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Invar joint
Vacuum 168 (2019) 108830
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Interfacial microstructure and mechanical properties of the TiC-Ni cermet/ Ag-Cu-Zn/Invar joint
T
Min Leia, Yulong Lia,b, Hua Zhanga,* a b
Key Lab for Robot and Welding Automation of Jiangxi Province, Mechanical and Electrical Engineering School, Nanchang University, 330031, Nanchang, China State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, 150001, Harbin, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ag-Cu-Zn Brazing joint Interfacial microstructure Evaporation
TiC-Ni cermet and Invar was brazed with Ag-Cu-Zn alloy in vacuum and the interfacial microstructure and mechanical properties were studied. The interfacial microstructure of the joint brazed at 910 °C for 10 min was TiC-Ni cermet/(Ag) + (Cu, Ni) + TiC/(Cu, Ni) + (Ag)/(Cu, Ni)/(Cu, Ni) + (Fe, Ni)/Invar alloy. When the temperature increased from 740 °C to 940 °C, (Cu, Ni) reactive layer at the cermet side dissolved into the brazing seam and became discontinuous. Moreover, TiC particles presented gradient distribution in the reactive region of the joint brazed between 850 °C and 940 °C. Meanwhile, the amount of (Cu, Ni) detaching from the cermet and entering the brazing seam increased. The shear strength firstly increased and then sharply decreased with temperature. The maximum shear strength of 161 MPa was obtained at 910 °C for 10 min and the joint was fractured on (Ag). The change trend of the shear strength between 820 °C and 940 °C was attributed to the increase thickness of the functional gradient material layer in the reactive region and the decrease of the (Cu, Ni) amount in the penetration region and the reactive region with the temperature increase.
1. Introduction Recently, TiC cermet has received extensive attention because of its outstanding wear resistance, good elevated-temperature mechanical property, and strong electrical and magnetic performance [1,2]. Invar alloy possesses good formability, high toughness, and low coefficient of thermal expansion, approximately one tens of that of the stainless steel, which makes it a promising candidate for structure demanding highdimension stability [3–5]. However, the strength of Invar alloy is not high enough to meet the requirement of its application [6]. In order to take advantage of both materials, it is necessary to join TiC cermet to Invar alloy. To date, some methods, such as brazing [7–10], diffusion bonding [11,12] and self-propagating high-temperature synthesis [13,14], have been selected to join cermet and metal. Among them, brazing is very popular because of its high quality and time-saving. As a brazing alloy with low liquidus temperature, excellent fluidity, and good electrochemical properties, Ag-Cu-Zn alloy attracts much attention [15–17]. Zn is easy to evaporate in vacuum because of its high vapor pressure [18]. Consequently, Ag-Cu-Zn alloy was regarded as a brazing alloy used in gas protected or air environment for a long time [19–22]. However, Liu reported that the sound joint of the TiAl-based alloy and 40Cr steel was achieved by vacuum brazing with Ag-Cu-Zn [23].
*
Subsequently, Ag-Cu-Zn-based alloys were widely used in vacuum brazing TiC-Ni cermet to steel [7,8], Ti(C, N)-based cermet to steel [24], TC4 alloy to austenitic stainless steel [25], and aluminum-bronze to austenitic martensitic stainless steel [26]. Recently, it was found that Zn evaporation promoted the formation of (Cu, Ni) during vacuum wetting of TiC-Ni cermet by the Ag-Cu-Zn alloy [27]. Nevertheless, how evaporative element influences the interfacial microstructure in vacuum brazing is still poorly understood. In this paper, TiC-Ni cermet was brazed to Invar alloy using the Ag-Cu-Zn alloy in vacuum and the effect of Zn evaporation on the interfacial microstructure of TiC-Ni cermet/Ag-Cu-Zn/Invar was clarified. In addition, the correlation between interfacial microstructure and mechanical properties of the TiC-Ni cermet and Invar joint was discussed. This investigation enriches the understanding of joining metal-matrix composites and metals with brazing alloy containing evaporative elements in vacuum. 2. Experiments TiC-Ni cermet was fabricated by self-propagating high-temperature synthesis. The composition of TiC-Ni cermet is 40 wt%Ni and 60 wt %TiC. Ni is the matrix phase and TiC particle is the strengthening phase, as shown in Fig. 1a. The dimension of TiC-Ni cermet is
Corresponding author. E-mail address: [email protected] (H. Zhang).
https://doi.org/10.1016/j.vacuum.2019.108830 Received 5 June 2019; Received in revised form 22 July 2019; Accepted 22 July 2019 Available online 23 July 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
Vacuum 168 (2019) 108830
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Fig. 1. Microstructure of the substrates (a) TiC-Ni cermet and (b) Invar alloy.
electron microscope (SEM, S-4700) provided with an energy dispersive X-ray spectrometer (EDS, TN4700) was used to study the microstructures. In addition, the phases were determined by the X-ray diffraction (XRD, JDX-3530 M, Cu-Ka). Shear strength of the joint was measured by the universe testing machine (INSTRON-1186) at room temperature. Fig. 2b illustrates the diagram of shear test. The average shear strength of joint was obtained from four different species at the same parameter.
