OBRABOTKAMETALLOV Vol. 27 No. 2 2025 technology During GTAW with a low-frequency pulse of 2 Hz at currents of 80–85 A (mode No. 4), a coarse scaling pattern formed on the surface of the weld bead under the dynamic effects of the pulsed current (Fig. 3, c). Since the effects of the pulsed current allows reducing the welding speed, it promotes more intensive fusion of the weld root at the same current values as in mode No. 1. Welded joints obtained in modes No. 5 and No. 6 were obtained using a high-frequency pulse of more than 100 Hz. These modes provided a fine scaling pattern on the surface of the weld bead; however, when the current is increased to 110…115 A, melting of the plate edges occurred. The weld width obtained with pulsed currents was 5…7 mm. The reduction in the weld width can be explained by the influence of the welding current, namely, the effect of the pulsed current reduces the average power in the heat heating spot [22]. A typical morphology of the weld cross-section is shown in Fig. 4. In the upper part of the weld, a wide fusion zone (FZ) of the metal formed (Fig. 4, a) because of the action of the electric arc [23]. The weld metal consists entirely of β-grains (Fig. 4, b). The intense heating of the weld metal caused by the electric arc and, as a consequence, the wide weld (5–8 mm) also led to the formation of a wide HAZ. Depending on the phase composition, the HAZ can be divided into HAZ1 and HAZ2. HAZ1 consists of large β-phase grains 150 ± 50 μm (Fig. 4, c), whereas HAZ2 consists of β (80 ± 30 μm) + α2 (4 ± 2 μm) grains. It is known that with a greater distance from the fusion line, the phase composition in the HAZ (β+ α2) also additionally contains the O-phase [24]. The weld metal formed at direct and pulsed currents possesses a microstructure in the form of columnar grains at the bead and globular grains in the root part of the weld (Fig. 5). Primary crystallization begins when heat is removed to the base material, originating along the FL. The primary crystallization of the weld metal proceeds periodically, and a layered structure can be found (Fig. 4, a). The main volume of the а b c d Fig. 4. BSE-analysis of a Ti–Al–Nb–(Zr, Mo)–Si alloy weld joint obtained by GTAW according to mode No.4: a – general view showing the locations of BSE and EBSD analyses (FL – fusion line; FZ – fusion zone; HAZ – heat-affected zone); b – FZ; c – HAZ with coarse β-grains (HAZ1); d – HAZ with β + α2 phase composition (HAZ2)
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