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Brazing TC4 alloy to Si3N4 ceramic using nano-Si3N4 reinforced AgCu composite filler
Materials and Design 76 (2015) 40–46
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Brazing TC4 alloy to Si3N4 ceramic using nano-Si3N4 reinforced AgCu composite filler Y.X. Zhao a, M.R. Wang a, J. Cao a, X.G. Song a,⇑, D.Y. Tang b, J.C. Feng a a b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China Department of Chemistry, School of Science, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 12 October 2014 Revised 19 March 2015 Accepted 24 March 2015 Available online 24 March 2015 Keywords: TC4 alloy Si3N4 ceramic Brazing Composite filler Interfacial microstructure
a b s t r a c t A novel particle reinforced AgCu composite filler (abbreviated as AgCuC filler) was developed by introducing nano-Si3N4 particles and micron-Ti particles into AgCu powder filler. Reliable brazing of TC4 alloy to Si3N4 ceramic was achieved by using the AgCuC filler. The interfacial microstructure of TC4/AgCuC/Si3N4 brazed joint was TC4/Ti–Cu intermetallic layers/Ag based composite reinforced by fine particles/TiN + Ti5Si3 layer/Si3N4. The addition of nano-Si3N4 in AgCuC filler improved the interfacial microstructure by suppressing the growth of continuous Ti–Cu intermetallic layers adjacent to TC4 alloy and promoting the formation of Ag based composite reinforced by particle phases in brazing seam. The continuous Ti–Cu intermetallic layers thicken and the TiCu2 particle phases coarsened gradually with an increasing brazing temperature. Brazing defects including micro-cracks and nano-Si3N4 aggregation were generated at higher brazing temperature. The highest average shear strength of 73.9 MPa was achieved when brazed at 880 °C for 10 min, which was higher than that of joints brazed using AgCu filler alone (49.2 MPa). Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Advanced ceramics have attracted great interest due to their outstanding properties such as high Young’s modulus, high temperature strength and hardness, and excellent corrosion resistance [1,2]. As one of the most promising ceramic materials, Si3N4 ceramics are considered as the next generation of wave-transparent materials applied in antenna radomes, which have been received extensive attentions [3–5]. In practical applications, antenna radomes must be firmly joined to a metallic holder made of Invar alloy, Kovar alloy or titanium alloys [6]. Therefore, reliable joining of Si3N4 ceramics to metal materials is absolutely essential to widen the applications of Si3N4 ceramics in antenna radomes. Various methods including brazing, diffusion bonding, and partial transient-liquid-phase (TLP) bonding have been developed to realize the joining of Si3N4 ceramics to metals [7–9]. Especially, brazing has become the most effective method due to its convenience and cost-effectiveness [10]. However, in the case of brazing Si3N4 ceramics to metals, the mismatch in coefficient of thermal expansion (CTE) and Young’s modulus among ceramic components, brazing seams and metal components can result in high residual stresses in brazed joints. Especially, the residual stress concentration at ⇑ Corresponding author. E-mail address: [email protected] (X.G. Song). http://dx.doi.org/10.1016/j.matdes.2015.03.046 0261-3069/Ó 2015 Elsevier Ltd. All rights reserved.
interface will reduce the joint properties, even possibly lead to early mechanical failures within brazing joints [11,12]. Therefore, it is imperative to reduce or eliminate the residual stresses in the interfacial region to improve the joining properties. In recent years, composite fillers developed by introducing tiny ceramic particles or fibers with low CTE into traditional brazing fillers have attracted great interest in brazing ceramics materials to metals. The use of composite fillers associated with optimized brazing process can produce metal-based composite reinforced by fine particles or fibers in brazing seam. Thus, the mismatch in CTE and Young’s modulus between ceramics and brazing seams can be reduced, and the interfacial microstructure and joining properties of ceramic/metal brazed joints could be improved significantly [13–15]. However, the ceramic particles or fibers added in composite fillers are within several to several ten microns in size, and the micron-sized reinforcements in composite fillers proved to be problematic in terms of void formation, poor flowability, cracks, etc. [16,17]. In this study, nano-Si3N4 particles reinforced AgCu composite filler (abbreviated as AgCuC filler) was developed to braze TC4 alloy and Si3N4 ceramic. The interfacial microstructure of brazed joints was characterized; especially the effect of nano-Si3N4 addition in AgCuC filler on the interfacial microstructure evolution was analyzed. In addition, the effect of brazing temperature on the interfacial microstructure and joining properties of brazed joints was investigated in detail.
Y.X. Zhao et al. / Materials and Design 76 (2015) 40–46
2. Experimental The substrate materials used in the experiment were commercially available TC4 alloy and Si3N4 ceramic. The Si3N4 ceramic used in this work was hot-pressed with small amounts of Al2O3 and Y2O3 as sintering additive. The raw ceramics were cut to blocks with the dimension of 5 mm 5 mm 5 mm using a diamond cutting machine. The dimensions of TC4 alloy which nominal compositions was Ti–6Al–4V (wt.%) were 8 mm 8 mm 3 mm and 20 mm 8 mm 3 mm for metallographic observation and shear strength test, respectively. Nano-Si3N4 particles (20 nm) and micron-Ti particles (20 lm) were added into AgCu eutectic powder filler (Ag–28Cu) and then the mixture was milled for 2 h in an argon atmosphere using a QM-SB planetary ball mill to prepare AgCuC filler. The content both of nano-Si3N4 and micron-Ti in AgCuC filler was 2 wt.%. Prior to joining, the brazing surfaces of the TC4 and Si3N4 samples were ground on SiC grit papers and then polished using diamond pastes. All of the polished samples were ultrasonically cleaned in acetone for 20 min and dried by air blowing. The AgCuC filler was placed between the brazing couples to form a sandwich type and then the brazing assemblies were carefully placed into the vacuum furnace. At the beginning of brazing process, the furnace was heated to 750 °C at a rate of 40 °C/min, then to the brazing temperatures (860–940 °C) at a rate of 20 °C/min. Subsequently, the brazing couples were held for 10 min at brazing temperature, then cooled down to 300 °C at a rate of 5 °C/min. Finally, the joints were spontaneously cooled down to room temperature in furnace. During brazing process, the vacuum was kept at 1.3–2.0 103 Pa and a pressure of 20 KPa was applied on each brazing assembly to ensure proper contact. The brazed specimens for metallographic observation were cross-sectioned, perpendicular to the brazed interface. The interfacial microstructure was characterized employing scanning electron microscopy (SEM, Quanta 200FEG). Componential analysis of various reaction phases in joints was carried out using an energy dispersive spectrometer (EDS) with the operation voltage of 15 KV and minimum spot size of 1 lm. Moreover, transmission electron microscopy characterization (TEM, Tecnai G2 F30) and selected area electron diffraction (SAED) analysis were conducted to determine the reaction phases formed in brazing seams. The brazed specimens for TEM observation were sliced to 0.5 mm in thickness perpendicular to the brazing seam using a diamond cutting machine. The slices were mechanically polished down to less than 80 lm using 600# diamond abrasives. The thin foil specimens were then cut to 3mm in diameter using an ultrasonic disk cutter (Model 601 Gatan, USA), further grinded using a dimple grinder (Model 656 Gatan, USA) and accomplished by ion milling using a precision ion polishing system (PIPS, Model 695 Gatan, USA). The shear tests were performed at a constant speed of 0.5 mm/min by using a universal testing machine (Instron 1186) at room
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temperature. A schematic of the shear test can be seen in Ref. [18]. For each set of experimental data, at least five samples were used to average the joints strength. After shear test, the fracture of brazed joints was inspected with optical microscope (OM). In addition, differential thermal analysis (DTA) was performed in order to determine the melting point of AgCuC filler.
