Arc welding
Arc welding is a welding process that is used to join metal to metal by using electricity to create enough heat to melt metal, and the melted metals, when cool, result in a binding of the metals. It is a type of welding that uses a welding power supply to create an electric arc between a metal stick ("electrode") and the base material to melt the metals at the point of contact. Arc welding power supplies can deliver either direct (DC) or alternating (AC) current to the work, while consumable or non-consumable electrodes are used.
The welding area is usually protected by some type of shielding gas (e.g. an inert gas), vapor, or slag. Arc welding processes may be manual, semi-automatic, or fully automated. First developed in the late part of the 19th century, arc welding became commercially important in shipbuilding during the Second World War. Today it remains an important process for the fabrication of steel structures and vehicles.
Power supplies
To supply the electrical energy necessary for arc welding processes, a number of different power supplies can be used. The most common classification is constant
The direction of current used in arc welding also plays an important role in welding. Consumable electrode processes such as shielded metal arc welding and gas metal arc welding generally use direct current, but the electrode can be charged either positively or negatively. In general, the positively charged anode will have a greater heat concentration (around 60%).[3] "Note that for stick welding in general, DC+ polarity is most commonly used. It produces a good bead profile with a higher level of penetration. DC− polarity results in less penetration and a higher electrode melt-off rate. It is sometimes used, for example, on thin sheet metal in an attempt to prevent burn-through."[4] "With few exceptions, electrode-positive (reversed polarity) results in deeper penetration. Electrode-negative (straight polarity) results in faster melt-off of the electrode and, therefore, faster deposition rate."[5] Non-consumable electrode processes, such as gas tungsten arc welding, can use either type of direct current (DC), as well as alternating current (AC). With direct current however, because the electrode only creates the arc and does not provide filler material, a positively charged electrode causes shallow welds, while a negatively charged electrode makes deeper welds.[6] Alternating current rapidly moves between these two, resulting in medium-penetration welds. One disadvantage of AC, the fact that the arc must be re-ignited after every zero crossing, has been addressed with the invention of special power units that produce a square wave pattern instead of the normal sine wave, eliminating low-voltage time after the zero crossings and minimizing the effects of the problem.[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. For example, an 80 A welder with a 60% duty cycle must be "rested" for at least 4 minutes after 6 minutes of continuous welding.[8] Failure to observe duty cycle limitations could damage the welder. Commercial- or professional-grade welders typically have a 100% duty cycle.
Consumable electrode methods
One of the most common types of arc welding is
Flux-cored arc welding (FCAW) is a variation of the GMAW technique. FCAW wire is actually a fine metal tube filled with powdered flux materials. An externally supplied shielding gas is sometimes used, but often the flux itself is relied upon to generate the necessary protection from the atmosphere. The process is widely used in construction because of its high welding speed and portability.
Submerged arc welding (SAW) is a high-productivity welding process in which the arc is struck beneath a covering layer of granular flux. This increases arc quality, since contaminants in the atmosphere are blocked by the flux. The slag that forms on the weld generally comes off by itself and, combined with the use of a continuous wire feed, the weld deposition rate is high. Working conditions are much improved over other arc welding processes since the flux hides the arc and no smoke is produced. The process is commonly used in industry, especially for large products.[12] As the arc is not visible, it is typically automated. SAW is only possible in the 1F (flat fillet), 2F (horizontal fillet), and 1G (flat groove) positions.
Non-consumable electrode methods
Gas tungsten arc welding (GTAW), or tungsten/inert-gas (TIG) welding, is a manual welding process that uses a non-consumable electrode made of tungsten, an inert or semi-inert gas mixture, and a separate filler material. Especially useful for welding thin materials, this method is characterized by a stable arc and high quality welds, but it requires significant operator skill and can only be accomplished at relatively low speeds. It can be used on nearly all weldable metals, though it is most often applied to stainless steel and light metals. It is often used when quality welds are extremely important, such as in bicycle, aircraft and marine applications.[13]
A related process, plasma arc welding, also uses a tungsten electrode but uses plasma gas to make the arc. The arc is more concentrated than the GTAW arc, making transverse control more critical and thus generally restricting the technique to a mechanized process. Because of its stable current, the method can be used on a wider range of material thicknesses than can the GTAW process and is much faster. 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. A variation of the process is plasma cutting, an efficient steel cutting process.[14]
Other arc welding processes include
Corrosion issues
Some materials, notably high-strength steels, aluminum, and titanium alloys, are susceptible to hydrogen embrittlement. 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. Stick electrodes for such materials, with special low-hydrogen coating, are delivered in sealed moisture-proof packaging. New electrodes can be used straight from the can, but when moisture absorption may be suspected, they have to be dried by baking (usually at 450 to 550 °C or 840 to 1,020 °F) in a drying oven. Flux used has to be kept dry as well.[15]
Some
Filler metal (electrode material) improperly chosen for the environmental conditions can make them corrosion-sensitive as well. There are also issues of galvanic corrosion if the electrode composition is sufficiently dissimilar to the materials welded, or the materials are dissimilar themselves. Even between different grades of nickel-based stainless steels, corrosion of welded joints can be severe, despite that they rarely undergo galvanic corrosion when mechanically joined.[17]
Safety issues
Welding can be a dangerous and unhealthy practice without the proper precautions; however, with the use of new technology and proper protection the risks of injury or death associated with welding can be greatly reduced.
