- Email: [email protected]
Diffusion bonding between Ti–6Al–4V alloy and ferritic stainless steel
Materials Letters 61 (2007) 1747 – 1750 www.elsevier.com/locate/matlet
Diffusion bonding between Ti–6Al–4V alloy and ferritic stainless steel Bulent Kurt a,⁎, Nuri Orhan a , Ertan Evin b , Adnan Çalik c a
b
University of Firat, Faculty of Technical Education, Department of Metallurgy Education, Elazig, Turkey University of Firat, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Elazig, Turkey c University of SDU, Faculty of Technical Education, Department of Machine Education, Isparta, Turkey Received 31 March 2006; accepted 22 July 2006 Available online 15 August 2006
Abstract In the present study, Ti–6Al–4V alloy was diffusion bonded to a ferritic stainless steel. The effect of bonding temperature on the microstructural development across the joint region was investigated. After diffusion bonding, microstructural analysis including metallographic examination, energy dispersive spectrograph (EDS) and shear strength was conduced. From the results, it was seen that bonding on the temperature was affecting the Fe and Ti mutual diffusion which controls the interface microstructure. The microstructure of the interface region was formed, consisting of Fe and Ti intermetallics. © 2006 Elsevier B.V. All rights reserved. Keywords: Diffusion bonding; Ti–6Al–4V; Ferritic stainless steel; Interface microstructure
1. Introduction Diffusion bonding (DB) is a joining process wherein the principal mechanism for joint formation is solid-state diffusion. Coalescence of the faying surfaces is accomplished through the application of pressure at elevated temperature. No melting and only limited macroscopic deformation or relative motion of the parts occur during welding [1,2]. Joining of dissimilar and similar materials has already been widely carried out by diffusion bonding with metals, such as aluminum and titanium alloys and stainless steels. It is generally used to join materials for special purposes where a relatively large contact area is involved [3,4]. The joints between Ti and stainless steel find wide applications in the nuclear industry. The conventional fusion welding of these two materials results in segregation, stress concentration sites and formation of intermetallics at interfaces such as Fe2Ti and FeTi. All these deleterious effects lead to failure of components in service condition [4]. It is well known that Ti alloys are prone to brittle intermetallic phases when they bonded to stainless steel [5–9]. These intermetallic phases have detrimental effects on the joint strength [10]. ⁎ Corresponding author. Tel.: +90 424 2370000/4254; fax: +90 424 2184674. E-mail address: [email protected] (B. Kurt). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.123
We thought that these detrimental intermetallic phases or their thickness can be reduced with the application of appropriate temperature and time for diffusion bonding. From this point, the Ti–6Al–4Valloy has been bonded to a ferritic stainless steel. The reason for choosing this couple is to combine the cheap and corrosion resistant properties of ferritic stainless steel with the excellent room-temperature or elevated-temperature strength, creep, or fracture toughness characteristics of Ti–6Al–4V. The microstructure of the interface was studied, and the mechanical strength of the joints was evaluated by shear tests. 2. Experimental procedure In the study, Ti–6Al–4V alloy and a ferritic stainless steel were used for diffusion bonding. Table 1 shows the chemical compositions of these materials. Ti–6Al–4V and ferritic stainless steel were received in the form of plates of 16 and 10 mm thickness, respectively. For diffusion bonding, specimens were cut into 10 × 10 × 10 mm dimensions from these materials using a sensitive specimen cutting machine. Prior to diffusion bonding, one face of each specimen was processed in order to achieve a predetermined degree of roughness using 1200 mesh grinding paper. The samples were then degreased in an ultrasonic bath using acetone.
1748
B. Kurt et al. / Materials Letters 61 (2007) 1747–1750
Table 1 Chemical composition of the specimen used for diffusion bonding Alloy
Ti–6Al–4V Ferritic S. S.
