VT6 Titanium Alloy Research Articles

Technological properties of sheet titanium alloys VT6

This paper presents a comprehensive study of the microstructure, mechanical and technological properties (formability and weldability in solid state) of sheet blanks made of titanium alloy VT6, developed specifically for superplastic forming and diffusion bonding (SPF / DB) in PJSC VSMPO-AVISMA Corporation using various technologies. The conducted studies have made it possible to establish that sheets of titanium alloy VT6 with different microstructure in the temperature range of 850 – 920°C exhibit good superplastic properties. Studies of the technological properties of experimental sheets show their good weldability and high formability under superplasticity conditions. The relative length of the pores in the zone of the solid-phase joints is from 5.3 to 1.8 %, the equivalent deformation during SPF is from 1500 to 2700 %. The technological properties of the VT6 alloy in different states are very similar. The results obtained allow us to recommend this experimental sheet alloy for the manufacture of hollow structures by the SPF / DB process under superplasticity.

Selecting a dependence for the approximation of experimental data on secondary creep and creep rupture strength

As a rule, for the evaluation of the mechanical characteristics of a metallic material by secondary creep and creep rupture strength, tests are carried out for uniaxial tension of cylindrical specimens under the influence of a stationary axial force. These mechanical characteristics include the experimental dependence of constant strain rate on nominal stress and the experimental dependence of rupture time on nominal stress. In order not to conduct a large number of experiments, so that these mechanical characteristics can be determined at any nominal stress, one of the two empirical dependencies is used, allowing the corresponding experimental dependences to be approximated with the smallest total error. As such empirical dependences, a power dependence with two material parameters and a fractional power dependence with four material parameters are considered, two of which acquire the definite physical meaning of starting creep stress (the maximum stress at which the strain rate is zero) and break creep stress (the minimum stress at which instantaneous rupture occurs). When choosing an empirical dependence, the author used experimental data obtained by him from mechanical tests for uniaxial tension of cylindrical VT5 and VT6 titanium alloy specimens at 650 °C. The calculated total errors testify that both empirical dependences satisfactorily approximate the considered experimental data.

Morphology and Corrosion Properties of Hydroxyapatite Coating on VT6 Titanium Alloy

Morphology and Corrosion Properties of Hydroxyapatite Coating on VT6 Titanium Alloy

Calculation and investigation of the phase composition of a composite intermetallic layer synthesized on the surface of VT6 titanium alloy from Cu–SiC and Al–SiC powders

The physical and chemical foundations of the technology of cold gas-dynamic spraying of coatings on titanium for two types of charge are considered, in one of which chemically inactive copper is used as a ductile metal in combination with abrasive silicon carbide powder, in the other – chemically active aluminum in combined with silicon carbide. A thermodynamic modeling technique has been developed to select the composition of the coating charge and predict the change in its phase composition at high temperature.

Titanium alloy fatigue strength and eigenfrequency stability

We conducted a study on fatigue in flat samples of the VT3-1 titanium alloy using “soft” cyclic beam bending tests. For this purpose, we developed an innovative electromagnetic test bench. The test bench's electromechanical system induces mechanical vibrations at a frequency that matches the eigenfrequency of the sample, ensuring that the cyclic load frequency remains constant. The electromagnetic force bends the sample while the elastic force unbends it, producing a quasi-sinusoidal cyclic load. Through our investigation, we determined the impact of this cyclic loading on both cyclic strength and durability. Our findings indicate that the VT3-1 titanium alloy possesses high resistance to fatigue and an endurance limit. Furthermore, we observed a low variability of the experimental fatigue resistance in relation to the approximating fatigue curve, suggesting the alloy has high structural stability. This finding indicates that the VT3-1 titanium alloy possesses high structural stability. To assess eigenfrequency stability, we subjected the alloy samples to cyclic tests, interrupting them at a reference number of 50 million cycles to evaluate changes in eigenfrequencies and stability under loads close to the fatigue limit. The results showed that the titanium alloy has a high level of eigenfrequency stability. Interruptions in cyclic tests resulted in jump-like increases in eigenfrequencies, which was not observed in continuous tests. Nevertheless, the total eigenfrequency deviations from the initial value at the end of the tests were similar in both cases

