The value of Wи for each atom cannot be precisely calculated. However, if we assume that the quantity Wи and depends linearly on T, then its value can be calculated by the formula
where Ti — the temperature at which almost all the atoms in the plasma are ionized, for example, when the degree of ionization reaches 0.95.
The Ui/Ti ratio for each class of atoms constantly and can be denoted by с, then the formula can be written as
Wi = Ui – cТ,
where с — the coefficient constant for each substance.
For argon, the value of c is с ≈ 8,2*10–4 eV/deg (Fig. 1 shows the graph of the degree of ionization (α), constructed from experimental data).
It is known that the plasma temperature in the argon arc is ~1,3 ・ 104К at the center of the arc, therefore, according to the formula Wи = 10,7 eV, and Wi at UiAr = 15,76 eV is Wi = 5,06 eV. From the data obtained, it follows that Wи and Wi in the welding arc vary appreciably from its cross section (see Fig. 2).
The same should be observed at the frontal surface of the drop located in the arc, in which the plasma flow going from the electrode to the product is slowed, which leads to the increase in its temperature. The similar phenomenon should, according to the above, be observed at the surface of the weld pool.
This phenomenon is especially important for the study of processes in the cathode region of welding arcs, since the external source of ionization-a beam of electrons emitted from the surface of the cathode-acts on the plasma in this region and consists of atoms of the material of the melting electrode. It follows from this that the energy of electrons in this beam during ionization of atoms is not spent on ionization of atoms with Wi = Ui, but on the ionization of atoms with a much smaller value of Wi, which requires the lower electron energy, and therefore a lower cathode voltage drop (Uк). Perhaps, this can explain the small value of the cathode voltage drop (Uк) for the argon arc burning between tungsten electrodes. The anode voltage drop (Ua) is independent of Ui (anode material) and is ~ 4.5 V. At the same time, for the argon arc burning between tungsten electrodes, Uк + Uа ≈ 10V, hence in this case Uк ≈ 5,5V, which corresponds to the Wi value defined above. The same can be said for the value of Uk for welding aluminum with the tungsten electrode in argon and in helium, for which Uк + Uа are ~ 10 V and ~ 11 V, respectively, and the ionization potential of the helium atom is Ui = 24.58 eV, which also allows It is also possible to explain the appreciably smaller value of Uк when welding in helium in comparison with its Ui (for helium Uк ≈ 0,25Ui).