Composition (wt.%) Al
V
Fe
O
C
N
Ti
Cr
Ni
Mo
Si
Mn
P
6.08 –
4.02 –
0.22 Bal.
0.18 –
0.02 0.24
0.01 –
Bal. –
– 17.7
– 0.24
– 0.02
– 0.3
– 0.2
– 0.01
Diffusion bonding was made in an argon atmosphere in a bonding chamber equipped with an induction heating unit. Coupled metals were heated to the bonding temperature with a heating rate of 40 °C min− 1. The bonds were made under a pressure of 5 MPa at three different temperatures, 885, 930 and 980 °C for 30 min. Once the bonding process was completed the samples were cooled at a rate of 15 °C/min to room temperature before removal from the chamber. After bonding, the bonded specimens were transversely cut through the bond and metallographically polished to 3 μm diamond finish for metallographic examination. For microstructural examination, Ti–6Al–4V was etched by Keller's reagent and the ferritic stainless steel was etched electrolytically in a solution of 50 ml HNO3 and 50 ml pure water. Metallographic observations were performed by SEM microscopy and changes in the interface region were investigated using
energy dispersive spectroscopy (EDS). The bonds were mechanically tested by using a shear test process. For this aim, the bonded specimens were further machined to produce shear test specimens with dimensions of 8 × 8 × 10 mm, to eliminate edge effects on test data. An Instron tensile testing machine set at a crosshead speed of 0.5 mm min− 1 was used for the shear tests. The microhardness values were measured on both sides of the bonded specimens using a 15 g load. 3. Results and discussion 3.1. Microstructure The micrographs for the specimens bonded at 885, 930 and 980 °C for 30 min are shown in Figs. 1, 2 and 3 respectively. As can be seen from the micrographs, all specimens exhibited a good bonding along
Fig. 1. SEM micrographs of the bond interface of the specimen bonded at 885 °C.
B. Kurt et al. / Materials Letters 61 (2007) 1747–1750
the interface. From the micrograph in Fig. 1a, it is seen that there occurred four different regions, namely: parent material microstructure (α+β Ti) of Ti–6Al–4V, stabilized β phase microstructure in Ti–6Al– 4V side, intermetallic phases in interface zone and parent material microstructure of stainless steel. Among them stabilized β phase microstructure in Ti–6Al–4V side is observed at all temperatures. Fe and Cr diffused from stainless steel to Ti alloy side are strong β stabilizers of Ti [2,5]. The widths of stabilized β phase regions were determined as 20, 30 and 40 μm with increasing test temperature. Concentration profiles of these elements across the diffusion zone obtained from EDS analysis are shown in Fig. 1a. A decrease is seen in the diffused amount of Ti into the stainless steel side as a result of probably binding of Ti to iron as a result of the formation of FeTi intermetallics in the interface region. From SEM micrographs of the diffusion zone two layers are observed within the interface zone (clearly seen in Figs. 1b, 2, and 3). I. Dark region consists of 63.71% Ti, 28.02% Fe, 4.42% Cr, 2.9% V and 0.95% Al and is approximately 2.5 μm wide as can be seen from the EDS analysis (Fig. 1b) in this field. This composition corresponds to the FeTi intermetallic phase. FeTi layer has been reduced with increasing test temperature (clearly seen in Figs. 1b, 2 and 3). II. Bright region consists of 33.2% Ti, 55.27% Fe, 10.95% Cr and 0.58% Al and is approximately 2 μm wide from the EDS analysis taken in this field (Fig. 1b). This composition was determined to be the Fe2Ti intermetallic phase by the Fe–Cr–Ti ternary phase diagram [7]. The widths of these phases have been measured from the interfaces in the SEM photographs (Figs. 1b, 2 and 3). These results show that the width of the FeTi phase is reduced but the width of Fe2Ti is raised by increasing the test temperature. 3.2. Shear testing Shear strength of the bonds was determined as 135, 180 and 187 MPa according to the increasing test temperature. The highest shear strength value was produced (187 MPa) at the test temperature of 980 °C. The shear strength of the bonds was raised by increasing the test temperature. From the interface micrographs in Figs. 1b, 2 and 3 it could be seen that the thickness of the FeTi intermetallic phase layer was decreased by increasing the process temperature, but the thickness of the Fe2Ti layer in the interface region was raised. From the literature it is well known that brittle intermetallic compounds in the reaction zone are mainly responsible for lowering the strength of the diffusion-
1749
Fig. 3. SEM micrographs of the bond interface of the specimen bonded at 980 °C.
bonded couples [5]. However, these results showed that the FeTi intermetallic phase has a more detrimental effect than the Fe2Ti intermetallic phase over the shear strength of the bonds. A further evaluation of the bonds formed at different temperatures was obtained by SEM examination of the mode of fracture of the shear test pieces. The failure at the bonding temperature of 885 °C was related to the FeTi intermetallic phase on the bonding line. This can be seen clearly from the EDS analysis taken on the fracture surface (Fig. 4).
