MIG (Metal Inert Gas) Welding
Also known as GMAW (Gas Metal Arc Welding).

Mig welding is the most common form of welding accounting for about 50% of all welds. The process uses a consumable wire electrode fed through the welding gun and an inert shielding gas also supplied through the welding gun. The circuit formed between the electrode and the parent metal forms an arc which simultaneously melts the parent metal and filler wire (electrode) forming the weld pool. As the wire is continuously fed from a spool the process is sometimes call semi automatic welding.

Mig, originally developed for welding aluminum, can also be used to weld mild steel and stainless steel. In general terms the filler wire must be the same metal as the parent metal.

Metal deposition in Mig

Short Circuit or Dip Mig Welding

In regular Mig welding the electrode makes contact with the weld pool by dipping into the pool causing a short circuit. This action melts the wire on contact. Short circuit is the “coldest” form of Mig welding and can sometimes lead to poor fusion of metals in thicker pieces. Also the nature of the arc leads to greater weld spatter.

Spray Transfer Mig Welding

In droplet or Spray Mig welding the higher voltage is used to melt the tip of the wire before it makes contact with the weld pool. The molten wire forms a spray of droplets about the size of the wire diameter which are projected across the arc. Care should be taken to avoid the molten wire simply dripping into the weld pool as this gives an uneven weld.

Pulsed Mig Welding

A variant on Spray Transfer welding where the current is pulsed to provide just enough force to project the molten wire droplet from the wire to the work piece. One droplet per pulse. The fluctuating current gives greater control leading to a smaller neater weld.

Shield gasses

The shielding gas forms the arc plasma, stabilizes the arc on the parent metal and ensures smooth transfer of welds droplets to the weld pool. Ferrous metals use mixes of CO2 and Argon while non ferrous metals typically use mixtures of Argon and Helium.

MIG Advantages

• High productivity, because you don’t have to stop to change rods or chip and brush the weld frequently.
• Easy to learn and makes reasonably neat welds.
• Can be used on thick plate (with several passes)
• Can weld on stainless, mild steel, and aluminum.
• Can weld in all positions including overhead
• No flux to remove as shielding comes from the gas
• High welding speed can be attained (up to 4 times faster than stick)

MIG Disadvantages

• Not very portable – due to the cumbersome bottle of shielding gas.
• Causes weld spatter and greater local heating which causes oxidation and metal distortion.
• Cannot be used in drafty situations as the shield gas is blown away which causes porosity.

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