Ti3Al intermetallic composite coatings on Ti6Al4V alloy by laser cladding process

Accepted Manuscript Microstructure and high temperature oxidation resistance of in-situ synthesized TiN/ Ti3Al intermetallic composite coatings on Ti6Al4V alloy by laser cladding process Hongxi Liu, Xiaowei Zhang, Yehua Jiang, Rong Zhou PII:

S0925-8388(15)31424-9

DOI:

10.1016/j.jallcom.2015.10.168

Reference:

JALCOM 35723

To appear in:

Journal of Alloys and Compounds

Received Date: 9 August 2015 Revised Date:

6 October 2015

Accepted Date: 19 October 2015

Please cite this article as: H. Liu, X. Zhang, Y. Jiang, R. Zhou, Microstructure and high temperature oxidation resistance of in-situ synthesized TiN/Ti3Al intermetallic composite coatings on Ti6Al4V alloy by laser cladding process, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.10.168. 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 proof before it is published in its final 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.

ACCEPTED MANUSCRIPT

Microstructure and high temperature oxidation resistance of in-situ synthesized TiN/Ti3Al intermetallic composite coatings on Ti6Al4V alloy by laser cladding process Hongxi Liu, Xiaowei Zhang, Yehua Jiang, Rong Zhou

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School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China

ABSTRACT

High temperature anti-oxidation TiN/Ti3Al intermetallic composite coatings were fabricated with the powder and

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AlN powder on Ti6Al4V titanium alloy surface by 6 kW transverse-flow CO2 laser apparatus. The chemical composition, morphology and microstructure of the TiN/Ti3Al composite coatings were characterized by optical

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microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). In order to evaluate the high temperature oxidation resistance of TiN/Ti3Al coating, the isothermal oxidation test was performed in a high temperature resistance furnace at 600

and 800

, respectively. The result shows that the

composite coating has a rapidly solidified fine microstructure consisting of TiN primary phase (granular-like, flake-like or dendrites), with an even distribution in Ti3Al matrix. It indicates that a physical and chemical reaction between Ti

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powder and AlN powder has completely occurred under the laser irradiation condition. In addition, the microhardness of the TiN/Ti3Al intermetallic composite coating is 3.4 times higher than that of the Ti6Al4V alloy substrate and reaches 844 HV0.2. The high temperature oxidation behavior test reveals that the high temperature oxidation resistance

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of TiN/Ti3Al composite coating is much better than that of titanium alloy substrate. The excellent high temperature oxidation resistance of TiN/Ti3Al intermetallic composite coating is attributed to the formation of reinforced phases TiN, Al2O3 and TiO2. The laser cladding TiN/Ti3Al intermetallic composite coating is anticipated to be a promising high

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temperature oxidation resistance coating for Ti6Al4V alloy. Key words: Laser cladding; Intermetallic composite coating; Titanium alloy; High temperature oxidation resistance; Microstructure

*

Corresponding author (Rong Zhou, Hongxi Liu). Tel.: 86-0871-65136755; Fax: 86-0871-65136755

E-mail address: [email protected] (Rong Zhou, Hongxi Liu)

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ACCEPTED MANUSCRIPT 1. Introduction Titanium and its alloys are widely used as important structural materials in aviation, aerospace, petrochemical and auto industries owing to its low density, high specific strength, exceptional corrosion resistance and excellent

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solderability [1-3]. However, titanium and its alloys also show some undesirable properties, such as poor high temperature oxidation resistance, lubrication, friction and wear behavior, which limit titanium alloy further application in high temperature (>500

) environment and the engineering tribological components field [4-6]. In order to solve

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these problems, various surface treatment techniques, especially the laser cladding surface modification technology has been widely used in the past decades [7-12].

