3.1 INTRODUCTION

After a brief review of welding processes let us go into the science of welding. This will help us in the understanding of the further discussions regarding the welding applications and tech­nologies that will follow. Most welding processes require the application of heat or pressure or both to produce a suitable bond between the pieces to be joined sufficient in strength to meet the demands of the task (the intended use).

Almost all the available and concievable high intensity heat sources have been used in welding. Externally used heat sources of technical importance include: arcs, electron beams, light beams. exothermic reactions and electrical resistance. A heat source must transfer suffi­cient energy at high intensity to produce local melting and fusion.

It has been the endeavour of welding engineers to evolve a welding heat source which provides high heat intensity (energy density per unit cross-sectional area of source—plasma arc, electron beam, laser beam, etc.) to cause melting. During welding, heat may be considered to be transferred from the source to the surface of the work and then by conduction, from the contact area to colder regions of the metal. These two processes are somewhat competitive. With high intensity heat sources, say electron beam, energy is delivered through the contact area so rapidly that local melting occurs before there is significant loss of heat by conduction. In Bunsen burner on the other extreme a large quantity of heat is lost by conduction to the workpiece without melting. Thus Bunsen burner is not suitable for welding.