The effect of the type of plasma gas on current constriction at the molten tip of an arc electrode

During welding, the Lorentz (electromagnetic) force is the major force that detaches the molten metal droplet from the electrode-anode. The magnitude of this force is determined by the current distribution within the droplet. Experiments show that the Lorentz force very much depends on the type of plasma gas used. No explanation of this very important fact exists at the present time. An explanation based on the influence of the negative voltage-current characteristic of the anode layer on the current distribution near the anode is proposed. The reason for the falling shape of the V-I curve of the anode layer is discussed in detail. It is shown qualitatively and by calculations that the negative voltage-current characteristic leads to the current constriction at the anode tip. The steeper the slope of the V-I curve the more the current constriction is pronounced. According to calculations, the Lorentz force is several times higher when argon (slowly falling V-I curve) is used as a plasma gas compared to the case when the plasma gas is helium (steeply falling V-I curve). An explanation of the small Lorentz force for an are in a molecular gas is proposed.