4. Conclusions The effects of bonding temperature on the microstructure and shear strength of diffusion bonds between Ti–6Al–4V alloy and ferritic stainless steel were investigated. The following results were obtained as a result of increasing the bonding temperatures. I. All bonds were microvoid and microcrack free. II. The bonding zone contains intermetallic phases such as FeTi and Fe2Ti in the diffusion-bonded joints. III. The width of FeTi intermetallic phase decreases with increasing test temperature while that of Fe2Ti rises. IV. The highest shear strength (187 MPa) was obtained for the bonding temperature of 980 °C. Acknowledgment This study was supported financially by the Firat University Scientific Research Fund. References
Fig. 2. SEM micrographs of the bond interface of the specimen bonded at 930 °C.
[1] M.M. Schawartz, D.F. Paulonýs, Welding Handbook V.3 Diffusion Welding and Brazing, 1984, pp. 312–336. [2] W.M. Murray, C.B. Cliff, ASM handbook online, Fundamentals of Diffusion Bonding, Rockwell International Science Center, 1993. [3] M.J. Cox, M.J. Kim, R.W. Carpenter, Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science 33 (2002) 437–442. [4] M. Ghosh, K. Bhanumurthy, G.B. Kale, J. Krishnan, S. Chatterjee, Journal of Nuclear Materials 322 (2003) 235–241.
1750
B. Kurt et al. / Materials Letters 61 (2007) 1747–1750
Fig. 4. Fractographs of the diffusion-bonded joints for the temperature of 885 °C. [5] A.M. Kliauga, D. Travessa, M. Ferrante, Materials Characterization 46 (2001) 65–74. [6] M. Ghosh, S. Chatterjee, Materials Characterization 54 (2005) 327–337. [7] M. Ghosh, S. Chatterjee, B. Mishra, Materials Science & Engineering. A, Structural Materials: Properties, Microstructure and Processing 363 (2003) 268–274.
[8] N. Orhan, T.I. Khan, M. Eroglu, Scripta Materialia 45 (2001) 441–446. [9] B. Aleman, I. Gultierrez, J.J. Urcola, Scripta Materialia 36 (1997) 509–515. [10] P. He, J.C. Feng, B.G. Zhang, Y.Y. Qian, Materials Characterization 48 (2002) 401–406.
Diffusion bonding between Ti–6Al–4V alloy and ferritic stainless steel Bulent Kurt a,⁎, Nuri Orhan a , Ertan Evin b , Adnan Çalik c a
b
University of Firat, Faculty of Technical Education, Department of Metallurgy Education, Elazig, Turkey University of Firat, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Elazig, Turkey c University of SDU, Faculty of Technical Education, Department of Machine Education, Isparta, Turkey Received 31 March 2006; accepted 22 July 2006 Available online 15 August 2006
Abstract In the present study, Ti–6Al–4V alloy was diffusion bonded to a ferritic stainless steel. The effect of bonding temperature on the microstructural development across the joint region was investigated. After diffusion bonding, microstructural analysis including metallographic examination, energy dispersive spectrograph (EDS) and shear strength was conduced. From the results, it was seen that bonding on the temperature was affecting the Fe and Ti mutual diffusion which controls the interface microstructure. The microstructure of the interface region was formed, consisting of Fe and Ti intermetallics. © 2006 Elsevier B.V. All rights reserved. Keywords: Diffusion bonding; Ti–6Al–4V; Ferritic stainless steel; Interface microstructure
1. Introduction Diffusion bonding (DB) is a joining process wherein the principal mechanism for joint formation is solid-state diffusion. Coalescence of the faying surfaces is accomplished through the application of pressure at elevated temperature. No melting and only limited macroscopic deformation or relative motion of the parts occur during welding [1,2]. Joining of dissimilar and similar materials has already been widely carried out by diffusion bonding with metals, such as aluminum and titanium alloys and stainless steels. It is generally used to join materials for special purposes where a relatively large contact area is involved [3,4]. The joints between Ti and stainless steel find wide applications in the nuclear industry. The conventional fusion welding of these two materials results in segregation, stress concentration sites and formation of intermetallics at interfaces such as Fe2Ti and FeTi. All these deleterious effects lead to failure of components in service condition [4]. It is well known that Ti alloys are prone to brittle intermetallic phases when they bonded to stainless steel [5–9]. These intermetallic phases have detrimental effects on the joint strength [10]. ⁎ Corresponding author. Tel.: +90 424 2370000/4254; fax: +90 424 2184674. E-mail address: [email protected] (B. Kurt). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.123
We thought that these detrimental intermetallic phases or their thickness can be reduced with the application of appropriate temperature and time for diffusion bonding. From this point, the Ti–6Al–4Valloy has been bonded to a ferritic stainless steel. The reason for choosing this couple is to combine the cheap and corrosion resistant properties of ferritic stainless steel with the excellent room-temperature or elevated-temperature strength, creep, or fracture toughness characteristics of Ti–6Al–4V. The microstructure of the interface was studied, and the mechanical strength of the joints was evaluated by shear tests. 2. Experimental procedure In the study, Ti–6Al–4V alloy and a ferritic stainless steel were used for diffusion bonding. Table 1 shows the chemical compositions of these materials. Ti–6Al–4V and ferritic stainless steel were received in the form of plates of 16 and 10 mm thickness, respectively. For diffusion bonding, specimens were cut into 10 × 10 × 10 mm dimensions from these materials using a sensitive specimen cutting machine. Prior to diffusion bonding, one face of each specimen was processed in order to achieve a predetermined degree of roughness using 1200 mesh grinding paper. The samples were then degreased in an ultrasonic bath using acetone.
