Russian Journal of Non-Ferrous Metals最新文献

An effective approach to increasing the fatigue resistance of metal products is to create residual compressive stresses on the surface of the product using surface plastic deformation (SPD) processing SPD. In the present study, with the help of the finite element analysis, one of the effective SPD methods, the process of abrasive-free ultrasonic finishing (AFUF), is studied. Another well-known approach to improving mechanical characteristics, including the fatigue resistance, is the creation of an ultrafine-grained (UFG) structural state in the product. This study is devoted to investigation of the stress–strain state of a UFG workpiece subjected to SPD by the AFUF method using the finite element analysis. Commercially pure Grade 4 titanium in the UFG state obtained by the equal channel angular pressing “conform” method (ECAP-C) is chosen as the workpiece material. In the course of the study, the stress–strain state of the deformation zone after a single impact of an indenter with subsequent unloading is analyzed in the elastoplastic formulation of the problem. The effect of the oscillation amplitude and geometrical characteristics of the indenter on residual radial stresses, including their depth of occurrence, average normal stress, and the accumulated effective strain, has been analyzed. It has been established that, with an increase in the indenter radius, the value of the accumulated effective strain (e) decreases. The behavior of distribution of the e parameter shows a gradient character with its values decreasing from the surface to the center of the workpiece. An analysis of the simulation results shows that the residual radial stresses in the region of the deformation zone are predominantly compressive stresses and, accordingly, allow increasing the fatigue resistance of the final product. It has been established that, with an increase in the indenter oscillation amplitude, the values of residual radial stresses also rise, with their maximum achieving 540 MPa at the amplitude of 75 µm and the depth of occurrence of these stresses reaching 0.3 mm. Increasing the indenter radius, or, in other words, in fact, the contact surface area, leads to an increase in the residual radial compressive stresses, which turns out to be an almost linear increase.

In investigation of the aggregative stability of disperse systems by sediment volumetry, a violation of the structure of water in the contact area causes formation of nanobubbles, whose coalescence leads to appearance of hydrophobic attraction forces. A change in the aggregative stability of aqueous dispersions of particles can be interpreted in such a way that ingress of water molecules having a high potential of interaction with molecules of the medium in the interfacial gap between particle surfaces and outflow of water molecules exhibiting high intensity of interaction with a solid surface from the interfacial gap between particle surfaces is difficult. Excess osmotic pressure between hydrophilic surfaces leads to their hydrophilic repulsion, and excess osmotic pressure of the surrounding water (reduced osmotic pressure between surfaces) leads to hydrophobic attraction of the surfaces. To change the result of flotation, it is sufficient to bring a heat flow to a nanoscale-thick liquid layer, within which action of forces of structural origin is localized, determining the stability of wetting films. To increase the temperature in the interfacial gap between the particle and the bubble using the heat of water vapor condensation, as a gas for flotation, a mixture of air and hot water vapor is proposed. The developed flotation method has been tested in flotation of gold ores. The efficient steam flow rate determined from the results of a factorial experiment is 10.7 × 10^–3 kg/(s m²), with the xanthate flow rate being 1.74 g/t. In the rough flotation operation, the jet method of constructing a flowsheet is used, which provides for combination of the initial feed and rough concentrate. In comparison with flotation of ores according to a factory scheme, the yield of a concentrate sent to hydrometallurgical processing is smaller by 23.4 rel. %, with the achieved level of gold recovery remaining the same.

Insufficient understanding of the nature of the interfacial interaction of reinforcing particles with the matrix alloy during repeated remelting of cast composite materials is one of the problems that limit the increase in the volume of their industrial application. This work is aimed at establishing the effect of repeated remelting of AK12 + 10 vol % SiC aluminum matrix composites on the retention and chemical stability of silicon carbide reinforcing particles. It is shown that an increase in the number of remelting iterations was not accompanied by the appearance of new phases at the interfaces between particles and the matrix, which indicates the stability of the SiC reinforcing phase in aluminum–silicon melts under the considered temperature–time and concentration conditions. During repeated remelting of aluminum matrix composites with silicon carbide, the degree of particle distribution uniformity shifts toward a more uniform distribution (on average 0.81046 at the first iteration of remelting, 0.6901 at the second, and 0.5609 at the third) and some decrease in their average sizes occurs (from 70.74 µm at the first iteration to 65.76 µm at the second and 61.21 µm at the third), apparently owing to particle fragmentation, leading to an increase in the amount of a finer fraction. At the same time, the share of the area occupied by particles in the segments of the section under consideration remains practically unchanged (10.9293, 10.9607, and 11.6483% in the first, second, and third iterations of remelting, respectively). In the course of repeated remelting of aluminum matrix composites of the Al–SiC system, processes of redistribution of reinforcing particles occur, leading to the destruction of agglomerates even in the absence of intensive mixing by an impeller. Because of this, the uniformity of particle distribution in the structure of ingots of secondary aluminum matrix composites can be significantly improved.

