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An investigation on microstructure evolution and mechanical properties during liquid state diffusion bonding of Al2024 to Ti–6Al–4V
Materials Characterization 98 (2014) 113–118
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
An investigation on microstructure evolution and mechanical properties during liquid state diffusion bonding of Al2024 to Ti–6Al–4V Majid Samavatian a,⁎, Ayoub Halvaee b, Ahmad Ali Amadeh b, Alireza Khodabandeh a a b
Department of Materials Engineering, Tehran Science and Research Branch, Islamic Azad University, Tehran, Iran School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 23 July 2014 Received in revised form 6 October 2014 Accepted 21 October 2014 Available online 23 October 2014 Keywords: Bonding Isothermal solidification Diffusion Intermetallic compound Interface
a b s t r a c t Joining mechanism of Ti/Al dissimilar alloys was studied during liquid state diffusion bonding process using Cu/ Sn/Cu interlayer at 510 °C under vacuum of 7.5 × 10−5 Torr for various bonding times. The microstructure and compositional changes in the joint zone were analyzed by scanning electron microscopy equipped with energy dispersive spectroscopy and X-ray diffraction. Microhardness and shear strength tests were also applied to study the mechanical properties of the joints. It was found that with an increase in bonding time, the elements of interlayer diffused into the parent metals and formed various intermetallic compounds at the interface. Diffusion process led to the isothermal solidification and the bonding evolution in the joint zone. The results from mechanical tests showed that microhardness and shear strength values have a straight relation with bonding time so that the maximum shear strength of joint was obtained for a bond made with 60 min bonding time. © 2014 Elsevier Inc. All rights reserved.
1. Introduction Ti and Al alloys are widely used materials in aviation industry [1,2]. In some specific locations, the supplementary characteristics of Ti and Al are requisite to obtain structures with lower weight, increased strength and lower expenditure [3]. Joining of Ti and Al is so difficult by conventional fusion welding processes because there are great differences in the performance and physical properties of these two alloys [4]. Hence, diffusion bonding process can be an acceptable method for Ti/Al couples [5]. Solid state diffusion bonding of Ti and Al was performed by Fukutomi et al. [6]. Their results showed that a brittle intermetallic compound (TiAl3) formed at the interface was detrimental to the mechanical properties of joint. Moreover, the formation of the brittle intermetallics such as TiAl, TiAl3 and Ti3Al was reported by other researchers [5,7]. On the other hand, Prescott R and Graham MJ [8] showed that the existence of oxide film on the aluminum surface impeded good metal to metal contact. In order to overcome these deficiencies arisen during the solid state diffusion bonding process, liquid state diffusion bonding was proponed. This process is highly tolerant to the existence of a faying surface oxide film [9]. Furthermore, the presence of an interlayer between the parent metals prevents from direct contact of Al and Ti, thereupon the formation of mentioned intermetallics can be controlled. Some researchers have studied liquid state diffusion bonding of Ti/Al joint. Woong et al. [10] showed that bonding proceeded by the removal ⁎ Corresponding author. E-mail address: [email protected] (M. Samavatian).
http://dx.doi.org/10.1016/j.matchar.2014.10.018 1044-5803/© 2014 Elsevier Inc. All rights reserved.
