Mechanism of constriction in a high frequency pulsed welding arc

Abstract

The phenomenon of arc constriction at high frequency pulsing was investigated experimentally using the monochromatic imaging method (MIM) on arcs generated with a thermionic electrode. Square pulses with 50% duty cycle between 75 A and 125 A (average of 100 A) were explored at frequencies of 1 kHz, 3 kHz, and 5 kHz. Constant current arcs of 75 A, 100 A, and 125 A were also investigated as a reference. The observations indicate that the high-temperature region of the arc (above 14,000 K) increase in volume during the pulse, to sizes larger than those corresponding to the pulse current applied in steady state (125 A), similarly during the background current, the size of the hot regions is smaller than under the same value of constant current (75 A). The size increase during the pulse is much more prominent than the decrease during the background. This net increase in the average size of the most electrically conductive region takes current away from the less conductive periphery of the arc. The pulse frequencies from 1 kHz to 5 kHz have significant effect on thermal delay from Joule region to convection region. The mechanisms identified explain the arc constriction observed experimentally. This new understanding allows for open arcs that can rival more complex plasma torches in heat concentration and in the ability to control the heat source from a welding arc using purely electronic means, without resorting to gas mixing or mechanical adjustments.