OBRABOTKAMETALLOV TECHNOLOGY Vol. 26 No. 4 2024 a b Fig. 8. Results of ultrasonic hardening of the top bead (a) and hardness distribution in the surface layer (b) voltage, welding current, nozzle-to-workpiece distance, shielding gas torch angle and step distance. The main problem faced by WAAM is to decide whether to increase the deposition rate of the metal being deposited or to reduce the heat input distribution. This is due to the fact that the wire feed is closely related to the required heat input [15–20] for its melting. In our experiments, we found that the optimal indicator is the range of wire feed of 5–6 m/min. At the same time, we saw that with such parameters there is another factor that should be taken into account; this factor is the porosity of the weld bead (Fig. 2). It is known that gas porosity is a typical defect that occurs in the WAAM process and should be eliminated because it negatively aff ects the mechanical properties [23, 24]. Initially, gas porosity leads to a decrease in the mechanical strength of the component due to damage from the formation of microcracks. In addition, it often leads to the fact that the deposited layer has worse fatigue properties due to the spatial distribution of structures of diff erent shapes and sizes [24]. Another factor that contributes to the formation of porosity in the layered structure is the prevalence of surface contaminants in the raw material, such as moisture, impurities, and grease. The gas pores are usually trapped in the uppermost layer of the fusion zone, causing it to spread toward the top of the solidifi ed weld pool. When thin oxide fi lms form quickly on the surface of the weld pool, it easily absorb molecular hydrogen and moisture from the air, which then increases the amount of hydrogen present in the upper part of each layer. As a result, there is usually more trapped hydrogen and small micropores in the fusion line zone of each layer, which can grow and coalesce into larger pores when exposed to high temperatures. Consequently, larger pores are often observed along the fusion line zone between layers. Porosity is one of the most common and undesirable defects that greatly degrades weld properties such as strength and fatigue. Our results show that the shielding gas fl ow rate aff ects the quality of the part. Increasing the gas fl ow rate reduces the porosity within the studied range. The single-scan track tests conducted in [24] show that even with single-scan tracks, the weld pool geometry is signifi cantly dependent on the shielding gas fl ow conditions. Our experiments (Fig. 3) show that increasing the shielding gas fl ow rate within 8–14 l/ min allows us to reduce the porosity in the deposited metal to almost zero. The pores trapped by the gas are spherical (Fig. 3). The pores develop throughout the process due to gas entrapment, supersaturation of dissolved gases, and chemical reaction within the weld pool, which leads to the formation of gaseous products [23, 24]. When the equilibrium gas pressure exceeds the sum of its hydrostatic, atmospheric, and capillary pressures, there is a high probability of the formation of trapped gas holes. Nucleating pores lead to vacancies [18, 21], allowing supersaturated gases to penetrate into the weld pool. When rapid cooling occurs, the pore nucleation sites can be captured by the weld pool. On the other hand, slower cooling rates allow these pores to enlarge and sometimes merge with neighboring pores. Further, our proposed mechanical grinding allows for further reduction of metal porosity by removing the defective layer after surfacing, as well as by removing the oxidized metal Fig. 4, 5.
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