Process used to fuse metal by using heat from an electrical arc
Gas metal arc weldingMan welding a metal structure in a newly constructed house in Bengaluru, India
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
Engine driven welder capable of AC/DC weldingA diesel powered welding generator (the electric generator is on the left) as used in Indonesia
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 current power supplies and constant voltage power supplies. In arc welding, the voltage is directly related to the length of the arc, and the current is related to the amount of heat input. Constant current power supplies are most often used for manual welding processes such as gas tungsten arc welding and shielded metal arc welding, because they maintain a relatively constant current even as the voltage varies. 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. Constant voltage power supplies hold the voltage constant and vary the current, and as a result, are most often used for automated welding processes such as gas metal arc welding, flux cored arc welding, and submerged arc welding. 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. For example, if the wire and the base material get too close, the current will rapidly increase, which in turn causes the heat to increase and the tip of the wire to melt, returning it to its original separation distance.[1] Under normal arc length conditions, a constant current power supply with a stick electrode operates at about 20 volts.[2]
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 80A 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.
One of the most common types of arc welding is shielded metal arc welding (SMAW), which is also known as manual metal arc welding (MMAW) or stick welding. An electric current is used to strike an arc between the base material and a consumable electrode rod or stick. The electrode rod is made of a material that is compatible with the base material being welded and is covered with a flux that gives off vapors that serve as a shielding gas and provide a layer of slag, both of which protect the weld area from atmospheric contamination. The electrode core itself acts as filler material, making a separate filler unnecessary. The process is very versatile, requiring little operator training and inexpensive equipment. However, weld times are rather slow, since the consumable electrodes must be frequently replaced and because slag, the residue from the flux, must be chipped away after welding.[9] Furthermore, the process is generally limited to welding ferrous materials, though specialty electrodes have made possible the welding of cast iron, nickel, aluminum, copper and other metals. The versatility of the method makes it popular in a number of applications including repair work and construction.[10]
Gas metal arc welding (GMAW), commonly called MIG (for metal/inert-gas), is a semi-automatic or automatic welding process with a continuously fed consumable wire acting as both electrode and filler metal, along with an inert or semi-inert shielding gas flowed around the wire to protect the weld site from contamination. Constant voltage, direct current power source is most commonly used with GMAW, but constant current alternating current are used as well. 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. Originally developed for welding aluminum and other non-ferrous materials in the 1940s, GMAW was soon economically applied to steels. Today, GMAW is commonly used in industries such as the automobile industry for its quality, versatility and speed. 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.[11]
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]
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 austeniticstainless steels and nickel-based alloys are prone to intergranular corrosion. When subjected to temperatures around 700°C (1,300°F) for too long a time, chromium reacts with carbon in the material, forming chromium carbide and depleting the crystal edges of chromium, impairing their corrosion resistance in a process called sensitization. Such sensitized steel undergoes corrosion in the areas near the welds where the temperature-time was favorable for forming the carbide. This kind of corrosion is often termed weld decay.
Knifeline attack (KLA) is another kind of corrosion affecting welds, impacting steels stabilized by niobium. Niobium and niobium carbide dissolves in steel at very high temperatures. At some cooling regimes, niobium carbide does not precipitate, and the steel then behaves like unstabilized steel, forming chromium carbide instead. 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. Structures made of such steels have to be heated in a whole to about 1,000°C (1,830°F), when the chromium carbide dissolves and niobium carbide forms. The cooling rate after this treatment is not important.[16]
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 safety checklist
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 sparks is significant. To prevent them, welders wear protective clothing in the form of heavy leathergloves and protective long sleeve jackets to avoid exposure to extreme heat, flames, and sparks. 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.[18]
Eye damage
Auto darkening welding hood with 90× 110mm cartridge and 3.78× 1.85in viewing area
Exposure to the brightness of the weld area leads to a condition called arc eye in which ultraviolet light causes inflammation of the cornea and can burn the retinas of the eyes. 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.[19] To protect bystanders, transparent welding curtains often surround the welding area. These curtains, made of a polyvinyl chloride plastic film, shield nearby workers from exposure to the UV light from the electric arc.[20]
Inhaled matter
Welders are also often exposed to dangerous gases and particulate matter. Processes like flux-cored arc welding and shielded metal arc welding produce smoke containing particles of various types of oxides. The size of the particles in question tends to influence the toxicity of the fumes, with smaller particles presenting a greater danger. Additionally, many processes produce various gases (most commonly carbon dioxide and ozone, but others as well) that can prove dangerous if ventilation is inadequate.
