Scientific principles for creating computer models of crystallization and computer thermal analysis of alloys in order to study and determine the optimal technological modes for obtaining castings with given properties have been developed, and computer models have been created for producing castings from aluminum alloys with the required structure and properties. Based on modern ideas about the processes of nucleation and growth of crystals, a computer model of crystallization of metal alloys under various cooling conditions and different methods of their modification has been developed. Such system allows to calculate the parameters of crystallization of alloys, to determine the grain size and distribution of elements in the casting. The adequacy of the created computer research methods is proved based on the example of crystallization of aluminum and its alloys, by comparing the results of modeling and experimental data.
The studies which were carried out by the proposed method with the developed simulation model have established that the cooling curve of pure aluminum at a cooling rate of 0.00025 deg/s has a supercooling section and temperature recalescence when solid phase particles appear in the liquid metal. This indicates that the pre-crystallization state of the metal, which is close to equilibrium, becomes unstable during the solidification of the casting even at low cooling rates. At low cooling rates, the dependence of the number of crystallization centers formed in the melt on time N = f(t) increases monotonically, and the time dependence of the rate of their formation has an extremum. With an increase of the cooling intensity of the alloy, «steps» appear on the dependence N = f(t), and the time dependence of the rate of formation of solid phase particles has several peaks. This indicates that crystallization centers in the melt arise both at the beginning of metal solidification and at the end. With a further increase of the cooling rate, many «steps» appear on the dependence N = f(t), and number of peaks increases on the dependence of the rate of formation of solid phase particles, with periodic appearance of crystallization centers in the melts.
It was found that during homogeneous crystallization of hypoeutectic Al-Si alloys, crystallization centers of a solid solution based on aluminum appear within a short time, and the mechanism of their formation is similar to the crystallization process of pure aluminum. After completion of the process of solid solution formation, crystallization of the eutectic component begins starting with the appearance of crystallization centers in it.
In hypoeutectic alloys of the Al-Cu system, the content of copper in which does not exceed its maximum solubility in a solid solution based on aluminum and which are cooled at a rate of 5.0–6.67 deg/s, crystallization centers are formed in two stages. At the beginning of crystallization, a solid solution forms. At the second stage, crystallization centers also appear in the liquid-solid alloy before the solid phase.
Information on the advisability of using computer thermal analysis systems for studying processes of crystallization and formation of properties in alloys is generalized. The main parameters of its measuring channel have been determined, and algorithms for mathematical processing of cooling curves for aluminum-based alloys have been optimized.
Methods for studying the processes of crystallization of pure metal and binary alloys are proposed, which make it possible to determine by cooling curves the ratio of the heat of crystallization to the specific heat capacity and the amount of solid phase in castings at any time of their solidification, and this is a further development of the theory of solidification of metals and alloys.
An algorithm is proposed for determining the degree of modification of the eutectic in aluminum casting alloys Al + (9.0–12.0) % Si and Al + (7.0–8.0) % Si + (0.4–0.5) % Mg using a computer thermal analysis system of their melts.
The results of industrial testing in production conditions of the State Enterprise «Metallurgy» (Kiev) and OJSC «Artemmash» (Kiev) confirm the effectiveness of the computer thermal analysis systems for express analysis of the state of aluminum melts. Information about the systems of imitation modeling of crystallization of metals and alloys is included in the educational process of the National Technical University of Ukraine «Igor Sikorsky Kiev Polytechnic Institute» (Kiev) and Sumy State University (Sumy). It is also advisable to use the research results in educational and scientific programs of higher educational institutions for training of specialists in foundry, metallurgy and materials science.
