The difference is that MIG welding uses inert gas to create a shield during the welding process, whereas MAG welding uses an active gas. Here's a quick look at these gases: Inert Gas – Includes noble gases such as helium and argon

The MIG and MAG welding methods differ from each other in that MIG (metal inert gas) welding uses an inert shielding gas, which does not participate in the welding process, while MAG (metal active gas) welding employs an active shielding gas that participates in the welding process.

MIG WELDING:

A chemically inert gas such as nitrogen,argon,helium,or a mixture of such gases is used as shielding gas in Metal Inert Gas(MIG) welding.

Because of pure inert gas,the shielding gas remains intact during welding(i.e. no disintegration takes place).

Shielding gas does not include any chemical element into the weld bead.

It cannot alter the chemical composition of weld bead or its properties.

It is not flexible in terms of shielding gas (as only inert gas can be used).

Industrially pure inert gas cylinder is somewhat costlier. Accordingly, MIG welding process is less economic.

MIG welding is preferred for joining non-ferrous metals like aluminium

MAG WELDING:

A mixture of chemically active gas(like oxygen or carbon dioxide) and inert gas(like nitrogen,argon or helium) is used as shielding gas in metal active gas(MAG) welding.

Due to presence of active gas,a part of shielding gas chemically disintegrates during welding due to intense heat of electric arc.

As the active shielding gas breaks down,certain chemical elements(like oxygen,carbon,etc.) are induced in to weld bead.

It is capable in changing chemical composition and thus properties of weld bead. This is sometimes undesirable,but sometimes it can be carried out intentionally to obtain intended properties of weld bead.

It allows usage of that particular gas mixture as shielding gas which can give desired features(such as deeper penetration,less spatter,etc.)

Shielding gas cylinder for MAG welding is comparatively cheaper.

MAG welding is preferred for joining ferrous metals (like mild steel or stainless steel).  

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