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Complex refurbishment of titanium turbine blades by applying heat-resistant coatings by direct metal deposition
Engineering Failure Analysis 86 (2018) 115–130
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Complex refurbishment of titanium turbine blades by applying heat-resistant coatings by direct metal deposition
T
A.I. Gorunov Kazan National Research Technical University named after Tupolev A.N. – KAI, Kazan 420111, Russia
AR TI CLE I NF O
AB S T R A CT
Keywords: Direct metal deposition Titanium alloy Coating Microstructure Chemical composition
The paper presents a comprehensive approach to the refurbishment of titanium turbine blades by direct laser deposition. Coatings based on titanium carbide with the introduction of boron carbide and tungsten carbide particles for airfoil shroud platform and based on the commercially pure Ti with the addition of fine particles of aluminum oxide for the turbine airfoil are proposed. It is shown that large refractory particles with a hardness of 1384 HV0.1 ÷ 5108 HV0.1 are crystallization centers and reinforcing particles, and melting fine particles form high-temperature phases of TiB and Ti3Al in the coating metal with a hardness of 520 HV0.1.
1. Introduction The modern aircraft turbojet engine – is, as it is known, the heart of the aircraft, and with its help, it takes the air, but possible troubles for the engine start from the ground. The turbojet engine consumes a lot of air during its operation. Until the aircraft did not get enough speed all the air is sucked into the engine from the surrounding environment and, along with it, the objects, that somehow find themselves in critical proximity near the air intake, can also get into the engine duct. In the case of unfavorable conditions, this will lead at least to considerable financial losses – this is the removal and subsequent refurbishment of the engine at the manufacturing plant or repair enterprise. Blades are the main elements exposed to wear and, in particular, the blade foot and butt end of the blade airfoil. The refurbishment of blades made of different materials has peculiarities [1–7], related to the materials application uniformity, blade metal protection, material choice and etc. For example, the refurbishment of the titanium alloys blades by laser cladding methods has some features related to the cladding zone protection from oxidation [8–9]. In some cases, the laser deposition technology of wear-resistant coatings is accompanied by the addition of a small number of particles based on heat-resistant alloys into materials with a prevailing particle quantity with significantly lower melting point to increase the heat resistance [10–15]. The efficiency of critical parts of aerospace engines increases with increasing the operating temperatures. A higher temperature level can be achieved by creating new heat-resistant alloys operating at higher temperatures. The most optimal in this regard is titanium aluminide. High melting point, low density, high elasticity modulus, increase in yield strength with increasing the temperature, resistance to oxidation and ignition, high strength/density ratio, heat resistance – all this creates favorable conditions for the application of this material as a constructive one for a new generation of aerospace engines. Concurrently, borides and other refractory boron compounds are increasingly used in industry and engineering. The high heat resistance of some borides makes them promising components for high-temperature alloys, especially composite materials reinforced
E-mail address: [email protected]. https://doi.org/10.1016/j.engfailanal.2018.01.001 Received 15 May 2017; Received in revised form 7 January 2018; Accepted 10 January 2018 1350-6307/ © 2018 Published by Elsevier Ltd.
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Table 1 Chemical composition of the titanium alloy VT8, %. Alloy blades
Ti
Al
Mo
Si
Fe
C
N
Zr
O
H
VT8
Осн.
6,5
3,3
0,2
0,2
0,1
0.05
0.5
0.1
0.01
with boride fibers or dispersion-strengthened borides. In this case, borides increase hardness and, as a consequence, the wear resistance of coatings is increased [16,17]. The creation of wear-resistant and heat-resistant coatings on the butt end of the blade airfoil is an urgent task, at that the issue of reducing the engine weight or at least maintaining the weight when applying a solid heat-resistant coating is equally significant. The use of inexpensive materials based on titanium carbide with the addition of small portions of solid heat-resistant metal particles will allow finding solutions for the development of new heat-resistant coatings for blade shroud platform. In this paper, we propose a comprehensive approach for solving the problem of refurbishing worn turbine blades which affects the most important engine-building problems, such as the refurbishment of the blade foot and the missing elements of the blade airfoil.
2. Materials and methods For the research, we selected the blade of the compressor's 5th stage of gas turbine engine made of a titanium alloy VT8. Table 1 presents the chemical composition of the blade material. The most characteristic sections exposed to considerable wear and requiring the refurbishment were cut from the blade for the study (Fig. 1a, b). The CT scanner XViewTM series model H5000 was applied to detect pores and cracks in the received samples due to the fact that most of the blades can have hidden defects (Fig. 1b). The bimodal structure of the VT8 alloy is represented by the primary α-phase and the β-transformed matrix (Fig. 1). The size of αgrains is 3–6 μm. The blades were refurbished using a laser cladding plant, which includes an IPG Photonics fiber laser with a power of up to 10 kW, with a radiation wavelength of 1064 nm. Powder material of a titanium alloy was supplied through the coaxial nozzle into the cladding zone. We used the TiC powder with a particle size of 20–100 μm (50 μm – 70%) with the addition of the WC part with a particle size of 50–150 μm and the BC with a particle size of 50–150 as the basis for the laser cladding of the shroud platform, and powders of commercially pure Ti with a particle size 20–50 and Al2O3 with a particle size ≈1 μm for refurbishment of the blade airfoil. Fig. 2 shows the powder particles. The cladding process was carried out using a constant (Fig. 3a), and a pulse (Fig. 3b) laser operation modes. The mixing of the powders was carried out in a tabletop machine Turbula Model T2F shaker. After the blades refurbishment by laser cladding method, they were cut, and microsections were prepared for metallographic analysis. Samples were filled with epoxy resin successively ground with a set of diamond grinding discs (120 grit, 220 grit, 500 grit) and polished using diamond suspensions (9 μm, 3 μm). Macro- and microstructure of the metal was identified by chemical etching in a prepared reagent HF – 15 cm3, HNO3–35 cm3, H2O – 200 cm3, glycerin – 100 cm3.
Fig. 1. The appearance of damaged blades a – schematic representation, b – image obtained using an X-ray tomography.
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Fig. 2. The duplex structure of the titanium alloy VT8 which is typical for the compressor blades of the 5th stage. SEM image of the thin section surface in secondary electrons (SEI).
Fig. 3. Type of powder material for laser cladding a – boron carbide, b – tungsten carbide, c – titanium carbide, d – pure titanium, e – alumina.
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Airfoil shroud platform
Turbine airfoil
Fig. 4. Scheme of coating: (a, b) – airfoil shroud platform; (c, d) – Turbine airfoil.