of surface oxide layer on Al alloy and wetting of the molten interlayer on Al, followed by subsequence isothermal solidification. The work by Alhazza and Khan [11] indicated that the liquid state diffusion bonding of Al alloy to Ti alloy using a 100 μm thick Sn-based interlayer was successful. They reported that the use of a thinner interlayer will limit the extent of intermetallics formed at the interface and promote mechanical properties of the joint. Hereupon, Kenevisi and Mousavi Khoie [12,13] studied Ti/Al bonding with a 50 μm thick Sn-based interlayer. Although their results showed that the bonding process was performed successfully, the shear strength of joint was lower than that of Alhazza and Khan [11]. Considering the works done by the researchers, it can be concluded that Sn is an appropriate interlayer to join Al alloy to Ti alloy, but interlayer thickness for a constant bonding time should be between 50 and 100 μm to obtain desired mechanical properties. In this work, liquid state diffusion bonding of Al2024 and Ti–6Al–4V with 80 μm thick pure Sn interlayer was carried out. The effect of bonding parameters on the microstructure of the joint zone and their corresponding effect on mechanical properties were studied. 2. Experimental procedure Pure Sn foil with the thickness of 80 μm was applied to join Ti–6Al– 4V and Al2024. Accurate compositions of parent metals are listed in Table 1. At first, the samples with dimensions of 10 × 10 × 5 mm3 were fabricated by wire cut machine and then ground up to 1500 grit SiC papers. A thin layer of Cu on the samples can prevent oxidation and increase the wettability of interlayer [11]. Hence, a 6 μm thick Cu layer
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Table 1 Chemical composition of parent metals. wt.% Alloys Al2024 Ti–6Al–4V
Al Base 6.2
Ti 0.03 Base
Cu 4.89 0
Table 2 EDS analysis (wt.%) of regions for bond made in 20 min shown in Fig. 1a. V 0 4.01
Mg 2.49 0
Fe 0.31 0.03
Si 0.28 0
Zn 0.1 0
Others b0.1 b0.2
A B C D
Sn
Al
Cu
Ti
Mg
91.55 93 0 0
5.85 3.2 51.2 50.35
2.6 2.41 46.13 47.30
0 1.39 0 0.3
0 0 2.67 2.05
was electrodeposited onto Al and Ti alloys [14,15]. The interlayer was kept between the base metals and a fixture was then employed for a uniaxial pressure of 2 MPa onto the samples. The bonding process was carried out in a furnace under vacuum of 7.5 × 10−5 Torr and temperature of 510 °C. The bonding time varied from 20 up to 60 min. The cross-sections of bonded samples were polished and etched by a reagent with the composition of 5 ml HNO3, 3 ml HCl, 2 ml HF, and 90 ml distilled water [16]. Scanning electron microscope (SEM), electron dispersive spectroscopy (EDS) and X-ray diffraction (XRD) were applied
Fig. 2. SEM micrograph for a bond made in 40 min.
Table 3 EDS analysis (wt. %) of regions for bond made in 40 min shown in Fig. 2.
A B C
Fig. 1. a) SEM micrograph for a bond made in 20 min and b) eutectic-like structure at the interface for a bond made in 20 min.
Sn
Al
Cu
Ti
Mg
88.5 1.59 21
6.24 49.13 52.35
3.81 48.75 5.76
1.12 0 20.89
0.33 0.53 0
Fig. 3. SEM micrograph for a bond made in 50 min.
M. Samavatian et al. / Materials Characterization 98 (2014) 113–118
Fig. 4. SEM micrograph for a bond made in 60 min.
Table 4 EDS analysis (wt.%) of regions for bond made in 60 min shown in Fig. 4.
A B
Sn
Al
Cu
r
0 0.23
40.7 11.75
1.01 3.72
58.29 84.3
to characterize the joint zone and identify intermetallic compounds at the interface. The shear strength of samples was evaluated according to ASTM standard D1002-99 [17] by the MTS30/MH tensile testing machine at a cross-head speed of 1 mm/min. Three samples were tested for each parameter. Microhardness testing was also conducted by the Mitutoyo HM tester with a load of 50 g and 12 second load time. 3. Results and discussion 3.1. Microstructure and compositional changes Fig. 1 shows the SEM micrograph of a bond made in 20 min. EDS analysis of selected regions in Fig. 1a is represented in Table 2. At first, the solid state diffusion of Cu into Ti alloy occurred; while according
115
