Microstructure of laser cladded carbide reinforced Inconel 625 alloy for turbine blade application

Author’s Accepted Manuscript Microstructure of laser cladded carbide reinforced Inconel 625 alloy for turbine blade application J. Huebner, D. Kata, J. Kusiński, P. Rutkowski, J. Lis www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(17)30570-9 http://dx.doi.org/10.1016/j.ceramint.2017.03.194 CERI14963

To appear in: Ceramics International Received date: 3 February 2017 Revised date: 27 March 2017 Accepted date: 27 March 2017 Cite this article as: J. Huebner, D. Kata, J. Kusiński, P. Rutkowski and J. Lis, Microstructure of laser cladded carbide reinforced Inconel 625 alloy for turbine blade application, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.03.194 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Title: Microstructure of laser cladded carbide reinforced Inconel 625 alloy for turbine blade application

Authors: J. Huebnera1*, D. Kataa, J. Kusińskib, P. Rutkowskia, J. Lisa a

Faculty of Materials Science and Ceramics, Department of Ceramics and Refractories, AGH

University of Science and Technology, Krakow, Poland b

Faculty of Metals Engineering and Industrial Computer Science, Department of Surface

Engineering and Materials Characterisation, AGH University of Science and Technology, Krakow, Poland

*

Corresponding author: Jan Huebner, phone number: +48 663 132 761; fax: +48 12 633 46 30.

e-mail: [email protected]

ABSTRACT: Inconel 625 - WC metal matrix composite is a very promising material for high temperature applications. In this study, microstructure investigation and phase composition of a mixture between Inconel 625 and fine tungsten carbide (φ ≈ 0.64 µm) was performed by means of XRD, SEM with EDS and TEM with EDS. Two powder mixtures were prepared: 20 wt % of WC and 30 wt % of WC and deposited on Inconel 625 substrate by laser cladding obtaining a crack and pore free material. The high temperature of the process resulted in partial dissolution of WC in Inconel 625 matrix. In sample containing 30 wt % of WC appearance of topologically closepacked (TCP) phases was observed at grain boundaries. WC, W2C, NbC, W6C2.54 and (W,Cr,Ni)23C6 were detected by XRD. Angular residual carbides and spherical oxide precipitates were visible in both types of samples. Processes occurring during laser action were explained. Keywords:

1

Postal address: ul. Smoluchowskiego 6/14, 30-069 Kraków.

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metal matrix composites; ceramic reinforcement; Inconel 625; tungsten carbide; additive manufacturing; microstructure;

1. Introduction The need of innovative materials for high temperature applications rose abruptly in the recent years [1][2][3]. Most of them due to the need of increased efficiency of various engines e. g. turbines. Higher work temperature is reflected in greater energy output. The main problems are mechanical properties of the used materials and their corrosion resistance. Because of work environment specification, turbine blades are exposed to chemical, temperature and abrasive factors that lead to their destruction. In order to obtain materials that are suitable for such an aggressive environment, both new materials and production methods had to be developed and adapted. Obtaining protective coatings on surface is cost-effective because of relatively small amount of material needed in comparison to producing the whole element [4]. Nickel based superalloys such as Inconel, are very important because of their excellent properties: high ductility, high toughness, very good oxidation- and wear-resistance at elevated temperatures (≈ 650oC). Additionally, Inconel alloys combines good pitting corrosion resistance in broad spectrum of temperatures with excellent weldability [5][6][7][8]. For the sake of further improvement of mechanical properties, metal matrix composites (MMC) were considered. Introduction of ceramics as reinforcement particles into the metal matrix allows for better mechanical performance. Typical ceramic properties of tungsten carbide: high hardness and good wear resistance is combined with its good wettability by nickel. Because of that it is suitable material for reinforcement in Inconel based MMCs. Rapid prototyping as a manufacturing technique has widely developed in recent years. High speed of process and versatility of material choice caused great rise in its popularity. Few limitations of geometrical orientation of produced elements are additional advantages of that method [9][10]. Laser cladding is the technique that allows to obtain chemically homogenous

