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Microstructure of Ni–NiCr laminated composite and Cr18–Ni8 steel joint by vacuum brazing
Vacuum 86 (2012) 2059e2063
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Microstructure of NieNiCr laminated composite and Cr18eNi8 steel joint by vacuum brazing Na Wu, Yajiang Li*, Juan Wang Key Laboratory for LiquideSolid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 November 2011 Received in revised form 27 April 2012 Accepted 23 May 2012
Vacuum brazing of NieNiCr laminated composite to Cr18eNi8 steel was carried out using NieCreSieB amorphous foil. Microstructure characteristic of the brazed joints was investigated by scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), field-emission scanning electron microscope (FE-SEM) and microsclerometer. NieCreSieB amorphous foil exhibited good wettability on NieNiCr laminated composite. The brazed region consisted of g-Ni solid solution and NieB eutectic. An excellent bonding with shear of 170 MPa was obtained. An interface of Ni3B precipitated in Ni cover layer near the brazed region. Microhardness of the interface was 500 HV. There was an interface consisting of fine boride granules in Cr18eNi8 steel close to the brazed region and microhardness of the interface was 250 HV. Bonding behavior of the brazed joint was described according to the microstructure characteristic. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Laminated composite Vacuum brazing Microstructure Elemental distribution
1. Introduction NieNiCr laminated composite is a newly-developed material composed of pure-Ni cover layers and NiCr base layer. The cover layer exhibits good resistance to high temperature and corrosion. NiCr base layer is featured with lower density, higher strength and lower cost compared with the cover layer. Therefore, NieNiCr laminated composite combines the merits of the above two layers, showing low density and good resistance in high temperature and corrosive environment [1,2]. With increasing the thrust to weight ratio, the modern aero-engines are demanded to have lighter weight and higher working temperature. So NieNiCr laminated composite is an attractive candidate for the modern aeroengines to improve the thrust to weight ratio. Usually, only a part of the component will withstand the effect of high temperature or corrosion. So joining the laminated composite with Cr18eNi8 stainless steel to form a composite structure will not only make full use of their respective performance advantages but also reduce the component weight. The structural applications of NieNiCr laminated composite require reliable joining techniques. Two main difficulties in the welding of NieNiCr laminate are identified. First, both Ni cover layer and NiCr base layer should have good bonding with weld metal. Second, it is important to keep the structure integrity of NieNiCr laminate after welding. The thickness of Ni cover layer is only 0.2e0.3 mm,
* Corresponding author. Tel./fax: þ86 531 88392924. E-mail address: [email protected] (Y. Li). 0042-207X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2012.05.027
making the welding of NieNiCr laminated composite different from the traditional large size composite plates (thickness of cover layer >1 mm) [3]. Gas tungsten arc welding (GTAW) shows arc stability, which is easy to realize automatic operation. However, it induced thermal stress and even micro-cracks in the welding of NieNiCr laminate [4]. In contrast, the whole assembly is heated in vacuum brazing, which leads to lower thermal stress. So vacuum brazing is more suitable for joining the complicated parts. Therefore, vacuum brazing of NieNiCr laminated composite to Cr18eNi8 steel was performed in this study. Microstructure, elemental distribution and bonding behavior were studied to evaluate the microstructure characteristic of the brazed joint.
2. Materials and experiment Materials used in this study were NieNiCr laminated composite and Cr18eNi8 austenitic stainless steel with the thickness of 2.6 mm. NieNiCr laminated composite was prepared by putting Ni80-Cr20 powder between the pure-Ni (Ni >99.5%) cover layers and then hot pressing in vacuum. Ni80-Cr20 powder was sintered into nichrome-based powder alloy and it was called “NiCr base layer”. The sandwich structure of NieNiCr laminated composite was 0.3 mm Ni cover layer þ 2 mm NiCr base layer þ 0.3 mm Ni cover layer. Microstructure of the laminated composite is shown in Fig. 1. NiCr base layer consisted of bone-shaped g-Ni(Cr) solid solution in white and porosities in black. Porosity fraction of NiCr base layer was 35.4% and the density was 6.72 g/cm3. The Cr18eNi8
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NieCreSieB amorphous foil was used as filler metal in this study and the melting temperature range was 970e1000 C. The chemical composition of Cr18eNi8 steel and NieCreSieB amorphous foil is shown in Table 1. The brazing process was conducted at 1080 C for 20 min under a vacuum of 104 Pa in Workhorse-II vacuum diffusion bonding installation. The cooling process was performed in the vacuum chamber by circulating water until the temperature reached 100 C. Then the brazed joints were cooled to 50 C in the chamber spontaneously. The heating pattern of the brazing process is illustrated in Fig. 4. Transverse section of the brazed joint was ground and polished, and finally etched by a solution of 80 ml HCl, 13 ml HF and 7 ml HNO3. Microstructure and elemental distribution of the brazed joint were analyzed by means of FEI-QUANTA2000 scanning electron microscopy (SEM) equipped with OXFORD Energy-dispersive Spectroscopy (EDS) and HITACHI SU-70 field-emission scanning electron microscope (FE-SEM) with EDS. The shearing strength of joint was measured using CMT-5105 testing machine. The Vickers microhardness was determined by DHV-1000 microsclerometer, with a constant load of 50 g for 10 s.