1748
B. Kurt et al. / Materials Letters 61 (2007) 1747–1750
Table 1 Chemical composition of the specimen used for diffusion bonding Alloy
Ti–6Al–4V Ferritic S. S.
Composition (wt.%) Al
V
Fe
O
C
N
Ti
Cr
Ni
Mo
Si
Mn
P
6.08 –
4.02 –
0.22 Bal.
0.18 –
0.02 0.24
0.01 –
Bal. –
– 17.7
– 0.24
– 0.02
– 0.3
– 0.2
– 0.01
Diffusion bonding was made in an argon atmosphere in a bonding chamber equipped with an induction heating unit. Coupled metals were heated to the bonding temperature with a heating rate of 40 °C min− 1. The bonds were made under a pressure of 5 MPa at three different temperatures, 885, 930 and 980 °C for 30 min. Once the bonding process was completed the samples were cooled at a rate of 15 °C/min to room temperature before removal from the chamber. After bonding, the bonded specimens were transversely cut through the bond and metallographically polished to 3 μm diamond finish for metallographic examination. For microstructural examination, Ti–6Al–4V was etched by Keller's reagent and the ferritic stainless steel was etched electrolytically in a solution of 50 ml HNO3 and 50 ml pure water. Metallographic observations were performed by SEM microscopy and changes in the interface region were investigated using
energy dispersive spectroscopy (EDS). The bonds were mechanically tested by using a shear test process. For this aim, the bonded specimens were further machined to produce shear test specimens with dimensions of 8 × 8 × 10 mm, to eliminate edge effects on test data. An Instron tensile testing machine set at a crosshead speed of 0.5 mm min− 1 was used for the shear tests. The microhardness values were measured on both sides of the bonded specimens using a 15 g load. 3. Results and discussion 3.1. Microstructure The micrographs for the specimens bonded at 885, 930 and 980 °C for 30 min are shown in Figs. 1, 2 and 3 respectively. As can be seen from the micrographs, all specimens exhibited a good bonding along
Fig. 1. SEM micrographs of the bond interface of the specimen bonded at 885 °C.
