An Inert gas, such as Argon, has no effect, or reaction, on or with the welding process, it simply performs the essential task of replacing Air/Oxygen from around the weld pool. Active gases do have an effect on the welding process. The effect of an active gas on MIG Welding is twofold. Firstly, the CO₂ content in an Argon/ CO₂ mix makes the gas slightly electrically conductive, this in turn raises the arc voltage, which increases penetration. The second effect is the CO₂ content breaks down the surface tension of the molten weld pool. Using CO₂ to break the surface tension of the molten weld pool allows the weld to flow and flatten slightly for the correct weld deposit profile
If an active gas is used for TIG Welding the raised arc voltage will increase hole blows and cause excessive burning of the Tungsten Electrode.

If an Inert gas is used for MIG Welding (except Aluminum and Mig Braze), higher machine power output will be needed to achieve penetration and the finished weld will look too tall because the surface tension has not been broken.

Keep reading, Happy welding

Thank you,

KP Bhatt