OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 (GOST 2169-69) and magnesium powder MPF-4 I (GOST 6001-79I). The powders differ in fusion temperatures, density, heat conductivity and other characteristics. The fusion temperature of aluminum is 660 °C, those of silicon and magnesium are 1,414 °C and 650 °C respectively. The density of aluminum is 2.7 g/cm3, the density of silicon is 2.35 g/cm3, the density of magnesium is 1.74 g/cm3. At the same time the heat capacity of aluminum, silicon and magnesium does not differ much. The molecular heat capacity of aluminum is 24.35 J/(K∙mol), those of silicon and magnesium are 20.16 J/(K∙mol) and 24.9 J/(K∙mol) respectively. The heat conductivity of aluminum under the room temperature is 237 W/(m∙K), the heat conductivity of silicon and magnesium under the room temperature is 149 W/(m∙K) and 156 W/(m∙K) respectively. The surface morphology of the particles was studied with the scanning electron microscope LEO EVO in Research Equipment Sharing Center «Hanotekh» ISPMS SB RAS under the following conditions: accelerating voltage – 20 kV, beam current – 1–2 nA, focal distance 8.5–10 mm, magnification – 100–2,000. The elemental composition of the surface of the sample was analyzed using the X-ray microanalyzer for dispersion analysis Oxford Instruments INCA350. The x-ray diffraction studies were conducted on X-ray diffractometer DRON-7 (Burevestnik, Russia) in CoKα-rays (λ = 0.1789 nm). The voltage supplied to the roentgen tube was 35 kV, current – 22 mA. The imaging was conducted in the Bragg-Brentano geometry (2theta-theta) at the 2θ angle range of 10–165° with the sampling interval of 0.05° and rotation of the sample. Exposure time for each point was 5 s. To complete the X-ray diffraction studies, due to the vertical fixation of the sample on the diffractometer goniometer, the powders were glued with transparent zapon enamel into the viniplast forms. The grain-size composition was established with the help of the screen test. The screen test ensures a simple approach to obtaining powder size-wise distribution by sieving the powder through the sieves (No. 0100, No. 0080, No. 0064, No. 0040, No. 0020) put on one another in the order of mesh size decreasing subjected to mechanical vibration during 60 min. Each screen holds the particles which are impassable for the next screen and this way each screen generates the size range of particles. The screen test of aluminum powder showed that the proportion of particles smaller than 20 µm is 6.5 wt. %, the proportion of particles smaller than 40 µm is 20 wt. %, 27 wt. % of particles are smaller than 64 µm in size, 17 wt. % of particles are smaller than 80 µm in size, 11 wt. % of particles are smaller than 100 µm in size, and particles over 100 µm make 25 wt. % of the powder. In the magnesium powder 3.6 % of particles are smaller than 20 µm in size, 15 % of particles are smaller than 40 µm in size, 27 % of particles are smaller than 64 µm in size, 26 % of particles are smaller than 80 µm in size, 25 % of particles are smaller than 100 µm in size. The powder loss when screening did not exceed 4%. The silicon powder was not sieved due to its poor flowability. To obtain the powder composition, particle size range of 20–64 μm was chosen as recommended for the selective laser melting. Smaller alloying elements ensure higher density of the powder layer, although the difference in the sizes of particles may result in undesired segregation. Three elementary powders were combined by weight Al – 91 wt. %, Si – 8 wt. %, Mg – 1 wt. % and then mixed with a globe mill during one hour in protective argon atmosphere. Balls of structural steel ShKh15 with a diameter of 5, 7 and 8 mm served as grinding medium. A globe mill is an economically viable and widely used method of mechanical alloying of a powder composition. The samples were grown at the selective laser melting unit VARISKAF-100MVS developed and made in Yurga Institute of Technology (branch) of Tomsk Polytechnic University [18, 19]. The conditions of the searching experiments were as follows: the constant laser power P = 80 W and the pulsed laser power P = 100 W, modulation m = 5,000 Hz, under the invariable mode parameters: the sampling interval S = 90 µm, the layer thickness t = 25 µm, and the varied laser beam scanning speed V = 100, 200, 300, 400 mm/s. The samples with a side of 10 cmwere built on the aluminum base plate in a chamber preliminarily heated to 200 °C and filled with argon after preliminary degassing. The metallographic samples were prepared by standard abrasive machining and diamond paste polishing to obtain the polished cross-section.
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