Fig. 1. Microstructure of NieNiCr laminated composite.
3. Results and discussion 3.1. Microstructure of the brazed joint There were porosities in NiCr base layer. The porosities in the porous metal play an important role in the brazing process and there are three cases. The porosities generate capillary force, which will pull the molten filler metal into the porosities. When the capillary force is great, an excessive filler metal will infiltrate into the porosities leaving voids in the brazed region [5]. When there is no infiltration into porosities, porosities have the potential to diminish the contact area between the filler metal and the porous base metal [6]. It is unfavorable for the bonding strength. Infiltration to short distances is advantageous in creating a strong bond by increasing the contact area [7]. There was a sufficient amount of brazing alloy remaining in the joint gap without voids. The NieCreSieB amorphous foil exhibited good wettability on Ni cover layer and NiCr base layer, especially on the interface between them.
Fig. 2. Microstructure of Cr18-Ni8 steel.
steel was hot-rolled and the microstructure is shown in Fig. 2. Specimens with dimension of 30 10 2.6 mm3 were cut from the plates via wire cut electrical discharge machining. Before brazing the faying surfaces were polished with 360# and 800# SiC paper to remove the oxide film and then cleaned in alcohol. The brazing specimens were arranged in a butt joint as shown in Fig. 3.
Fig. 3. Schematic diagram of joint assembly (unit in mm).
Fig. 4. Heating pattern of the brazing process.
Table 1 Chemical composition of Cr18-Ni8 steel and NieCreSieB amorphous foil (wt.%). Materials
Chemical composition Ni
Cr
Fe
Si
B
C
Mn
Ti
S
P
Cr18-Ni8 steel NieCreSieB amorphous foil
8.0e11.0 Bal.
17.0e19.0 6.0e8.0
Bal. 2.5e3.5
1.0 4.0e5.0
e 2.75e3.5
0.12 0.02
2.0 e
0.5e0.7 e
0.03 e
0.035 e
N. Wu et al. / Vacuum 86 (2012) 2059e2063
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Fig. 5. Microstructure of (a) brazed joint, (b) magnified SEM morphology of zone “A”, (c) magnified SEM morphology of zone “B” and (d) brazed region of NiCr base layer.
Microstructure of the brazed joint of NieNiCr laminated composite to Cr18eNi8 steel is shown in Fig. 5. Chemical composition of the special phases in the brazed region was detected by EDS as shown in Table 2. There was a chain of small shapeless blocks (point 1 in Fig. 5a) formed in Ni cover layer near the brazed region. EDS result indicates that the Ni content of the blocks was 85.84% and the B content was 12.23%. So the precipitation in Ni cover layer was identified as Ni3B [8]. Many white particles (point 2 in Fig. 5b) precipitated in NiCr base layer under the influence of heating pattern. The Ni:Cr ratio of the particles was 2:1. Thus, the white precipitates were determined to be Ni2Cr based on NieCr phase diagram [9]. There was an interface (about 10 mm) consisting of fine granules (point 3 in Fig. 5c) in the diffusion affected zone of Cr18eNi8 steel. The fine granules were rich in B, Cr and Fe and it were identified as (Fe, Cr)B precipitates [10]. Point 4 in the brazed region of NiCr base layer was rich in Si and it was considered to be g-Ni(Si) solid solution. The dark gray phase (point 5 in Fig. 5d) in the brazed region was rich in B (28.82%). According to the binary NieB phase diagram [11], the solubility of B in Ni is (0.3, at.%). Therefore, the dark gray phase was identified as Ni3B. Point 6 was identified as g-Ni(Si) solid solution. But the Si content in point 6 was higher than point 4. The Ni3B in the center of the brazed region and the g-Ni(Si) solid solution around (point 6) composed the divorced eutectic [12,13]. Therefore, the brazed region consisted of g-Ni solid solution and NieB divorced eutectic. There was B remained in the brazed region of NiCr base layer with the formation of Ni3B. However, B preferred to diffuse into Ni
cover layer along the grain boundaries. The diffusion of B from the filler metal into Ni cover layer was faster than that from the filler metal into NiCr base layer. There was not enough B remaining in the brazed region of Ni cover layer to form the Ni3B. So the brazed region consisted of g-Ni solid solution completely. 3.2. Elemental distribution Elemental distribution across the brazed region of NiCr base layer was measured by EDS and the result is shown in Fig. 6.
