It's not just a matter of MIG or TIG that determines the shielding gas used. Most commonly MIG is only being used for mild steel and in that case the 25/75 mix is used. If however you are set up to weld aluminum with your MIG then you would need to switch to the ARGON just as you would if TIG welding. That is one of the aspects of TIG welding that makes it so versitile is that you are set up to weld many different types of metals without otally reconfiguring your equipment.