The automated durometer “Tukon 2500” was used to measure the microhardness. Structure analysis of the surface of metallographic samples was performed using “Axiovert-200M” a universal inverted microscope. Electron microscopic image of the samples surface was obtained on the workstation Auriga CrossBeam. X-ray diffraction analysis was carried out using a multifunctional diffractometer Rigaku SmartLab. Diffraction patterns were registered according to the Bragg–Brentano scheme and in the geometry of the parallel beams under the following conditions: The remaining parameters for registering the diffraction patterns (divergence slits, scan interval and step, filming time taken at the point) were selected individually in each particular case. The measurement of the surface distribution of the temperature was carried out using the FLIR A6500S thermal imager. 3. Experimental part 3.1. Selection of the cladding regime and composition of the powder mixture The mixing technology of a powder composition consisting mainly of fine particles of titanium carbide was proposed to create a heat-resistant coating with high hardness, as well as, adding larger particles of tungsten carbide and boron carbide into it to increase the wear resistance and heat resistance of the coating. The WC and BC additives not exceeding 2 wt% of titanium carbide were the optimal powder composition. Such a distribution was chosen purposefully to avoid the formation of components with high stoichiometric coefficients which subsequently could embrittle the melt matrix. As shown in Fig. 4 an idea to form a coating based on titanium carbide so that the coating weight was not high was proposed. The power of laser radiation in the pulsed mode has been preliminarily selected so that particles smaller than 30 μm were melted and large particles remained unmelted. According to this theory, the task was to obtain a molten titanium carbide layer with uniformly distributed inclusions of WC, BC, TiC over the cross-section, which would be the crystallization centers and would allow creating a highly dispersed metal structure of the base. The second method for applying the coating on the blade airfoil is most likely not suitable for the shroud platform. This is due to the fact that for a given thickness of the blade airfoil the significant heat input would result in the melting not only of the solid carbide metal particles of the coating but of the finest blade airfoil. Therefore, the mode in which the laser output power in the pulsed mode would not produce a significant remelting of the blade airfoil butt end was initially selected. After that, a classical scheme for laser deposition of the titanium alloy was proposed, but a material that would reduce the weight of the structure, while increasing the strength and hardness of the coating, was used as the additive. Al2O3 was chosen as an additive to the pure titanium, but in this case, the particle size was 1–2 μm so that the refractory alumina particles could form the new phases in the coating and accelerate the crystallization rate. Previously, it was shown that the formation of coatings based on titanium and its alloys, as well as the culturing of products by direct laser deposition methods requires ample protection in a gas environment or deep vacuuming. This necessity is primarily due to the fact that the metal is in an overheated state for a long time. In this regard, to realize the minimum exposure time of laser radiation on the metal melt, this work was carried out using the pulsed mode of laser radiation Fig. 5. The laser modes were selected from the conditions to not only provide laser cladding without causing a significant remelting of the base metal surface but also to reduce the residence time of the weld metal and metal base in the overheated state which would prevent the formation of golden nitride films or embrittled alpha-phase layer. Since one of the main objectives of this work was to develop a cheap turbine blade refurbishment technology that would help to avoid the use of expensive powder material and the construction of gas chambers to protect the laser impact zone from the atmospheric air during laser cladding. The shape and size of the thermal field that creates the laser spot during the laser cladding were recorded with the help of the 118
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W,
105 Airfoil shroud platform
84 63
Turbine airfoil
42 21 0 10 20 30 40 50 60 70 80 t, msec Fig. 5. Laser operation mode to refurbish the airfoil shroud platform and turbine airfoil.
thermal imager to select the laser operation mode (Fig. 6). In the “tail” which is formed behind the laser spot zone, it can be seen that under a steady-state mode of the laser exposure there are regions of increased maximum temperatures in this tail as well as in the center of the laser spot itself. This fact makes it necessary to provide additional protection for the cladding zone because titanium begins to react rapidly with the air atmosphere since the nozzle with the outflowing gas-powder mixture continues to move away from the cladding zone applying new metal layers. In this case, the mode at which the pulse time was less than 50 msec was the optimal one. As a result of the work, it was found that for a lesser influence and formation of the so-called tail after the spot it is necessary to observe the exposure time corresponding to the pictures in Fig. 7a, b, c. In this case the pulsed operation mode of the laser is implemented which allows applying the coating by the direct laser deposition method with a maximum heat input and minimal impact on the treatment zone. At the same time for laser cladding of refractory components such as TiC, WC, BC it is necessary to carry out laser deposition under a pulsed laser operation but at a power of 8 kW because of the high energy dissipation. In this case, the cladding should be carried out on the butt ends of the blade airfoil with the application of a constant mode of laser radiation using a permanent laser. We can see from the figure that it is desirable that the pulse time during the cladding does not exceed 50 msec. In this case, there will be no formation of the so-called “tail” behind the laser spot, which leads to oxidation of the deposited surface and to contact with the surrounding atmosphere. The mode selection of the laser cladding on the butt ends of the blade airfoil, and shroud platform was performed by varying the
Fig. 6. Distribution of heat energy over the sample surface at different time periods: a – run, b – 50 msec, c – 100 msec, d – 150 msec, e – 200 msec, f – 500 msec, g – 1 s, h – 2 s, i – 3 s.
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Fig. 7. Selection of the titanium alloy laser cladding mode: a – (120 msec, 2 kW); b – (60 msec, 5 kW); c – (60 msec, 7 kW); c, d, e, f, g – laser cladding on the blade airfoil; h, i, − the appearance of the deposited metal Ti (pure) + 2 wt% Al2O3; j, k – the external appearance of the deposited metal TiC + WC 2 wt% + BC 2 wt%.
laser radiation power and the time between pulses on the surface of the titanium alloy – substrate. On the micrograph, we can see that the spot has a characteristic shape, but the thermal imaging camera did not show this. Therefore, to eliminate all possible drawbacks, the distance between the overlaying spots was chosen with an overlap of 10%. Moreover, an increase in the laser radiation power makes it possible to obtain a fairly simple oval shape of the laser spot, which allowed applying the coatings investigated in this paper. Fig. 7 d–g shows, the bright flashes of the powder material that ignites at high temperature interacting with the air atmosphere but the cladding zone remains protected by a generous flow of argon protective gas. After such cladding, the surface of the deposited metal has a silvery color without the formation of a yellow nitride film (Fig. 7 h–k). This technology makes it possible to apply coatings based on titanium by laser cladding, without the use of a deep vacuuming and utilization of bulky protective chambers filled with argon. 3.1.1. Research of cladding of the shroud platform The coating on the surface of the titanium alloy was obtained as a result of the cladding of the mixture based on TiC with the addition of BC and WC (Fig. 4a). In this case, we can see that the pulsed laser radiation at the impact on the powder composition resulted in the formation of a matrix based on titanium carbide and solid refractory inclusions based on tungsten carbide and boron carbide. Fig. 8 shows the image of the coating microstructures with zones assigned for chemical elemental analysis. The characteristic sections of the zones of the cladded metal matrix (Fig. 8, b) and the transition zone between the base metal and the deposited metal (Fig. 8 c, d) were selected for the elemental analysis. The chemical analysis in Fig. 9 showed that the melting of the particles of both the powder material and titanium base has occurred despite the pulsed laser radiation mode. The particles of boron carbide took part in the remelting process of powder material based on titanium carbide the tungsten carbide particles remained as inclusions. The presence of molybdenum and aluminum among the detectable elements (Fig. 10) indicates that the cladding was carried out in the pulsed laser action mode; a significant heat input of 8 kW resulted in a melting of the substrate surface and mixing with the deposited metal. Therefore, on the boundary of the deposited metal and substrate (Fig. 10) we see the double peaks of titanium introduced from titanium carbide and the substrate. 120
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Fig. 8. SEM images of the section surface in the secondary electrons (SEI) and the back-scattered electrons – compositional contrast (COMPO) at different magnifications. The boundary between the “cladding” zone and the “metal-base”.
Fig. 9. Local elemental analysis of the cladded metal matrix. Auger electron spectra obtained on the section surface in the analysis zones see Fig. 8.
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Fig. 10. Elemental analysis of the boundary between cladded metal and metal-base. Auger electron spectra obtained on the section surface in the analysis zones see Fig. 8.
Since the high laser radiation power was required to melt fine particles of titanium, tungsten and boron carbides and laser spot focus was raised above the substrate surface by 500 μm, it has led to the involvement of the substrate metal masses in the cladding process. There was a partial mixing of the base metal with the deposited metal which is confirmed by the presence of molybdenum in the
Fig. 11. SEM images of inclusions on the section surface cut from the coating (TiC + 2 wt% BC + 2 wt% WC) in secondary electrons (SEI) and the topographical contrast (TOPO).
