Neduzhyi A. Regularities of the formation of the primary aluminum phase in the obtaining of castings from hypoeutectic silumins by methods of rheo- and thixocasting

The dissertation is devoted to the work up of the scientific and technological bases for obtaining castings from hypoeutectic partially crystallized silumins with a non-dendritic structure of the primary aluminum phase under the influence of various flow effects on the crystallized metal. The use of a cold flat-bottomed cooling slope for casting a metal slurry into steel moulds makes it possible to obtain castings from an AK7 alloy with a non-dendritic primary alfa-phase structure with a crystal size of 30...54 mkm. With an increase in the length of the cooling slope from 100 to 550 mm, at the temperatures of casting the alloy on the cooling slope 620 and 630 °C the structure of the alloy is more uniform, the crystals of the primary solid solution of aluminum have a more rounded shape, and their size decreases. The process of thixocasting with the use of billets with a rosette-like structure of the primary aluminum phase provides the production of shaped castings with a globular structure of a solid solution of aluminum. The globular structure of the primary alfa-phase, which is formed in the castings, is uniform and uniformly distributed throughout the cross section of the samples. The increase in the size of globular crystals of a solid solution of aluminum from 87 to 285 ?m practically does not change the Brinell hardness, which is from 58 to 60 units. The change in the morphology of the alfa-phase from dendritic to non-dendritic promotes a certain decrease in the friction coefficient from 0,35 to 0,30. Based on the revealed regularities of structure formation, changes in the morphology of the primary alfa-phase, their interrelation with the characteristics of mechanical properties, a new technological process of rheocasting was developed using a sectional cooling slope with a warmed "pocket" and mechanical stirring the metal slurry in it with a turbine stirrer. This ensured the production of castings with a non-dendrite globular structure of the primary aluminum phase with a crystal size of 40...80 mkm and an increased level of plasticity (the elongation increases from 2 % to 9 %, with a relative narrowing of 10...12 %). The use of a new technology rheocasting contributes to an increase in the durability of steel chill moulds when obtaining castings from alloys of the AK7 type by 13...14 % due to a decrease in the melt casting temperature by 90...100 °С.