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Effect of CuCe alloy addition on the microstructure and mechanical performance of brazed diamonds with NiCr alloy
International Journal of Refractory Metals & Hard Materials 80 (2019) 253–258
Contents lists available at ScienceDirect
International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effect of CueCe alloy addition on the microstructure and mechanical performance of brazed diamonds with NieCr alloy Duanzhi Duana,b, a b
T
⁎,1
, Lin Suna,1, Qijing Lina, Xudong Fanga, Changsheng Lia, Zhuangde Jianga
State Key Laboratory of Mechanical Manufacturing Systems Engineering, Xi'an Jiaotong University, Xi'an 710049, China School of Mechatronics Engineering, Nanchang University, Nanchang 330031, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Brazed diamonds Ni–Cr alloy Cu–Ce alloy Microstructure Mechanical performance
This study was aimed to reduce unwanted thermal damage to diamond grains during brazing. To achieve this goal, the diamonds were brazed using NieCr alloy with the addition of CueCe alloy (i.e. NieCr composite solder). Surface morphology and interfacial microstructure of the brazed samples using NieCr composite solder was characterized. The thermally induced residual stresses of brazed diamond grains were examined by Raman spectroscopy. The static pressure strength and impact toughness of the brazed diamond grains were examined. Constitutional phases of NieCr composite solder after brazing were detected. The results show that the thermal residual stresses values of the brazed diamond grains using NieCr composite solder, in which 2 wt% and 5 wt% CueCe alloy were used respectively, were decreased by 6.4% and 9.7% respectively, meanwhile, the static pressure strength values increased by 4.9% and 13.4% respectively as well as impact toughness values increased by 4.0% and 9.2% respectively, compared with that of the brazed diamond grains using NieCr alloy. Chromium carbides (Cr3C2 and Cr7C3) were obtained at the bonding interface between diamonds and composite solder. The constitutional phases containing Ce2Ni7, which could be beneficial to reduce the thermal damage, were formed in the solder alloy after brazing.
1. Introduction Given its extreme hardness, abrasive resistance, and high chemical stability, diamond has become very popular in fabricating diamond tools (e.g. blade saw, grinding wheel, and drill bit) to process the hardfragile materials (e.g. natural stone, ceramics, and concrete) [1]. However, due to their chemical inertness, diamond grains are very hard to wet by most metallic elements (e.g. copper, iron, and nickel). Hence, the diamond grains are hard to be well bonded to the metallic matrix by sintering and electroplating, which could appear limitations when attempting to acquire sufficient bond strength. As for monolayer brazed diamond tools, diamond grains can be firmly held onto a steel substrate by active brazing technology. Because of the presence of strong chemical combination between diamonds and solder alloy, brazed diamond tools exhibit many advantages such as high protrusion of abrasives and large chip-storage space [2–6]. Recently, many studies [7–12] reported the progress of brazed diamond tools using active solders based on AgeCu, CueSn and NieCr alloys. The addition of carbide forming elements (e.g. Ti, Cr, or V) into the solder alloys must be helpful to enhance the interfacial bonding
strength. NieCr alloy have become a popular choice of fabricating brazed diamond tools because of strong mechanical strength and high abrasive resistance. But NieCr alloy could cause thermal damage to the diamonds because the element Ni is a catalyst that can promote the transformation of diamond to graphite under normal pressure and at high temperature (above 800 °C). The brazing is generally carried out by using NieCr alloy at the brazing temperature of 1050 °C for the dwell time of 10 min [13]. Hence, it would weaken the mechanical performance of diamonds and deteriorate the machining performance of diamond tools [13–15]. Up to now there are very few researches on reducing the thermal damage of the diamonds during high temperature brazing. To explore methods of solving the problem mentioned above, diamonds were directly brazed onto a substrate using NieCr composite solder (i.e. addition of CueCe alloy into NieCr alloy) in this study. In the previous studies, the surface-metallized diamond grains were fabricated using NieCr composite solder to only achieve higher mechanical strength compared with the ones fabricated using NieCr alloy and its principle was not investigated [16]. In this study, three kinds of brazed samples were prepared using NieCr alloy and NieCr composite
⁎
Corresponding author. E-mail address: [email protected] (D. Duan). 1 These authors contributed equally to this work and should be considered as co-first authors. https://doi.org/10.1016/j.ijrmhm.2019.01.022 Received 2 December 2018; Received in revised form 20 January 2019; Accepted 29 January 2019 Available online 30 January 2019 0263-4368/ © 2019 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 80 (2019) 253–258
D. Duan et al.
Table 1 Compositions of three kinds of solder alloys (wt%). Type
Ni–Cr alloy
Cu–Ce alloy
Ni–Cr alloy NO.1 NieCr composite solder NO.2 NieCr composite solder
100 98 95
0 2 5
solder, respectively. Subsequently, surface morphology and interface microstructure of the brazed samples were studied. Then the residual stress of the brazed samples was analyzed. The static pressure strength and impact toughness of the above three kinds of brazed diamonds were measured at last.
