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Correlation between microstructure, mechanical properties, and brazing temperature of steel to titanium joint
Journal of Alloys and Compounds 487 (2009) 639–645
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Correlation between microstructure, mechanical properties, and brazing temperature of steel to titanium joint Ahmed Elrefaey ∗ , Wolfgang Tillmann Institute of Materials Engineering, Faculty of Mechanical Engineering, Dortmund University of Technology, Leonhard-Euler-Str.2. 44227, Dortmund, Germany
a r t i c l e
i n f o
Article history: Received 15 April 2009 Received in revised form 6 August 2009 Accepted 8 August 2009 Available online 15 August 2009 Keywords: Steel Titanium Brazing Microstructure Mechanical properties X-ray diffraction
a b s t r a c t An attempt was made to simultaneously correlate the microstructure, mechanical properties, and brazing temperature of steel to titanium brazed joint. Brazing was conducted by means of silver-based filler (Incusil ABA) at a temperature range of 650–850 ◦ C for 15 min in a high vacuum furnace. It has been observed that the main interaction layer at the steel/incusil interfacial layer was the FeTi phase. Meanwhile, at the titanium/incusil side, ␣–Ti in addition to Ti–Cu phases were observed. The maximum shear strength of the joints was 113 MPa for the specimens brazed at 750 ◦ C. At this temperature, the fracture morphology presented a ductile–brittle fracture through the Ag-rich area, that is contradictory to the joint brazed at higher temperatures, which showed a brittle fracture at the titanium/incusil interfacial area owing to the presence of harmful Ti–Cu intermetalic phases. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The light weight, high strength-to-weight ratio, and excellent corrosion resistance of titanium and its alloys has led to the use of these materials in aerospace, chemical, marine, and medical industries. These properties also can enhance the performance of many weapon systems, such as ships and combat vehicles. The major disadvantages of this metal are its high cost and difficulty in joining it with other materials. Among the methods for overcoming these disadvantages is joining titanium to steel. These joints, if successfully made, will posses all the requirements for high quality joints, and also reduce the amount of materials needed thus making them economically attractive and viable. Titanium–steel combination represent one of the combinations which are difficult to weld, since a number of brittle intermetallics are formed due to the limited solubility of Fe and Ti in the solid state and hence, bond strength deteriorated [1–3]. Brazing is one of the most suitable ways to obtain good welds of titanium to steel because it involves only the melting of the filler material. Thus, problems which typically occur when dissimilar metals are fused are avoided. However, titanium is a highly reactive element, on which a non-wetting scale is readily generated when exposed to elevated temperatures and it reacts readily with many elements to form brittle intermetallics. To avoid the intru-
sion of detrimental impurities such as oxygen and nitrogen, it is mandatory to join titanium alloys in an inert environment. Hence, titanium/steel joints are brazed in a vacuum furnace. It has been reported that pure silver, silver base alloys, titanium base alloys, and copper base alloys have been used to braze titanium and titanium alloys [4–7]. Since the melting point of pure silver greatly decreased by alloying copper into silver matrix and a lower brazing temperature is preferred for most brazing processes, especially for brazing titanium, incusil ABA braze alloy was chosen as filler metal for vacuum brazing of titanium to steel. This filler metal has solidus 605 ◦ C and liquidus 715 ◦ C, which are significantly lower values than those of traditional silver based alloys. It is reported that In and Ti are added in addition to other elements to adjust the alloy behaviour, e.g., to improve the activity of the element, reduce the melting temperature, and increase the alloy fluidity [8]. The objective of this study is to evaluate incusil-ABA filler metal by brazing commercially pure titanium to steel in a vacuum furnace. The relation between brazing temperatures, interfacial microstructure, and the resultant mechanical properties of the brazed joints were extensively studied. 2. Experimental 2.1. Materials
∗ Corresponding author. E-mail address: [email protected] (A. Elrefaey). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.08.029
The materials employed in this investigation were 2 mm thick plates of commercially pure titanium (ASTM grade II) strengthened by small additions of oxygen, nitrogen, carbon, hydrogen, and iron, and low carbon steel. The nominal composi-
640
A. Elrefaey, W. Tillmann / Journal of Alloys and Compounds 487 (2009) 639–645
Fig. 3. Microstructure of brazed joint at 650 ◦ C.
before brazing and then sandwiched between the overlapped areas of the parent metals. 2.2. Brazing condition
Fig. 1. Schematic illustration of the brazed joints; (a) joint for microstructure analysis and (b) joint for shear test.