Table 1 Composition of Invar alloy (wt.%). Ni
Mn
Si
S
C
P
Fe
35.0–37.0
0.2–0.6
≤0.2
≤0.02
≤0.02
≤0.05
balance
3. Results and discussion 3.1. Microstructure of the TiC-Ni cermet/Ag-Cu-Zn/invar interface Zn in the Ag-Cu-Zn alloy evaporates completely after the temperature increases to 810 °C, according to the result obtained from the wetting experiment [27]. In order to explore the influence of Zn evaporation on the interfacial microstructure of the brazing joint, TiC-Ni cermet and Invar alloy was firstly brazed at a lower temperature (740 °C) for 10 min. The interfacial microstructure and composition of the characteristic phase are given in Fig. 3 and Table 2, respectively. The result shows that some Zn still exists in the brazing seam and the reactive layer. The brazing seam consists of (Ag) and (Cu). Some (Fe, Ni) detaches from the Invar substrate and the ratio of Fe/Ni approaches that in the Invar alloy. In addition, continuous (Cu, Ni) forms at both sides of interface. Besides, the ratio of Fe/Ni in the (Cu, Ni) layer at the Invar alloy side is far lower than that in the Invar alloy. It indicates that dissolution of the Invar alloy is much weaker than that of the Ni matrix, which is in accordance with the much lower solubility of Fe in the AgCu based alloy than that of Ni [20]. Also, it's noted that some (Ag) appears in the (Cu, Ni) layer at the cermet side in the brazing configuration, which is much more than that formed during the vacuum wetting process [27]. Moreover, there are a lot of pores in the brazing seam. Similar phenomenon was also presented in the stainless steel/Ag-Cu-Zn/Ti-6Al4V joint brazed in vacuum [25]. The presence of pores is because of the Zn evaporation. On the one side, Zn evaporates rapidly at high temperature due to its high vapor pressure [18]. However, only some Zn
Fig. 2. Schematic diagram of (a) the assemebly sample and (b) shear test.
4 mm × 4 mm × 4 mm. Invar alloy is the commercial Fe-Ni alloy and composed of austenite structure, as shown in Fig. 1b. The composition is presented in Table 1. The dimension of Invar alloy is 10 mm × 20 mm × 3 mm. The substrates were ground with SiC paper up to grit 1000. A mixture powder of Zn and Ag-28Cu (wt.%) with a proportion of 1: 3 was smelted by induction method to fabricate the AgCu-Zn alloy. The alloy starts to melt in vacuum around 670 °C, according to the result obtained from the wetting experiment reported in Ref. [27]. The addition of Zn greatly decreases the melting point of the Ag-Cu-Zn alloy, causing a lower brazing temperature than other element addition, such as Ni [28]. Then the brazing alloy was cut and ground to a final thickness of 150 μm. Prior to experiments, the specimens were cleaned in an ultrasonic acetone bath and then were carefully assembled. The schematic diagram of the assembly sample is shown in Fig. 2a. After the furnace reaching a vacuum of 10−2 Pa, the brazing couple was first heated to 600 °C at a rate of 20 °C/min, then to the peak temperature (740 °C–940 °C) at a rate of 10 °C/min. After held for 5 miñ15 min, the joints were cooled to 400 °C at a rate of 5 °C/min. Finally, it was cooled to room temperature in the furnace. A scanning
Fig. 3. (a) Interfacial microstructure of the joint brazed at 740 °C for 10 min (b) Enlarged area at the Invar side. 2
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Table 2 Composition of the characteristic phase in Fig. 3 (at. %).
Table 3 Composition of the characteristic phase in Fig. 4 (at.%).
Point
Ag
Cu
Zn
Ni
Fe
Phase
Point
Ag
Cu
Zn
Ni
Fe
Ti
C
Possible phases
A B C D E
2.1 5 75.5 2.8 10.7
74.3 85.9 17.2 68.2 5.4
7.9 6.2 5.4 13.9 1
13.1 1.9 1.1 10.7 30
2.6 1 0.8 4.4 52.9
(Cu, Ni) (Cu) (Ag) (Cu, Ni) (Fe, Ni)
A B C D E F G
0.4 0.6 79.7 – 1.5 0.4 0.5
22.6 53.9 8.9 47.3 70.3 27.3 2.6
– 0.5 4 1.2 – 0.5 –
19.4 34.1 2.9 42.9 23.5 54.7 38.6
3.3 6.7 3 7.7 4.7 17.1 58.3
14 4.2 1.5 0.9 – – –
40.3 – – – – – –
TiC+(Cu, Ni) (Cu, Ni) (Ag) (Cu, Ni) (Cu, Ni) (Cu, Ni)+(Fe, Ni) (Fe, Ni)
atoms can leave the liquid because of the small liquid-gas interface in the brazing configuration. As a result, a lot of vapor bubbles form and remain in the brazing seam. On the other side, the solidus temperature of Ag-Cu-Zn alloy rises during the evaporation process, determined from the Ag-Cu-Zn ternary phase diagram [29]. As a result, some liquid solidifies during the holding process. The vapor bubble could not escape from the solidified alloy, so pores form in the brazing seam. As the temperature increases, the Zn evaporation becomes stronger, which causes the decrease of Zn concentration. Finally, Zn in the brazing alloy evaporates completely [27]. The characteristic interfacial microstructure of the joint was investigated at 910 °C for 10 min, as shown in Fig. 4. The whole interface consists of four different characteristic regions: the penetration region in the TiC-Ni cermet (Region I), the reactive region adjacent to the cermet (Region II), the brazing seam (Region III), and the reactive layer at the Invar side (Region IV). Table 3 gives the composition of each characteristic phase in Fig. 4. Little Zn was detected in the residual brazing alloy and the reactive phases, which shows that the Zn element has evaporated completely during the brazing process. Both Region I and Region II are composed of (Ag), (Cu, Ni) and TiC particle. Region I in TiC-Ni cermet is formed by penetration of the liquid into the cermet, which was also observed in the joint brazed with Ag-31Cu-23Zn at 950 °C for 15 min [7], but not found in the joint brazed with Ag-54Cu-33Zn up to 950 °C for 15 min [8]. Such phenomenon should be attributed to the better flowability of the near eutectic liquid. The (Cu, Ni) layer in Region II changes to discontinuous. Also, some liquid alloy flows into the grain boundary, which results into the detachment of some (Cu, Ni). Besides, due to the serious dissolution process, many TiC particles float into Region II. More noticeably, the TiC particle gradually decreases from 60 wt% to 0 in the reactive region, leading to the gradient distribution of TiC particles in the (Ag) and (Cu, Ni) matrix in Region II, as seen in the FGM layer marked in Fig. 4a. The microstructure of the brazing seam (Region III) is composed of (Ag) and stripe-like (Cu, Ni), which departs from the eutectic pattern. Region IV is composed of two kinds of reactive layer, as shown in Fig. 4b. The reactive layer at point E is rich in Cu and Ni, which is supposed to be (Cu, Ni). The reactive layer adjacent to the Invar alloy at point F consists of Fe, Cu and Ni, which should be the mixture of (Cu, Ni) and (Fe, Ni), deduced from the Cu-Fe-Ni ternary phase diagram [30]. Point G has a similar composition with the Invar alloy. Fig. 5 presents the XRD results of the products of the joint brazed at 910 °C for 10 min at the cermet side (above) and at the Invar side
Fig. 5. XRD results of the reactive layer.
