OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 Results and discussion In the recent years many studies have been concentrating on the optimization of the modes of selective laser melting of powder metallic alloys mainly made up of spherical particles. The problem of SLM alloy formation from metallic powdered materials made up of non-spherical particles is understudied. The properties of powders change both at the stage of preparation of the powder composition and during the SLM process under the influence of the environment, mechanical and thermal influences. All this affect the quality of the produced items. To determine the optimal SLM modes we need to know the particle distribution in terms of its size and chemical composition of the surface. Pollution of the powders is the major problem in SLM, especially when treating high-reactive materials, such as magnesium, titanium and aluminum alloys. Long-term exposure of the powders to the natural air leads to its oxidation and, as a result, to non-stable SLM process. Oxide layers prevent wetting and cause porosity. Consequently, to understand the process of alloy formation in SLM we need to know the initial characteristics of the powders as it has a significant effect upon the quality of produced items. The important characteristics of the powder material to be used in SLM are morphology, grain sizing, surface chemistry, bulk density, rheology and thermal properties which are known to influence the material behavior when the material is exposed to the laser action [20]. Scanning electron microscopy, X-ray and computer tomography are used to study the form and the surface morphology of the powder particles. In Figure 1 we provide scanning electron microscopy images (SEM) of the surfaces of aluminum, silicon and magnesium powders obtained when imaging the sample. The aluminum powder is formed by conglomerates of irregular particles 1–20 µm in size and larger particles 30–140 µm in size (Fig. 1, a, b). Single-phased magnesium powder is a mixture of particles with “scaly” structure, its size varies within the range of 30–400 µm (Fig. 1, c, d). Its particles have irregular form with rough texture of the surface which leads to reduced flowability. The element composition of the powder corresponds to magnesium with oxygen content less than 2 wt. %. Single-phased magnesium powder consists of conglomerates 0.5–45 µm in size (Fig. 1, e, f). The proportion of large conglomerates in the powder does not exceed 15 vol. %. The powder also contains small amounts of aluminum, titanium, calcium and oxygen (not more than 4 %). Figures 2–4 present X-ray diffraction patterns with the completed phase identification of magnesium, aluminum and silicon samples respectively. The phase compositions correspond to the single phases of Mg, Al, Si. To form the Al-Si-Mg powder composition, the powders were subjected to mechanical mixing by placing the initial powders in a globe mill at the ratio Al – 91 wt. %, Si – 8 wt. %, Mg – 1 wt. % and activation in the protective argon atmosphere during one and two hours. Balls 5, 7 and 8 cm in diameter made of structural steel ShKh15 served as the grinding bodies at a mass ratio “powder-balls” of 1:10. The acceleration of the grinding bodies in the process of mechanical alloying was 40g. The scanning electron images of the mixture of aluminum, magnesium and silicon powders obtained as a result of imaging the samples after one hour of mechanical activation are shown in Figure 5. The powder is presented by spherical and ellipsoidal aluminum particles which sizes vary within the range of 1–40 µm (Figure 5, a–d). There are also conglomerates of spherical particles 30 to 50 µm in size. The silicon particles in the powder mixture are presented by irregular agglomerates which sizes vary from 3 to 40 µm. The particles of magnesium powder are distributed throughout the powder (Fig. 6, a). A large amount of deformed powder particles is observed after one hour of mechanical activation of the powders (Fig. 5, a, b). After two hours of mechanical activation the powder is also presented by spherical and ellipsoidal (elongated) aluminum particles which sizes vary within the range of 1–50 µm (Fig. 5 c, d, Fig. 6, b). There are conglomerates of spherical particles 30 to 50 µm in size. Silicon particles in the powder mixture are also presented by irregular agglomerates which sizes vary from 3 to 70 µm (Fig. 6, b). According to the mapping element analysis the magnesium particles are distributed throughout the powder and are also presented in the form of large (up to 70 µm) conglomerates (Fig. 6, a). After two hours of treatment the powder was
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