OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 4 2023 Plasma cutting modes for sheet metal Alloy S, mm Mode No. I, А U, V V, m/min E, kJ/m AA5056 10 1 170 125 3.4 6.3 AA5056 10 2 170 125 3.0 7.1 AA5056 10 3 170 125 2.7 7.9 AA5056 10 4 170 125 3.7 5.7 AA5056 10 5 170 125 4.1 5.2 AA2024 10 1 170 125 4.2 5.1 AA2024 10 2 170 125 3.8 5.6 AA2024 10 3 170 125 3.3 6.4 AA2024 10 4 170 125 4.6 4.6 AA2024 10 5 170 125 5.0 4.3 Grade2 10 1 170 125 4.1 5.2 Grade2 10 2 170 125 3.4 6.3 Grade2 10 3 170 125 3.0 7.1 Grade2 10 4 170 125 2.7 7.9 Grade2 10 5 170 125 2.4 8.9 When cutting specimens of A5056 alloy with a thickness of 10 mm, such feature led to the formation of a characteristic relief in the lower part of the cut (fig. 2 c, f). The distance between the projections above the cut surface is about 200 μm, the size of the projections is up to 180–200 μm. In the central and upper parts of the cutting area, the relief is more chaotic and characterized by a large size of irregularities. The size of projections above the surface reaches more than 450–500 μm. Significant differences in the structure of the cutting surface at different modes were not revealed, for the majority of specimens the features of the structure of the cutting surface shown in fig. 2 are preserved. When the A2024 alloy specimens are cut according to the modes used, no regular relief formation is observed on the surface (fig. 3). The structure of the cut surface in the upper, central and lower parts of the cut is quite close. The size of the projections above the cut surface is up to 400–450 μm. This structure is also characteristic of most modes and does not change significantly from one specimen to another. When the Grade2 alloy specimens are cut, a smoother relief is formed on the cut surface (fig. 4). The average size of the irregularities above the cut surface is up to 200 μm. Although there are differences in the morphology of the cut surface in the upper, lower, and central parts of the cut, it is related more to the orientation of the relief elements than to the size of the irregularities. The structure of A5056 alloy specimens (fig. 5) in the surface layers after cutting is mainly represented by the fusion zone (FZ) and the heat affected zone (HAZ), gradually transitioning to the base metal zone (BM). The magnitude of macro distortion of the cut surface varies depends on the mode. The smallest distortion (up to 1,000–1,200 μm) is characteristic of specimens obtained by mode No. 2 at a relatively low (3.0 m/min) cutting speed and above average (7.1 kJ/m) heat input during cutting (fig. 5 a–d). An increase in the cutting speed from these values results in a significant decrease in cut quality, and a decrease in the cutting speed does not result in an increase in cutting accuracy. The depth of the fusion zone is rather small and does not exceed 150 µm from the cutting surface (fig. 5 g, h). The structure in this area is represented by a dendritic structure typical of cast metal, formed during crystallization from the melt. The size of the heat affected zone on the surface of metallographic sections is not revealed, the structure in it is practically identical to the base metal (fig. 5 f–h). This is due to the sufficiently high resistance of the non-heat-treatable ductile aluminum alloy A5056 to structural changes with increasing temperature. The structure of the A2024 alloy specimens after plasma cutting differs significantly from that described above (fig. 6). In this case, the value of macro distortions of the cutting zone reaches a rather significant
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