(below), respectively. The XRD results determine that the products are (Ag), TiC and (Cu, Ni) at the cermet side, and are (Cu, Ni) and mixture of (Fe, Ni) and (Cu, Ni) at the Invar side. The existence of Fe in the mixture of (Cu, Ni) and (Fe, Ni) causes the characteristic peaks to slightly move to the right. Fig. 6 displays the interface of the joint brazed at temperature ranging from 820 °C to 940 °C for 10 min. As the temperature is 820 °C, the liquid alloy penetrates into the cermet. The layered (Cu, Ni) changes to discontinuous. The brazing seam consists of (Ag) and (Cu, Ni). As the temperature increases, the thickness of the penetration region (Region I) gradually increases from 6.2 μm at 820 °C to 31.1 μm at 940 °C, and the amount of (Ag) in the TiC-Ni cermet increases. Moreover, TiC particles presents gradient distribution in the reactive region (Region II) of the joint brazed between 850 °C and 940 °C; and the thickness of Region II increases with the temperature. Meanwhile, the amount of (Cu, Ni) detaching from the cermet and entering the brazing seam increases. Notably, most of the (Cu, Ni) goes into the brazing seam at 940 °C. Similar phenomenon was observed in the TiC-Ni cermet/Ag31Cu-23Zn/steel brazed at 950 °C for 15 min [7]. The interface change is closely related to the Zn evaporation. The forming of large amount of (Cu, Ni) induced by Zn evaporation [27] consumes a lot of Cu element, which causes a copper-poor and silverrich brazing alloy near the cermet and forms (Ag) in the (Cu, Ni) layer, as shown in Fig. 3a. The (Ag) re-melts and forms a lot of micro-channel along the grain boundary for liquid flow as the temperature rises. Additionally, it was found that the dissolution along the grain boundary had a faster rate than that of the bulk phase in the Ag-Cu/Ni couple [31], so the liquid alloy prefers to penetrate along the grain boundary
Fig. 4. (a) Interfacial microstructure of the joint brazed at 910 °C for 10 min (b) Enlarged area in (a). 3
Vacuum 168 (2019) 108830
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Fig. 6. Interface of the joint brazed at brazed at different temperature for 10 min (a) 820 °C (b) 850 °C (c) 880 °C (d) 940 °C.
possesses the highest strength. Fig. 9 presents the fracture surface of joint at the cermet side brazed at 940 °C for 10 min. According to the morphology, the fracture surface is divided into two characteristic regions: Region i and Region ii. The results of the element composition in each region are shown in Table 4. The results show that Region i consists of (Ag), (Cu, Ni) and some granular TiC particles, which indicates that it is the penetration region in the joint. Similarly, Region ii consists of (Ag), (Cu, Ni) and some flat TiC particles, which shows that it is adjacent to the reactive region of the joint. The fracture morphology of joint brazed at 740 °C for 10 min is presented in Fig. 10. It consists of two regions. Similar as Region ii in Fig. 9, Region a in Fig. 10 is composed of (Ag), (Cu, Ni) and the flat TiC particle, which shows that it is adjacent to the reactive region of the joint. Region b consists of (Ag) and (Cu), which shows that it is the brazing seam. Besides, some pores present in the brazing seam, which is also observed in Fig. 3. The pores are potential crack source under the shear test process, which are detrimental to the strength of the joint. Therefore, the joint obtained at 740 °C for 10 min has the minimum shear strength. At a higher brazing temperature, the pores caused by Zn evaporation are not observed in the brazing alloy. And the adhesion between the TiC and the brazing alloy becomes the weak part of the joint. The adhesive strength between the TiC and the alloy can be evaluated by the work of adhesion Wad, which is determined by the Young-Dupre equation:
and dissolve the bulk phase adjacent to the grain boundary. As a result, the reactive layer transforms from continuous to discontinuous, finally detaching from substrate. Besides, temperature increase promotes liquid penetration along the boundary between the TiC particle and metal phase, which increases the (Ag) in the original cermet as the temperature is increased. As the temperature continues increasing, the solubility of Ni in the Ag-Cu alloy gradually rises [31]. So some (Cu, Ni) in the reactive region dissolves into the brazing alloy, which results in the increase of (Cu, Ni) in the brazing seam with temperature. 3.2. Mechanical evaluation of the brazing joint The shear strength of joint brazed at different temperature for 10 min is exhibited in Fig. 7. The strength firstly increases and then decreases with temperature. And the maximum value of 161 MPa is obtained at 910 °C for 10 min. The strength maintains at a high level between 820 °C and 910 °C. However, when the temperature increases up to 940 °C, the strength is rapidly decreased. Fig. 8 shows the fracture surface of the joint brazed at 910 °C for 10 min at the cermet side. The macroscopic fracture surface is quite flat, which means that there is no deflection during the crack propagation process. And the microscopic fracture surface consists of tiny quasicleavage planes and tearing ridges. Additionally, there is linear plastic deformation on the microscopic fracture surface, indicative of good ductility and toughness. Obviously, it is beneficial to obtain the joint with high strength. The EDS result shows that the fracture surface consists of (Ag), which indicates that it is the brazing seam. Because of the good ductility and toughness of (Ag), the joint fractured on (Ag)
Wad = σlv(1+cosθ) Where, σlv is the surface tension of the liquid alloy and θ is the equilibrium contact angle of the liquid alloy on the TiC ceramic. The equilibrium contact angle of Ag, Cu and Ni liquid on TiC cermet is 120° [32], 88° [32] and 28° [33], respectively. It indicates that the wettability of TiC ceramic by the liquid Ag is much worse than Cu or Ni. And the surface tension of Ag, Cu and Ni is 827 mN/m [34], 1186 mN/ m [34], and 1768 mN/m [33], respectively. According to the YoungDupre equation, the work of adhesion of different liquid alloys on the TiC ceramic is calculated to be 414 mJ/m2, 1227 mJ/m2, and 3329 mJ/ m2, which manifests that the work of adhesion between TiC ceramic and Ag is much smaller than that between TiC ceramic and Cu or Ni. In other words, it demonstrates that (Cu, Ni) at the interface can greatly improve the adhesive strength between the cermet substrate and the
Fig. 7. Shear strength of the joint brazed at different temperature for 10 min. 4
Vacuum 168 (2019) 108830
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Fig. 8. (a) Fracture surface of the joint at the cermet side brazed at 910 °C for 10 min (b) Enlarged views of the fracture surface.