3. Results and discussions 3.1. Characterization of AgCuC filler Fig. 1 shows the morphologies of the original AgCu eutectic filler and the as-prepared AgCuC filler. It can be clear seen in Fig. 1(a) that the AgCu eutectic filler contained a large number of AgCu eutectic filler balls. The tiny AgCu balls had smooth surfaces and their diameters were within 50 lm. The microscopic morphology of AgCuC filler shown in Fig. 1(b) displayed that the size of filler balls increased slightly and their surfaces became rougher. These changes were mainly caused by the ball milling process, during which, the nano-Si3N4 particles were adhered densely on the surfaces of AgCu eutectic filler balls. X-ray diffraction analysis was carried out on AgCu eutectic filler and AgCuC filler to verify whether metallurgical reactions occurred during the ball milling process. By comparing the two XRD patterns shown in Fig. 2, it is concluded that both AgCu eutectic filler and AgCuC filler consisted of Ag and Cu. There was no new peaks appeared in the XRD pattern of AgCuC filler besides some peaks of Ti and Si3N4, indicating that no metallurgical reaction occurred during ball milling process. there was The DTA curves illustrated in Fig. 3 reveals that both AgCu eutectic filler and AgCuC filler
Fig. 2. XRD patterns of powder fillers: (a) AgCu eutectic filler and (b) nano-Si3N4 reinforced AgCu composite filler (AgCuC filler).
Fig. 1. Microscopic morphologies of powder fillers: (a) AgCu eutectic filler and (b) nano-Si3N4 reinforced AgCu composite filler (AgCuC filler).
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Fig. 3. DTA curves of powder fillers: (a) AgCu eutectic filler and (b) nano-Si3N4 reinforced AgCu composite filler (AgCuC filler).
melted at 783 °C, which means that the nano-Si3N4 addition had no effect on the melting point of AgCu eutectic filler. The DTA curve shown in Fig. 3(a) indicates that the melting of AgCu eutectic filler is an endothermic process. While in the case of AgCuC filler, the Ti particles could dissolve into molten AgCu filler and reacted with nano-Si3N4 during the melting process. The exothermic reaction between active Ti and nano-Si3N4 released heat which resulted in an obvious reduction of endothermic peak of AgCuC filler, as shown in Fig. 3(b). 3.2. Typical interfacial microstructure of the TC4/AgCuC/Si3N4 brazed joints Fig. 4 shows the typical interfacial microstructure and the elemental distribution of TC4/AgCuC/Si3N4 joint brazed at 880 °C for 10 min. The joint consisted of three reaction zones which were classified by the difference in microscopic morphology, as shown in Fig. 4(a): Zone I (reaction zone adjacent to TC4 substrate),
Zone II (brazing seam) and Zone III (continuous thin reaction layer adjacent to Si3N4 substrate). Fig. 4(b) displays that the element Ag was mainly distributed in the brazing seam and there were some small second phases dispersed in it. According to the distributions of elements Cu and Ti shown in Fig. 4(c) and (d) respectively, both the Zone I and the second phases dispersed in Zone II mainly contained elements Ti and Cu. Additionally, it is noted that a continuous thin Ti-rich layer (Zone III) was formed adjacent to Si3N4 ceramic, indicating that intensive diffusion of element Ti from TC4 alloy or AgCuC filler had occurred during brazing. In order to further investigate the interfacial microstructure, more details are presented in a larger visual field in backscattered electron mode, as shown in Fig. 5, and major elements at each spot in Fig. 5 detected by EDS are listed in Table 1. According to the elemental contents and Ti–Cu binary alloy phase diagram [19], the three reaction layers in Zone I shown in Fig. 5(a) were: Ti2Cu intermetallic phase (layer A), TiCu intermetallic phase (layer B) and TiCu2 intermetallic phase (layer C). The white phase marked as D
Fig. 4. Typical interfacial microstructure and elemental distribution of TC4/AgCuC/Si3N4 joint brazed at 880 °C for 10 min. (a) Interfacial microstructure of TC4/AgCuC/Si3N4 joint and EDS elemental distribution maps of (b) Ag, (c) Cu, and (d) Ti.
Y.X. Zhao et al. / Materials and Design 76 (2015) 40–46
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Fig. 5. High magnification BE images of the interfacial microstructure of TC4/AgCuC/Si3N4 joint brazed at 880 °C for 10 min. (a) Zone I and (b) Zone III.
