Elemental argon gas 

LOL

A lot

Well they fill those cylinders to like 2000 psi and that's almost no psi above atmospheric it doesn't take 2000 psi but that's how they store enough

Mig welding with aluminum wires

Thanks! :)

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it's a shielding gas that prevent oxidation aka impurities from forming hot and molten metal loves oxygen and was oxidize and become rust.