2. Material and methods The experimental materials adopted in this research included Ni–Cr–B–Si alloy powder, Cu–20wt.%Ce powder, diamonds as well as steel substrates. The CueCe powder alloy was sieved with a 200-mesh screen. The size of diamonds (Huanghe Whirl-wind Co. Ltd., China) was ranged from 420 μm to 500 μm. The compositions of two kinds of NieCr composite solders adopted in this study are displayed in Table 1. Firstly, diamond grains were ultrasonically cleaned in ethanol and steel substrates were polished mechanically. Secondly, the solder alloy was uniformly spread on the steel substrate with a thickness of approximately 300 μm. The diamonds were subsequently arranged onto the solder alloy. Then the samples were fixed into a furnace and a heating treatment was performed at the brazing temperature of 1050 °C for the dwell time of 10 min with a vacuum degree below 2 × 10 −2 Pa. Finally, brazed samples were obtained after brazing and cooling process. The brazed samples were marked differently according to the different kinds of solder alloys adopted. For example, the brazed samples using NieCr alloy were marked as conventional brazed samples. The brazed samples using No.1 NieCr composite solder were marked as No.1 brazed samples. A Hitachi scanning electron microscope (SEM, SU3500) accompanied with energy dispersive spectrometry (EDS) and A Zeiss SEM (Gemini 500) were used to observe the morphology and interface microstructure of the brazed samples. The interfacial resultant around the brazed diamond surface after etching and the chemical composition of NieCr composite solder after brazing were detected by X-ray diffraction analysis (Bruker, D8 Advance X-ray diffractometer) respectively. Additionally, the brazed diamonds were studied by Raman spectroscopy (HORIBA LabRAM HR Evolution) in the range from 600 to 1700 cm−1. The vibrational spectrum feature of materials can be reflected by the Raman scattering peak. When a directed laser-beam is focused upon a certain point onto the brazed diamond grain surface under compressive stress or tensile stress, the Raman spectral peak position changes over to a higher or a lower wave-number, respectively, with reference to the peak wave-number of Raman spectra of a diamond under stress-free. Therefore, the Raman peak position can reflect the stress state of the brazed diamonds [17]. The three kinds of brazed specimens were strongly corroded to separate the brazed diamonds. For example, the brazed diamonds obtained from conventional brazed samples after corroding were marked as conventional brazed diamonds. The brazed diamonds obtained from No.2 brazed samples after corroding were marked as No.2 brazed diamonds, as displayed in Fig. 1. A static strength measuring instrument (DLY-92) was used to measure the static pressure strength of the three kinds of brazed diamonds. An impact toughness measuring instrument (CMC-II) for super-hard abrasives was used to measure the toughness of the three kinds of brazed diamonds by ball mill method. The measurement methods of the static pressure strength and impact toughness of the three kinds brazed diamonds have been described in the literature [16].
Fig. 1. Micrograph of No.2 brazed diamonds after corroding.
Fig. 2. Micrograph of the brazed diamond grains using NieCr alloy.
Fig. 3. Micrograph of the brazed diamond grains using NieCr alloy.
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elements Cu, Ni, Cr, Si and C changed respectively, indicating a slow transition tendency at the bonding interface, as shown in Fig. 5(b). An enrichment of carbide forming element Cr migrating toward the diamond region implies the segregation of Cr from NieCr composite solder [18], which suggests that it is possible to form carbides. One side of the No.2 brazed sample was polished and then etched with a mixed solution containing blue copperas, hydrochloric acid and sulphuric acid. Fig. 6(a) and (b) illustrate an SEM micrograph of interfacial microstructure between solder alloy and steel substrate and the interfacial main elements line scanning by EDS, respectively. As seen in Fig. 6(a), a bonding joint, of which the thickness was about 5–10 μm, was formed at the interface. As shown in Fig. 6(b), the curves connecting the elements Fe, Ni, Cr and Si changed respectively at the interface, suggesting a formation layer may containing Fe3Si and Ni3Si intermetallic compounds and (FeNi) solid-solution. It indicated that the addition of CueCe alloy cannot prevent the metallurgical bonding between solder alloy and steel substrate. The No.2 brazed sample was put into chloroazotic acid to completely remove the solder alloy and steel substrate. Fig. 7 shows an SEM micrograph of No.2 brazed diamonds after etching. As seen in Fig. 7, the diamond surface was entirely covered with resultants. Some resultants were prismatic. Fig. 8 displays an XRD pattern of No.2 brazed diamonds. It can be seen the resultants on the diamond surface mainly contained Cr3C2 and Cr7C3, confirming the chemical combination developed between diamonds and NieCr composite solder [19,20]. It indicated that the addition of CueCe alloy in the present study had no effect on the chemical reaction between Cr and diamonds. Therefore, NieCr composite solder exhibited a good surface wettability to diamonds, as shown in Fig. 4.
Fig. 4. Micrograph of the brazed diamond grains using NieCr composite solder.
3. Experimental results and discussion 3.1. Effects of CueCe alloy addition on Morphology Figs. 2 and 3 illustrate the SEM micrographs of the brazed samples fabricated using NieCr alloy (i.e. conventional brazed samples). As seen from Fig. 2, there were some big etch pits on the brazed diamond surfaces. As seen from Fig. 3, there were a few microcracks around the diamonds. Fig. 4 illustrates an SEM micrograph of the brazed diamond samples using No.2 NieCr composite solder. As seen from Fig. 4, the NieCr composite solder climbed up along the diamond surface, suggesting the NieCr composite solder exhibiting a good surface wettability to diamonds. At the same time, there were few etch pits on the brazed diamond surfaces and microcracks adjacent to the diamonds. Obviously, the surface morphology of the brazed samples using NieCr composite solder was better.
3.3. Effect of CueCe alloy on residual stresses Different coefficients of thermal expansion of diamond grains, solder alloy, and steel substrate would cause residual stresses adjacent to the diamonds during brazing [21,22]. The high residual stress might cause the microcracks around the diamond or originating from the diamond surface. The thermally induced residual stresses in diamond grains can be measured by Raman spectroscopy. The residual stresses in the sample are expressed by a wave-number shift of the Raman spectral peak compared with that of the Raman spectral peak of the sample under stress-free [17]. The residual stresses were measured along path I shown in Fig. 9 with an interval of 25 μm from point O (i.e. at the center on the top surface of diamond grain) during the experiment. The peak wave-
3.2. Interfacial microstructure characterization An SEM interface micrograph of the No.2 brazed diamond is displayed in Fig. 5(a), while the interfacial main elements line scanning by EDS is displayed in the Fig. 5(b). As seen in Fig. 5(a), a well-bonding joint was demonstrated at the interface. And the curves connecting the
Fig. 5. Interfacial microstructure of No.2 brazed diamond. (a) Interface micrograph and (b) interfacial main elements line scanning. 255
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Fig. 6. Interfacial microstructure of No.2 brazed sample between solder alloy and steel substrate. (a) Interface micrograph and (b) interfacial main elements line scanning.
Fig. 9. Measurement schematic of residual stresses by Raman spectroscopy.
Fig. 7. Micrograph of the brazed diamonds using NieCr composite solder after etching.
