The article discusses the fundamentals of fusion welding processes with an emphasis on the underlying scientific principles. It describes how surface temperature varies on steel with surface power densities that range from 400 to 8000 W/cm2. The article illustrates the spectrum of practical heat intensities used for fusion welding. It contains tables that present information on the thermal diffusivities of common elements and alloys from 20 to 100 °C.

Three types of energy are used primarily as direct heat sources for fusion welding: electric arcs, laser beams, and electron beams. This article reviews the physical phenomena that influence...