Hong BIAN, Ynyu SONG, Duo LIU,*, Yuzhen LEI, Xioguo SONG,Jin CAO

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

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

KEYWORDS

Abstract Nano-Al2O3 particles modified AgCuNi filler was adopted to braze the SiO2 ceramic and TC4. The effects of filler size as well as the brazing temperature on the interfacial microstructure and mechanical property of the joints were investigated.Nanoscale filler reduced the phases dimension and promoted the homogeneous distribution of microstructure, obtaining a higher joint strength when compared to microscale filler. The increase of brazing temperature made the accelerating dissolution and diffusion of Ti, which promoted the increase of thickness of Ti4O7+TiSi2 layer adjacent to SiO2 ceramic and diffusion layer zone nearby TC4 alloy. The hypoeutectic structure was produced in the brazing seam due to the high Ti content.The maximum shear strength of～40 MPa was obtained at 950°C for 10 min.

1. Introduction

SiO2ceramic is the promising structural material due to its significant fracture toughness, excellent resistance of heat and wear, high-temperature viscosity and excellent thermal stability,which are beneficial to applying in industries such as aerospace, optics, bioengineering and electronics.1–3However, it is quite difficult for SiO2ceramic to manufacture the structure with complicated shape and large scale due to its inherent high brittleness and hardness. So the joining of SiO2ceramic with metals or alloys has great potential to expand the applications of SiO2ceramic. Ti-6Al-4V (TC4) alloy, as the most popular material used in aerospace field, is often utilized to join ceramic due to its excellent fracture toughness, low weight and high mechanical resistance.4–6Among the various joining technologies, the active brazing method is considered as the most effective means to join SiO2ceramic and TC4 due to its convenience,cost-effectiveness and high-quality process.7–9In recent years, most of the focus has been on the brazing of SiO2ceramic,many of which are directed toward the use of AgCu eutectic filler alloys containing active elements as Ti, Ni, Zr, etc. to realize a reliable joining.1,10,11Nevertheless, the mismatch in coefficient of thermal expansion (CTE) between the ceramic and metals or alloys would cause the generation of a large residual stress in the joint, affecting the brazing quality and even resulting in the failure joining.12–14Introducing the low CTE materials such as ceramic particles (SiC, Si3N4, Al2O3,etc.) and carbon nanomaterials (carbon fiber, graphene nanoplatelets, carbon nanotubes, etc.) into the brazing alloys can alleviate the residual stress to a large extent,thereby obtaining the high strength joints.15–22In addition, the size of particle additives in brazing alloys can exert great influence on the interfacial microstructure of the joints.23The joint brazed using micron-sized reinforcements additives are prone to defects(void,poor flow ability,cracks,etc.)compare with that brazed using nanosized reinforcements.24,25Therefore, it is essential to reduce the residual stress in the joints to enhance joints’ strength while maintaining integrity of the interface.

Previous work focused on the joining process of SiO2ceramic with metal,few studies have been done on the microstructure evolution and mechanical properties evaluation of SiO2ceramic/metal joint brazed by particles reinforced composite filler.10It is reported that the ceramic/metal joints prepared by particles reinforcing materials have more effective reinforcing impact compare to sole fillers for improving mechanical performance of the joints.26–28Consequently, filler matrix of AgCuNi in nanosized was adopted in this paper, and nano-Al2O3particles with low CTE was used as reinforcement to mitigate residual stress of the joints. The effect of filler alloy with different particle size on the joint microstructure and strength was investigated. Furthermore, the effect of brazing temperature on the microstructure evolution and mechanical property of the joints was studied.

2. Experimental

The materials to be joined in this study are SiO2ceramic and TC4 alloy; their microstructures and chemical compositions are shown in Fig.1 and Table 1, respectively. SiO2ceramic with density of 3.7 g/cm3through the doping of Al2O3and MgO additives was fabricated by using the pressureless sintering technique. TC4 was mainly composed of matrix α phase and discontinuous β phase. The adopted filler consisted of nanoscale Ag, Cu, Ni and Al2O3powders (99.95% in purity),as shown in Fig.2. The fabrication process of composite filler involved: Nano-Ni particles (35 in wt%) were dispersed into alcoholic solution of AgCu eutectic (Ag-28Cu (wt%)) to stir and ultrasonic for 15 min. And then nano-Al2O3particles(0.3 in wt%) were slowly added into the mixed solution with a constant stirring and mixed with the AgCuNi solution by further sonication. The obtained mixture solution was kept in an oven at 80°C for 30 min.

Table 1 Chemical compositions of SiO2 ceramic and TC4(wt.%).