Fig. 10. Raman spectrum of brazed diamonds using NieCr composite solder.
number of the Raman spectrum of original diamonds without residual stresses was 1332.22 cm−1 in the experiment. Fig. 10 displays the Raman spectrum of No.1 brazed diamond at point O. As shown in Fig. 10, the peak wave-number of the Raman spectrum was 1331.56 cm−1, implying that the brazed diamond enduring residual stress. The peak wave-number of the Raman spectrum of No.2 brazed
Fig. 8. An XRD pattern of the brazed diamonds after etching.
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Fig. 12. Static pressure strength and impact toughness values of three kinds of brazed diamonds. Fig. 11. Residual stress of three kinds of brazing diamonds.
pressure strength values of No. 1 and No. 2 brazed diamonds were increased by 4.9% and 13.4% respectively, as well as the impact toughness values of No. 1 and No. 2 brazed diamonds increased by 4% and 9.2% respectively, compared with that of the brazed diamonds using NieCr alloy (i.e. conventional brazed diamonds). The decrease in the mechanical strength of the brazed diamond grains was attributed to the thermal damage of diamond grains. It is noted that the increasing proportions of static pressure strength and impact toughness were both higher than that of weight proportion of CueCe alloy, suggesting the addition of Ce could reduce the thermal damage, which was consistent with the results reported in Section 3.3.
diamond at point O was 1331.66 cm−1. Because of the shape of regular cubo-octahedron or icosahedron of diamonds and the geometry of the specimen in the present study (Fig. 9), allowing unrestrained thermal expansion (or contraction) of diamonds in the Z direction, an equi-biaxial (EB) stress state can be assumed. According to the literature [23], the equi-biaxial thermal residual stress σ can be calculated as follows:
σ = K·(ω − ω0)
(1)
Where ω is the Raman spectral peak position of the brazed diamonds, ω0 is the Raman spectral peak position of original diamonds under stress-free, and the proportionality factor K is −0.43 GPa/cm−1 in the present study. Fig. 11 summarizes the comparison of three kinds of measured residual stresses in this study. The residual stress values were the average results of three measurements at the same position for a kind of brazed diamond. As seen from Fig. 11, obviously within the distance from approximately 25 μm close to the top surface, tensile stresses were detected, and afterward it became compressive stresses. At the same time the compressive stress values all increased with increasing depth. It is noted that the residual compressive stress values of the brazed diamond grains using NieCr composite solder were lower than that of the brazed diamond grains using NieCr alloy at the same measuring distance. When the depth reached 200 μm, the residual compressive stress values of the brazed diamonds using No.1 and No.2 NieCr composite solder were −1.16 GPa and −1.12 GPa respectively, which were decreased about 6.4% and 9.7% respectively compared with that of the brazed diamonds using NieCr alloy. From the above Raman analysis, it indicated that the addition of CueCe alloy could reduce the residual stresses to the diamonds during high-temperature brazing.
3.5. Constitutional phases and discussion No.2 NieCr composite solder powders were filled in the graphite mould and put into the furnace to fabricate an alloy block. The heat treatment parameters were the same as that reported in Section 2. Fig. 13 shows an XRD pattern of the alloy block. As shown in Fig. 13, the dominating phases were CeNi2, Ce2Ni7, Ni3Si, Ni4B3, CrB, and NieCr phase. The Gibbs free energy for Equations 2Ce+7Ni → Ce2Ni7 at the brazing temperature(1050 °C, 1323.15 K) is given by −30.86 kJ/ mol [24,25]. The values are negative, indicating that the reaction could happen spontaneously during the heat-treatment process. The melting point of Ce2Ni7 is 1060 ± 5 °C [25,26], which is much higher than the heat-treatment temperature (1050 °C). Therefore, the phase Ce2Ni7 was
3.4. The mechanical strength of the diamonds To study the influence of the CueCe alloy addition on the mechanical strength of diamonds, the static pressure strength and impact toughness of the brazed diamonds after etching are investigated. Fig. 12 displays the static pressure strength and impact toughness values of three kinds of brazed diamond grains after etching. Apparently, the CueCe alloy had a positive influence on the static pressure strength and impact toughness of the diamond grains. The more the content of CueCe alloy addition into the NieCr composite solder, the higher the static pressure strength and impact toughness of the brazed diamond grains. The weight proportion of CueCe alloy in No.1 and No.2 NieCr composite solder were 2% and 5% respectively. Meanwhile, the static
Fig. 13. An XRD pattern of the alloy block fabricated using No.2 NieCr composite solder. 257
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Acknowledgment The authors gratefully acknowledge the financial support of this research by the National Natural Science Foundation of China (No. 51720105016), Postdoctoral Innovative Talents Support Program of China (No. BX20180250), Major National Science and Technology Project (2017ZX04021001-005-001), Natural Science Foundation of Jiangxi province (No.20171BAB216032). References [1] J.V. Naidich, G.A. Kolesnichenko, Surface Phenomena in Metallurgical Processes, Consultants Bureau, New York, 1965. [2] A.K. Chattopadhyay, L. Chollet, H.E. Hintermann, On performance of brazed monolayer diamond grinding wheel, CIRP Ann. Manu. Tech. 40 (1991) 347–350. [3] H.E. Hintermann, A.K. Chattopadhyay, New generation superabrasive tool with monolayer configuration, Diam. Relat. Mater. (12) (1992) 1131–1143. [4] C.M. Sung, Brazed diamond grid: a revolutionary design for diamond saws, Diam. Relat. Mater. 8 (1997) 1540–1543. [5] B. Xiao, H.J. Xu, Z.B. Wu, Investigation on brazing of single-layer superabrasive wheel, Key Eng. Mater. 202 (2001) 155–158. [6] J.C. Sung, M. Sung, The brazing of diamond, Int. J. Refract. Met. Hard Mater. 27 (2009) 382–393. [7] P. Mukhopadhyay, A. Ghosh, On bond wear, grit-alloy interfacial chemistry and joint strength of synthetic diamond brazed with Ni–Cr–B–Si–Fe and Ti activated AgCu filler alloys, Int. J. Refract. Met. Hard Mater. 