Chapter 2 from the Welding Handbook, Tenth Edition, Volume 1, “Welding Science and Technology,” has been selected by the AWS Product Development Committee as a service to industry professionals. The physics of welding deals with the phenomena associated with welding processes and the formation of weld bonds. When perfectly flat and clean metal surfaces contact each other in pristine vacuum or inert gas protected conditions, atomic bonds can form spontaneously and welding may occur without any heat or pressure input. However, under practical conditions, roughness of material surfaces and the presence of other materials such as stable oxides prevent the close atom-to-atom proximity needed for weld bonds to form between metal surfaces. For fusion welding, the close contact needed for formation of atomic bonds is obtained by melting adjoining materials, whereas for solid-state welding it is obtained by application of heat and/or pressure, resulting in plastic deformation and diffusion so that weld bonds may form without melting The physics of welding deals with the phenomena associated with welding processes and the formation of weld bonds. When perfectly flat and clean metal surfaces contact each other in pristine vacuum or inert gas protected conditions, atomic bonds can form spontaneously and welding may occur without any heat or pressure input. However, under practical conditions, roughness of material surfaces and the presence of other materials such as stable oxides prevent the close atom-to-atom proximity needed for weld bonds to form between metal surfaces. For fusion welding, the close contact needed for formation of atomic bonds is obtained by melting adjoining materials, whereas for solid-state welding it is obtained by application of heat and/or pressure, resulting in plastic deformation and diffusion so that weld bonds may form without melting..
