The influence of the structure and properties of powder heat-resistant alloys on the features of 3D printing of products from them

. Methods of fractional analysis of powders based on heat -resistant nickel alloys, electron microscopy and elemental analysis, study of bulk density of powder fractions, as well as approaches of mathematical modeling of packing density were applied in the work. Spherical powders based on heat - resistant nickel alloys of two different fractions: 50...200 μm and ≤ 63 μm were studied. The morphology of the surface of spherical particles of powders based on heat -resistant nickel alloys of different fractional composition, as well as their granulometric characteristics, was researched . It is shown that as the fraction al composition of powder fractions decreases, their homogeneity and bulk density increase. At the same time, it was established that more finely dispersed fractions are characterized by worse fluidity indicators. According to the simulation results, the optim al fractional composition of the powder for filling the previously specified volume was determined. It is shown that as the size of the investigated particles decreases, their


Introduction
A variety of materials, including powders of metal alloys, are used for the manufacture of products on laser 3D printers [1,2].In particular, powders of heat-resistant nickel and chrome-nickel alloys CrNi50WMoTiAlNb, CrNi60WTi are often used for the aerospace industry.They combine high heat resistance, heat resistance and satisfactory resistance to gas corrosion [3].
In railway manufacturing, 3D printing and powder metallurgy alloys are pivotal for precision and robust component production [4].These technologies facilitate intricate element creation, including connecting parts and rail systems, through rapid prototyping, fostering innovation and design refinement.Powder metallurgy alloys, such as nickel or cobalt alloys, ensure high thermal resistance and component strength while maintaining lightweight properties [5,6].
This technology allows on-demand component production, reducing spare part storage costs and enhancing manufacturing scalability.Efficient restoration and repair of railway equipment are achieved through 3D printing of powder metallurgy alloys for replacement parts.Overall, these technologies drive material exploration and innovation, advancing the railway transport sector for heightened reliability and efficiency.[4,7].
There are two main approaches to producing parts by 3D printing: fixing the applied powder by melting or sintering, or direct layer-by-layer application of the molten powder on the substrate [8].Regardless of the chosen approach, the parameters of the technological process of obtaining powders from the studied alloys should be optimized [9].It is important to achieve the most uniform form of spherical powder particles, eliminate defects on their surface, and arrange them according to their fractional composition [10,11].
However, one of the key problems that arises in additive manufacturing is the presence of pores in printed products, which are difficult to eliminate during layer-by-layer build-up [12].This leads to a decrease in the mechanical properties of the final product.The ability of the powder to sinter is related to the packing density while optimizing the particle size.Therefore, during the layer-by-layer build-up of layers, it is important to ensure a correlation between the ratio of the volume of powder particles of different fractions, their surface microrelief and the level of defects [13].
Metallographic research was performed on an EVO-40XVP scanning electron microscope, elemental analysis was performed on an OXFORD INCA Energy 350 energy dispersive spectrometer.Determination of the granulometric characteristics of the powders was carried out using ImageJ image analysis software [14].The bulk density parameters of the powders were set in accordance with ISO 3923-2-81 standards.
Modeling of the packing density of the powder fraction of the investigated alloys was performed using the «Spherestest» program [15].

Microstructure research
Based on the results of metallography, it was established that the studied fractions of the powder are characterized by a spherical shape of particles with a small discrepancy in size (Fig. 1).It should be noted that the surface of the particles has a typical dendritic structure, and defects in the form of "satellites" are formed on some of them.However, their number and size are insignificant, so they will not affect the quality of the printed product.

Research of elemental composition
The conformity of the chemical composition of the studied alloys for each fraction of the powder after the spheroidization process was evaluated on an Inca Energy 350 energy dispersive X-ray spectrometer.
The microstructure of the powders with the designation of the places where the elemental analysis was carried out are presented in Figure 2.
Table 1 shows the elemental composition of the studied powders of heat-resistant alloys of different fractions.
The results of the elemental analysis confirmed the full compliance of the chemical composition of the researched powder alloys with the brands of CrNi50WMoTiAlNb and CrNi60WTi alloys, respectively.It should be noted that they were obtained by the method of centrifugal plasma spraying of an electrode of the appropriate chemical composition.

