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Many different [|energy sources] can be used for welding, including a gas [|flame], an [|electric arc], a [|laser], an [|electron beam], [|friction], and [|ultrasound]. While often an industrial process, welding can be done in many different environments, including open air, [|under water] and in [|outer space]. Regardless of location, however, welding remains dangerous, and precautions are taken to avoid burns, [|electric shock], eye damage, poisonous fumes, and overexposure to [|ultraviolet light]. Until the end of the 19th century, the only welding process was [|forge welding], which blacksmiths had used for centuries to join iron and steel by heating and hammering them. [|Arc welding] and [|oxyfuel welding] were among the first processes to develop late in the century, and [|resistance welding] followed soon after. Welding technology advanced quickly during the early 20th century as
 * Welding** is a [|fabrication] or [|sculptural] [|process] that joins materials, usually [|metals] or [|thermoplastics], by causing [|coalescence]. This is often done by [|melting] the workpieces and adding a filler material to form a pool of molten material (the //weld pool//) that cools to become a strong joint, with [|pressure] sometimes used in conjunction with [|heat], or by itself, to produce the weld. This is in contrast with [|soldering] and [|brazing], which involve melting a lower-melting-point material between the workpieces to form a bond between them, without melting the workpieces.

The history of joining metals goes back several millennia, with the earliest examples of welding from the [|Bronze Age] and the [|Iron Age] in [|Europe] and the [|Middle East]. Welding was used in the construction of the [|iron pillar] in [|Delhi], [|India], erected about 310 AD and weighing 5.4 [|metric tons].[|[1]] The [|Middle Ages] brought advances in [|forge welding], in which blacksmiths pounded heated metal repeatedly until bonding occurred. In 1540, [|Vannoccio Biringuccio] published //[|De la pirotechnia]//, which includes descriptions of the forging operation. [|Renaissance] craftsmen were skilled in the process, and the industry continued to grow during the following centuries.[|[2]] Welding, however, was transformed during the 19th century. In 1802, Russian scientist [|Vasily Petrov] discovered the [|electric arc][|[3]] and subsequently proposed its possible practical applications, including welding. In 1881–82 a Russian inventor [|Nikolai Benardos] created the first electric arc welding method known as [|carbon arc welding], using carbon electrodes. The advances in arc welding continued with the invention of metal electrodes in the late 1800s by a Russian, [|Nikolai Slavyanov] (1888), and an American, [|C. L. Coffin] (1890). Around 1900, [|A. P. Strohmenger] released a coated metal electrode in [|Britain], 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.[|[4]] [|Resistance welding] was also developed during the final decades of the 19th century, with the first patents going to [|Elihu Thomson] in 1885, who produced further advances over the next 15 years. [|Thermite welding] was invented in 1893, and around that time another process, [|oxyfuel welding], became well established. [|Acetylene] was discovered in 1836 by [|Edmund Davy], but its use was not practical in welding until about 1900, when a suitable [|blowtorch] was developed.[|[5]] At first, oxyfuel welding was one of the more popular welding methods due to its portability and relatively low cost. As the 20th century progressed, however, it fell out of favor for industrial applications. It was largely replaced with arc welding, as metal coverings (known as [|flux]) for the electrode that stabilize the arc and shield the base material from impurities continued to be developed.[|[6]] [|World War I] caused a major surge in the use of welding processes, with the various military powers attempting to determine which of the several new welding processes would be best. The British primarily used arc welding, even constructing a ship, the //[|Fulagar]//, with an entirely welded hull. Arc welding was first applied to aircraft during the war as well, as some German airplane fuselages were constructed using the process.[|[7]] Also noteworthy is the first welded road [|bridge] in the world, designed by [|Stefan Bryła] of the [|Warsaw University of Technology] in 1927, and built across the river [|Słudwia Maurzyce] near [|Łowicz, Poland] in 1929.[|[8]] During the 1920s, major advances were made in welding technology, including the introduction of automatic welding in 1920, in which electrode wire was fed continuously. [|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.[|[9]] During the following decade, further advances allowed for the welding of reactive metals like [|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].[|[10]] During the middle of the century, many new welding methods were invented. 1930 saw the release of [|stud welding], which soon became popular in shipbuilding and construction. [|Submerged arc welding] was invented the same year 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. [|Shielded metal arc welding] was developed during the 1950s, using a flux-coated consumable electrode, and 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, and that same year, [|plasma arc welding] was invented. [|Electroslag welding] was introduced in 1958, and it was followed by its cousin, [|electrogas welding], in 1961.[|[11]] In 1953 the Soviet scientist N. F. Kazakov proposed the [|diffusion bonding] method.[|[12]] Other recent developments in welding include the 1958 breakthrough of [|electron beam welding], making deep and narrow welding possible through the concentrated heat source. Following the invention of the [|laser] in 1960, [|laser beam welding] debuted several decades later, and has proved to be especially useful in high-speed, automated welding. In 1991 [|friction stir welding] was invented in the UK and found high-quality applications all over the world. All of these three new processes, however, continue to be quite expensive due the high cost of the necessary equipment, and this has limited their applications.[|[13]]