Пути повышения свойств заготовок деталей ГТД из жаропрочных титановых сплавов, полученных методом прямого лазерного выращивания

Today, additive technologies for the production of billets from heat-resistant titanium alloys are used in aircraft engine building and the aerospace industry. However, the mechanisms of structure formation and the level of mechanical properties in such blanks are poorly understood and are of interest. Currently, available data are not sufficient for the confident application of additive technologies in the production of aircraft engines. This paper evaluates the microstructure and substantiates the possibility of improving the mechanical properties of workpieces of GTE parts made of VT20 titanium alloy, obtained by laser-powder additive surfacing. For research, workpieces with a size of 215x120x4 mm 120x85x14 mm were grown. As a "building" material, a powder of spherical shape of particles was used, the chemical composition of which corresponds to the alloy VT20. The research results showed that during laser-powder growth in the VT20 alloy, the formation of "hardening" structures is observed, due to the accelerated heat removal into the previously formed layer of cast metal. In this regard, the strength of the alloy sharply increases, and its impact toughness and relative narrowing decrease. Note that stress relief annealing after growing (T = 750 ± 10 ° C, holding time - 1.5 ... 2.0 h.) does not significantly affect the level of mechanical properties. When the annealing temperatures rise to critical levels, significant structural changes are observed in the VT20 alloy, which in turn affects its mechanical properties. Based on the analysis of microstructures and the results of mechanical tests after distinct types of heat treatment (800 ° C, 850 ° C, 920 ° C, holding time - 60...75 min.), it was found that an increase in the plastic properties of the VT20 alloy is observed when the samples are heated to temperatures close to the temperature of the polymorphic transformation (960...1000 ° C). Annealing of workpieces after surfacing according to mode T = 920 ± 10 ° C, holding time - 60...75 min. allows to somewhat reduce the strength characteristics of the VT20 alloy, while increasing its impact strength and relative narrowing.

Determination of properties of non-spherical VT20 alloy powders for modelling packing density

The study of unfavorable titanium alloy powders of VT20 grades was carried out and the methods of computer analysis were applied to determine the parameters of their optimal packaging. Metallographic studies were performed on a scanning electron microscope EVO-40XVP, and elemental analysis was performed using an energy dispersion spectrometer OXFORD INCA Energy 350. Determination of particle size distribution of powders was performed using image analysis software ImageJ. The surface morphology of non-spherical particles of VT20 alloy powder was studied for three different fractions: 100 ... 160 μm, 160 ... 200 μm and 200 ... 250 μm. It is shown that the powder particles are characterized by a nonspherical shape and a small difference in size. There is a tendency according to which when the particle size of the powder of the investigated alloy decreases, their shape approaches spherical. According to the results of particle size analysis, it was found that the usual sieve analysis does not allow to fully assess the distribution of powder by fractions. It was found that for the fraction 200 ... 250 μm the dominant particles are with an average diameter of 226 μm, for the fraction 160 ... 200 μm - 177 μm and for the fraction 100 ... 160 μm - 114 μm, respectively. Thus, for the fraction of titanium powder of the BT20 brand 200 ... 250 the polydispersity is 6.4%, for the fraction 160 ... 200 - 8.3%, and for the fraction 100 ... 160 - 9.1%. It is established that the fluidity of titanium alloy powders of the BT20 brand is: for the fraction 200 ... 250 μm - 62.35 s, for the fraction 160 ... 200 μm - 65.44 s, and for the fraction 100 ... 160 - 68, 73 s. That is, the highest value of fluidity is characterized by the powder with the largest particle size. Simulation of the pre-defined volume filling was performed using the "Spheres test" program. The average radii of particles of VT20 titanium alloy powder particles and the probability of the sizes of each of fractions of powder which is necessary at filling of the set volume was calculatedthe possibility of their precipitation have been established. Based on the obtained results, the packing density of VT20 titanium alloy powders was calculated depending on their fractional composition. It is confirmed that as the particle size of the powder decreases, their packing density increases. The surface morphology of non-spherical particles of VT20 alloy powder of different fractional composition and their particle size characteristics were studied. It is shown that with decreasing fractional composition of powder fractions, their homogeneity and bulk density increase. It was found that finer fractions are characterized by poorer fluidity. The simulation results determine the optimal fractional composition of the powder to fill the pre-specified volume. It is shown that as the size of the test particles decreases, their packing density increases. Keywords: additive production, titanium, microstructure, particle size distribution, bulk density, fluidity, packing density modelingmodelling.

