プラズマ・アーク溶接における電流密度とシリーズ・アークの発生

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In plasma arc welding, an important role is played by the current density of the arc. The current density, however, is decreased greatly if the so-called series arc starts in parallel with the main arc. In this paper, the factors which predominate series arcing are experimentally investigated. Results are summarized as follows : (1) When the nozzle end of the plasma torch is covered with oxide film or is in oxidizing atmosphere, the cathode spot of series arc is easily formed with the help of oxide film, resulting in the great reduction of current capacity I_c. For example, if there is a sufficient quantity of oxide film on the nozzle end, I_c falls by half. But the series arc itself has the cathodic cleaning action of oxide film, so that by repeating the series arcing of short time (about 1 second) in an inert gas atmosphere, I_c increases according to the extent of oxide elimination, up to the value obtained without oxide film. (2) In case that the nozzle end is cleaned and is in an inert gas atmosphere, the series arc is started by the temperature rise of the nozzle exit up to its melting point, at every nozzle diameter. This means that if the temperature of the exit rises up to the melting point, the cathode spot is formed very easily. As a matter of course, after the series arc is started, the nozzle exit is melted or burnt off by the cathode energy of this arc. With increase of the nozzle diameter d_n, however, I_c increases roughly in proportional to d_n and not to the sectional area of the nozzle. (3) When some kind of metals of low melting point like solder is present at the end of the nozzle, the series arc starts very easily and consequently I_c decreases remarkably, for example, by half. (4) With use of tungsten for the nozzle duct, I_c is considerably lowered than with a copper nozzle. This can be considered to be caused by the easiness of the temperature rise of the tungsten nozzle, up to the extent at which hot cathode is formed, due to its low thermal conductivity. (5) With increase of working gas flow rate Q in the range relevant to plasma arc welding, I_c increases considerably, in spite of the remarkable rise of arc voltage with Q. This can be attributed to the fall of the nozzle temperature caused by increasing Q.
社団法人溶接学会の論文
1970-04-25

プラズマ・アーク溶接における電流密度とシリーズ・アークの発生

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