to Al2XXX-Cu diagram obtained by Yan et al. [18], due to the diffusion of Cu into Al alloy side, the formation of eutectic liquid phases can be the initial stage of bonding process at Al/Cu interface. As marked in Fig. 1a, region A is rich in tin. This indicates the molten tin could diffuse along the grain boundaries of Al alloy. Detecting the remaining interlayer in the joint zone (see region B), of course, a significant amount of tin was identified by EDS. Region C includes Al, Cu and Mg elements. Considering the ternary phase diagram of these elements acquired by Styles et al. [19], the formation of θ-Al2Cu and S-Al2CuMg eutectic phases across the grain boundaries can be expected. Also, region D has a chemical composition similar to region C. This region is located along the interface and has an interesting eutectic-like structure (see Fig. 1b). Fig. 2 represents the SEM micrograph of a bond made in 40 min. EDS analysis of different regions in Fig. 2 is listed in Table 3. The substantial presence of tin in region A indicates that the low amount of interlayer is likewise remained at the interface after 40 min bonding time. Region B mainly consists of Al and Cu. According to the Al–Cu binary phase diagram [20], existence of Al2Cu in this region can be proved. Region C is consistently placed across the Ti alloy interface. The presence of various elements such as Al, Cu, Ti and Sn in this region suggests the formation of varied intermetallic compounds. With an increase in bonding time, complete diffusion of molten Sn into the parent metals occurred. Thereupon, the diffusion mechanism is dominated at a rate somewhere between the diffusivity of liquid and solid in the joint zone. This phenomenon is called isothermal solidification. The bonding process is continued until all of the liquid (molten Sn) has disappeared (see Fig. 3). With completion of the bonding process, the width of the joint zone decreases to the minimum value and just a thin intermetallic layer remains at the interface. SEM micrograph of a bond made in 60 min is shown in Fig. 4. Two distinct regions were identified at the interface whose analyses are listed in Table 4. Region A mainly contains Al and Ti suggesting the formation of the intermetallics such as TiAl in the joint zone. This region is located along the interface and has a thickness of about 1 μm indicating fiddling formation of this kind of intermetallic compound in the joint zone. The presence of Cu and significant amount of Ti in region B separated this region from region A and caused the formation of a different intermetallic at the interface. Fig. 5 illustrates the distribution of elements perpendicular to the interface for a bond made in 60 min. Considering the figure, the width of the joint zone reaches about 10–12 μm. Substantial diffusion of molten tin into the parent metals and subsequently isothermal solidification during the bonding process are the most important reasons for reduction in the joint width after 60 min bonding time. Comparing the line scanning result with other works done by researchers indicates that the primary interlayer thickness has a straight relation with the final
Fig. 5. EDS line scan of a bond made in 60 min.
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joint width [11–13]. A thin interlayer may provide less intermetallics to produce a sound metallurgical bond at the interface while increasing interlayer thickness leads to the formation of a thick reaction layer in the joint zone. 3.2. Identifying intermetallic compounds The presence of several elements namely Al, Cu, Ti, Sn and Mg in the joint zone might lead to the formation of various intermetallic compounds at the interface. Recognition of these intermetallics is not possible by EDS analysis and therefore, XRD was used for this purpose. Fractured surfaces of a bond made at 60 min bonding time were analyzed by XRD (See Fig. 6). The results from the XRD spectrums show that varied intermetallic compounds such as Al3Cu12Sn, Cu3Sn, Al2Cu, Al2CuMg, Ti3Al, Sn3Ti5, TiAl and Cu3Ti were produced at the interface. Also, there are several peaks indicating the existence of Al and Ti on
the fractured area. As it is expected, weak peak intensities of TiAl and Ti3Al were observed in the XRD spectrums signifying low amount of these brittle intermetallic compounds in the joint zone. Direct contact of Ti alloy and Al alloy in solid state diffusion bonding leads to uncontrollable propagation of these intermetallics in the joint zone which can be harmful for mechanical properties of the bond [6,7], whereas the presence of an interlayer between parent metals in liquid state diffusion bonding process is the most important reason for reduction in the formation of TiAl and Ti3Al at the interface. 3.3. Mechanical characterization of the joints Microhardness profiles of the bonds made at 20, 40 and 60 min are illustrated in Fig. 7. Considering the general disposition of microhardness profiles, it can be determined that the hardness of joint zone increases with an increase in bonding time. Low hardness value
Fig. 6. XRD spectrum of a) fractured Al alloy surface and b) fractured Ti alloy surface.
M. Samavatian et al. / Materials Characterization 98 (2014) 113–118
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Fig. 7. Microhardness profiles of the bonds made in 20, 40 and 60 min.