Table 2 EDS result of chemical composition in the brazed joint (at.%). Point
Ni
Cr
Fe
Si
B
Possible phase
1 2 3 4 5 6
85.84 62.03 e 79.70 64.83 76.90
e 37.97 26.32 12.40 5.29 11.31
e e 34.42 2.74 1.05 2.28
1.93 e e 5.16 e 9.51
12.23
B-enriched g-Ni Ni2Cr (Fe, Cr)B Si-enriched g-Ni NieB eutectic Si-enriched g-Ni
39.26 e 28.82 e
Fig. 6. Elemental distribution across the brazed region of NiCr base layer (SSZ e solid solution zone; BZ e brittle zone).
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Cr18eNi8 steel due to the nonsymmetrical interdiffusion as shown in Fig. 5a. 3.3. Microhardness of the brazed joint
Fig. 7. Microhardness distribution (50 g) through the brazed joint.
There were enrichment of Si in the solid solution zone and depletion of Si in the brittle zone. Due to the solubility of Si (17, at.%) in Ni is higher than B (0.3, at.%), the g-Ni solid solution was rich in Si and B was ejected into the leaving melt [14]. The Ni, Cr, Fe and Si distributions transformed gradually from the brazed region to the NiCr base layer. An excellent bonding between the brazed region and NiCr base layer was obtained without the formation of intermetallics at the interface. Fe and Cr diffused from Cr18eNi8 steel to the brazed region while Ni, Si and B diffused in the opposite direction. Due to the low solubility of B in Fe, boride precipitated in Cr18eNi8 steel near the brazed region. According to the microstructure (Fig. 5), B diffused inside the crystals of Cr18eNi8 near the brazed region and diffused along the grain boundaries far away from the brazed region. However, Si and B diffused much faster than Fe and Cr due to their smaller atomic radius, resulting in the nonsymmetrical interdiffusion [10]. Microporosities formed in the brazed region close to
Microhardness distribution through the brazed joint of NieNiCr laminated composite to Cr18eNi8 steel (Fig. 7) was measured via DHV-1000 microsclerometer, with a constant load of 50 g for 10 s. Microhardness of Cr18eNi8 steel was 200 HV. Microhardness of the interface in Cr18eNi8 steel near the brazed region was 250 HV, which was higher than Cr18eNi8 steel. The microhardness increment was due to the precipitation of fine boride granules. Microhardness of the g-Ni solid solution was 200 HV. Microhardness of the brittle Ni3B in the brazed region was 530 HV. The brittle phase played an important in the shear strength of the brazed region. Shear strength of the NieNiCr laminated composite/Cr18eNi8 steel joint brazed with NieCreSieB filler metal was 170 MPa. However, shear strength of the joint brazed with NieCreP filler metal was 137 MPa [15]. It was for this reason that Si and B diffused faster than P resulting in the less brittle phase in the brazed region. Brazed region of Ni cover layer was composed of Ni solid solution and the microhardness was 200 HV. Microhardness of the interfacial region (500 HV) was significantly higher than Ni cover layer (80 HV). It was attributed to the existence of Ni3B at the interfacial region. 3.4. Bonding behavior Fig. 8 describes the bonding behavior for the vacuum brazed joint of NieNiCr laminated composite to Cr18eNi8 steel using NieCreSieB amorphous foil. It can be described as four stages. In the first stage, the filler metal became liquid due to its low melting point. There were dissolution and diffusion between the filler metal and base metals. The melting point depressant elements Si and B diffused outward from the molten filler metal to base materials while Fe, Ni and Cr diffused in the opposite direction as shown in Fig. 8a. Therefore, the melting point of the molten filler
Fig. 8. Bonding behavior for vacuum brazed joint of NieNiCr laminated composite to Cr18-Ni8 steel: (a) elemental diffusion; (b) isothermal solidification stage; athermal solidification stage for the formation of divorced eutectic: (c) g-Ni and (d) Ni3B.