Fig. 9. (a) Fracture surface of the joint at the cermet side brazed at 940 °C for 10 min (b) and (c) Enlarged views of the fracture surface at Region i and ii in (a).
1. Dissolution of the Ni matrix in the cermet is much higher than that of the Invar alloy in the joint brazed at 740 °C for 10 min. It causes the formation of continuous (Cu, Ni) layer at both the cermet and Invar alloy sides. In addition, a lot of pores form in the brazing seam because of the incomplete evaporation of Zn into the vacuum circumstance. 2. The interfacial microstructure of the joint brazed at 910 °C for 10 min is TiC-Ni cermet/(Ag)+(Cu, Ni)+TiC/(Ag)+(Cu, Ni)/(Cu, Ni)/(Cu, Ni)+(Fe, Ni)/Invar alloy. The formation of large amount of (Cu, Ni) induced by the Zn evaporation causes a copper-poor and silver-rich brazing alloy near the cermet and forms (Ag) in the (Cu, Ni) layer. No pores are observed in the joint after complete Zn evaporation. 3. When the temperature rises from 740 °C to 820 °C, (Cu, Ni) dissolves in the brazing alloy and become discontinuous. When the temperature further increases to 940 °C, the amount of (Ag) in the cermet increases and the thickness of the penetration region gradually increases from 6.2 μm at 820 °C to 31.1 μm at 940 °C. Moreover, TiC particles presents gradient distribution in the reactive region (Region II) of the joint brazed between 850 °C and 940 °C; and the thickness of reactive layer adjacent to the cermet increases with temperature. Meanwhile, the amount of (Cu, Ni) detaching from the cermet and entering the brazing seam increases. 4. The shear strength firstly increases and then decreases with temperature. The maximum shear strength of 161 MPa is obtained at 910 °C for 10min and the joint is fractured on (Ag). The shear strength maintains at a high level in a wide range of brazing
Table 4 Element composition at each point in Fig. 9 (at. %). Point
Ag
Cu
Zn
Ni
Fe
Ti
Phases
A B C
41.06 2.14 20.44
34.33 5.11 38.51
2.05 1.17 0.84
15.96 5.02 29.45
3.51 1.57 5.69
3.09 85.00 5.06
(Ag)+(Cu, Ni) TiC (Ag)+(Cu, Ni)
Ag-Cu-Zn alloy, and (Ag) in the penetration region and the reactive region decreases the adhesive strength. On the other hand, it was proposed that the thickness of the gradient layer close to the substrate is the most important for releasing the residual stress in the joint; and the effect is the best when it is the thickest [35]. Seen from Figs. 4 and 6, the thickness of the functional gradient material layer (marked as FGM) increases as the temperature increases, which is beneficial for the release of the residual stress. So the shear strength of the joint firstly increases and then decreases under both effects of (Cu, Ni) and FGM layer in the joint as the temperature increases from 820 °C to 940 °C.
4. Conclusion TiC-Ni cermet and Invar alloy was brazed with the Ag-Cu-Zn filler metal in vacuum. The effects of heating parameters on the interfacial microstructure and mechanical properties were studied. And the following conclusions can be obtained. 5
Vacuum 168 (2019) 108830
M. Lei, et al.
Fig. 10. (a) Fracture surface of the joint at the cermet side brazed at 740 °C for 10 min (b) and (c) Enlarged views of the fracture surface at Region a and b in (a).
parameter. 5. The low shear strength at 740 °C for 10 min is attributed to the pores in the brazing alloy. And the change trend between 820 °C and 940 °C is attributed to the increase of the FGM layer in the reactive region and the decrease of the amount of (Cu, Ni) in the penetration region and the reactive region with the temperature.