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]
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, Humphry Davy discovered the short pulsed electric arcs.[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.[28] Arc welding was first developed when Nikolai Benardos presented arc welding of metals using a carbon electrode at the International Exposition of Electricity, Paris in 1881, which was patented together with Stanisław Olszewski in 1887.[29] In the same year, French electrical inventor Auguste de Méritens also invented a carbon arc welding method, patented in 1881, which was successfully used for welding lead in the manufacture of lead–acid batteries.[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. Around 1900, A. P. Strohmenger released in Britain a coated metal electrode which gave a more stable arc. In 1905, Russian scientist Vladimir Mitkevich proposed the usage of three-phase electric arc for welding. In 1919, alternating current welding was invented by C. J. Holslag but did not become popular for another decade.[31]
Competing welding processes such as resistance welding and oxyfuel welding were developed during this time as well;[32] but both, especially the latter, faced stiff competition from arc welding especially after metal coverings (known as flux) for the electrode, to stabilize the arc and shield the base material from impurities, continued to be developed.[33]
A young woman arc welding in a munitions factory in Australia in 1943
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. Porosity and brittleness were the primary problems and the solutions that developed included the use of hydrogen, argon, and helium as welding atmospheres.[38] During the following decade, further advances allowed for the welding of reactive metals such as aluminum and magnesium. This, in conjunction with developments in automatic welding, alternating current, and fluxes fed a major expansion of arc welding during the 1930s and then during World War II.[39]
During the middle of the century, many new welding methods were invented. Submerged arc welding was invented in 1930 and continues to be popular today. In 1932, a Russian, Konstantin Khrenov successfully implemented the first underwater electric arc welding. Gas tungsten arc welding, after decades of development, was finally perfected in 1941 and gas metal arc welding followed in 1948, allowing for fast welding of non-ferrous materials but requiring expensive shielding gases. Using a consumable electrode and a carbon dioxide atmosphere as a shielding gas, it quickly became the most popular metal arc welding process. In 1957, the flux-cored arc welding process debuted in which the self-shielded wire electrode could be used with automatic equipment, resulting in greatly increased welding speeds. In that same year, plasma arc welding was invented. Electroslag welding was released in 1958 and was followed by its cousin, electrogas welding, in 1961.[40]
See also
Robot welding– use of mechanized programmable tools, which completely automate a welding process by both performing the weld and handling the partPages displaying wikidata descriptions as a fallback
Welding is a fabrication process that joins materials, usually metals or thermoplastics, primarily by using high temperature to melt the parts together and allow them to cool, causing fusion. Common alternative methods include solvent welding using chemicals to melt materials being bonded without heat, and solid-state welding processes which bond without melting, such as pressure, cold welding, and diffusion bonding.
A welder is a person or equipment that fuses materials together. The term welder refers to the operator, the machine is referred to as the welding power supply. The materials to be joined can be metals or varieties of plastic or polymer. Welders typically have to have good dexterity and attention to detail, as well as technical knowledge about the materials being joined and best practices in the field.
Shielded metal arc welding (SMAW), also known as manual metal arc welding, flux shielded arc welding or informally as stick welding, is a manual arc welding process that uses a consumable electrode covered with a flux to lay the weld.
Submerged arc welding (SAW) is a common arc welding process. The first SAW patent was taken out in 1935. The process requires a continuously fed consumable solid or tubular electrode. The molten weld and the arc zone are protected from atmospheric contamination by being "submerged" under a blanket of granular fusible flux consisting of lime, silica, manganese oxide, calcium fluoride, and other compounds. When molten, the flux becomes conductive, and provides a current path between the electrode and the work. This thick layer of flux completely covers the molten metal thus preventing spatter and sparks as well as suppressing the intense ultraviolet radiation and fumes that are a part of the shielded metal arc welding (SMAW) process.