N. Wu et al. / Vacuum 86 (2012) 2059e2063
metal near the base materials increased, which induced the isothermal solidification (Fig. 8b). Isothermal solidification involved the formation of g-Ni solid solution. The isothermal solidification stage continued as the outward diffusion process continued during the holding brazing time. The solubility of Si (17, at.%) in Ni is higher than B (0.3, at.%). Thus, the g-Ni solid solution was rich in Si and the remaining melt was rich in B. The NiCr base layer dissolved in the melting filler metal. Due to the same matrix of them, the interface was not clear and an excellent bonding was obtained. Si and B preferred to diffuse along grain boundaries in Ni cover layer. However, due to the small size of B, the intragranular diffusion of B was essential [14]. As the diffusion continued, Ni3B precipitated in the Ni cover layer due to the low solubility of B in Ni. It was rich in Fe, Cr and B in Cr18eNi8 steel near the brazed region, so an interface consisting of (Fe, Cr)B precipitates was formed. Many white Ni2Cr particles precipitated in NiCr base layer [9]. As the temperature decreased in the cooling stage, the athermally solidified stage occurred. The g-Ni solid solution of the divorced eutectic grew on the existing g-Ni solid solution as shown in Fig. 8c. In the last stage (Fig. 8d), the leaving melt was further enriched in boron and solidified as blocky Ni3B. 4. Conclusions NieNiCr laminated composite is featured with low density and good resistance in high temperature and corrosive environment. It is an attractive candidate for the modern aero-engines to improve the thrust to weight ratio. Brazed joint of NieNiCr laminated composite to Cr18eNi8 steel was obtained by vacuum brazing with NieCreSieB amorphous foil at 1080 C for 20 min in a vacuum of 104 Pa. The brazed region consisted of g-Ni solid solution and NieB divorced eutectic. There were mainly dissolution and diffusion between filler metal and NiCr base layer. An excellent bonding with the shear strength of 170 MPa was obtained. An interface of blocky Ni3B formed in Ni cover layer close to the brazed region. Many (Fe, Cr)B granules precipitated and composed an interface of 10 mm in Cr18eNi8 steel close to the brazed region. Ni2Cr particles
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precipitated in NiCr base layer under the influence of heating pattern. Acknowledgements This project was supported by the National Natural Science Foundation of China (51175303) and Doctoral Fund of Ministry of Education of China (200804220020). References [1] Kim JK, Yu TX. Forming and failure behavior of coated, laminated and sandwiched sheet metals: a review. J Mater Process Technol 1997;63(1e3):33e42. [2] Chen YL. Microlaminated composite: the next materials of turbine blade. Aviat Maint Eng 2001;5:10e2. [3] Missori S, Murdolo F, Sili A. Single-pass laser beam welding of clad steel plate. Weld J 2004;83(2):65e71. [4] Li YJ, Xia CZ, Puchkov UA, Wang J. Weldability of pure-Ni/NiCr laminated composite and 18-8 stainless steel. Trans China Weld Inst 2010;31(2): 13e6. [5] Hamill J. Weld techniques give powder metal a different dimension. Met Powder Rep 2007;62(5):22e4. 26e27, 29e31. [6] Gao HY, He YH, Shen PZ, Jiang Y, Huang BY, Xu NP. Welding of FeAl porous materials and stainless steel. Chin J Nonferrous Met 2009;19(1):90e5. [7] Peter K. Sokolowski, Thomas F. Murphy, Bruce A. Lindsley. Considerations in sinter-brazing PM components. In: Powder Metal World Congress and Exhibition. Hollywood, Florida; 2010. [8] Gale WF, Wallach ER. Microstructure development in transient liquid-phase bonding. Metall Trans A 1991;22:2451e7. [9] Shankar V, Bhanu K, Rao S, Mannan SL. Microstructure and mechanical properties of Inconel 625 purealloy. J Nucl Mater 2001;288:222e32. [10] Ou CL, Liaw DW, Du YC, Shiue RK. Brazing of 422 stainless steel using the AWS classification BNi-2 Braze alloy. J Mater Sci 2006;41(19):6353e61. [11] Nagasaki Seizo, Hirabayashi Makoto. Binary alloy phase-diagrams; 2002. Tokyo, Japan. [12] Tung SK, Lim LC, Lai MO. Microstructural evolution and control in BNi-4 brazed joints of nickel 270. Scripta Mater 1995;33(8):1253e9. [13] Pouranvari M, Ekrami A, Kokabi AH. Microstructure development during transient liquid phase bonding of GTD-111 nickel-based superalloy. J Alloys Compd 2008;461(1e2):641e7. [14] Yuan XJ, Kang CY, Kim MB. Microstructure and XRD analysis of brazing joint for duplex stainless steel using a Ni-Si-B filler metal. Mater Character 2009; 60(9):923e31. [15] Wu N, Li YJ, Wang J, Puchkov UA. Vacuum brazing of super-Ni/NiCr laminated composite to Cr8-Ni8 steel with NiCrP filler metal. J Mater Process Technol 2012;212(4):794e800.