72 (2018) 236–243. [8] B.J. Sun, B. Xiao, Effects of process parameters on interfacial microstructure, residual stresses, and properties of tunnel furnace brazed diamonds, Diam. Relat. Mater. 85 (2018) 98–103. [9] J.C. Chen, D. Mu, X.J. Liao, G.Q. Huang, H. Huang, X.P. Xu, H. Huang, Interfacial microstructure and mechanical properties of synthetic diamond brazed by Ni–Cr–P filler alloy, Int. J. Refract. Met. Hard Mater. 74 (2018) 52–60. [10] S.X. Liu, B. Xiao, Z.Y. Zhang, D.Z. Duan, Microstructural characterization of diamond/CBN grains steel braze joint interface using Cu–Sn–Ti active filler alloy, Int. J. Refract. Met. Hard Mater. 54 (2016) 54–59. [11] D.Z. Duan, B. Xiao, B. Wang, P. Han, W.J. Li, S.W. Xia, Microstructure and mechanical properties of pre-brazed diamond abrasive grains using Cu–Sn–Ti alloy, Int. J. Refract. Met. Hard Mater. 48 (2015) 427–432. [12] C. Artini, M.L. Muolo, A. Passerone, Diamond-metal interfaces in cutting tools: a review, J. Mater. Sci. 47 (2012) 3252–3264. [13] Y. Chen, Y.C. Fu, H.H. Su, J.H. Xu, H.J. Xu, The effects of solder alloys on the morphologies and mechanical properties of brazed diamond grains, Int. J. Refract. Met. Hard Mater. 42 (2014) 23–29. [14] P. Mukhopadhyay, D.R. Simhan, A. Ghosh, Challenges in brazing large synthetic diamond grain by Ni-based filler alloy, J. Mater. Process. Technol. 250 (2017) 390–400. [15] F.L. Meng, A.G. Liu, H.H. Sun, M.H. Guo, Finite element analysis of thermal effects on brazed cold diamond grain, Trans. Indian Inst. Metals 68 (2015) 829–838. [16] D.Z. Duan, B. Xiao, W. Wang, Z.Y. Zhang, B. Wang, P. Han, X.Y. Ding, Interface characteristics and performance of pre-brazed diamond grains with Ni–Cr composite alloy, J. Alloys Compd. 644 (2015) 626–631. [17] S. Buhl, C. Leinenbach, R. Spolenak, K. Wegener, Influence of the brazing parameters on microstructure, residual stresses and shear strength of diamond-metal joints, J. Mater. Sci. 45 (2010) 4358–4368. [18] B. Xiao, H.J. Xu, Y.C. Fu, J.H. Xu, Form and distribution characterization of reaction products at the brazing interface between Ni–Cr alloy and diamond, Key Eng. Mater. 259 (2004) 151–153. [19] C.Y. Wang, Y.M. Zhou, F.L. Zhang, Z.C. Xu, Interfacial microstructure and performance of brazed diamond grains with Ni–Cr–P alloy, J. Alloys Compd. 476 (2009) 884–888. [20] A. Trenker, H. Seidemann, High-vacuum brazing of diamond tools, Ind. Diam. Rev. 62 (2002) 49–51. [21] U.E. Klotz, C. Liu, F.A. Khalid, H.-R. Elsener, Influence of brazing parameters and alloy composition on interface morphology of brazed diamond, Mater. Sci. Eng. A 495 (2008) 265–270. [22] M. Akbari, S. Buhl, C. Leinenbach, R. Spolenak, K. Wegener, Thermomechanical analysis of residual stresses in brazed diamond metal joints using Raman spectroscopy and finite element simulation, Mech. Mater. 52 (2012) 69–77. [23] S. Buhl, C. Leinenbach, R. Spolenak, K. Wegener, Microstructure, residual stresses and shear strength of diamond-steel-joints brazed with a Cu–Sn-based active filler alloy, Int. J. Refract. Met. Hard Mater. 30 (2012) 16–24. [24] I. Barin, O. Knacke, Thermochemical Properties of Inorganic Substances, Springer, Berlin, 1973. [25] M. Palumbo, G. Borzone, S. Delsante, N. Parodi, G. Cacciamani, R. Ferro, L. Battezzati, M. Baricco, Thermodynamic analysis and assessment of the Ce–Ni system, Intermetallics 12 (2004) 1367–1372. [26] W. Xiong, Y. Du, X.G. Lu, J.C. Schuster, H.L. Chen, Reassessment of the Ce–Ni binary system supported by key experiments and ab initio calculations, Intermetallics 15 (2007) 1401–1408. [27] H.Y. Zhou, C.Y. Tang, M.M. Tong, Z.F. Gu, Q.R. Yao, G.F. Rao, Experimental investigation of the Ce–Cu phase diagram, J. Alloys Compd. 511 (2012) 262–267. [28] C.M. Sung, M.F. Tai, Reactivities of transition metals with carbon: Implications to the mechanism of diamond synthesis under high pressure, Int. J. Refract. Met. Hard Mater. 15 (1997) 237–256.
Fig. 14. The CueCe phase diagram [27].
stable at the brazing temperature. The CueCe phase diagram [27] was displayed in Fig. 14. As seen from Fig. 14, the brazing temperature (1050 °C, 1323.15 K) was higher than the liquidus temperature of the CueCe alloy used in the present study. Therefore, there was no CuxCe (x = 6,5,4,2 or 1) phase formation at the brazing temperature. At the same time the NieCr liquid and the CueCe liquid could be syncretized with each other and lead to the formation of Ce2Ni7. The thermal damage results from the interfacial thermal residual stress caused by the large differences in the material properties among the three materials (i.e. diamonds, steel substrate, and solder alloys) and the erosion of diamonds mainly caused by Ni, which can resolve the carbon element and promote chemical reaction between NieCr alloy and carbon [15,28]. As shown in Fig. 13, Ni also induced the formation of hard phases (e.g. Ni3Si and Ni4B3) with B and Si. It was probable that the formation of Ce2Ni7 caused a decrease of liquid Ni content in the solder alloy at the brazing temperature and thereby inhibited the hard phases growth. So the residual stress existing close to the interface between diamonds and solder alloy was reduced. The formation of Ce2Ni7 also could weaken the effect of Ni on the diamonds and thereby significantly reduce the erosion of the brazed diamonds. Therefore, the mechanical performance of the brazed diamond grains was improved. 4. Conclusions (1) The surface morphology of the brazed samples using NieCr composite solder, in which 2 wt% or 5 wt% CueCe alloy addition was adopted, was better than that of the brazed samples using NieCr alloy. Chromium carbides (Cr3C2 and Cr7C3) was obtained, confirming the chemical combination developed between diamonds and NieCr composite solder. (2) The residual stress values of the brazed diamonds using NieCr composite solder were lower than that of the brazed diamond grains using NieCr alloy at the same measuring distance. The static pressure strength values of the brazed diamonds using NieCr composite solder, in which 2 wt% and 5 wt% CueCe alloy were used respectively, were decreased by 4.9% and 13.4% respectively as well as impact toughness values increased by 4% and 9.2% respectively, compared with that of the brazed diamond grains using NieCr alloy. (3) The constitutional phases containing CeNi2, Ce2Ni7, Ni3Si, Ni4B3, and CrB were formed in the solder alloy after brazing. The formation of Ce2Ni7 could be beneficial to reduce the thermal damage and improve the mechanical performance of the diamonds.
