Metallurgy of Rare and Precious Metals
This review is devoted to the review of current trends in the use of rare-earth metals (REM) in two major scientific and technical fields – the production of magnetic and luminescent materials. The reviews show that it is REM that gives this product unique properties. The information on the content of matrix and alloying components, their influence on achieving the required characteristics of the most popular magnetic materials is systematized. The prospects of new combinations of rare-earth metals in the further progress of the production of magnetic materials for various purposes are shown. Along with the traditional cobalt-samarium and neodymium-iron-boron compositions, new magnetic materials with increased hysteresis properties and temperature-time stability have been developed, phases with variable valence have been synthesized, which are used as memory elements in information systems. The article also reviews and summarizes the results of studies in another important area of REM application – the creation of luminescent materials. Phosphors based on compounds of rare earth metals are used in the production of high-pressure mercury lamps with improved characteristics, X-ray screens, high and low pressure fluorescent lamps, screens for electron-optical converters. Narrow-band phosphors based on REM compounds are of interest for lamps used in plant growing, especially for areas with a cold climate, where year-round plant growth is possible only with the use of additional radiation sources.The trends in the synthesis of luminescent materials using various rare-earth metals and their combinations are revealed. Attention is turned to the need to use chemically pure precursors of rare-earth metals in the creation of such materials. The prospects of creating nanophosphors, as well as the improvement of synthesis methods and diagnostic methods, are noted.
Pressure Treatment of Metals
The process of hot die forging of AK4-1 aluminum alloy billets for the piston of an internal combustion engine (ICE) for an unmanned aerial vehicle (UAV) was simulated using the Deform-3D software package. The object of research was an ICE piston mounted on one of the UAV types of Russian production. Simulation was performed using the following parameters: tooling and billet temperature was 450 °C, ambient temperature was 20 °C, punch speed was 5 mm/s, and Siebel friction index was 0.4. Rigid plastic medium was chosen as a material model. The number of elements (6000) was selected so that at least 3 elements fit in the narrowest section of the part. Thus, as illustrated by the piston die forging, computer simulation in the Deform-3D software makes it possible to develop hot die forging processes for making aluminum alloy billets for UAV ICE pistons. At the same time, computer simulation can be used to evaluate the power parameters of the hot die forging process, study the nature of billet forming in die forging, make necessary adjustments to the virtual process, and develop the design of a die forging tool in order to select the most effective process solutions when designing a real process. The described computer simulation technique can be extended to other aluminum alloy die forgings.
A comparative simulation of hot radial shear rolling (RSR) of billets made of a superelastic Ti–Zr–Nb and a commercial VT6 alloy was performed using the QForm finite element modeling program. Rolling in 48 modes with a variable feed angle and elongation ratio at 4 levels and initial rolling temperature at 3 levels was investigated for each alloy. The Ti–Zr–Nb alloy rheology during hot deformation was determined experimentally by hot upset forging and imported into the QForm program. The presence of maxima on the flow curves at the initial stage of deformation, which are absent in the VT6 alloy, is revealed. Simulation results are presented in the form of fields of the stiffness coefficient, strain rate intensity, cumulative strain degree in the maximum reduction section depending on the rolling mode. General regularities of the Ti–Zr–Nb and VT6 behavior in RSR are similar. The gradient of the fields studied decreases, and the roll pressure and torque increase with an increase in the feed angle and elongation ratio. The initial rolling temperature does not significantly affect the deformation pattern, but it significantly affects the roll pressure and torque. At the same time, the experimental alloy demonstrated the greater tendency to localize deforming forces in the near-contact zone and to increase the gradient of stress-strain state parameters over the billet section. The study of the tightening shape and depth of rolled billet ends showed that the Ti–Zr–Nb alloy has a 3.5–9.6 % greater tightening depth. It is shown that experimental alloy rolling requires 1.6–2.4 times higher roll pressure and torque as compared to the commercial alloy.
The article proposes a variant of the rheological model of hot deformation – the law of hyperbolic sine, which, in contrast to the standard one, takes into account not only the strain rate and process temperature, but also the strain ratio. Material constants included in the law of hyperbolic sine are replaced by polynomial functions of the strain ratio with coefficients calculated using the corresponding method developed. The paper describes applications of the rheological model proposed in low-density aluminum-lithium alloys 1424 of the Al–Mg–Li–Zn system and V-1461 of the Al–Cu–Li–Zn system, for which flow curves in the temperature range 400–480 °C and strain rate range 1–60 s–1 up to a strain ratio of 0.6 are defined by physical simulation at the Gleeble 3800 unit. The influence of the initial material state was also investigated – samples were taken from both the ingot and hot-rolled plates. Constants were determined for the rheological model of hot deformation including the Zener–Hollomon parameter and the law of hyperbolic sine for the entire range of stresses and strains. After approximating the dependences of the model parameters on true strains with a 4th degree polynomial law, a rheological model was created that describes the alloy behavior in the temperature-rate range under study. The features of changes in hyperbolic sine law parameters depending on the strain ratio were established. It was shown that, in general, parameters for the cast material are higher than for the rolled one. A comparison between the standard and proposed models showed that the use of the standard model over the entire strain interval leads to too high flow stress values (up to 12 %).