(78 VHN) in the center of a joint made in 20 min indicates the presence of remaining interlayer at the interface while with increasing bonding time to 60 min; hardness value goes up to 154 VHN. This can be attributed to the formation of various intermetallic compounds at the interface [5]. The hardness trend up to near 100 μm inside the Al alloy for the bond made in 20 min is inordinate. In some places the hardness values have lowered to about 75 VHN and in some other places it has raised to Al2024 hardness. Probably, the diffusion of Sn into the grain boundaries of the Al alloy and the formation of local Sn-rich regions in the vicinity of interface causes this disorder in hardness value. As it is observed, increasing bonding time up to 60 min makes the hardness value in this region (100 μm inside Al2024) be increased. This is due to the formation of solid solutions and isothermal solidification during the bonding process. The hardness values at the 20 μm of Ti alloy for the bonds made at 20, 40 and 60 min are 301, 338 and 344 VHN, respectively. This indicates that with increasing bonding time and interdiffusion of Ti and Sn, solid solutions form at the Ti side and joint width decreases. The results from hardness distribution across the joint zone are compatible with those observed in the published works [11,12]. Fig. 8 illustrates the shear strength of joints as a function of bonding time. The attained data shows that increasing bonding time to 40 min
caused a swift rise in shear strength value but then it stayed almost constant at 36 MPa. This indicates that a minimum bonding time of 40 min is required to obtain acceptable bond strength. This is compatible with microstructure analysis (Fig. 2) showing that the significant amount of molten Sn diffuses into the parent metals and the bonding evolution proceeds considerably. The maximum shear strength in this work was higher than the values achieved by Kenevisi and Mousavi Khoie [13] (30 MPa) and Alhazza et al. (19 MPa) [21]. The fractured surface for Ti–6Al–4V and Al2024 sides with bonding time of 60 min is shown in Fig. 9. The fractographs suggest a brittle fracture mode. Also, the formation of the voids and the crack propagation occurred along the intermetallic phases which were formed in the bond interface. 4. Conclusion 1- From this work, it can be concluded that the liquid state diffusion bonding of Ti–6Al–4V to Al2024 using Cu coatings and pure Sn as an interlayer was performed successfully. 2- Diffusion of Cu and Sn into parent metals led to the formation of various intermetallic compounds namely, Al3Cu12Sn, Cu3Sn, Al2Cu,
Fig. 8. Shear strength of the bonds as a function of bonding time.
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4- With increasing bonding time, the shear strength of joint increases which is due to the diffusion of interlayer into parent metals and completion of bonding process.
References
Fig. 9. SEM fractographs of the bond made in 60 min: a) Al2024 surface and b) Ti–6Al–4V surface.
Al2CuMg, Ti3Al, Sn3Ti5 and TiAl at the interface and these were recognized by XRD analysis. 3- Hardness value at the interface has a straight relation with bonding time. An increase in bonding time leads to the formation of intermetallic compounds in the joint zone which can be the most important reason for the increase of hardness value.
[1] A. Heinz, A. Haszler, C. Keidel, S. Moldenhauer, R. Benedictus, W. Miller, Recent development in aluminium alloys for aerospace applications, Mater. Sci. Eng. A 280 (2000) 102–107. [2] K.H. Rendigs, Aluminum structures used in aerospace — status and prospects, Mater. Sci. Forum 242 (1997) 11–24. [3] J.P. Immarigeon, R.P. Holt, A.K. Koul, L. Zhao, W. Wallace, J.C. Beddoes, Lightweight materials for aircraft applications, Mater. Charact. 35 (1995) 41–67. [4] Yanni Wei, Jinglong Li, Jiangtao Xiong, Huang Fu, Fusheng Zhang, Syed Hamid Raza, Joining aluminum to titanium alloy by friction stir lap welding with cutting pin, Mater. Charact. 