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The addition of titanium nitride to fabricate Ti3Al intermetallic matrix composite coating has been proved to be a promising method for improving the high temperature oxidation resistance and tribological properties of titanium alloy because of the high hardness, low density and excellent stability of Ti3Al intermetallic composite coating under high temperature conditions [13-16]. But the main problems in the development of ex-situ processing method are interfacial

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reactions between reinforcement and matrix, and poor wettability between reinforcement phase and matrix. To avoid the inherent problems of ex-situ processing method, in-situ processing technique has been developed owing to its

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unique advantages [17-21].

In recent years, the literatures regarding laser in-situ processing technique have been gradually increasing. However,

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most reports have focused on the surface tribological properties of machine parts [22-26]. Investigation rarely involves the high temperature oxidation resistance of industrial parts after being treated by laser in-situ synthesized technique. In this paper, high temperature anti-oxidation TiN/Ti3Al intermetallic composite coating was synthesized on Ti6Al4V alloy with different ratios of Ti and AlN two kinds of mixed powders in nitrogen atmosphere by laser cladding. The chemical composition, geometric morphology and microstructure characterization of TiN/Ti3Al laser cladding coatings were analyzed by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). The high temperature oxidation resistance of the composite coating was also 2

ACCEPTED MANUSCRIPT investigated under different high temperature conditions.

2. Experimental procedure A commercial alpha (α) and beta (β) two-phase Ti6Al4V titanium alloy was used as the substrate materials in this

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study. The principal chemical composition (wt. %) was listed in Table 1. The flat samples, 80 mm×40 mm×3 mm in size, were machined by mechanical polishing and cleaned repeatedly with anhydrous ethanol and acetone to remove remaining oil, oxides or other impurities on titanium alloy substrate surface. The commercial pure Ti powder (75 µm

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particle size, 99.5% purity) and pure AlN powder (9.6 µm particle size, 99.5% purity) were selected as the laser cladding materials. Three groups of mixed powder samples with various molar ratios of 4Ti:3AlN, 4Ti:2AlN and

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4Ti:1AlN were homogeneously ground in an agate mortar and then pasted on the titanium alloy substrate surface by an organic binder (5% polyvinyl alcohol solution) with a thickness of 1.0 mm. All samples were dried in a drying cabinet to vaporize water moisture in the pre-placed coatings at room temperature.

Laser in-situ synthesis experiment was carried out on a 6 kW transverse-flow continuous-wave CO2 laser apparatus

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with a five-axis CNC (computer numerical controlled) machine tool. Nitrogen gas was blown into the molten pool as shielding gas and reaction medium with a flow rate of 10 L·h-1 during laser cladding. The single-track laser cladding

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parameters were as follows: laser power 4.0 kW, laser scanning speed 6 mm·s-1, and laser beam circular spot diameter 5 mm. For convenience, the three kinds of laser cladding samples and untreated titanium alloy substrate sample were

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marked with symbol S1 (4Ti:3AlN), S2 (4Ti:2AlN), S3 (4Ti:1AlN) and S0 (substrate), respectively. After laser cladding, the composite coatings were sectioned along the perpendicular line to the scanning direction by wire-electrode cutting. One side of each coupon was ground with SiC abrasive paper No.600, 800, 1000 grits sequentially, followed by polishing with diamond paste (average size 1.0 µm). The metallographic sample was prepared using standard mechanical polishing procedures and etched in HF, HNO3 and H2O mixture solution (with a volume ration of HF:HNO3:H2O=2:1:17) at room temperature for approximately 15 s before OM and SEM observation. The microstructure of the laser cladding composite coating was analyzed using OM and EPMA-8705 scanning electron 3

ACCEPTED MANUSCRIPT microscope (SEM). The phase composition of laser cladding composite coating was analyzed using a D/max-3B X-ray diffractometer (Cu Kα radiation and a continuous scanning mode) by scanning angle in the 2θ = 20~100° range. The microhardness of the profile along the depth direction of the laser cladding composite coating was measured by an

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HVS-1000 microhardness tester with a load of 1.96 N and duration 15 s. The starting point was 200 µm from the coating surface, and the interval distance between each two points was 100 µm. Three points were measured at each position and the average of the three points as the microhardness value. and 800

