Vol 23, No 2 (2021)

Introduction. The final quality of products is formed during finishing operations, which include the grinding process. It is known that when grinding brittle materials, the cost of grinding work increases significantly. It is possible to reduce the scatter of product quality indicators when grinding brittle materials, as well as to increase the reliability and efficiency of the operation, by choosing the optimal parameters of the technological system based on dynamic models of the process. However, to describe the regularities of the removal of particles of a brittle non-metallic material and the wear of the surface of the grinding wheel in the contact zone, the known models do not allow taking into account the peculiarities of the process in which micro-cutting and brittle chipping of the material are combined. Purpose of the work: to create a new probabilistic model for removing the surface layer when grinding brittle non-metallic materials. The task is to study the laws governing the removal of particles of brittle non-metallic material in the contact zone. In this work, the removal of material in the contact zone as a result of microcutting and brittle chipping is considered as a random event. The research methods are mathematical and physical simulation using the basic provisions of the theory of probability, the laws of distribution of random variables, as well as the theory of cutting and the theory of a deformable solid. Results and discussion. The developed mathematical models make it possible to trace the effect on material removal of the overlap of single cuts on each other when grinding holes in ceramic materials. The proposed dependences show the regularity of stock removal within the arc of contact of the grinding wheel with the workpiece. The considered features of the change in the probability of material removal upon contact of the treated surface with an abrasive tool and the proposed analytical dependences are valid for a wide range of grinding modes, wheel characteristics and a number of other technological factors. The obtained expressions make it possible to find the amount of material removal also for schemes of end, flat and circular external grinding, for which it is necessary to know the amount of removal increment due to brittle fracture during the development of microcracks in the surface layer. One of the ways to determine the magnitude of this increment is to simulate the crack formation process using a computer. The presented results confirm the prospects of the developed approach to simulate the processes of mechanical processing of brittle non-metallic materials.

Introduction. Finishing operations, in particular, cylindrical grinding, essentially form the quality parameters of products, its performance characteristics and functional suitability. At the same time, the cost of grinding work increases significantly in comparison with grinding metals, reaching an average of 20 ... 28% of the total cost of manufacturing products. The selection of the optimal parameters of the technological system based on the process simulation can improve the reliability, productivity and economic efficiency. To describe the processing of brittle nonmetallic materials, empirical dependences are mainly used, and the existing analytical models do not take into account the stochastic nature of the grinding operation and the combination of microcutting and brittle chipping when removing particles of brittle nonmetallic material and wear of the surface of the grinding tool. Purpose of the work: simulation of stock removal in the contact zone during internal grinding of brittle non-metallic materials. The task is to study the features and patterns of change in the probability of material removal when the treated surface comes into contact with an abrasive tool. In the work, the theoretical and probabilistic models are obtained, allowing to reveal the patterns of material removal in the contact zone. The models make it possible to trace the regularities of the interaction of cutting and piercing grains on the surface of the workpiece and the process of removing the allowance in the contact zone due to a combination of the phenomena of microcutting and brittle chipping, considered as a random event. The research methods are mathematical and physical simulation using the basic provisions of the theory of probability, the laws of distribution of random variables, as well as the theory of cutting and the theory of a deformable solid. Results and discussion. Data are obtained that provide a clear illustration of the patterns of material removal along the contact zone at various levels. Analysis of the results obtained shows that the peripheral speed of the tool and the rotation speed of the workpiece, which are directly included in the equation for calculating the probability of material removal, significantly affect the rate of material removal. The cross feed also has a significant effect on stock removal. A qualitative picture of the change in the probability of material removal in the contact zone during grinding of holes in brittle nonmetallic materials is obtained. The obtained patterns of change in the probability of material removal when the machined surface is in contact with an abrasive tool and analytical dependences are valid for a wide range of grinding modes, tool characteristics and other technological factors.

