Why 100% Argon or Argon/CO2 75/25%

[jakeru]

You don't want to use CO2 when TIG'ing because it will oxidize your super (white) hot tungsten electrode. You need to keep that electrode oxidation-free, to keep your TIG welds clean and happy.

"MIG" welding with a reactive gas like CO2 would technically be called "MAG" welding (not MIG) due to the active CO2 gas.

For wire-feed welding use, as the electrode gets rapidly consumed by design, steel welding is tolerant of some oxidation, and also given how high the travel speeds and heat input of the wire-feed process are, the extra heat imparted by mixing in some CO2 is generally desirable. CO2 is also inexpensive.

CO2 is not desirable for wire-feed welding metals that are not tolerant of oxidation, such as aluminum (which would call for an inert gasses like pure argon, or an argon/Helium mix, such as would be used for TIG.)

So in summary, if you wanted one gas with the most versatility, and didn't care about cost, it would probably be Argon/Helium mix, because Helium also provides extra heat, while simultaneously being completely inert. Too bad Helium is very expensive. If the reason you just want to pick "one gas" is to save cost, you probably won't acbhieve that goal with Ar/He mix. If you have some other goal for picking "one gas" like portability of space savings, etc, Argon/Helium may achieve your goal. I love the stuff for TIG'ing aluminum.