) of all samples was evaluated in a chamber type

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The high temperature oxidation resistance (at 600

electric resistance furnace. Laser cladding composite coatings were cut into a thickness of 0.8 mm from the treated

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samples and the substrate was also cut into the same thickness. The test parameters: heating time for 15 hours and weighing once per 5 hours with a 10-5 g accuracy electronic balance, oxidation temperature for 600

and 800

,

respectively. The high temperature oxidation resistance of the laser cladding coating samples was characterized by three ways: Firstly, the oxidation kinetics curves; Secondly, the relative oxidation resistance, i.e. the ratio of oxidation weight

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increase of the original titanium alloy to that of the laser cladding samples; Finally, the oxidized coating color variation of the sample surface and the characteristics of oxidized specimen images were observed by OM.

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3. Results and discussion

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3.1 Microstructure of TiN/Ti3Al composite coatings

Fig.1 shows the surface macro-morphology of laser cladding in-situ synthesized TiN/Ti3Al intermetallic composite coatings. It can be seen that the powder in sample S1 and S2 (especially S1) are insufficiently combusted and in the laser coatings contain a large number of pores, while sample S3 exhibits a relatively sufficient combustion surface. The main reason is that there is not enough laser energy on the surface of sample S2; only the mixed powders bed was combusted and condensed into particles as a result of surface tension. When the prepared Ti/AlN powder bed was irradiated by high energy laser beam in the nitrogen environment, the Ti/AlN mixed powder effectively absorbed the laser energy, which led to the melt of Ti powder and AlN powder and the chemical reaction of a liquid-solid solution 4

ACCEPTED MANUSCRIPT occurred immediately (see Equation 1). In addition, there was another probable chemical reaction between Ti and N under the high energy laser beam irradiation (see Equation 2), from which the TiN phase was produced due to the good affinity of titanium and nitrogen [1, 5].

4 Ti + AlN → Ti 3 Al + TiN 1 N 2 → TiN 2

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Ti +

(1) (2)

The XRD patterns of the three kinds of laser cladding samples are shown in Fig.2. It can be seen that sample S2 and

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sample S3 were mainly composed of TiN reinforced phase and Ti3Al intermetallic matrix phase. From Fig.2, the diffraction peak intensity of TiN and Ti3Al phases decreased with the decrease of the AlN powder molar ratio. In

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addition, the TiN phase in sample S3 shows an obvious preferential orientation in (111) plane, which is mainly related to the TiN content. The existence of TiN and Ti3Al phases indicated that the reaction between Ti powder and AlN powder can be occurred in the laser cladding process [27, 28]. Fig.3 shows the overview microstructure on the cross section of TiN/Ti3Al intermetallic compound composite coatings. It can be clearly seen that the TiN/Ti3Al intermetallic

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compound composite coatings indicated a good metallurgical bonding with the Ti6Al4V titanium alloy substrate. Comparison of the results of sample S2 and sample S3, it reveals that the microstructure of sample S3 is more dense

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and homogeneity than that of sample S2, and there are no crack or pore defects, which is in good agreement with the macro-morphology results in Fig.1.

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Fig.4 shows the typical SEM morphologies of TiN/Ti3Al intermetallic composite coatings with different molar ratio of AlN powder. It can be seen from Fig.4, the volume fraction of the reinforced phase in TiN/Ti3Al intermetallic composite coating increased with the decrease of the AlN powder molar ratio. As also can be seen clearly from Fig.4, the TiN phase in sample S2, being granular-like and undeveloped dendrites, was embedded in Ti3Al intermetallic matrix phase. The granular-like and dendrite phases were dense in some areas in sample S2, which showed a non-uniform distribution of the two kinds of particles in TiN/Ti3Al intermetallic compound composite coatings. For sample S3, the microscopic morphology of TiN phase changed from granular-like to undeveloped and developed dendrites, and a lot of 5

