OBRABOTKAMETALLOV Vol. 27 No. 2 2025 technology When welding in low-frequency pulsed modes, the FZ structure does not differ from the structure of welds obtained in modes No. 1 and No. 3. The Lmax and Hmax of dendrites in the weld, obtained in the low-frequency pulse mode (mode No. 4), are ≈ 0.98 mm and 0.27 mm, respectively, and the average size of the globular grains is ≈ 190 μm (Fig. 5, c). However, when using high-frequency pulsed current (mode No. 5), the Lmax and Hmax of dendrites in the weld decreases to ≈ 0.61 mm and 0.23 mm, and the average size of the globular grains is ≈ 140 μm (Fig. 5, d). In welding mode No. 5, a localized lack of fusion of 100…150 μm can be observed (Fig. 5, d), which is smaller than at the same welding current parameters in mode No. 1 (300…500 μm). With an increase in the high-frequency current values (mode No. 6), the Lmax (1.06 mm) and Hmax (0.33 mm) of the β-phase dendrites increases, while the average size of the globular β-grains in the lower part of the weld remains at ≈ 130 ± 45 μm (Fig. 5, e), which is smaller than at the same currents without the pulsed mode No. 3. The above results show that the pulsed welding current demonstrates periodic changes in the dynamic effects of the arc, which can enhance the effect of mixing the molten pool and thereby reduce the grain size in the weld [25]. The microhardness profile in the cross-section of welded joints obtained with direct current along the entire length is at the same level, since the weld width varies within 5…8 mm and the values correspond to the microhardness of the FZ and the HAZ1 with globular β-phase grains (Fig. 6, a, b). The range of values varies from 335 to 390 HV0.2. The range of microhardness values of welds obtained with pulsed currents is in the range of 340…380 HV0.2 (Fig. 6, c, d, e). Tensile tests of welded joints obtained GTAW (Fig. 7) showed that the strength level of the joints welded in mode No. 6 reaches ≈ 90 % of the base metal (σu = 1,100 MPa, δ = 1.1 %), and is not lower than 80 % of the base metal in modes No. 4 and No. 5 (σu = 1070 MPa, δ = 1.49 %) (Fig. 7, a). However, the ductility of the welded joint obtained in mode No. 5 is absent (σu = 960 MPa, δ = 0 %), and for the welded joints obtained in modes No. 4 and No. 6, it drops by 2–3 times in relation to the base metal and amounts to no more than 2 % (Fig. 7, b). a b c d e Fig. 6. Cross-sectional microhardness profiles of Ti–Al–Nb–(Zr, Mo)–Si alloy GTAW joints according to: a, b – DC mode; c, d, e – pulsed mode
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