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Microstructure of NieNiCr laminated composite and Cr18eNi8 steel joint by vacuum brazing Na Wu, Yajiang Li*, Juan Wang Key Laboratory for LiquideSolid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 November 2011 Received in revised form 27 April 2012 Accepted 23 May 2012
Vacuum brazing of NieNiCr laminated composite to Cr18eNi8 steel was carried out using NieCreSieB amorphous foil. Microstructure characteristic of the brazed joints was investigated by scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), field-emission scanning electron microscope (FE-SEM) and microsclerometer. NieCreSieB amorphous foil exhibited good wettability on NieNiCr laminated composite. The brazed region consisted of g-Ni solid solution and NieB eutectic. An excellent bonding with shear of 170 MPa was obtained. An interface of Ni3B precipitated in Ni cover layer near the brazed region. Microhardness of the interface was 500 HV. There was an interface consisting of fine boride granules in Cr18eNi8 steel close to the brazed region and microhardness of the interface was 250 HV. Bonding behavior of the brazed joint was described according to the microstructure characteristic. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Laminated composite Vacuum brazing Microstructure Elemental distribution
1. Introduction NieNiCr laminated composite is a newly-developed material composed of pure-Ni cover layers and NiCr base layer. The cover layer exhibits good resistance to high temperature and corrosion. NiCr base layer is featured with lower density, higher strength and lower cost compared with the cover layer. Therefore, NieNiCr laminated composite combines the merits of the above two layers, showing low density and good resistance in high temperature and corrosive environment [1,2]. With increasing the thrust to weight ratio, the modern aero-engines are demanded to have lighter weight and higher working temperature. So NieNiCr laminated composite is an attractive candidate for the modern aeroengines to improve the thrust to weight ratio. Usually, only a part of the component will withstand the effect of high temperature or corrosion. So joining the laminated composite with Cr18eNi8 stainless steel to form a composite structure will not only make full use of their respective performance advantages but also reduce the component weight. The structural applications of NieNiCr laminated composite require reliable joining techniques. Two main difficulties in the welding of NieNiCr laminate are identified. First, both Ni cover layer and NiCr base layer should have good bonding with weld metal. Second, it is important to keep the structure integrity of NieNiCr laminate after welding. The thickness of Ni cover layer is only 0.2e0.3 mm,
* Corresponding author. Tel./fax: þ86 531 88392924. E-mail address: [email protected] (Y. Li). 0042-207X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2012.05.027
making the welding of NieNiCr laminated composite different from the traditional large size composite plates (thickness of cover layer >1 mm) [3]. Gas tungsten arc welding (GTAW) shows arc stability, which is easy to realize automatic operation. However, it induced thermal stress and even micro-cracks in the welding of NieNiCr laminate [4]. In contrast, the whole assembly is heated in vacuum brazing, which leads to lower thermal stress. So vacuum brazing is more suitable for joining the complicated parts. Therefore, vacuum brazing of NieNiCr laminated composite to Cr18eNi8 steel was performed in this study. Microstructure, elemental distribution and bonding behavior were studied to evaluate the microstructure characteristic of the brazed joint.
2. Materials and experiment Materials used in this study were NieNiCr laminated composite and Cr18eNi8 austenitic stainless steel with the thickness of 2.6 mm. NieNiCr laminated composite was prepared by putting Ni80-Cr20 powder between the pure-Ni (Ni >99.5%) cover layers and then hot pressing in vacuum. Ni80-Cr20 powder was sintered into nichrome-based powder alloy and it was called “NiCr base layer”. The sandwich structure of NieNiCr laminated composite was 0.3 mm Ni cover layer þ 2 mm NiCr base layer þ 0.3 mm Ni cover layer. Microstructure of the laminated composite is shown in Fig. 1. NiCr base layer consisted of bone-shaped g-Ni(Cr) solid solution in white and porosities in black. Porosity fraction of NiCr base layer was 35.4% and the density was 6.72 g/cm3. The Cr18eNi8
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NieCreSieB amorphous foil was used as filler metal in this study and the melting temperature range was 970e1000 C. The chemical composition of Cr18eNi8 steel and NieCreSieB amorphous foil is shown in Table 1. The brazing process was conducted at 1080 C for 20 min under a vacuum of 104 Pa in Workhorse-II vacuum diffusion bonding installation. The cooling process was performed in the vacuum chamber by circulating water until the temperature reached 100 C. Then the brazed joints were cooled to 50 C in the chamber spontaneously. The heating pattern of the brazing process is illustrated in Fig. 4. Transverse section of the brazed joint was ground and polished, and finally etched by a solution of 80 ml HCl, 13 ml HF and 7 ml HNO3. Microstructure and elemental distribution of the brazed joint were analyzed by means of FEI-QUANTA2000 scanning electron microscopy (SEM) equipped with OXFORD Energy-dispersive Spectroscopy (EDS) and HITACHI SU-70 field-emission scanning electron microscope (FE-SEM) with EDS. The shearing strength of joint was measured using CMT-5105 testing machine. The Vickers microhardness was determined by DHV-1000 microsclerometer, with a constant load of 50 g for 10 s.