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Acknowledgment The authors appreciate the financial support of National Natural Science Foundation of China (NSFC, Grant number 51665038), the Academic and Technical Leaders Founding Project of Major Disciplines of Jiangxi Province (20182BCB22001) and Key Project of the Natural Science Foundation of Jiangxi Province (20171ACB21011). References [1] J. Li, G. Sheng, L. Huang, Additional active metal Nb in Cu-Ni system filler metal for brazing of TiC cermet/steel, Mater. Lett. 156 (2015) 10–13. [2] X. Zhang, L. Guo, F. Yang, 3D gel printing of graded TiC-high manganese steel cermet, J. Mater. Sci. 54 (3) (2019) 2122–2132. [3] X.H. Zhan, Y. Meng, J.J. Zhou, C.Q. Qi, C.L. Zhang, D.D. Gu, Quantitative research on microstructure and thermal physical mechanism in laser melting deposition for Invar alloy, J. Manuf. Process. 31 (2018) 221–231. [4] T. Wang, T. Ivas, W. Lee, C. Leinenbach, J. Zhang, Relief of the residual stresses in Si3N4/ Invar joints by multi-layered braze structure-Experiments and simulation, Ceram. Int. 42 (2016) 7080–7087. [5] Y. Wang, Z.W. Yang, L.X. Zhang, J.C. Feng, Microstructure and mechanical properties of invar alloy and Si3N4 ceramic brazed joints, Rare Metal Mater. Eng. 44 (2015) 339–343. [6] A. Vinogradov, S. Hashimoto, V.I. Kopylov, Enhanced strength and fatigue life of ultrafine grain Fe-36Ni Invar alloy, Mat. Sci. Eng. A-Struct. 355 (2003) 277–285. [7] J.C. Feng, L.X. Zhang, Interface structure and mechanical properties of the brazed joint of TiC cermet and steel, J. Eur. Ceram. Soc. 26 (2006) 1287–1292. [8] L.X. Zhang, J.C. Feng, P. He, Brazing temperature and time effects on the mechanical properties of TiC cermet/Ag-Cu-Zn/steel joints, Mat. Sci. Eng. A-Struct. 428 (2006) 24–33. [9] F.Z. Wang, Q.Z. Wang, B.H. Yu, B.L. Xiao, Z.Y. Ma, Interface structure and mechanical properties of Ti(C,N)-based cermet and 17-4PH stainless steel joint brazed with nickelbase filler metal BNi-2, J. Mater. Process. Technol. 211 (2011) 1804–1809. [10] G.H. Han, H. Bian, H.Y. Zhao, X.G. Song, Y. Li, D. Liu, J. Cao, J.C. Feng, Interfacial microstructure and mechanical properties of TZM alloy and ZrC particle reinforced tungsten composite joint brazed using Ti-61Ni filler, J. Alloy. Comp. 747 (2018) 266–275. [11] M.I. Barrena, J.M.G. de Salazar, L. Matesanz, Ni-Cu alloy for diffusion bonding cermet/ steel in air, Mater. Lett. 63 (2009) 2142–2145. [12] J. Cao, X.G. Song, L.Z. Wu, J.L. Qi, J.C. Feng, Characterization of Al/Ni multilayers and their application in diffusion bonding of TiAl to TiC cermet, Thin Solid Films 520 (2012) 3528–3531. [13] J. Wang, J. Cheng, P. Bai, Y. Li, Investigation of joining Al-C-Ti cermets and Ti6Al4V by combustion synthesis, Mater. Sci. Eng. B-Adv. 177 (2012) 1703–1706.
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Interfacial microstructure and mechanical properties of the TiC-Ni cermet/ Ag-Cu-Zn/Invar joint
T
Min Leia, Yulong Lia,b, Hua Zhanga,* a b
Key Lab for Robot and Welding Automation of Jiangxi Province, Mechanical and Electrical Engineering School, Nanchang University, 330031, Nanchang, China State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, 150001, Harbin, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ag-Cu-Zn Brazing joint Interfacial microstructure Evaporation
TiC-Ni cermet and Invar was brazed with Ag-Cu-Zn alloy in vacuum and the interfacial microstructure and mechanical properties were studied. The interfacial microstructure of the joint brazed at 910 °C for 10 min was TiC-Ni cermet/(Ag) + (Cu, Ni) + TiC/(Cu, Ni) + (Ag)/(Cu, Ni)/(Cu, Ni) + (Fe, Ni)/Invar alloy. When the temperature increased from 740 °C to 940 °C, (Cu, Ni) reactive layer at the cermet side dissolved into the brazing seam and became discontinuous. Moreover, TiC particles presented gradient distribution in the reactive region of the joint brazed between 850 °C and 940 °C. Meanwhile, the amount of (Cu, Ni) detaching from the cermet and entering the brazing seam increased. The shear strength firstly increased and then sharply decreased with temperature. The maximum shear strength of 161 MPa was obtained at 910 °C for 10 min and the joint was fractured on (Ag). The change trend of the shear strength between 820 °C and 940 °C was attributed to the increase thickness of the functional gradient material layer in the reactive region and the decrease of the (Cu, Ni) amount in the penetration region and the reactive region with the temperature increase.
1. Introduction Recently, TiC cermet has received extensive attention because of its outstanding wear resistance, good elevated-temperature mechanical property, and strong electrical and magnetic performance [1,2]. Invar alloy possesses good formability, high toughness, and low coefficient of thermal expansion, approximately one tens of that of the stainless steel, which makes it a promising candidate for structure demanding highdimension stability [3–5]. However, the strength of Invar alloy is not high enough to meet the requirement of its application [6]. In order to take advantage of both materials, it is necessary to join TiC cermet to Invar alloy. To date, some methods, such as brazing [7–10], diffusion bonding [11,12] and self-propagating high-temperature synthesis [13,14], have been selected to join cermet and metal. Among them, brazing is very popular because of its high quality and time-saving. As a brazing alloy with low liquidus temperature, excellent fluidity, and good electrochemical properties, Ag-Cu-Zn alloy attracts much attention [15–17]. Zn is easy to evaporate in vacuum because of its high vapor pressure [18]. Consequently, Ag-Cu-Zn alloy was regarded as a brazing alloy used in gas protected or air environment for a long time [19–22]. However, Liu reported that the sound joint of the TiAl-based alloy and 40Cr steel was achieved by vacuum brazing with Ag-Cu-Zn [23].
*
Subsequently, Ag-Cu-Zn-based alloys were widely used in vacuum brazing TiC-Ni cermet to steel [7,8], Ti(C, N)-based cermet to steel [24], TC4 alloy to austenitic stainless steel [25], and aluminum-bronze to austenitic martensitic stainless steel [26]. Recently, it was found that Zn evaporation promoted the formation of (Cu, Ni) during vacuum wetting of TiC-Ni cermet by the Ag-Cu-Zn alloy [27]. Nevertheless, how evaporative element influences the interfacial microstructure in vacuum brazing is still poorly understood. In this paper, TiC-Ni cermet was brazed to Invar alloy using the Ag-Cu-Zn alloy in vacuum and the effect of Zn evaporation on the interfacial microstructure of TiC-Ni cermet/Ag-Cu-Zn/Invar was clarified. In addition, the correlation between interfacial microstructure and mechanical properties of the TiC-Ni cermet and Invar joint was discussed. This investigation enriches the understanding of joining metal-matrix composites and metals with brazing alloy containing evaporative elements in vacuum. 2. Experiments TiC-Ni cermet was fabricated by self-propagating high-temperature synthesis. The composition of TiC-Ni cermet is 40 wt%Ni and 60 wt %TiC. Ni is the matrix phase and TiC particle is the strengthening phase, as shown in Fig. 1a. The dimension of TiC-Ni cermet is
Corresponding author. E-mail address: [email protected] (H. Zhang).
https://doi.org/10.1016/j.vacuum.2019.108830 Received 5 June 2019; Received in revised form 22 July 2019; Accepted 22 July 2019 Available online 23 July 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Microstructure of the substrates (a) TiC-Ni cermet and (b) Invar alloy.
electron microscope (SEM, S-4700) provided with an energy dispersive X-ray spectrometer (EDS, TN4700) was used to study the microstructures. In addition, the phases were determined by the X-ray diffraction (XRD, JDX-3530 M, Cu-Ka). Shear strength of the joint was measured by the universe testing machine (INSTRON-1186) at room temperature. Fig. 2b illustrates the diagram of shear test. The average shear strength of joint was obtained from four different species at the same parameter.