B. Kurt et al. / Materials Letters 61 (2007) 1747–1750
the interface. From the micrograph in Fig. 1a, it is seen that there occurred four different regions, namely: parent material microstructure (α+β Ti) of Ti–6Al–4V, stabilized β phase microstructure in Ti–6Al– 4V side, intermetallic phases in interface zone and parent material microstructure of stainless steel. Among them stabilized β phase microstructure in Ti–6Al–4V side is observed at all temperatures. Fe and Cr diffused from stainless steel to Ti alloy side are strong β stabilizers of Ti [2,5]. The widths of stabilized β phase regions were determined as 20, 30 and 40 μm with increasing test temperature. Concentration profiles of these elements across the diffusion zone obtained from EDS analysis are shown in Fig. 1a. A decrease is seen in the diffused amount of Ti into the stainless steel side as a result of probably binding of Ti to iron as a result of the formation of FeTi intermetallics in the interface region. From SEM micrographs of the diffusion zone two layers are observed within the interface zone (clearly seen in Figs. 1b, 2, and 3). I. Dark region consists of 63.71% Ti, 28.02% Fe, 4.42% Cr, 2.9% V and 0.95% Al and is approximately 2.5 μm wide as can be seen from the EDS analysis (Fig. 1b) in this field. This composition corresponds to the FeTi intermetallic phase. FeTi layer has been reduced with increasing test temperature (clearly seen in Figs. 1b, 2 and 3). II. Bright region consists of 33.2% Ti, 55.27% Fe, 10.95% Cr and 0.58% Al and is approximately 2 μm wide from the EDS analysis taken in this field (Fig. 1b). This composition was determined to be the Fe2Ti intermetallic phase by the Fe–Cr–Ti ternary phase diagram [7]. The widths of these phases have been measured from the interfaces in the SEM photographs (Figs. 1b, 2 and 3). These results show that the width of the FeTi phase is reduced but the width of Fe2Ti is raised by increasing the test temperature. 3.2. Shear testing Shear strength of the bonds was determined as 135, 180 and 187 MPa according to the increasing test temperature. The highest shear strength value was produced (187 MPa) at the test temperature of 980 °C. The shear strength of the bonds was raised by increasing the test temperature. From the interface micrographs in Figs. 1b, 2 and 3 it could be seen that the thickness of the FeTi intermetallic phase layer was decreased by increasing the process temperature, but the thickness of the Fe2Ti layer in the interface region was raised. From the literature it is well known that brittle intermetallic compounds in the reaction zone are mainly responsible for lowering the strength of the diffusion-
1749
Fig. 3. SEM micrographs of the bond interface of the specimen bonded at 980 °C.
bonded couples [5]. However, these results showed that the FeTi intermetallic phase has a more detrimental effect than the Fe2Ti intermetallic phase over the shear strength of the bonds. A further evaluation of the bonds formed at different temperatures was obtained by SEM examination of the mode of fracture of the shear test pieces. The failure at the bonding temperature of 885 °C was related to the FeTi intermetallic phase on the bonding line. This can be seen clearly from the EDS analysis taken on the fracture surface (Fig. 4).
4. Conclusions The effects of bonding temperature on the microstructure and shear strength of diffusion bonds between Ti–6Al–4V alloy and ferritic stainless steel were investigated. The following results were obtained as a result of increasing the bonding temperatures. I. All bonds were microvoid and microcrack free. II. The bonding zone contains intermetallic phases such as FeTi and Fe2Ti in the diffusion-bonded joints. III. The width of FeTi intermetallic phase decreases with increasing test temperature while that of Fe2Ti rises. IV. The highest shear strength (187 MPa) was obtained for the bonding temperature of 980 °C. Acknowledgment This study was supported financially by the Firat University Scientific Research Fund. References
Fig. 2. SEM micrographs of the bond interface of the specimen bonded at 930 °C.
[1] M.M. Schawartz, D.F. Paulonýs, Welding Handbook V.3 Diffusion Welding and Brazing, 1984, pp. 312–336. [2] W.M. Murray, C.B. Cliff, ASM handbook online, Fundamentals of Diffusion Bonding, Rockwell International Science Center, 1993. [3] M.J. Cox, M.J. Kim, R.W. Carpenter, Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science 33 (2002) 437–442. [4] M. Ghosh, K. Bhanumurthy, G.B. Kale, J. Krishnan, S. Chatterjee, Journal of Nuclear Materials 322 (2003) 235–241.
1750
B. Kurt et al. / Materials Letters 61 (2007) 1747–1750
Fig. 4. Fractographs of the diffusion-bonded joints for the temperature of 885 °C. [5] A.M. Kliauga, D. Travessa, M. Ferrante, Materials Characterization 46 (2001) 65–74. [6] M. Ghosh, S. Chatterjee, Materials Characterization 54 (2005) 327–337. [7] M. Ghosh, S. Chatterjee, B. Mishra, Materials Science & Engineering. A, Structural Materials: Properties, Microstructure and Processing 363 (2003) 268–274.
[8] N. Orhan, T.I. Khan, M. Eroglu, Scripta Materialia 45 (2001) 441–446. [9] B. Aleman, I. Gultierrez, J.J. Urcola, Scripta Materialia 36 (1997) 509–515. [10] P. He, J.C. Feng, B.G. Zhang, Y.Y. Qian, Materials Characterization 48 (2002) 401–406.