Fig. 1. Microstructure of NieNiCr laminated composite.
3. Results and discussion 3.1. Microstructure of the brazed joint There were porosities in NiCr base layer. The porosities in the porous metal play an important role in the brazing process and there are three cases. The porosities generate capillary force, which will pull the molten filler metal into the porosities. When the capillary force is great, an excessive filler metal will infiltrate into the porosities leaving voids in the brazed region [5]. When there is no infiltration into porosities, porosities have the potential to diminish the contact area between the filler metal and the porous base metal [6]. It is unfavorable for the bonding strength. Infiltration to short distances is advantageous in creating a strong bond by increasing the contact area [7]. There was a sufficient amount of brazing alloy remaining in the joint gap without voids. The NieCreSieB amorphous foil exhibited good wettability on Ni cover layer and NiCr base layer, especially on the interface between them.
Fig. 2. Microstructure of Cr18-Ni8 steel.
steel was hot-rolled and the microstructure is shown in Fig. 2. Specimens with dimension of 30 10 2.6 mm3 were cut from the plates via wire cut electrical discharge machining. Before brazing the faying surfaces were polished with 360# and 800# SiC paper to remove the oxide film and then cleaned in alcohol. The brazing specimens were arranged in a butt joint as shown in Fig. 3.
Fig. 3. Schematic diagram of joint assembly (unit in mm).
Fig. 4. Heating pattern of the brazing process.
Table 1 Chemical composition of Cr18-Ni8 steel and NieCreSieB amorphous foil (wt.%). Materials
Chemical composition Ni
Cr
Fe
Si
B
C
Mn
Ti
S
P
Cr18-Ni8 steel NieCreSieB amorphous foil
8.0e11.0 Bal.
17.0e19.0 6.0e8.0
Bal. 2.5e3.5
1.0 4.0e5.0
e 2.75e3.5
0.12 0.02
2.0 e
0.5e0.7 e
0.03 e
0.035 e
N. Wu et al. / Vacuum 86 (2012) 2059e2063
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Fig. 5. Microstructure of (a) brazed joint, (b) magnified SEM morphology of zone “A”, (c) magnified SEM morphology of zone “B” and (d) brazed region of NiCr base layer.
Microstructure of the brazed joint of NieNiCr laminated composite to Cr18eNi8 steel is shown in Fig. 5. Chemical composition of the special phases in the brazed region was detected by EDS as shown in Table 2. There was a chain of small shapeless blocks (point 1 in Fig. 5a) formed in Ni cover layer near the brazed region. EDS result indicates that the Ni content of the blocks was 85.84% and the B content was 12.23%. So the precipitation in Ni cover layer was identified as Ni3B [8]. Many white particles (point 2 in Fig. 5b) precipitated in NiCr base layer under the influence of heating pattern. The Ni:Cr ratio of the particles was 2:1. Thus, the white precipitates were determined to be Ni2Cr based on NieCr phase diagram [9]. There was an interface (about 10 mm) consisting of fine granules (point 3 in Fig. 5c) in the diffusion affected zone of Cr18eNi8 steel. The fine granules were rich in B, Cr and Fe and it were identified as (Fe, Cr)B precipitates [10]. Point 4 in the brazed region of NiCr base layer was rich in Si and it was considered to be g-Ni(Si) solid solution. The dark gray phase (point 5 in Fig. 5d) in the brazed region was rich in B (28.82%). According to the binary NieB phase diagram [11], the solubility of B in Ni is (0.3, at.%). Therefore, the dark gray phase was identified as Ni3B. Point 6 was identified as g-Ni(Si) solid solution. But the Si content in point 6 was higher than point 4. The Ni3B in the center of the brazed region and the g-Ni(Si) solid solution around (point 6) composed the divorced eutectic [12,13]. Therefore, the brazed region consisted of g-Ni solid solution and NieB divorced eutectic. There was B remained in the brazed region of NiCr base layer with the formation of Ni3B. However, B preferred to diffuse into Ni
cover layer along the grain boundaries. The diffusion of B from the filler metal into Ni cover layer was faster than that from the filler metal into NiCr base layer. There was not enough B remaining in the brazed region of Ni cover layer to form the Ni3B. So the brazed region consisted of g-Ni solid solution completely. 3.2. Elemental distribution Elemental distribution across the brazed region of NiCr base layer was measured by EDS and the result is shown in Fig. 6.