Flux-cored arc welding is a semi-automatic or automatic arc welding process. FCAW requires a continuously-fed consumable tubular electrode containing a flux and a constant-voltage or, less commonly, a constant-current welding power supply. An externally supplied shielding gas is sometimes used, but often the flux itself is relied upon to generate the necessary protection from the atmosphere, producing both gaseous protection and liquid slag protecting the weld.
Gas tungsten arc welding is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area and electrode are protected from oxidation or other atmospheric contamination by an inert shielding gas. A filler metal is normally used, though some welds, known as 'autogenous welds', or 'fusion welds' do not require it. A constant-current welding power supply produces electrical energy, which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma.
Plasma arc welding (PAW) is an arc welding process similar to gas tungsten arc welding (GTAW). The electric arc is formed between an electrode and the workpiece. The key difference from GTAW is that in PAW, the electrode is positioned within the body of the torch, so the plasma arc is separated from the shielding gas envelope. The plasma is then forced through a fine-bore copper nozzle which constricts the arc and the plasma exits the orifice at high velocities and a temperature approaching 28,000 °C (50,000 °F) or higher.
Electric resistance welding (ERW) is a welding process in which metal parts in contact are permanently joined by heating them with an electric current, melting the metal at the joint. Electric resistance welding is widely used, for example, in manufacture of steel pipe and in assembly of bodies for automobiles. The electric current can be supplied to electrodes that also apply clamping pressure, or may be induced by an external magnetic field. The electric resistance welding process can be further classified by the geometry of the weld and the method of applying pressure to the joint: spot welding, seam welding, flash welding, projection welding, for example. Some factors influencing heat or welding temperatures are the proportions of the workpieces, the metal coating or the lack of coating, the electrode materials, electrode geometry, electrode pressing force, electric current and length of welding time. Small pools of molten metal are formed at the point of most electrical resistance as an electric current is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are limited to relatively thin materials.
A welding power supply is a device that provides or modulates an electric current to perform arc welding. There are multiple arc welding processes ranging from Shielded Metal Arc Welding (SMAW) to inert shielding gas like Gas metal arc welding (GMAW) or Gas tungsten arc welding (GTAW). Welding power supplies primarily serve as devices that allow a welder to exercise control over whether current is alternating current (AC) or direct current (DC), as well as the amount of current and voltage.
Shielding gases are inert or semi-inert gases that are commonly used in several welding processes, most notably gas metal arc welding and gas tungsten arc welding. Their purpose is to protect the weld area from oxygen, and water vapour. Depending on the materials being welded, these atmospheric gases can reduce the quality of the weld or make the welding more difficult. Other arc welding processes use alternative methods of protecting the weld from the atmosphere as well – shielded metal arc welding, for example, uses an electrode covered in a flux that produces carbon dioxide when consumed, a semi-inert gas that is an acceptable shielding gas for welding steel.
In metalworking, a filler metal is a metal added in the making of a joint through welding, brazing, or soldering.
Atomic hydrogen welding is an arc welding process that uses an arc between two tungsten electrodes in a shielding atmosphere of hydrogen. The process was invented by Irving Langmuir in the course of his studies of atomic hydrogen. The electric arc efficiently breaks up the hydrogen molecules, which later recombine with tremendous release of heat, reaching temperatures from 3400 to 4000 °C. Without the arc, an oxyhydrogen torch can only reach 2800 °C. This is the third-hottest flame after dicyanoacetylene at 4987 °C and cyanogen at 4525 °C. An acetylene torch merely reaches 3300 °C. This device may be called an atomic hydrogen torch, nascent hydrogen torch or Langmuir torch. The process was also known as arc-atom welding.