Heat, fire, and explosion hazard
Because many common welding procedures involve an open electric arc or flame, the risk of burns from heat and
Eye damage
Exposure to the brightness of the weld area leads to a condition called
Inhaled matter
Welders are also often exposed to dangerous gases and
Electrical safety
While the open-circuit voltage of an arc welding machine may be only a few tens of volts up to about 120 volts, even these low voltages can present a hazard of electric shock for the operators. Locations such as ship's hulls, storage tanks, metal structural steel, or in wet areas are usually at earth ground potential and operators may be standing or resting on these surfaces during operating of the electric arc. Welding machines operating off AC power distribution systems must isolate the arc circuit from earth ground to prevent insulation faults in the machine from exposing operators to high voltage. 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.[21] Welding operators are careful to install return clamps so that welding current cannot pass through the bearings of electric motors, conveyor rollers, or other rotating components, which would cause damage to bearings. Welding on electrical buswork connected to transformers presents a danger of the low welding voltage being "stepped up" to much higher voltages, so extra grounding cables may be required.
Interference with pacemakers
Certain welding machines which use a high frequency alternating current component have been found to affect pacemaker operation when within 2 meters of the power unit and 1 meter of the weld site.[22]
History
While examples of forge welding go back to the Bronze Age and the Iron Age, arc welding did not come into practice until much later.
In 1800
Competing welding processes such as
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.[34] Arc welding was first applied to aircraft during the war as well, and some German airplane fuselages were constructed using this process.[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.[37]
During the 1920s, major advances were made in welding technology, including the 1920 introduction of automatic welding in which electrode wire was continuously fed. Shielding gas became a subject receiving much attention as scientists attempted to protect welds from the effects of oxygen and nitrogen in the atmosphere.
During the middle of the century, many new welding methods were invented.
See also
- Robot welding – use of mechanized programmable tools, which completely automate a welding process by both performing the weld and handling the part
- Sensors for arc welding
- Weld quality assurance
References
Notes
- ^ Cary & Helzer 2005, pp. 246–249
- ^ "Selecting a Constant Current (CC) DC Welder for Training Purposes". Miller Electric Mfg. LLC. 1 December 2007.
- ^ "Welding Metallurgy: Arc Physics and Weld Pool Behaviour" (PDF). Canteach.
- ^ "DC vs. AC Polarity for SMAW". Lincoln Electric. Retrieved 20 November 2017.
- ^ "AC/DC: Understanding Polarity". Retrieved 20 November 2017.
- ^ Lincoln Electric 1994, p. 5.4.5
- ^ Weman 2003, p. 16
- ^ What does welder "duty cycle" mean? http://www.zena.net/htdocs/FAQ/dutycycle.shtml
- ^ Weman 2003, p. 63
- ^ Cary & Helzer 2005, p. 103
- ^ Lincoln Electric 1994, p. 5.4.3
- ^ Weman 2003, p. 68
- ^ Weman 2003, p. 31
- ^ Weman 2003, pp. 37–38
- ^ Drive Off Moisture and Get Better Welds Archived March 15, 2006, at the Wayback Machine
- ^ Intergranular Corrosion Archived 2006-04-21 at the Wayback Machine
- ^ Galvanic Corrosion
- ^ Cary & Helzer 2005, pp. 52–62
- ^ "Through a Glass, Darkly -".
- ^ Cary & Helzer 2005, pp. 42, 49–51
- ^ W. Fordham Cooper (ed).), Electrical Safety Engineering Third Edition, Butterworth-Heinemann, 1983 ISBN 0750616458, page 531
- S2CID 24234010.
- ^ Hertha Ayrton. The Electric Arc, pp. 20 and 94. D. Van Nostrand Co., New York, 1902.
- ^ S2CID 11047670.
- ^ "Дуговой разряд" [electric arc], Большая советская энциклопедия [Great Soviet Encyclopedia] (in Russian)
- (PDF) from the original on 2011-02-11.
- ISBN 978-90-277-1402-2.
- ^ "Encyclopedia.com. Complete Dictionary of Scientific Biography". Charles Scribner's Sons. 2008. Retrieved 9 October 2014.
- ^ "Beginnings of submerged arc welding" (PDF). Archived from the original (PDF) on 2016-03-04.
- ISBN 978-0-521-05341-9.
- ^ Cary & Helzer 2005, pp. 5–6
- ^ Cary & Helzer 2005, p. 6
- ^ Weman 2003, p. 26
- ^ "Weld It!". Time. 1941-12-15. Archived from the original on February 2, 2009. Retrieved 2008-11-07.
- ^ Lincoln Electric 1994, pp. 1.1–5
- ^ Royal Naval & World Events time line
- ^ Case Studies on Shipbuilding Archived February 3, 2009, at the Wayback Machine
- ^ Cary & Helzer 2005, p. 7
- ^ Lincoln Electric 1994, pp. 1.1–6
- ^ Cary & Helzer 2005, p. 9
Sources
- Cary, Howard B.; Helzer, Scott C. (2005), Modern Welding Technology, Upper Saddle River, New Jersey: Pearson Education, ISBN 978-0-13-113029-6
- Kalpakjian, Serope; Schmid, Steven R. (2001), Manufacturing Engineering and Technology, Prentice-Hall, ISBN 978-0-201-36131-5
- ISBN 978-99949-25-82-7
- Weman, Klas (2003), Welding processes handbook, New York: CRC Press, ISBN 978-0-8493-1773-6
Further reading
- ISBN 0-87170-780-2
- Blunt, Jane and Nigel C. Balchin (2002). Health and Safety in Welding and Allied Processes. ISBN 1-85573-538-5.
- Hicks, John (1999). Welded Joint Design. ISBN 0-8311-3130-6.
External links
- Arc Flash Awareness video (25:39) from U.S. National Institute for Occupational Safety and Health