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and crack- and pore-free coatings. Due to the high cooling rate, fine structure of material can be easily obtained. Properties of material can be modified by changing process parameters such as power density, laser scanning velocity, powder/wire feed rate etc. In this study, powder laser cladding is investigated because of better connection between coating and substrate than in wire laser cladding [11][12]. Previous experiments [13][14] confirmed that addition of tungsten carbide to Inconel 625 leads to better mechanical performance. However, most of researches were focused on relatively big WC particles (φ = 50 - 150 μm) introduced separately with Inconel 625 powder [15][16]. In this study, fine WC powder (φ ≈ 0.64 μm) was previously mixed with Inconel 625 powder. Microstructure of obtained samples were observed and investigated in order to better understanding of processes occurring in material during laser cladding. Due to the high energy density of laser beam, temperatures during process are very high. It can be the reason of material cracking because of thermal stresses appearance. Dissolution of carbide particles in metal matrix leads to formation of secondary carbides by reaction with the alloying elements. Some authors proved that segregation of specific elements in a dendrite axis are conditioned by partition coefficient k [17][18], defined by:

k  Ccore / C0 (Equation 1) where Ccore – concentration of element in cell or dendrite core, C0 – average concentration of element in the analyzed area of the material. Elements which partition coefficient k < 1 shows tendency to segregation at grain boundaries. While elements which partition coefficient k > 1 segregates in dendrite axis. For nickel-based superalloys, elements with the diameter of an atom similar to Ni, such as Cr, Fe and Co - have partition coefficient k value close to 1. For molybdenum and niobium partition coefficient k < 1 - respectively kMo ≈ 0.85 and kNb ≈ 0.5 for nickel binary alloys, and keeps getting lower with increased amount of Fe in material [18]. For tungsten, partition coefficient kW ≈ 1.67 for nickel binary alloy [19]. Because of segregation and supercooling during laser cladding, formations of MxCy carbides and intermetallic phases are possible due to eutectic reaction occurring by rapid solidification [20]. This is the reason why

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obtained material is enriched with both Mo and Nb. In the final stage of solidification topologically close-packed (TCP) phases are expected to appear. 2. Results 2.1. Samples preparation Commercially produced Inconel 625 powder having spherical particles of average diameter φ = 104 μm was used. The elemental composition of the powder is given in Table 1. The tungsten carbide powder having angular grains of average diameter φ ≈ 0.64 µm was used as reinforcement in the composite. The WC particle size is ranged from 0.1 µm to 8.0 µm as shown in Fig. 1. Bi-modal character of grain size distribution is visible. The mode of particles appeared at 0.8 µm with 10% of vol. fraction, however the second small peak coming from 0.15 µm grains was registered as well. Two different powder mixtures were prepared, namely Inconel 625 + 20 wt % of WC and Inconel 625 + 30 wt % of WC. To produce 100 g of mixture, set amount of powders were put inside milling chamber with addition of 100 ml of isopropyl alcohol. Tungsten carbide balls were used as grinding media. Small amount of resin - 0.025 g - was added to provide better adhesion of WC powder onto Inconel 625 particles. Homogenization process lasted for 90 minutes with the intensity of 500 rpm. The morphology of homogenized powder is shown in Fig. 2A. Inconel 625 large grains are covered by fine WC particles as it shown in higher magnification in Fig. 2B. Thus it ensures a good contact between Inconel and WC particles for further laser processing. Laser cladding was performed using apparatus supplied by JK Laser Company model JK2000FL equipped with ytterbium doped wire fiber. The laser beam with wavelength of 1063 ± 10 was used. Powder was deposited onto substrate material by using 4 nozzles located around cladding head. High energy density of laser beam caused Inconel 625 particles to create melt pool during the laser action. As the cladding head moved forward, molten material was solidified and welded to the substrate. Scheme of the process is shown in Fig. 3.

