Both MIG (metal inert gas) and MAG (metal active gas) welding create heat between metal pieces and a wire electrode to form a joint. The difference is the type of gas used to shield the weld area (pool). MIG employs inert gases (argon and helium) and is mostly used for work with non-ferrous metals like aluminum. MAG uses active gas (mixtures of argon, carbon dioxide and oxygen) to weld steel. These processes are very versatile and can be used in most situations.