ACCEPTED MANUSCRIPT granular-like phases were distributed evenly in needle-like phases. According to the above-mentioned XRD results (see Fig.2) and the following EDS analysis data (see Table 2) can be drawn a conclusion that the granular-like and dendrites phases are TiN reinforced phases and the needle-like phase is Ti3Al intermetallic matrix phase. For more clarity, Fig.5

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showed the high magnification SEM images of TiN/Ti3Al laser cladding composite coatings corresponding to Fig.4. The content of Ti, N and Al elements in granular and dendrite reinforced phases of TiN/Ti3Al intermetallic compound composite coatings with different molar ration of AlN powder was determined by EDS analysis, and the results of

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points 1, 2 and 3 in Fig.5 were shown in Table 2. As can be seen from Table 2, the Al, N and Ti element contained in point 1 (corresponding Fig.5a) was 71.40 at.%, 21.78 at.% and 5.20 at.%, respectively. But the atomic percentage of Al

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in TiN phase, AlN phase and Ti3Al phase was 0.00, 65.85 at.% and 15.88 at.%, respectively. Therefore, considering the XRD patterns and the Al content in granular-like and dendrite reinforced phases in TiN/Ti3Al intermetallic composite coatings, the granular-like and dendrite reinforced phases were mainly composed of TiN phase. Similarly, the chemical composition of point 2 (corresponding to Fig.5a and Fig.5b) was also mainly composed of TiN phase. But for point 3 in

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Fig.5a and point 1, point 3 in Fig.5b, the needle-like phase was mainly composed of Ti3Al. All these proved that in-situ TiN/Ti3Al intermetallic compound composite coating can be synthesized on the Ti6Al4V alloy surface by the laser

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cladding process. The cladding coating prepared by a higher molar ratio of Ti and AlN powder is more uniform than that of the lower molar ratio. The main reason is that the reaction between 4 mol Ti and 1 mol AlN need less laser

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energy than that of between 4 mol Ti and 2 mol AlN and 4 mol Ti and 3 mol AlN.

3.2 High-temperature oxidation resistance of TiN/Ti3Al composite coating The oxidation kinetics curves of untreated titanium alloy and TiN/Ti3Al intermetallic compound composite coating samples are exhibited in Fig.6. These curves depicted the oxidation weight gain of different samples with the oxidation time. The corresponding results of oxidation weight gain summarized in Table 3, which indicated the relative oxidation resistance for three groups of specimens at 600

and 800 . Comparison of the results between bare titanium alloy

and the samples of in-situ synthesized TiN/Ti3Al intermetallic compound composite coatings, it can be found that the 6

ACCEPTED MANUSCRIPT oxidation kinetics curves for sample S3 is approximately linear increase within 15 hours at 600 relative oxidation resistance for sample S3 reached 6.83 at 600

and 1.94 at 800

and 800

, and the

, respectively.

For sample S2, the oxidation kinetics curve is also approximately linear increase at 600

, while at 800

, it is

relative oxidation resistance at 600

and 800

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similar to the oxidation kinetics curve of S0, which shows a rapid increase in the beginning 5 hours. In addition, the for sample S2 is 5.63 and 1.39. This proved that the high temperature

oxidation resistance of bare titanium alloy was poor whether at 600

or 800

. Whereas, the high temperature

were much better than that of bare Ti6Al4V titanium alloy at 600

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oxidation resistance for in-situ synthesized TiN/Ti3Al intermetallic compound composite coatings by laser cladding and 800

.

and 800

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Fig.7 is the XRD patterns of unoxidized TiN/Ti3Al laser cladding coating (sample S3) and oxidized coatings at 600 . It can be seen clearly that after 15 hours oxidized at 600

, the chemical composition change of in-situ

synthesized TiN/Ti3Al intermetallic composite coatings is not obvious, which is still composed of TiN reinforced phase and Ti3Al matrix phase. This indicated that the high temperature oxidation resistance of titanium alloy had been