71 (2012) 1–5. [5] R. Jiangwei, L. Yajiang, F. Tao, Microstructure characteristics in the interface zone of Ti/Al diffusion bonding, Mater. Lett. 56 (2002) 647–652. [6] H. Fukutomi, M. Nakamura, T. Suzuki, S. Takagi, S. Kikuchi, Void formation by the reactive diffusion of titanium and aluminum foils, Mater. Trans. JIM 41 (2000) 1244–1246. [7] Y. Wei, W. Aiping, Z. Guisheng, R. Jialie, Formation process of the bonding joint in Ti/Al diffusion bonding, Mater. Sci. Eng. A 480 (2008) 456–463. [8] R. Prescott, M.J. Graham, Formation of aluminum oxide scales on high-temperature alloys, Oxid. Met. 38 (1992) 233–254. [9] O.C. Grant, D.S. Carl, Overview of transient liquid phase and partial transient liquid phase bonding, J. Mater. Sci. 46 (2011) 5305–5323. [10] H.S. Woong, H.B. Ha, H.H. Soon, Microstructure and bonding mechanism of Al/Ti bonded joint using Al–10Si–1Mg filler metal, Mater. Sci. Eng. A 355 (2003) 231–240. [11] A.N. Alhazaa, T.I. Khan, Diffusion bonding of Al7075 to Ti–6Al–4V using Cu coatings and Sn–3.6Ag–1Cu interlayers, J. Alloys Compd. 494 (2010) 351–358. [12] M.S. Kenevisi, S.M. Mousavi Khoie, An investigation on microstructure and mechanical properties of Al7075 to Ti–6Al–4V Transient Liquid Phase (TLP) bonded joint, Mater. Des. 38 (2012) 19–25. [13] M.S. Kenevisi, S.M. Mousavi Khoie, A study on the effect of bonding time on the properties of Al7075 to Ti–6Al–4V diffusion bonded joint, Mater. Lett. 76 (2012) 144–146. [14] M. Zheng, M. Willey, H. Song, A.C. West, Copper Electrodeposition Onto Titanium and Other Substrates: Effect of Bath Composition, San Antonio, TX, United States, 2004. 264–272. [15] F.A. Lowenheim, Modern Electroplating, third edition Wiley-Interscience Publication, New York, 1974. [16] George F. Vander Voort, ASM Handbook, Metallography and Microstructures, vol. 9, ASM International, 2004. [17] ASTM standard D1002, Standard Test Method for Apparent Shear Strength of Singlelap-joint Adhesively Bonded Metal Specimens by Tension Loading (Metal-to-metal), ASTM International, 1999. [18] X.Y. Yan, Y.A. Chang, F.Y. Xie, S.L. Chen, F. Zhang, S. Daniel, Calculated phase diagrams of aluminum alloys from binary Al–Cu to multicomponent commercial alloys, J. Alloys Compd. 320 (2001) 151–160. [19] M.J. Styles, C.R. Hutchinson, Y. Chen, A. Deschamps, T.J. Bastow, The coexistence of two S (Al2CuMg) phases in Al–Cu–Mg alloys, Acta Mater. 60 (2012) 6940–6951. [20] J.L. Murray, The aluminum–copper system, Int. Mater. Rev. 30 (1985) 211–234. [21] A. Alhazaa, T. Khan, I. Haq, Transient liquid phase (TLP) bonding of Al7075 to Ti–6Al–4V alloy, Mater. Charact. 61 (2010) 312–313.
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
An investigation on microstructure evolution and mechanical properties during liquid state diffusion bonding of Al2024 to Ti–6Al–4V Majid Samavatian a,⁎, Ayoub Halvaee b, Ahmad Ali Amadeh b, Alireza Khodabandeh a a b
Department of Materials Engineering, Tehran Science and Research Branch, Islamic Azad University, Tehran, Iran School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 23 July 2014 Received in revised form 6 October 2014 Accepted 21 October 2014 Available online 23 October 2014 Keywords: Bonding Isothermal solidification Diffusion Intermetallic compound Interface
a b s t r a c t Joining mechanism of Ti/Al dissimilar alloys was studied during liquid state diffusion bonding process using Cu/ Sn/Cu interlayer at 510 °C under vacuum of 7.5 × 10−5 Torr for various bonding times. The microstructure and compositional changes in the joint zone were analyzed by scanning electron microscopy equipped with energy dispersive spectroscopy and X-ray diffraction. Microhardness and shear strength tests were also applied to study the mechanical properties of the joints. It was found that with an increase in bonding time, the elements of interlayer diffused into the parent metals and formed various intermetallic compounds at the interface. Diffusion process led to the isothermal solidification and the bonding evolution in the joint zone. The results from mechanical tests showed that microhardness and shear strength values have a straight relation with bonding time so that the maximum shear strength of joint was obtained for a bond made with 60 min bonding time. © 2014 Elsevier Inc. All rights reserved.
