Wetting behavior of AgCu-4.5Ti filler reinforced by carbon nanotubes on C/C composite

Xingyi LI, Ling LIU, Ynyu SONG, Duo LIU,*, Shengpeng HU,Xioguo SONG, Jin CAO

a State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China

b Shandong Provincial Key Laboratory of Special Welding Technology, Harbin Institute of Technology at Weihai, Weihai 264209, China

c Hunan Marketing Company, PetroChina Company Limited, Changsha 410005, China

KEYWORDS C/C composite;Carbon Nanotubes (CNTs);Composite filler;Microstructure;Wetting

Abstract The effect of Carbon Nanotubes (CNTs) content on the wettability of AgCu-4.5Ti+x CNTs (wt%) composite filler alloys on C/C composite was investigated. The results show that the added CNTs reacted with element Ti in the filler and produced the dispersed fine in situ synthesized TiC particles,which increased the consumption of element Ti and provided the nucleus for the growth of Ti-Cu compounds simultaneously.The above effects of introducing CNTs,inhibited the formation of Ti-Cu compounds, also changed the distribution of compounds, which dramatically influenced the interfacial microstructure and characteristics of wetting behavior. The increase of CNTs content refined and dispersed coarse Ti-Cu compounds, decreased the initial spreading temperature, and improved the wettability, but high content of CNTs (more than 0.3wt%) decreased the wettability of the filler alloy.The wetting interfacial microstructure of corresponding composite filler alloys were analyzed by Scanning Electron Microscope(SEM),Energy Dispersive X-ray Spectrometer (EDS) and Transmission Electron Microscope (TEM), which consisted of TiC, TiCu,TiCu2 and TiCu4 compound. The typical wetting behavior of AgCu-4.5Ti+0.3wt% CNTs composite filler on C/C composite was divided into four stages.The effect mechanism of CNTs content on the wetting behavior was proposed.

1. Introduction

Due to the poor formability, C/C composite needs to be connected with the metal such as TC4 titanium alloy for manufacturing large scale composite components.One of the methods to realize a reliable joining between dissimilar materials is brazing, which is simple in operation, strong in adaptability,low cost, and excellent in performance of the obtained joint.9-12The wetting of liquid filler alloy on C/C composite is the critical process for brazing, which influences the quality of joint as well.13-15

The high residual stress in the joint caused by the thermal expansion coefficient mismatch between the C/C composite and TC4 titanium alloy was the main reason of the deterioration of joint strength, and the fracture even occurred during the cooling. The fine ceramic particles, intermetallic compounds particles, and carbon materials were added to conventional filler alloy as the reinforcement phase, which contributed to release the residual stress.16-19The additional reinforcement phase in composite filler alloy improves the fluidity and wettability of the filler on the substrate,decreases the thermal expansion coefficient of filler alloy and also changes the size and distribution of intermetallic compounds, which effectively improves the joint quality. Owing the properties of extremely high elastic modulus, high tensile strength, low density, low thermal expansion coefficient and high temperature resistance,20-22the CNTs were considered to be the appropriate reinforcement phase added to the filler alloy,which were beneficial for the wetting behavior on C/C composite.

In this work, CNTs were added into the AgCu-4.5Ti filler alloy,and the wettability of composite filler alloy with different CNTs contents on C/C composite at elevating process was investigated.The contact angle and spreading area of the composite filler alloys were measured and the corresponding interfacial microstructures were analyzed. The wetting mechanism of the composite filler alloy on C/C composite was proposed.

2. Experimental

2.1. Experimental materials

The selected substrate was three-dimensional C/C composite with carbon content greater than 99.89wt%, machined into 17 mm×15 mm×5 mm. The wetting surfaces were mechanically ground with 600-1200 mesh SiC abrasive paper and polished by 2.5 mm diamond paste, then ultrasonic cleaned in ethanol for 5 min and dried in air atmosphere, the micromorphology of C/C composite was shown in Fig. 1.

The AgCu-4.5Ti filler alloy was mechanically mixed by AgCu eutectic powder (99.98wt%, ～20 μm) and Ti powder(99.95wt%, ～40 μm) by planetary ball milling for 5 h, then with addition of x wt% CNTs (x=0.1, 0.2, 0.3, 0.4) and dispersed by acetone solution with ultrasonically mixed and heated until the acetone evaporated. The dried powder was cold-press compacted into a cylinder with diameter of 2 mm under a pressure of 3 MPa,and ground into cubic pieces about 50 mg by SiC abrasive paper for wetting experiments.

2.2. Experimental process and characterization methods

Fig. 1 Micromorphology of C/C composite.

