Selective Synthesis Of Heat-Resistant Nickel Alloy: Structural Aspects
The "three-dimensional printing" technology appeared in the late 80’s of XX century. The pioneer company in this field was 3D Systems, which developed its first commercial stereolithography apparatus (1986). Wide distribution of digital technologies in the field of design, modeling and machining has stimulated the explosive nature of 3D-printing technologies development, and now it is extremely difficult to indicate the area of material production where 3D printers would not have been used to some extent . Additive technologies (3D printing) presuppose the fabrication (construction) of a physical object (a part) by layer-by-layer application method, in contracts to conventional methods of forming a part, by removing material from the work-piece array. In ASTM F2792.1549323-1 standard, additive technologies are defined as a process of combining a material to create an object from 3D model data, usually layer by layer, in contrast to "subtractive" production technologies . For powder materials, as well as for metallopowder compositions, the techniques related to the layer formation method for layer-by-layer deposition technology are used, "bed deposition", and assume the presence of a certain surface ("bed") on which a layer is firstly formed, and then the building material is selectively cured (fixed) in this layer. The term "selective synthesis" or "selective laser sintering" (SLS), if the "curing" tool is a laser, corresponds quite precisely to this technology.
For critical aviation components manufactured using additive technologies, it is especially important to obtain a defect-free material with high performance characteristics. Despite the high technological and economic attractiveness of additive technologies, this technology is not ideal for all metal materials. Low weldability and complexity of the chemical composition of alloys lead to the formation of hot cracks and high porosity of the material obtained by selective fusion. In the course of selective synthesis, rapid crystallization of the material occurs, which is accompanied by a high cooling rate, as a result of which a specific structure is formed in the material, which has a non-equilibrium nature. The structural-phase state of such a material may be controlled by various technological factors. One of the most effective levers of influence on the structure of the material is the laser energy parameters [3–5].
The peculiarities of formation of non-equilibrium structures in the course of selective laser sintering and subsequent heat treatment using Ni-Al-W-Co-Cr-Ti-Mo high-temperature nickel alloy as an example are considered in this article.
THE ESSENCE OF THE PROCESS
ZhS6K-VI high-temperature nickel alloy selected for research is used in the industry to manufacture gas turbine engine blades using equiaxed crystallization technology, i. e., the finished product, GTE blade, has a molded structure. It is known that ZhS6K-VI alloy is practically not welded, but in the heat-treated state it is strengthened by the γ′ (Ni3Al) phase. In view of this, when the granules of the alloy ZhS6K-VI are sintered, the resulting piece may contain cracks formed during the cooling of the material from the molten state. In order to establish the relationship between the quality of the resulting material and the features of its structure at all levels, from macro to nano, the samples of ZhS6K-VI alloy, synthesized at different laser processing parameters, such as power and scanning speed, were examined. Control over visible defects (cracks), in other words, the quality of the material, makes evident the resulting structural dependencies. Quality, and therefore high performance characteristics of the synthesized material, depends on the quality of the powder as well. First of all, it is necessary to obtain granules that are minimally defective and of the correct size (Fig. 1). The powder is produced by spraying a liquid melt jet in HERMIGA10/100VI atomizer (gas atomization). The size of the granules used in the SLS process is from 10 to 40 µm.
The samples for the research were obtained by the method of selective laser sintering (SLS) on Concept Laser M2 Cusing unit. In an inert nitrogen atmosphere, successive sintering of the powder layers with a laser beam 50 µm in diameter occurred. In addition to the original powder, the quality of the resulting material is influenced by the laser beam scanning path. Depending on the geometry of the final product and the properties of the source material, several types of ruling are used: simple lined (solid unidirectional ruling), discrete diagonal (ruling by separate areas, the direction of tracks is the same) and unloading (checkered) ruling, "meanders". The sintering of the powder material takes place in the form of a track (tracks) having the form of a semicircle in the plane perpendicular to the construction plane, which is the crystallization front or the molten pool (Fig. 2).
