OBRABOTKAMETALLOV technology Vol. 24 No. 4 2022 Introduction Plasma cutting of metals is an integral part of production processes in various engineering industries. Although the cut quality of plasma cutting may be inferior to, for example, waterjet or laser cutting [1], but its advantage is the optimal combination of technological capabilities, simplicity of equipment setup and productivity, also when cutting metal with a thickness of more than 100 mm [2]. To date, a number of research in the field of plasma metal cutting are being conducted. An important area of research is to obtain a metal cut surface characterized by minimal roughness and geometric variations [3–6]. Also, minimizing changes in metal structure under cut surface caused by temperature effects of plasma jet, including dross formation, is important as well [7–10]. These trends form the main task of research: obtaining a quality cut, as geometric and structural changes in material are usually removed by further processing, the minimization of allowances for which determines the effectiveness of the plasma cutting process. To solve this problem, researchers have proposed a number of methods related both to changes in equipment realization of a cutting process, and to optimization of its parameters [11–14]. Methods of cutting parameters optimization involve the application of various methods of mathematical modeling, establishing a relationship between geometric and structural parameters of a material in the area of cutting and a range of parameters of a cutting process. Among the main parameters that determine the quality of the cut, plasma arc current and voltage, cutting height, and cutting speed are considered [15–17]. However, all studies are carried out, mainly, with consideration of cut metals with a thickness of up to 20 mm and insufficient attention is paid to cutting of metals with greater thicknesses. According to the authors, this is due, primarily, to the limitations associated with the operating conditions of cutting plasmatrons. The most widely used plasmatrons with thermochemical cathodes and operating at direct current polarity have limitations in terms of capacity and the number of switchings, which is associated with the temperature mode of operation, as well as the wear of cathode inserts made of relatively expensive and rare metals [18– 20]. For cutting metals of large thicknesses the method of cutting with currents of reversed polarity seems promising, in which the supporting spot of the cutting arc is significantly deepened in the cutting cavity, and the distribution of heat input to the cutting front edge along the height of it is more uniform. Due to this, it becomes possible to cut metals of large thicknesses, a better quality of cut along the bevel of edges, and a smaller width of the cutting cavity [21–22]. Considering the above, the main purpose of the present work is to develop methods of plasma cutting of copper, titanium and aluminum alloy sheets with thickness up to 40 mm using plasmatron, working with currents of reverse polarity. An additional task is to determine the influence of sheet thickness and nonstandard arrangement of the plates on the structure of the cutting edge. Research methodology Experimental research was carried out on the production site in LLC “ITS-Siberia”. Cutting was carried out on a plasmatron with reverse polarity. The exterior of the plasma cutter is shown in Fig. 1. The machine consists of a work table for placing workpieces, a plasmatron, a moving carriage and guides to move a plasmatron. It also includes a gas preparation unit and a power unit. Nitrogen is used as protective gas. Rolled sheets of copper C1220 (Cu ≥99.96 %) with thickness of 40 mm, aluminum alloy AA2024 (Al 90.9–94.7 %; Cu 3.8–4.9 %; Mg 1.2–1.8 %; Mn 0.3–0.9 %) with thickness of 12 and 40 mm, and titanium alloy Ti-1.5Al-1.0Mn (Ti 94.33–97.5 %; Al 1.5–2.5 %; Mn 0.7–2.0 %) with thickness of 5 and 10 mm were used as experimental material. In order to form specimens from a 10 mm thick titanium alloy, two 5 mm thick sheets stacked together were used. This was done to further reveal the specifics of cutting packages of sheets, which change significantly in the presence of the interface between the sheets being cut. The cutting process parameters used in the research are given in Table 1. Plasma cutting process parameters were determined empirically based on typical parameters used for cutting metals and alloys on conventional equipment. Length of a cutting cut was varied from 100 to 300 mm. Parameters were adjusted until a relatively uniform cut was achieved, which was determined
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