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What is Gas Metal Arc Welding? (MIG Welding / MAG Welding)


Metal Inert Gas (MIG) welding and Metal Active Gas (MAG) welding, process numbers 131 and 135 respectively in accordance with ISO 4063, are both variations of the Gas Metal Arc Welding (GMAW) process, which they are more commonly referred to as in USA and some other countries. These use heat created from an electric arc between a consumable metal electrode and a workpiece, creating a weld pool and fusing them together, forming a joint. The arc and weld pool is protected from the environment and contaminants by a shielding gas.

This is part of a series of TWI Frequently Asked Questions (FAQs).

MIG/MAG is similar to other arc welding processes, e.g. MMA, in that heat for welding is produced by forming an arc between a consumable metal electrode and the workpiece; the electrode melts to form the weld bead. The main difference is that the metal electrode is a small diameter wire fed continuously through the contact tip of the welding torch from a wire spool, while a shielding gas is fed through the welding torch. As the wire is continuously fed, the manual process is sometimes referred to as semi-automatic welding. MIG and MAG welding both use a gas supply to provide arc shielding, unlike MMA where the flux on the electrode is melted 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 make-up of the shielding gas is important as it has a significant effect on the stability of the arc, metal transfer, weld profile, penetration, and the degree of spatter.

MIG (Metal Inert Gas) welding: This process uses inert gases or gas mixtures as the shielding gas. Argon and helium or Ar / He mixes are inert gases and typically used for the MIG welding of non-ferrous metals such as aluminium. Inert gases do not react with the filler material or weld pool.

MAG (Metal Active Gas) welding: This process uses active shielding gases. These gases can react with filler metal transferring across the arc and the weld pool, affecting its chemistry and/or resulting mechanical properties.

Active  shielding gases used for the welding of steels are carbon dioxide or mixtures of argon, carbon dioxide and oxygen. Examples of these 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 specialised gases.

Metal Transfer Mode

The manner, or mode, in which the metal transfers from the filler wire to the weld pool largely determines the operating features of the process. There are four principal metal transfer modes as defined in ISO 4063:

  • Short circuiting (dip transfer)
  • Globular transfer
  • Spray transfer
  • Pulsed transfer

Short-circuiting metal transfer are used for low heat input operation, and skill is required to avoid lack of fusion. In short-circuiting or 'dip' transfer, the molten metal forming on the tip of the wire is transferred by the wire dipping into the weld pool. This is achieved by setting a low voltage. Care in setting the voltage and the inductance in relation to the wire feed speed is essential to minimise spatter. Inductance is used to control the surge in current which occurs when the wire dips into the weld pool.

For spray transfer, a  higher voltage and current are required, producing a higher heat input. The molten metal at the tip of the wire transfers to the weld pool in the form of a spray of small droplets (less than the diameter of the wire). However, there is a minimum current level or threshold, below which droplets are not forcibly projected across the arc; this is globular transfer. If welding is attempted much below the threshold current level, the low arc forces are insufficient to prevent large droplets forming at the tip of the wire. These droplets transfer erratically across the arc under normal gravitational force, often producing large amounts of spatter, and occasionally dip transfer may also occur.

The pulsed mode was developed as a means of reducing the heat input of spray transfer whilst maintaining its advantages. Spray type metal transfer is achieved by applying pulses of high current, each pulse having sufficient force to detach a droplet of weld metal.

Conventional MIG/MAG welding is carried out using a constant voltage power source which provides an inherently stable 'self-adjusting' arc. 

What are the Advantages of MIG / MAG Welding?

  • can be operated in several ways, including semi and fully automatically, including robotically
  • allows for the fast production of high quality welds
  • due to a lack of flux being used, there is no chance of slag being trapped in the weld metal
  • is a versatile process that can be used to join a variety of metals and alloys
  • MAG welding can be performed in all positions, making it one of the most widely-used welding processes. 

What are the Disadvantages of MIG / MAG Welding?

  • for vertical or overhead welding short circuit transfer is required. With no fast freezing flux, there is nothing to hold the fluid weld pool in position
  • welding cannot be performed outdoors without enclosures as the welding gas needs to be protected from the wind
  • with limited deoxidants available in the process, all rust must be removed from the workpiece before welding commences.
  • flux cored arc welding (MAG welding with flux cored wires) may be more suitable for positional welding and outdoor applications. As with all arc processes, proper PPI must be worn and, in particular, eye protection

MIG and MAG Welding Expertise

TWI has a considerable amount of knowledge and experience in the development and qualification of MIG / MAG welding procedures for a variety of applications across industry.

Related Frequently Asked Questions (FAQs)

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