The C/C composite substrate was placed on a boron nitride ceramic plate inside the vacuum chamber and adjusted to a horizontal position. The filler alloy cube was preplaced on the surface of C/C composite substrate before the wetting experiment. Firstly, the vacuum chamber was evacuated to 5×10-4Pa at room temperature and preheated to 200°C.Subsequently, the vacuum chamber was heated to 780°C at the rate of 10°C/min and kept for 10 min to obtain a homogeneous heated sample. Finally, the temperature rose to 900°C at the rate of 4°C/min.The wetting process was photographed by a camera at a rate of 12 frames/min. At least three repetitions were performed to ensure the repeatability of experimental results.

After wetting experiments, the sample was furnace-cooled down to room temperature.The contact angles were measured by drop-analysis software FTA 32 and the spreading areas were measured by image analysis software Image J. The samples were cut perpendicularly to the wetting interface and polished for microstructure analysis by field emission Scanning Electron Microscopy (SEM, MERLIN Compact, ZEISS)equipped Energy Dispersive X-ray Spectroscopy (EDS,OCTANE PLUS, EDAX).

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3. Results and discussion

3.1. Characteristics of wetting behavior

Fig. 2 shows the DSC curves of AgCu-4.5Ti+x CNTs(wt%) composite filler alloys. The melting point of AgCu-4.5Ti+0.1wt% CNTs was almost the same as AgCu-4.5Ti filler alloy (approximately 780°C). With the increases of CNTs content below 0.4wt%,the melting point of composite fillers decreases.

Fig. 2 DSC curves of AgCu-4.5Ti+x CNTs (wt%) composite filler alloy.

The initial process of melting always takes place at the crystal surface or plane with high energy,due to the metastable of CNTs surface with characteristics of high surface energy and specific surface area, the introduction of CNTs increases the non-equilibrium melting zones,then the melting point of composite filler is decreased. As the content of CNTs above 0.3wt%, the agglomeration effect of CNTs will increase the melting point.

Fig. 3 shows the variations of contact angle and droplet height with temperature for AgCu-4.5Ti filler and AgCu-4.5Ti+0.3wt%CNTs composite filler, respectively. The contact angle and droplet height of AgCu-4.5Ti were stable at 860°C. For AgCu-4.5Ti+0.3wt% CNTs composite filler,the contact angle was stable at 850°C with the droplet height indeclinable at about 855°C.As shown in Fig.3(a),the initial spreading temperature of the AgCu-4.5Ti filler alloy on the C/C composite was 809°C, the final contact angle was about 8°, and the final height of the droplet was less than 0.5 mm.In contrast,the spreading temperature of composite filler with addition of 0.3wt% CNTs was 800°C, the final contact angle was about 7°, and the droplet height dropped to 0.3 mm, as shown in Fig. 3(b).

The wetting process of AgCu-4.5Ti+0.3wt%CNTs could be divided into four stages defined by the rate of change of droplet height,as shown in Fig.3(b).(A)Stage I(800-810°C):After the solid filler alloy melted gradually, the contact angle decreased rapidly and the droplet height dropped sharply.The final contact angle of this stage had dropped below 40°.(B)Stage II(810-845°C):Compared with Stage I,the spreading speed of this stage was lower,the wetting behavior reached a state of equilibrium,and the variations of contact angle and the wetting temperature show a good linearity relationship.The contact angle was decreased to about 11°,and the droplet height was reduced to about 0.8 mm. (C) Stage III (845-855°C): During this process, the droplet height dropped sharply again. (D) Stage IV(855-880°C): The spreading of composite filler alloy was finished,contact angle and the droplet height were no longer changed.The final contact angle was about 7°,and the droplet height was reduced to about 0.3 mm.

Fig. 4 shows the variations of contact angle and droplet height of composite filler with different CNT contents. The images of corresponding spreading area were shown in Fig. 5.

The worst wettability of composite filler on C/C composite substrate was obtained when the content of CNTs was 0.4wt%. The spreading process was completed at 855°C.The final contact angle was approximately 15°,and the droplet height was 0.97 mm with the spreading area 37.03 mm2.When the content of CNTs decreased to 0.3wt%, the optimal wettability of composite filler was obtained with the lowest droplet height of 0.3 mm, contact angle of about 7°, and spreading area of 96.16 mm2. The introduction of CNTs (below 0.4wt%)into AgCu-4.5Ti filler alloy decreased the initial spreading temperature from 809°C to 804°C. During the final stage of the heating, the wetting process was completed, the contact angle and the droplet height were no longer changed.

