Solid State Phenomena Vol. 313

Abstract: Formation of nanostructure states on the surface of materials exposed to concentrated flows of energy is one of the relevant problems of modern materials processing. In the paper the authors describe the mechanism of the micro-scale droplets formation based on the study and modeling of the physical processes and technological aspects of the interaction between the heterogenic plasma flows and the molten substance at the electrode tip. The authors show new physical mechanisms and criteria for micro-and nanoparticles origination, develop a physical-mathematical model of the interaction between the molten metal and the plasma discharge with imposed high-frequency pulse action.

Abstract: The change in ultrasound rate in the plastic deformation of high-chromium X39Cr13 stainless steel with ferrite–carbide structure (initially), martensite structure (after quenching), and sorbite structure (after high tempering) is investigated. The loading curve is different for each state. In the initial state, the loading curve is practically parabolic. In the martensitic state, linear strain hardening is the only stage. In the sorbitic state, a three-stage curve is observed. The structure of the steel after different types of heat treatment is studied by optical and scanning probe microscopy. In parallel with the recording of the loading curve, the change in properties of the ultrasound surface waves (the Rayleigh waves) in the steel under tension is measured. The structure of the steel determines not only the type of deformation curve in uniaxial extension but also the dependence of the ultrasound rate on the strain.

Abstract: One of the well-known methods for increasing wear resistance, especially for friction pairs, is surfacing wear-resistant materials on the working surfaces of mating parts [1, 2, 3]. Less expensive grades of steel can be used as the main material in the manufacture of parts, and the surfacing materials in this case must have increased characteristics: mechanical, corrosion and radiation resistance, heat resistance, wear resistance; good anti-friction properties, i.e. more expensive [4, 5]. This significantly reduces the cost of manufacturing or repairing parts, especially in mass production, since cheap grades of steel are used for their manufacture [5, 6]. Improvement of parts by gas powder laser cladding is carried out in shipbuilding, energy, oil and gas and mining industries, in the aviation industry and others. Note that although the method of coating appeared a long time ago, various methods of applying surfaced coatings are still being developed and refined [5,6]. In gas powder laser cladding, coatings are obtained by forcing the powder flow directly into the laser radiation zone [7, 8]. The powder particles are heated in the laser radiation zone and fall on the treated surface (substrate). It is known that the powder particles melt only after they hit the substrate [3, 4], but at the same time the surface layer of the base metal melts. After heating and melting the substrate, a liquid melt bath is formed, which, along with the molten powder, contains a significant part of the base metal components [7, 8]. The intensity of saturation of the surfacing metal with the substrate components is characterized by the proportion of the base metal in the cross section of the track, which is determined by the mixing ratio [9, 10, 11]. It is equal to the ratio of the area of the melted substrate to the sum of the areas of the melted substrate and the track as a percentage and depends on the cladding modes, which is determined after metallographic studies of the structure of protective coatings [8].

Abstract: In order to restore worn parts during repair work, it is often used to press the repair bushings. While assembling joints, cracks may form in the spanning part (bushing), as it may have surface and structure defects. Therefore, an urgent task is to increase the crack resistance of joints with tension during its assembly. The paper examines the influence of the geometry of the spanning part on the process of cracking. To assess the effect of defects on the crack formation, the method of modeling defects with a surface crack of a semi elliptical shape was used. The crack resistance is estimated using the force criterion of fracture mechanics – the stress intensity coefficient. As a result, it was found that the ratio between the wall thickness of the enclosing sleeve and its outer diameter has a significant effect on the crack resistance of the joint with tension, which allows reducing the origin probability and crack development during its pressing by varying these parameters. While assembling joints, the following methods are often used: mechanical-using a press and thermal with heating of the covering part. When the method of joint with cooling of the covered part, the greatest strength of the joint is achieved, but the probability of cracking increases. This is because when the temperature of the enclosing sleeve decreases in contact with the cooled shaft, the critical stress intensity coefficient (fracture toughness), which is a mechanical characteristic of crack resistance, decreases. To reduce the effect of cooling the sleeve, you can use a combined method of assembling the joint, in which the covered part is cooled and the covering part is heated. It is shown that to assess the fracture resistance at low temperatures, it is optimal to conduct full-scale tests, where the cooling and heating temperatures of the press joint parts are the experimental factors.

