why argon?

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For tig mostly argon is used, I was curious as to that, why not co2 or nitrogen?
Reply:Argon gives a more stable arc, which is desired in TIG.  CO2 conducts heat better, but it also isn't completely inert.  It's used in MIG to achieve better penetration.  With TIG, one has more control over the arc and the filler metal, so an aid to penetration really isn't required.  Nitrogen is too reactive to be used as a shielding gas.-Heath
Reply:thanks for the info.