Welding is a manufacturing or carving process, which connects materials, usually metals or thermoplastics, by coalescence. This is usually done by melting the workpiece and adding filler material to form a pool of molten material (welding pool), which becomes a strong joint after cooling, and pressure is sometimes used together with heat or alone to produce welding. This is different from soldering and brazing, which involve melting low-melting materials between workpieces to form a bond between them without melting the workpieces.

Arc welding Many different energy sources can be used for welding, including gas flame, arc, laser, electron beam, friction and ultrasound. Although welding is usually an industrial process, it can be carried out in many different environments, including open air, underwater and outer space. However, no matter where it is, welding is still dangerous, and preventive measures must be taken to avoid burns, electric shocks, eye damage, toxic fumes and excessive exposure to ultraviolet rays.

Until the end of19th century, the only welding process was forging welding, which blacksmiths have used for centuries to connect metals by heating and beating. Arc welding and oxygen fuel welding are the first batch of processes developed at the end of this century, followed by resistance welding. At the beginning of the 20th century, with the outbreak of World War I and World War II, welding technology developed rapidly, which promoted the demand for reliable and cheap connection methods. After the war, several modern welding technologies were developed, including manual methods, such as shielded metal arc welding, one of the most popular welding methods, and semi-automatic and automatic processes, such as gas shielded metal arc welding, submerged arc welding, flux-cored wire arc welding and electroslag welding. In the second half century, with the invention of laser beam welding and electron beam welding, the development continued. Today, science continues to progress. Robot welding is becoming more and more common in industrial environment. Researchers continue to develop new welding methods and have a better understanding of welding quality and performance.