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Fig. 12. Diffractogram of the deposited layer in the range of 2θ 25–90°, where the diffraction peaks α-Ti (ICDD PDF-2 01-089-5009), β-Ti (ICDD PDF-2 01-077-3482), TiC (ICDD PDF-2 03-065-0242), Ti2C (ICDD PDF-2 01-071-6257), WC (ICDD PDF-2 00-025-1047), SiO2 (ICDD PDF-2 01-083-2470) and TiB (ICDD PDF-2 01-0732148) are marked (a), in the range of 2θ 32–49°, (b), in the range of 2θ 50–80° (c).
matrix of the deposited metal and aluminum. There is a presence of boron, the appearance of which is probably because the fine particles of the element melted and participated in the formation of the matrix coating. In the micrograph (Fig. 11) it can be seen that large unmelted TiC, WC, BC particles are distributed in the central part of the deposited metal. Previously, it was found that under the influence of constant laser radiation all large particles float to the coating surface and are not fixed in its central part and on the boundary with the substrate. Fig. 12 shows the diffractogram of the deposited metal matrix to determine the possible options for the formation of phase composition. The registration of the diffractograms occurred in the Bragg–Brentano geometry under the following conditions: slits in the primary beam: 15 mm; 0.1° (divergence slit). The hardness of these particles was WC - 1384 HV0.1, TiC - 2579 HV0.1, WC - 5108 HV0.1 at the hardness of the coating matrix of 520 HV0.1. The X-ray phase analysis of the sample in the deposited layer region (Fig. 10 a, b, c) showed the presence of a series of different phases, namely: α-Ti (ICDD PDF-2 01-089-5009), β-Ti (ICDD PDF- 2 01-077-3482), TiC (ICDD PDF-2 03-065-0242), Ti2C (ICDD PDF2 01-071-6257), WC (ICDD PDF-2 00-025-1047). The small diffraction peak in the region of 26.58° (Fig. 3), probably belongs to silicon oxide (SiO2, ICDD PDF-2 01-083-2470) which was introduced as a result of sample preparation for analysis. Low-intensity peaks in the regions of 29.21° and 52.3° belong to TiB (ICDD PDF-2 01-073-2148). 123
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Fig. 13. Phase diagram of Ti-B.
When synthesizing the titanium borides, the temperatures of 1200 ÷ 1800 °C are required to provide the completeness of the solid-phase reactions behavior Ti (solid) + B (solid) → TiBk (solid) and to form the perfect and pure crystalline borides structures. The components and reaction products (В, Ti, TiВk) have higher affinity for O2, N2, H2O and carbon. Therefore, the impurities formation of oxides, nitrides, carbides and other phases is possible. Identification of the obtained new boride phases requires the use of elaborate analytical and crystal-chemical methods. The fulfillment of these conditions at the first stages of research is difficult. However, judging by the diffractogram shown in Fig. 12 it can be seen that the formation of TiB occurs at cladding. If you make evaluation according to the phase diagram shown in the Fig. 13 (green line) the cladding temperature exceeded 1700 °C but not more than 2800 °C. Under the conditions that the boron carbide melted during the laser cladding, the temperature fluctuated within 2300 °C (the melting temperature of boron carbide). However, as it is known, there is a general pattern at rapid heating of titanium alloys, namely, at rapid heating and cooling there is a temperature exceeding by 200–300 °C. Titanium boride TiB has a face-centered cubic lattice of the ZnS type [19]. The formation of zigzag chains of boron atoms is observed in it (synthesis of metals and boron). These reactions are carried out either by fusing the metal and boron or by sintering the corresponding metals with boron: xMe + yB → MexBy. The TiB properties surpass those of similar substance titanium carbide TiC [20] exceptionally high hardness (~25–35 GPa at room temperature), high melting point (3225 °C), high thermal diffusivity (60–120 W/(mK)), high electrical conductivity (~105 S/cm). As a result of this combined mixing of the powder material a matrix of the melt based on titanium carbide was formed, melting of its fine particles when interacting with boron carbide resulted in the formation of TiB. In this case, the matrix of the melt is saturated with titanium carbide particles that have exceeded the size of 50 μm and became the crystallization centers and reinforcing particles. This occurred because the temperature was insufficient for melting the particles of this size, as well as larger particles and boron carbide. The tungsten carbide particles served as crystallization centers that influenced the formation of the highly dispersed structure of the matrix of the melt. 3.1.2. Investigation of cladding of the blade airfoil butt end Fig. 14 shows photomicrographs of coatings obtained on the substrate of titanium alloy VT8 by the cladding of the pure titanium alloy (Fig. 14 a, b) and by the cladding of the pure titanium alloy with the addition of 20 wt% alumina (Fig. 14 c, d). Fig. 14a shows that after the laser cladding of pure titanium a classical lamellar structure typical for a casting titanium alloy is observed in the structure. This results in the formation of pores in the coating with a size of ≈100 μm and between the layers 20 μm. Fig. 14 c, d show the formation of large cracks in the coating material when adding more than 20 wt% of aluminum oxide it is inadmissible for these materials in their operation. Previously, in [18] it was shown that the addition of alumina portions leads to cracking of the deposited metal. This is true in the case when such an addition exceeds 5 wt% of powder material. However, the pulsed laser operation mode made it possible to form a coating without an additional protective chamber at that the pore size was 3 ÷ 5 μm. In our case, the alumina particle size did not exceed 1 μm which allowed its melting together with pure titanium particles. This technology has allowed avoiding the partial floating of aluminum particles on the coating surface due to the density difference. The formation of new phases with nitrogen was not detected in the diffractograms, which indicates the correctness of the 124
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Fig. 14. Type of deposited metal: a, b – Ti (pure) a constant laser operation mode; c, d – Ti + 20 wt% Al2O3 pulsed laser operation mode.
technology. The presence of a small amount of aluminum was detected in the central part of the weld bead using chemical analysis on an electron microscope in Fig. 15. Fig. 16 presents the elemental analysis of the matrix of cladded metal. Alumina in the original form of the powder granules is very difficult to fix in the central regions of the deposited pure titanium. Because of the different densities of alumina and titanium oxide, Al2O3 like solid inclusions of titanium, tungsten and boron carbides tends to float and to concentrate in the upper layers of the deposited metal. Therefore, the fixing of the said elements in the deposited metal was achieved by applying a pulsed laser operation mode. This allowed minimizing the time of overheating. The study of the elemental composition of deposited commercially pure titanium with the addition of 2 wt% of alumina with the fraction size of 1–2 μm allowed us to establish the absence of additional elements which are part of the blade-base metal in the deposited metal, which suggests that the pulsed mode of laser radiation action did not result in the mixing of the deposited metal with the blade-substrate metal. However, in this case, with the statistical data set of 10 scans, one can see the barely manifested peaks of aluminum in the diagram of Fig. 16. Alumina particles do not appear in the electronic photographs. Fig. 17 shows the diffractograms of the deposited metal and metal of the base (blade). The registration of the diffractograms occurred in the Bragg–Brentano geometry under the following conditions: the diffractograms were recorded from four regions with a successive shift of the sample by 0.5 mm before each analysis. The analysis area width is approximately equal to 0.7 mm, and its length corresponds to the selected slit (10 mm). The slits in the primary beam: 15 mm; 0.025° (divergence slit), the slits on the diffracted beam: 0.025° (anti-scattering slit), 0.3 mm, scanning interval – 20–90°, scanning step – 0.02°, filming time at the point – 5 s. In the diffractogram (Fig. 17) made in different sections of the deposited metal it can be clearly seen that because of its small size the alumina melted and dissolving in the pure titanium formed Ti3Al compound. When passing from the base metal (Fig. 17 a–c) to the deposited metal a slight change in the intensity of the phases typical for the metal of the base – substrate α-Ti, β-Ti occurs. When passing to the deposited metal region (Fig. 16 c–d), the appearance of additional peaks corresponding to the Ti3Al can be seen quite distinctly. Fig. 17 a, b, c shows the characteristic peaks of the formation of α + β-Ti two-phase structure and Fig. 17 d presents the formation of an additional Ti3Al phase. Not only the coating cracking occurs when adding more than 20 wt% of alumina but also the formation of compounds like TiAl3 and TiAl2 and TiAl, which forming solid films prevent the penetration of aluminum into the inner layers which leads to a high concentration of brittle areas. The initial stage of the structure formation of titanium aluminides is the aluminum melting caused by the heat impulse and its further spreading over the channels of the capillary-porous medium. Subsequent diffusion of aluminum atoms into the lattice of titanium particles leads to the origination of an intermetallic TiAl3 125
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Fig. 15. Elemental analysis of the matrix of cladded metal. Zones of areal analysis: Zone 1, 2 – Ø = 30 μm. For the spectra see Fig. 15.