1. Introduction Ti and Al alloys are widely used materials in aviation industry [1,2]. In some specific locations, the supplementary characteristics of Ti and Al are requisite to obtain structures with lower weight, increased strength and lower expenditure [3]. Joining of Ti and Al is so difficult by conventional fusion welding processes because there are great differences in the performance and physical properties of these two alloys [4]. Hence, diffusion bonding process can be an acceptable method for Ti/Al couples [5]. Solid state diffusion bonding of Ti and Al was performed by Fukutomi et al. [6]. Their results showed that a brittle intermetallic compound (TiAl3) formed at the interface was detrimental to the mechanical properties of joint. Moreover, the formation of the brittle intermetallics such as TiAl, TiAl3 and Ti3Al was reported by other researchers [5,7]. On the other hand, Prescott R and Graham MJ [8] showed that the existence of oxide film on the aluminum surface impeded good metal to metal contact. In order to overcome these deficiencies arisen during the solid state diffusion bonding process, liquid state diffusion bonding was proponed. This process is highly tolerant to the existence of a faying surface oxide film [9]. Furthermore, the presence of an interlayer between the parent metals prevents from direct contact of Al and Ti, thereupon the formation of mentioned intermetallics can be controlled. Some researchers have studied liquid state diffusion bonding of Ti/Al joint. Woong et al. [10] showed that bonding proceeded by the removal ⁎ Corresponding author. E-mail address: [email protected] (M. Samavatian).
http://dx.doi.org/10.1016/j.matchar.2014.10.018 1044-5803/© 2014 Elsevier Inc. All rights reserved.
of surface oxide layer on Al alloy and wetting of the molten interlayer on Al, followed by subsequence isothermal solidification. The work by Alhazza and Khan [11] indicated that the liquid state diffusion bonding of Al alloy to Ti alloy using a 100 μm thick Sn-based interlayer was successful. They reported that the use of a thinner interlayer will limit the extent of intermetallics formed at the interface and promote mechanical properties of the joint. Hereupon, Kenevisi and Mousavi Khoie [12,13] studied Ti/Al bonding with a 50 μm thick Sn-based interlayer. Although their results showed that the bonding process was performed successfully, the shear strength of joint was lower than that of Alhazza and Khan [11]. Considering the works done by the researchers, it can be concluded that Sn is an appropriate interlayer to join Al alloy to Ti alloy, but interlayer thickness for a constant bonding time should be between 50 and 100 μm to obtain desired mechanical properties. In this work, liquid state diffusion bonding of Al2024 and Ti–6Al–4V with 80 μm thick pure Sn interlayer was carried out. The effect of bonding parameters on the microstructure of the joint zone and their corresponding effect on mechanical properties were studied. 2. Experimental procedure Pure Sn foil with the thickness of 80 μm was applied to join Ti–6Al– 4V and Al2024. Accurate compositions of parent metals are listed in Table 1. At first, the samples with dimensions of 10 × 10 × 5 mm3 were fabricated by wire cut machine and then ground up to 1500 grit SiC papers. A thin layer of Cu on the samples can prevent oxidation and increase the wettability of interlayer [11]. Hence, a 6 μm thick Cu layer
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Table 1 Chemical composition of parent metals. wt.% Alloys Al2024 Ti–6Al–4V
Al Base 6.2
Ti 0.03 Base
Cu 4.89 0
Table 2 EDS analysis (wt.%) of regions for bond made in 20 min shown in Fig. 1a. V 0 4.01
Mg 2.49 0
Fe 0.31 0.03
Si 0.28 0
Zn 0.1 0
Others b0.1 b0.2
A B C D
Sn
Al
Cu
Ti
Mg
91.55 93 0 0
5.85 3.2 51.2 50.35
2.6 2.41 46.13 47.30
0 1.39 0 0.3
0 0 2.67 2.05
was electrodeposited onto Al and Ti alloys [14,15]. The interlayer was kept between the base metals and a fixture was then employed for a uniaxial pressure of 2 MPa onto the samples. The bonding process was carried out in a furnace under vacuum of 7.5 × 10−5 Torr and temperature of 510 °C. The bonding time varied from 20 up to 60 min. The cross-sections of bonded samples were polished and etched by a reagent with the composition of 5 ml HNO3, 3 ml HCl, 2 ml HF, and 90 ml distilled water [16]. Scanning electron microscope (SEM), electron dispersive spectroscopy (EDS) and X-ray diffraction (XRD) were applied
Fig. 2. SEM micrograph for a bond made in 40 min.
Table 3 EDS analysis (wt. %) of regions for bond made in 40 min shown in Fig. 2.
A B C
Fig. 1. a) SEM micrograph for a bond made in 20 min and b) eutectic-like structure at the interface for a bond made in 20 min.
Sn
Al
Cu
Ti
Mg
88.5 1.59 21
6.24 49.13 52.35
3.81 48.75 5.76
1.12 0 20.89
0.33 0.53 0
Fig. 3. SEM micrograph for a bond made in 50 min.