Prior to the brazing, the SiO2ceramic was cut into pieces with the dimension of 5 mm×5 mm×5 mm. The sizes of TC4 alloy were 20 mm×10 mm×2 mm and 8 mm×8 mm×2 mm, the former used for metallographic observation,the latter used for mechanical property test.After that, the sample surfaces of the SiO2ceramic and TC4 were sanded by SiC grit papers and then polished using diamond paste. The polished samples were ultrasonic cleaning in acetone for 15 min. The composite filler with the thickness of 100 μm was placed between SiO2ceramic and TC4, then the assembly was placed in a vacuum resistance furnace. When the furnace vacuum dipped below 5×10-3Pa, the brazing procedure was executed and the process was as follows: The temperature was first increased to 750°C with a rate of 20°C/min and held for 10 min. Then the temperature was increased to the 950°C with a rate of 10°C/min and held for 10 min. Finally, the temperature was reduced to 300°C with a rate of 5°C and then the brazed samples were furnacecooled down to the room temperature.

The cross section of brazed samples for metallographic observation was examined by a field emission scanning electron microscopy(SEM,MERLIN Compact,Zeiss).The chemical compositions of the interfacial microstructure were characterized using SEM equipped with an energy dispersive spectrometer (EDS, OCTANE PLUS, EDAX). To investigate the mechanical property of the brazed samples, shear strength tests were conducted on a universal material testing machine(Instron 5967). The as-prepared brazed samples were kept stationary in the machine and then exerted the load. The sliding speed was set at 0.5 mm/min during the shear strength tests.And at least five set of the shear strength values for each set of experimental data was obtained to evaluate the mechanical property of the brazed joint.

Fig.1 Microstructures of the base materials.

Fig.2 Microstructures of powder particles.

3. Results and discussion

Fig.3 Interfacial microstructure and high-magnification BSE images of SiO2/TC4 joint brazed using micron-filler and nano-filler.

Fig.3 shows the interfacial microstructures of the SiO2/TC4 joint brazed at 950°C for 10 min via using the microsized filler(micron-filler)and nanosized filler(nano-filler),respectively.It can be clearly seen from the joint brazed with micron-filler shown in Fig.3(a) that plenty of block-like phases distributed in the brazing seam nearby SiO2ceramic, whereas the brazing seam of SiO2/TC4 joint brazed with nano-filler shown in Fig.3(b) varied obviously from the joint brazed with micron-filler.As shown in Fig.3(b), the reaction products seen in Fig.3(a)with the morphology of block shaped disappeared markedly and the obtained microstructure possessed homogenous distribution. Nano-particles could provide more dispersive nucleation sites for reaction products, suggesting that the use of nano-filler had more effective microstructure dispersing impact compared to micron-filler usage. Additionally, the magnified images of interfacial microstructure in the selection boxes 1(shown in Fig.3(a))and 2(shown in Fig.3(b))were presented in Fig.3(c)and(d),respectively.The size of the phases reduced significantly for the joint brazed using nano-filler, indicating that nano-filler possessed the favorable role for refining the interfacial microstructure.

Fig.4 Shear strength of the joints brazed using micron-filler and nano-filler.

Fig.4 shows the shear strength of the SiO2/TC4 joint brazed with microscale and nanoscale filler, respectively. The average shear strength for the joint brazed using nano-filler was ～40 MPa,which is almost 110.5%higher than that brazed with micron-filler.By comparing the interfacial microstructure as well as the mechanical property of the joints brazed with nano-filler and micron-filler, the scale of filler has the important effects in optimizing interfacial structure and strengthening joint. The nano-filler could lead to the reduction in reaction phases volume and promote its homogeneous distribution, which was conducive to improving the mechanical property of the brazed joint.

Fig.5(a) shows the typical interfacial microstructure of SiO2/TC4 joint brazed using AgCuNi+Al2O3composite filler at 950°C for 10 min, and corresponding main elements distribution are shown in Fig.5(b)–(e).The brazed joint was divided into three areas marked by area I (diffusion area nearby TC4 alloy), area II (the brazing seam) and area III (continuous reaction layers adjacent to SiO2ceramic). The interface of the joint was almost occupied by the filler matrix element Ag, as shown in Fig.5(b). Area I was mainly distributed with base metal matrix element Ti, and elements Cu and Ni were found at the same position due to the diffusion within the scope of elements.It indicates that some Ti-Cu-Ni compounds were dispersed upon the Ti substrate. In area II, the contents of Cu and Ni increased while the Ti content decreased compared to the elemental contents in area I,which suggesting that the quantity of Ti-Cu-Ni compounds showed a progressive trend from area I to II.According to the Ti distribution shown in Fig.5(e), Ti migrated to SiO2ceramic from TC4 under the driving force of concentration gradient and accumulated at the surface of the ceramic. A Ti-rich reaction layer was observed adjacent to SiO2ceramic, indicating that interfacial reaction between Ti and SiO2ceramic had occurred during brazing.

