Welding is a fabrication process that joins materials, usually metals or thermoplastics, primarily by using high temperatures to melt the parts together and allow them to cool, causing fusion. Common alternative methods include solvent welding (of thermoplastics) 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 (parent 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 (the weld pool) that cools to form a joint that can be stronger than the base material. Welding processes use some form of shielding to protect the melting metals from being contaminated or oxidized.
Many different energy sources can be used for welding, including a gas flame (chemical), an electric arc (electrical), a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding may be performed in many different environments, including in the open air, underwater, 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 a Scandinavian word and is possibly related to the Swedish (to well) or Danish (to well up or overflow). In the 8th century AD, a period known as the Viking Age began, and Scandinavians settled across England, resulting in loan words, shifts in word meaning, and grammatical changes. In many languages, the terms for welding are related to those for boiling or bubbling. In Old Swedish, can mean to boil, to bubble up, to overflow, to boil over, or to melt. The Swedish word for the raw material used to make wrought iron is , literally meaning "boiling iron".
The Old English word for welding iron together was . Derived from the Old English weallan, weld entered usage in Middle English as an alternative form of the verb well (), meaning 'to spring up' (as if from a fountain) or 'to boil'. The added 'd' was excrescent, a consonant added without etymological rationale. It came to mean to boil, to heat to a high degree, or to beat iron after heating.
The word was first recorded in English in 1590. A 14th-century translation of the Christian Bible into English by John Wycliffe translated Isaiah 2:4 as "" (they shall beat together their swords into plowshares). In a 1590 version, this was changed to "" (they shall weld together their swords into plowshares), suggesting that this use of the word became popular in English sometime between these periods.
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 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 examples include the iron objects found with Tutankhamun including an iron headrest and dagger. The dagger was forged from meteoric iron at temperatures below .
Iron metallurgy was independently developed at least twice: in Anatolia by 1800 BC and in China by around 900 BC. 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. By the 7th century BCE, blast furnaces were developed in China, but the technology traveled westward only during the Islamic Golden Age. By the time they arrived in Europe, blast furnaces were being independently developed in modern-day Germany.
The European 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. 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, French electrical engineer Auguste de Méritens used an arc to join the lead plates in a lead–acid battery. Two inventors from the Russian empire working in a French laboratory, Polish engineer Stanisław Olszewski and de Méritens' pupil Nikolai Benardos, developed the first electric arc welding method, known as carbon arc welding using carbon electrodes, receiving a British patent in 1885 and an American patent in 1887. 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 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, oxy–fuel 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, oxy–fuel 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 coatings (known as flux) 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 increased demand for military weaponry and caused a surge in industrial welding. Dutch aviation pioneer Anthony Fokker's business welded fuselages for the German Luftstreitkräfte. Due to a gas shortage, the British primarily used arc welding, and British shipbuilder Cammell Laird constructed the first ocean-going ship with an entirely welded hull, the Fullagar. American shipbuilders experimented with resistance welding before seeing the success that the British had with arc welding.
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.