Table 1 Composition of Invar alloy (wt.%). Ni
Mn
Si
S
C
P
Fe
35.0–37.0
0.2–0.6
≤0.2
≤0.02
≤0.02
≤0.05
balance
3. Results and discussion 3.1. Microstructure of the TiC-Ni cermet/Ag-Cu-Zn/invar interface Zn in the Ag-Cu-Zn alloy evaporates completely after the temperature increases to 810 °C, according to the result obtained from the wetting experiment [27]. In order to explore the influence of Zn evaporation on the interfacial microstructure of the brazing joint, TiC-Ni cermet and Invar alloy was firstly brazed at a lower temperature (740 °C) for 10 min. The interfacial microstructure and composition of the characteristic phase are given in Fig. 3 and Table 2, respectively. The result shows that some Zn still exists in the brazing seam and the reactive layer. The brazing seam consists of (Ag) and (Cu). Some (Fe, Ni) detaches from the Invar substrate and the ratio of Fe/Ni approaches that in the Invar alloy. In addition, continuous (Cu, Ni) forms at both sides of interface. Besides, the ratio of Fe/Ni in the (Cu, Ni) layer at the Invar alloy side is far lower than that in the Invar alloy. It indicates that dissolution of the Invar alloy is much weaker than that of the Ni matrix, which is in accordance with the much lower solubility of Fe in the AgCu based alloy than that of Ni [20]. Also, it's noted that some (Ag) appears in the (Cu, Ni) layer at the cermet side in the brazing configuration, which is much more than that formed during the vacuum wetting process [27]. Moreover, there are a lot of pores in the brazing seam. Similar phenomenon was also presented in the stainless steel/Ag-Cu-Zn/Ti-6Al4V joint brazed in vacuum [25]. The presence of pores is because of the Zn evaporation. On the one side, Zn evaporates rapidly at high temperature due to its high vapor pressure [18]. However, only some Zn
Fig. 2. Schematic diagram of (a) the assemebly sample and (b) shear test.
4 mm × 4 mm × 4 mm. Invar alloy is the commercial Fe-Ni alloy and composed of austenite structure, as shown in Fig. 1b. The composition is presented in Table 1. The dimension of Invar alloy is 10 mm × 20 mm × 3 mm. The substrates were ground with SiC paper up to grit 1000. A mixture powder of Zn and Ag-28Cu (wt.%) with a proportion of 1: 3 was smelted by induction method to fabricate the AgCu-Zn alloy. The alloy starts to melt in vacuum around 670 °C, according to the result obtained from the wetting experiment reported in Ref. [27]. The addition of Zn greatly decreases the melting point of the Ag-Cu-Zn alloy, causing a lower brazing temperature than other element addition, such as Ni [28]. Then the brazing alloy was cut and ground to a final thickness of 150 μm. Prior to experiments, the specimens were cleaned in an ultrasonic acetone bath and then were carefully assembled. The schematic diagram of the assembly sample is shown in Fig. 2a. After the furnace reaching a vacuum of 10−2 Pa, the brazing couple was first heated to 600 °C at a rate of 20 °C/min, then to the peak temperature (740 °C–940 °C) at a rate of 10 °C/min. After held for 5 miñ15 min, the joints were cooled to 400 °C at a rate of 5 °C/min. Finally, it was cooled to room temperature in the furnace. A scanning
Fig. 3. (a) Interfacial microstructure of the joint brazed at 740 °C for 10 min (b) Enlarged area at the Invar side. 2
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Table 2 Composition of the characteristic phase in Fig. 3 (at. %).
Table 3 Composition of the characteristic phase in Fig. 4 (at.%).
