Hollow cathode arc discharge as an effective energy source for welding processes in vacuum

Abstract

This paper presents the results of an investigation of thermal and physical properties of the hollow cathode arc discharge (HCAD) with respect to its application for welding processes in vacuum. The following main parameters of the arc discharge were studied: the external voltage–current (V–I) characteristics; plasma parameters inside the cathode cavity and in the arc external column and the radial heat flux density distribution into the anode. Langmuir electrical probes have been utilized to investigate plasma parameters. Electron energy distribution function was determined from the probe V–I characteristics by the computation of an inverse ill-posed problem. It was shown that, depending on welding parameters, HCAD can exist in two different forms: diffusive or constricted. At currents below 60 A, HCAD has the diffusive form, and with the increase in the current it changes to the constricted form. The discharge constriction phenomenon, we believe, could be explained by the appearance in the external plasma of high velocity electrons with energies from 12 to 22 eV. Parameters of the heat flux into the anode were investigated with spot and split-anode calorimeters. The heat flux density on the anode of the diffusive form of the discharge has a Gaussian distribution. The heat flux of the constricted form is significantly different from the diffusive one and can be approximated by the sum of two combined normal-circular heat sources with different power concentration coefficients. It was also found that the efficiency parameter of the discharge energy transfer to the anode can reach 0.7–0.86 of the discharge voltage, which confirmed that HCAD is a highly effective energy source for welding processes in vacuum. Examples of industrial applications of HCAD for welding, brazing and alloying in vacuum are presented.