OBRABOTKAMETALLOV technology Vol. 27 No. 2 2025 The microstructure was evaluated using scanning electronmicroscopy (BSE and EBSD analysis), namely, evaluation of the presence of internal defects (pores, cracks, lack of fusion), identification of phases, and geometric dimensions of the microstructure elements in the weld metal and heat-affected zone (HAZ). BSE and EBSD analysis was performed using a Q600 3D instrument (FEI, Czech Republic) using TSL OIM Analysis 9 software with a scanning step of 3 μm and an accelerating voltage of 30 kV. Results and discussion Fig. 3 shows the appearance of welded joints. The welded joints obtained in the selected modes (Table 2) do not have external cracks, pores or heat colors. The presence of defects at the beginning and end of the welds is attributed to the absence of run-off plates (Fig. 3). At currents of 80–85 A (mode No. 1), a fine scaling pattern is observed on the weld bead. A localized lack of fusion was found in the weld root, which could potentially be eliminated by a more uniform welding speed. In speed-up welding condition at I = 150…155 A (mode No. 2), burn-through of the plates occurred; furthermore, the liquid metal locally accumulated under the force of surface tension, forming a drop-shaped weld bead. Thus, burn-through occurred on part of the plate, while a wide weld formed on another part. This mode is not suitable for GTAW of Ti–Al–Nb–(Zr, Mo)–Si alloy plates 2 mm thick. At increased currents of 110…115 A (mode No. 3), defects did not form, ensuring a uniform, fine-ripple weld with full fusion of the plates (Fig. 3, b). However, the weld width (mode No. 3) is 1.5 times wider compared to mode No. 1, and the melting of the plate edges is more intense. On average, the weld width obtained with direct current was 5…8 mm. Ta b l e 2 Modes of GTAW for Ti–Al–Nb–(Zr, Mo)–Si alloy plates No. Current, A Pulse frequency, Hz Shielding gas, L/min Blowing L/min Comments 1 80–85 – 12 2 local lack of fusion 2 150–155 – 12 2 burn-through 3 110–115 – 12 2 – 4 80–85 2 12 2 coarse scaling 5 80–85 >100 12 2 fine scaling 6 110–115 >100 15 2 fine scaling a b c d e Fig. 3. Appearance of Ti–Al–Nb–(Zr, Mo)–Si alloy weld joints produced by GTAW under: a – direct current (mode No. 1); b – direct current (mode No. 3); c – low-frequency pulse (mode No. 4); d – highfrequency pulse (mode No. 5); e – high-frequency pulse (mode No. 6)
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