Welding
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.
Metal welding is distinct from lower temperature bonding techniques such as brazing and soldering, which do not melt the base metal and instead require flowing a filler metal to solidify their bonds.
In addition to melting the base metal in welding, a filler material is typically added to the joint to form a pool of molten material that cools to form a joint that can be stronger than the base material. Welding also requires a form of shield to protect the filler metals or melted metals from being contaminated or oxidized.
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 may be performed in many different environments, including in open air, under water, and in outer space. Welding is a hazardous undertaking and precautions are required to avoid burns, electric shock, vision damage, inhalation of poisonous gases and fumes, and exposure to intense ultraviolet radiation.
Until the end of the 19th century, the only welding process was forge welding, which blacksmiths had used for millennia to join iron and steel by heating and hammering. Arc welding and oxy-fuel welding were among the first processes to develop late in the century, and electric resistance welding followed soon after. Welding technology advanced quickly during the early 20th century, as world wars 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, now one of the most popular welding methods, as well as semi-automatic and automatic processes such as gas metal arc welding, submerged arc welding, flux-cored arc welding and electroslag welding. Developments continued with the invention of laser beam welding, electron beam welding, magnetic pulse welding, and friction stir welding in the latter half of the century. Today, as the science continues to advance, robot welding is commonplace in industrial settings, and researchers continue to develop new welding methods and gain greater understanding of weld quality.
Etymology
The term weld is derived from the Middle English verb well or welling, meaning 'to heat' ; 'to bring to a boil'. The modern word was probably derived from the past-tense participle welled, with the addition of d for this purpose being common in the Germanic languages of the Angles and Saxons. It was first recorded in English in 1590. A fourteenth century translation of the Christian Bible into English by John Wycliffe translates Isaiah 2:4 as "...thei shul bete togidere their swerdes into shares...". In the 1590 version this was changed to "", suggesting this particular use of the word probably became popular in English sometime between these periods.The Old English word for welding iron was samod or samodwellung.
The word is related to the Old Swedish word valla, meaning 'to boil', which could refer to joining metals, as in valla järn. Sweden was a large exporter of iron during the Middle Ages, so the word may have entered English from the Swedish iron trade, or may have been imported with the thousands of Viking settlements that arrived in England before and during the Viking Age, as more than half of the most common English words in everyday use are Scandinavian in origin.
History
The history of joining metals goes back several millennia. Fusion welding processes that join metals by melting them were not widely used in pre-industrial welding. Early welding techniques used pressure to join to the metals, often with heat not sufficient to fully melt the base metals. One notable exception was a technique to join sections of large statues. In Greek and Roman lost-wax casting, the statues were cast as smaller pieces and molten bronze was poured into the joints with temperatures sufficient to create fusion welds.The earliest known welding dates to the Bronze Age. Gold is soft enough to be pressure welded with little to no heat, and archaeologists have found small boxes made by pressure welding overlapping sheets of gold. In the Iron Age, Mediterranean societies developed forge welding. In forge welding, metal is heated to the point that it becomes soft enough for a blacksmith to hammer separate pieces together. Very early notable examples are the iron objects found with Tutankhamun including an iron headrest and dagger. The dagger was forged from meteoric iron at temperatures below. Typically, wrought iron is forged at around. The ancient Greek historian Herodotus credits Glaucus of Chios with discovering "iron welding". Glaucus is known for an iron pedestal welded to hold a silver krater at Delphi.
The Middle Ages brought advances in forge welding, in which blacksmiths hammered heated metal repeatedly until bonding occurred. In Europe and Africa, forging shifted from open charcoal fires to bloomeries. China developed the blast furnace late in the first millennia. Forge welding was used in the construction of the Iron pillar of Delhi, erected in Delhi, India about 310 AD and weighing 5.4 metric tons. 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.
In 1800, Sir Humphry Davy discovered the short-pulse electrical arc and presented his results in 1801. In 1802, Russian scientist Vasily Petrov created the continuous electric arc, and subsequently published "News of Galvanic-Voltaic Experiments" in 1803, in which he described experiments carried out in 1802. Of great importance in this work was the description of a stable arc discharge and the indication of its possible use for many applications, one being melting metals. In 1808, Davy, who was unaware of Petrov's work, rediscovered the continuous electric arc. In 1881–82 inventors Nikolai Benardos and Stanisław Olszewski 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, and an American, C. L. Coffin. 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 using a three-phase electric arc for welding. Alternating current welding was invented by C. J. Holslag in 1919, but did not become popular for another decade.
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 torch was developed. 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 advances in metal coverings were made. Flux covering the electrode primarily shields the base material from impurities, but also stabilizes the arc and can add alloying components to the weld metal.
World War I caused a major surge in the use of welding, 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 "Fullagar" 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.
During the middle of the century, many new welding methods were invented, including the introduction of automatic welding in 1920, in which electrode wire was fed continuously. Shielding gas received 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. Testing methods were introduced for weld integrity. First vibration testing was done using a hammer and stethoscope; later, X-ray tests were developed to see into the weld. During the 1930s, 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. Russian inventor Konstantin Khrenov implemented the first underwater electric arc welding. In 1930, Kyle Taylor was responsible for the release of stud welding, which soon became popular in shipbuilding and construction. Submerged arc welding was invented the same year. During World War II, submerged arc welding was widely used for ship-building because it allowed certain types of welds to be done twenty times faster than earlier techniques.
Image:Maurzyce 2009.jpg|thumb|Bridge of Maurzyce
Improvements to welding processes opened up new possibilities for construction. Previously, large metal structures had been made from metals joined mechanically with rivets, along with bolts, screws, and belts. These connected but unfused metal structures had inherent weaknesses. The steamboat Sultana killed over a thousand passengers when its riveted boiler failed under pressure. Titanic sank due in part to failures in its riveted hull. In 1930, the first all-welded merchant vessel, M/S Carolinian, was launched. The strength of welded steel also allowed for the creation of entirely new types of ships, notably the liquefied natural gas tanker. The ASME Boiler and Pressure Vessel Code, created in response to deadly boiler failures was used to develop the spherical tanks that contain LNG during transport. Also noteworthy is the first welded road bridge in the world, the Maurzyce Bridge in Poland. Early skyscrapers and steel truss bridges were built from riveted steel beams. Welding allows for stronger and lighter structures and greater range of shapes. The Sydney Opera House's icon shape is built on a stud-welded steel frame.
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 by Robert Gage. Electroslag welding was introduced in 1958, and it was followed by its cousin, electrogas welding, in 1961. In 1953, the Soviet scientist N. F. Kazakov proposed the diffusion bonding method.
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. Magnetic pulse welding has been industrially used since 1967. Friction stir welding was invented in 1991 by Wayne Thomas at The Welding Institute and found high-quality applications all over the world. All of these four new processes continue to be quite expensive due to the high cost of the necessary equipment, and this has limited their applications.