258
Contents lists available at ScienceDirect
International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effect of CueCe alloy addition on the microstructure and mechanical performance of brazed diamonds with NieCr alloy Duanzhi Duana,b, a b
T
⁎,1
, Lin Suna,1, Qijing Lina, Xudong Fanga, Changsheng Lia, Zhuangde Jianga
State Key Laboratory of Mechanical Manufacturing Systems Engineering, Xi'an Jiaotong University, Xi'an 710049, China School of Mechatronics Engineering, Nanchang University, Nanchang 330031, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Brazed diamonds Ni–Cr alloy Cu–Ce alloy Microstructure Mechanical performance
This study was aimed to reduce unwanted thermal damage to diamond grains during brazing. To achieve this goal, the diamonds were brazed using NieCr alloy with the addition of CueCe alloy (i.e. NieCr composite solder). Surface morphology and interfacial microstructure of the brazed samples using NieCr composite solder was characterized. The thermally induced residual stresses of brazed diamond grains were examined by Raman spectroscopy. The static pressure strength and impact toughness of the brazed diamond grains were examined. Constitutional phases of NieCr composite solder after brazing were detected. The results show that the thermal residual stresses values of the brazed diamond grains using NieCr composite solder, in which 2 wt% and 5 wt% CueCe alloy were used respectively, were decreased by 6.4% and 9.7% respectively, meanwhile, the static pressure strength values increased by 4.9% and 13.4% respectively as well as impact toughness values increased by 4.0% and 9.2% respectively, compared with that of the brazed diamond grains using NieCr alloy. Chromium carbides (Cr3C2 and Cr7C3) were obtained at the bonding interface between diamonds and composite solder. The constitutional phases containing Ce2Ni7, which could be beneficial to reduce the thermal damage, were formed in the solder alloy after brazing.
1. Introduction Given its extreme hardness, abrasive resistance, and high chemical stability, diamond has become very popular in fabricating diamond tools (e.g. blade saw, grinding wheel, and drill bit) to process the hardfragile materials (e.g. natural stone, ceramics, and concrete) [1]. However, due to their chemical inertness, diamond grains are very hard to wet by most metallic elements (e.g. copper, iron, and nickel). Hence, the diamond grains are hard to be well bonded to the metallic matrix by sintering and electroplating, which could appear limitations when attempting to acquire sufficient bond strength. As for monolayer brazed diamond tools, diamond grains can be firmly held onto a steel substrate by active brazing technology. Because of the presence of strong chemical combination between diamonds and solder alloy, brazed diamond tools exhibit many advantages such as high protrusion of abrasives and large chip-storage space [2–6]. Recently, many studies [7–12] reported the progress of brazed diamond tools using active solders based on AgeCu, CueSn and NieCr alloys. The addition of carbide forming elements (e.g. Ti, Cr, or V) into the solder alloys must be helpful to enhance the interfacial bonding
strength. NieCr alloy have become a popular choice of fabricating brazed diamond tools because of strong mechanical strength and high abrasive resistance. But NieCr alloy could cause thermal damage to the diamonds because the element Ni is a catalyst that can promote the transformation of diamond to graphite under normal pressure and at high temperature (above 800 °C). The brazing is generally carried out by using NieCr alloy at the brazing temperature of 1050 °C for the dwell time of 10 min [13]. Hence, it would weaken the mechanical performance of diamonds and deteriorate the machining performance of diamond tools [13–15]. Up to now there are very few researches on reducing the thermal damage of the diamonds during high temperature brazing. To explore methods of solving the problem mentioned above, diamonds were directly brazed onto a substrate using NieCr composite solder (i.e. addition of CueCe alloy into NieCr alloy) in this study. In the previous studies, the surface-metallized diamond grains were fabricated using NieCr composite solder to only achieve higher mechanical strength compared with the ones fabricated using NieCr alloy and its principle was not investigated [16]. In this study, three kinds of brazed samples were prepared using NieCr alloy and NieCr composite
⁎
Corresponding author. E-mail address: [email protected] (D. Duan). 1 These authors contributed equally to this work and should be considered as co-first authors. https://doi.org/10.1016/j.ijrmhm.2019.01.022 Received 2 December 2018; Received in revised form 20 January 2019; Accepted 29 January 2019 Available online 30 January 2019 0263-4368/ © 2019 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 80 (2019) 253–258
D. Duan et al.
Table 1 Compositions of three kinds of solder alloys (wt%). Type
Ni–Cr alloy
Cu–Ce alloy
Ni–Cr alloy NO.1 NieCr composite solder NO.2 NieCr composite solder
100 98 95
0 2 5
solder, respectively. Subsequently, surface morphology and interface microstructure of the brazed samples were studied. Then the residual stress of the brazed samples was analyzed. The static pressure strength and impact toughness of the above three kinds of brazed diamonds were measured at last.
