Fundamental Stadies on Cutting Phenomena in Plasma Arc Cutting (3rd Report)

Kerf Formation Phenomena in Plasma Arc Cutting

Abstract

The kerf formation in a plasma arc cutting is influenced by the behavior of molten metal within the cutting groove in addition to the heat distribution on the cutting front described in the report 2, as well as known in welding bead formation. So, the authors deduced the kerf formation mechanism by the investigations of heat input character and molten metal flow pattern from analysis of the cutting front shapes and the traces of insert metal flow, respectively.
The heat distribution within the groove in a palsma arc cutting can be roughly approximated to that in a plasma jet cutting, in which heat flux from plasma stream to the cutting groove exponentially decreases along the center of cutting front in regardless of division into three regions I-III described in the report 2, since the most of energy given to the groove is generated in the constricted nozzle part within torch.
While, the molten metal within the groove indicates complex flow patterns in each region I-III contrary to the expectation from the plasma stream. In the region I and II, the molten metal flows toward side-and-downward due to surface tension in addition to the momentum of plasma stream. Especially, in the region II at straight polarity, the sideward metal flow is remarkable, since the most of anode spot locates on the center part of cutting front. The region III is divided into two parts from the difference in flow pattern. In the upper part of III (region III-a), the molten metal flows along or slightly toward the cutting front. In the lower part of in (region III-b), the molten metal strongly flows toward sideward, too, because of the momentum change of the stream due to change inclination of cutting front.
Such metal flows toward and in sideward act to spreading the kerf width, so that the kerf width in the region II is wider than that in the region I or III-a, and lower part widening kerf is formed in the region III-b. The kerf spreading phenomenon in the region I is suppressed by using the reverse polarity torch, since the cathode spot wanders near and on the soldification line (drag line).