Table 2 EDS result of chemical composition in the brazed joint (at.%). Point
Ni
Cr
Fe
Si
B
Possible phase
1 2 3 4 5 6
85.84 62.03 e 79.70 64.83 76.90
e 37.97 26.32 12.40 5.29 11.31
e e 34.42 2.74 1.05 2.28
1.93 e e 5.16 e 9.51
12.23
B-enriched g-Ni Ni2Cr (Fe, Cr)B Si-enriched g-Ni NieB eutectic Si-enriched g-Ni
39.26 e 28.82 e
Fig. 6. Elemental distribution across the brazed region of NiCr base layer (SSZ e solid solution zone; BZ e brittle zone).
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Cr18eNi8 steel due to the nonsymmetrical interdiffusion as shown in Fig. 5a. 3.3. Microhardness of the brazed joint
Fig. 7. Microhardness distribution (50 g) through the brazed joint.
There were enrichment of Si in the solid solution zone and depletion of Si in the brittle zone. Due to the solubility of Si (17, at.%) in Ni is higher than B (0.3, at.%), the g-Ni solid solution was rich in Si and B was ejected into the leaving melt [14]. The Ni, Cr, Fe and Si distributions transformed gradually from the brazed region to the NiCr base layer. An excellent bonding between the brazed region and NiCr base layer was obtained without the formation of intermetallics at the interface. Fe and Cr diffused from Cr18eNi8 steel to the brazed region while Ni, Si and B diffused in the opposite direction. Due to the low solubility of B in Fe, boride precipitated in Cr18eNi8 steel near the brazed region. According to the microstructure (Fig. 5), B diffused inside the crystals of Cr18eNi8 near the brazed region and diffused along the grain boundaries far away from the brazed region. However, Si and B diffused much faster than Fe and Cr due to their smaller atomic radius, resulting in the nonsymmetrical interdiffusion [10]. Microporosities formed in the brazed region close to
Microhardness distribution through the brazed joint of NieNiCr laminated composite to Cr18eNi8 steel (Fig. 7) was measured via DHV-1000 microsclerometer, with a constant load of 50 g for 10 s. Microhardness of Cr18eNi8 steel was 200 HV. Microhardness of the interface in Cr18eNi8 steel near the brazed region was 250 HV, which was higher than Cr18eNi8 steel. The microhardness increment was due to the precipitation of fine boride granules. Microhardness of the g-Ni solid solution was 200 HV. Microhardness of the brittle Ni3B in the brazed region was 530 HV. The brittle phase played an important in the shear strength of the brazed region. Shear strength of the NieNiCr laminated composite/Cr18eNi8 steel joint brazed with NieCreSieB filler metal was 170 MPa. However, shear strength of the joint brazed with NieCreP filler metal was 137 MPa [15]. It was for this reason that Si and B diffused faster than P resulting in the less brittle phase in the brazed region. Brazed region of Ni cover layer was composed of Ni solid solution and the microhardness was 200 HV. Microhardness of the interfacial region (500 HV) was significantly higher than Ni cover layer (80 HV). It was attributed to the existence of Ni3B at the interfacial region. 3.4. Bonding behavior Fig. 8 describes the bonding behavior for the vacuum brazed joint of NieNiCr laminated composite to Cr18eNi8 steel using NieCreSieB amorphous foil. It can be described as four stages. In the first stage, the filler metal became liquid due to its low melting point. There were dissolution and diffusion between the filler metal and base metals. The melting point depressant elements Si and B diffused outward from the molten filler metal to base materials while Fe, Ni and Cr diffused in the opposite direction as shown in Fig. 8a. Therefore, the melting point of the molten filler
Fig. 8. Bonding behavior for vacuum brazed joint of NieNiCr laminated composite to Cr18-Ni8 steel: (a) elemental diffusion; (b) isothermal solidification stage; athermal solidification stage for the formation of divorced eutectic: (c) g-Ni and (d) Ni3B.