Effect of Heat-Treatment Conditions on the Microstructure and the Static Fracture Toughness of an α + β VT23 Titanium Alloy

Effect of Heat-Treatment Conditions on the Microstructure and the Static Fracture Toughness of an α + β VT23 Titanium Alloy

Structure and Properties of Electroexplosion Molybdenum Coating Deposited on Titanium Alloy VT6

Structure and Properties of Electroexplosion Molybdenum Coating Deposited on Titanium Alloy VT6

Determining the influence of irradiation swelling on creep and damage in elements with orthotropic material properties

This paper reports a method that numerically models the deformation and accumulation of hidden damage in structural elements that are in an inhomogeneous thermal field and are exposed to radiation. Materials considered are those exhibiting orthotropy (transversal isotropy) of long-term properties. The problem is stated as an boundary – initial value one. To solve it, the finite element method and the initial-difference method of time integration are used. To simulate the anisotropy of the process of accumulation of hidden damage, a damage tensor is applied. The development of irradiation swelling strains is described using the equation for a limited temperature range and a specific fluence value. The results of numerical modeling of creep and damage in plates in tension with circular notches, which are in an inhomogeneous temperature field, are considered. The material of the plates is a titanium alloy VT1-0. It was found that the effect of irradiation significantly, up to 6‒7 %, increases the level of deformation in the plate. Radiation significantly, by almost 4 times, reduces the time until the completion of the hidden fracture of the plates. It was found that orthotropy of radiation swelling properties leads to redistribution of areas with significant strains and damage values. It has been established that the effect of irradiation swelling also qualitatively changes the nature of the distribution of maximum damage in the plate, which extends to a fairly large area. Such results are due to the additional effect of irradiation swelling strains on the rate of general irreversible deformation and redistribution of stresses

Creation and study of intermetallic coating of the Ni–Ti system reinforced with tungsten carbide to increase wear resistance of titanium alloy

The results of studying the intermetallic coating of the Ni–Ti system with the addition of tungsten carbide are presented. The coating was synthesized on the surface of titanium alloy VT6 using an integrated approach – preliminary deposition of a precursor coating from monometallic nickel by cold gas-dynamic spraying and subsequent laser processing. It is shown that the introduction of tungsten carbide into the intermetallic matrix provides an increase in hardness by a factor of three or more, as well as a decrease in wear intensity by a factor of 80 compared to the wear resistance of VT6 titanium alloy. A technology has been developed and a batch of steam turbine blades with a wear-resistant coating on the surface of shrouds has been manufactured.

Superplastic Behavior and Microstructural Features of the VT6 Titanium Alloy with an Ultrafine-Grained Structure during Upsetting

In this paper, the superplastic behavior of the two-phase titanium alloy VT6 with an ultrafine-grained (UFG) structure produced by equal-channel angular pressing is examined. The deformation of specimens with a UFG structure was performed by upsetting in a temperature range of 650-750 °C and strain rate range of 1 × 10-4-5 × 10-1 s-1. Under these conditions, an increased strain-rate sensitivity coefficient m was observed. The calculation of apparent activation energy showed values in a range of 160-200 kJ/mol while the superplastic flow of the VT6 alloy was occurring. When superplastic behavior (SPB) was impeded, the energy Q grew considerably, indicating a change in mechanism from grain-boundary sliding (GBS) to bulk diffusion. A change in temperature and strain rate influenced the development of superplastic flow and the balance of relaxation processes. Microstructural analysis shows that the UFG state is preserved at upsetting temperatures of 650 and 700 °C. A decrease in strain rate and/or an increase in upsetting temperature promoted a more active development of recrystallization and grain growth, as well as α2-phase formation. In a certain temperature and strain-rate range of the UFG VT6 alloy, α2-phase plates were found, the formation of which was controlled by diffusion. The effect of the α2-phase on the alloy's mechanical behavior is discussed.