3.2. Microstructure of wetting interface

Fig.6 shows the interfacial microstructure of AgCu-4.5Ti filler on C/C composite substrate, and two different regions could be observed in Fig. 6(a) obviously, the magnified images were shown in Fig. 6(b) and (c). Region I consisted of white light phase with dispersive grey phase and Region II consisted of dark grey phase with dispersive black particles. The results of EDS analysis on Points A-D in Fig. 6(b)-(d) were shown in Table 1. It could be concluded the continuous grey phase existed in Region II was Ti-Cu compounds, while the white light phase and the dark grey phase in Region I were Ag(solid solution) and Cu (solid solution) respectively, which could be determined as AgCu eutectic. As shown in Fig. 6(d) and (e),a continuous reaction layer existed on both central region and edge region. AgCu filler alloy could not spread on C/C composite,23the addition of the element Ti reacted with element C and formed a TiC reaction layer on the surface of the C/C composite, which reduced the surface energy of the C/C composite,and this was the key point to improve the wettability of AgCu liquid phase on substrate.

Fig. 7 shows the High Resolution Transmission Electron Microscope (HRTEM) images of AgCu-4.5Ti+0.3wt%CNTs filler alloy after the wetting experiments. Lots of light grey fine particles arranged in stripes could be observed in Region I of Fig.7(a).According to the Selected Area Electron Diffraction(SAED)pattern of Region I,the fine particles were determined as TiC. After the reaction between element Ti and CNTs, TiC compound formed and connected to each other,which still kept the original tubes-like structure of the CNTs,as shown in Region I in Fig.7(a).These show that the element Ti reacted with CNTs to generate the fine TiC particles in situ,which achieved the transformation from the original CNTs into the tubes-like structure of TiC particles.

Fig. 3 Variations of contact angle and droplet height with temperature at elevating process.

Fig. 4 Variations of contact angle and droplet height with temperature for different CNTs contents.

Fig. 5 Spreading area of different CNTs contents.

Fig. 6 Wetting interface of AgCu-4.5Ti on C/C composite.

Table 1 EDS results of spots marked in Fig. 6.

Fig. 7 HRTEM images of AgCu-4.5Ti+0.3wt% CNTs filler after wetting experiment.

Fig. 8 Wetting interface of AgCu-4.5Ti+0.3wt%CNTs on C/C composite.

Fig.8 shows the interfacial microstructure of AgCu-4.5Ti+0.3wt% CNTs filler on the surface of C/C composite substrate.Similar to Fig.6(b)and(c),Region I and Region II could also be found in Fig. 8(b) and (c). The AgCu eutectic of Region I in Fig. 8(b) was finer than that of in Fig. 6(b). TiC particles and Ti-Cu intermetallic compounds of Region II in Fig. 8(c),which determined by the EDS results shown in Table 2,were dispersed.As the previous study proved that the coarse Ti-Cu intermetallic compounds reduced the liquid fluidity, which inhibited the wetting process.24The additional CNTs consumed more element Ti and formed the in situ synthesized TiC through the reaction between the element Ti and CNTs, which inhibited the formation of Ti-Cu intermetallic compounds. The dispersed in situ synthesized TiC became the new nucleus for Ti-Cu intermetallic compounds to generate around,which refined the original coarse Ti-Cu intermetallic compounds.The refined and dispersed Ti-Cu intermetallic compounds reduce the viscosity of the droplets,which promoted the spreading process.The thickness of reaction layer in Fig.8(d)was thicker than that of reaction layer in Fig.8(e)apparently,and TiC+TiCu2and TiCu4were determined in Point A of Fig. 8(c) and Point B of Fig. 8(d) respectively.

To investigate the wetting process of composite filler on C/C composite substrate with different CNTs contents,the interfacial microstructure of composite filler/(C/C) composite was observed and the evolution of interfacial microstructure was shown in Fig.9.The droplet height of the central region gradually decreased with the increase of CNTs content when the spreading area firstly increased and then decreased. The aggregative compounds could be observed in central region of AgCu-4.5Ti filler alloy, which inhibited the dispersion of droplet. As the content of CNTs increased, the extent of agglomeration of Ti-Cu compounds were alleviated effectively.The best wettability was obtained when the CNTs content was 0.3wt%, while the maximum spread area of 96.16 mm2and lowest droplet height of 0.3 mm. With the CNTs content exceeded 0.3wt%, the wettability of the liquid filler alloy on the C/C composite substrate deteriorated. Because the excessive additional CNTs existed in the filler alloy with the formof clusters,resulted in the aggregative compounds,which made the fluidity and wetting of the droplets worse.

Table 2 EDS results of spots marked in Fig. 8.

Fig. 9 Macrographs of cross-sectional droplet.

