"The advent of high intensity low momentum surface heating devices is having a natural impact on the field of materials processing@ particularly fusion welding. The laser and electron beam offer potential capability for precise control of energy and location which cannot be approached by older sources such as arcs and flames@ and the fact that under certain operating conditions they transmit little or no thrust to the material being worked is an important advantage. However@ the extreme intensity of these sources@ expressed in dimensions of power per unit area@ presents problems as well as advantages when addressed to the objective of local melting as in fusion welding. With more conventional welding heat sources@ there was seldom@ if ever@ any question of too high an intensity. Indeed@ the problem has generally been quite the reverse@ in that there is a tendency for metals having relatively high thermal conductivity to conduct the heat away from the weld region almost as fast as it is supplied. In this sense@ the term ""melting efficiency"" has been used to express the fraction of the total heat which is actually used for melting@ and with a typical open arc@ this melting efficiency will generally fall below 50%. Melting efficiency is directly related to the intensity of the heat source@ and as the intensity increases@ the melting efficiency also increases."