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This article addresses improvement of the mechanical properties of Al–Si alloys, in particular the complex-alloyed silumin AL25, by hot deformation. The aim of the study was to assess the effect of hot-deformation temperature and strain rate on the grain size of the aluminum-based solid-solution matrix, the size of silicon and intermetallic particles, and the amount of defects in the form of microcracks and micropores in AL25 alloy. AL25 alloy billets (composition, wt. %: 12.0 Si, 3.0 Cu, 1.0 Mg, 1.2 Ni, 0.7 Mn, 0.7 Fe, balance Al) were produced by permanent-mold casting. Microstructural analysis was performed using a Neophot-2 metallographic microscope and a Tescan Mira 3LHM scanning electron microscope. The billets were deformed by upsetting between f lat dies in an isothermal die on a hydraulic press and were tensile-tested at temperatures of 350–500 °C over a strain-rate range of 10 –4 –10 1 s –1 using an Instron universal electromechanical testing machine. To evaluate the effect of deformation on the structure and properties of the alloy, the initial billets were deformed at 400–500 °C and strain rates of 10 –4 and 10 –2 s –1 . Heat treatment was carried out according to the following schedule: quenching from 515 °C and aging at 210 °C for 10 h. It was shown that, after all deformation conditions followed by quenching and aging, the solid-solution structure was fine grained and recrystallized, with an average grain size of 7–15 μm. Recrystallization occurred during heating prior to quenching when deformation was performed at 350–480 °C, and also before reheating, as observed after deformation at 500 °C. The grain structure of the solid solution was heterogeneous throughout the alloy volume because of the nonuniform distribution of silicon particles and intermetallics. The smallest grains were observed in eutectic colonies, where the alloy exhibited a microduplex-type structure. Hot upsetting of AL25 alloy caused fragmentation of silicon particles and intermetallics. This process was accompanied by crack initiation within the particles; the cracks then widened, separating the newly formed fragments. Cracks in eutectic silicon crystals and intermetallics formed at all deformation temperatures. In primary crystals, cracks were observed only at a high strain rate of – 10 1 с –1 . Fragmentation of silicon particles and intermetallics was governed mainly by the degree of deformation. The formation of defects in the form of microcracks and micropores also depended on temperature and degree of deformation. As the degree of deformation increased, the total area occupied by defects, their average area, and their total number increased. A correlation was established between alloy structure and mechanical properties. Optimal temperature–strain-rate conditions were determined that promoted microcrack healing of and increased long-term strength.