Fig. 16. Elemental analysis of the matrix of cladded metal. Fig. 15 shows auger electron spectra obtained on the section surface in the analysis zones.
compound in the diffusion zone. The internal compressive and external contracting stresses arise at the formation of the intermetallic compound it can lead to its destruction [21,22]. In a system containing 39,6 wt% of Al, the previously formed layer limits the movement of aluminum atoms into the titanium material. At that, there is a buildup of the TiAl3 layer which leads to a depletion of the aluminum mass and subsequent origination of titanium monoaluminide. When the process spreads into the depth of the titanium mass, the aluminum concentration decreases, and it causes the origination of the intermetallic Ti3Al compound (Fig. 18). The rapid diffusion of aluminum atoms into titanium occurs because of the pulsed mode with high power, which results in the formation of Ti3Al, high cooling rates fix it in the matrix making its 126
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Fig. 17. Diffractograms of the sample cut from the blade with a Ti + 2 wt% Al203 deposited metal in the range of 2θ 30–90° obtained from different analysis regions (a and b – base region, c – transition region, d – deposited layer region). The diffraction peaks α-Ti, β-Ti and Ti3Al are marked in the diffractogram.
distribution homogeneous over the coating cross-section. However, when using the pulsed mode, unfortunately, most of the laser radiation energy is dissipated and therefore a laser with a power of up to 10 kW is required for melting such refractory inclusions as Al2O3 the titanium, boron and tungsten carbide. 3.1.3. Comparison of microhardness and structure of applied coatings The addition of alumina into the deposited pure titanium alloy results in the formation of the coating with a hardness that is 1.6 times the substrate hardness and 1.2 times higher than the that of deposited pure titanium (Fig. 19). Thus it can be observed that the pulsed laser radiation mode leads to the formation of a thermal influence zone. Due to the rapid heat dissipation from the thin wall of the blade airfoil, the significant hardness drop does not occur in the HAZ (Fig. 19 a). The use of a laser radiation mode at a power of 8 kW resulted in the formation of several extensive HAZ with a significant drop of hardness. This effect is caused by a large power density supplied to the substrate surface. In this case, the hardness of the obtained matrix of the deposited metal based on TiC, WC, and BC was 2 times greater than that of the base metal Fig. 19 b. The hardness is distributed uniformly in the deposited metal (Fig. 19 a, b) and its cross-section. In this case, a gradual decrease in hardness is observed in the transition zone between the deposited metal and the base metal down to the heat affected zone. In both cases when studying the distribution of size and form of the structural components of the deposited metal (Fig. 20) it is seen that in the case of cladding the powder composition where the boron, titanium, and tungsten carbides were the crystallization centers (Fig. 20, e). In this case the average diameter and the size of the matrix structural components of the melt were significantly smaller than that of pure titanium with titanium oxide inclusions (Fig. 20, f). It becomes obvious that the crystallization centers, in this case, played a crucial role in the structure formation, in spite of the fact that in this case the mode with a higher laser power was used and the deposited metal was for a long time in the overheated state. As it is known, the high cooling rate leads to the formation of a highly dispersed structure. This occurs when the number of crystallization centers exceeds the rate of their growth at cooling of the molten metal. In our case, the introduction of additional crystallization centers allowed exceeding the growth rate of dendritic crystals in the matrix of the melt. In addition, these crystallization centers (TiC, BC, WC) have hardness 5 times greater than that of the deposited metal matrix and blade base metal. 127
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Fig. 18. Phase diagram of Ti-Al.
Fig. 19. Distribution of hardness between the deposited metal and the substrate, a – butt end of blade airfoil, b – section of the metal of the shroud platform.
The process of structure formation in the metal of the coatings based on titanium carbide with the addition of boron and tungsten carbides as well as commercially pure titanium with the addition of alumina occurs depending on both the qualitative and quantitative components of the initial powder composition and the technology of the cladding, which significantly affects the physical–chemical properties of the resulting intermetallic compound.
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Fig. 20. Analysis of the deposited metal structure: a, b, e – butt end of blade airfoil; c, d, f – metal section of the shroud platform.
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4. Conclusions 1. It is found that the technology of laser cladding is fully capable of controlling the process of structure formation, has a broad range of advantages and can be used to produce titanium aluminides and titanium borides with the desired chemical and physical properties. 2. It was established that the addition of 5 wt% BC and 5 wt% WC into the TiC powder composition makes it possible to obtain a coating with the titanium borides in its matrix with a uniform distribution of large solid heat-resistant BC and WC particles. This combination is achieved by melting fine powder particles, and coarse particles are the crystallization centers to obtain a highly dispersed matrix structure. 3. Mixing of the base metal results in the formation of the coating matrix with a hardness of 520 HV0.1, which is important when blades operate under alternating loads, at that the hardness of the inclusion particles was: WC = 1384 HV0.1, TiC = 2579 HV0.1, WC = 5108 HV0.1. 4. The addition of 2 wt% of Al2O3 with a particle size of 1–2 μm to commercially pure Ti makes it possible to obtain a coating for the blade airfoil with the hardness of 1.6 times higher than that of the base metal. At the same time, alumina particles melt, and Ti3Al phase is formed in the titanium matrix. The base metal and the deposited coating do not mix. Acknowledgments The authors are grateful to the Ministry of Education of the Russian Federation for supported research projects no. 14.578.21.0245 and 9.3236.2017/4.6. References [1] Boris Rottwinkel, Christian Nölke, Stefan Kaierle, Volker Wesling, Crack repair of single crystal turbine blades using laser cladding technology, Procedia CIRP 22 (2014) 263–267. [2] Eckart Uhlmann, Robert Kersting, Tiago Borsoi Klein, Marcio Fernando Cruz, Anderson Vicente Borille, Additive manufacturing of titanium alloy for aircraft components, Procedia CIRP 35 (2015) 55–60. [3] Baohai Wu, Jian Wang, Ying Zhang, Ming Luo, Adaptive location of repaired blade for multi-axis milling, J. Comput. Des. Eng. 2 (2015) 261–267. [4] P. Bendeich, N. Alam, M. Brandt, D. Carr, K. Short, R. Blevins, C. Curfs, O. Kirstein, G. Atkinson, T. Holden, R. Rogge, Residual stress measurements in laser clad repaired low pressure turbine blades for the power industry, Mater. Sci. Eng. A 437 (2006) 70–74. [5] Y.P. Kathuria, Some aspects of laser surface cladding in the turbine industry, Surf. Coat. Technol. 132 (2000) 262–269. [6] L. Shepeleva, B. Medres, W.D. Kaplan, M. Bamberger, A. Weisheit, Laser cladding of turbine blades, Surf. Coat. Technol. 125 (2000) 45–48. [7] Xiong Zheng, Guang-xia Chen, Xiao-yan Zeng, Effects of process variables on interfacial quality of laser cladding on aeroengine blade material GH4133, J. Mater. Process. Technol. 209 (2009) 930–936. [8] Ryan Cottam, Milan Brandt, Laser cladding of Ti-6Al-4V powder on Ti-6Al-4V substrate: effect of laser cladding parameters on microstructure, Phys. Procedia 12 (2011) 323–329. [9] Z.H.A.N.G. Qiang, C.H.E.N. Jing, T.A.N. Hua, L.I.N. Xin, H.U.A.N.G. Wei-dong, Microstructure evolution and mechanical properties of laser additive manufactured Ti−5Al−2Sn−2Zr−4Mo−4Cr alloy, Trans. Nonferrous Metals Soc. China 26 (2016) 2058–2066. [10] Shengfeng Zhou, Xiaoyan Zeng, Growth characteristics and mechanism of carbides precipitated in WC–Fe composite coatings by laser induction hybrid rapid cladding, J. Alloys Compd. 505 (2010) 685–691. [11] Dongsheng Wang, Zongjun Tian, Lida Shen SonglinWang, Zhidong Liu, Microstructural characterization of Al2O3–13 wt.% TiO2 ceramic coatings prepared by squash presetting laser cladding on GH4169 superalloy, Surf. Coat. Technol. 254 (2014) 195–201. [12] M. Sharifitabar, J. Vahdati Khaki, M. Haddad Sabzevar, Microstructure and wear resistance of in-situ TiC–Al2O3 particles reinforced Fe-based coatings produced by gas tungsten arc cladding, Surf. Coat. Technol. 285 (2016) 47–56. [13] Jyotsna DuttaMajumdar, Lin Li, Development of titanium boride (TiB) dispersed titanium (Ti) matrix composite by direct laser cladding, Mater. Lett. 64 (2010) 1010–1012. [14] Yunxiao Chen, Wu Dongjiang, Guangyi Ma, Lu Weifeng, Dongming Guo, Coaxial laser cladding of Al2O3-13%TiO2 powders on Ti-6Al-4 V alloy, Surf. Coat. Technol. 228 (2013) S452–S455. [15] Manoj Masanta, S.M. Shariff, A. Roy Choudhury, Microstructure and properties of TiB2–TiC–Al2O3 coating prepared by laser assisted SHS and subsequent cladding with micro-/nano-TiO2 as precursor constituent, Mater. Des. 90 (2016) 307–317. [16] A.I. Gorunov, Fabrication of coatings and bulk products made of a nickel-based material by additive technology laser metal deposition, Russ. Metall. (Metally) 7 (2016) 663–668. [17] Yinghua Lin, Jianhua Yao, Yongping Lei, Fu Hanguang, Liang Wang, Microstructure and properties of TiB2–TiB reinforced titanium matrix composite coating by laser cladding, Opt. Lasers Eng. 86 (November 2016) 216–227. [18] Igor Shishkovsky, Igor Smurov, Titanium base functional graded coating via 3D laser cladding, Mater. Lett. 73 (15) (April 2012) 32–35. [19] J.L. Murray, The B-Ti (boron-titanium) system, Bull. Alloy Phase Diagr. 7 (6) (1986) 550–555. [20] B. Basu, et al., Processing and properties of monolithic TiB2 based materials, Int. Mater. Rev. 51 (2006) 352. [21] C. Sims, N. Stoloff, W. Nagel, Superalloys II (N.Y.), (1987). [22] Y.-W. Kim, F. Froes, Physical Metallurgy of Titanium Aluminides (Warrendale, PA), (1990), pp. 451–465.
