Gas tungsten arc welding: Difference between revisions

Revision as of 07:54, 3 October 2006

Revision as of 07:55, 3 October 2006

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GTAW is most commonly used to weld thin sections of [[stainless steel]] and light metals such as [[Aluminium|aluminum]], [[magnesium]], and [[copper]] alloys. The process grants the operator greater control over the weld than competing procedures such as [[shielded metal arc welding]] and [[gas metal arc welding]], allowing for stronger, higher quality welds. However, GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques. A related process, [[plasma arc welding]], uses a slightly different welding torch to create a more focused welding arc and as a result is often automated.<ref>Weman, 31, 37-38</ref>

==Development==

After the discovery of the [[electric arc]] in 1800 by [[Humphry Davy]], arc welding developed slowly. [[C. L. Coffin]] had the idea of welding in an inert gas atmosphere in 1890, but even in the early 1900s, welding non-ferrous materials like aluminum and magnesium remained difficult, because these metals reacted rapidly with the air, resulting in porous and [[dross]]-filled welds.<ref>Cary and Helzer, 5-8</ref> Processes using flux covered electrodes did not satisfactorily protect the weld area from contamination. To solve the problem, bottled [[inert]] gases were used in the beginning of the 1930s. A few years later, a [[direct current]], gas-shielded welding process emerged in the aircraft industry for welding magnesium.

This process was perfected in 1941, and became known as heliarc or tungsten inert gas welding, because it utilized a tungsten electrode and helium as a shielding gas. Initially, the electrode overheated quickly, and in spite of tungsten's high [[Melting point|melting temperature]], particles of tungsten were transferred to the weld. To address this problem, the polarity of the electrode was changed from positive to negative, but this made it unsuitable for welding many non-ferrous materials. Finally, the development of [[alternating current]] made it possible to stabilize the arc and produce high quality aluminum and magnesium welds.<ref>Lincoln Electric, 1.1-7&ndash;1.1-8</ref>

Developments continued during the following decades. Linde Air Products developed water-cooled torches that helped to prevent overheating when welding with high currents.<ref>Cary and Helzer, 8</ref> Additionally, during the 1950s, as the process continued to gain popularity, some users turned to [[carbon dioxide]] as an alternative to the more expensive welding atmospheres consisting of [[argon]] and [[helium]]. However, this proved unacceptable for welding aluminum and magnesium because it reduced weld quality, and as a result, it is rarely used with GTAW today.

In 1953, a new process based on GTAW was developed, called [[plasma arc welding]]. It affords greater control and improves weld quality by using a nozzle to focus the electric arc, but is largely limited to automated systems, whereas GTAW remains primarily a manual, hand-held method.<ref>Lincoln Electric, 1.1-8</ref> Development within the GTAW process has continued as well, and today a number of variations exist. Among the most popular are the pulsed-current, manual programmed, hot-wire, dabber, and increased penetration GTAW methods.<ref>Cary and Helzer, 75</ref>

==Operation==

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Welders often develop a technique of rapidly alternating between moving the torch forward (to advance the weld pool) and adding filler metal. The filler rod is withdrawn from the weld pool each time the electrode advances, but it is never removed from the gas shield to prevent oxidation of its surface and contamination of the weld. Filler rods composed of metals with low melting temperature, such as aluminum, require that the operator maintain some distance from the arc while staying inside the gas shield. If held too close to the arc, the filler rod can melt before it makes contact with the weld puddle. As the weld nears completion, the arc current is often gradually reduced to prevent the formation of a crater at the end of the weld.<ref>Jeffus, 378</ref><ref>Lincoln Electric, 9.4-7</ref>

===Safety===

Like other arc welding processes, GTAW can be dangerous if proper precautions are not taken. [[Welder]]s wear [[protective clothing]], including heavy [[leather]] [[glove]]s and protective long sleeve jackets, to avoid exposure to extreme heat and flames. Due to the absence of smoke in GTAW, the electric arc can seem brighter than in [[shielded metal arc welding]], making operators especially susceptible to [[arc eye]] and skin irritations not unlike [[sunburn]]. [[Helmet]]s with dark face plates are worn to prevent this exposure to [[ultraviolet light]], and in recent years, new helmets often feature a [[liquid crystal]]-type face plate that self-darkens upon exposure to high amounts of UV light. Transparent welding curtains, made of a [[polyvinyl chloride]] plastic film, are often used to shield nearby workers and bystanders from exposure to the UV light from the electric arc.

Welders are also often exposed to dangerous gases and [[particulate]] matter. Shielding gases can displace oxygen and lead to [[asphyxiation]], and while smoke is not produced, the brightness of the arc in GTAW can cause surrounding air to break down and form [[ozone]]. Similarly, the brightness and heat can cause poisonous fumes to form from cleaning and degreasing materials. Cleaning operations using these agents should not be performed near the site of welding, and proper ventilation is necessary to protect the welder.<ref>Cary and Helzer, 42, 75</ref>

===Applications===