Table 1 Chemical compositions and possible phases of each spot marked in Fig. 5 (at.%) Spot
Si
N
Ag
Cu
Ti
Al
V
Possible phase
A B C D E F
– – – 2.56 2.20 22.89
– – – 9.65 8.56 36.99
2.52 0.87 0.86 77.62 0.69 1.46
30.80 48.88 60.48 3.62 58.63 4.31
63.06 47.70 36.76 5.12 28.71 33.45
2.32 1.47 1.22 0.86 0.77 0.54
1.30 1.08 0.68 0.57 0.44 0.36
Ti2Cu TiCu TiCu2 Ag(s,s) TiCu2 TiN, Ti5Si3
and gray phase marked as E in Fig. 5(b) were Ag based solid solution and TiCu2 intermetallic compound respectively. Moreover, the continuous thin reaction layer adjacent to Si3N4 ceramic shown in Fig. 5(b) was composed of TiN and Ti5Si3 compounds (Layer F). Actually, during brazing, element Ti both from TC4 alloy and from AgCuC filler dissolved in molten AgCu filler and then diffused toward Si3N4 substrate, finally reacted with the ceramic to form a TiN + Ti5Si3 reaction layer. The TiN + Ti5Si3 reaction layer was also found not only in our previous studies where Si3N4 ceramic was brazed to TiAl alloy using AgCuTi active filler, but also in other researches where active Ti containing fillers were selected to braze Si3N4 ceramics [20–23]. It is worth noting that Ag based composite reinforced by gray TiCu2 phases and a large number of fine black particles was formed in brazing seam, as shown in Fig. 5. Fig. 6 shows the bright field image and selected area electron diffraction patterns from a TEM sample extracted from Zone II of brazing seam. According to the results of TEM and SAED, the tiny particles dispersed in Ag matrix or in TiCu2 phases were TiN and Ti5Si3, which were formed by the reaction between nano-Si3N4 and active Ti during brazing. As analyzed above, the interfacial microstructure of TC4/AgCuC/Si3N4
brazed joint was TC4/Ti–Cu intermetallic layers/Ag based composite reinforced by TiCu2, TiN and Ti5Si3 particles/TiN+Ti5Si3 layer/ Si3N4. In order to investigate the effect of nano-Si3N4 addition in AgCuC filler on the interfacial microstructure of TC4/Si3N4 joints, AgCu eutectic powder filler was processed by mechanical treatment in a ball mill in the same modes as the nano-Si3N4 reinforced composite filler and then used to braze TC4 alloy to Si3N4 ceramic. Fig. 7 shows the typical interfacial microstructure of TC4/AgCu/ Si3N4 joint brazed at 880 °C for 10 min. It clearly demonstrates that the joint also consisted of three zones: Ti–Cu intermetallic layers (Zone I), brazing seam (Zone II) and TiN + Ti5Si3 layer (Zone III), which is similar to that of TC4/AgCuC/Si3N4 joint illustrated in Fig. 4(a). However, comparing Figs. 7 and 4(a) reveals that the average thickness of continuous Ti–Cu intermetallic layers adjacent to
Fig. 7. Typical interfacial microstructure TC4/Si3N4 joint brazed at 880 °C for 10 min using AgCu eutectic filler processed by mechanical treatment in the same modes as the AgCuC filler.
Fig. 6. Transmission electron microscopy (TEM) characterization and selected area electron diffraction (SAED) analysis of the microstructure in Zone II. (a) TEM image of the microstructure in Zone II, (b) SAED pattern of TiN and (c) SAED pattern of Ti5Si3.
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TC4 substrate was reduced from 25 lm to 10 lm. Meanwhile, the TiN + Ti5Si3 layer adjacent to Si3N4 substrate thinned significantly. These changes indicate that the rates of reactions between molten filler and base materials decreased when AgCuC filler was used. So it is deduced that the nano-Si3N4 addition in AgCuC filler suppressed the growth of Ti–Cu intermetallic layer TiN + Ti5Si3 layer to some extent. In addition, Ag based composite reinforced by fine particles was formed in brazing seam when AgCuC filler was used, which replaced the bright Ag solid solution. The reinforcements such as TiCu2, TiN and Ti5Si3 dispersed in Ag matrix not only strengthened the brazing seam but also reduced its CTE. Therefore, it is anticipated that the use of AgCuC filler could improve the joint properties in a certain extent. Moreover, a diffusion layer which could be observed in TC4/AgCu/Si3N4 joint disappeared in TC4/AgCuC/Si3N4 joint. Based on the analysis above, the brazing mechanism can be described as follow. The AgCu eutectic balls in AgCuC filler began to melt when brazing temperature exceeded its melting point. A semi-solid mixture containing molten AgCu filler, nano-Si3N4 and micron-Ti was formed gradually in the gap between TC4 alloy and Si3N4 ceramic. Element Cu diffused to TC4 alloy and reacted with it, which resulted in the formation of continuous Ti–Cu intermetallic layers adjacent to TC4 substrate. Simultaneously, active element Ti both from TC4 substrate and from micro-Ti particles dissolved into liquid filler, diffused towards Si3N4 side and reacted with it to form a TiN + Ti5Si3 reaction layer. The reaction mechanism between Si3N4 and active Ti was studied by Tunckan et al. [24] and Singh et al. [25] who reported the reaction equations as below.
phases in brazing seam and thus the TiCu2 particle phases were also dispersed within the Ag matrix, as shown in Fig. 5. In fact, the nano-Si3N4 addition acted as a barrier to the diffusion of Cu towards TC4 side and to the diffusion of active Ti towards Si3N4 side, which suppressed the growth both of the continuous Ti–Cu intermetallic layers and the TiN + Ti5Si3 layer. Therefore, compared with TC4/AgCu/Si3N4 joint, TC4/AgCuC/Si3N4 joint exhibited relatively thin reaction layers, as shown in Fig. 4(a). 3.3. Effect of brazing temperature on the interfacial microstructure of TC4/AgCuC/Si3N4 joints Fig. 8 shows the interfacial microstructure of TC4/AgCuC/Si3N4 joints brazed at different temperatures for 10 min. It is found that the brazing temperature had a significant effect on the interfacial microstructure. Both the Ti–Cu intermetallic layers adjacent to TC4 alloy and the Ti5Si3 + TiN layer adjacent to Si3N4 ceramic thickened gradually as the brazing temperature increased. The width of
1
Ti þ 1=4Si3 N4 ! TiN þ 3=4Si DG ðJ mol Þ ¼ 613; 000 þ 40:8 T 5Ti þ 3Si ! Ti5 Si3
1
DG ðJ mol Þ ¼ 194; 140 þ 16:74 T
ð1Þ ð2Þ
Additionally, the reaction between nano-Si3N4 and active Ti also occurred in liquid filler, which caused that tiny TiN and Ti5Si3 particles were dispersed in Ag matrix in brazing seam. These tiny particles could act as nucleation sites for the precipitation of TiCu2
Fig. 9. Effect of brazing temperature on the shear strength of TC4/AgCuC/Si3N4 brazed joints.
Fig. 8. Interfacial microstructure of TC4/AgCuC/Si3N4 joints brazed at different temperature for 10 min. (a) 860 °C, (b) 880 °C, (c) 900 °C, (d) 920 °C and (e) 940 °C.