tion of the commercially pure titanium, in weight percent, was 0.08% C, 0.20% Fe, 0.03% N, 0.18% O, 0.02% H, and balance Ti. On the other hand, the composition of steel, in weight percent, was 0.12% C, 0.05% P, 0.05% S, 0.05% Mn, and balance Fe. The plates were cut into 30 mm × 25 mm × 2 mm chips for shear strength testing and 14mm × 10 mm × 2 mm chips for microstructure analysis. The schematic diagram of the specimens is shown in Fig. 1. The brazing was carried out using 200 m thick foil of incusil ABA (Ag–27.25Cu–12.5In–1.25Ti, wt.%). The parent metals were first polished with SiC papers up to 1000 grit and subsequently cleaned by an ultrasonic bath using acetone as the solvent prior to furnace brazing. The brazing foils were cleaned in acetone
Titanium to steel overlap width was kept at 6 mm since it is recommended to use lap width of no more than 3 times the thickness of the base metals to achieve high strength for the joint [9]. The joints were fixed with stainless steel clamp, and then carefully placed into a vacuum furnace (SCHMETZ GmbH). Initially, the samples were heated up to a temperature 50 K below the solidus temperature of the filler alloy for a dwell time of 5 min. This step aimed at achieving the thermal equilibrium of the couple. The sample was then heated up to the brazing temperature, as schematically illustrated in Fig. 2. Brazing experiments were carried out at 650, 750, 800, and 850 ◦ C to study the effect of the brazing temperature on the metallurgical and mechanical properties of the joint. At all brazing temperatures, the dwell time was 15 min. 2.3. Microstructure observation and mechanical testing The polished microstructures of the joints were examined with a light optical microscope and a scanning electron microscope (SEM), equipped with an energy dispersive X-ray (EDX) for chemical analysis. The hardness measurement was performed with the help of a Vickers hardness testing machine with 25 s impressing time. Tensile shear specimens were machined from brazed lap joints as suggested by Harvey et al. [10]. The test was carried out at room temperature and the displacement rate was 0.5 mm/s. Three samples were used to calculate the average shear strength of the joint. X-ray diffraction (XRD) analysis of the fracture surfaces was used to detect and analyse the reaction products. The X-ray scan rate was set at 4◦ /min, and its range was between 20◦ and 150◦ .
3. Results and discussion 3.1. Microstructure and phase analysis
Fig. 2. Schematic illustration of the brazing temperature and time profile.
Preliminary studies to braze at 650 ◦ C led to a higher tendency to form big pores or voids in the brazed seam, especially at steel/incusil interface. Although not all joints brazed at a temperature of 650 ◦ C have showed such tendency, the pores were clearly observed in some samples. A typical form of this defect is shown in Fig. 3. The brazing filler metal could not melt completely at a braz-
Fig. 4. Microstructure of brazed joint at 750 ◦ C (a), 800 ◦ C (b), and 850 ◦ C (c).
A. Elrefaey, W. Tillmann / Journal of Alloys and Compounds 487 (2009) 639–645
641
Table 2 Chemical analysis of areas shown in Fig. 6, at.%.
Table 1 Chemical analysis of areas shown in Fig. 5, at.%. Average chemical analysis
Suggested phase
Ag
Ti
Cu
In
Fe
73.13 3.20 1.04
– 4.33 23.35
19.47 90.12 72.45
6.22 2.35 3.16
1.17 – –
Ag solid solution Cu solid solution Cu4 Ti
ing temperature of 650 ◦ C since this temperature lies in between the solidus and liquidus temperature of the filler metal. A representative cross section microstructure of the brazed joints at brazing temperatures of 750, 800, and 850 ◦ C is shown
Symbol
A B C D E
A B C D E
Fig. 6. SEM microstructure of titanium/steel joint brazed at 750 ◦ C.
Suggested phase
Ag
Ti
Cu
In
1.86 5.21 5.48 2.77 1.38
1.74 26.24 77.24 40.50 32.61
2.67 25.11 13.08 56.43 65.72
– 3.71 4.20 0.30 0.29
Fe 93.73 39.73 – – –
␣Fe + (Cu) FeTi + Cu ␣–Ti Cu4 Ti3 Cu2 Ti
Table 3 Chemical analysis of areas shown in Fig. 8, at.%. Symbol
Fig. 5. Microstructure of the braze material in the joint brazed at 750 ◦ C.
