Metal inert gas (MIG), metal active gas (MAG) including CO2 and mixed gas processes
The MIG semi-automatic and automatic processes are increasing in use and are displacing some of the more traditional oxy-acetylene and MMA uses.
For repair work on thin sheet as in the motor trade, semi-automatic MIG using argon–CO2 mixtures has displaced the traditional oxyacetylene methods because of the reduced heat input and narrower HAZ, thus reducing distortion. For larger fabrication work, mechanical handling equipment with automatic MIG welding heads has revolutionized the fabrication industry, while the advent or robots, which are program controlled and use a fully automated MIG welding head with selfcontained wire feed, make less demands on the skilled welder.
Argon could not be used alone as a shielding gas for mild, low-alloy and stainless steel because of arc instability but now sophisticated gas mixtures of argon, helium, CO2 and oxygen have greatly increased the use of the process. Much research is proceeding regarding the welding of stainless and 9% nickel steels by this method, using magnetic arc oscillation and various gas combinations to obtain positional welds of great reliability and freedom from defects.
The process has very many applications and should be studied by the student as one of the major processes of the future.
It is convenient to consider, under this heading, those applications which involve shielding the arc with argon, helium and carbon dioxide (CO2) and mixtures of argon with oxygen and/or CO2 and helium, since the power source and equipment are essentially similar except for the gas supply.