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Laser cladding parameters were optimized and presented in Table 2. Thus, it is allowed to produce homogenous, crack-free coatings. In order to avoid heat stress and sample distortion, entire coating was prepared using layer by layer technique [11][12]. The total amount of 6 thin sublayers were deposited on each sample to obtain final thickness of about 1.0 mm. Temperature of melt pool was measured by radiation pyrometer during the laser action and reached maximum of 1750 ± 5oC. To avoid appearance of non-alloying elements in samples, Inconel 625 made by Cold Metal Transfer (CMT) technique was used as base material (called further substrate material) for laser cladding. The technique was adapted and thoroughly described by Kusiński et al. [17]. Cleaning in ultrasonic cleaner in isopropanol was performed to remove dust and grease from substrate material. Samples were cut by diamond saw parallelly to the surface. Cross section was grinded using 80 to 1200 grit diamond discs, then gradually polished by different diamond suspensions from 6 µm to 1 µm. The samples were cleaned in ultrasonic cleaner and electrochemically etched in 10% CrO3 water solution, by voltage of 1.6 V for 10 seconds. Scanning electron microscopy examination (SEM) and energy dispersive X-ray analysis were performed by NOVA NANO SEM 200 apparatus equipped with EDAX EDS analyzer. EDS analysis was performed using standardless normalized ZAF correction. Boundary between deposited material and substrate was examined by HITACHI S-3500N equipped with EDS NORAN 986B-1SPS EDS

analyzer. Phase composition of samples were checked by X-ray diffraction analysis using PANalitycal X-ray Diffractor (XRD) and X-pert HighScore software. The transmission electron microscopy (TEM) observations using 200 kV JEAOL JEM-2010ARP microscope were performed. Additionally TEM EDS analysis of selected samples were performed to find out elemental composition of spherical precipitates. 2.2. X-ray diffraction analysis The XRD analysis were performed on two types of samples described previously in Table 2. As shown in Fig. 4, phase composition were significantly different for each type. Samples containing lower amount of tungsten carbide - 20 wt % WC, showed presence of austenitic

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structure γ-Ni phase and small amount of pure WC - Fig. 4A. In samples containing 30 wt % WC - Fig. 4B, appearance of secondary carbides with higher amount of Nb, Cr and W was observed: WC, W2C, NbC, W6C2.54 and (W,Cr,Ni)23C6. Because of fine size of WC grains (φ ≈ 0.64 µm), secondary carbides had formed due to partial WC dissolution in the nickel-based alloy matrix. The amount of tungsten carbide addition in samples containing 30 wt % of WC, resulted in formation of secondary carbides with alloying elements - Nb, that tend to segregate at grain boundaries. 2.3. Microstructure Observation The boundary between deposited material and substrate is shown in Fig. 5. Base material was melted together with powder mixture by laser beam forming melt pool. As the laser moves further material solidified. The welded area is uniform and homogenous without any porosity. Deposited

Inconel 625 - WC composite material is characterized by crack and pore-free

microstructure. Figures 6 and 7 represents SEM image of sample containing 20 wt % WC and Fig. 8 and 9 shows sample containing 30 wt % of WC. Two separate phases were observed. Metallic grains - called further phase A, is visible as gray area. The second phase, located at the grain boundaries - called further phase B is shown as white area. The EDS and mapping were performed.. Table 3 represents composition for 20 wt % WC sample, while Table 4 represents 30 wt % WC sample. As shown in Table 3, phase A contains higher amount of Cr, Ni and W. In contrast, analysis for phase B shows increased amount of Nb and Mo. Elemental maps presented in Fig. 7 shows clearly that molybdenum and niobium segregates to grain boundaries. It confirms results obtained by other authors regarding element segregation in nickel-based alloys [17][18]. It seems that during laser cladding process, tungsten carbide particles were partially dissolved in metal matrix. Despite this secondary carbides were not detected by XRD – as shown in Fig. 4A.