In this research, the effects of different stirring speeds on the molten pool flow and the internal flow field of the molten pool were studied by water simulation and numerical simulation. The software ANSYS ICEM, ANSYS FLUENT, and Tecplot 360 were used to model, compute, and post-process, respectively. A physical model made of plexiglass was used to conduct the water simulation experiment. The advantages of vortex smelting reduction were analyzed theoretically and verified by high-temperature experiments. At a stirring speed of approximately 150 rpm, vortex smelting reduction was not only beneficial to the uniform dispersion of the material particles in the molten pool, increasing the reduction reaction rate, but it was also beneficial to the separation of slag and metal. The experimental results confirmed the advantages of vortex smelting reduction, contributing to the recovery of iron in high-iron red mud.

Additive manufacturing, which includes a set of technologies for manufacturing complex-shaped products with the required set of properties, is currently being widely developed. Most additive technologies are associated with the manufacture of the product by melting and fusion of metal powder particles by means of laser radiation. Eutectic aluminum alloys of the Al–Ca, Al–Ce, Al–La, and Al–Ni systems, which have excellent casting properties, are supposedly promising for use in additive technologies. However, there is very little information in the literature on the effect of laser processing on such eutectic structures. In this regard, the work investigated the effect of laser radiation on the structure and mechanical properties of samples from eutectic compositions, namely, Al–8% Ca, Al–10% La, Al–10% Ce, and Al–6% Ni. To do this, the continuous laser modification of their surfaces was carried out. The level of hardening was evaluated by measuring the microhardness of the modified surface. The mechanisms of fracture of specimens under tensile testing have been established. It is shown that, in the structure of the modified surfaces of samples of four alloys, the distribution of the second component becomes more uniform compared to the structure of the base metal. In the Al–8% Ca alloy, the greatest hardening effect is observed, which, however, contributes to embrittlement under tensile stress. However, the modified Al–8% Ca alloy is of interest because of its increased hardness and therefore possibly increased wear resistance. On the contrary, laser modification of the surfaces of the Al–10% Ce, Al–10% La, and Al–6% Ni alloy samples provides a lower hardening effect, but increases their tensile strength with the formation of a ductile or mixed ductile and brittle fracture. The results obtained confirm the prospects of using alloys of the Al–Ca, Al–Ce, Al–La, and Al–Ni systems in additive manufacturing.

The paper is devoted to the study of the combustion kinetics and mechanisms of elemental mixtures in the Zr–Mo–Si–B system, as well as the analysis of phase and structural transformations in the combustion wave. A thermodynamic analysis of possible chemical reactions occurring in combustion wave is carried out. In the range of 298–2500 K, the reaction of ZrB₂ formation is preferred. Above 2200 K, the formation of MoB becomes more thermodynamically advantageous compared to MoSi₂. Estimates of the phase stability of synthesis products have shown that the phases ZrB₂, MoSi₂, and MoB are in equilibrium. The experimental dependences T_c(T₀) and U_c(T₀) are linear, which implies an unchanged combustion mechanism at T₀ = 298–800 K. Preheating leads to an increase in U_c. Similarly, the increase in the proportion of Zr and B in the mixture has a similar effect, which increases heat emission and T_c. With a minimum content of Zr and B, the interaction of Mo with Si with the formation of MoSi₂ by the mechanism of reaction diffusion is decisive. With an increase in the proportion of Zr and B, the rise of T₀ to 750 K does not affect the T_c. The E_eff values (50–196 kJ/mol) confirm the significant influence of liquid-phase processes on the combustion kinetics. The mechanism of structure formation has been studied. In the combustion front, a Si–Zr–Mo melt is formed. The primary grains of ZrB₂ and MoB crystallize from this melt as it is saturated with boron. At the same time, the melt spreads over the surface of Zr and Mo particles. This leads to formation of ZrSi_x and MoSi_x films. Core-shell structures are formed behind the combustion front, which disappear as they move towards the post-combustion zone. The phase composition of the products is formed in the combustion front in less than 0.25 s.

