Gas metal arc welding (GMAW) is a method of welding that causes metal objects to merge into a single body. An arc of electricity is used to heat the metal and an uninterrupted, consumable electrode wire. A shielding gas is used to prevent the nitrogen and oxygen in the air from causing problems with the weld; the choice of gas varies depending on the metals being welded, but gas mixtures may include carbon dioxide, argon, and helium. The nature of this type of welding makes it impractical to use outdoors.
The methods and techniques involved in gas metal arc welding are widely used in the sheet metal industry. Due to their extensive utilization, many of the major automobile manufacturing companies make use of gas metal arc welding, particularly for arc spot welding. It has also been accepted in the field of automated welding, where robots handle the work pieces and the welding gun to speed up the manufacturing process.
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There are four basic types of gas metal arc welding: spray, pulsed-spray, globular metal transfer, and short-circuiting. Each type differs based on how the metal from the electrode is transferred. Globular and short-circuiting can only be used on ferrous metals, which contain iron.
Spray was the first type developed, and requires high heat to vaporize the metal and transfer it along the arc. Although this method produces a high-quality weld, the heat required makes it best for thicker metals and a larger weld area. Pulse-spray, a more recent development, operates in a similar way, but uses pulses of current and does not require the same level of heat; this makes it more practical for a wider range of applications.
Globular metal transfer is generally considered to be the least desirable method of GMAW, as the electrode drips from the wire and can spatter. It tends to be more cost-efficient, however, requiring less expensive carbon dioxide shielding gas. Short-circuiting also transfers the metal in droplets, but a lower current and slower feed rate mean less spatter and often a better weld. It can be slower than other methods, however, and may not work as well on thicker materials.
Gas metal arc welding developed as an attempt to improve gas tungsten arc welding (GTAW). This process, which used a non-consumable tungsten electrode instead of a consumable wire, was somewhat slow, particularly on thicker pieces of metal. By switching to a different type of electrode, the process could be accomplished more quickly, and thus more profitably.
In its early development, the gas used in gas metal arc welding was inert, as it had been with GTAW. Inert gasses, like argon, are expensive, however, and it was not until reactive shielding gases started to become commonplace with carbon steels that the processed gained popularity. Originally, aluminum electrode wires were used, but the process has improved to also include titanium, manganese, and silicon.
Gas metal arc welding is considered a relatively easy process to learn, especially when compared to other types of welding like GTAW. The welding gun feeds the electrode wire automatically, which helps simplify the process. All welding is potentially dangerous, however, and proper training and safety precautions are always required. The welder may be exposed to toxic gasses and high levels of heat, so the proper protective clothing should always be worn.