Which combination of gases might be used for welding both stainless and mild steels in MIG welding?

The correct choice is a mix of CO2 with argon, which is commonly used in MIG welding for both stainless and mild steels. This mixture enhances the welding process by providing better control of the arc stability and improving penetration. Argon acts as a shielding gas that provides a protective atmosphere to prevent oxidation during the welding process. When mixed with CO2, the mixture helps to produce a more fluid weld pool and contributes to a cleaner weld.

Using solely argon could be effective for stainless steel but may not provide adequate penetration or heat for mild steels. While helium and nitrogen can play roles in certain processes, they are not optimal for the desired outcomes in welding these specific materials. Carbon monoxide and carbon dioxide mixtures are not standard for MIG welding; instead, CO2 in conjunction with argon is preferred for its benefits in controlling the welding characteristics of various steels.