The retrograde velocity of vacuum arcs in a transverse magnetic field was investigated theoretically and experimentally. Using two opto-transistors the retrograde velocities of vacuum arcs were measured. The influence of cathode surface contamination on the spot velocity was eliminated. The retrograde velocities were measured as functions of the magnetic induction, the arc current, the electrode spacing and the cathode material (copper, titanium and 316 stainless steel). With copper as the cathode material, the arc voltage as functions of magnetic induction, arc current and electrode spacing were also measured. A model for the retrograde motion mechanism of vacuum arcs is developed. The arc velocity calculated according to this model agrees quantitatively with the reported measurements as well as qualitatively with the previously available experimental data.

This paper proposes a novel triple-wire gas indirect arc welding process. The welding system consists of two power sources and three wires. The effects of the power source mode and the wire configuration on arc stability and behavior are studied. The metal transfer is analyzed and bead-on-plate welding is employed. Results show that two direct current power sources cannot produce a stable process, but the combination of a direct current with a pulsed direct current can produce a stable process. The reason is that the pulsed direct current can boost and stabilize the metal transfer. For the wire configuration, a smaller contact angle between the main wire and the side wire is more desirable.Compared with the traditional gas metal arc welding, this novel process has the advantages of high wire melting rate, low penetration depth, and low dilution rate. Compared with twin-wire gas indirect arc welding, it provides a broader range of applicable currents with sufficient heat input.

KEYWORDS: MAG-TIG double-arc welding, Thick mild plate, Backing (root) welding, One side welding with back formation MAG-TIG (metal active gas-Tungsten inert gas) double-arc welding process is applied to make stable back beads in the first layer during one-side multilayer welding without backing plate. To explain the reason why stable back beads can be formed, a comparison was made between single MAG welding and MAG-TIG double-arc welding. Results indicated that during the optimized condition in MAG-TIG double-arc welding, about one-third heat energy of the heat source acts on the root face, and about two-third heat energy acts on the molten pool. The distribution of temperature and surface tension in the molten pool must be more balanced and reasonable, in order to obtain the ideal back weld bead formation. This process is a new welding process giving high quality and efficiency in root welding of thick plate.

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