Gas Metal-Arc Welding
Gas metal-arc welding, formerly called metal inert gas (MIG) welding, employs shielding gases, namely argon, helium, carbon dioxide and their mixtures. In this process, a consumable metallic electrode is fed automatically during welding which gives much faster speeds of welding. More common methods of gas metal-arc welding process may be divided into the following two distinct groups.
(a) Metal inert gas (MIG) welding
(b) CO, welding or metal active gas (MAG) welding
(a)Metal inert gas (MIG) welding:
Arc is established between a continuously fed consumable metal electrode or wire and the workpiece (Fig.). Arc is shielded by inert gas like argon, helium, carbon dioxide or mixture of these gases. Electrode wire is fed from a coil rotated with a constant speed motor and the are length is maintained constant by using (a) self-adjusted arc for semi-automatic plants or (b) self-controlled arc in fully automatic MIG plants.
MIG welding employs power sources with flat or drooping characteristics. Mostly, DC generator or AC transformer with rectifier is used. AC is not generally preferred. because of unequal/burn off rates in positive and negative half cycles. DCRP gives deeper penetration and is used for thicker jobs.. Shielding gases used include argon, helium, CO, or nitrogen and the mixtures. thereof.
Argon or helium is used for welding aluminium, magnesium and copper, and carbon dioxide for mild steel and nitrogen for copper. Argon plus CO₂ are used for welding mild steel, low alloy steel and stainless steels, whereas a mixture of argon, helium and CO₂ is used for welding austenitic stainless steel.
MIG, with short-circuiting metal transfer mode, is popular for welding thin sheets and sections (less than 6 mm) of carbon steels, low alloy steels, stainless steels, aluminium, magnesium, copper, nickel and their alloys and also for welding tool steels and die steels. MIG with globular transfer mode gives deeper penetration and finds applications in aircraft, pressure vessels and ship building industry.
Under similar conditions, MIG welding is a much faster process compared to TIG welding with advantages such as no flux needed, high welding speeds possible, difficult to weld metals like aluminium, and stainless steels are welded effectively. The process can be automated.
(b) CO₂ welding or metal active gas (MAG) welding:
Gas metal-arc welding with CO₂ gives smooth and high speed welds. The shielding gas is a carbon dioxide gas (CO₂) which dissociates at temperatures about 7760°C during welding and by using deoxidants (like silicon and aluminium), the free oxygen (produced during dissociation of CO₂) combines with these deoxidants and forms minute islands of slag that float out to the surface of the molten metal and protect the weld bead.
In addition, CO₂ is cheaper (one-ninth the cost of argon gas) with non-turbulent flow and it covers well over weld area. The CO, welding is done in the similar way as MIG. The process is very popular and economical in welding light gauge steels. Because of good penetration capability, CO, welding is also effectively used for fillet and butt welds in metals 0.63 to 38.10 mm thick.
It is extensively used for welding carbon steels and alloy steels. It is also faster and a competitor to shielded metal arc welding for steels.
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