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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
Metal Inert Gas (MIG) welding is an arc welding process that uses a continuous solid wire
electrode heated and fed into the weld pool from a welding gun. The two base materials are
melted together forming a joint. The gun feeds a shielding gas alongside the electrode
helping protect the weld pool from airborne contaminants.
MIG/MAG welding is a versatile technique suitable for both thin sheet and thick section components. An arc is struck between the end of a wire electrode and the workpiece, melting both of them to form a weld pool. The wire serves as both heat source (via the arc at the wire tip) and filler metal for the welding joint. The wire is fed through a copper contact tube (contact tip) which conducts welding current into the wire. The weld pool is protected from the surrounding atmosphere by a shielding gas fed through a nozzle surrounding the wire. Shielding gas selection depends on the material being welded and the application. The wire is fed from a reel by a motor drive, and the welder moves the welding torch along the joint line. Wires may be solid (simple drawn wires), or cored (composites formed from a metal sheath with a powdered flux or metal filling). Consumables are generally competitively priced compared with those for other processes. The process offers high productivity, as the wire is continuously fed.
Manual MIG/MAG welding is often referred as a semi-automatic process, as the wire feed rate and arc length are controlled by the power source, but the travel speed and wire position are under manual control. The process can also be mechanized when all the process parameters are not directly controlled by a welder, but might still require manual adjustment during welding. When no manual intervention is needed during welding, the process can be referred to as automatic.
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.
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 and pulsed metal transfer are used for low current operation while spray metal transfer is only used with high welding currents. 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; for a 1.2mm diameter wire, arc voltage varies from about 17V (100A) to 22V (200A). Care in setting the voltage and the inductance in relation to the wire feed speed is essential to minimize spatter. Inductance is used to control the surge in current which occurs when the wire dips into the weld pool.
For droplet or spray transfer, a much higher voltage is necessary to ensure that the wire does not make contact i.e. short-circuit, with the weld pool; for a 1.2mm diameter wire, the arc voltage varies from approximately 27V (250A) to 35V (400A). The molten metal at the tip of the wire transfers to the weld pool in the form of a spray of small droplets (about the diameter of the wire and smaller). However, there is a minimum current level, threshold, below which droplets are not forcibly projected across the arc. If an open arc technique is attempted much below the threshold current level, the low arc forces would be insufficient to prevent large droplets forming at the tip of the wire. These droplets would transfer erratically across the arc under normal gravitational forces. The pulsed mode was developed as a means of stabilising the open arc at low current levels i.e. below the threshold level, to avoid short-circuiting and spatter. Metal transfer is achieved by applying pulses of current, each pulse having sufficient force to detach a droplet. Synergic pulsed MIG refers to a special type of controller which enables the power source to be tuned (pulse parameters) for the wire composition and diameter, and the pulse frequency to be set according to the wire feed speed.
In addition to general shielding of the arc and the weld pool, the shielding gas performs a number of important functions:
Thus, the shielding gas will have a substantial effect on the stability of the arc and metal transfer and the behaviour of the weld pool, in particular, its penetration. General purpose shielding gases for MIG welding are mixtures of argon, oxygen and CO2, and special gas mixtures may contain helium. The gases which are normally used for the various materials are:
Argon based gases, compared with CO2, are generally more tolerant to parameter settings and generate lower spatter levels with the dip transfer mode. However, there is a greater risk of lack of fusion defects because these gases are colder. As CO2 cannot be used in the open arc (pulsed or spray transfer) modes due to high back-plasma forces, argon based gases containing oxygen or CO2 are normally employed.
MIG/MAG is widely used in most industry sectors and accounts for more than 50% of all weld metal deposited. Compared to MMA, MIG/MAG has the advantage in terms of flexibility, deposition rates and suitability for mechanization. However, it should be noted that while MIG/MAG is ideal for ‘squirting’ metal, a high degree of manipulative skill is demanded of the MIG welder.
Tunaak Metals Nigeria Limited is a structural steel fabrication company, which specializes in fabrication of all type of structures, ranging from storage tanks, industrial and marine pipping and miscellaneous steel structures in the civil, industrial and marine sectors. With innovative ideas in steel fabrication we build world class products for a better community. We provide essential and specialize training for our staff to enable us go beyond the expectations of our clients, while implementing adequate safety measures which protects the environment and persons.