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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Complex refurbishment of titanium turbine blades by applying heat-resistant coatings by direct metal deposition
T
A.I. Gorunov Kazan National Research Technical University named after Tupolev A.N. – KAI, Kazan 420111, Russia
AR TI CLE I NF O
AB S T R A CT
Keywords: Direct metal deposition Titanium alloy Coating Microstructure Chemical composition
The paper presents a comprehensive approach to the refurbishment of titanium turbine blades by direct laser deposition. Coatings based on titanium carbide with the introduction of boron carbide and tungsten carbide particles for airfoil shroud platform and based on the commercially pure Ti with the addition of fine particles of aluminum oxide for the turbine airfoil are proposed. It is shown that large refractory particles with a hardness of 1384 HV0.1 ÷ 5108 HV0.1 are crystallization centers and reinforcing particles, and melting fine particles form high-temperature phases of TiB and Ti3Al in the coating metal with a hardness of 520 HV0.1.
1. Introduction The modern aircraft turbojet engine – is, as it is known, the heart of the aircraft, and with its help, it takes the air, but possible troubles for the engine start from the ground. The turbojet engine consumes a lot of air during its operation. Until the aircraft did not get enough speed all the air is sucked into the engine from the surrounding environment and, along with it, the objects, that somehow find themselves in critical proximity near the air intake, can also get into the engine duct. In the case of unfavorable conditions, this will lead at least to considerable financial losses – this is the removal and subsequent refurbishment of the engine at the manufacturing plant or repair enterprise. Blades are the main elements exposed to wear and, in particular, the blade foot and butt end of the blade airfoil. The refurbishment of blades made of different materials has peculiarities [1–7], related to the materials application uniformity, blade metal protection, material choice and etc. For example, the refurbishment of the titanium alloys blades by laser cladding methods has some features related to the cladding zone protection from oxidation [8–9]. In some cases, the laser deposition technology of wear-resistant coatings is accompanied by the addition of a small number of particles based on heat-resistant alloys into materials with a prevailing particle quantity with significantly lower melting point to increase the heat resistance [10–15]. The efficiency of critical parts of aerospace engines increases with increasing the operating temperatures. A higher temperature level can be achieved by creating new heat-resistant alloys operating at higher temperatures. The most optimal in this regard is titanium aluminide. High melting point, low density, high elasticity modulus, increase in yield strength with increasing the temperature, resistance to oxidation and ignition, high strength/density ratio, heat resistance – all this creates favorable conditions for the application of this material as a constructive one for a new generation of aerospace engines. Concurrently, borides and other refractory boron compounds are increasingly used in industry and engineering. The high heat resistance of some borides makes them promising components for high-temperature alloys, especially composite materials reinforced
E-mail address: [email protected]. https://doi.org/10.1016/j.engfailanal.2018.01.001 Received 15 May 2017; Received in revised form 7 January 2018; Accepted 10 January 2018 1350-6307/ © 2018 Published by Elsevier Ltd.
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Table 1 Chemical composition of the titanium alloy VT8, %. Alloy blades
Ti
Al
Mo
Si
Fe
C
N
Zr
O
H
VT8
Осн.
6,5
3,3
0,2
0,2
0,1
0.05
0.5
0.1
0.01
with boride fibers or dispersion-strengthened borides. In this case, borides increase hardness and, as a consequence, the wear resistance of coatings is increased [16,17]. The creation of wear-resistant and heat-resistant coatings on the butt end of the blade airfoil is an urgent task, at that the issue of reducing the engine weight or at least maintaining the weight when applying a solid heat-resistant coating is equally significant. The use of inexpensive materials based on titanium carbide with the addition of small portions of solid heat-resistant metal particles will allow finding solutions for the development of new heat-resistant coatings for blade shroud platform. In this paper, we propose a comprehensive approach for solving the problem of refurbishing worn turbine blades which affects the most important engine-building problems, such as the refurbishment of the blade foot and the missing elements of the blade airfoil.
2. Materials and methods For the research, we selected the blade of the compressor's 5th stage of gas turbine engine made of a titanium alloy VT8. Table 1 presents the chemical composition of the blade material. The most characteristic sections exposed to considerable wear and requiring the refurbishment were cut from the blade for the study (Fig. 1a, b). The CT scanner XViewTM series model H5000 was applied to detect pores and cracks in the received samples due to the fact that most of the blades can have hidden defects (Fig. 1b). The bimodal structure of the VT8 alloy is represented by the primary α-phase and the β-transformed matrix (Fig. 1). The size of αgrains is 3–6 μm. The blades were refurbished using a laser cladding plant, which includes an IPG Photonics fiber laser with a power of up to 10 kW, with a radiation wavelength of 1064 nm. Powder material of a titanium alloy was supplied through the coaxial nozzle into the cladding zone. We used the TiC powder with a particle size of 20–100 μm (50 μm – 70%) with the addition of the WC part with a particle size of 50–150 μm and the BC with a particle size of 50–150 as the basis for the laser cladding of the shroud platform, and powders of commercially pure Ti with a particle size 20–50 and Al2O3 with a particle size ≈1 μm for refurbishment of the blade airfoil. Fig. 2 shows the powder particles. The cladding process was carried out using a constant (Fig. 3a), and a pulse (Fig. 3b) laser operation modes. The mixing of the powders was carried out in a tabletop machine Turbula Model T2F shaker. After the blades refurbishment by laser cladding method, they were cut, and microsections were prepared for metallographic analysis. Samples were filled with epoxy resin successively ground with a set of diamond grinding discs (120 grit, 220 grit, 500 grit) and polished using diamond suspensions (9 μm, 3 μm). Macro- and microstructure of the metal was identified by chemical etching in a prepared reagent HF – 15 cm3, HNO3–35 cm3, H2O – 200 cm3, glycerin – 100 cm3.