Electroslag welding(ESW) is a highly productive, single pass welding process for thick (greater than 25 mm up to about 300 mm) materials in a vertical or close to vertical position. (ESW) is similar to electrogas welding, but the main difference is the arc starts in a different location. An electric arc is initially struck by wire that is fed into the desired weld location and then flux is added. Additional flux is added until the molten slag, reaching the tip of the electrode, extinguishes the arc. The wire is then continuously fed through a consumable guide tube (can oscillate if desired) into the surfaces of the metal workpieces and the filler metal are then melted using the electrical resistance of the molten slag to cause coalescence. The wire and tube then move up along the workpiece while a copper retaining shoe that was put into place before starting (can be water-cooled if desired) is used to keep the weld between the plates that are being welded. Electroslag welding is used mainly to join low carbon steel plates and/or sections that are very thick. It can also be used on structural steel if certain precautions are observed, and for large cross-section aluminium busbars. This process uses a direct current (DC) voltage usually ranging from about 600 A and 40-50 V, higher currents are needed for thicker materials. Because the arc is extinguished, this is not an arc process.
Electrogas welding (EGW) is a continuous vertical-position arc welding process developed in 1961 in which an arc is struck between a consumable electrode and the workpiece. A shielding gas is sometimes used, but pressure is not applied. A major difference between EGW and its cousin, electroslag welding, is that the arc in EGW is not extinguished but instead remains struck throughout the welding process. It is used to make square-groove welds for butt and t-joints, especially in the shipbuilding industry and in the construction of storage tanks.
Hyperbaric welding is the process of extreme welding at elevated pressures, normally underwater. Hyperbaric welding can either take place wet in the water itself or dry inside a specially constructed positive pressure enclosure and hence a dry environment. It is predominantly referred to as "hyperbaric welding" when used in a dry environment, and "underwater welding" when in a wet environment. The applications of hyperbaric welding are diverse—it is often used to repair ships, offshore oil platforms, and pipelines. Steel is the most common material welded.
Hardfacing is a metalworking process where harder or tougher material is applied to a base metal. It is welded to the base material, and generally takes the form of specialized electrodes for arc welding or filler rod for oxyacetylene and gas tungsten arc welding. Powder metal alloys are used in plasma-transferred arc (PTA), also called powder plasma welding, and thermal spray processes like high-velocity oxygen fuel coating, plasma spray, spray and fuse, etc. Submerged arc welding, flux core arc welding (FCAW) and metal inert gas (MIG) / metal active gas (MAG) use continuously fed wire varying in diameter depending on the process and current. The strip cladding process uses strips from 50 mm wide to 125 mm with a thickness of 0.5mm. Open arc welding uses a continuously fed tubular electrode which may or may not contain flux.
Orbital welding is a specialized area of welding whereby the arc is rotated mechanically through 360° around a static workpiece, an object such as a pipe, in a continuous process. The process was developed to address the issue of operator error in gas tungsten arc welding processes (GTAW), to support uniform welding around a pipe that would be significantly more difficult using a manual welding process, and to ensure high quality repeatable welds that would meet more stringent weld criteria set by ASME. In orbital welding, computer-controlled process runs with little intervention from the operator.
In metalworking, a welding defect is any flaw that compromises the usefulness of a weldment. There are many different types of welding defects, which are classified according to ISO 6520, while acceptable limits for welds are specified in ISO 5817 and ISO 10042.
Gas metal arc welding (GMAW), sometimes referred to by its subtypes metal inert gas (MIG) and metal active gas (MAG) is a welding process in which an electric arc forms between a consumable MIG wire electrode and the workpiece metal(s), which heats the workpiece metal(s), causing them to fuse. Along with the wire electrode, a shielding gas feeds through the welding gun, which shields the process from atmospheric contamination.
Gas blending is the process of mixing gases for a specific purpose where the composition of the resulting mixture is specified and controlled. A wide range of applications include scientific and industrial processes, food production and storage and breathing gases.
↑ Shea, William R., ed. (1983). Nature mathematized: historical and philosophical case studies in classical modern natural philosophy. Dordrecht: Reidel. p.282. ISBN978-90-277-1402-2.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