Abstract: The structure and misorientations of grain boundaries of ultrafine-grained nickel subjected to rolling and forging at liquid nitrogen temperature are studied. It is shown that as a result of rolling in UFG nickel obtained by the ECAP the forming of a band fragmented structure with the formation of special twin boundaries Σ3 is observed. An increase in the strain rate (forging) leads to the appearance of localized deformation bands in which the formation of new small grains is observed through dynamic recrystallization. The development of recrystallization results in increase up to 7% in UFG nickel the fraction of special twin boundaries Σ3 which are similar in nature to annealing twins.

Abstract: This paper presents the results of the investigation of the grain structure formation in the intermetallic compound Ni₃Al under conditions of its high-temperature synthesis under pressure in a powder mixture of nickel and aluminum of stoichiometric composition and the effect of grain size on the strength properties of the synthesized intermetallic compound. The grain structure was investigated by optical metallography, transmission electron microscopy, and EBSD analysis; the ultimate tensile strength of the intermetallic compound was investigated under the tension of the samples in the temperature range from 20 to 1000 °C. It was found that with a decrease in the grain size, not only does the tensile strength of the intermetallic compound multiply increases but also on the anomalous temperature dependence of the intermetallic compound strength there is a significant shift in the maximum strength value to the region of higher temperatures.

Abstract: Heat resistant cobalt-based alloys have found a specific niche in the present-day mechanical engineering due to their unique properties. To begin with, cobalt-based alloys are used as corrosion, heat and wear resistant materials intended for aggressive environments and operation at extreme temperatures, e.g. blades, nozzles, swirlers, rings and other elements of turbines and internal combustion engines. Traditional molding methods applied in the mechanical engineering fail to provide necessary operational and technological characteristics of aforementioned machine parts. Owing to selective laser melting it is possible to reduce a production time and manufacturing costs for machine elements with a complex physical configuration and generate an alloy with an extraordinary structure, which is not found in traditionally combined compounds. A structure of cobalt exists in two crystal modifications: a hexagonal close-packed epsilon phase, a low-temperature phase and a face-centered cubic lattice gamma phase, a high-temperature phase. The alloy hardness is directly related to an amount of a low-temperature phase. The laser melting shortens a laser beam impact time on a powder composition due to a higher power and laser travelling speed. A high value of heat conductivity seems to be the reason for rapid solidification and cooling, which, in their turn, increase a percent of an alpha-martensite phase in an alloy and improve the hardness and wear resistance of machine parts. The reported paper summarizes studies aimed at the development of a stable phase structure three-component alloy (Сo-66 mass % Cr-6 mass % Mo) based on the cobalt-chromium-molybdenum system and mixed up via selective laser melting.

Abstract: The reported paper presents a field pattern of sliding lines in a plastic zone of flow chip formation in the process of cutting metals considered for front and rear angles on the cutting blade other than zero. Equations of sliding lines for these conditions are proposed. Using calculation data, plasticity zone border lines are plotted for positive and negative front angles. The paper suggests methods and calculation data on average stress and plane stress state components in nodal points of the plastic zone of chip formation. The study provides data for plotting distribution diagrams of normal and tangential stresses of contact stresses on work surfaces of a cutting blade.

Abstract: Wear of abrasive tools, including coated abrasives, is an important performance feature. A large number of theoretical works and practical studies are devoted to wear assessment issues. In practice, there are many direct and indirect approaches to measuring wear of an abrasive tool. At the same time, many of them have significant shortcomings such as assessment complexity, high time costs, a significant share of manual labor, low measurement accuracy, the need to use specialized equipment, etc. Based on the foregoing, the development of an effective technique for assessing the coated abrasive wear with the use of information and analytical technologies is very relevant and promising. This work is devoted to the development of an original technique for determining the wear of coated abrasives and studying of the tool surface texture. Some results of studies on the wear of coated abrasives of various grain sizes and manufacturers are presented in the article.