In order to improve the overall properties of cast Al–Cu–Mn alloy, the effect of Zr content on the microstructure and corrosion resistance of Al–Cu–Mn alloy was investigated. The microstructures of the alloy were analyzed by scanning electron microscopy and transmission electron microscopy, and the corrosion resistance of the alloy was tested by exfoliation corrosion (EXCO), intergranular corrosion (IGC) and electrochemical corrosion tests. The results show that the corrosion resistance of Al–Cu–Mn alloy with 0.2 wt % Zr is superior to other alloys. Specifically, the EXCO phenomenon of the alloy is lighter, and the rating of EXCO is EA. The IGC depth of the alloy can reach the lower value (56.3 μm). The electrochemical self-corrosion potential (E_corr), corrosion current (i_corr) and corrosion rate (R_corr) are –0.8046 V, 0.0028 mA m^–2 and 0.1558 mm a^–1, respectively. The improved corrosion resistance of the Al–Cu–Mn alloy with 0.2 wt % Zr resulted from the formation of the T phase and more uniform dispersive distribution of the Al₃Zr phase in the alloy.

The effect of warm rolling and three aging treatments on microstructure and mechanical properties of 7075 alloy was investigated via optical microscopy, electron backscattered diffraction, transmission electron microscopy and tensile tests. 7075 alloy was warm rolled and then were solution treated followed by three different aging processes viz., one-step, two-step and three-step aging. Results show that the ductility of 7075 alloy is improved during warm rolling and the strength increases with increasing total warm-rolled reduction. The average grain size decreases after aging compared with that in the solution state. The microstructure of the one-stage aged sample consists of elongated grains and equiaxed grains while almost completely equiaxed microstructure is obtained in solutionized, two-stage and three-stage aged samples. The distributions of dislocations, precipitates and grain refinement influence mechanical properties together. The one-stage aged sample possesses a combination of acceptable strength and excellent ductility. Two-stage and three-stage aged samples have higher elongations but lower strengths.

The results of studies of interaction between titanium melts and silica-containing casting mold are presented. Pure silicon and compounds of titanium oxides and silicides have been detected by X-ray diffraction analysis in the contact zone. The problem of negative influence of the mold on the casting is solved by using thermochemically resistant monocorundic forms on an alumina sol binder and corundum filler. For casting according to meltable models, a composition of a refractory suspension with special additives has been developed, which will improve the wetting of models with suspension, as well as increase the strength of the mold. In the article, there are studies of sedimentation properties of suspension. A method has been developed for accelerated curing of sequentially applied layers of refractory suspension by drying in vacuum and subsequent chemical curing with a gaseous reagent. The formation time of one layer is reduced from 3–5 h to 20–30 min. Comparative studies of the kinetics of convective drying and dehydration in vacuum of alumina sol binder have been conducted. The process of removing moisture increases by 2–6 times once in a vacuum of 5–10 kPa. The method of X-ray phase analysis has made it possible to study the conversion of alumina sol during high-temperature heating. The stable phase α-Al₂O₃ in the mold shell is obtained when the calcination temperature rises to 1300–1350°C, and the strength of 9–12 MPa is also achieved when sintering additives are added to the suspension. Recommendations are given for additional protection of refractory ceramic layers after evacuation and drying: treatment of the last layer with gaseous hardeners and application of a polyvinyl acetal solution with a density of 1100–1200 kg/m³. The proposed technological solutions will make it possible to increase both the efficiency of the technological process of forming and casting of titanium alloys and the quality of castings.

Adequate heat input provided by the proper combination of friction stir welding (FSW) parameters is critical to sound welding. Optimum parameter setting requires exhaustive trials and extensive experiments, which require considerable time, resources, and cost. This study uses simulation and modelling approaches to generate three significant tool-work heat flux generating interfaces (tool shoulder, lateral and bottom surfaces of the pin). The temperature data was acquired by performing nine experiments on 4 mm thick AA6060-T5 sheets. The effects of significant FSW parameters (Tool Rotational Speed (TRS) and welding speed (WS)) on the heat input were modelled. The calculated heat input rates at the shoulder and pin surfaces (Q₁, Q₂, and Q₃) were numerically estimated. The experimental data was converted into a mathematical model using the response surface method to study the effect of welding parameters on heat input from each of the three surfaces. The analysis of the results showed that among three interfaces, the shoulder provides the most significant heat input due to the immense friction between this surface and the parts to be welded. The interaction between the main factors produced little heat on the three surfaces. The ANOVA test showed that the three models are a good approximation of the results of both experiments and theories.