Introduction. A fair number of technical structures have gaps (backlashes) which can be contingently divided into two types. One of them is the gaps in connections between substructures which are introduced so that the connections may operate correctly. Sizes of such gaps are usually normalized. Another type is the backlashes which occur during operation. Due to the normalized gaps usually expand while operating, both of the types may lead to increased loading and wear of mechanical parts, an alteration in dynamical characteristics and a deterioration in a technical state of mechanical structures. It explains the necessity to control the gaps. When the ground vibration testing of the structures is performed, it seems appropriate to use these tests to detect such gaps. Research Objective: developing the method to control the gaps in the technical structures during the ground vibration testing based on distortions of portraits of forced oscillations. Research Technique. The steady-state forced oscillations of the technical structures, which were measured by acceleration sensors, are excited by means of shakers. The sensor signals are represented as the portraits: the vertical scanning is proportional to the signal and the horizontal scanning – to its first harmonic with the phase shift of π/2. In case of a linear system, the portraits are circles. The presence of the gaps distorts the portraits of oscillations specifically. To estimate the distortions numerically, the first harmonic is subtracted from the Fourier series of the portrait of oscillations, the absolute maximum of the residue is calculated over the oscillation period and used subsequently as the distortion parameter Ψ. The value of the parameter Ψ is normalized and denoted as ξ. The ξ distributions are plotted on controlled objects. The locations of the gaps are determined through the positions of the local maxima of the distortions. While calculating the parameter ξ, the two types of normalization, which were conditionally named the global and local ones, are being used. In case of the global normalization, the value of Ψ is related to the amplitude of the first harmonic at the control point of the structure. The local normalization means that the magnitude of Ψ is related to the amplitude of the first harmonic of the sensor where that parameter was previously calculated. The global normalization is required to analyze the distortion distribution of the portraits of oscillations of the entire technical structure. The local normalization of the distortions of the portraits of oscillations is utilized to establish the locations of the gaps in the mechanical parts and structural connections. The ground vibration tests were carried out via Test.Lab software. The subprogram is integrated into the software interface in order to analyze the portraits of oscillations. It enabled one to calculate the distortions of the portraits of oscillations, plot the distortion distributions of the structure and save it for further use. It allowed one to control the gaps during vibration strength tests, as well as while the structures being used, by means of comparing the distortion distributions of the parameter ξ related to different states of the structure. Additionally, the plotting of the distortion distributions of the portraits of oscillations for each structural component is added to the subprogram so as to control the defects subsequently. Not only the locations of the gaps are determined in the force-displacement application systems but also the equation is given to calculate its magnitudes. The practical recommendations on using that equation are presented. Results and Discussion. The possibility of detecting the gaps by the distortions of the portraits of oscillations is illustrated with the example of the diagnostics of the layout of the control wiring and the airplanes during the ground vibration testing as well as the open-type spacecraft structures. It is shown that the developed method enables one to detect all the gaps in the testing object which distort the portraits of oscillations.

Introduction. Difficult-to-machine materials with enhanced physical and mechanical properties are increasingly being used in various industries. Such materials are used in mechanical engineering for the manufacture of parts and assemblies of machines and mechanisms, in the production and processing of food products where increased operational requirements are required. In modern production, along with traditional methods of intensifying technological operations, combined and hybrid processing technologies are used. For the finishing of products, abrasive grinding with a diamond tool is used. One of the problems hindering the wide practical application of this method in industry is the fact that it has a high prime cost caused by the cost of materials used in the manufacture and the laboriousness of the tool shaping process. This leads to the need to develop a new technology for manufacturing diamond tools. The purpose of the work is to increase the efficiency of the end diamond abrasive tool with a metal bond by using carbon steels as a body material, increasing the strength of the connection between the body and the diamond-bearing part, as well as choosing an effective tool manufacturing technology. Research methodology. To gain this task, a technology for manufacturing end diamond abrasive tools is developed and tested. Allowing using the technology of capacitor welding to connect the diamond-bearing part with the shank and use medium-carbon hardened high-quality steels with a hardness of 45-60 HRC as the shank material. The strength of the connection of the body with the working diamond-bearing part of the grinding head samples is determined by tensile testing on a 1958U10 tensile machine with maximum load 100 kN. The quality of the joint is assessed visually by the presence of discontinuities in the joint, as well as by examining the microstructure and measuring the microhardness of the weld and heat-affected zones. The microhardness of the welded joint is measured using an HMV-G21ST semi-automatic microhardness tester (Shimadzu, Japan) at a load of 50 g. Results and discussion. Thus, the results of comparative studies allow us to assert that the strength of the connection between the shank and the working diamond-bearing part according to the proposed technology surpasses similar characteristics of the strength of the connection between the shank and the diamond-bearing layer of grinding heads obtained by the method selected by the prototype. Conclusions. The proposed technology for the manufacture of diamond heads increases the strength of the connection between the body and the diamond-bearing working part, reduces the cost of manufacturing the grinding heads due to the use of hardened medium-carbon steels as the material of the tool body instead of high-speed steel grades, the technology is simplified and the possibility of automating the manufacture of tools appears.

