OBRABOTKAMETALLOV Vol. 24 No. 4 2022 technology Fig. 5. Cut faces appearance of specimens of copper with a thickness of 40 mm: 1 – bottom of the cut face; 2 – top of the cut face; 3 – a flowed metal In the heat-affected zone, a heating above the temperature of polymorphic transformation and subsequent hardening occurs with the formation of a needle-like structure, which is close for all three cutting modes (Fig. 6, e). The zone of metal melting which is mainly located in the area of flowed metal is represented by a dendritic structure (Fig. 6, f) formed with sufficiently rapid crystallization from the liquid state, which leads to formation of rather fine dendrites. The described changes in the cut zone structure inevitably lead to changes in the mechanical properties of the material, which can be unacceptable in various conditions. To investigate changes of mechanical properties of a cut surface further in the work, measurements of microhardness of a near-surface zone were carried out. The results of measurements of microhardness in specimens show, that in the heat-affected zone there is an increase in microhardness of the material (Fig. 6, g, h) both in the top, and in the bottom parts of the specimen. At distance of up to 2,000 µm from the cut surface, the microhardness values are at a level close to the base metal. In general, all of the three selected modes are quite well suited for obtaining parts by plasma cutting. From the viewpoint of the smallest values of the allowance for the further processing, mode No. 2 can be considered more optimal, characterized by an average cutting speed and the smallest depth of the heat-affected zone (up to 880 μm). It should be noted that during cutting of titanium alloy a metal hardening occurs in the heat-affected zone with an increase in microhardness, which can reduce the machinability of the material during further edge milling. The structure of the cutting zone of the titanium alloy Ti-1.5Al-1.0Mn specimens consisting of two plates with thickness of 5 mm, stacked in a package, is similar enough to that described earlier (Fig. 7, a). In the upper part of the upper plate a significant distortion of macrogeometry is observed, and the heat affected zone increases to the bottom of both plates (Fig. 7, a–d). At the same time, the lower plate is characterized by a rather uniform shape of the cut edge. In this case, only a small amount of remelted material is located on the cut surface of the upper plate, while the cut surface of the lower plate may have a rather thick layer with a dendritic structure (Fig. 7, a, d, e). The size of the heat-affected zone in the upper part of the upper plate is 550–700 µm, in the bottom part – 1,150–1,300 µm, in the upper part of the lower plate – 800–950 µm, in its bottom part – 1,900–2,300 µm. The smallest sizes of the heat-affected zone are characteristic for the cutting mode No. 1. The melted zone metal is fairly unevenly distributed over the cut surface. Between the molten metal and the base metal of the specimen, the formation of defects in the form of pores
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