Fracture of aircraft titanium alloys under high-frequency loading

The paper is devoted to the problem of fatigue failure of two-phase titanium alloy VT3-1 under the action of high-frequency cyclic loading. Samples for the tests were made from the compressor disk of aviation engine D30-Ku installed on Tupolev Tu-154 planes. Repeated alternating loading was carried out by displacements of small amplitude at 20 kHz, coinciding with the first harmonic of longitudinal elastic vibrations of the sample. Analysis of specimen fracture surfaces shows predominant subsurface fracture. Analysis of areas of fatigue nucleation allows identifying elements of microstructure associated with crack nucleation. The results of the study can be used in the interpretation of fracture scenarios of structural elements in operation. Increasing the resource of products under such loads is possible through the use of damping layers or tapes, where possible.

Influence of electron‑beam surfacing modes on thickness and uniformity of single‑layer coating when restoring parts made of titanium alloys

The influence of the beam current, the distance between the beads of the clad material and the wire feeding intensity during electron‑beam (EB) surfacing of titanium alloy VT9 on the thickness and uniformity of the resulting restoring coating was investigated. As the beam current strength increases, the width of the deposited layer increases and the height decreases. The optimal distance between the beads should be considered an offset that does not exceed the width of the bead. It is possible to control the height of the deposited layer in a wide range by precise dosing of the wire feed intensity. The uniformity of surfacing is ensured by a balanced energy impact. Chemical composition of the clad layer meets the requirements of normative documents on the content of impurity gases for products from medical materials, obtained by additive technologies. The results of the study are used in the restoration of the surface of the landing gear element of aviation equipment.

Monitoring of vacuum diffusion welding process of titanium alloy using mathematical modelling

The results of evaluation and analysis of technological modes of diffusion welding in vacuum of titanium alloy VT6 using mathematical modeling are presented. Within the framework of the general concept of mathematical modeling existing in the description of powder metallurgy processes, a mathematical model was used to solve planar problems of the plastic flow of metal, which allows us to study the stress-strain state of real materials in the process of their formation. To solve the flat problems of plastic metal flow, a developed technique was used, which allows calculating and having experimental rheological dependencies, as well as mechanical test data, to choose the deformation modes optimal from the point of view of the quality of joints when they are obtained by diffusion welding in vacuum. The results obtained made it possible to optimize the parameters of diffusion welding of titanium powder blanks.

Research of the influence of the kinetics of a hydrogen-containing medium on the stress-strain state of a cylindrical shell made of titanium alloy

This paper considers a mathematical model of the stress-strain state of a circular cylindrical shell made of titanium alloy VT1-0, operating in an aggressive hydrogen-containing environment. The shell is under the action of an internal evenly distributed pressure. To formulate the problem and carry out calculations, a model with triple nonlinearity is used, formulated within the approach associated with normalized stress spaces. The resulting algorithm for solving the problem of studying the effect of an aggressive medium on titanium alloy shells effectively takes into account the substantially nonlinear change in the parameters of the stress-strain state depending on the time factor. In the study, a system of nonlinear resolving equations for calculating the shell was formulated, results were obtained for the key parameters of the effect of a hydrogen medium on the stress-strain state of a cylindrical shell, taking into account embrittlement.

Зносостійкість титанового сплаву ВТ1-0 з модифікованою поверхнею в умовах абразивного впливу