Introduction. The development of hydrogen energy implies a decrease in the dependence of various human activities on fossil energy sources and a significant reduction in carbon dioxide emission into the atmosphere. Therefore, the requirements for the quality of structural materials, which have the prospect of being used for storage and transportation of hydrogen, as well as for the creation of infrastructure facilities for hydrogen energy, are increasing. Therefore, the scientific researches on the hydrogen-assisted microstructure and mechanical behavior of structural materials in various loading schemes are of great importance. The aim of this work is to establish the effect of chemical-deformation treatment, including rolling combined with hydrogen saturation, on the microstructure, phase composition, and mechanical properties of 316L-type austenitic stainless steel. Methods. Transmission electron microscopy and backscattered electron diffraction, X-ray diffraction, X-ray phase and magnetic phase analysis, microindentation and uniaxial static tension are utilized. Results and Discussion. It is shown experimentally that after rolling with 25 and 50 % upset, the morphology of the defect structure and the phase composition of 316L steel substantially depends on the deformation temperature (at room temperature or with the cooling of the samples in the liquid nitrogen) and on hydrogen saturation rate (for 5 hours at a current density of 200 mA/cm2). The main deformation mechanisms of the steel in rolling are slip, twinning, and microlocalization of plastic flow, which all provide the formation of ultrafine grain-subgrain structure in the samples. In addition, deformation-induced ε and α' martensitic phases are formed in the structure of the rolled samples. Regardless of the regime of chemical-deformation processing, grain-subgrain structures with a high density of deformation defects are formed in steel, but its morphologies are dependent on the processing regime. The experimental data indicate that both preliminary hydrogen saturation and a decrease in the deformation temperature contribute to the more active development of mechanical twinning and deformation-induced phase transformations during rolling. Despite the discovered effects on the influence of hydrogen saturation on the deformation mechanisms and the morphology of a defective microstructure formed during rolling, preliminary hydrogenation has little effect on the mechanical properties of steel at a fixed degree and temperature of deformation.These data indicate that irrespective of the morphology of the defective grain-subgrain structure, grain refinement, accumulation of deformation defects and an increase in internal stresses lead to an increase in the strength characteristics of the steel.

This paper provides a review of studies on the development and characterization of high-entropy alloys (HEAs). It is structured in the following way. Alloys’; design strategy based on entropy approach. Expectations and modern perceptions. This section describes the initial principles of multicomponent alloys design which provide stable structure and mechanical properties. It is noted that the role of high mixing entropy in the formation of disordered solid solutions and the suppression of the brittle intermetallic phases formation have been significantly reconsidered over time. Currently, obtaining a single-phase solid solution structure is not the main requirement for HEAs. The composition of HEAs. This section describes some typical multicomponent alloys having different elemental compositions. It is shown, that at present time the most studied alloys are based on 3-d transition elements. Using alloys of this group the possibility of providing both high and low values of strength and ductility is shown. Fabrication methods of HEAs. This section describes the methods for the fabrication of high-entropy alloys. It is noted that the most commonly used methods are based on the melting of the initial materials and its subsequent crystallization. Such methods of HEAs fabrication as powder metallurgy, magnetron sputtering, self-propagating high-temperature synthesis, melt spinning, and diffusion welding are also discussed. Structure of HEAs. This section provides the data on HEAs possessing multiphase structure and containing fine nanosized precipitates. Besides, the studies in which HEAs have been obtained in the form of metallic glasses, carbides, oxides, and borides are reviewed. The factors that can affect the structural state of the multicomponent alloys are discussed. The ambiguity of opinions of different research groups is noted. Properties of HEAs. This section mainly concentrates on the mechanical properties of HEAs. However, some other promising properties of HEAs like high wear resistance and reduced diffusivity are also discussed. Plastic deformation of HEAs. This section describes the evolution of the structure and properties of HEAs caused by thermal and mechanical processing. Characterization methods of HEAs. This section lists the characterization techniques, which are most frequently used to study HEAs. The structure of these alloys is mainly studied by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and optical microscopy. The methods for properties measurements are also briefly reviewed. Application of HEAs. This section describes the promising fields of HEAs application. It can be utilized in the aerospace, aircraft, and nuclear industries as well as for car manufacturing, acoustelectronics, and in the design of microwave devices. Russian-language publications on HEAs. This section lists the studies, published in the Russian language as well as the thesis, done in Russian universities.