As the sample is formed by the 3D printing method, ultra-rapid heating of local regions occurs, followed by uneven cooling of the fused layers. This leads to the formation of non-equilibrium disperse structures. Stabilization of these structures, as well as optimization of the technological process, including an adequate selection of the laser energy parameters, is the basis for obtaining a defect-free material with high mechanical characteristics. In order to determine the most effective optimization methods, it is necessary to have a clear idea of the processes that take place "inside" the material, both during synthesis and subsequent heat treatments.
MATERIAL INTERNAL ARCHITECTURE
AND ITS CONTROL
The evolution of structural elements on both micro- and nano-level has a number of regularities. For example, the structure for all SLS materials has a certain scale classification. The molten pool (crystallization front) is detected metallographically, its curvature and depth depend on the heat sink, i. e., directly related to the amount of energy injected. The columnar crystals grow on the front border, which are grouped in so-called fragments. Within the fragment, all columns are unidirectional. To scale of one fragment in the cross section, the structure is cellular (Fig. 3). The size and the equiaxity of the cells also depend on the energy impact of the laser during the SLS.
The energy parameters of the laser, along with the type of ruling, affect the distribution of stresses arising during the sintering of the material, which can be estimated by texture, i. e., mutual orientation of fragments [7, 8]. The use of subsequent heat treatment also changes the material texture condition. Special methods for studying crystalline materials, which also include metals, allow us to describe the changes in the state of a material under various laser beam effects. The widely known method of X-ray structural analysis (X-ray diffraction method) makes it possible to evaluate the prevailing orientation of the blocks, constituting the material after sintering by constructing the orientation distribution in the crystal volume (pole figures). The method of electron back scattered diffraction (EBSD), allows us to visualize the relative arrangement of structural elements by means of color coding. Approach to one of the three pure colors (red, blue or green) in the image corresponds to the turns to the main directions of the crystal lattice of individual material blocks (Fig. 4). By combining these methods, one can obtain complete information about the crystallographic misorientations in the material and assess the perfection of the structure (the fraction of unidirectional fragments, the number of newly born grains, and the fraction of boundaries between the blocks).
Even more detailed studies of structural elements, such as local mutual orientation of fragments consisting of columnar crystals, boundary zones, types of phases formed, and the distribution of chemical elements in the volume of columnar microcrystals (cells) are carried out using high-resolution transmission electron microscopy (TEM). The method has successfully proved itself for solving problems related to the study of nanoscale objects, and allows us to study the structure of materials down to the atomic level (Fig. 5b). The phenomenon of diffraction and electron interference, as the basis of the TEM method, makes it possible to obtain images of the elements of the crystal structure in ultrahigh resolution (up to 1 Е). For example, using TEM methods, it was possible to establish that the boundaries of columnar crystals inside the fragments are decorated with carbide interlayers and separate inclusions of additional elements. Such formations on the boundaries of cells restrain the flow of material under load, increasing the operational capabilities of the material. Furthermore, unlike the macroblocks observed above, there is no visible crystallographic misorientation of submicron cells (Fig. 5a).
The use of X-ray spectral microanalysis together with obtaining TEM images of the structure allowed us to establish that microheterogeneity of major alloying elements of the alloy are found within the scope of the columnar crystallites. The cell boundary is enriched in chromium, titanium, tungsten and molybdenum, which are part of the main carbides, and the heterogeneous distribution of aluminum is observed in the cell volume as the most mobile element among the listed ones (Fig. 6).
EVOLUTION OF THE STRUCTURE IN THE PROCESS OF SLS AND SUBSEQUENT ANNEALING
Talking about structure formation of the materials, hardened by dispersed non-equilibrium particles of γ′-phase of such alloy, as ZhS6K-VI, one cannot help but mention the features of phase transformations directly in the course of layer-by-layer synthesis with characteristic heating of the layers adjacent to the laser impact zone and also after the subsequent heat treatment [9–11]. In the course of the studies carried out by TEM method, very dispersed decomposition with the isolation of γ′-phase was observed on the samples obtained under various SLS modes. Its size varies somewhat depending on the SLS mode, and also uneven in the volume of the cell, which confirms the data of X-ray spectral microanalysis.