2. Material and methods The experimental materials adopted in this research included Ni–Cr–B–Si alloy powder, Cu–20wt.%Ce powder, diamonds as well as steel substrates. The CueCe powder alloy was sieved with a 200-mesh screen. The size of diamonds (Huanghe Whirl-wind Co. Ltd., China) was ranged from 420 μm to 500 μm. The compositions of two kinds of NieCr composite solders adopted in this study are displayed in Table 1. Firstly, diamond grains were ultrasonically cleaned in ethanol and steel substrates were polished mechanically. Secondly, the solder alloy was uniformly spread on the steel substrate with a thickness of approximately 300 μm. The diamonds were subsequently arranged onto the solder alloy. Then the samples were fixed into a furnace and a heating treatment was performed at the brazing temperature of 1050 °C for the dwell time of 10 min with a vacuum degree below 2 × 10 −2 Pa. Finally, brazed samples were obtained after brazing and cooling process. The brazed samples were marked differently according to the different kinds of solder alloys adopted. For example, the brazed samples using NieCr alloy were marked as conventional brazed samples. The brazed samples using No.1 NieCr composite solder were marked as No.1 brazed samples. A Hitachi scanning electron microscope (SEM, SU3500) accompanied with energy dispersive spectrometry (EDS) and A Zeiss SEM (Gemini 500) were used to observe the morphology and interface microstructure of the brazed samples. The interfacial resultant around the brazed diamond surface after etching and the chemical composition of NieCr composite solder after brazing were detected by X-ray diffraction analysis (Bruker, D8 Advance X-ray diffractometer) respectively. Additionally, the brazed diamonds were studied by Raman spectroscopy (HORIBA LabRAM HR Evolution) in the range from 600 to 1700 cm−1. The vibrational spectrum feature of materials can be reflected by the Raman scattering peak. When a directed laser-beam is focused upon a certain point onto the brazed diamond grain surface under compressive stress or tensile stress, the Raman spectral peak position changes over to a higher or a lower wave-number, respectively, with reference to the peak wave-number of Raman spectra of a diamond under stress-free. Therefore, the Raman peak position can reflect the stress state of the brazed diamonds [17]. The three kinds of brazed specimens were strongly corroded to separate the brazed diamonds. For example, the brazed diamonds obtained from conventional brazed samples after corroding were marked as conventional brazed diamonds. The brazed diamonds obtained from No.2 brazed samples after corroding were marked as No.2 brazed diamonds, as displayed in Fig. 1. A static strength measuring instrument (DLY-92) was used to measure the static pressure strength of the three kinds of brazed diamonds. An impact toughness measuring instrument (CMC-II) for super-hard abrasives was used to measure the toughness of the three kinds of brazed diamonds by ball mill method. The measurement methods of the static pressure strength and impact toughness of the three kinds brazed diamonds have been described in the literature [16].
Fig. 1. Micrograph of No.2 brazed diamonds after corroding.
Fig. 2. Micrograph of the brazed diamond grains using NieCr alloy.
Fig. 3. Micrograph of the brazed diamond grains using NieCr alloy.
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elements Cu, Ni, Cr, Si and C changed respectively, indicating a slow transition tendency at the bonding interface, as shown in Fig. 5(b). An enrichment of carbide forming element Cr migrating toward the diamond region implies the segregation of Cr from NieCr composite solder [18], which suggests that it is possible to form carbides. One side of the No.2 brazed sample was polished and then etched with a mixed solution containing blue copperas, hydrochloric acid and sulphuric acid. Fig. 6(a) and (b) illustrate an SEM micrograph of interfacial microstructure between solder alloy and steel substrate and the interfacial main elements line scanning by EDS, respectively. As seen in Fig. 6(a), a bonding joint, of which the thickness was about 5–10 μm, was formed at the interface. As shown in Fig. 6(b), the curves connecting the elements Fe, Ni, Cr and Si changed respectively at the interface, suggesting a formation layer may containing Fe3Si and Ni3Si intermetallic compounds and (FeNi) solid-solution. It indicated that the addition of CueCe alloy cannot prevent the metallurgical bonding between solder alloy and steel substrate. The No.2 brazed sample was put into chloroazotic acid to completely remove the solder alloy and steel substrate. Fig. 7 shows an SEM micrograph of No.2 brazed diamonds after etching. As seen in Fig. 7, the diamond surface was entirely covered with resultants. Some resultants were prismatic. Fig. 8 displays an XRD pattern of No.2 brazed diamonds. It can be seen the resultants on the diamond surface mainly contained Cr3C2 and Cr7C3, confirming the chemical combination developed between diamonds and NieCr composite solder [19,20]. It indicated that the addition of CueCe alloy in the present study had no effect on the chemical reaction between Cr and diamonds. Therefore, NieCr composite solder exhibited a good surface wettability to diamonds, as shown in Fig. 4.
Fig. 4. Micrograph of the brazed diamond grains using NieCr composite solder.
3. Experimental results and discussion 3.1. Effects of CueCe alloy addition on Morphology Figs. 2 and 3 illustrate the SEM micrographs of the brazed samples fabricated using NieCr alloy (i.e. conventional brazed samples). As seen from Fig. 2, there were some big etch pits on the brazed diamond surfaces. As seen from Fig. 3, there were a few microcracks around the diamonds. Fig. 4 illustrates an SEM micrograph of the brazed diamond samples using No.2 NieCr composite solder. As seen from Fig. 4, the NieCr composite solder climbed up along the diamond surface, suggesting the NieCr composite solder exhibiting a good surface wettability to diamonds. At the same time, there were few etch pits on the brazed diamond surfaces and microcracks adjacent to the diamonds. Obviously, the surface morphology of the brazed samples using NieCr composite solder was better.
3.3. Effect of CueCe alloy on residual stresses Different coefficients of thermal expansion of diamond grains, solder alloy, and steel substrate would cause residual stresses adjacent to the diamonds during brazing [21,22]. The high residual stress might cause the microcracks around the diamond or originating from the diamond surface. The thermally induced residual stresses in diamond grains can be measured by Raman spectroscopy. The residual stresses in the sample are expressed by a wave-number shift of the Raman spectral peak compared with that of the Raman spectral peak of the sample under stress-free [17]. The residual stresses were measured along path I shown in Fig. 9 with an interval of 25 μm from point O (i.e. at the center on the top surface of diamond grain) during the experiment. The peak wave-
3.2. Interfacial microstructure characterization An SEM interface micrograph of the No.2 brazed diamond is displayed in Fig. 5(a), while the interfacial main elements line scanning by EDS is displayed in the Fig. 5(b). As seen in Fig. 5(a), a well-bonding joint was demonstrated at the interface. And the curves connecting the
Fig. 5. Interfacial microstructure of No.2 brazed diamond. (a) Interface micrograph and (b) interfacial main elements line scanning. 255
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Fig. 6. Interfacial microstructure of No.2 brazed sample between solder alloy and steel substrate. (a) Interface micrograph and (b) interfacial main elements line scanning.
