No 3 (2021)

At modern mechanical facilities, the friction-welded joints are getting widespread as the most advanced production technique characterized by high efficiency, processability, cost-effectiveness, and safety. Moreover, it allows producing high-quality joints of a large number of different analogous and opposite metals and alloys. Despite all these advantages, one should consider that metal, in the process of welded joint formation, suffers a local thermo-deformational effect, which causes the gradient nature of the structure and residual strains of a welded joint. These factors directly influence the structure’s working ability and durability under fatigue loads, which are the most common cause for parts failure. The paper contains the assessment of the post-weld tempering influence on the cyclic life of welded joints of 32G2 and 40HN steels produced using the rotational friction welding technique. The authors tested laboratory specimens with welded joints under the high-cycle fatigue using the simulation machine with the two-point fastening of a revolving specimen under the action of even twisting moment. The study involved the statistical processing of the obtained results of cyclic life. Based on the metallographic analysis, the authors identified the weak points in welded points where the fatigue cracks initiation and progress occurred in the initial state and after tempering. The paper presents the fractographs illustrating the fracture mechanism of specimens under the study. The authors identified the influence of different tempering temperature modes on the cyclic life of the studied welded joints and the nature of their fracture. The study shows that tempering at the temperature over 400 °C promotes fracture acceleration under the effect of fatigue loads due to the development of return and polygonization processes in the vulnerable area of the thermomechanical action zone.

The use of epoxy antifriction coatings can significantly reduce thermal stress in the friction zone and expand the coating working temperature interval while keeping high wear resistance. The paper considers the effect of non-activated and activated by surfactants silicate filler – rice husk ash on the physicochemical and mechanical properties of epoxy materials applied as antifriction coatings. All studied samples of rice husk ash, both initial and activated with surfactants, have an alkaline surface nature. The study identified that all cationic quaternary ammonium salts (QAS) reduce the pH of rice husk ash. At the same time, nonionic OXIPAV increases this indicator. Activation of the rice husk ash surface, both by the quaternary ammonium salts and aminosilanes, significantly reduces the porosity of this silicate. In this case, the average pore diameter does not change significantly, and their specific surface area decreases significantly, to a lesser extent, when activated by nonionic quaternary ammonium salts. The application of quaternary ammonium salts and aminosilanes in the amount of 33 % for activation of the surface of the investigated silicate filler reduces its modifying effect in epoxy compositions, regardless of the chemical structure of the surfactants used, which is not a typical effect. Therefore, the authors assumed that the suboptimal concentration of quaternary ammonium salts and aminosilanes was used. The study identified that the optimal concentration of 50 % alcohol solution of KATAPAV is 14.7–21 %. In this range of the QAS content, there is a significant increase in hardness (about 40 %), a slight decrease in wear (about 10 %), and a significant decrease in the coefficient of static friction (up to 2 times). At the same time, the authors observed an increase in adhesion to metal up to 3 times and bending strength up to 25 %. Thus, rice husk ash activated with an optimal amount of quaternary ammonium salts is an effective modifier of epoxy coatings, which improves their antifriction properties and increases wear resistance, hardness, strength, and adhesion characteristics.

The development of the mechanical treatment workflow CAP system is aimed at the solution of a crucial task of reduction of terms and the improvement of quality of multiproduct machining manufactures work preparation, as the existing workflow CAP systems have not got the possibility of fast response to changes in a production situation often arising within the multiproduct manufacture. The authors of this paper developed the workflow CAP system, which contains the requirements of the design activity full automation, design solution multivariance, and the feedback with the engineering process implementation subsystem. The paper deals with the development of a mathematical model and the technique of searching for sustainable levels of selecting design alternatives depending on the production situation for the whole design procedures of the workflow CAP system. The authors prove the application of a mathematical tool of genetic algorithms; describe the mathematical model using its terms. As a gene, the level of selection in a separate project procedure is specified. A chromosome is a set of genes according to the project procedures. The objective function determines the minimum total time of processing of the specified nomenclature of parts based on the ranges of gene aggregates resulting from crossing and mutation operations. The result of the work is the mathematical model and the technique for identifying the sustainable levels of selection in each project procedure ensuring the possibility of self-adjustment of the workflow CAP system depending on the production situation.

It is known that titanium and its alloys are one of the promising materials in the industry, especially in medicine, due to their excellent biocompatibility and corrosion resistance. The latest modern equipment and instruments used in traumatology, orthopedics, dentistry, etc. demand increasingly higher mechanical properties for materials. In comparison with commercially pure titanium, alloys do not have such high corrosion-resistant properties and biocompatibility. In this regard, improving the mechanical characteristics of a pure material is an urgent issue. The authors studied the effect of annealing on the structure and properties of commercially pure grade 4 titanium in the coarse-grained and ultrafine-grained states. The ultrafine-grained state was obtained using high-pressure torsion (HPT) under the pressure of 6 GPa at N=10 revolutions at room temperature. In the microstructure investigated using transmission electron microscopy, the authors could detect particles of precipitated phases after annealing, which had different morphologies. Deformation leads to an increase in the precipitated particles after annealing. The authors carried out an X-ray phase analysis, which showed the approximation of the lattice parameters of the α-phase after deformation and annealing at 700 °C to the values of the parameters of pure titanium. Thus, aging processes occur in the material, accompanied by the decomposition of the supersaturated solid solution and the release of particles of the second phase. The paper shows the results of titanium microhardness measurements in different states. The combined treatment, consisting of HPT at N=5 revolutions, annealing at 700 °C, and additional HPT deformation at N=5 revolutions, allowed obtaining the record strength for commercially pure grade 4 titanium.

Recently, the interest of researchers has focused on a new FCC class (FCC – face-centered cubic lattice) high-entropy alloys (HEA), due to their unique properties – high values of the strain hardening coefficient, good plasticity, and ductile fracture at low test temperatures. Such a combination of properties in an FCC of HEA is achieved by mixing five or more elements in equal atomic proportions. Due to the strong temperature dependence of stresses at the σ_0.1(T) yield point, these alloys have low σ_0.1 values at temperatures above room temperature, which hinders their practical application. A precipitation hardening is an effective way to achieve high strength and is successfully used for hardening HEA FCC. The paper studied the influence of ageing at 923 K for 4 hours and at 1073 K for 18 and 30 hours on the mechanical behavior of single crystals of (CoCrFeNi)₉₄Ti₂Al₄ (at.%) HEA FCC oriented along the [001] direction under tension. Ageing at 923 K for 4 hours and at 1073 K for 18 and 30 hours leads to the precipitation of γ′-phase particles, the size and volume fraction of which depend on the ageing temperature and time. The γ′-phase particles precipitation leads to an increase in stresses at the yield point from 47 MPa (ageing at 923 K, 4 hours) to 226 MPa (ageing at 1073 K, 30 hours) relative to quenched crystals at 296 K. The study identified the dependence of the strain hardening coefficient, plasticity, and the maximum stress level before fracture on heat treatment. The author discussed the reasons for the growth of stresses at the yield point and the strain hardening coefficient upon precipitation of γ′-phase particles.