Fig. 3 — Heat measurement experiment. Fig. 5 — Arc pressure distribution under different arc lengths in Experiment 2. CP, 50 A; CG, 50 A; plasma gas flow rate, 3 L/min. pressure distribution for the separated arc in different conditions. The results are shown in Figs. 4–6. The results from Experiment 1 are shown in Fig. 4. In each legend of the figure, the first number refers to the CP and the second represents the CG. As such, 100-0 implies the CP and CG at 100 A and 0, respectively. In all the experiments, the sum of the currents was 100 A, i.e., the current flowing through the plasma torch was unchanged. As the electron flow separated from the arc plasma, it gradually increased until completed. The plasma gas flow rate remained 3 L/min and the arc length remained 4 mm. As known, the ionization starts at the cathode and continues in the arc column. An electric field was thus formed. The charged particles accelerate in the electric field. As the electron flow was separated from the arc plasma, the ionizability of the arc plasma decreased, and the number of the charged particles decreased. The electric field intensity thus decreased and the velocity of charged particles decreased when they reached the arc plasma receiver. Hence, the pressure of the arc plasma was reduced. As can be seen in Fig. 4, the pressure of the arc plasma decreased as the CG (the electron flow that was separated from the arc plasma) increased. With the increase in the separated electron flow, both of the peak pressure and the distribution radius decreased. When the arc length was changed at three levels, i.e., 4, 6, and 8 mm, the results are shown in Fig. 5. The CP and the CG remained 50 A, respectively, and the plasma gas flow rate remained 3.0 L/min. When other conditions were unchanged, the temperature of the arc plasma decreased, the density and the voltage of the arc plasma increased as the arc length increased. The velocity of the arc plasma was influenced by voltage and temperature, and the effect of the voltage depended on the polarity of charged particles and the direction of the electric field. Based on the fundamental law of electricity that like charges repel and opposite charges attract, the acceleration direction of the cations was toward the cathode, and the acceleration direction of the negatively charged particles was toward the anode. The temperature had a negative effect on the velocity of all the particles in the arc plasma as the temperature decreased. In this case, compared to temperature, the voltage had a smaller influence on the velocity of the arc plasma. The velocity of the arc plasma thus decreased as the arc length increased. As shown in Fig. 5, the peak of the pressure curve decreased as the arc length increased. The results from Experiment 3 are shown in Fig. 6. The plasma gas flow rate was changed at three levels, 2.5, 3.0, and 3.5 L/min. The arc length remained 4 mm and the CP and the CG remained 50 A, respectively. When other conditions were unchanged, the initial velocity of the arc plasma increased as the plasma gas flow rate increased, based on the fluid dynamics. In addition, the running distance of the arc plasma remained unchanged. The final velocity of the arc plasma, when reaching the workpiece, in- WELDING RESEARCH 222-s WELDING JOURNAL / JUNE 2016, VOL. 95 Fig. 4 — Arc pressure distribution under different currents in Experiment 1. Arc length, 4 mm; plasma gas flow rate, 3 L/min. The currents are given as CPCG.
Welding Journal | June 2016
To see the actual publication please follow the link above