Welding is defined as the permanent joining of two materials by coalescence. Coalescence is produced by heat and/or pressure. The amount of heat and/or pressure is dependent on the welding processes. As defined by the American Welding Society (AWS), there are five welding categories: (a) oxy fuel gas welding; (b) arc welding; (c) electric resistance welding; (d) solid state; and (e) unique processes. Oxy fuel gas welding, arc welding, electric resistance welding, and unique processes also can be categorized as fusion welding processes. Fusing welding implies that the parent materials and possibly a filler material melt together to form the welded joint. Ultrasonic welding (USW) is the joining of materials by clamping the components together and applying high-frequency (10,000–100,000 Hz), oscillating shear stresses parallel to the part interface. Unique welding processes are fusion welding processes that do not easily fit into one of the other welding categories listed in the beginning of this chapter. There are many, such as laser-beam welding, electron-beam welding, thermit welding, and flash welding. However, only laser-beam welding is discussed in this section. Brazing is defined by the American Welding Society (AWS) as “A group of welding processes that produces coalescence of materials by heating them in the presence of a filler metal having a liquidus above 840° F (449° C) and below the solidus of the base metal. Soldering is defined by the American Welding Society as “A group of welding processes that produces coalescence of materials by heating them to a suitable temperature and using a filler metal having a liquidus below 840° F (449° C) and below the solidus of the base material.” In adhesive bonding, one surface is the adhesive while the other is the adhered.