The cost of rebuilding machine parts as a result of wear is enormous and rising every year. At a US symposium on reducing wear in machinery, the general consensus was that wear management is central to solving national problems such as energy conservation, material reduction, and ensuring the reliability and safety of mechanical systems. Nitriding significantly increases the wear resistance of metals and alloys. The formation of chemical compounds in titanium alloys by introducing nitrogen or increasing its concentration limit changes the rate of chemical reactions and the kinetics of oxide film growth, and increases their adhesion to the substrate. This leads to a decrease in the intensity of adhesive node formation and improves the tribological characteristics of titanium alloys. Therefore, it is necessary to study a titanium alloy with a hardened nitrided layer to obtain experimental results to determine the regularity of the influence of diffusion saturation parameters on wear resistance under abrasive conditions. The universal installation "VIPA-1" was used for vacuum ion nitriding in the pulse mode and the formation of diffusion layers on the surface. Technological parameters of vacuum ion nitriding in the pulse mode: temperature - 550°C, pressure - 25-150 Pa, processing time - 10 hours, ratio of reaction gases - 80% Ag + 20% N2. For experimental studies, samples of VT1-0 titanium alloy with dimensions of 30x30 mm and a thickness of 5 mm were used. The surface hardening of the samples was carried out uniformly around the entire perimeter, which ensures a uniform thickness of the diffusion layer. The analysis of the operating conditions of the friction surfaces of most structural elements made it possible to use the installation for bench tests according to the scheme of wear with a free abrasive (Brinell method). Wear tests were carried out on an experimental setup in accordance with GOST 23.208-79 (Fig. 2). The friction process was modelled in the presence of a free, not rigidly fixed abrasive, which coincides with the American standard ASTM C 6568. The experiment was carried out at a sliding speed of 0.158 m/s, a load of 20 kg (with a shoulder of 272 mm) and a friction path of 50 m. Steel 45, hardened to a hardness of 480-500 HB, was used as a reference. The wear body was a disc made of VT1-0 titanium alloy with a diameter of 100 mm and a thickness of 3.5 mm. On the basis of experimental studies, it has been established that the main mechanisms for increasing the wear resistance of titanium alloys as a result of the process of diffusion saturation of the surface with nitrogen during vacuum ion nitriding in the pulse mode are strengthening of the surface layers; creation of a favourable residual stress pattern; change in the patterns of deformation of the surface layers; change in the chemical and adhesive properties of the surface; transfer of diffusing nitrogen atoms into the depth of the matrix during friction due to the tribodiffusion effect. Conclusions. 1. Maximum wear rate of titanium alloy VT1-0 without hardening. 2. The effect of thermocyclic nitriding increases the wear resistance of VT1-0 alloy: in sand - 3 times; in water + sand - 3.5 times; in salt + sand - 2.5 times. 3. The effect of isothermal nitriding increases the wear resistance of VT1-0 alloy: in sand - 4 times; in water + sand - 3.5 times; in salt + sand - 2.5 times.

Мікроструктура поверхневого шару титанового сплаву модифікованого вакуумним іонним азотуванням в імпульсному режимі