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Fig. 10. Fractographies of TC4/AgCuC/Si3N4 joints brazed at (a) 860 °C, (b) 880 °C and (c) 940 °C.
brazing seam kept about 60 lm when brazing temperature was below 900 °C (shown in Fig. 8(a) and (b)), and increased to 75 lm when brazing temperature reached 900 °C, as shown in Fig. 8(c). However, it was reduced greatly to no more than 30 lm when brazing temperature exceeded 900 °C (shown in Fig. 8(d) and (e)). The reduction was due to the intensive diffusion of elements in molten filler such as Ag, Cu into TC4 substrate and the expulsion of molten filler by capillarity out of the joint gap at higher temperatures. Actually, the intensive diffusion at higher brazing temperature produced a diffusion layer in TC4 substrate, which gradually thickened with the increasing brazing temperature, as shown in Fig. 8(c)–(e). It is noted that the microstructure of brazing seam changed obviously with the increasing brazing temperature. At lower brazing temperature (860 °C), the reaction between nano-Si3N4 and active Ti in molten filler was incomplete, which resulted in an amount of nano-Si3N4 remained and aggregated in brazing seam after brazing process and thus the amount of tiny Ti5Si3 and TiN particles dispersed in Ag matrix was reduced, as shown in Fig. 8(a). Fig. 8(b) and (c) shows desirable microstructure could be obtained when brazed at 880 °C and 900 °C, although slight growth and coarsening of TiCu2 phases were taken place in Ag matrix during brazing at 900 °C. When brazing temperature increased to 920 °C, the Ag based composite was deteriorated due to the coalescence of TiCu2 phase in brazing seam and the formation of micro-cracks in Si3N4 substrate, as shown in Fig. 8(d). Further increasing the brazing temperature to 940 °C, the micrograph shown in Fig. 8(e) reveals that brazing defects including nano-Si3N4 aggregation, micro-cracks were generated in the brazing seam. These defects were mainly caused by an intensive mutual diffusion of the elements occurred between TC4 alloy and molten filler that promoted the excessive growth of Ti–Cu intermetallics in brazing seam. Meanwhile, the reaction between nano-Si3N4 and active Ti in molten filler was suppressed and the redistribution of tiny Ti5Si3 and TiN particles could not be achieved, which resulted in a large number of unreacted nano-Si3N4 aggregated in brazing seam.
3.4. Effect of brazing temperature on the shear strength of TC4/AgCuC/ Si3N4 joints
insufficient. While the specimens brazed at 880 °C and 900 °C possessed high shear strength. The maximum average shear strength of 73.9 MPa was obtained when brazing was conducted at 880 °C, which is 24.7 MPa (50%) higher than that of the TC4/ Si3N4 joints brazed using AgCu alone (49.2 MPa). Interfacial observations in Section 3.3 reveal that the desirable Ag based composite reinforced by tiny Ti5Si3 and TiN particles as well as TiCu2 phases was formed in brazing seam. The reinforcements with lower CTE in Ag matrix not only strengthened the brazing seam but also reduced its CTE. Thus, the mismatch in CTE among TC4 substrate, brazing seam and Si3N4 substrate was reduced and the residual stresses in interfacial region could be relieved to some extent. So, the joining properties were improved finally. However, when brazing temperature was further increased to 900 °C or 920 °C, the shear strength decreased sharply. This result could be attributed to the formation of brazing defects including micro-cracks and nano-Si3N4 aggregation shown in Fig. 8(d) and (e), which inevitably resulted in a weak bonding between TC4 alloy and Si3N4 ceramic. Fracture analysis was performed by optical microscope to investigate the fracture locations. Fig. 10 shows three kinds of fracture mode. Failure of TC4/AgCuC/Si3N4 joints brazed at 860 °C and 920 °C always occurred in Si3N4 ceramic substrate near the braze interface and a bowed crack path was observed in Si3N4 substrate, as shown in Fig. 10(a), which indicated that high residual stresses were generated in Si3N4 substrate close to the interface and the residual stresses reduced the joint strength. In fact, the formation of micro-cracks within Si3N4 substrate before shear test shown in Fig. 8(d) was mainly due to the presence of considerable residual stresses in interfacial region. For the shear test of specimens brazed at 880 °C and 900 °C, cracks propagated in the brazing seam and finally ruptured in Si3N4 substrate, as illustrated in Fig. 10(b). This fracture mode suggested that the residual stresses was relieved because of the improved interfacial microstructure shown in Fig. 8(b) and (c). In addition, all of the TC4/AgCuC/Si3N4 joints brazed at 940 °C fractured along the brazing seam, the typical fractography can be seen in Fig. 10(c). This fracture mode was mainly due to a weak bonding formed in brazing seams that was caused by a large amount of defects.
4. Conclusions Fig. 9 shows the room-temperature shear strength of TC4/ AgCuC/Si3N4 joints brazed at different temperature for 10 min. As a result, the shear strength of specimens brazed at 860 °C was
Reliable brazing of TC4 alloy and Si3N4 ceramic was achieved by using nano-Si3N4 reinforced AgCu composite filler (AgCuC filler).
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The interfacial microstructure and joining properties of TC4/AgCuC/ Si3N4 brazed joint were investigated in this study. Primary conclusions are summarized as follows. (1) No metallurgical reaction occurred among AgCu eutectic powder, nano-Si3N4 and micron-Ti particles during ball milling process. The addition of nano-Si3N4 in AgCuC filler changed its melting point marginally. (2) The typical interfacial microstructure of TC4/AgCuC/Si3N4 brazed joint was TC4/Ti–Cu intermetallic layers/Ag based composite reinforced by TiCu2, TiN and Ti5Si3 particles/ TiN + Ti5Si3 layer/Si3N4. Nano-Si3N4 addition in AgCuC filler improved the interfacial microstructure by providing dispersive nucleation sites for Ti–Cu intermetallic compounds in brazing seam and suppressing the growth of continuous Ti–Cu intermetallic layers adjacent to TC4 alloy. (3) Increasing brazing temperature accelerated the diffusion and reaction of elements in molten filler, which resulted in an increase in thickness both of Ti–Cu intermetallic layers and TiN + Ti5Si3 reaction layer. When brazing temperature exceeded 900 °C, brazing defects including micro-cracks and nano-Si3N4 aggregation were generated in brazed joints. (4) Brazing temperature had a strong influence on the shear strength of TC4/AgCuC/Si3N4 brazed joints. The highest average shear strength reached 73.9 MPa when brazed at 880 °C for 10 min. Nano-Si3N4 addition in AgCuC filler significantly improved the interfacial microstructure and joining properties of TC4/AgCuC/Si3N4 brazed joints.