Fig. 9. Measurement schematic of residual stresses by Raman spectroscopy.
Fig. 7. Micrograph of the brazed diamonds using NieCr composite solder after etching.
Fig. 10. Raman spectrum of brazed diamonds using NieCr composite solder.
number of the Raman spectrum of original diamonds without residual stresses was 1332.22 cm−1 in the experiment. Fig. 10 displays the Raman spectrum of No.1 brazed diamond at point O. As shown in Fig. 10, the peak wave-number of the Raman spectrum was 1331.56 cm−1, implying that the brazed diamond enduring residual stress. The peak wave-number of the Raman spectrum of No.2 brazed
Fig. 8. An XRD pattern of the brazed diamonds after etching.
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Fig. 12. Static pressure strength and impact toughness values of three kinds of brazed diamonds. Fig. 11. Residual stress of three kinds of brazing diamonds.
pressure strength values of No. 1 and No. 2 brazed diamonds were increased by 4.9% and 13.4% respectively, as well as the impact toughness values of No. 1 and No. 2 brazed diamonds increased by 4% and 9.2% respectively, compared with that of the brazed diamonds using NieCr alloy (i.e. conventional brazed diamonds). The decrease in the mechanical strength of the brazed diamond grains was attributed to the thermal damage of diamond grains. It is noted that the increasing proportions of static pressure strength and impact toughness were both higher than that of weight proportion of CueCe alloy, suggesting the addition of Ce could reduce the thermal damage, which was consistent with the results reported in Section 3.3.
diamond at point O was 1331.66 cm−1. Because of the shape of regular cubo-octahedron or icosahedron of diamonds and the geometry of the specimen in the present study (Fig. 9), allowing unrestrained thermal expansion (or contraction) of diamonds in the Z direction, an equi-biaxial (EB) stress state can be assumed. According to the literature [23], the equi-biaxial thermal residual stress σ can be calculated as follows:
σ = K·(ω − ω0)
(1)
Where ω is the Raman spectral peak position of the brazed diamonds, ω0 is the Raman spectral peak position of original diamonds under stress-free, and the proportionality factor K is −0.43 GPa/cm−1 in the present study. Fig. 11 summarizes the comparison of three kinds of measured residual stresses in this study. The residual stress values were the average results of three measurements at the same position for a kind of brazed diamond. As seen from Fig. 11, obviously within the distance from approximately 25 μm close to the top surface, tensile stresses were detected, and afterward it became compressive stresses. At the same time the compressive stress values all increased with increasing depth. It is noted that the residual compressive stress values of the brazed diamond grains using NieCr composite solder were lower than that of the brazed diamond grains using NieCr alloy at the same measuring distance. When the depth reached 200 μm, the residual compressive stress values of the brazed diamonds using No.1 and No.2 NieCr composite solder were −1.16 GPa and −1.12 GPa respectively, which were decreased about 6.4% and 9.7% respectively compared with that of the brazed diamonds using NieCr alloy. From the above Raman analysis, it indicated that the addition of CueCe alloy could reduce the residual stresses to the diamonds during high-temperature brazing.
3.5. Constitutional phases and discussion No.2 NieCr composite solder powders were filled in the graphite mould and put into the furnace to fabricate an alloy block. The heat treatment parameters were the same as that reported in Section 2. Fig. 13 shows an XRD pattern of the alloy block. As shown in Fig. 13, the dominating phases were CeNi2, Ce2Ni7, Ni3Si, Ni4B3, CrB, and NieCr phase. The Gibbs free energy for Equations 2Ce+7Ni → Ce2Ni7 at the brazing temperature(1050 °C, 1323.15 K) is given by −30.86 kJ/ mol [24,25]. The values are negative, indicating that the reaction could happen spontaneously during the heat-treatment process. The melting point of Ce2Ni7 is 1060 ± 5 °C [25,26], which is much higher than the heat-treatment temperature (1050 °C). Therefore, the phase Ce2Ni7 was
3.4. The mechanical strength of the diamonds To study the influence of the CueCe alloy addition on the mechanical strength of diamonds, the static pressure strength and impact toughness of the brazed diamonds after etching are investigated. Fig. 12 displays the static pressure strength and impact toughness values of three kinds of brazed diamond grains after etching. Apparently, the CueCe alloy had a positive influence on the static pressure strength and impact toughness of the diamond grains. The more the content of CueCe alloy addition into the NieCr composite solder, the higher the static pressure strength and impact toughness of the brazed diamond grains. The weight proportion of CueCe alloy in No.1 and No.2 NieCr composite solder were 2% and 5% respectively. Meanwhile, the static
Fig. 13. An XRD pattern of the alloy block fabricated using No.2 NieCr composite solder. 257