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The Fig. 8 shows microstructure and EDS analysis of Inconel 625 - 30 wt % WC samples. The microstructure is quite different than for the samples containing lower amount of WC. As shown in Fig. 8, phase B is located at grain boundaries. For Inconel 625 - 30 wt % WC sample, phases A and B have similar amount of Cr, Fe and W. As presented in Table 4, slightly higher concentration of Ni was detected in phase A. Increased amount of Nb and Mo was detected in the phase B as a result of segregation. Maps present in Fig. 9 shows that molybdenum and niobium are located at grain boundaries. Tungsten diffused into Inconel 625 grains during laser processing. Additionally, typical eutectic structure of phase B was observed - Fig. 8. This indicates that there could be more than one product during solidification which coincides with the results of XRD analysis - Fig. 4. Carbon content in matrix is not shown, because its real content in matrix is low and EDS does not allow reliable estimate of its value. The transmission electron microscope images presented in Figures 10-14 are showing bright and dark field microstructure appearance, typical for each type of samples. Fig. 10A shows sample containing 20 wt % of WC. Spherical precipitates were confirmed as oxides (Fig. 12) while angular - were most likely residual WC carbides. Fig. 10B shows microstructure of sample containing 30 wt % of WC. Observed microstructure was different than for 20 wt % of WC sample. Precipitates of topologically close-packed (TCP) phases observed at grain boundaries are marked by arrow. TCP phases contains γ, η and Laves phase. According to DuPont et al. [6,18], formation of Laves phase occurs during crystallization of Inconel 625 in 1200oC by the following scheme:

Liquid  Liquid    Liquid    NbC    NbC  Laves

Laves phase is typically forming in nickel based alloys due to high amount of Nb in Inconel 625. Small amount of angular residual WC carbides was also observed in this sample. Figure 11 shows TEM images of Inconel 625 - 20 wt % WC sample. As presented in Fig. 11 spherical precipitates were observed. Figure 11B presents dark field image recorded in marked (in inset) reflection from TCP phase. Reflections originating from metal matrix are much

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stronger than reflections from TCP phase. They are visible on diffraction pattern and represents γ-Ni matrix. Because of lower addition of fine tungsten carbide powder, amount of γ, η and Laves phases was not detected by X-ray diffraction analysis. Small precipitates of TCP phases were visible at the grain boundaries. Amount of carbon originated from WC in the system, allowed formation of TCP phases only in some areas of the material. To check elemental composition of spherical precipitate visible in Fig. 12, TEM EDS analysis was performed as shown in Table 5. Diameter of spherical particle was measured to be about 600 nm. Presence of 16 wt % of oxygen and 40 wt % of chromium indicates that this is oxide indeed. Increased concentration of 7 wt % of Ti, 4 wt % of Si, 5 wt % of Al and 6 wt % of Mn - typically very low for Inconel 625 (Table 1) were also detected. Measured 22 wt % of Ni may originate from matrix. Despite using argon protective atmosphere during laser deposition of coating small amount of oxygen from powder impurities was present. High chemical affinity between oxygen and alloying elements was the reason of oxides’ appearance. Figure 13 presents Inconel 625 - 30 wt % WC sample. Large amount of dislocations was observed in γ-phase as well as TCP phase precipitates. In comparison to 20 wt % of WC samples, this type of sample has increased amount of TCP phases consisting of carbides (Fig. 4B) and Laves phase. Fig. 13B shows dark field image of TCP phase. Precipitates of TCP phase were formed at grain boundaries together with carbides during rapid solidification characteristic for laser cladding process. In Fig. 14A, three different crystallites were observed. Black TCP phases precipitates are visible at the grain boundaries - marked by arrows. Area diffraction pattern in Fig. 14B, shows that three visible crystallites have almost the same crystallographic orientation. This indicates that growth of these crystallites proceeded in similar direction. 3. Discussion Presented results shows how addition of fine tungsten carbide particles affects Inconel 625 microstructure. Laser cladding as material deposition technique allowed preparation of coatings that are characterized by fine structure. Because of high thermal conductivity of

