Inert gas property

Inert gases like argon or helium are chemically inert. So they don't react with the molten metal, they only displace oxygen and nitrogen from the air. This prevents oxidation and porosity in the weld. Without this protection, alloying elements burn off and the weld becomes porous and brittle. In TIG welding, the gas is therefore mandatory, but in MAG welding it also protects the melt until it solidifies. Make sure the gas flow is correct and the torch is not too far from the weld pool. Drafts can blow away the shielding gas, then you'll immediately get black deposits and a bad weld.