Fig. 1. The appearance of damaged blades a – schematic representation, b – image obtained using an X-ray tomography.
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Fig. 2. The duplex structure of the titanium alloy VT8 which is typical for the compressor blades of the 5th stage. SEM image of the thin section surface in secondary electrons (SEI).
Fig. 3. Type of powder material for laser cladding a – boron carbide, b – tungsten carbide, c – titanium carbide, d – pure titanium, e – alumina.
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Airfoil shroud platform
Turbine airfoil
Fig. 4. Scheme of coating: (a, b) – airfoil shroud platform; (c, d) – Turbine airfoil.
The automated durometer “Tukon 2500” was used to measure the microhardness. Structure analysis of the surface of metallographic samples was performed using “Axiovert-200M” a universal inverted microscope. Electron microscopic image of the samples surface was obtained on the workstation Auriga CrossBeam. X-ray diffraction analysis was carried out using a multifunctional diffractometer Rigaku SmartLab. Diffraction patterns were registered according to the Bragg–Brentano scheme and in the geometry of the parallel beams under the following conditions: The remaining parameters for registering the diffraction patterns (divergence slits, scan interval and step, filming time taken at the point) were selected individually in each particular case. The measurement of the surface distribution of the temperature was carried out using the FLIR A6500S thermal imager. 3. Experimental part 3.1. Selection of the cladding regime and composition of the powder mixture The mixing technology of a powder composition consisting mainly of fine particles of titanium carbide was proposed to create a heat-resistant coating with high hardness, as well as, adding larger particles of tungsten carbide and boron carbide into it to increase the wear resistance and heat resistance of the coating. The WC and BC additives not exceeding 2 wt% of titanium carbide were the optimal powder composition. Such a distribution was chosen purposefully to avoid the formation of components with high stoichiometric coefficients which subsequently could embrittle the melt matrix. As shown in Fig. 4 an idea to form a coating based on titanium carbide so that the coating weight was not high was proposed. The power of laser radiation in the pulsed mode has been preliminarily selected so that particles smaller than 30 μm were melted and large particles remained unmelted. According to this theory, the task was to obtain a molten titanium carbide layer with uniformly distributed inclusions of WC, BC, TiC over the cross-section, which would be the crystallization centers and would allow creating a highly dispersed metal structure of the base. The second method for applying the coating on the blade airfoil is most likely not suitable for the shroud platform. This is due to the fact that for a given thickness of the blade airfoil the significant heat input would result in the melting not only of the solid carbide metal particles of the coating but of the finest blade airfoil. Therefore, the mode in which the laser output power in the pulsed mode would not produce a significant remelting of the blade airfoil butt end was initially selected. After that, a classical scheme for laser deposition of the titanium alloy was proposed, but a material that would reduce the weight of the structure, while increasing the strength and hardness of the coating, was used as the additive. Al2O3 was chosen as an additive to the pure titanium, but in this case, the particle size was 1–2 μm so that the refractory alumina particles could form the new phases in the coating and accelerate the crystallization rate. Previously, it was shown that the formation of coatings based on titanium and its alloys, as well as the culturing of products by direct laser deposition methods requires ample protection in a gas environment or deep vacuuming. This necessity is primarily due to the fact that the metal is in an overheated state for a long time. In this regard, to realize the minimum exposure time of laser radiation on the metal melt, this work was carried out using the pulsed mode of laser radiation Fig. 5. The laser modes were selected from the conditions to not only provide laser cladding without causing a significant remelting of the base metal surface but also to reduce the residence time of the weld metal and metal base in the overheated state which would prevent the formation of golden nitride films or embrittled alpha-phase layer. Since one of the main objectives of this work was to develop a cheap turbine blade refurbishment technology that would help to avoid the use of expensive powder material and the construction of gas chambers to protect the laser impact zone from the atmospheric air during laser cladding. The shape and size of the thermal field that creates the laser spot during the laser cladding were recorded with the help of the 118
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W,
105 Airfoil shroud platform
84 63
Turbine airfoil
42 21 0 10 20 30 40 50 60 70 80 t, msec Fig. 5. Laser operation mode to refurbish the airfoil shroud platform and turbine airfoil.
thermal imager to select the laser operation mode (Fig. 6). In the “tail” which is formed behind the laser spot zone, it can be seen that under a steady-state mode of the laser exposure there are regions of increased maximum temperatures in this tail as well as in the center of the laser spot itself. This fact makes it necessary to provide additional protection for the cladding zone because titanium begins to react rapidly with the air atmosphere since the nozzle with the outflowing gas-powder mixture continues to move away from the cladding zone applying new metal layers. In this case, the mode at which the pulse time was less than 50 msec was the optimal one. As a result of the work, it was found that for a lesser influence and formation of the so-called tail after the spot it is necessary to observe the exposure time corresponding to the pictures in Fig. 7a, b, c. In this case the pulsed operation mode of the laser is implemented which allows applying the coating by the direct laser deposition method with a maximum heat input and minimal impact on the treatment zone. At the same time for laser cladding of refractory components such as TiC, WC, BC it is necessary to carry out laser deposition under a pulsed laser operation but at a power of 8 kW because of the high energy dissipation. In this case, the cladding should be carried out on the butt ends of the blade airfoil with the application of a constant mode of laser radiation using a permanent laser. We can see from the figure that it is desirable that the pulse time during the cladding does not exceed 50 msec. In this case, there will be no formation of the so-called “tail” behind the laser spot, which leads to oxidation of the deposited surface and to contact with the surrounding atmosphere. The mode selection of the laser cladding on the butt ends of the blade airfoil, and shroud platform was performed by varying the
Fig. 6. Distribution of heat energy over the sample surface at different time periods: a – run, b – 50 msec, c – 100 msec, d – 150 msec, e – 200 msec, f – 500 msec, g – 1 s, h – 2 s, i – 3 s.
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Fig. 7. Selection of the titanium alloy laser cladding mode: a – (120 msec, 2 kW); b – (60 msec, 5 kW); c – (60 msec, 7 kW); c, d, e, f, g – laser cladding on the blade airfoil; h, i, − the appearance of the deposited metal Ti (pure) + 2 wt% Al2O3; j, k – the external appearance of the deposited metal TiC + WC 2 wt% + BC 2 wt%.
laser radiation power and the time between pulses on the surface of the titanium alloy – substrate. On the micrograph, we can see that the spot has a characteristic shape, but the thermal imaging camera did not show this. Therefore, to eliminate all possible drawbacks, the distance between the overlaying spots was chosen with an overlap of 10%. Moreover, an increase in the laser radiation power makes it possible to obtain a fairly simple oval shape of the laser spot, which allowed applying the coatings investigated in this paper. Fig. 7 d–g shows, the bright flashes of the powder material that ignites at high temperature interacting with the air atmosphere but the cladding zone remains protected by a generous flow of argon protective gas. After such cladding, the surface of the deposited metal has a silvery color without the formation of a yellow nitride film (Fig. 7 h–k). This technology makes it possible to apply coatings based on titanium by laser cladding, without the use of a deep vacuuming and utilization of bulky protective chambers filled with argon. 3.1.1. Research of cladding of the shroud platform The coating on the surface of the titanium alloy was obtained as a result of the cladding of the mixture based on TiC with the addition of BC and WC (Fig. 4a). In this case, we can see that the pulsed laser radiation at the impact on the powder composition resulted in the formation of a matrix based on titanium carbide and solid refractory inclusions based on tungsten carbide and boron carbide. Fig. 8 shows the image of the coating microstructures with zones assigned for chemical elemental analysis. The characteristic sections of the zones of the cladded metal matrix (Fig. 8, b) and the transition zone between the base metal and the deposited metal (Fig. 8 c, d) were selected for the elemental analysis. The chemical analysis in Fig. 9 showed that the melting of the particles of both the powder material and titanium base has occurred despite the pulsed laser radiation mode. The particles of boron carbide took part in the remelting process of powder material based on titanium carbide the tungsten carbide particles remained as inclusions. The presence of molybdenum and aluminum among the detectable elements (Fig. 10) indicates that the cladding was carried out in the pulsed laser action mode; a significant heat input of 8 kW resulted in a melting of the substrate surface and mixing with the deposited metal. Therefore, on the boundary of the deposited metal and substrate (Fig. 10) we see the double peaks of titanium introduced from titanium carbide and the substrate. 120
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Fig. 8. SEM images of the section surface in the secondary electrons (SEI) and the back-scattered electrons – compositional contrast (COMPO) at different magnifications. The boundary between the “cladding” zone and the “metal-base”.