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improved significantly by the laser cladding process. However, an oxide scale consisted of anatase-TiO2 as the major phase and α-Al2O3 as the minor phase formed on the surface of the coating after oxidation at 800

, and the diffraction

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peak intensity of TiO2 was much higher than that of α-Al2O3. The main reason, on the one hand, is that the affinity ability between Ti and O is stronger than that of Al and O, on the other hand is that the content of Ti in laser cladding

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composite coating is higher than that of Al. In addition, Ti diffuses faster than Al towards the surface, thus a major TiO2 phase at the surface layer of the oxide composite coating samples can be formed [29]. Fig.8 and Fig.9 showed the surface oxide layer images of titanium alloy substrate and TiN/Ti3Al intermetallic compound composite coatings after oxidizing at 600

and 800

for 15 hours. At 600

, the surface of the bare

titanium alloy was multicolored (Fig.8a), while that of the TiN/Ti3Al laser cladding coating sample was slightly changed. From Fig.8b, it can be observed that a lot of brittle and loose TiO2 hole formed on the Ti6Al4V titanium alloy surface. The presence of these TiO2 hole caused the oxidation weight gain of Ti6Al4V titanium alloy. However, from 7

ACCEPTED MANUSCRIPT Fig.8c and Fig.8d, there is no oxide on the surface of laser cladding TiN/Ti3Al intermetallic composite coating. These phenomena are in agreement with the above-mentioned oxidation kinetics curves analysis results. At 800

, from Fig.9a, the surface of the bare Ti6Al4V titanium alloy presented a large amount of oxide area and

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was severely damaged. The color of sample S2 became dark significantly. This indicated that the oxidation of the two samples had occurred obviously, as shown in Fig.9b and c. For sample S3, surface oxide also be generated at 800

for

15 hours, but the surface oxide is not obvious. The morphology of oxide area is uniform and dense due to the

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appearance of Al2O3, which can be clearly seen from Fig. 9d. According to literature [30], during the oxidation process, the affinity between Al and O is stronger than that of Ti and O. So the Al2O3 oxide area would be preferentially formed

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on the laser in-situ synthesized TiN/Ti3Al composite coating. Therefore, the TiN/Ti3Al laser cladding composite coating showed an excellent high temperature oxidation resistance.

According to the above results and the distinct microstructure characteristics of laser in-situ synthesized TiN/Ti3Al intermetallic compound composite coating, a hybrid dense and continuous (Al2O3/TiO2) oxide area can be formed under

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long-time oxidized at high temperature, which reduces the growth velocity of porous TiO2 oxide and slows down the further oxidation. Therefore, the laser cladding composite coating can exhibit excellent high temperature oxidation

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resistance. Combining with the above-mentioned Fig.5 and Table 2, we can draw a conclusion that in-situ synthesized TiN/Ti3Al intermetallic compound composite coating with a molar ratio of 4Ti:1AlN shows better high temperature

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oxidation resistance.

3.3 Microhardness of TiN/Ti3Al composite coating Surface microhardness distribution curves of bare titanium alloy and in-situ synthesized TiN/Ti3Al intermetallic composite coatings along the depth direction are plotted in Fig.10. The microhardness profile of every area, including cladding zone (CZ), bonding zone (BZ), heat affected zone (HAZ) and substrate (SUB), are illustrated from left to right hand side, respectively. It can be seen that the microhardness of laser cladding composite coating gradually decreased from the top surface to the bottom, and was much higher than that of titanium alloy substrate. For sample S2, the 8

ACCEPTED MANUSCRIPT microhardness changed drastically and presented a jagged fluctuation distribution except in coating/substrate bonding zone. The laser cladding TiN/Ti3Al intermetallic compound composite coating showed a very inhomogeneous microhardness distribution corresponding well with inhomogeneous microstructure and distribution of hard TiN phase

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(Fig.4a). In addition, the microhardness of TiN/Ti3Al intermetallic composite coating is between 478 HV0.2 and 1448 HV0.2, which is about 2-6 times of the Ti6Al4V titanium alloy substrate hardness (240-250 HV0.2). Dispersed TiN particles and refined grains enhance the TiN/Ti3Al composite coatings and significantly improve its microhardness.

