Welding

A weld is defined as a blend or coalescence of two or metals (or nonmetals) by heating them until they reach a critical temperature and flow together. Upon cooling, the metal becomes hard. The piece of metal to be welded is called the base metal, workpiece, or work. The edges of the base metal are often specially prepared for welding by, for example, machining, shearing, or gouging. There are five basic weld joints: butt, lap, corner, "T," and edge. The American Welding Society has developed a system of symbols that are added to mechanical drawings, to convey precisely how a welding site should be prepared, what type of weld should be made, and any other considerations.

Fusion welding, a heat process that sometimes requires the use of a filler metal, uses either electricity (arc welding) or gas (gas welding) as its source of heat energy. Solid state processes, such as friction welding and ultrasonic welding, weld metals at a temperature below their melting points, without the addition of a filler metal. Pressure is always used to achieve a weld with this method. When most metals are heated, a reaction takes place between the base metal and the surrounding atmosphere. For example, some metals oxidize when melted, which can interfere with the quality of the weld. Other common atmospheric contaminants are nitrogen and hydrogen. To control this problem, fluxes and inert gases are used to rid the welding area of impurities, and to protect the area from the atmospheric gases by displacing the surrounding air from the weld site. Welding is done mechanically or manually with welding guns or torches, and can also be performed by robots.