Fig.5 Interfacial microstructure and elemental distribution images of SiO2/TC4 joint brazed using AgCuNi+Al2O3 composite filler at 950°C for 10 min.

Fig.6 Microstructure and high-magnification BSE images of SiO2/TC4 joint brazed using AgCuNi+Al2O3 composite filler at 950°C for 10 min.

To further identify the interfacial microstructure of the brazed joint, each area was magnified to observe as shown in Fig.6 and the EDS was used to analyze the chemical contents on phase composition of the interface.Area I was the diffusion layer zone adjacent to TC4 that diffused and reacted during brazing. Its characteristics of morphologies could be classified into three characteristic zones marked in selection boxes 1, 2 and 3. According to the EDS results listed in Table 2, the coarse dark grey acicular-like phase(marked as A)in selection box 1 was α-Ti,the strip-like phases(marked as B,C,D and E)in selection boxes 2 and 3 were Ti2(Ni,Cu) and Ti2(Cu,Ni)respectively. The equiaxed α-Ti in selection box 1 first transformed into β-Ti at heating stage and then transformed into acicular α-Ti during cooling period.29Refs. [29,30] show that the structure in selection boxes 2 and 3 was formed by the hypereutectoid reaction β-Ti →α-Ti+Ti2(Ni,Cu)+Ti2(Cu,Ni) and hypoeutectic reaction L →α-Ti+Ti2(Ni,Cu)+Ti2(-Cu,Ni). So an area consisted of α-Ti, α-Ti+Ti2(Ni,Cu)+Ti2(Cu,Ni) hypereutectoid structure and α-Ti+Ti2(Ni,Cu)+Ti2(Cu,Ni) hypoeutectic structure was obtained. Area II,situated at the central region of the joint;its microstructure mainly consisted of Ti2Cu, Ti2Ni and α-Ti+Ti2(Ni,Cu)+Ti2(Cu,Ni) hypoeutectic structure according to the EDS analysis.The compositions in this area is similar to that of area I, whereas the phases with relatively large volume wereobtained in area II as presented in Fig.6(e) and (f). It is because that there was a difference for the contents of Ni and Cu between varied areas, leading to the different dimensions. Area III is the reaction area of SiO2ceramic substrate and active element Ti,which can be marked as the interaction layer area. As shown in Fig.6(g), it can be clearly seen that a continuous reaction layer formed at the surface of SiO2ceramic.EDS results show that the interaction layer was composed of Ti4O7and TiSi2,corresponding to the Ref.[31].As a result,the interfacial microstructure of SiO2/AgCuNi+Al2O3/TC4 brazed joint could be described as: TC4/α-Ti /α-Ti+Ti2(Ni,Cu)+Ti2(Cu,Ni) hypereutectoid structure/α-Ti+Ti2(Ni,Cu)+Ti2(Cu,Ni) hypoeutectic structure/Ti2(Ni,Cu)+Ti2(-Cu,Ni)/α-Ti+Ti2(Ni,Cu)+Ti2(Cu,Ni) hypoeutectic structure/Ti4O7+TiSi2/SiO2ceramic.

Table 2 EDS results of each spot marked in Fig.6 (at.%).

In order to validate the effect of brazing temperature, the SiO2ceramic and TC4 were brazed at different temperature for 10 min using AgCuNi+Al2O3composite filler. Fig.7 shows the interfacial microstructure evolution of brazed joints with temperature. It is clear that the microstructure varied significantly with the increase of the brazing temperature. For the joint brazed at 910°C, the insufficient driving force for the atomic diffusion in case of the relatively low temperature would result in a low thickness of diffusion layer zone with only 45.6 μm, as shown in Fig.7(a). Moreover, it can be observed that the brazing seam was almost occupied by the Ti2(Ni,Cu)+Ti2(Cu,Ni) intermetallic compounds.As to the joints brazed at 930°C, the increased brazing temperature promoted the dissolution of TC4 and diffusion of atoms. As shown in Fig.7(b), the thickness of diffusion layer zone increased slightly due to the increase in the reactions between the filler and TC4. Moreover, the diffusion of Cu and Ni atoms into TC4 substrate reduced the β-transus temperature and acicular α-Ti appeared adjacent to TC4 in area I.32Furthermore, partial Ti2(Ni,Cu)+Ti2(Cu,Ni) intermetallic compounds transformed into the α-Ti+Ti2(Ni,Cu)+Ti2(Cu,Ni) hypoeutectic structure due to the high Ti content in the brazing seam at elevated temperature. As further increasing the brazing temperature to 950°C or above, the migration of atoms was strengthened. Increased the mutual diffusion of Ti, Cu and Ni would lead to the further increase of the thickness of diffusion layer zone while the thickness of brazing seam was reduced, as shown in Fig.7(c) and (d). In addition, the more diffusion of Cu and Ni towards TC4 substrate further reduced the β-transus temperature and coarsen the acicular α-Ti.