Point
Ag
Cu
Zn
Ni
Fe
Phase
Point
Ag
Cu
Zn
Ni
Fe
Ti
C
Possible phases
A B C D E
2.1 5 75.5 2.8 10.7
74.3 85.9 17.2 68.2 5.4
7.9 6.2 5.4 13.9 1
13.1 1.9 1.1 10.7 30
2.6 1 0.8 4.4 52.9
(Cu, Ni) (Cu) (Ag) (Cu, Ni) (Fe, Ni)
A B C D E F G
0.4 0.6 79.7 – 1.5 0.4 0.5
22.6 53.9 8.9 47.3 70.3 27.3 2.6
– 0.5 4 1.2 – 0.5 –
19.4 34.1 2.9 42.9 23.5 54.7 38.6
3.3 6.7 3 7.7 4.7 17.1 58.3
14 4.2 1.5 0.9 – – –
40.3 – – – – – –
TiC+(Cu, Ni) (Cu, Ni) (Ag) (Cu, Ni) (Cu, Ni) (Cu, Ni)+(Fe, Ni) (Fe, Ni)
atoms can leave the liquid because of the small liquid-gas interface in the brazing configuration. As a result, a lot of vapor bubbles form and remain in the brazing seam. On the other side, the solidus temperature of Ag-Cu-Zn alloy rises during the evaporation process, determined from the Ag-Cu-Zn ternary phase diagram [29]. As a result, some liquid solidifies during the holding process. The vapor bubble could not escape from the solidified alloy, so pores form in the brazing seam. As the temperature increases, the Zn evaporation becomes stronger, which causes the decrease of Zn concentration. Finally, Zn in the brazing alloy evaporates completely [27]. The characteristic interfacial microstructure of the joint was investigated at 910 °C for 10 min, as shown in Fig. 4. The whole interface consists of four different characteristic regions: the penetration region in the TiC-Ni cermet (Region I), the reactive region adjacent to the cermet (Region II), the brazing seam (Region III), and the reactive layer at the Invar side (Region IV). Table 3 gives the composition of each characteristic phase in Fig. 4. Little Zn was detected in the residual brazing alloy and the reactive phases, which shows that the Zn element has evaporated completely during the brazing process. Both Region I and Region II are composed of (Ag), (Cu, Ni) and TiC particle. Region I in TiC-Ni cermet is formed by penetration of the liquid into the cermet, which was also observed in the joint brazed with Ag-31Cu-23Zn at 950 °C for 15 min [7], but not found in the joint brazed with Ag-54Cu-33Zn up to 950 °C for 15 min [8]. Such phenomenon should be attributed to the better flowability of the near eutectic liquid. The (Cu, Ni) layer in Region II changes to discontinuous. Also, some liquid alloy flows into the grain boundary, which results into the detachment of some (Cu, Ni). Besides, due to the serious dissolution process, many TiC particles float into Region II. More noticeably, the TiC particle gradually decreases from 60 wt% to 0 in the reactive region, leading to the gradient distribution of TiC particles in the (Ag) and (Cu, Ni) matrix in Region II, as seen in the FGM layer marked in Fig. 4a. The microstructure of the brazing seam (Region III) is composed of (Ag) and stripe-like (Cu, Ni), which departs from the eutectic pattern. Region IV is composed of two kinds of reactive layer, as shown in Fig. 4b. The reactive layer at point E is rich in Cu and Ni, which is supposed to be (Cu, Ni). The reactive layer adjacent to the Invar alloy at point F consists of Fe, Cu and Ni, which should be the mixture of (Cu, Ni) and (Fe, Ni), deduced from the Cu-Fe-Ni ternary phase diagram [30]. Point G has a similar composition with the Invar alloy. Fig. 5 presents the XRD results of the products of the joint brazed at 910 °C for 10 min at the cermet side (above) and at the Invar side
Fig. 5. XRD results of the reactive layer.
(below), respectively. The XRD results determine that the products are (Ag), TiC and (Cu, Ni) at the cermet side, and are (Cu, Ni) and mixture of (Fe, Ni) and (Cu, Ni) at the Invar side. The existence of Fe in the mixture of (Cu, Ni) and (Fe, Ni) causes the characteristic peaks to slightly move to the right. Fig. 6 displays the interface of the joint brazed at temperature ranging from 820 °C to 940 °C for 10 min. As the temperature is 820 °C, the liquid alloy penetrates into the cermet. The layered (Cu, Ni) changes to discontinuous. The brazing seam consists of (Ag) and (Cu, Ni). As the temperature increases, the thickness of the penetration region (Region I) gradually increases from 6.2 μm at 820 °C to 31.1 μm at 940 °C, and the amount of (Ag) in the TiC-Ni cermet increases. Moreover, TiC particles presents gradient distribution in the reactive region (Region II) of the joint brazed between 850 °C and 940 °C; and the thickness of Region II increases with the temperature. Meanwhile, the amount of (Cu, Ni) detaching from the cermet and entering the brazing seam increases. Notably, most of the (Cu, Ni) goes into the brazing seam at 940 °C. Similar phenomenon was observed in the TiC-Ni cermet/Ag31Cu-23Zn/steel brazed at 950 °C for 15 min [7]. The interface change is closely related to the Zn evaporation. The forming of large amount of (Cu, Ni) induced by Zn evaporation [27] consumes a lot of Cu element, which causes a copper-poor and silverrich brazing alloy near the cermet and forms (Ag) in the (Cu, Ni) layer, as shown in Fig. 3a. The (Ag) re-melts and forms a lot of micro-channel along the grain boundary for liquid flow as the temperature rises. Additionally, it was found that the dissolution along the grain boundary had a faster rate than that of the bulk phase in the Ag-Cu/Ni couple [31], so the liquid alloy prefers to penetrate along the grain boundary
Fig. 4. (a) Interfacial microstructure of the joint brazed at 910 °C for 10 min (b) Enlarged area in (a). 3
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Fig. 6. Interface of the joint brazed at brazed at different temperature for 10 min (a) 820 °C (b) 850 °C (c) 880 °C (d) 940 °C.
