A B C D 100%. There was a narrow gap (0.5 mm) to separate the two water-cooled copper blocks, which had ceramic coatings on their facing surfaces to furtheensure insulation. A pressure sensor was placed under the arc plasma receiver to receive the arc plasma pressure signal. A calculation based on the circular segment principle was used to analyze the distribution of pressure. A high-speed video system was used to image the arc separation phenomenon occurring during the experiment. For the heat measurement, the receivers received the majority of the heat from the arc plasma, except for radiation loss of the arc column, and all the heat from the electron flow. However, these measurements represented the actual case for welding where the radiation was also lost. The method for the calculation of the heat distribution was detailed in our previous study (Ref. 19). Two watercooling circulatory systems were built for the two copper blocks, respectively. To minimize the heat loss to better ensure accuracy, enough thermal insulation was used to enclose the water flow pipes. Each water-cooling circulatory system was installed with two sensors to measure the temperature of the cooling water. Further, one sensor was installed near the outlet of the pump, and the other was fixed near the outlet of the water-cooled copper block. A multichannel oscilloscope was used to collect the signals and a comprehensive algorithm was used to analyze the data. Experimental Procedure In all experiments, the plasma torch had a 3-mm orifice diameter, 4.8-mm tungsten diameter, and 4- mm tungsten setback. Pure argon (99.99%) was used for the shielding gas and plasma gas, and the flow rate for the shielding gas was 12 L/min. The major parameters for the designed experiments are given in Table 1, including the current of the PAW (CP), the current of the GTAW (CG), plasma gas flow rate, and arc length. The current flowing through the plasma torch was 100 A. The electron flow was separated from the arc plasma, with the CG from 0 to 100 A representing conventional PAW, partial separation at different degrees, and full separation. The CP reduced from 100 to 0 A, accordingly. WELDING RESEARCH In the pressure measurement experiments, the arc was ignited on the electron flow receiver without contact to the arc plasma receiver — Fig. 2A. After being ignited, the arc moved at a speed of 8 mm/min. The arc of the PAW power source started immediately when the fringe of the arc contacted the border-right of the arc plasma receiver. The torch kept moving until it arrived in the middle of the plasma arc receiver — Fig. 2D. The force sensor under the arc plasma receiver was sampled at 1 Hz. The data received was filtered and then calculated based on the circular segment principle. In the heat measurement experiments, a standstill welding arc was used, as shown in Fig. 3. In order to better understand the heat property of the arc plasma, which contains part of the electron flow, the CG was increased and the CP was decreased, and the arc was ignited by starting the PAW power source and GTAW power source at the same time. The distance from the border-left of the electron flow receiver to the axis of the plasma torch was 6 mm. The heating time was 500 s after the arc was ignited. The initial temperature of the cooling water was 18°C, the flow rate of each water circulation was 11 L/min, and the inner diameter of the water flow pipes was 8.5 mm. The heat was calculated based on the heat balance equation (energy Equation 1), Q = cmDT (1) where DT is the temperature elevation, c is the specific heat capacity of water, and m is the total water mass in circulation. In all experiments, the c and m were the same. The energy output (P) of the arc plasma can be calculated based on power Equation 2, P = Q/t (2) where Q is the heat of the workpieces received and t is the heating time. Measurement Results and Discussion Pressure Distribution As shown in Table 1, Experiments 1–3 were designed to measure the JUNE 2016 / WELDING JOURNAL 221-s Fig. 2 — Pressure measurement experiment. A — Starting the GTAW power; B — starting the transfer arc of PAW; C — crossing; D — ending.
Welding Journal | June 2016
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