No 3 (2023)

Current trends in the development of mechanical engineering impose increasingly stringent requirements for the performance characteristics of manufactured goods. The main parameters characterizing the quality of a product as a whole are the physical, mechanical, and geometric indicators of the working surfaces of the compound units. In domestic practice, a machined surface is mainly characterized by a rather limited number of parameters (no more than 6), such as the average microroughness height, the microroughness height at 10 points, etc. However, their use is not enough to manufacture competitive products in the modern conditions. For example, international ISO/ASME/DIN standards include a much broader set of parameters required to accurately describe the performance properties of a surface. The paper analyzes the approaches to the formation of requirements for the microgeometry of the working surfaces of parts used in modern mechanical engineering. Based on the analysis, the author proposed and mathematically substantiated a general approach to modelling surface texture characteristics, which allows describing adequately the surface using a new parameter – the profile physical coefficient, since it is virtually impossible to directly compare the technologies developed in Russia with foreign analogues based on the current standards. First, the profile physical coefficient was determined at the section level. Next, it was decomposed into a Fourier series for the two-dimensional and three-dimensional cases. The paper presents the analysis of the new parameter applicability on the example of a product obtained by honing. The author concluded about the applicability of this parameter and the necessity to develop a comprehensive methodology based on it for evaluating the surface after machining.

Analysis of the processing equipment structures when designing according to the temperature criterion is a necessary guarantee of ensuring the required performance characteristics. The presence of a significant number of parts in the processing equipment units and mechanisms requires, when designing, the prediction of the heat flow passage through the joints. When simulating contact thermal resistance, the variety of requirements for a joint can be taken into account by introducing a pseudolayer into the contact zone. The paper presents test results of the proposed regression dependence of the temperature change when the heat flow goes through the pseudolayer obtained considering four significant factors: the pseudolayer thickness, the nominal pressure, the material yield strength, and the actual contact zone location. The adequacy of the specified regression dependence was verified experimentally and applying numerical simulation using large-block finite elements. To describe the process of heat transfer in the thermal model elements, the authors determined contact thermal resistances for several conditions for the heat flow propagation: from one finite element to another within one part; from one finite element to another located in an adjacent part; heat flow passing through closed cavities; heat flow propagation into the environment for finite elements located on the outer (free) contour of the part. The experiments showed a good agreement between the experimental data and the simulation results. The application of large-block finite elements based on the proposed contact thermal resistance model allowed bringing the FE simulation technique to engineering use without complex software.

Resistance welding in large-scale manufacturing is carried out with a significant number of disturbances, the cumulative effect of which may exceed the capabilities of modern control equipment. Most resistance welding control systems used in industry to compensate for existing disturbances provide welding current phase control depending on the measured parameters characterizing the process of welded joint formation. The efficiency of such controllers is largely determined by the accuracy of measuring and setting the phase control parameters, which include the opening and conduction angles of welding thyristors. The paper shows that when switching on a contact machine, a phase shift of the mains voltage occurs in the load mode relative to the mains voltage in the idle mode. Using a simplified electric equivalent circuit of a contact welding machine, the paper describes the nature of the phase shift of the mains voltage. Circuit active resistance and inductance are selected as parasitic parameters of the mains. The authors simulated the electrical processes in the contact machine according to the three-loop equivalent circuit. The study shows the influence of mains parasitic parameters on the phase regulation stability, the features of the obtained current and voltage oscillograms. Depending on the mains and contact welding machine parameters, the phase shift magnitude ranges from fractions to units of an electrical degree. With welding current parametric stabilization by the mains voltage, the influence of mains parasitic parameters can be neglected. When the regulator operates in the mode of maintaining the secondary current numerical value, a decrease in the generated current relative to the specified one is observed. The authors proposed and tested a technique for determining the parasitic parameters of the supply mains based on the results of a short circuit test.