Fig. 9. Local elemental analysis of the cladded metal matrix. Auger electron spectra obtained on the section surface in the analysis zones see Fig. 8.
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Fig. 10. Elemental analysis of the boundary between cladded metal and metal-base. Auger electron spectra obtained on the section surface in the analysis zones see Fig. 8.
Since the high laser radiation power was required to melt fine particles of titanium, tungsten and boron carbides and laser spot focus was raised above the substrate surface by 500 μm, it has led to the involvement of the substrate metal masses in the cladding process. There was a partial mixing of the base metal with the deposited metal which is confirmed by the presence of molybdenum in the
Fig. 11. SEM images of inclusions on the section surface cut from the coating (TiC + 2 wt% BC + 2 wt% WC) in secondary electrons (SEI) and the topographical contrast (TOPO).
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Fig. 12. Diffractogram of the deposited layer in the range of 2θ 25–90°, where the diffraction peaks α-Ti (ICDD PDF-2 01-089-5009), β-Ti (ICDD PDF-2 01-077-3482), TiC (ICDD PDF-2 03-065-0242), Ti2C (ICDD PDF-2 01-071-6257), WC (ICDD PDF-2 00-025-1047), SiO2 (ICDD PDF-2 01-083-2470) and TiB (ICDD PDF-2 01-0732148) are marked (a), in the range of 2θ 32–49°, (b), in the range of 2θ 50–80° (c).
matrix of the deposited metal and aluminum. There is a presence of boron, the appearance of which is probably because the fine particles of the element melted and participated in the formation of the matrix coating. In the micrograph (Fig. 11) it can be seen that large unmelted TiC, WC, BC particles are distributed in the central part of the deposited metal. Previously, it was found that under the influence of constant laser radiation all large particles float to the coating surface and are not fixed in its central part and on the boundary with the substrate. Fig. 12 shows the diffractogram of the deposited metal matrix to determine the possible options for the formation of phase composition. The registration of the diffractograms occurred in the Bragg–Brentano geometry under the following conditions: slits in the primary beam: 15 mm; 0.1° (divergence slit). The hardness of these particles was WC - 1384 HV0.1, TiC - 2579 HV0.1, WC - 5108 HV0.1 at the hardness of the coating matrix of 520 HV0.1. The X-ray phase analysis of the sample in the deposited layer region (Fig. 10 a, b, c) showed the presence of a series of different phases, namely: α-Ti (ICDD PDF-2 01-089-5009), β-Ti (ICDD PDF- 2 01-077-3482), TiC (ICDD PDF-2 03-065-0242), Ti2C (ICDD PDF2 01-071-6257), WC (ICDD PDF-2 00-025-1047). The small diffraction peak in the region of 26.58° (Fig. 3), probably belongs to silicon oxide (SiO2, ICDD PDF-2 01-083-2470) which was introduced as a result of sample preparation for analysis. Low-intensity peaks in the regions of 29.21° and 52.3° belong to TiB (ICDD PDF-2 01-073-2148). 123
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Fig. 13. Phase diagram of Ti-B.
When synthesizing the titanium borides, the temperatures of 1200 ÷ 1800 °C are required to provide the completeness of the solid-phase reactions behavior Ti (solid) + B (solid) → TiBk (solid) and to form the perfect and pure crystalline borides structures. The components and reaction products (В, Ti, TiВk) have higher affinity for O2, N2, H2O and carbon. Therefore, the impurities formation of oxides, nitrides, carbides and other phases is possible. Identification of the obtained new boride phases requires the use of elaborate analytical and crystal-chemical methods. The fulfillment of these conditions at the first stages of research is difficult. However, judging by the diffractogram shown in Fig. 12 it can be seen that the formation of TiB occurs at cladding. If you make evaluation according to the phase diagram shown in the Fig. 13 (green line) the cladding temperature exceeded 1700 °C but not more than 2800 °C. Under the conditions that the boron carbide melted during the laser cladding, the temperature fluctuated within 2300 °C (the melting temperature of boron carbide). However, as it is known, there is a general pattern at rapid heating of titanium alloys, namely, at rapid heating and cooling there is a temperature exceeding by 200–300 °C. Titanium boride TiB has a face-centered cubic lattice of the ZnS type [19]. The formation of zigzag chains of boron atoms is observed in it (synthesis of metals and boron). These reactions are carried out either by fusing the metal and boron or by sintering the corresponding metals with boron: xMe + yB → MexBy. The TiB properties surpass those of similar substance titanium carbide TiC [20] exceptionally high hardness (~25–35 GPa at room temperature), high melting point (3225 °C), high thermal diffusivity (60–120 W/(mK)), high electrical conductivity (~105 S/cm). As a result of this combined mixing of the powder material a matrix of the melt based on titanium carbide was formed, melting of its fine particles when interacting with boron carbide resulted in the formation of TiB. In this case, the matrix of the melt is saturated with titanium carbide particles that have exceeded the size of 50 μm and became the crystallization centers and reinforcing particles. This occurred because the temperature was insufficient for melting the particles of this size, as well as larger particles and boron carbide. The tungsten carbide particles served as crystallization centers that influenced the formation of the highly dispersed structure of the matrix of the melt. 3.1.2. Investigation of cladding of the blade airfoil butt end Fig. 14 shows photomicrographs of coatings obtained on the substrate of titanium alloy VT8 by the cladding of the pure titanium alloy (Fig. 14 a, b) and by the cladding of the pure titanium alloy with the addition of 20 wt% alumina (Fig. 14 c, d). Fig. 14a shows that after the laser cladding of pure titanium a classical lamellar structure typical for a casting titanium alloy is observed in the structure. This results in the formation of pores in the coating with a size of ≈100 μm and between the layers 20 μm. Fig. 14 c, d show the formation of large cracks in the coating material when adding more than 20 wt% of aluminum oxide it is inadmissible for these materials in their operation. Previously, in [18] it was shown that the addition of alumina portions leads to cracking of the deposited metal. This is true in the case when such an addition exceeds 5 wt% of powder material. However, the pulsed laser operation mode made it possible to form a coating without an additional protective chamber at that the pore size was 3 ÷ 5 μm. In our case, the alumina particle size did not exceed 1 μm which allowed its melting together with pure titanium particles. This technology has allowed avoiding the partial floating of aluminum particles on the coating surface due to the density difference. The formation of new phases with nitrogen was not detected in the diffractograms, which indicates the correctness of the 124
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Fig. 14. Type of deposited metal: a, b – Ti (pure) a constant laser operation mode; c, d – Ti + 20 wt% Al2O3 pulsed laser operation mode.
