当前位置：焊接技术首页 >> 焊接资料 >> Welding is a fabrication>fabrication process that joins materials, usually metals>metals or thermoplastics>thermoplastics, by causing coalescence>coalescence. This is often done by melting>melting the workpieces and adding a filler material to form a pool of molten material (the weld puddle) that cools to become a strong joint, but sometimes pressure>pressure is used in conjunction with heat>heat, or by itself, to produce the weld. This is in contrast with soldering>soldering and brazing>brazing, which involve melting a lower-melting-point material between the workpieces to form a bond between them, without melting the workpieces.
Many different energy sources>energy sources can be used for welding, including a gas flame>flame, an electric arc>electric arc, a laser>laser, an electron>electron beam, friction>friction, and ultrasound>ultrasound. While often an industrial process, welding can be done in many different environments, including open air, underwater>underwater and in space>space. Regardless of location, however, welding remains dangerous, and precautions must be taken to avoid burns, electric shock>electric shock, poisonous fumes, and overexposure to ultraviolet light>ultraviolet light.
Until the end of the 19th century, the only welding process was forge welding>forge welding, which blacksmiths had used for centuries to join metals by heating and pounding them. Arc welding>Arc welding and oxyfuel welding>oxyfuel welding were among the first processes to develop during the 1800s, and resistance welding>resistance welding followed soon after. Welding technology advanced quickly during the early 20th century as World War I>World War I and World War II>World War II drove the demand for reliable and inexpensive joining methods. Following the wars, several modern welding techniques were developed, including manual methods like shielded metal arc welding>shielded metal arc welding, now one of the most popular welding methods, as well as semi-automatic and automatic processes such as gas metal arc welding>gas metal arc welding, submerged arc welding>submerged arc welding and flux-cored arc welding>flux-cored arc welding. Developments continued with the invention of laser beam welding>laser beam welding and electron beam welding>electron beam welding in the latter half of the century. Today, the science continues to advance. Robot welding>Robot welding is becoming more commonplace in industrial settings, and researchers continue to develop new welding methods and gain greater understanding of weld quality and properties.
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The history of joining metals goes back several millennia, with the earliest examples of welding from the Bronze Age>Bronze Age and the Iron Age>Iron Age in Europe>Europe and the Middle East>Middle East. Welding was used in the construction of the Iron pillar>Iron pillar in Delhi>Delhi, India>India, erected about 310 and weighing 5.4 metric tons>metric tons.[1] The Middle Ages>Middle Ages brought advances in forge welding>forge welding, in which blacksmiths pounded heated metal repeatedly until bonding occurred. In 1540, Vannoccio Biringuccio>Vannoccio Biringuccio published De la pirotechnia>De la pirotechnia, which includes descriptions of the forging operation. Renaissance>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 1800, Sir Humphrey Davy>Sir Humphrey Davy discovered the electric arc>electric arc, and advances in arc welding continued with the inventions of metal electrodes by a Russian, Nikolai Slavyanov, and an American, C.L. Coffin in the late 1800s, even as carbon arc welding>carbon arc welding, which used a carbon electrode, gained popularity. Around 1900, A. P. Strohmenger released a coated metal electrode in Britain>Britain, which gave a more stable arc, and in 1919, alternating current>alternating current welding was invented by C.J. Holslag, but did not become popular for another decade.[3]
Resistance welding>Resistance welding was also developed during the final decades of the 19th century, with the first patents going to Elihu Thompson in 1885, and he produced further advances over the next 15 years. Thermite>Thermite welding was invented in 1893, and around that time, another process, oxyfuel welding>oxyfuel welding, became well established as well. Acetylene>Acetylene was discovered in 1836 by Edmund Davy, but its use was not practical in welding until about 1900, when a suitable blowtorch>blowtorch was developed.[4] 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>flux) for the electrode that stabilize the arc and shield the base material from impurities continued to be developed.[5]
World War I>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. The Americans were more hesitant, but began to recognize the benefits of arc welding when the process allowed them to repair their ships quickly after a German>German attack in the New York Harbor>New York Harbor at the beginning of the war. Arc welding was first applied to aircraft during the war as well, as some German airplane fuselages were constructed using the process.[6]
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>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>hydrogen, argon>argon, and helium>helium as welding atmospheres.[7] During the following decade, further advances allowed for the welding of reactive metals like aluminum>aluminum and magnesium>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>World War II.[8]
During the middle of the century, many new welding methods were invented. 1930 saw the release of stud welding>stud welding, which soon became popular in shipbuilding and construction. Submerged arc welding>Submerged arc welding was invented the same year, and continues to be popular today. Gas tungsten arc welding>Gas tungsten arc welding, after decades of development, was finally perfected in 1941, and gas metal arc welding>gas metal arc welding followed in 1948, allowing for fast welding of non-ferrous>ferrous materials but requiring expensive shielding gases. Shielded metal arc welding>Shielded metal arc welding was developed during the 1950s, using a consumable electrode and a carbon dioxide>carbon dioxide atmosphere as a shielding gas, and it quickly became the most popular metal arc welding process. In 1957, the flux-cored arc welding>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>plasma arc welding was invented. Electroslag welding>Electroslag welding was released in 1958, and it was followed by its cousin, electrogas welding>electrogas welding, in 1961.[9]
Other recent developments in welding include the 1958 breakthrough of electron beam welding>electron beam welding, making deep and narrow welding possible through the concentrated heat source. Following the invention of the laser>laser in 1960, laser beam welding>laser beam welding debuted several decades later, and has proved to be especially useful in high-speed, automated welding. Both of these processes, however, continue to be quite expensive due the high cost of the necessary equipment, and this has limited their applications.[10]
Arc welding>Arc welding processes use a welding power supply>welding power supply to create and maintain an electric arc between an electrode and the base material to melt metals at the welding point. They can use either direct>direct (DC) or alternating>alternating (AC) current, and consumable or non-consumable electrodes>electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas>inert gas, known as a shielding gas>shielding gas, and filler material is sometimes used as well.current>current power supplies and constant voltage>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.[11]
The type 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 welding, the positively charged anode>anode will have a greater heat concentration, and as a result, changing the polarity of the electrode has an impact on weld properties. If the electrode is positively charged, it will melt more quickly, increasing weld penetration and welding speed. Alternatively, a negatively charged electrode results in more shallow welds.[12] Nonconsumable electrode processes, such as gas tungsten arc welding, can use either type of direct current, as well as alternating current. However, with direct current, 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.[13] 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>square wave pattern instead of the normal sine wave>sine wave, making rapid zero crossings possible and minimizing the effects of the problem.[14]
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