OBRABOTKAMETALLOV Vol. 23 No. 3 2021 MATERIAL SCIENCE EQUIPMENT. INSTRUMENTS 6 4 solenoid 15, additionally forming a magnetic field to focus the plasma flow and electric arc. The inner body of the plasma torch 16 with water and air supply channels is made of fluoroplastic, and the outer body 17 is made of steel. The working electrode 14 (Figure 1, b) and nozzle 7 (Figure 1, c) are made of M1 copper. Operation of the plasma torch in the standard mode is associated with the formation of a plasma jet around the plasma-forming arc (Figure 1, d). Water supply to the cutting zone leads to the formation of “water mist” 18 (Figure 1, d). The presence of water mist during cutting accelerates the process of material cooling and makes it possible to cut aluminum alloys without a protective atmosphere in the form of nitrogen, since the oxidation of the edge is minimal and the quality of the cut is high enough [19, 20]. The main difficulties in cutting arose at the start of the process, when the starting arc is ignited and then the working arc 19 with the plasma column is formed (Figure 1, e). In this case, if the process is normal, the plasma column is formed, the arc is short-circuited between the sheet and the electrode, and the plasma jet is turned off (Figure 1, d). If there are problems at the start, the effect of external arcing can be realized (Figure 1, f), when the ignition of the working arc does not support the formation of a plasma jet. Cutting was carried out according to the modes typical for aluminum and titanium alloy plates 60–100 mm thick. The development and optimization of cutting modes for hot-rolled non-ferrous metal plates was carried out earlier in [13–16]. The current of the electric arc ranged from 300 to 370A, the voltage from 300 to 400 V, the height of the plasma torch above the surface of the plate during the cutting process was from 16 to 25 mm. The gas pressure was from 2.0 to 4.0 bar, the water pressure in the system before entering the plasma torch cooling circuit was 6 bar, the gap between the nozzle and the electrode was from 0.5 to 2.0 mm. Cutting speed was from 250 to 3,000 mm/min. Air was used as a plasma-forming gas. The main Fig. 1. The operational scheme of the reverse-polarity plasma torch (a), the appearance of the working electrode (b) and nozzle (c), the cutting process under normal conditions (d), the start of the cutting process (e), the process of external arc burning (f) and cutting with excess speed (g): 1 – plate; 2 – plasma jet; 3 – gas flow; 4 – starting arc; 5 – working arc; 6 – flow of plasma-forming and protective gas; 7 – nozzle; 8 – external nut; 9 – vortex flows of gas and plasma; 10 – swirl ring; 11 – water supply to the hollow electrode; 12 – supply of cooling water to the plasma torch body; 13 – water cooling channels; 14 – electrode; 15 – solenoid; 16 – inner casing made of fluoroplastic; 17 – outer steel casing; 18 – “water mist”; 19 – arc burning at the moment of starting; 20 – external arc burning
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