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metal near the base materials increased, which induced the isothermal solidification (Fig. 8b). Isothermal solidification involved the formation of g-Ni solid solution. The isothermal solidification stage continued as the outward diffusion process continued during the holding brazing time. The solubility of Si (17, at.%) in Ni is higher than B (0.3, at.%). Thus, the g-Ni solid solution was rich in Si and the remaining melt was rich in B. The NiCr base layer dissolved in the melting filler metal. Due to the same matrix of them, the interface was not clear and an excellent bonding was obtained. Si and B preferred to diffuse along grain boundaries in Ni cover layer. However, due to the small size of B, the intragranular diffusion of B was essential [14]. As the diffusion continued, Ni3B precipitated in the Ni cover layer due to the low solubility of B in Ni. It was rich in Fe, Cr and B in Cr18eNi8 steel near the brazed region, so an interface consisting of (Fe, Cr)B precipitates was formed. Many white Ni2Cr particles precipitated in NiCr base layer [9]. As the temperature decreased in the cooling stage, the athermally solidified stage occurred. The g-Ni solid solution of the divorced eutectic grew on the existing g-Ni solid solution as shown in Fig. 8c. In the last stage (Fig. 8d), the leaving melt was further enriched in boron and solidified as blocky Ni3B. 4. Conclusions NieNiCr laminated composite is featured with low density and good resistance in high temperature and corrosive environment. It is an attractive candidate for the modern aero-engines to improve the thrust to weight ratio. Brazed joint of NieNiCr laminated composite to Cr18eNi8 steel was obtained by vacuum brazing with NieCreSieB amorphous foil at 1080 C for 20 min in a vacuum of 104 Pa. The brazed region consisted of g-Ni solid solution and NieB divorced eutectic. There were mainly dissolution and diffusion between filler metal and NiCr base layer. An excellent bonding with the shear strength of 170 MPa was obtained. An interface of blocky Ni3B formed in Ni cover layer close to the brazed region. Many (Fe, Cr)B granules precipitated and composed an interface of 10 mm in Cr18eNi8 steel close to the brazed region. Ni2Cr particles
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precipitated in NiCr base layer under the influence of heating pattern. Acknowledgements This project was supported by the National Natural Science Foundation of China (51175303) and Doctoral Fund of Ministry of Education of China (200804220020). References [1] Kim JK, Yu TX. Forming and failure behavior of coated, laminated and sandwiched sheet metals: a review. J Mater Process Technol 1997;63(1e3):33e42. [2] Chen YL. Microlaminated composite: the next materials of turbine blade. Aviat Maint Eng 2001;5:10e2. [3] Missori S, Murdolo F, Sili A. Single-pass laser beam welding of clad steel plate. Weld J 2004;83(2):65e71. [4] Li YJ, Xia CZ, Puchkov UA, Wang J. Weldability of pure-Ni/NiCr laminated composite and 18-8 stainless steel. Trans China Weld Inst 2010;31(2): 13e6. [5] Hamill J. Weld techniques give powder metal a different dimension. Met Powder Rep 2007;62(5):22e4. 26e27, 29e31. [6] Gao HY, He YH, Shen PZ, Jiang Y, Huang BY, Xu NP. Welding of FeAl porous materials and stainless steel. Chin J Nonferrous Met 2009;19(1):90e5. [7] Peter K. Sokolowski, Thomas F. Murphy, Bruce A. Lindsley. Considerations in sinter-brazing PM components. In: Powder Metal World Congress and Exhibition. Hollywood, Florida; 2010. [8] Gale WF, Wallach ER. Microstructure development in transient liquid-phase bonding. Metall Trans A 1991;22:2451e7. [9] Shankar V, Bhanu K, Rao S, Mannan SL. Microstructure and mechanical properties of Inconel 625 purealloy. J Nucl Mater 2001;288:222e32. [10] Ou CL, Liaw DW, Du YC, Shiue RK. Brazing of 422 stainless steel using the AWS classification BNi-2 Braze alloy. J Mater Sci 2006;41(19):6353e61. [11] Nagasaki Seizo, Hirabayashi Makoto. Binary alloy phase-diagrams; 2002. Tokyo, Japan. [12] Tung SK, Lim LC, Lai MO. Microstructural evolution and control in BNi-4 brazed joints of nickel 270. Scripta Mater 1995;33(8):1253e9. [13] Pouranvari M, Ekrami A, Kokabi AH. Microstructure development during transient liquid phase bonding of GTD-111 nickel-based superalloy. J Alloys Compd 2008;461(1e2):641e7. [14] Yuan XJ, Kang CY, Kim MB. Microstructure and XRD analysis of brazing joint for duplex stainless steel using a Ni-Si-B filler metal. Mater Character 2009; 60(9):923e31. [15] Wu N, Li YJ, Wang J, Puchkov UA. Vacuum brazing of super-Ni/NiCr laminated composite to Cr8-Ni8 steel with NiCrP filler metal. J Mater Process Technol 2012;212(4):794e800.