Magnesium alloys are promising materials for aviation, automotive engineering, and medicine, however, due to the low resistance to stress corrosion cracking (SCC), their wide application is limited. To create alloys with high resistance to SCC, a comprehensive study of this phenomenon nature is required. Previously, it was suggested that diffusible hydrogen and corrosion products formed on the magnesium surface can play an important role in the SCC mechanism. However, the contribution of each of these factors to the SCC-induced embrittlement of magnesium and its alloys is understudied. Since the influence of diffusible hydrogen on the mechanical properties of metals increases with the strain rate decrease, the study of the strain rate sensitivity of the SCC-susceptibility of magnesium alloys is a critical task. In this work, the authors studied the effect of the strain rate in the range from 5·10⁻⁶ to 5·10⁻⁴ s⁻¹ on the mechanical properties, the state of the side and fracture surfaces of the as-cast commercially pure magnesium and the AZ31 alloy before and after exposure to a corrosive environment and after removal of corrosion products. The study identified that the preliminary exposure to a corrosive medium leads to the AZ31 alloy embrittlement, but does not affect the mechanical properties and the fracture mode of pure magnesium. The authors found that the AZ31 alloy embrittlement caused by the preliminary exposure to a corrosive medium appears extensively only at the low strain rate and only if the layer of corrosion products is present on the specimens’ surface. The study shows that a change in the strain rate has little effect on the mechanical properties of pure magnesium. The authors concluded that the main cause of the AZ31 alloy embrittlement after soaking in a corrosive medium is the corrosion products layer, which presumably contains the embrittling agents such as hydrogen and residual corrosive medium.

In the process of formation of composite coatings, partial dissolution of strengthening particles (most often carbides) in the matrix is possible; therefore, in some cases, the material creation mode is chosen taking into account the volume fraction of primary carbides not dissolved during coating deposition. The methods currently widely used for calculating the volume fraction of carbides in the structure of composite coatings (manual point method and programs implementing classical computer vision methods) have limitations in terms of the possibility of automation. It is expected that performing semantic segmentation using convolutional neural networks will improve both the performance of the process and the accuracy of carbide detection. In the work, multiclass semantic segmentation was carried out including the classification on the image of pores and areas that are not a microstructure. The authors used two neural networks based on DeepLab-v3 trained with different loss functions (IoU Loss and Dice Loss). The initial data were images of various sizes from electron and optical microscopes, with spherical and angular carbides darker and lighter than the matrix, in some cases with pores and areas not related to the microstructure. The paper presents mask images consisting of four classes, created manually and by two trained neural networks. The study shows that the networks recognize pores, areas not related to the microstructure, and perfectly segment spherical carbides in images, regardless of their color relative to the matrix and the presence of pores in the structure. The authors compared the proportion of carbides in the microstructure of coatings determined by two neural networks and a manual point method.

Friction stir processing is one of the modern methods of local modification of the surface of aluminum alloys in the solid-phase state, which provides the dispersion of structural components. In heat-hardened aluminum alloys with a matrix type structure, heat treatment following after friction stir processing can lead to abnormal grain growth in the stir zone. However, in alloys with the structure close to microduplex type, a fine-grained structure can be formed after friction stir processing and heat treatment. This work is aimed at evaluating the possibility of increasing the microstructure thermal stability of the AK4-1 (Al–Cu–Mg–Fe–Si–Ni) matrix-type aluminum alloy. For this purpose, AK12D (Al–Si–Cu–Ni–Mg) aluminum alloy with the structure close to microduplex type was locally mixed into the studied alloy by friction stir processing. Subsequent Т6 heat treatment was carried out according to the standard mode for the AK4-1 alloy. Studies showed that the stir zone had an elliptical shape with an onion-ring structure. This structure comprised alternating rings with different amounts and sizes of excess phases. At the same time, in the stir zone center, the width of rings and the average area of excess phases were larger compared to the stir zone periphery, where the width of rings and the average area of particles were smaller. The average area of excess phases in the rings with their higher content was smaller than in the rings with their lower content. This distribution of excess phases leads to the formation of a fine-grained microstructure, where the average size of grains depends on the interparticle distance in the α-Al solid solution.