OBRABOTKAMETALLOV Vol. 24 No. 4 2022 technology In the upper part of the specimen the height of roughness peaks above the cut surface is 70–75 µm in average, while in the lower part it is more than 245–275 µm depending on the cutting mode (the lowest values are characteristic for the mode with the cutting speed of 2,000 mm/min according to Table 1). These peaks are formed by flows of molten metal blown by the gas jet from the cutting zone, quickly solidifying when the plasmatron withdraws from the cutting point. As can be seen from the optical photographs and from the results of laser scanning microscopy, the peaks in the upper part of the cut, in addition to its smaller size, can be characterized by a smaller distance between it in comparison with the peaks in the lower part of the specimen. On the cut surface in both the upper and lower parts of the specimen, it is possible to see quite obvious features of oxidation (Fig. 2, a), despite the use of shielding gas in the cutting process. In the lower part of the cut, it is possible to distinguish an irregularity of the edge, which is formed during cutting as a result of blowing of metal from the area of the cut, and its crystallization with formation of a small amount of flowed metal. During plasma cutting of 10 mm thick specimens of titanium alloy Ti-1.5Al-1.0Mn, due to the increased material thickness and superimposed sheets, displacement of molten metal from the cutting zone was difficult. For this reason, the differences in the features of the structure of the upper and lower parts of the cut become even more significant. Figure 2, b shows images of the cut surface of the upper and lower plate after cutting under mode No. 1 (Table 1). The surface of the top plate after cutting is quite close to that previously observed on specimens with a thickness of 5 mm. In the upper part of the upper plate, the height of irregularities above the cut surface is not more than 110–150 µm, while at the bottom of the plate it can reach 205–215 µm and more. Significantly larger irregularities and heterogeneity of the surface structure are characteristic for the lower plate. In the upper part of the lower plate, the size of irregularities is up to 200–305 µm, and in the lower part it is up to 330–680 µm. The smallest sizes of peaks and valleys are characteristic for the specimens obtained by the mode No. 1. Periodicity of the formed peaks on the cut surface changes from the upper to the lower part of the plates. The smallest distance between the peaks can be observed in the upper part of the upper plate. Further towards the lower part of the cut there is an increase in the distance between irregularities, which reaches a maximum in the lower plate. In spite of the largest size of irregularities on the cut surface in the lower plate, the cut with the most homogeneous distribution of peaks and valleys is formed on it. In the lower plate, the cut is almost unchanged in height or along the length of the plate. The direction of the formed peaks on the cut surface of the lower plate also does not change, while in the upper plate it significantly changes from the top to the bottom. On the cutting surfaces of both the upper and lower plates, it is possible to identify marks of oxidation of the material (Fig. 2, b). Moreover, oxidation, according to the appearance of the surface, varying degrees occurs at the top and bottom plate. In the lower part of the plates, formation of small amount of flowed metal in both plates is detected, but in the lower plate the overlap is significantly smaller and the lower edge of the cut is more uniform along the length of the plate. Parameters of plasma cutting modes of AA2024 aluminum alloy with thickness of 12 mm were determined using the parameters given in Table 1. Low cutting speed (mode No. 3) of AA2024 alloy specimens with thickness of 12 mm causes low quality of the cut surface (Fig. 3, a). In this case, there are drastic differences between the upper and lower cutting zones, with the presence of small and quasi-periodic relief elements in the upper part and large elements in the lower part. At higher cutting speeds (mode No. 4) a rather homogeneous cut surface is obtained (Fig. 3, b). The size of irregularities above the cut surface was 50–150 µm in the upper part of the cut and 80–260 µm in the lower part. The smallest irregularities are characteristic for the specimens obtained according to mode No. 4. No clearly visible oxides or any inclusions of other character were identified on the cut surfaces as well. During plasma cutting of specimens of aluminum alloy AA2024 with thickness of 40 mm a number of specific features were detected. In this case, the heating of the material of the cutting zone is of great importance, as a result of which at the beginning of the cut almost all samples have a deviation of the cutting axis location from the preset position (Fig. 4). There are a large number of relief elements on the cut surface, and its arrangement in the upper part is more ordered than in the lower one of the sample. In the lower part of the cutting zone, a small amount of metal is noted, which forms local areas of flowed metal, which
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