3.3. CNTs effect mechanism on wetting behavior

Fig. 10 shows the interface of AgCu-4.5Ti+x CNTs (wt%)filler alloy/(C/C) composite. The reaction layer in Region I mainly consisted of Ti-Cu compounds was the left side on Fig. 10(b)-(e), the right side was the reaction layer in Region II mainly composed by AgCu eutectic. The reaction layer in edge region was slightly thicker than that in central region obviously. And with the content of CNTs increased, the discrepancy of thickness between central and edge was enlarged.The main reasons for this phenomenon were as the following.When the composite filler alloy melted before the wetting process, element Ti was consumed to form the in situ synthesized TiC and Ti-Cu compounds in central region. So, the inadequate element Ti resulted that the initial reaction layer formed was thin.Due to the dispersion of Ti-Cu compounds refined by the in situ synthesized TiC,the fluidity of liquid filler alloy was increased, and the spreading behavior was promoted. During the spreading process, at the edge of the droplet, element Ti was mainly consumed to form the new reaction layer, which established a large concentration gradient of element Ti between the central region and the edge of droplet. The concentration gradient driven element Ti to diffuse toward the edge, meanwhile the diffusion between the liquid filler alloy and the surface of C/C composite substrate in the central region was inhibited by the Ti-Cu compounds which caused a relatively thin TiC reaction layer.More element Ti preferred to diffuse toward the edge resulted that the reaction layer in edge region was thicker than that in central region.

The wetting behavior of AgCu-4.5Ti+CNTs composite filler on C/C composite could be schematized in Fig. 11. During the stage as shown in Fig.11(a)and(b),the active element Ti began to dissolve into AgCu liquid and reacted with dispersed CNTs to generate the in situ synthesized TiC, which provided the nucleus for Ti-Cu compounds. Meanwhile, element Ti also diffused toward C/C composite substrate to form a TiC reaction layer at the surface of substrate progressively,which promoted the spreading of liquid filler alloy, resulted that the contact angle and droplet height decreased sharply during this stage. As shown in Fig. 11(b) and (c), element Ti was consumed to form the TiC and Ti-Cu compounds, while the formation of new reaction layer in the edge region established a concentration gradient of element Ti,which promoted the diffusion of element Ti from the central region to the edge continuously. The spreading process of this stage continued steady. During the stage as shown in Fig. 11(c) and (d), the CNTs were consumed and production of Ti-Cu compounds was finished. The formation and growth of new reaction layer consumed remaining element Ti, so the droplet height decreased sharply again.Finally,the element Ti was consumed and the growth of TiC reaction layer was finished, the contact angle and the drop height were no longer changed.

Fig. 10 Reaction layer of different composite fillers on C/C composite.

Fig. 11 Schematic of wetting behavior.

Combining with the wetting spreading process and the analysis of the reaction characteristics of the composite filler alloys, the mechanism of CNTs promoting the wetting behavior was mainly included as following. The addition of an appropriate amount of CNTs could decrease the melting point of the composite filler alloy,21,25so that the diffusion between the C/C composite and composite filler alloy could start at a relatively lower temperature (804°C). With the addition of CNTs below 0.3 wt%, the fluidity of the liquid alloy was improved due to the formation of in situ synthesized TiC through the reaction between element Ti and CNTs, which refined and dispersed Ti-Cu compounds in central region of droplet and made the formation of new reaction layer at edge of spreading droplet easier. Meanwhile the concentration gradient of element Ti between central region and edge of droplet was established by the formation of new reaction layer at the edge of droplet. So, the diffusion of element Ti from central to edge of droplet was promoted.

4. Conclusions

The effect of CNTs content on the wettability behavior of composite filler alloy/(C/C)composite system was investigated with elevating temperature process.The interfacial microstructure of composite filler alloys on the C/C composite substrate was analyzed. Several conclusions drawn from this study were listed as following:

(1) The wetting behavior of AgCu-4.5Ti+0.3 wt% CNTs composite filler on C/C composite was divided into four stages. In the first stage, TiC reaction layer formed on the surface of C/C composite, the spreading process was promoted. Meanwhile, the contact angle and droplet height decreased sharply.In the second stage,the filler alloy spreaded on the C/C matrix uniformly and TiC reaction layer grew simultaneously. In the third stage,the formation of in situ TiC and Ti-Cu compounds finished, the droplet height decreased quickly. Finally, the element Ti was consumed, the wetting behavior completed, the droplet height and contact angle no longer changed.

(2) The CNTs content influenced the wettability of AgCu-4.5Ti on the C/C composite intensely. With addition of CNTs below 0.4 wt%, the initial spreading temperature decreased. The variation of contact angle showed the tendency of firstly decreasing and then increasing with the increase of CNTs content. When the content of CNTs was 0.3 wt%,the best wettability was obtained with final droplet height of 0.3 mm, contact angle of 7°and the spreading area of 96.16 mm2.

(3) The element Ti was consumed by the dispersed CNTs in the filler alloy, enlarging the concentration gradient between the central region and the edge of droplet,promoting the spreading process. And the in situ synthesized fine TiC refined the coarse Ti-Cu compounds which increased the fluidity and wettability of filler alloy.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This project is supported by National Natural Science Foundation of China (Nos. 51875130 and 51775138) and Natural Science Foundation of Shandong Province, China (No.ZR2019MEE091).