It is notable that the interaction layer zone changed greatly at the brazing temperature ranging from 910°C to 970°C.To investigate the effect of brazing temperature on microstructure evolution in interaction layer zone, the details were shown in Fig.8. It can be clearly seen that the thickness of interaction layer zone increased as brazing temperature rising. With the increase of brazing temperature, more Ti atoms accumulated on the surface adjacent to SiO2ceramic side through promoting the dissolution of TC4. The vigorous reaction between Ti and SiO2ceramic occurred during brazing, which promoted the growth of continuous Ti4O7+TiSi2layer on the surface of SiO2ceramic. An interaction layer with approximate 0.74 μm in thickness was formed at the interface between brazing seam and SiO2ceramic at 970°C.

Fig.7 Interfacial microstructure of SiO2/AgCuNi+Al2O3/TC4 joints brazed for 10 min at different temperature.

Fig.8 Microstructure of interaction layer brazed for 10 min at different temperature.

Fig.9 Shear strength of SiO2/AgCuNi+Al2O3/TC4 joints brazed at various temperature.

Fig.9 shows the shear strength of SiO2/TC4 joints brazed at different brazing temperature for 10 min. The mechanical testing indicates that the shear strength of joints increased first and then decreased with the increase of brazing temperature.It could be concluded that the interfacial microstructure as a function of brazing temperature had a great influence on the joints’property.The mismatch in CTE between the SiO2ceramic(5×10-6/K)and TC4(10×10-6/K)would cause the high residual stress and thus decrease the mechanical property of the joints. The CTE of nano-Al2O3particles (7.4×10-6/K)was between the SiO2ceramic and TC4. So added nano-Al2O3particles could adjust the CTE mismatch to a certain extent, contributing to reduce the residual stress. While for the joint brazed at 910°C, a large number of Ti2(Ni,Cu) and Ti2(Cu,Ni)brittle intermetallic compounds formed in the brazing seam. So the shear strength of the joint was low. The α-Ti possessed the good plasticity compared to Ti2(Ni,Cu)and Ti2(-Cu,Ni)for alleviating stress of the joints.30Therefore,the ductility of the brazed joints depended on the quantity of the α-Ti precipitation on the interface to a large extent. As seen from the Fig.7,with the increase of brazing temperature,the dimension of diffusion layer zone increased and partial hypoeutectic structure formed in the brazing seam; it was beneficial to relieving the residual stress in the joints through the plastic deformation of α-Ti. Thus the shear strength of the joint increased with the brazing temperature and the maximum shear strength of 40 MPa was obtained at 950°C. Nevertheless, when the brazing temperature increased to 970°C, rising temperature decreased the joint strength.The coarse α-Ti adjacent to the TC4 obtained at this temperature was harmful to the joint property.30In addition, the thicker interaction layer zone was not conducive to stress release of the joint.So the relative low shear strength of the joint was obtained.

4. Conclusions

In this work, SiO2ceramic and TC4 alloy were successfully brazed using nano-AgCuNi+Al2O3composite filler. The comparisons of nanosized and microsized fillers as well as the effects of brazing temperature on the interfacial microstructure and mechanical property of the joints were investigated.The experimental results show that the nanoscale particles could provide dispersive nucleation sites for reaction phases, and the obtained microstructure possessed homogenous distribution. The joint brazed using nano-filler possessed the higher shear strength compared with that brazed using micron-filler. The low brazing temperature was disadvantaged for the atomic diffusion and interfacial reaction,leading to the relative low shear strength of the joints. With the increase of brazing temperature,the reaction between Ti and SiO2ceramic was proceeded adequately. The increased temperature promoted the dissolution of TC4 and more Ti diffused into the filler, the thickness of the diffusion layer zone increased and the hypoeutectic structure formed in the brazing seam, which was beneficial to relieving the residual stress. The maximum shear strength of the joints brazed with nano-filler reached～40 MPa at 950°C for 10 min.

Acknowledgment

This project is supported by National Natural Science Foundation of China (Grant Nos. 51505105, 51875130 and 51775138) and the Key Research & Development Program of Shandong Province (No. 2017GGX40103).