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Inconel 625 in temperatures above 600oC - 21.3 W/m·K, distribution of heat generated by laser action proceeds extremely fast. Rapid solidification resulted in fine microstructure. As shown in Fig. 15 there are 3 main steps of laser cladding process in Inconel 625 -WC system. A - powder mixture consisted of Inconel 625 particles with average diameter of φ = 104 µm, homogenously covered by angular WC grains with average diameter of 0.64 µm was spread on the substrate surface. B - Due to the laser action temperature increased rapidly to maximum of about 1750oC which caused partial dissolution of WC particles in metal matrix. Because tungsten partition coefficient k is higher than 1 it started to diffuse into liquid metal matrix. On the other hand partition coefficients for niobium and molybdenum are lower than 1 which resulted in segregation at grain boundaries during solidification. C - solidification of coating finished. Chromium, niobium and molybdenum formed TCP phases consisting of MxCy carbides and Laves phase. They were observed together with residual WC at grain boundaries. Because of diffusion at elevated temperature, recrystallized metallic grains were enriched with tungsten. Refined microstructure appeared after laser processing. It is the result of melting and rapid solidification of Inconel 625 which led to formation of dense polycrystals. Average grain size of obtained coating was about 3 - 15 µm. Eutectic reaction during supercooling of material resulted in formation of characteristic structure between metallic grains as shown in Fig. 8 and Fig. 13. This kind of structure is typical for TCP phases. XRD analysis of samples containing 30 wt % of WC, detected following carbides: WC, W2C, NbC, W6C2.54 and (W,Cr,Ni)23C6. They were not detected in samples containing 20 wt % of WC. Microstructure observations shows presence of spherical precipitates in both types of samples. EDS analysis confirms that they have higher concentration of chromium, silica, titanium and oxygen in comparison to whole volume of sample. Due to chemical affinity between mentioned metals and oxygen, they formed oxides. Angular precipitates of WC was also observed in both types of samples. 4. Conclusions

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Summing up the presented results: 1) Inconel 625 - WC metal matrix composite coatings were successfully deposited on Inconel 625 substrate. No significant structural defects, such as cracks and pores were detected. 2) Laser cladding of Inconel 625 - WC allowed to obtain fine microstructure of deposited material. Due to rapid solidification average size of grains in coatings were about 3 - 15 µm. 3) Changing composition of mixtures of Inconel 625 - fine tungsten carbide powders, resulted in different microstructure of obtained material. Samples containing 30 wt % WC shows appearance of secondary carbides. High temperature of about 1750oC during laser cladding process resulted in partial dissolution of WC in metal. This allowed formation of topologically close-packed (TCP) phases that consisted different carbides: WC, W2C, NbC, W6C2.54, (W,Cr,Ni)23C6 and Laves phase. Characteristic structure of TCP phases was observed. 4) Because of chemical affinity between oxygen and alloying elements - Ti, Si, Al, Mn, Cr, spherical oxide precipitates were observed. Observed oxides had diameter of about 500 - 600 nm. 5) Both TCP phase and carbides are expected to improve mechanical properties of the obtained metal matrix composite coating. The effect of addition of fine WC on hardness, wear resistance etc. needs to be further investigated. Acknowledgements: This work was performed within the framework of funding for statutory activities of AGH University of Science and Technology in Cracow, Faculty of Materials Science and Ceramics (11.11.160.617).