Acknowledgments This project is supported by National Natural Science Foundation of China (Grant No. 51405099) and Shandong Province Outstanding Young Scientist Award Fund (Grant No. BS2013CL028). References [1] Q.F. Zeng, L.T. Zhang, Y.D. Xu, L.F. Cheng, X.T. Yan, J.K. Zu, G.Z. Dai, Designing experts system for in situ thoughened Si3N4 based on adaptive neural fuzzy inference system and genetic algorithms, Mater. Des. 30 (2) (2009) 256–259. [2] A.H. Song, S.E. Yoon, J. Choi, B.K. Kim, J.S. Park, A high-performance ceramic composite anode for protonic ceramic fuel cells based on lanthanum strontium vanadate, Int. J. Hydrogen Energy 39 (29) (2014) 16534–16540. [3] Y.J. Zhao, Y.J. Zhang, H.Y. Gong, H.B. Sun, Q.S. Li, Gas pressure sintering of BN/ Si3N4 wave-transparent material with Y2O3–MgO nanopowders addition, Ceram. Int. 40 (8) (2014) 13537–13541. [4] Y.F. Xia, Y.P. Zeng, D.L. Jiang, Dielectric and mechanical properties of porous Si3N4 ceramics prepared via low temperature sintering, Ceram. Int. 35 (4) (2009) 1699–1703.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Brazing TC4 alloy to Si3N4 ceramic using nano-Si3N4 reinforced AgCu composite filler Y.X. Zhao a, M.R. Wang a, J. Cao a, X.G. Song a,⇑, D.Y. Tang b, J.C. Feng a a b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China Department of Chemistry, School of Science, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 12 October 2014 Revised 19 March 2015 Accepted 24 March 2015 Available online 24 March 2015 Keywords: TC4 alloy Si3N4 ceramic Brazing Composite filler Interfacial microstructure
a b s t r a c t A novel particle reinforced AgCu composite filler (abbreviated as AgCuC filler) was developed by introducing nano-Si3N4 particles and micron-Ti particles into AgCu powder filler. Reliable brazing of TC4 alloy to Si3N4 ceramic was achieved by using the AgCuC filler. The interfacial microstructure of TC4/AgCuC/Si3N4 brazed joint was TC4/Ti–Cu intermetallic layers/Ag based composite reinforced by fine particles/TiN + Ti5Si3 layer/Si3N4. The addition of nano-Si3N4 in AgCuC filler improved the interfacial microstructure by suppressing the growth of continuous Ti–Cu intermetallic layers adjacent to TC4 alloy and promoting the formation of Ag based composite reinforced by particle phases in brazing seam. The continuous Ti–Cu intermetallic layers thicken and the TiCu2 particle phases coarsened gradually with an increasing brazing temperature. Brazing defects including micro-cracks and nano-Si3N4 aggregation were generated at higher brazing temperature. The highest average shear strength of 73.9 MPa was achieved when brazed at 880 °C for 10 min, which was higher than that of joints brazed using AgCu filler alone (49.2 MPa). Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Advanced ceramics have attracted great interest due to their outstanding properties such as high Young’s modulus, high temperature strength and hardness, and excellent corrosion resistance [1,2]. As one of the most promising ceramic materials, Si3N4 ceramics are considered as the next generation of wave-transparent materials applied in antenna radomes, which have been received extensive attentions [3–5]. In practical applications, antenna radomes must be firmly joined to a metallic holder made of Invar alloy, Kovar alloy or titanium alloys [6]. Therefore, reliable joining of Si3N4 ceramics to metal materials is absolutely essential to widen the applications of Si3N4 ceramics in antenna radomes. Various methods including brazing, diffusion bonding, and partial transient-liquid-phase (TLP) bonding have been developed to realize the joining of Si3N4 ceramics to metals [7–9]. Especially, brazing has become the most effective method due to its convenience and cost-effectiveness [10]. However, in the case of brazing Si3N4 ceramics to metals, the mismatch in coefficient of thermal expansion (CTE) and Young’s modulus among ceramic components, brazing seams and metal components can result in high residual stresses in brazed joints. Especially, the residual stress concentration at ⇑ Corresponding author. E-mail address: [email protected] (X.G. Song). http://dx.doi.org/10.1016/j.matdes.2015.03.046 0261-3069/Ó 2015 Elsevier Ltd. All rights reserved.
interface will reduce the joint properties, even possibly lead to early mechanical failures within brazing joints [11,12]. Therefore, it is imperative to reduce or eliminate the residual stresses in the interfacial region to improve the joining properties. In recent years, composite fillers developed by introducing tiny ceramic particles or fibers with low CTE into traditional brazing fillers have attracted great interest in brazing ceramics materials to metals. The use of composite fillers associated with optimized brazing process can produce metal-based composite reinforced by fine particles or fibers in brazing seam. Thus, the mismatch in CTE and Young’s modulus between ceramics and brazing seams can be reduced, and the interfacial microstructure and joining properties of ceramic/metal brazed joints could be improved significantly [13–15]. However, the ceramic particles or fibers added in composite fillers are within several to several ten microns in size, and the micron-sized reinforcements in composite fillers proved to be problematic in terms of void formation, poor flowability, cracks, etc. [16,17]. In this study, nano-Si3N4 particles reinforced AgCu composite filler (abbreviated as AgCuC filler) was developed to braze TC4 alloy and Si3N4 ceramic. The interfacial microstructure of brazed joints was characterized; especially the effect of nano-Si3N4 addition in AgCuC filler on the interfacial microstructure evolution was analyzed. In addition, the effect of brazing temperature on the interfacial microstructure and joining properties of brazed joints was investigated in detail.