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Compared with sample S2, sample S3 has an average microhardness 844 HV0.2 and 3.4 times higher than that of bare titanium alloy, then the microhardness decreases gradually to the substrate, which is mainly attributed to the uniform

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distribution of hard TiN phase (Fig.4b). Therefore, the molar ratio of Ti:AlN has an important effect on the microstructure and mechanical property of TiN/Ti3Al intermetallic compound composite coating.

4. Conclusions

High temperature anti-oxidation TiN/Ti3Al intermetallic compound composite coating was fabricated on Ti6Al4V

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titanium alloy surface by laser cladding in-situ synthesized with Ti/AlN mixed powder. The laser cladding coating was mainly composed of α-Ti, TiN and Ti3Al phases. TiN/Ti3Al intermetallic compound composite coating produced by the

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molar ratio of 4Ti:1AlN has a dense microstructure with a uniform distribution of primary TiN phase in Ti3Al intermetallic matrix. The relative oxidation resistance of TiN/Ti3Al coating at 600

and 800

reaches 6.83 and 1.94,

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respectively. The microhardness of TiN/Ti3Al intermetallic composite coating has a 3.4 times higher than Ti6Al4V titanium alloy substrate. It proved that laser cladding TiN/Ti3Al intermetallic composite coating can effectively improve the high temperature oxidation resistance and mechanical property of the Ti6Al4V titanium alloy.

Acknowledgements This research was jointly supported financially by the National Nature Science Foundation of China (Grant No.61368003 and No.51165015). The authors would like to thank Qingming Chen and Chunjian Wang in Kunming University of Science and Technology for the XRD and SEM measurements. 9

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properties of in situ synthesized TiC, TiN, and SiC reinforced Ti3Al intermetallic matrix composite coatings

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Tables

Table 1 Chemical composition of Ti6Al4V titanium alloy Al

V

Fe

C

N

H

Content (wt. %)

6.02

4.0

0.30

0.08

0.05

0.015

O 0.20

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Ti

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Element

Bal.

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Table 2 EDS analysis results (atomic fractions, at.%) of points 1, 2 and 3 in Fig.5a and Fig.5b Fig. 5a. 2

Fig. 5a. 3

Fig. 5b. 1

Fig. 5b. 2

Fig. 5b. 3

Ti

71.40

69.65

85.93

84.29

78.16

88.74

N

21.78

28.38

/

/

20.56

/

Al

5.20

1.97

11.12

13.23

1.29

8.78

AC C

EP

TE D

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SC

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Fig. 5a. 1

13

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Table 3 Relative oxidation resistance of different samples at 600 Sample No.

and 800 S0

S2

S3

)

1.00

5.63

6.83

Relative oxidation resistance (800

)

1.00

1.39

1.94

AC C

EP

TE D

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SC

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Relative oxidation resistance (600

14

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Figure captions

Fig.1 Macro-morphology of laser in-situ synthesized TiN/Ti3Al intermetallic compound composite coatings in different molar ratios of Ti and AlN (a) Sample S1 (4Ti:3AlN); (b) Sample S2 (4Ti:2AlN) and (c) Sample S3 (4Ti:1AlN)

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Fig.2 XRD patterns of Ti6Al4V titanium alloy and TiN/Ti3Al intermetallic compound composite coatings

Fig.3 Overview microstructures on the cross section of TiN/Ti3Al intermetallic compound composite coatings (a) Sample S2; (b) Sample S3

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Fig.4 SEM morphologies of TiN/Ti3Al intermetallic compound composite coatings

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(a) Sample S2; (b) Sample S3

Fig.5 High magnification SEM images of TiN/Ti3Al intermetallic compound composite coatings corresponding to Fig.4

