Удосконалення процесу алмазного шліфування кругами на керамічній зв'язці за рахунок забезпечення їх самозаточування

Abstract

Dissertation for the degree of Candidate of Technical Sciences in the specialty 05.03.01 – "Processes of mechanical processing, machines and tools" (13 – mechanical engineering) – National Technical University "Kharkov Polytechnic Institute", Kharkov, 2021. The aim of the work is to increase the efficiency of diamond grinding through the implementation of rational self-sharpening of circles on ceramic binders by scientifically sound determination of their characteristics at the stage of tool design, manufacture and operation. The object of research is the processes of formation of a diamond-abrasive tool on ceramic bonds and grinding in the self-sharpening mode. The subject of research is the determination of rational characteristics and technological parameters for the manufacture of diamond wheels, as well as grinding modes that ensure their rational self-sharpening. The dissertation research is devoted to solving an urgent scientific and applied problem of developing theoretical and technological principles for ensuring the conditions for self-sharpening of diamond wheels on ceramic bonds. The introduction substantiates the relevance of the work, formulates its goals and objectives, the object, subject and methods of research, discloses the scientific novelty and practical significance of the work, provides data on the results approbation, scientific publications and the structure of the work. The first chapter is devoted to a comprehensive analysis of scientific and technical information on promising directions for the development of modern diamond abrasive tool (DAT) production, the main factors that determine the processes of diamond wheels destruction, as well as ways to increase their stability. The theoretical and technological prerequisites for self-sharpening of diamond wheels are considered, it is noted that at the moment there is no single view of the nature of self-sharpening, which complicates the operational methods development of influencing the diamond wheel working surface. It is shown that the use of DAT on ceramic bonds is promising, which are distinguished by high quality of finishing, hardness, resistance to high temperatures and chemical influences, make it possible to manufacture tools with different types of structure and, under certain grinding conditions, are demonstrate the effect of self-sharpening. It is noted that today glass-ceramic bonds for the manufacture of DAT are almost not used due to the high sintering temperature of the ceramic-matrix abrasive tool layer, which causes thermal degradation of diamond grains. So, to realize the potential of DAT on ceramic bonds, a scientifically grounded approach to the creation of diamond wheels on low-melting bonds with a set of specified physical, mechanical and thermophysical properties is necessary. Modern approaches to the study of diamond wheels making and operation processes are analyzed. It is concluded that the most effective way to solve the problem of optimizing the characteristics of the components of the diamond-bearing layer of the wheel, the technological parameters of its sintering and grinding in the mode of rational self-sharpening is the finite element modeling of the corresponding processes. Based on the analysis of literature data, the goal and objectives of the research are formulated and the ways of their solution are determined. The second chapter provides a description of the general research methodology. An improved methodology for modeling the process of sintering a diamond-bearing layer of DAT and grinding in a self-sharpening mode, based on finite element calculations, is described, software products and general conditions for conducting model experiments are described. The materials used in the development of glass-ceramic bonds and diamondcontaining composites based on them are characterized, and the methods and equipment for experimental tests are described. The determination of the features of the grinding process and the quality of the processed surface was carried out according to generally accepted methods and included measurements of the 3D topography of the working surface of the wheel using the laser scanning method. Investigation of the processes of phase formation, structure and phase composition of materials was carried out using methods of physicochemical analysis: X-ray phase, differential thermal, electron-microscopic. The research involved the instruments and equipment of the interdepartmental laboratory of NTU "KhPI", University of Applied Sciences: Technology, Business and Design (Hochschule Wismar), Magdeburg Technical University, Texas A&M University. The third chapter is devoted to theoretical studies of the stress-strain state of the diamond wheel components at the stage of DAT manufacturing. The results of finite element analysis and 3D modeling of the sintering process of a diamond-bearing layer are discussed, on the basis of which recommendations are given on the rational characteristics of a ceramic- matrix diamond composite. In this direction, the following results were obtained: – methodology for diamond wheels designing based on 3D modeling of diamond wheels capable of self-sharpening when grinding difficult-to-machine materials were improved; for the first time, the model of the sintering processes a diamond-bearing layer and grinding was carried out, taking into account the temperature dependences of the main properties of the structural components of the circle, as well as the porosity of the bond; – it was found that the main reasons restraining the wide using of diamond wheels on existing ceramic bonds is the discrepancy between their high melting temperature (~ 700-800 °C) and the strength of diamond grains, which leads to a rapid decrease in cutting ability by reason of the thermal destruction of grains even at the stage making circles; – the relationship between the state of grains during sintering of the diamond layer and their ability to restore cutting properties during grinding has been revealed; it was found that for rational self-sharpening the grains must maintain their integrity and be in condition, which is characterized by the presence of microcracks inherited after sintering of the diamond-bearing layer; – the influence of the technological parameters of the process and the characteristics of the diamond wheel on the integrity of the grains during sintering was established, on the basis of which practical recommendations were given for the selection of diamond compositions with rational properties. The fourth chapter presents the results of finite element modeling of the grinding process with diamond wheels in the self-sharpening mode. The influence of the characteristics of the diamond-bearing layer and the properties of various difficult-to-machine materials on the change in the stress-strain state in the cutting zone has been investigated. Research in this direction gave the following results: – for the first time, the optimal combinations of physical-mechanical and thermophysical properties of a ceramic bond were determined depending on the porosity of the diamond-bearing layer, the concentration and granularity of powders, which provide rational self-sharpening when grinding difficult-to-machine materials; – the influence of the characteristics of diamond powders, their concentration, bond properties, the depth of the grain in the bond and the size of the wear areas formed on the grains during grinding on the SSS of the system was determined; – a dynamic analysis of the grinding process was carried out in the LS-Dyna solver; the volumes of destroyed grains and processing material in different periods of grinding have been determined. Thе fifth chapter contains the results of research aimed at the development of lowmelting glass-ceramic bonds with a given set of properties and ceramic-matrix diamondcontaining composites with rational characteristics. The results of research in this direction made it possible to determine the following: – a low-melting glass-ceramic bond has been developed, which ensures the production of a diamond-bearing layer at a sintering temperature of 600-620 °C and, in terms of a set of properties, satisfies certain conditions for rational self-sharpening of wheels. It was found that the modification of the basic eutectic composition of the Na₂O–B₂O₃–SiO₂ system with Li₂O and TiO₂ oxides provides the formation of lithium titanates, which increase the heat resistance, wear resistance and water resistance of the glass-ceramic bond; – it has been shown that the use of pore-forming additives (2.0–2.5 wt.% of Li₂CO₃) or a structure-forming filler (20–30 wt.% of technogenic microspheres) makes it possible to increase the total porosity of the diamond wheel to 25–33% and 45–55% accordingly, and to increase the efficiency of the tool due to rational self-sharpening; – an algorithm and methodology for the selection of rational characteristics of the diamond-bearing layer has been developed, which makes it possible to increase the tool life and productivity of grinding difficult-to-machine materials. The effectiveness of the proposed technological solutions is confirmed by the results of laboratory and industrial tests under the conditions of LLC «Instail», which indicate an increase in the grinding productivity by 1.7 times and a decrease in the cost of diamond abrasive processing by 1.5 times due to a reduction in the consumption of diamond grains by 20-40%.