An introduction to physical phenomena in arc welding processes

Abstract

As is well known, arc discharges have been applied to various processes such as welding, cutting, spray coating, melting and refining. Unlike electrodeless discharge methods such as high frequency discharge of inductive coupling type (eg. RF discharge), arc discharge is a polarized discharge in which the arc is generated between the positive and negative electrodes.¹ Accordingly, when the arc discharge is applied to welding processes, the material becomes one of the electrodes. In TIG welding, the material, that is, the molten pool, generally acts as the anode to the tungsten cathode. As shown in Fig. 1, TIG welding processes are based on the close energy balance between the ‘electrode–arc plasma–molten pool’. On the other hand, for the formation of the molten pool, energy transfer from the arc is also important, but energy transfer in the molten pool after that is extremely important, too. In TIG welding of steels, in which energy transfer by convection current becomes dominant rather than thermal conduction,² the penetration at the weld joint has hugely different geometries according to the difference in convective current phenomena at the molten pool. As a driving force of convective current in the molten pool in TIG welding, four forces have been considered,^2–6 as shown in Fig. 1. They are the drag force (friction force) caused by plasma jet (cathode jet) generated by the arc, the buoyancy force induced by the density difference inside the molten pool, the electromagnetic force induced by the current flow inside the molten pool and Marangoni Force induced by the surface tension gradient of the molten pool. These four forces can be said to be also based on the close force balance of the ‘electrode–arc plasma–molten pool’, as with the energy balance.