technology. The presence of a small amount of aluminum was detected in the central part of the weld bead using chemical analysis on an electron microscope in Fig. 15. Fig. 16 presents the elemental analysis of the matrix of cladded metal. Alumina in the original form of the powder granules is very difficult to fix in the central regions of the deposited pure titanium. Because of the different densities of alumina and titanium oxide, Al2O3 like solid inclusions of titanium, tungsten and boron carbides tends to float and to concentrate in the upper layers of the deposited metal. Therefore, the fixing of the said elements in the deposited metal was achieved by applying a pulsed laser operation mode. This allowed minimizing the time of overheating. The study of the elemental composition of deposited commercially pure titanium with the addition of 2 wt% of alumina with the fraction size of 1–2 μm allowed us to establish the absence of additional elements which are part of the blade-base metal in the deposited metal, which suggests that the pulsed mode of laser radiation action did not result in the mixing of the deposited metal with the blade-substrate metal. However, in this case, with the statistical data set of 10 scans, one can see the barely manifested peaks of aluminum in the diagram of Fig. 16. Alumina particles do not appear in the electronic photographs. Fig. 17 shows the diffractograms of the deposited metal and metal of the base (blade). The registration of the diffractograms occurred in the Bragg–Brentano geometry under the following conditions: the diffractograms were recorded from four regions with a successive shift of the sample by 0.5 mm before each analysis. The analysis area width is approximately equal to 0.7 mm, and its length corresponds to the selected slit (10 mm). The slits in the primary beam: 15 mm; 0.025° (divergence slit), the slits on the diffracted beam: 0.025° (anti-scattering slit), 0.3 mm, scanning interval – 20–90°, scanning step – 0.02°, filming time at the point – 5 s. In the diffractogram (Fig. 17) made in different sections of the deposited metal it can be clearly seen that because of its small size the alumina melted and dissolving in the pure titanium formed Ti3Al compound. When passing from the base metal (Fig. 17 a–c) to the deposited metal a slight change in the intensity of the phases typical for the metal of the base – substrate α-Ti, β-Ti occurs. When passing to the deposited metal region (Fig. 16 c–d), the appearance of additional peaks corresponding to the Ti3Al can be seen quite distinctly. Fig. 17 a, b, c shows the characteristic peaks of the formation of α + β-Ti two-phase structure and Fig. 17 d presents the formation of an additional Ti3Al phase. Not only the coating cracking occurs when adding more than 20 wt% of alumina but also the formation of compounds like TiAl3 and TiAl2 and TiAl, which forming solid films prevent the penetration of aluminum into the inner layers which leads to a high concentration of brittle areas. The initial stage of the structure formation of titanium aluminides is the aluminum melting caused by the heat impulse and its further spreading over the channels of the capillary-porous medium. Subsequent diffusion of aluminum atoms into the lattice of titanium particles leads to the origination of an intermetallic TiAl3 125
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Fig. 15. Elemental analysis of the matrix of cladded metal. Zones of areal analysis: Zone 1, 2 – Ø = 30 μm. For the spectra see Fig. 15.
Fig. 16. Elemental analysis of the matrix of cladded metal. Fig. 15 shows auger electron spectra obtained on the section surface in the analysis zones.
compound in the diffusion zone. The internal compressive and external contracting stresses arise at the formation of the intermetallic compound it can lead to its destruction [21,22]. In a system containing 39,6 wt% of Al, the previously formed layer limits the movement of aluminum atoms into the titanium material. At that, there is a buildup of the TiAl3 layer which leads to a depletion of the aluminum mass and subsequent origination of titanium monoaluminide. When the process spreads into the depth of the titanium mass, the aluminum concentration decreases, and it causes the origination of the intermetallic Ti3Al compound (Fig. 18). The rapid diffusion of aluminum atoms into titanium occurs because of the pulsed mode with high power, which results in the formation of Ti3Al, high cooling rates fix it in the matrix making its 126
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Fig. 17. Diffractograms of the sample cut from the blade with a Ti + 2 wt% Al203 deposited metal in the range of 2θ 30–90° obtained from different analysis regions (a and b – base region, c – transition region, d – deposited layer region). The diffraction peaks α-Ti, β-Ti and Ti3Al are marked in the diffractogram.
distribution homogeneous over the coating cross-section. However, when using the pulsed mode, unfortunately, most of the laser radiation energy is dissipated and therefore a laser with a power of up to 10 kW is required for melting such refractory inclusions as Al2O3 the titanium, boron and tungsten carbide. 3.1.3. Comparison of microhardness and structure of applied coatings The addition of alumina into the deposited pure titanium alloy results in the formation of the coating with a hardness that is 1.6 times the substrate hardness and 1.2 times higher than the that of deposited pure titanium (Fig. 19). Thus it can be observed that the pulsed laser radiation mode leads to the formation of a thermal influence zone. Due to the rapid heat dissipation from the thin wall of the blade airfoil, the significant hardness drop does not occur in the HAZ (Fig. 19 a). The use of a laser radiation mode at a power of 8 kW resulted in the formation of several extensive HAZ with a significant drop of hardness. This effect is caused by a large power density supplied to the substrate surface. In this case, the hardness of the obtained matrix of the deposited metal based on TiC, WC, and BC was 2 times greater than that of the base metal Fig. 19 b. The hardness is distributed uniformly in the deposited metal (Fig. 19 a, b) and its cross-section. In this case, a gradual decrease in hardness is observed in the transition zone between the deposited metal and the base metal down to the heat affected zone. In both cases when studying the distribution of size and form of the structural components of the deposited metal (Fig. 20) it is seen that in the case of cladding the powder composition where the boron, titanium, and tungsten carbides were the crystallization centers (Fig. 20, e). In this case the average diameter and the size of the matrix structural components of the melt were significantly smaller than that of pure titanium with titanium oxide inclusions (Fig. 20, f). It becomes obvious that the crystallization centers, in this case, played a crucial role in the structure formation, in spite of the fact that in this case the mode with a higher laser power was used and the deposited metal was for a long time in the overheated state. As it is known, the high cooling rate leads to the formation of a highly dispersed structure. This occurs when the number of crystallization centers exceeds the rate of their growth at cooling of the molten metal. In our case, the introduction of additional crystallization centers allowed exceeding the growth rate of dendritic crystals in the matrix of the melt. In addition, these crystallization centers (TiC, BC, WC) have hardness 5 times greater than that of the deposited metal matrix and blade base metal. 127
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Fig. 18. Phase diagram of Ti-Al.
Fig. 19. Distribution of hardness between the deposited metal and the substrate, a – butt end of blade airfoil, b – section of the metal of the shroud platform.
The process of structure formation in the metal of the coatings based on titanium carbide with the addition of boron and tungsten carbides as well as commercially pure titanium with the addition of alumina occurs depending on both the qualitative and quantitative components of the initial powder composition and the technology of the cladding, which significantly affects the physical–chemical properties of the resulting intermetallic compound.
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Fig. 20. Analysis of the deposited metal structure: a, b, e – butt end of blade airfoil; c, d, f – metal section of the shroud platform.
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4. Conclusions 1. It is found that the technology of laser cladding is fully capable of controlling the process of structure formation, has a broad range of advantages and can be used to produce titanium aluminides and titanium borides with the desired chemical and physical properties. 2. It was established that the addition of 5 wt% BC and 5 wt% WC into the TiC powder composition makes it possible to obtain a coating with the titanium borides in its matrix with a uniform distribution of large solid heat-resistant BC and WC particles. This combination is achieved by melting fine powder particles, and coarse particles are the crystallization centers to obtain a highly dispersed matrix structure. 3. Mixing of the base metal results in the formation of the coating matrix with a hardness of 520 HV0.1, which is important when blades operate under alternating loads, at that the hardness of the inclusion particles was: WC = 1384 HV0.1, TiC = 2579 HV0.1, WC = 5108 HV0.1. 4. The addition of 2 wt% of Al2O3 with a particle size of 1–2 μm to commercially pure Ti makes it possible to obtain a coating for the blade airfoil with the hardness of 1.6 times higher than that of the base metal. At the same time, alumina particles melt, and Ti3Al phase is formed in the titanium matrix. The base metal and the deposited coating do not mix. Acknowledgments The authors are grateful to the Ministry of Education of the Russian Federation for supported research projects no. 14.578.21.0245 and 9.3236.2017/4.6. References [1] Boris Rottwinkel, Christian Nölke, Stefan Kaierle, Volker Wesling, Crack repair of single crystal turbine blades using laser cladding technology, Procedia CIRP 22 (2014) 263–267. [2] Eckart Uhlmann, Robert Kersting, Tiago Borsoi Klein, Marcio Fernando Cruz, Anderson Vicente Borille, Additive manufacturing of titanium alloy for aircraft components, Procedia CIRP 35 (2015) 55–60. 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