Y.X. Zhao et al. / Materials and Design 76 (2015) 40–46
2. Experimental The substrate materials used in the experiment were commercially available TC4 alloy and Si3N4 ceramic. The Si3N4 ceramic used in this work was hot-pressed with small amounts of Al2O3 and Y2O3 as sintering additive. The raw ceramics were cut to blocks with the dimension of 5 mm 5 mm 5 mm using a diamond cutting machine. The dimensions of TC4 alloy which nominal compositions was Ti–6Al–4V (wt.%) were 8 mm 8 mm 3 mm and 20 mm 8 mm 3 mm for metallographic observation and shear strength test, respectively. Nano-Si3N4 particles (20 nm) and micron-Ti particles (20 lm) were added into AgCu eutectic powder filler (Ag–28Cu) and then the mixture was milled for 2 h in an argon atmosphere using a QM-SB planetary ball mill to prepare AgCuC filler. The content both of nano-Si3N4 and micron-Ti in AgCuC filler was 2 wt.%. Prior to joining, the brazing surfaces of the TC4 and Si3N4 samples were ground on SiC grit papers and then polished using diamond pastes. All of the polished samples were ultrasonically cleaned in acetone for 20 min and dried by air blowing. The AgCuC filler was placed between the brazing couples to form a sandwich type and then the brazing assemblies were carefully placed into the vacuum furnace. At the beginning of brazing process, the furnace was heated to 750 °C at a rate of 40 °C/min, then to the brazing temperatures (860–940 °C) at a rate of 20 °C/min. Subsequently, the brazing couples were held for 10 min at brazing temperature, then cooled down to 300 °C at a rate of 5 °C/min. Finally, the joints were spontaneously cooled down to room temperature in furnace. During brazing process, the vacuum was kept at 1.3–2.0 103 Pa and a pressure of 20 KPa was applied on each brazing assembly to ensure proper contact. The brazed specimens for metallographic observation were cross-sectioned, perpendicular to the brazed interface. The interfacial microstructure was characterized employing scanning electron microscopy (SEM, Quanta 200FEG). Componential analysis of various reaction phases in joints was carried out using an energy dispersive spectrometer (EDS) with the operation voltage of 15 KV and minimum spot size of 1 lm. Moreover, transmission electron microscopy characterization (TEM, Tecnai G2 F30) and selected area electron diffraction (SAED) analysis were conducted to determine the reaction phases formed in brazing seams. The brazed specimens for TEM observation were sliced to 0.5 mm in thickness perpendicular to the brazing seam using a diamond cutting machine. The slices were mechanically polished down to less than 80 lm using 600# diamond abrasives. The thin foil specimens were then cut to 3mm in diameter using an ultrasonic disk cutter (Model 601 Gatan, USA), further grinded using a dimple grinder (Model 656 Gatan, USA) and accomplished by ion milling using a precision ion polishing system (PIPS, Model 695 Gatan, USA). The shear tests were performed at a constant speed of 0.5 mm/min by using a universal testing machine (Instron 1186) at room
41
temperature. A schematic of the shear test can be seen in Ref. [18]. For each set of experimental data, at least five samples were used to average the joints strength. After shear test, the fracture of brazed joints was inspected with optical microscope (OM). In addition, differential thermal analysis (DTA) was performed in order to determine the melting point of AgCuC filler.
3. Results and discussions 3.1. Characterization of AgCuC filler Fig. 1 shows the morphologies of the original AgCu eutectic filler and the as-prepared AgCuC filler. It can be clear seen in Fig. 1(a) that the AgCu eutectic filler contained a large number of AgCu eutectic filler balls. The tiny AgCu balls had smooth surfaces and their diameters were within 50 lm. The microscopic morphology of AgCuC filler shown in Fig. 1(b) displayed that the size of filler balls increased slightly and their surfaces became rougher. These changes were mainly caused by the ball milling process, during which, the nano-Si3N4 particles were adhered densely on the surfaces of AgCu eutectic filler balls. X-ray diffraction analysis was carried out on AgCu eutectic filler and AgCuC filler to verify whether metallurgical reactions occurred during the ball milling process. By comparing the two XRD patterns shown in Fig. 2, it is concluded that both AgCu eutectic filler and AgCuC filler consisted of Ag and Cu. There was no new peaks appeared in the XRD pattern of AgCuC filler besides some peaks of Ti and Si3N4, indicating that no metallurgical reaction occurred during ball milling process. there was The DTA curves illustrated in Fig. 3 reveals that both AgCu eutectic filler and AgCuC filler
Fig. 2. XRD patterns of powder fillers: (a) AgCu eutectic filler and (b) nano-Si3N4 reinforced AgCu composite filler (AgCuC filler).
Fig. 1. Microscopic morphologies of powder fillers: (a) AgCu eutectic filler and (b) nano-Si3N4 reinforced AgCu composite filler (AgCuC filler).
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Fig. 3. DTA curves of powder fillers: (a) AgCu eutectic filler and (b) nano-Si3N4 reinforced AgCu composite filler (AgCuC filler).
melted at 783 °C, which means that the nano-Si3N4 addition had no effect on the melting point of AgCu eutectic filler. The DTA curve shown in Fig. 3(a) indicates that the melting of AgCu eutectic filler is an endothermic process. While in the case of AgCuC filler, the Ti particles could dissolve into molten AgCu filler and reacted with nano-Si3N4 during the melting process. The exothermic reaction between active Ti and nano-Si3N4 released heat which resulted in an obvious reduction of endothermic peak of AgCuC filler, as shown in Fig. 3(b). 3.2. Typical interfacial microstructure of the TC4/AgCuC/Si3N4 brazed joints Fig. 4 shows the typical interfacial microstructure and the elemental distribution of TC4/AgCuC/Si3N4 joint brazed at 880 °C for 10 min. The joint consisted of three reaction zones which were classified by the difference in microscopic morphology, as shown in Fig. 4(a): Zone I (reaction zone adjacent to TC4 substrate),
Zone II (brazing seam) and Zone III (continuous thin reaction layer adjacent to Si3N4 substrate). Fig. 4(b) displays that the element Ag was mainly distributed in the brazing seam and there were some small second phases dispersed in it. According to the distributions of elements Cu and Ti shown in Fig. 4(c) and (d) respectively, both the Zone I and the second phases dispersed in Zone II mainly contained elements Ti and Cu. Additionally, it is noted that a continuous thin Ti-rich layer (Zone III) was formed adjacent to Si3N4 ceramic, indicating that intensive diffusion of element Ti from TC4 alloy or AgCuC filler had occurred during brazing. In order to further investigate the interfacial microstructure, more details are presented in a larger visual field in backscattered electron mode, as shown in Fig. 5, and major elements at each spot in Fig. 5 detected by EDS are listed in Table 1. According to the elemental contents and Ti–Cu binary alloy phase diagram [19], the three reaction layers in Zone I shown in Fig. 5(a) were: Ti2Cu intermetallic phase (layer A), TiCu intermetallic phase (layer B) and TiCu2 intermetallic phase (layer C). The white phase marked as D
Fig. 4. Typical interfacial microstructure and elemental distribution of TC4/AgCuC/Si3N4 joint brazed at 880 °C for 10 min. (a) Interfacial microstructure of TC4/AgCuC/Si3N4 joint and EDS elemental distribution maps of (b) Ag, (c) Cu, and (d) Ti.
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Fig. 5. High magnification BE images of the interfacial microstructure of TC4/AgCuC/Si3N4 joint brazed at 880 °C for 10 min. (a) Zone I and (b) Zone III.