The mechanical strength and durability of structural elements is ensured by the correct selection of the materials from which they are made and the special functional properties of the surface layer, which are provided by applying coatings or by changing the structural state of the surface layers of the material by means of their modification. The use of titanium alloys provides unique properties, including high specific strength, corrosion resistance and a fairly high melting point. However, the use of these materials is limited by their low hardness, extremely low wear resistance, high tendency to sticking, high coefficient of friction in pairs with almost all materials, high chemical activity during heating above 500ᵒС and sensitivity to stress concentrators. To reduce the disadvantages of titanium alloys, various technological processes of strengthening are used by modifying the surface layers: by implantation of ions of various metals; surface treatment with concentrated energy flows; plasma flows; surface plastic deformation; ultrasonic treatment; laser processing; diffusion saturation and chemical-thermal treatment. Of all the modification methods, such a method of HTO as nitriding has become the most widespread, however, it involves long-term high-temperature heating, which causes fragility of the obtained nitrided layers, leads to a change in the metal structure and a decrease in strength characteristics. Task statement: conduct metallographic studies of the microstructure of modified surfaces with determination of the structure and depth of saturation of the diffusion layer with nitrogen Microsands for metallographic studies were produced on the "Alpha & Beta" installation (manufacturer of TM BUEHLER) according to the standard method (preventing tempering and overheating) with subsequent detection of the microstructure by chemical etching in a herbarium: НNO3 : HF : H2O (7 ml. : 2 ml. : 50 ml.). Metalohrafichni doslidzhennya zrazkiv provodyly v pozdovzhnʹomu peretyni po tovshchyni na optychnomu invertovanomu mikroskopi «AXIOVERT 40 MAT» z fiksatsiyeyu mikrostruktur tsyfrovoyu fotokameroyu AXIOCAM 305 v prohrami «ZEN core v3.1» Yak pokazaly doslidzhennya, tovshchyna nitrydnoho sharu pry vakuumnomu ionnomu azotuvanni v impulʹsnomu rezhymi tytanovoho splavu VT1-0 skladaye ne bilʹshe 1520 mkm, pid yakym znakhodytʹsya zona vnutrishnʹoho azotuvannya menshoyi tverdosti, yaka postupovo zmenshuyetʹsya po eksponentsiyniy zalezhnosti do tverdosti osnovy. Metallographic studies of the samples were carried out in a longitudinal cross-section by thickness on an optical inverted microscope "AXIOVERT 40 MAT" with fixation of microstructures by a digital camera АXIOCAM 305 in the program "ZEN core v3.1". As the studies showed, the thickness of the nitride layer during vacuum ion nitriding in the pulse mode of titanium alloy VT1-0 is no more than 1520 μm, under which there is a zone of internal nitriding of lower hardness, which gradually decreases exponentially depending on the hardness of the base. The structure of the base material does not change during low-temperature nitriding. A nitrided layer is formed on the surface with a nitride zone of varying thickness, which depends on the mode of nitriding and is weakly etched by the herbarium. The microstructure of the samples is an equiaxed polyhedral structure of the α-phase with a liquation band in the axial zone. Microhardness was determined under a load of 50, 100, and 200 g. The holding time under the load was 10 seconds. Microhardness was measured on the sandpapers in the radial direction of the segment with a step of 0.05 mm. The load depended on the dispersion and thickness of the reinforced layer. Conclusions. Nitriding (up to 900ᵒС) in the glow discharge of titanium VT1-0 and VT1-00 allows to significantly increase the microhardness due to the properties of nitrogen and its compounds with titanium, while preserving the original mechanical properties of the base material. A change in the parameters of the nitriding process (temperature, pressure, composition of the saturating medium and nitriding time) causes a change in the physical and mechanical characteristics, structure, thickness, phase and chemical composition of the surface nitriding layer: microhardness of the surface up to 10500 MPa; the thickness of the nitrided layer is up to 500 microns; the thickness of the nitride layer is up to 25 microns; phase shift difference TiN, Ti2N, Ti(N); different gradient of hardness in depth, which allows to optimize the power of the surface of the ball in specific conditions of operation.

A COMPREHENSIVE STUDY OF FATIGUE CRACK INITIATION AND GROWTH UNDER VERY HIGH CYCLE TORSIONAL FATIGUE LOADING

This paper investigates the fatigue behavior of smooth specimens made of VT3-1 titanium alloy under fully reversed loading conditions. Mathematical modeling results are presented for the fatigue fracture of smooth specimens under high cycle torsional fatigue loading. The fatigue quasi-crack initiation and growth are calculated using a multimode two-parameter model of fatigue damage accumulation. The surface and subsurface quasi-crack initiation under torsional loading is studied. The numerical results are in good agreement with experimental data and can be used to predict the change in crack growth mechanisms under complex multiaxial loadings such as torsion.

Creep and long-term strength of hydrogen-containing VT6 titanium alloy with a piecewise constant dependence of tensile stress on time

Рассматривается ползучесть наводороженного стержня из титанового сплава ВТ6 при кусочно-постоянной зависимости напряжения от времени вплоть до разрушения. Обсуждаются результаты экспериментально-теоретического исследования влияния концентрации предварительно внедренного водорода на ползучесть и длительную прочность растягиваемых стержней из титанового сплава ВТ6 при температуре 600 $^\circ$C и постоянных номинальных растягивающих напряжениях в диапазоне от 47 до 217 МПа.

Surface saturation monitoring of forming products from titanium alloy VT20 with additives

In the publication, the saturation of the surface of a large size of stamped articles made of VT20 alloy with interstitial impurities and the formation of non-metallic compounds on it were investigated. The process of gas saturation in several directions depending on external parameters: heating temperature, medium, holding time is analyzed.

A Mesoscale Study of Fatigue Fracture of Near β Titanium Alloy VT22 after Radial Shear Rolling with Subsequent Aging

A Mesoscale Study of Fatigue Fracture of Near β Titanium Alloy VT22 after Radial Shear Rolling with Subsequent Aging