possesses the highest strength. Fig. 9 presents the fracture surface of joint at the cermet side brazed at 940 °C for 10 min. According to the morphology, the fracture surface is divided into two characteristic regions: Region i and Region ii. The results of the element composition in each region are shown in Table 4. The results show that Region i consists of (Ag), (Cu, Ni) and some granular TiC particles, which indicates that it is the penetration region in the joint. Similarly, Region ii consists of (Ag), (Cu, Ni) and some flat TiC particles, which shows that it is adjacent to the reactive region of the joint. The fracture morphology of joint brazed at 740 °C for 10 min is presented in Fig. 10. It consists of two regions. Similar as Region ii in Fig. 9, Region a in Fig. 10 is composed of (Ag), (Cu, Ni) and the flat TiC particle, which shows that it is adjacent to the reactive region of the joint. Region b consists of (Ag) and (Cu), which shows that it is the brazing seam. Besides, some pores present in the brazing seam, which is also observed in Fig. 3. The pores are potential crack source under the shear test process, which are detrimental to the strength of the joint. Therefore, the joint obtained at 740 °C for 10 min has the minimum shear strength. At a higher brazing temperature, the pores caused by Zn evaporation are not observed in the brazing alloy. And the adhesion between the TiC and the brazing alloy becomes the weak part of the joint. The adhesive strength between the TiC and the alloy can be evaluated by the work of adhesion Wad, which is determined by the Young-Dupre equation:
and dissolve the bulk phase adjacent to the grain boundary. As a result, the reactive layer transforms from continuous to discontinuous, finally detaching from substrate. Besides, temperature increase promotes liquid penetration along the boundary between the TiC particle and metal phase, which increases the (Ag) in the original cermet as the temperature is increased. As the temperature continues increasing, the solubility of Ni in the Ag-Cu alloy gradually rises [31]. So some (Cu, Ni) in the reactive region dissolves into the brazing alloy, which results in the increase of (Cu, Ni) in the brazing seam with temperature. 3.2. Mechanical evaluation of the brazing joint The shear strength of joint brazed at different temperature for 10 min is exhibited in Fig. 7. The strength firstly increases and then decreases with temperature. And the maximum value of 161 MPa is obtained at 910 °C for 10 min. The strength maintains at a high level between 820 °C and 910 °C. However, when the temperature increases up to 940 °C, the strength is rapidly decreased. Fig. 8 shows the fracture surface of the joint brazed at 910 °C for 10 min at the cermet side. The macroscopic fracture surface is quite flat, which means that there is no deflection during the crack propagation process. And the microscopic fracture surface consists of tiny quasicleavage planes and tearing ridges. Additionally, there is linear plastic deformation on the microscopic fracture surface, indicative of good ductility and toughness. Obviously, it is beneficial to obtain the joint with high strength. The EDS result shows that the fracture surface consists of (Ag), which indicates that it is the brazing seam. Because of the good ductility and toughness of (Ag), the joint fractured on (Ag)
Wad = σlv(1+cosθ) Where, σlv is the surface tension of the liquid alloy and θ is the equilibrium contact angle of the liquid alloy on the TiC ceramic. The equilibrium contact angle of Ag, Cu and Ni liquid on TiC cermet is 120° [32], 88° [32] and 28° [33], respectively. It indicates that the wettability of TiC ceramic by the liquid Ag is much worse than Cu or Ni. And the surface tension of Ag, Cu and Ni is 827 mN/m [34], 1186 mN/ m [34], and 1768 mN/m [33], respectively. According to the YoungDupre equation, the work of adhesion of different liquid alloys on the TiC ceramic is calculated to be 414 mJ/m2, 1227 mJ/m2, and 3329 mJ/ m2, which manifests that the work of adhesion between TiC ceramic and Ag is much smaller than that between TiC ceramic and Cu or Ni. In other words, it demonstrates that (Cu, Ni) at the interface can greatly improve the adhesive strength between the cermet substrate and the
Fig. 7. Shear strength of the joint brazed at different temperature for 10 min. 4
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Fig. 8. (a) Fracture surface of the joint at the cermet side brazed at 910 °C for 10 min (b) Enlarged views of the fracture surface.
Fig. 9. (a) Fracture surface of the joint at the cermet side brazed at 940 °C for 10 min (b) and (c) Enlarged views of the fracture surface at Region i and ii in (a).
1. Dissolution of the Ni matrix in the cermet is much higher than that of the Invar alloy in the joint brazed at 740 °C for 10 min. It causes the formation of continuous (Cu, Ni) layer at both the cermet and Invar alloy sides. In addition, a lot of pores form in the brazing seam because of the incomplete evaporation of Zn into the vacuum circumstance. 2. The interfacial microstructure of the joint brazed at 910 °C for 10 min is TiC-Ni cermet/(Ag)+(Cu, Ni)+TiC/(Ag)+(Cu, Ni)/(Cu, Ni)/(Cu, Ni)+(Fe, Ni)/Invar alloy. The formation of large amount of (Cu, Ni) induced by the Zn evaporation causes a copper-poor and silver-rich brazing alloy near the cermet and forms (Ag) in the (Cu, Ni) layer. No pores are observed in the joint after complete Zn evaporation. 3. When the temperature rises from 740 °C to 820 °C, (Cu, Ni) dissolves in the brazing alloy and become discontinuous. When the temperature further increases to 940 °C, the amount of (Ag) in the cermet increases and the thickness of the penetration region gradually increases from 6.2 μm at 820 °C to 31.1 μm at 940 °C. Moreover, TiC particles presents gradient distribution in the reactive region (Region II) of the joint brazed between 850 °C and 940 °C; and the thickness of reactive layer adjacent to the cermet increases with temperature. Meanwhile, the amount of (Cu, Ni) detaching from the cermet and entering the brazing seam increases. 4. The shear strength firstly increases and then decreases with temperature. The maximum shear strength of 161 MPa is obtained at 910 °C for 10min and the joint is fractured on (Ag). The shear strength maintains at a high level in a wide range of brazing
Table 4 Element composition at each point in Fig. 9 (at. %). Point
Ag
Cu
Zn
Ni
Fe
Ti
Phases
A B C
41.06 2.14 20.44
34.33 5.11 38.51
2.05 1.17 0.84
15.96 5.02 29.45
3.51 1.57 5.69
3.09 85.00 5.06
(Ag)+(Cu, Ni) TiC (Ag)+(Cu, Ni)
Ag-Cu-Zn alloy, and (Ag) in the penetration region and the reactive region decreases the adhesive strength. On the other hand, it was proposed that the thickness of the gradient layer close to the substrate is the most important for releasing the residual stress in the joint; and the effect is the best when it is the thickest [35]. Seen from Figs. 4 and 6, the thickness of the functional gradient material layer (marked as FGM) increases as the temperature increases, which is beneficial for the release of the residual stress. So the shear strength of the joint firstly increases and then decreases under both effects of (Cu, Ni) and FGM layer in the joint as the temperature increases from 820 °C to 940 °C.
4. Conclusion TiC-Ni cermet and Invar alloy was brazed with the Ag-Cu-Zn filler metal in vacuum. The effects of heating parameters on the interfacial microstructure and mechanical properties were studied. And the following conclusions can be obtained. 5
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Fig. 10. (a) Fracture surface of the joint at the cermet side brazed at 740 °C for 10 min (b) and (c) Enlarged views of the fracture surface at Region a and b in (a).
parameter. 5. The low shear strength at 740 °C for 10 min is attributed to the pores in the brazing alloy. And the change trend between 820 °C and 940 °C is attributed to the increase of the FGM layer in the reactive region and the decrease of the amount of (Cu, Ni) in the penetration region and the reactive region with the temperature.
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