Table 1 Chemical compositions and possible phases of each spot marked in Fig. 5 (at.%) Spot
Si
N
Ag
Cu
Ti
Al
V
Possible phase
A B C D E F
– – – 2.56 2.20 22.89
– – – 9.65 8.56 36.99
2.52 0.87 0.86 77.62 0.69 1.46
30.80 48.88 60.48 3.62 58.63 4.31
63.06 47.70 36.76 5.12 28.71 33.45
2.32 1.47 1.22 0.86 0.77 0.54
1.30 1.08 0.68 0.57 0.44 0.36
Ti2Cu TiCu TiCu2 Ag(s,s) TiCu2 TiN, Ti5Si3
and gray phase marked as E in Fig. 5(b) were Ag based solid solution and TiCu2 intermetallic compound respectively. Moreover, the continuous thin reaction layer adjacent to Si3N4 ceramic shown in Fig. 5(b) was composed of TiN and Ti5Si3 compounds (Layer F). Actually, during brazing, element Ti both from TC4 alloy and from AgCuC filler dissolved in molten AgCu filler and then diffused toward Si3N4 substrate, finally reacted with the ceramic to form a TiN + Ti5Si3 reaction layer. The TiN + Ti5Si3 reaction layer was also found not only in our previous studies where Si3N4 ceramic was brazed to TiAl alloy using AgCuTi active filler, but also in other researches where active Ti containing fillers were selected to braze Si3N4 ceramics [20–23]. It is worth noting that Ag based composite reinforced by gray TiCu2 phases and a large number of fine black particles was formed in brazing seam, as shown in Fig. 5. Fig. 6 shows the bright field image and selected area electron diffraction patterns from a TEM sample extracted from Zone II of brazing seam. According to the results of TEM and SAED, the tiny particles dispersed in Ag matrix or in TiCu2 phases were TiN and Ti5Si3, which were formed by the reaction between nano-Si3N4 and active Ti during brazing. As analyzed above, the interfacial microstructure of TC4/AgCuC/Si3N4
brazed joint was TC4/Ti–Cu intermetallic layers/Ag based composite reinforced by TiCu2, TiN and Ti5Si3 particles/TiN+Ti5Si3 layer/ Si3N4. In order to investigate the effect of nano-Si3N4 addition in AgCuC filler on the interfacial microstructure of TC4/Si3N4 joints, AgCu eutectic powder filler was processed by mechanical treatment in a ball mill in the same modes as the nano-Si3N4 reinforced composite filler and then used to braze TC4 alloy to Si3N4 ceramic. Fig. 7 shows the typical interfacial microstructure of TC4/AgCu/ Si3N4 joint brazed at 880 °C for 10 min. It clearly demonstrates that the joint also consisted of three zones: Ti–Cu intermetallic layers (Zone I), brazing seam (Zone II) and TiN + Ti5Si3 layer (Zone III), which is similar to that of TC4/AgCuC/Si3N4 joint illustrated in Fig. 4(a). However, comparing Figs. 7 and 4(a) reveals that the average thickness of continuous Ti–Cu intermetallic layers adjacent to
Fig. 7. Typical interfacial microstructure TC4/Si3N4 joint brazed at 880 °C for 10 min using AgCu eutectic filler processed by mechanical treatment in the same modes as the AgCuC filler.
Fig. 6. Transmission electron microscopy (TEM) characterization and selected area electron diffraction (SAED) analysis of the microstructure in Zone II. (a) TEM image of the microstructure in Zone II, (b) SAED pattern of TiN and (c) SAED pattern of Ti5Si3.
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TC4 substrate was reduced from 25 lm to 10 lm. Meanwhile, the TiN + Ti5Si3 layer adjacent to Si3N4 substrate thinned significantly. These changes indicate that the rates of reactions between molten filler and base materials decreased when AgCuC filler was used. So it is deduced that the nano-Si3N4 addition in AgCuC filler suppressed the growth of Ti–Cu intermetallic layer TiN + Ti5Si3 layer to some extent. In addition, Ag based composite reinforced by fine particles was formed in brazing seam when AgCuC filler was used, which replaced the bright Ag solid solution. The reinforcements such as TiCu2, TiN and Ti5Si3 dispersed in Ag matrix not only strengthened the brazing seam but also reduced its CTE. Therefore, it is anticipated that the use of AgCuC filler could improve the joint properties in a certain extent. Moreover, a diffusion layer which could be observed in TC4/AgCu/Si3N4 joint disappeared in TC4/AgCuC/Si3N4 joint. Based on the analysis above, the brazing mechanism can be described as follow. The AgCu eutectic balls in AgCuC filler began to melt when brazing temperature exceeded its melting point. A semi-solid mixture containing molten AgCu filler, nano-Si3N4 and micron-Ti was formed gradually in the gap between TC4 alloy and Si3N4 ceramic. Element Cu diffused to TC4 alloy and reacted with it, which resulted in the formation of continuous Ti–Cu intermetallic layers adjacent to TC4 substrate. Simultaneously, active element Ti both from TC4 substrate and from micro-Ti particles dissolved into liquid filler, diffused towards Si3N4 side and reacted with it to form a TiN + Ti5Si3 reaction layer. The reaction mechanism between Si3N4 and active Ti was studied by Tunckan et al. [24] and Singh et al. [25] who reported the reaction equations as below.
phases in brazing seam and thus the TiCu2 particle phases were also dispersed within the Ag matrix, as shown in Fig. 5. In fact, the nano-Si3N4 addition acted as a barrier to the diffusion of Cu towards TC4 side and to the diffusion of active Ti towards Si3N4 side, which suppressed the growth both of the continuous Ti–Cu intermetallic layers and the TiN + Ti5Si3 layer. Therefore, compared with TC4/AgCu/Si3N4 joint, TC4/AgCuC/Si3N4 joint exhibited relatively thin reaction layers, as shown in Fig. 4(a). 3.3. Effect of brazing temperature on the interfacial microstructure of TC4/AgCuC/Si3N4 joints Fig. 8 shows the interfacial microstructure of TC4/AgCuC/Si3N4 joints brazed at different temperatures for 10 min. It is found that the brazing temperature had a significant effect on the interfacial microstructure. Both the Ti–Cu intermetallic layers adjacent to TC4 alloy and the Ti5Si3 + TiN layer adjacent to Si3N4 ceramic thickened gradually as the brazing temperature increased. The width of
1
Ti þ 1=4Si3 N4 ! TiN þ 3=4Si DG ðJ mol Þ ¼ 613; 000 þ 40:8 T 5Ti þ 3Si ! Ti5 Si3
1
DG ðJ mol Þ ¼ 194; 140 þ 16:74 T
ð1Þ ð2Þ
Additionally, the reaction between nano-Si3N4 and active Ti also occurred in liquid filler, which caused that tiny TiN and Ti5Si3 particles were dispersed in Ag matrix in brazing seam. These tiny particles could act as nucleation sites for the precipitation of TiCu2
Fig. 9. Effect of brazing temperature on the shear strength of TC4/AgCuC/Si3N4 brazed joints.
Fig. 8. Interfacial microstructure of TC4/AgCuC/Si3N4 joints brazed at different temperature for 10 min. (a) 860 °C, (b) 880 °C, (c) 900 °C, (d) 920 °C and (e) 940 °C.
