The purpose of a shielding gas when welding is to protect the molten weld from oxygen, nitrogen and hydrogen in the air. When the weld pool interacts with these elements, it can cause problems such as pores in the weld or increased splatter. Shielding gases for MIG welders are either inert or active. The most commonly used inert gas is argon, which can be used for most metals including aluminum. An argon/carbon dioxide mix is also a common gas mix to use with a MIG welder as it has active properties and so increases penetration, although it is not effective with aluminum.
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