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Acknowledgment The authors gratefully acknowledge the financial support of this research by the National Natural Science Foundation of China (No. 51720105016), Postdoctoral Innovative Talents Support Program of China (No. BX20180250), Major National Science and Technology Project (2017ZX04021001-005-001), Natural Science Foundation of Jiangxi province (No.20171BAB216032). References [1] J.V. Naidich, G.A. Kolesnichenko, Surface Phenomena in Metallurgical Processes, Consultants Bureau, New York, 1965. [2] A.K. Chattopadhyay, L. Chollet, H.E. Hintermann, On performance of brazed monolayer diamond grinding wheel, CIRP Ann. Manu. Tech. 40 (1991) 347–350. [3] H.E. Hintermann, A.K. Chattopadhyay, New generation superabrasive tool with monolayer configuration, Diam. Relat. Mater. (12) (1992) 1131–1143. [4] C.M. Sung, Brazed diamond grid: a revolutionary design for diamond saws, Diam. Relat. Mater. 8 (1997) 1540–1543. [5] B. Xiao, H.J. Xu, Z.B. Wu, Investigation on brazing of single-layer superabrasive wheel, Key Eng. Mater. 202 (2001) 155–158. [6] J.C. Sung, M. Sung, The brazing of diamond, Int. J. Refract. Met. Hard Mater. 27 (2009) 382–393. [7] P. Mukhopadhyay, A. Ghosh, On bond wear, grit-alloy interfacial chemistry and joint strength of synthetic diamond brazed with Ni–Cr–B–Si–Fe and Ti activated AgCu filler alloys, Int. J. Refract. Met. Hard Mater. 72 (2018) 236–243. [8] B.J. Sun, B. Xiao, Effects of process parameters on interfacial microstructure, residual stresses, and properties of tunnel furnace brazed diamonds, Diam. Relat. Mater. 85 (2018) 98–103. [9] J.C. Chen, D. Mu, X.J. Liao, G.Q. Huang, H. Huang, X.P. Xu, H. Huang, Interfacial microstructure and mechanical properties of synthetic diamond brazed by Ni–Cr–P filler alloy, Int. J. Refract. Met. Hard Mater. 74 (2018) 52–60. [10] S.X. Liu, B. Xiao, Z.Y. Zhang, D.Z. 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Guo, Finite element analysis of thermal effects on brazed cold diamond grain, Trans. Indian Inst. Metals 68 (2015) 829–838. [16] D.Z. Duan, B. Xiao, W. Wang, Z.Y. Zhang, B. Wang, P. Han, X.Y. Ding, Interface characteristics and performance of pre-brazed diamond grains with Ni–Cr composite alloy, J. Alloys Compd. 644 (2015) 626–631. [17] S. Buhl, C. Leinenbach, R. Spolenak, K. Wegener, Influence of the brazing parameters on microstructure, residual stresses and shear strength of diamond-metal joints, J. Mater. Sci. 45 (2010) 4358–4368. [18] B. Xiao, H.J. Xu, Y.C. Fu, J.H. Xu, Form and distribution characterization of reaction products at the brazing interface between Ni–Cr alloy and diamond, Key Eng. Mater. 259 (2004) 151–153. [19] C.Y. Wang, Y.M. Zhou, F.L. Zhang, Z.C. Xu, Interfacial microstructure and performance of brazed diamond grains with Ni–Cr–P alloy, J. Alloys Compd. 476 (2009) 884–888. [20] A. Trenker, H. Seidemann, High-vacuum brazing of diamond tools, Ind. Diam. Rev. 62 (2002) 49–51. [21] U.E. Klotz, C. Liu, F.A. Khalid, H.-R. Elsener, Influence of brazing parameters and alloy composition on interface morphology of brazed diamond, Mater. Sci. Eng. A 495 (2008) 265–270. [22] M. Akbari, S. Buhl, C. Leinenbach, R. Spolenak, K. Wegener, Thermomechanical analysis of residual stresses in brazed diamond metal joints using Raman spectroscopy and finite element simulation, Mech. Mater. 52 (2012) 69–77. [23] S. Buhl, C. Leinenbach, R. Spolenak, K. Wegener, Microstructure, residual stresses and shear strength of diamond-steel-joints brazed with a Cu–Sn-based active filler alloy, Int. J. Refract. Met. Hard Mater. 30 (2012) 16–24. [24] I. Barin, O. Knacke, Thermochemical Properties of Inorganic Substances, Springer, Berlin, 1973. [25] M. Palumbo, G. Borzone, S. Delsante, N. Parodi, G. Cacciamani, R. Ferro, L. Battezzati, M. Baricco, Thermodynamic analysis and assessment of the Ce–Ni system, Intermetallics 12 (2004) 1367–1372. [26] W. Xiong, Y. Du, X.G. 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Fig. 14. The CueCe phase diagram [27].
stable at the brazing temperature. The CueCe phase diagram [27] was displayed in Fig. 14. As seen from Fig. 14, the brazing temperature (1050 °C, 1323.15 K) was higher than the liquidus temperature of the CueCe alloy used in the present study. Therefore, there was no CuxCe (x = 6,5,4,2 or 1) phase formation at the brazing temperature. At the same time the NieCr liquid and the CueCe liquid could be syncretized with each other and lead to the formation of Ce2Ni7. The thermal damage results from the interfacial thermal residual stress caused by the large differences in the material properties among the three materials (i.e. diamonds, steel substrate, and solder alloys) and the erosion of diamonds mainly caused by Ni, which can resolve the carbon element and promote chemical reaction between NieCr alloy and carbon [15,28]. As shown in Fig. 13, Ni also induced the formation of hard phases (e.g. Ni3Si and Ni4B3) with B and Si. It was probable that the formation of Ce2Ni7 caused a decrease of liquid Ni content in the solder alloy at the brazing temperature and thereby inhibited the hard phases growth. So the residual stress existing close to the interface between diamonds and solder alloy was reduced. The formation of Ce2Ni7 also could weaken the effect of Ni on the diamonds and thereby significantly reduce the erosion of the brazed diamonds. Therefore, the mechanical performance of the brazed diamond grains was improved. 4. Conclusions (1) The surface morphology of the brazed samples using NieCr composite solder, in which 2 wt% or 5 wt% CueCe alloy addition was adopted, was better than that of the brazed samples using NieCr alloy. Chromium carbides (Cr3C2 and Cr7C3) was obtained, confirming the chemical combination developed between diamonds and NieCr composite solder. (2) The residual stress values of the brazed diamonds using NieCr composite solder were lower than that of the brazed diamond grains using NieCr alloy at the same measuring distance. The static pressure strength values of the brazed diamonds using NieCr composite solder, in which 2 wt% and 5 wt% CueCe alloy were used respectively, were decreased by 4.9% and 13.4% respectively as well as impact toughness values increased by 4% and 9.2% respectively, compared with that of the brazed diamond grains using NieCr alloy. (3) The constitutional phases containing CeNi2, Ce2Ni7, Ni3Si, Ni4B3, and CrB were formed in the solder alloy after brazing. The formation of Ce2Ni7 could be beneficial to reduce the thermal damage and improve the mechanical performance of the diamonds.
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