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Fig. 10. Fractographies of TC4/AgCuC/Si3N4 joints brazed at (a) 860 °C, (b) 880 °C and (c) 940 °C.
brazing seam kept about 60 lm when brazing temperature was below 900 °C (shown in Fig. 8(a) and (b)), and increased to 75 lm when brazing temperature reached 900 °C, as shown in Fig. 8(c). However, it was reduced greatly to no more than 30 lm when brazing temperature exceeded 900 °C (shown in Fig. 8(d) and (e)). The reduction was due to the intensive diffusion of elements in molten filler such as Ag, Cu into TC4 substrate and the expulsion of molten filler by capillarity out of the joint gap at higher temperatures. Actually, the intensive diffusion at higher brazing temperature produced a diffusion layer in TC4 substrate, which gradually thickened with the increasing brazing temperature, as shown in Fig. 8(c)–(e). It is noted that the microstructure of brazing seam changed obviously with the increasing brazing temperature. At lower brazing temperature (860 °C), the reaction between nano-Si3N4 and active Ti in molten filler was incomplete, which resulted in an amount of nano-Si3N4 remained and aggregated in brazing seam after brazing process and thus the amount of tiny Ti5Si3 and TiN particles dispersed in Ag matrix was reduced, as shown in Fig. 8(a). Fig. 8(b) and (c) shows desirable microstructure could be obtained when brazed at 880 °C and 900 °C, although slight growth and coarsening of TiCu2 phases were taken place in Ag matrix during brazing at 900 °C. When brazing temperature increased to 920 °C, the Ag based composite was deteriorated due to the coalescence of TiCu2 phase in brazing seam and the formation of micro-cracks in Si3N4 substrate, as shown in Fig. 8(d). Further increasing the brazing temperature to 940 °C, the micrograph shown in Fig. 8(e) reveals that brazing defects including nano-Si3N4 aggregation, micro-cracks were generated in the brazing seam. These defects were mainly caused by an intensive mutual diffusion of the elements occurred between TC4 alloy and molten filler that promoted the excessive growth of Ti–Cu intermetallics in brazing seam. Meanwhile, the reaction between nano-Si3N4 and active Ti in molten filler was suppressed and the redistribution of tiny Ti5Si3 and TiN particles could not be achieved, which resulted in a large number of unreacted nano-Si3N4 aggregated in brazing seam.
3.4. Effect of brazing temperature on the shear strength of TC4/AgCuC/ Si3N4 joints
insufficient. While the specimens brazed at 880 °C and 900 °C possessed high shear strength. The maximum average shear strength of 73.9 MPa was obtained when brazing was conducted at 880 °C, which is 24.7 MPa (50%) higher than that of the TC4/ Si3N4 joints brazed using AgCu alone (49.2 MPa). Interfacial observations in Section 3.3 reveal that the desirable Ag based composite reinforced by tiny Ti5Si3 and TiN particles as well as TiCu2 phases was formed in brazing seam. The reinforcements with lower CTE in Ag matrix not only strengthened the brazing seam but also reduced its CTE. Thus, the mismatch in CTE among TC4 substrate, brazing seam and Si3N4 substrate was reduced and the residual stresses in interfacial region could be relieved to some extent. So, the joining properties were improved finally. However, when brazing temperature was further increased to 900 °C or 920 °C, the shear strength decreased sharply. This result could be attributed to the formation of brazing defects including micro-cracks and nano-Si3N4 aggregation shown in Fig. 8(d) and (e), which inevitably resulted in a weak bonding between TC4 alloy and Si3N4 ceramic. Fracture analysis was performed by optical microscope to investigate the fracture locations. Fig. 10 shows three kinds of fracture mode. Failure of TC4/AgCuC/Si3N4 joints brazed at 860 °C and 920 °C always occurred in Si3N4 ceramic substrate near the braze interface and a bowed crack path was observed in Si3N4 substrate, as shown in Fig. 10(a), which indicated that high residual stresses were generated in Si3N4 substrate close to the interface and the residual stresses reduced the joint strength. In fact, the formation of micro-cracks within Si3N4 substrate before shear test shown in Fig. 8(d) was mainly due to the presence of considerable residual stresses in interfacial region. For the shear test of specimens brazed at 880 °C and 900 °C, cracks propagated in the brazing seam and finally ruptured in Si3N4 substrate, as illustrated in Fig. 10(b). This fracture mode suggested that the residual stresses was relieved because of the improved interfacial microstructure shown in Fig. 8(b) and (c). In addition, all of the TC4/AgCuC/Si3N4 joints brazed at 940 °C fractured along the brazing seam, the typical fractography can be seen in Fig. 10(c). This fracture mode was mainly due to a weak bonding formed in brazing seams that was caused by a large amount of defects.
4. Conclusions Fig. 9 shows the room-temperature shear strength of TC4/ AgCuC/Si3N4 joints brazed at different temperature for 10 min. As a result, the shear strength of specimens brazed at 860 °C was
Reliable brazing of TC4 alloy and Si3N4 ceramic was achieved by using nano-Si3N4 reinforced AgCu composite filler (AgCuC filler).
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The interfacial microstructure and joining properties of TC4/AgCuC/ Si3N4 brazed joint were investigated in this study. Primary conclusions are summarized as follows. (1) No metallurgical reaction occurred among AgCu eutectic powder, nano-Si3N4 and micron-Ti particles during ball milling process. The addition of nano-Si3N4 in AgCuC filler changed its melting point marginally. (2) The typical interfacial microstructure of TC4/AgCuC/Si3N4 brazed joint was TC4/Ti–Cu intermetallic layers/Ag based composite reinforced by TiCu2, TiN and Ti5Si3 particles/ TiN + Ti5Si3 layer/Si3N4. Nano-Si3N4 addition in AgCuC filler improved the interfacial microstructure by providing dispersive nucleation sites for Ti–Cu intermetallic compounds in brazing seam and suppressing the growth of continuous Ti–Cu intermetallic layers adjacent to TC4 alloy. (3) Increasing brazing temperature accelerated the diffusion and reaction of elements in molten filler, which resulted in an increase in thickness both of Ti–Cu intermetallic layers and TiN + Ti5Si3 reaction layer. When brazing temperature exceeded 900 °C, brazing defects including micro-cracks and nano-Si3N4 aggregation were generated in brazed joints. (4) Brazing temperature had a strong influence on the shear strength of TC4/AgCuC/Si3N4 brazed joints. The highest average shear strength reached 73.9 MPa when brazed at 880 °C for 10 min. Nano-Si3N4 addition in AgCuC filler significantly improved the interfacial microstructure and joining properties of TC4/AgCuC/Si3N4 brazed joints.
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