No 4 (2022)

The paper covers the development and analysis of a model of the true viscous lubricant movement in the working gap of a radial sliding bearing with a non-standard support profile having a fluoroplastic composite polymer coating with a groove on the shaft surface. The authors obtained new models based on the classical equation in the approximation for the “thin layer” and the continuity equation describing the laminar pattern of movement of a lubricant with the viscous rheological properties. The results of the numerical analysis of the developed models allowed obtaining a quantitative assessment of the efficiency of the bearing bush support profile and the polymer-coated shaft with an axial groove. To complete the set of studies and verify theoretical insights, the authors carried out the experimental research. The novelty of the work lies in the development of an engineering calculation technique that allows determining the magnitude of the main tribotechnical parameters of a radial sliding bearing (hydrodynamic pressure, load capacity, and friction ratio) and expanding the area of practical application of the developed engineering calculations. The design of the radial bearing with a fluoroplastic antifriction composite polymer coating, a 3 mm wide groove, and a special support profile ensured the stable shaft ascent on the hydrodynamic wedge, which experimentally confirmed the correctness of the results of theoretical studies of sliding bearings with a diameter of 40 mm at a sliding speed of 0.3–3 m/s and a load of 13–65 MPa.

High-pressure torsion deformation (HPT) is an effective method for transforming the structure of metallic materials, forming a nanostructural state in them, and significantly improving their strength. However, deformation achieved during HPT can be much less than expected due to the slippage. The study of the slippage effect during HPT of various materials is a topical issue. Previously, the authors proposed a simple and illustrative method for assessing slippage and the actual degree of torsion deformation achieved during HPT. Zr–1%Nb alloys, on which many studies of the HPT effect previously have been carried out, are good material for studying the slippage effect during HPT. Therefore, it is possible to compare obtained data with the results of other authors. The paper investigates the HPT impact on the structure and properties of the Zr–1%Nb alloy and demonstrates the slippage effect. The initial disk, prepared for HPT, was cut into two half-disks that were jointly placed on the strikers and exposed to joint HPT for n=¼ revolutions of anvils. The authors evaluated the slippage effect from the view of halves. The study showed that even at the initial HPT stages at n=¼ revolutions, there is a significant slippage of strikers and a sample, and the torsion deformation does not accumulate as expected. The authors analyzed the influence of various HPT modes on the microhardness, structure, and phase composition of the Zr–1%Nb alloy. The study shows that, despite the slippage effect, the Zr–1%Nb alloy is strongly hardened during HPT for one revolution (n=1) and HPT with n=10; the microhardness and tensile strength increase significantly, and up to 90 % of high-pressure ω-phases is formed in the sample. The authors conclude that during HPT, the deformation is implemented not by simple torsion but by the more complex modes.

Oxide layers formed by plasma-electrolytic oxidation (PEO) are characterized by a sufficiently high porosity, which influences almost the whole complex of service characteristics. However, the known data on the integral porosity of PEO-produced layers are rather contradictory, and the pore size distribution in these layers remains understudied.  Pore size distribution in the range of 10 nm to 10 µm (pore geometry was approximated by a spherical shape) was obtained by using analysis of scanning electron microscopy (SEM) images in a wide range of magnifications. Lognormal distribution function fits the shape of pore size distribution sufficiently well. Such distribution indicates the nature of pore formation, which can be related to the thermally activated process of gas emission from a liquid melt, the volume and average temperature of which, in turn, depend on the micro-arc discharge energy. The results of the oxide layer phase composition and crystallites sizes by the X-ray crystallography were described in the present paper. The amorphous component phase composition was estimated by the comparing of the of X-ray spectral microanalysis and X-ray crystallography methods. The thermal conductivity of the intact oxide layer and the polished layer (after the removal of its highly-porous outer part) was evaluated by using of the steady-state method and the laser flash method. The porosity values calculated based on the analysis of SEM-images, and the results of determining the phase composition, including amorphous phases, allowed evaluating the oxide layer thermal conductivity with use of four known analytical models. The results of calculating the thermal conductivity using the Loeb model demonstrate the good convergence with the experimental results obtained in this paper. Modeling results the size of crystallites effect on the oxide layer thermal conductivity significantly less than the porosity and amorphous phase.

One of the promising research areas developing in recent times in the materials science is the development and research of high-entropy alloys containing several metal elements with the concentration close to equiatomic. The interest to them is generated by the fact that such alloys demonstrate the improved mechanical and functional properties. Another promising area improving strength of metallic materials is grain refinement using the severe plastic deformation methods. This work uses both approaches to form an ultrafine-grained (UFG) structure in the high-entropy Fe₃₀Ni₃₀Mn₃₀Cr₁₀ alloy. The paper presents the structure, strength, thermal stability, and corrosion resistance of a high-entropy alloy subjected to the high pressure torsion (HPT). The study of the structure carried out by scanning electron microscopy showed that the application of the HPT deformation leads to the formation of an UFG structure with an average grain diameter less than 200 nm depending on temperature of HPT processing. Microhardness measuring and tensile tests at room temperature showed that after grain refinement, an increase in microhardness and ultimate tensile strength occurs in a high-entropy alloy, which is more than three times higher compared to the initial coarse-grained sample. At the same time, the UFG samples of a high-entropy alloy manifested thermal stability of microhardness after annealing up to temperature of 500 °С. The electrochemical tests carried out in an aqueous solution of 3.5 % NaCl at the temperature of 37 °С demonstrated a high corrosion resistance of the UFG high-entropy alloy.

Linear friction welding is an advanced technology for manufacturing titanium blisks for gas-turbine engine compressors, which are subjected to stringent requirements for cyclic strength and dimensional accuracy. Substitution of conventional butt joints with more technological T-shape joints is a promising area, which provides reducing of the pre-welding machining costs. The introduction of T-form joints requires additional research of thermal distribution specifics and strain-stress state formation in the welding process and after its end. Therefore, the study of residual stresses in titanium alloy T-shape joints produced by linear friction welding is topical. The paper investigates the residual stresses in imitating welded blisk joints. The authors consider the results of welding where the blade imitator has a reamed relief of a smaller section. The finite element model covering forging, cooling, and disassembly of welded specimens is offered. The authors developed the model in ANSYS Workbench to describe the strain-stress state of welded specimens, which allows for estimating the residual stress levels and spreading. The main distinctive feature of the model is an accounting of asymmetric temperature distribution obtained by finite-difference solving of a T-shape joint thermal problem and weld shape simulation obtained as a result of welded joints metallographic research. The presented model allows the evaluation of the residual stresses in joints. The distribution of residual stresses in T-shaped welded joints is specific – compressive stresses existing in a weld are balanced by tensile stresses acting at a distance of 1 mm from the joint. The formation of compressive stresses in a weld is caused by plastic deformation due to the forging force action.