Physical Metallurgy and Heat Treatment
Titanium alloys have been used for medical purposes for over 60 years. They are used in the manufacture of artificial heart valves, stents of blood vessels, endoprostheses of bones and joints (shoulder, knee, hip, elbow), for auricle reconstruction, in facial surgery, and also as dental implants. In first-generation materials (such as commercially pure titanium or VT6 alloys), the matrix consisted of the α-Ti phase or α-Ti and β-Ti mixture. Unfortunately, implants made of first-generation materials require replacement after 10–15 years of usage. This is due to the degradation of implants and loss of contact with the bone. Recently, these materials have been replaced by β-Ti alloys. These second- generation materials make it possible to exclude the harmful effect of aluminum and vanadium ions released during the gradual implant corrosion, and their elastic modulus is closer to the values for living bone than those for α and α+β alloys. Important areas in the development of β-Ti alloys include increasing their mechanical strength, fatigue strength, corrosion resistance and biocompatibility. New methods for the production and thermo-mechanical processing of titanium alloys arise and develop such as additive technologies or severe plastic deformation. Expensive alloying elements (such as tantalum, zirconium or niobium) are quite successfully replaced with cheaper ones (for example, chromium and manganese). As a result, the properties of titanium implants are gradually getting closer to that of the human bone, and their service life is steadily increasing. Therefore, this paper describes a comparative analysis conducted in relation to β-titanium-based alloys for medical applications.
The study covers the influence of electromechanical surface treatment (EMT), non-abrasive ultrasonic finishing (NAUF), their complex influence with subsequent aging on the fatigue life and surface microhardness changes. Samples for research were made of VT22 transition alloy rods after standard thermomechanical treatment. EMT was carried out by sample surface rolling with a roller and applying a high density current between them. As a result, surface thermomechanical treatment was carried out with the local fast surface heating and cooling. NAUF were implemented by shock treatment with an ultrasonic emitter striking on the treated surface. This revealed 1.8 times higher fatigue life when loading by rotational bending (with amplitude of 0.5σв) for samples after NAUF in comparison with the untreated initial state together with a slight increase in microhardness (up to 16 %). EMT reduces microhardness and fatigue life by almost 20 % and 70 %, respectively. EMT + NAUF complex processing has an insignificant effect on microhardness, but it increases fatigue life by 40 % with respect to EMT. Aging at 450 °C for 5 hours increases microhardness after EMT by 30–40 % with a simultaneous increase in fatigue life by 2 times. The aging of samples subjected to EMT + NAUF revealed virtually no increase in microhardness, but increased fatigue life by almost 3 times (as compared to EMT). According to fractography results, the reduction in fatigue life after EMT is associated with a reduction in the crack initiation stage, which virtually excludes this stage of fatigue damage accumulation from the overall sample fatigue life.
The paper studies specific features of the Al–2.5%Fe–1.5%Mn alloy microstructure formation depending on the cooling rate during casting and laser melting. As-cast microstructure analysis showed that with an increase in the cooling rate during crystallization from 0.5 to 940 K/s, the primary crystallization of the Al6(Mn,Fe) phase is almost completely suppressed with the non-equilibrium eutectic volume increasing to 43 %. The Al–2.5%Fe–1.5%Mn alloy microstructure after laser melting features by the presence of dendritic-type aluminum matrix crystals with an average cell size of 0.56 μm surrounded by an iron-manganese phase of eutectic origin with an average plate size of 0.28 μm. The primary crystallization of the Al6(Mn,Fe) phase is completely suppressed. Such a microstructure is formed at cooling rates of 1.1·104 to 2.5·104 K/s, which corresponds to the cooling rates implemented in additive technologies. Regions consisting of Al6(Mn,Fe) phase primary crystals formed by the epitaxial growth mechanism were revealed at the boundary between the track and the base metal and at the remelting boundary. The smaller the eutectic plates and dendritic cell located in the epitaxial layer, the more disperse the primary crystals in the remelting zone. The Al–2.5%Fe–1.5%Mn alloy after laser melting has high hardness at room temperature (93 HV) and good thermal stability after heating up to 300 °C (hardness slightly decreases to 85 HV), and its calculated yield strength is 227 MPa. Combined with the ultra-fine microstructure formed, high processibility during laser melting, hardness at room temperature, and high calculated yield strength, Al–2.5%Fe–1.5%Mn is a promising alloy for use in additive technologies.
For the first time, an intermetallic alloy based on the Heusler phase – Cu2TiAl – was obtained by self-propagating high-temperature synthesis (SHS) in the Cu–Ti–Al reaction mixture. The frontal combustion modes of green mixture compositions and phase formation processes during synthesis were studied. The products obtained were studied by X-ray diffraction analysis including high-temperature diffractometry with stage heating up to 900 K, scanning electron microscopy, differential thermal analysis (DTA), and some physical properties were studied. Also, electrophysical and magnetic measurements were carried out for the obtained alloy. The results of X-ray analysis and SEM using energy-dispersive analysis (EDA) showed that the Heusler phase content in the synthesized product is at least 82 %. The product also contains copper (Cu9Al4) and titanium (Ti3Al2) aluminides. The temperature dependence of the synthesized product electrical resistivity was measured for a wide temperature range of 90–1000 K, which was 0.3 μmm at T = 300 K. The metallic type of the conductivity for the samples obtained and the abnormal behavior of the electrical resistance temperature curve in the region of Т = 770÷790 K were revealed. Thermal analysis was used to measure the melting point of the synthesized product and to reveal additional heat effects at Т = 788, 848 and 1248 К associated with possible phase transitions in the Cu2TiAl intermetallic compound. A possible mechanism of phase transitions is considered in accordance with the Cu–Ti–Al system phase diagram. Magnetic measurements results showed that intermetallic samples obtained by the SHS method feature by weak ferromagnetic properties with residual magnetization of 0.069 A·m2/kg.
ISSN 2412-8783 (Online)