About Gas Metal Arc Welding
It is a known fact that one of the strongest methods of joining metal is through fusion by arc welding. In this welding process, an electric current allows an electric arc to melt two metal pieces. A filler material mixes with the two metals, and as it cools, a strong, solid bond forms.
Gas metal arc welding (GMAW), is quite often referred to as Metal Inert Gas (MIG) welding or Metal Active Gas (MAG) welding. Gas metal arc welding may be described as a semi-automatic or automatic arc welding process in which a continuous wire electrode and a shielding gas are fed through a welding gun. GMAW is also popularly called wire welding, as the filler/electrode material comes as spools of wire.
Welding professionals know that GMAW was initially developed only for welding aluminum and other non-ferrous metal pieces. GMAW later spread to welding steel because it allowed for lower welding time compared to other welding processes. GMAW has indeed revolutionized the field of welding as welds can now be done much faster and with greater degree of consistency. GMAW is currently used is nearly all assembly line processes, such as welding vehicle frames, pipe welding and structural welding, just to mention a few.
GMAW has quite a few advantages including high rate of filler metal deposition, narrow weld bead, minimum distortion of work piece and clean, precise welds. The demerits of GMAW are the equipments are complex and expensive and setting up equipments is cumbersome and tedious. To perform gas metal arc welding, the basic necessary equipment is a welding gun, a wire feed unit, required power supply, an electrode wire and a shielding gas supply.
The GMAW welding gun consists of control switch, contact tip, power cable, gas nozzle, an electrode conduit and liner and a gas hose. When the operator presses the control switch, he initiates the wire feed, then the electric power and the shielding gas flow, causing an electric arc to be struck. The contact tip is usually made of copper, though at times chemically treated to reduce spatter.
The gas nozzle is meant to evenly direct the shielding gas into the welding zone. Larger nozzles for greater shielding gas flow, is used for high current welding operations. Sometimes, a water hose is also built into the welding gun, to cool during high heat operations. Most machines provide the wire at a constant feed rate, but certain advanced machines are capable of varying the feed rate in response to the arc length and voltage.
Most applications of GMAW use a constant voltage power supply. As a result, any change in arc length results in a large change in heat input and current. Thus the welders keep the arc length consistent even when manually welding with hand-held welding guns. Alternating current is sedom used with GMAW and the electrode is generally positively charged. The polarity can be reversed only when special emissive-coated electrode wires are used.
As regards electrode selection, it depends on the composition of the metal being welded, the process variation being used, the joint design, and the material surface conditions. As a matter of fact, the finished weld metal should have mechanical properties similar to those of the base material. For GMAW, it is necessary to choose the equipment, wire electrode, shielding gas, and welding conditions that are capable of producing high-quality welds at a low cost.