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FIGURE CAPTIONS: Fig. 1. Particle size distribution of tungsten carbide powder characterized by bi-modal distribution Fig. 2. The SEM images showing Inconel 625 - WC powder mixture after 90 minutes of homogenization in ball mill Fig. 3. Schematic illustration of laser cladding powder deposition Fig. 4. XRD analysis of Inconel 625 - WC coatings: A - 20 wt % of WC sample shows presence of γ-Ni and WC; B - 30 wt % of WC sample shows presence of γ-Ni and various carbides: WC, W2C, NbC, W6C2.54 and (W,Cr,Ni)23C6 Fig 5. SEM image of cross-section showing boundary between deposited Inconel 625 - WC composite and substrate material - Inconel 625 Fig. 6. SEM image of Inconel 625 - 20 wt % WC sample Fig. 7. SEM elemental maps of Inconel 625 - 20 wt % WC sample Fig. 8. SEM image of Inconel 625 - 30 wt % WC sample Fig. 9. SEM elemental maps of Inconel 625 - 30 wt % WC sample Fig. 10.TEM images of typical microstructure of samples: A - residual WC carbide; B topologically close-packed (TCP) phases present at grain boundaries Fig. 11. TEM images of Inconel 625 - 20 wt % WC sample; A - spherical oxide precipitates and TCP phases; B - dark field image recorded in marked in inset TCP phase reflection Fig. 12. TEM image of spherical precipitate - confirmed to be oxide by EDS analysis in TABLE 5 Fig. 13. TEM images of Inconel 625 - 30 wt % WC sample. A - TCP phases with large amount of visible dislocations; B - area diffraction pattern of shining TCP phases

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Fig. 14. TEM images of Inconel 625 - 30 wt % WC sample. A - 3 metallic crystallites with TCP phases at the grain boundaries; B - area diffraction pattern of TCP phase, 3 similar crystallographic orientations for metallic crystallites are visible Fig. 15. Schematic illustration of processes during laser cladding of Inconel 625 - WC system

FIGURES Figure 1.

Figure 2.

15

Figure 3.

Figure 4.

16

Figure 5.

Figure 6.

17

Figure 7.

Figure 8.

Figure 9.

18

Figure 10.

Figure 11.

19

Figure 12.

Figure 13.

Figure 14.

20

Figure 15.

21

TABLES TABLE 1: Nominal composition of Inconel 625 in weight %

Element

Concentration [wt %]

Ni

> 58.0

Cr

20.0 - 23.0

Mo

8.0 - 10.0

Nb

3.15 - 4.15

Fe

< 5.0

C, Mn, Si, Al, Ti

< 0.5

TABLE 2: Laser cladding process parameters

Parameter

20 wt % WC

30 wt % WC

Laser beam diameter [µm]

500

500

Nominal laser power [W]

320

320

Laser work mode

Continuous

Continuous

Scanning velocity [mm/s]

10

10

Powder feed rate [g/min]

8.09

8.16

Number of tracks in sublayer

10

10

Number of sublayers

6

6

Distance between center of tracks [mm]

0.8

0.8

Track length [mm]

10.4

10.3

Clad thickness [mm]

1.03

1.05

22

TABLE 3: SEM EDS analysis of Inconel 625 - 20 wt % WC sample presented on Fig. 6 (weight %)

20 wt % WC

Nb

Mo

Cr

Fe

Ni

W

Phase A - metallic grains

2

7

17

1

57

9

Phase B - TCP phases

26

12

12

1

30

5

TABLE 4: SEM EDS analysis of Inconel 625 - 30 wt % WC sample presented on Fig. 8 (weight %)

30 wt % WC

Nb

Mo

Cr

Fe

Ni

W

Phase A - metallic grains

1

6

17

2

56

12

Phase B - TCP phases

10

9

16

1

44

13

TABLE 5: TEM EDS analysis results of spherical precipitate presented in Fig. 12

Element

Weight %

Atomic %

O

16

36

Al

5

6

Si

4

6

Ti

7

6

Cr

40

28

Mn

6

4

Ni

22

14

Total

100

100

23