M. Samavatian et al. / Materials Characterization 98 (2014) 113–118
Fig. 4. SEM micrograph for a bond made in 60 min.
Table 4 EDS analysis (wt.%) of regions for bond made in 60 min shown in Fig. 4.
A B
Sn
Al
Cu
r
0 0.23
40.7 11.75
1.01 3.72
58.29 84.3
to characterize the joint zone and identify intermetallic compounds at the interface. The shear strength of samples was evaluated according to ASTM standard D1002-99 [17] by the MTS30/MH tensile testing machine at a cross-head speed of 1 mm/min. Three samples were tested for each parameter. Microhardness testing was also conducted by the Mitutoyo HM tester with a load of 50 g and 12 second load time. 3. Results and discussion 3.1. Microstructure and compositional changes Fig. 1 shows the SEM micrograph of a bond made in 20 min. EDS analysis of selected regions in Fig. 1a is represented in Table 2. At first, the solid state diffusion of Cu into Ti alloy occurred; while according
115
to Al2XXX-Cu diagram obtained by Yan et al. [18], due to the diffusion of Cu into Al alloy side, the formation of eutectic liquid phases can be the initial stage of bonding process at Al/Cu interface. As marked in Fig. 1a, region A is rich in tin. This indicates the molten tin could diffuse along the grain boundaries of Al alloy. Detecting the remaining interlayer in the joint zone (see region B), of course, a significant amount of tin was identified by EDS. Region C includes Al, Cu and Mg elements. Considering the ternary phase diagram of these elements acquired by Styles et al. [19], the formation of θ-Al2Cu and S-Al2CuMg eutectic phases across the grain boundaries can be expected. Also, region D has a chemical composition similar to region C. This region is located along the interface and has an interesting eutectic-like structure (see Fig. 1b). Fig. 2 represents the SEM micrograph of a bond made in 40 min. EDS analysis of different regions in Fig. 2 is listed in Table 3. The substantial presence of tin in region A indicates that the low amount of interlayer is likewise remained at the interface after 40 min bonding time. Region B mainly consists of Al and Cu. According to the Al–Cu binary phase diagram [20], existence of Al2Cu in this region can be proved. Region C is consistently placed across the Ti alloy interface. The presence of various elements such as Al, Cu, Ti and Sn in this region suggests the formation of varied intermetallic compounds. With an increase in bonding time, complete diffusion of molten Sn into the parent metals occurred. Thereupon, the diffusion mechanism is dominated at a rate somewhere between the diffusivity of liquid and solid in the joint zone. This phenomenon is called isothermal solidification. The bonding process is continued until all of the liquid (molten Sn) has disappeared (see Fig. 3). With completion of the bonding process, the width of the joint zone decreases to the minimum value and just a thin intermetallic layer remains at the interface. SEM micrograph of a bond made in 60 min is shown in Fig. 4. Two distinct regions were identified at the interface whose analyses are listed in Table 4. Region A mainly contains Al and Ti suggesting the formation of the intermetallics such as TiAl in the joint zone. This region is located along the interface and has a thickness of about 1 μm indicating fiddling formation of this kind of intermetallic compound in the joint zone. The presence of Cu and significant amount of Ti in region B separated this region from region A and caused the formation of a different intermetallic at the interface. Fig. 5 illustrates the distribution of elements perpendicular to the interface for a bond made in 60 min. Considering the figure, the width of the joint zone reaches about 10–12 μm. Substantial diffusion of molten tin into the parent metals and subsequently isothermal solidification during the bonding process are the most important reasons for reduction in the joint width after 60 min bonding time. Comparing the line scanning result with other works done by researchers indicates that the primary interlayer thickness has a straight relation with the final
Fig. 5. EDS line scan of a bond made in 60 min.
