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generally accepted that far from the electrodes the plasma in the arc column is in
equilibrium, small gradients of temperature, concentration and potential do not disturb
the local thermodynamic equilibrium significantly [refs. 6 and 14]. Because of its
extremely small size, the cathode region is rather autonomous: any changes in arc
geometry, or current redistribution in the column or near the anode do not affect the
cathode region significantly. In contrast, the anode region presents a negative voltage-
current density characteristic or, in other words, the voltage drop across the region
decreases when current density increases [ref. 14]. A typical voltage distribution
across the arc can be viewed in Figure 2.5.
The type of the plasma gas will play an important role on how the anode
region negative voltage-current density characteristic (V-J curve) will affect the arc.
According to Nemchinsky [ref. 14], the Lorentz force3, which is the major force that
detaches the molten metal droplet from the electrode-anode at high currents, depends
very much on the current distribution inside the pendant droplet. This, in turn, will
depend on the level of current constriction at the anode tip (near anode plasma)4: the
more pronounced is the current constriction the less effective will be the Lorentz
force in detaching the droplet. The author [ref. 14] also states that some gas
characteristics such as high electron-to-atom (molecule) mass ratio, high gas thermal
conductivity and high electron-atom (molecule) collision cross section make the V-J
curve fall more steeply, increasing the current constriction in the near anode plasma
and, therefore, decreasing the effective Lorentz force. This was in concordance with
the results obtained by Rhee and Kannatey-Asibu [ref. 15] on the variation of the
spray transition current' for different shielding gases (see Figure 2.6). It should be
noted that the above mentioned explanation was proposed by Nemchisky [ref. 14] as
an attempt to explain how the different gases affect the behaviour of the welding arc.
It should also be noted that the same author used a rather confusing notation by using
the voltage-current (V-I) terminology to denote voltage-current density (V-J), giving
rise to dubious interpretation.
2.1.4 Weld pool behaviour
The weld pool physical behaviour affects the final quality of the weld bead.
According to Paton et al. [ref. 16], the molten metal in a weld pool must be in a
-dynamic equilibrium for a weld free of defects to form. The defects that could occur
normally take the form of undercut, lack of fusion and inconsistent bead cross section.
The weld pool dynamic equilibrium can be expressed by the equation: [ref. 16]
' Lorentz force is an electromagnetic force which results from the interaction between the current
density J passing through a conductor and the magnetic field B, induced by the same current density:
F= JxB, force per unit
When the current flows in a compound body consisting of two differently conducting media, the
distribution of the current inside the body is determined by the conduction conditions in the worst
conducting part of this body. Since plasma conductivity is much smaller than metal conductivity, the
current distribution inside the droplet adjusts itself to the current distribution in the plasma [ref. 14].
5 Spray Transition Current is defined as the welding current level at which the transition from
globular to spray transfer mode occurs (see Figure 2.6). Its value depends on the filler material size
and composition, and also, on the composition of the shielding gas [ref. 3].
7