In general, it is customary to use heat treatment (annealing) for foundry heat-resistant nickel alloys. This is justified by the fact that the system is non-equilibrium (i. e., the decomposition with the release of the strengthening phase has not been completed, and it is likely that this will happen during operation, which is highly undesirable, since it can lead to an uncontrolled change in material properties). By applying the standard heat treatment to the material obtained using the SLS technology, it turned out that the structure, namely γ′-phase, has an inhomogeneous character of isolation and imperfect shape. This indicates that the temperatures of complete dissolution of γ′-phase for the SLS material and classical heat-resistant alloy are different. To determine the temperature of phase dissolution and isolation, or temperatures of phase transitions in other words, differential-thermal analysis was conducted. The temperature of complete dissolution phase and alloy melting start temperature was established. The heat treatment according to the mode was chosen based on the interval between these two temperatures, and differs somewhat from the standard temperature of the classical alloy and makes it possible to obtain a homogeneous structure of the SLS material containing cuboidal γ′-phase without visible coagulation processes (particle aggregation) (Fig. 7).
"HEALING" IMPERFECTIONS AFTER SYNTHESIS
During heat treatment, the cellular structure is transformed into a homogeneous γ/γ′-structure, but the nature of the carbide distribution repeats the shape of the original cells, forming a regular "skeleton" structure. Thus, meshes of dispersed carbides of basically round shape together with γ′-phase particles form a unique structure, which, while possessing high strength, has all the grounds to remain highly plastic.
Complex studies described above, were applied to the samples synthesized using different laser parameters. As a result, it was possible to establish that with a decrease in the amount of energy pumped into the material and when using the most unloading types of ruling, the volume fraction of cracks in the material decreases, they become narrower. Furthermore, it was noted that during the annealing, a part of the cracks is tightened following diffusion laws, and the increase in the annealing time intensifies this process (Fig. 8).
Summarizing all of the above, we can draw the following conclusions:
• SLS material is characterized by the cellular structure, the cells are the cross sections of columnar crystals,
• in the absence of misorientations between the cells within the fragments, the boundaries of columnar crystals are formations containing carbides predominantly based on Ti, intermetallide phases, which include Cr and Al, as well as dislocation clusters in the vicinity of inclusions,
• the material with the least volume fraction of cracks corresponds to a structure with an equiaxial cell less than a micron in size, the boundary of which is determined by the dispersed inclusions of carbide and intermetallic phases. In the volume of such a cell, the inhomogeneities with respect to γ′-forming elements of Ti and Al are minimal, the distribution of the particles of γ′-phase in the volume is homogeneous,
• to stabilize the structure of high-temperature nickel alloys obtained by the SLS technology, heat treatment is required which is performed in the area of alloy homogenization temperatures (annealing). The peculiarity of the SLS material is that at the same time the original carbide structure is kept, which is characterized by a relatively even distribution of fine particles in the material volume. As a result, carbide particles, when heat treated, become a substrate for the formation of particles of hardening γ′-phase, creating an original internal architecture of the alloy, which in turn determines the unique complex of properties of the synthesized material. Furthermore, the additional effect of the final annealing of the material is the diffusion "healing" of a part of cracks,
• special attention in optimizing the laser energy parameters should be paid to the correlation between the amount of the energy supplied into the material, crystallographic texture, high-angle boundaries and the specific number of cracks in the material. The more energy is supplied, the higher are the thermal stresses, relaxed by the formation of cracks, and the sharper is the crystallographic texture. Thus, the texture condition of the material is a structurally sensitive parameter for assessing the fracture toughness of the material.