Arc welding
Arc welding power supplies can deliver either direct (DC) or alternating (AC) current to the work, while consumable or non-consumable electrodes are used.
This is important because in manual welding, it can be difficult to hold the electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate.
In these processes, arc length is kept constant, since any fluctuation in the distance between the wire and the base material is quickly rectified by a large change in current.
[1] Under normal arc length conditions, a constant current power supply with a stick electrode operates at about 20 volts.
[7] Duty cycle is a welding equipment specification which defines the number of minutes, within a 10-minute period, during which a given arc welder can safely be used.
With continuously fed filler electrodes, GMAW offers relatively high welding speeds; however the more complicated equipment reduces convenience and versatility in comparison to the SMAW process.
Because of the need to maintain a stable shroud of shielding gas around the weld site, it can be problematic to use the GMAW process in areas of high air movement such as outdoors.
It can be applied to all of the same materials as GTAW except magnesium; automated welding of stainless steel is one important application of the process.
If the electrodes used for welding contain traces of moisture, the water decomposes in the heat of the arc and the liberated hydrogen enters the lattice of the material, causing its brittleness.
Knifeline attack (KLA) is another kind of corrosion affecting welds, impacting steels stabilized by niobium.
This affects only a thin zone several millimeters wide in the very vicinity of the weld, making it difficult to spot and increasing the corrosion speed.
Because many common welding procedures involve an open electric arc or flame, the risk of burns from heat and sparks is significant.
The use of compressed gases and flames in many welding processes also pose an explosion and fire risk; some common precautions include limiting the amount of oxygen in the air and keeping combustible materials away from the workplace.
Welding goggles and helmets with dark face plates—much darker than those in sunglasses or oxy-fuel goggles—are worn to prevent this exposure.
In recent years, new helmet models have been produced featuring a face plate which automatically self-darkens electronically.
These curtains, made of a polyvinyl chloride plastic film, shield nearby workers from exposure to the UV light from the electric arc.
Additionally, many processes produce various gases (most commonly carbon dioxide and ozone, but others as well) that can prove dangerous if ventilation is inadequate.
The return clamp of the welding machine is located near to the work area, to reduce the risk of stray current traveling a long way to create heating hazards or electric shock exposure, or to cause damage to sensitive electronic devices.
[23][24] Independently, a Russian physicist named Vasily Petrov discovered the continuous electric arc in 1802[24][25][26][27] and subsequently proposed its possible practical applications, including welding.
[30] The advances in arc welding continued with the invention of metal electrodes in the late 19th century by a Russian, Nikolai Slavyanov (1888), and an American, C. L. Coffin.
[33] During World War I, welding started to be used in shipbuilding in Great Britain in place of riveted steel plates.
The Americans also became more accepting of the new technology when the process allowed them to repair their ships quickly after a German attack in the New York Harbor at the beginning of the war.
[35] In 1919, the British shipbuilder Cammell Laird started construction of a merchant ship, the Fullagar, with an entirely welded hull;[36] she was launched in 1921.
Shielding gas became a subject receiving much attention as scientists attempted to protect welds from the effects of oxygen and nitrogen in the atmosphere.
Porosity and brittleness were the primary problems and the solutions that developed included the use of hydrogen, argon, and helium as welding atmospheres.
Using a consumable electrode and a carbon dioxide atmosphere as a shielding gas, it quickly became the most popular metal arc welding process.