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joint width [11–13]. A thin interlayer may provide less intermetallics to produce a sound metallurgical bond at the interface while increasing interlayer thickness leads to the formation of a thick reaction layer in the joint zone. 3.2. Identifying intermetallic compounds The presence of several elements namely Al, Cu, Ti, Sn and Mg in the joint zone might lead to the formation of various intermetallic compounds at the interface. Recognition of these intermetallics is not possible by EDS analysis and therefore, XRD was used for this purpose. Fractured surfaces of a bond made at 60 min bonding time were analyzed by XRD (See Fig. 6). The results from the XRD spectrums show that varied intermetallic compounds such as Al3Cu12Sn, Cu3Sn, Al2Cu, Al2CuMg, Ti3Al, Sn3Ti5, TiAl and Cu3Ti were produced at the interface. Also, there are several peaks indicating the existence of Al and Ti on
the fractured area. As it is expected, weak peak intensities of TiAl and Ti3Al were observed in the XRD spectrums signifying low amount of these brittle intermetallic compounds in the joint zone. Direct contact of Ti alloy and Al alloy in solid state diffusion bonding leads to uncontrollable propagation of these intermetallics in the joint zone which can be harmful for mechanical properties of the bond [6,7], whereas the presence of an interlayer between parent metals in liquid state diffusion bonding process is the most important reason for reduction in the formation of TiAl and Ti3Al at the interface. 3.3. Mechanical characterization of the joints Microhardness profiles of the bonds made at 20, 40 and 60 min are illustrated in Fig. 7. Considering the general disposition of microhardness profiles, it can be determined that the hardness of joint zone increases with an increase in bonding time. Low hardness value
Fig. 6. XRD spectrum of a) fractured Al alloy surface and b) fractured Ti alloy surface.
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Fig. 7. Microhardness profiles of the bonds made in 20, 40 and 60 min.
(78 VHN) in the center of a joint made in 20 min indicates the presence of remaining interlayer at the interface while with increasing bonding time to 60 min; hardness value goes up to 154 VHN. This can be attributed to the formation of various intermetallic compounds at the interface [5]. The hardness trend up to near 100 μm inside the Al alloy for the bond made in 20 min is inordinate. In some places the hardness values have lowered to about 75 VHN and in some other places it has raised to Al2024 hardness. Probably, the diffusion of Sn into the grain boundaries of the Al alloy and the formation of local Sn-rich regions in the vicinity of interface causes this disorder in hardness value. As it is observed, increasing bonding time up to 60 min makes the hardness value in this region (100 μm inside Al2024) be increased. This is due to the formation of solid solutions and isothermal solidification during the bonding process. The hardness values at the 20 μm of Ti alloy for the bonds made at 20, 40 and 60 min are 301, 338 and 344 VHN, respectively. This indicates that with increasing bonding time and interdiffusion of Ti and Sn, solid solutions form at the Ti side and joint width decreases. The results from hardness distribution across the joint zone are compatible with those observed in the published works [11,12]. Fig. 8 illustrates the shear strength of joints as a function of bonding time. The attained data shows that increasing bonding time to 40 min
caused a swift rise in shear strength value but then it stayed almost constant at 36 MPa. This indicates that a minimum bonding time of 40 min is required to obtain acceptable bond strength. This is compatible with microstructure analysis (Fig. 2) showing that the significant amount of molten Sn diffuses into the parent metals and the bonding evolution proceeds considerably. The maximum shear strength in this work was higher than the values achieved by Kenevisi and Mousavi Khoie [13] (30 MPa) and Alhazza et al. (19 MPa) [21]. The fractured surface for Ti–6Al–4V and Al2024 sides with bonding time of 60 min is shown in Fig. 9. The fractographs suggest a brittle fracture mode. Also, the formation of the voids and the crack propagation occurred along the intermetallic phases which were formed in the bond interface. 4. Conclusion 1- From this work, it can be concluded that the liquid state diffusion bonding of Ti–6Al–4V to Al2024 using Cu coatings and pure Sn as an interlayer was performed successfully. 2- Diffusion of Cu and Sn into parent metals led to the formation of various intermetallic compounds namely, Al3Cu12Sn, Cu3Sn, Al2Cu,
Fig. 8. Shear strength of the bonds as a function of bonding time.
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4- With increasing bonding time, the shear strength of joint increases which is due to the diffusion of interlayer into parent metals and completion of bonding process.
References
Fig. 9. SEM fractographs of the bond made in 60 min: a) Al2024 surface and b) Ti–6Al–4V surface.
Al2CuMg, Ti3Al, Sn3Ti5 and TiAl at the interface and these were recognized by XRD analysis. 3- Hardness value at the interface has a straight relation with bonding time. An increase in bonding time leads to the formation of intermetallic compounds in the joint zone which can be the most important reason for the increase of hardness value.
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