Research of granulometric characteristics
Ensuring the high quality of a printed product made of powders of heat-resistant alloys requires a multifactorial analysis that monitors compliance with the conditions of the technological process [16].Optimization includes such aspects as the selection of parameters of the technological process, the input control of the quality of raw materials and the analysis of the physical and chemical properties of the obtained powder at all stages of production.At the same time, it is important for the final product to ensure the formation of a microstructure in the form of homogeneous spherical powder particles that are evenly distributed throughout the volume of the material [17].This can be achieved by working out the technology on the studied alloys, using special diagnostic equipment and certified control methods at all stages of production.A procedure for removing surface defects on powder particles should be provided, including ultrasonic cleaning methods or application of special coatings that prevent oxidation and adsorption processes [18].Therefore, the optimization of the technology for obtaining powders of heat-resistant alloys includes a comprehensive approach that includes measures to ensure the uniformity of particles and eliminate defects on their surface.This makes it possible to achieve high quality of the initial powder, which in turn contributes to obtaining high-quality products [19].
We conducted a granulometric analysis of powders of heat-resistant alloys CrNi50WMoTiAlNb and CrNi60WTi.The obtained results are presented in Figure 3 and Table 2.
As can be seen from the presented histograms of the distribution of powder particles of the studied CrNi50WMoTiAlNb alloys, particles with an average diameter of 136 µm dominate for the fraction 50...200 µm, and for the fraction ≤63 µm -with an average diameter of 45.6 µm.It should be noted that for the CrNi60WTi alloy, the fraction 50...200 μm is characterized by dominant particles with an average diameter of 124 μm, and the fraction ≤63 μm -29.7 μm.
The obtained results indicate that the average particle sizes of CrNi50WMoTiAlNb and CrNi60WTi alloy powders depend on the fluctuations of their fractional composition.This makes it possible not only to determine their parameters, but also to predict the procedure for optimizing the process of manufacturing powders, providing the necessary dimensions, fractional composition and technological properties for 3D printing of high-quality products.

Results of determination of bulk density
Bulk density is an important parameter that determines the degree of powder packing.A high bulk density indicates a compact arrangement of powder particles, which is important for the formation of products.The obtained data on the bulk density are necessary for choosing the optimal parameters of the fractional composition of powders and improving the technological processes of 3D printing of products made on their basis.Table 3 shows the results of determining the bulk density of powders of heat-resistant alloys of the brands CrNi50WMoTiAlNb and CrNi60WTi of different fractional composition.The obtained results indicate that CrNi60WTi alloy powders are characterized by a higher bulk density value than CrNi50WMoTiAlNb alloy powders.This may be related to the peculiarities of the phase state, polydispersity and the tendency of individual phases to coagulate.

Determination of packing density of powders using "Spherestest" software
One of the key problems that arise in the production of parts by the additive manufacturing method is the minimization of pore formation, which is achieved by the maximum possible packing density of powder particles in each of the sequentially applied (printed) layers.This directly affects the formation of the microstructure, which will ensure the required mechanical and physical properties of the parts [20].
Therefore, the analysis of the distribution of powder particles according to their sizes, compliance with a certain ratio of volume fractions of powder of different fractions became a key step in achieving the quality of additive manufacturing products.For this purpose, average values of average diameters and percentage ratio of powder particles within each studied fraction were selected experimentally.The obtained data made it possible to form an initial database for modeling the packing density of powders of heat-resistant alloys CrNi50WMoTiAlNb and CrNi60WTi with spherical particles.
The results of the simulation when filling the predefined volume are presented in Table 4.These data make it possible to apply a regulatory mechanism when choosing the fractional composition of powder materials and modes of the technological process of 3D printing depending on the shape and geometric dimensions of the products.
In the base on the obtained data, the packing density of the powder particles was calculated depending on their granulometric characteristics.The results are presented in Table 5.It is shown that the investigated CrNi60WTi alloy powders are characterized by a higher packing density than the CrNi50WMoTiAlNb alloy powders.This is explained by differences in their microstructure and phase composition.

Conclusions
In the base on the obtained experimental data and conducted modeling, it was established that the quality of printed products is determined not so much by the actual size of the powder particles of heat-resistant nickel and chromium-nickel alloys, but by the ability to adjust the phase and fractional composition.This makes it possible to achieve the maximum possible packing density of products during 3D printing and minimize the process of pore formation in them.
To improve the physical and mechanical properties of printed parts, it is necessary to maintain the stability of the microstructure and the fixed phase composition of the powders.
It is shown that the CrNi60WTi alloy has a higher packing density of powder particles compared to the CrNi50WMoTiAlNb alloy.This is explained by the different phase composition and different properties of these phases.
Optimization of the phase and fractional composition of powder heat-resistant alloys, their dimensional factors and modeling of packing density is a strategy for achieving high quality of products manufactured by the method of additive manufacturing.

Table 1 .
Statistical data of elemental analysis of spherical powders of heat-resistant alloys of different fractions.

Table 2 .
Polydispersity of powder particles of

Table 3 .
Results of determination of bulk density.

Table 4 .
Statistical average particle diameters of the studied powder alloys and the probability of "wrong decisions" when filling the predetermined volume of the product.

Table 5 .
Particle packing density of the studied powders according to simulation results.