\begingroup$ So then only the tip contains Th? Or is it alloyed throughout the electrode? $\endgroup$ – docscience Apr 16 '15 at 22:00
  • $\begingroup$ I imagine it has to be throughout - as the electrode erodes you want the work function to be unaltered. $\endgroup$ – Floris Apr 16 '15 at 22:01

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