Investigation of the interphase interaction at the interface in the Ti-C system with domestic titanium alloys of the a + β and pseudo a classes

Interaction of the Ti-C with titanium alloys of a + β and pseudo a classes and formation of the reaction layer at the interface have been investigated. We used titanium a + b alloys VT6 (Ti-Al-V) and VT8 (Ti-Al-Mo-Si) as well as pseudo a alloy VT20 (Ti-Al-Zr-Mo-V). The structure and composition of the interfaces were investigated by means of TEM in the scanning beam mode and energy dispersive spectroscopy. It is ascertained that already at the stage of production of the samples by thermal diffusion joining, interphase chemical interaction and formation of the reaction layers occurred. The reaction layer consists of distinct regions of small crystals (nanocrystals TiC of 10-50 nm in size) and large grains of Ti8C5 of 100-500 nm in size. Most of the reaction layer consists of large grains ofTi8C5. It was found that the average thickness of the reaction layer varies depending on the Ti alloy type and is ~0.89 μm (VT6 alloy), ~0.97 μm (VT8 alloy), and ~0.51 μm (VT20 alloy). Additional heat treatment of the samples leads to increasing the thickness of the reaction layer in all Ti-C/Ti alloy systems due to the growth of large grains of titanium carbide.

Investigation of structural-geometric parameters and elemental composition of spherical VT20 alloy powders

Purpose: Identification of structural-geometrical parameters, technological properties and elemental composition of spherical powders in a wide fraction range with respect to the VT20 alloy has been carried out. This is important for evaluating the optimum filling of a given volume by mixture of powders of different fractions during 3D printing. Design/methodology/approach: During the investigation of spherical Ti-alloy powders, a comprehensive approach was performed using Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), Dynamic Light Scattering (DLS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The surface morphology of the powders was studied on a Tuescan Vega 3 Scanning Electron Microscope. Using the Quantax energy dispersive spectrometer, element distribution maps were obtained and histograms of element distribution in the investigated powders were constructed. ICP-MS analysis was performed to clarify the elemental composition. DLS analysis using Malvern's Zetasizer Nano-ZS equipment allowed us to determine the functional parameters (hydrodynamic radius – Rh, zeta potential – z and specific conductivity) of particles of titanium alloy powder that indirectly indicate a tendency to form conglomerates. Findings: According to the microscopic examinations, the VT20 alloy powder consists of globular-shaped particles with the lamellar traces on their surfaces. The uniformity of the chemical element distribution within each fraction of the investigated powders was confirmed by EDS, and the full conformity of the powder fractions with the elemental composition of the VT20 alloy was confirmed by ICP-MS. The DLS method allowed to establish that the formation of conglomerates would not occur within the studied fractions of the VT20 alloy powder. Research limitations/implications: The use of high sensitive investigation methods gives understanding of the mechanisms of fine structure formation and possibility to control the processes of powder coagulation in the stage of electrostatic interactions. Practical implications: The obtained results can be used for the formation of fine spherical particles of the powder, but at the same time, these technologies can be extended for the particles of non-spherical shape. Originality/value: The DLS method allowed to establish that the formation of conglomerates would not occur within the studied fractions of the VT20 alloy powder. This, in turn, will improve powder melting during 3D printing. The measured zeta potential values allowed us to reveal mechanisms of fine structure formation and to control the processes of powder coagulation in the stage of electrostatic interactions.