Metal Inert Gas (MIG) Welding and Metal Active Gas (MAG) welding (the process numbers are 131 and 135 respectively as per ISO 4063) are two variations of Gas Metal Arc Welding (GMAW) which is what they are typically called in the USA and some other countries.
They use heat from an electric arc running between the consumable metal electrode and the workpiece, making a weld pool and fusing the two parts together, creating a joint. The arc and weld pool are protected from the environment and any contaminants by a shielding gas.
MIG/MAG is similar to other arc welding processes (e.g. MMA) in that for both processes we produce the heat for welding by establishing an arc between a consumable metal electrode and the workpiece, the metal electrode then melts and makes the weld bead.
However, with MIG and MAG welding instead of having a metal electrode that is of a certain diameter and may be fed manually to establish the weld, we have a small diameter continuous wire electrode fed from a wire spool through the contact tip of the welding torch, and the gas is fed through the welding torch.
This means that the feed and welding process is continuous, which is generally why the manual version is referred to as semi-automatic welding.
Unlike MMA welding where the flux covering of the electrode melts to provide arc shielding, both MIG and MAG welding utilize a gas supply to provide arc shielding.
What is the Difference Between MIG and MAG?
The only difference between MIG and MAG is the type of shielding gas used. The makeup of the shielding gas is critical because the composition has a direct influence on the arc stability, metal transfer, weld profile, penetration, and spatter.
MIG (Metal Inert Gas) welding: In MIG welding, inert gases or gas mixtures are used as shielding gases.
Argon and helium or Ar/He mixtures are inert gases and are commonly used for the MIG welding of non-ferrous metals including aluminium. Inert gases do not react with the filler material or the weld pool.
MAG (Metal Active Gas) welding: In MAG welding, the active shielding gases have the ability to react with filler metal flowing across the arc and the weld pool and can affect the chemistry of the weld or the resulting mechanical properties of the weld.
Active shielding gases for welding steels are carbon dioxide or mixtures of argon, carbon dioxide and oxygen. Examples of active gases include CO2, Ar + 2 to 5% O2, Ar + 5 to 25% CO2 and Ar + CO2 + O2.
Gases for other materials may include hydrogen, nitrogen or other specialized gases.
Metal Transfer Mode
The metal transfer mode, or the way the metal is travelling from the filler wire to the weld pool, has a large influence on the operating characteristics of the process.
According to ISO 4063, there are four primary modes of metal transfer:
- Short circuiting (or dip transfer)
- Globular transfer
- Spray transfer
- Pulsed transfer
Short-circuiting metal transfer is used where low heat input operation is required, skill is needed by the welder to avoid a lack of fusion. In short-circuiting or dip transfer, the molten metal that forms on the end of the wire is transferred into the weld pool by the wire dipping into the weld pool.
The voltage is set low to achieve this. It is important that the sufficient tension is set voltage and inductance proportional to the wire feed speed to reduce the spatter. The inductance is used to control the current surge when the wire dips into the weld pool.
For spray transfer, a higher arc voltage and current is needed, producing higher heat input. The molten metal at the tip of the wire transfers to the weld pool as a spray of droplets of small diameter (less than the wire diameter).
However, there is a minimum current level or threshold, below which droplets are not projected forcibly across the arc; this is globular transfer.
If welding attempts are made far below the threshold level, the low arc forces do not prevent the formation of larger droplets at the wire tip; these droplets transfer across the arc in an uncontrolled manner, under standard gravity causing large amounts of spatter, also, there may also be instances of dip transfer.
The pulsed mode was developed as a method of reducing the heat input of spray transfer while at the same time retaining the advantages of spray transfer. Spray type transfer is achieved by applying a pulse of high current and the pulse has sufficient force to detach a droplet of weld metal.
With conventional MIG/MAG welding, a constant voltage power source is used in operation, providing an inherently stable ‘self adjusting’ arc.
Advantages of MIG / MAG Welding:
- several options for operation, including semi and fully automatic (robotic)
- high production speed to produce high quality welds
- as it is not using flux, there is no potential for slag inclusion in the weld metal
- is a versatile process to join a variety of metals and alloys
- MAG welding is done in all positions, making it the most used welding process.
Disadvantages of MIG / MAG Welding:
- for vertical or overhead welding short circuit transfer is to be used. As there is no fast freezing flux there is nothing to normally hold the fluid weld pool in position.
- cannot be performed outdoor with no encloures as the gas shielding has to be protected from the effects of wind.
- limited deoxidants available in the welding process, all rust must be removed from the workpiece before welding begins.
- flux cored arc welding (MAG welding using flux cored wires), may be a better option for positional and outdoor applications. As with all arc processes, proper PPE must be worn and in particular eye protection