OBRABOTKAMETALLOV Vol. 26 No. 4 2024 TECHNOLOGY Our experiments have shown that the wire feed aff ects the characteristics of the weld bead, including the weld bead height, weld bead width, and contact angle (Fig. 4, b). The eff ect of the wire feed on the weld bead width is diffi cult to control. Initially, the weld bead width reaches a maximum value, but then begins to decrease as the wire feed increases. This is because the contact angle continues to increase with wire feed, so insuffi cient wetting will have a negative eff ect on the weld bead. Arc current and voltage are other important elements in the WAAM optimization process. These are the main process control parameters that regulate the amount of heat supplied and dissipated, thereby aff ecting the weld deposition. It is important to prevent uneven bead deposition and poor surface roughness. In addition, excessive heat input may cause re-melting of previously deposited layers, which will adversely aff ect the microstructure, bead size, and its mechanical properties. It is important to optimize the current and voltage to improve the overall effi ciency of the process. This allows for the optimal amount of heat required to melt the metals in the WAAM process. This minimizes defects such as wavy surface morphology, uneven layer deposits, and the time required for post-processing. The microstructure of the weld beads’ material shown in Fig. 6 consists of polygonal ferrite (PF) and intergranular lamellar pearlite (P), which is consistent with the work of other authors on WAAM with low-carbon wires [25]. Therefore, grain size analysis was performed on the WAAM material based on the micrographs of Fig. 6, b. The grain size was 25–35 μm, compared to traditional welding with Sv-08G2S wire [24–25]. The grain size usually increases as the beads grow with increasing distance from the base plate; this is due to the slower cooling away from the plate due to the decreasing infl uence of the heat removal eff ect [3–5, 22]. In WAAM, solidifi cation is a major processing issue due to the advancement of the microstructure containing large columnar grains. The mechanical properties of the weld beads are presented in Table 1. It is evident that additional machining of each bead improves the mechanical properties. Table 2 presents the impact toughness test results for the test pieces. The impact toughness values with additional machining are higher due to the elimination of the deposited metal porosity, Fig. 7. The results of ultrasonic hardening of the last deposited layer (Fig. 8) showed that it can smooth the weld tip profi le, improve its microstructure, increase microhardness and introduce useful compressive residual stress into the surface layer. It is known that additional processing of welds allows creating compressive stresses in the surface layer, thereby increasing fatigue strength [28, 29]. The work [28] shows that residual compressive stress was created in the area of ultrasonic impact at a depth of 1.5–1.7 mm and a width of 15 mm. Thus, our studies have shown that the use of an intermediate operation of machining of the weld bead improves the quality of the metal, and the fi nal operation of ultrasonic processing of the upper bead (closing the additive growth process) increases the hardness of the surface layer. According to the results of studies [28, 29], it was found that ultrasonic processing of welds and weld beads reduces the quantitative values of technological residual stresses. In our further studies, we will continue the study in this matter in order to clarify the optimal processing parameters. Conclusion 1. The relationship between the geometric dimensions of the surfacing beads and the porosity in the metal with and without interpass machining is established. It is shown that the metal formed by the new combined WAAM process has a higher set of mechanical properties compared to the metal obtained by the traditional WAAM technology. 2. It is established that the yield strength and ultimate strength of the metal formed by the combined WAAM process are 15–30 % higher than those obtained by the traditional WAAM process. The impact toughness values in impact bending tests of the metal formed by the combined WAAM process are 15–25 % higher than those obtained by the traditional WAAM process. 3. It is shown that ultrasonic hardening of the last deposited layer has a positive eff ect due to an increase in the microhardness of the surface layer of the metal and the formation of compressive stresses in it.
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