Fig.6 Oxidation kinetics curves of titanium alloy substrate and TiN/Ti3Al intermetallic compound composite coatings at 600 (a) and 800 (b)

and 800

TE D

Fig.7 XRD patterns of unoxidized cladding coating and oxidized cladding coatings at 600

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Fig.8 Surface oxide layer images of titanium alloy substrate and TiN/Ti3Al intermetallic compound composite coatings samples after oxidizing at 600 for 15 hours (a) Surface oxide color; (b) Sample S0; (c) Sample S2 and (d) Sample S3

AC C

Fig.9 Surface images of the oxide layer on titanium alloy substrate and TiN/Ti3Al intermetallic compound composite coatings after exposure at 800 for 15 hours (a) Surface oxide color; (b) Sample S0; (c) Sample S2 and (d) Sample S3

Fig.10 Microhardness distribution of untreated and laser cladding samples

15

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a)

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b)

c)

AC C

EP

TE D

Fig.1

16

AC C EP TE D

20 30

S3

40 50

Fig.2

17

60

2θ (°)

70

80

Ti(112) Ti(201) Ti(004)

Ti(103)

Ti(110)

Ti(102)

Ti(100) Ti(101)

Ti(002)

Ti(104)

Ti(202)

RI PT Ti3Al(104)

SC

TiN(311) TiN(222)

Ti3Al(203)

Ti3Al(220)

TiN(220)

Ti3Al(202)

TiN(200)

Ti3Al(201)

Ti3Al(002)

TiN(111)

S0

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S2 Ti3Al(200)

Intensity (a.u.)

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90 100

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Substrate

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Coating

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a)

AC C

EP

TE D

Fig.3a

18

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Coating

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Substrate

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b)

AC C

EP

TE D

Fig.3b

19

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M AN U

SC

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a)

AC C

EP

TE D

Fig.4a

20

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M AN U

SC

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b)

AC C

EP

TE D

Fig.4b

21

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M AN U

SC

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a)

AC C

EP

TE D

Fig.5a

22

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SC

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b)

AC C

EP

TE D

Fig.5b

23

a)

12

-2

S3 S2 S0

-2

10

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Oxidation weight gain(10 mg.mm )

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8 6

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4

0

0

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2

5

10 Oxidation time (h)

AC C

EP

TE D

Fig.6a

24

15

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b)

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S3 S2 S0

12 10 8 6

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4 2 0

0

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-2

-2

Oxidation weat gain(10 mg.mm )

14

5

10 Oxidation time (h)

AC C

EP

TE D

Fig.6b

25

15

AC C EP TE D

20 S3

30   

S3

40

2θ (°)

Fig.7

 





50

60

26

70


(220) (116)
(002)
(310) (214) (300)
(301)
(112)







80 90


(211)

TiN  Ti3Al


(110)

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(312)

(226)


(222)
(3301)


(321)
(400)


(202)

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  (024)


(101)

(110)
(200)
(111)  (113)
(210)



S3





(104)

(012)

Intensity (a.u.)

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TiO2

 Al2O3





100

AC C

EP

TE D

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SC

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Fig.8

Fig.8

27

AC C

EP

TE D

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Fig.9

28

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1600 S3 S2 S0

1200

CZ

BZ

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Microhardness, HV0.2 / ×9.8MPa

1400 Substrate

HAZ

1000

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800 600

200 200

400

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400

600

800

1000

Distance from the surface (µm)

AC C

EP

TE D

Fig.10

29

1200

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Research Highlights

> In-situ TiN/Ti3Al composite coating was synthesized on Ti6Al4V alloy by laser cladding.

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>The influence of Ti and AlN molar ratio on the microstructure of the coating was studied.

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> The TiN/Ti3Al intermetallic coating is mainly composed of α-Ti, TiN and Ti3Al phases.

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>The relative oxidation resistance of the coating at 600℃ and 800℃ is 6.83 and 1.94.

AC C

EP

TE D

>The microhardness of the coating reaches 844HV0.2, 3.4 times higher than the bare substrate.