Average chemical analysis
Average chemical analysis
Suggested phase
Ag
Ti
Cu
In
1.23 2.03 0.42 2.76 3.28
2.16 38.56 92.38 45.57 38.27
3.23 18.87 7.02 51.67 54.60
0.95 – 0.17 – 3.85
Fe 92.43 40.54 – – –
␣Fe + (Cu) FeTi + Cu ␣–Ti CuTi Cu4 Ti3
in Fig. 4. It is clear that a continuous reaction layer between the brazed seam and the titanium substrate was formed. Besides, the reaction layer has increased in thickness after the brazing temperature had been prolonged. At the same time, the steel/braze interface showed tiny interaction layers at all brazing temperatures. Additionally, coarse grain structure was formed at the steel boundary to the silver brazed foil. This coarse grain structure results from diffusion growth accompanied by recrystallization of the steel substrate at high temperatures. It is also important to note that the thickness of the brazed area gradually decreased by increasing the temperature due to the increase in overflow of the Ag-rich liquid during brazing. Brazing is usually performed through melting of the braze alloy, dissolution of parent metals, and solidification of the braze metal. In respect to our study, the main elements in concern are silver and copper, which are derived from the braze alloy and titanium, derived from the titanium substrate. Iron seems to be of no relevance in this process, owing to its very low solubility in silver and copper. It may only react with titanium to form Fe–Ti phases. Due to a wide immiscibility region in the Ag–Cu–Ti ternary system [11],
Fig. 7. The Cu–Ti binary alloy phase diagram.
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Fig. 9. XRD from fractured surfaces of the titanium side at brazing temperatures of 750 ◦ C (a) and 800 ◦ C (b).
Fig. 8. SEM microstructure of titanium/steel joint brazed at 800 ◦ C.
transformations are located close to the boundary of the binary Cu–Ti system and in a narrow Ag-rich region. Fig. 5 illustrates the microstructure of the braze material in the joint brazed at 750 ◦ C. Based on the EDX analysis results, see Table 1, the joints primarily consist of a matrix of Ag solid solution (Ag) alloyed with Cu and In. The ternary Ag–Cu–In phase diagram [12] indicates the presence
of this phase under equilibrium condition. Additionally, a mixed structure containing copper solid solution phase (Cu) and Cu4 Ti was clearly formed in the Ag matrix. Tiny precipitates are randomly distributed in the Ag solid solution matrix. This phase is too fine to be accurately analyzed by EDS. However, its chemical analysis suggested that it could be Ti3 In since the incorporation of titanium in braze metal increases at high temperatures and the solubility of In in Ti is quite high at high temperatures and decreases at lower temperatures according to Ti–In binary phase diagram [13]. In order to characterize the reaction layers of the joint brazed at 750 ◦ C, an SEM image was performed at the steel/incusil and tita-
Fig. 10. Hardness distributions along the joint produced at a temperature of 750 ◦ C (a) and 800 ◦ C (b).
A. Elrefaey, W. Tillmann / Journal of Alloys and Compounds 487 (2009) 639–645
nium/incusil interfacial layers as it is shown in Fig. 6. Moreover, a quantitative overview of the chemical analyses for the different areas is listed in Table 2. The microstructure of the steel/incusil alloy revealed two distinct phases. The diffusion of copper into the steel substrate produces a solid solution of limited solubility [14]. It is assumed that eutectoid transformation ␥ → ␣Fe + (Cu) took place with a high content of ␣Fe with respect to (Cu). The narrow eutectoid structure (∼ =5 m in width) is clearly shown in Fig. 6a. This is indicated by area A which presented high percentages of Fe in respect to Cu. Besides, a very thin interaction layer (area B) was formed close to area A. Based on the chemical analysis of this area, it is expected to contain FeTi + Cu intermetallic phase. According to Van Beek et al. [15], the Fe–Ti–Cu ternary alloy phase diagram suggests that nearly 38 at.% Cu could be dissolved in FeTi. At the same time, titanium/braze alloy revealed three phases as it is shown in Fig. 6b. It is worth noting that the migration of Cu in the titanium substrate lowers the eutectoid transformation temperature of Ti and a thin ␣–Ti phase forms by the decomposition of -Ti during cooling, as reported by Voort [16]. The ␣–Ti phase is shown by region C in Fig. 6b. The ratios between Ti and Cu in layer D and E are close to the Cu4 Ti3 and Cu2 Ti phases which formed at the interface. Based on the Cu–Ti binary alloy phase diagram shown in Fig. 7 [14], a series of invariant reactions upon cooling of the liquid are listed below: CuTi + L ↔ Cu4 Ti3 (peritectic, 925 ± 10 ◦ C).
643
Fig. 11. Relation between average shear strength of the joint and brazing temperature.
Cu4 Ti3 + L ↔ Cu2 Ti (peritectic, 890 ± 10 ◦ C). L ↔ Cu2 Ti + Cu4 Ti (eutectic, 875 ± 10 ◦ C). The formation of interfacial Cu4 Ti3 phase is caused by the peritectic reaction between the molten braze and the CuTi phase. Accordingly, the interfacial Cu4 Ti3 phase subsequently reacts with the residual molten braze upon the cooling cycle, and the Cu2 Ti phase is formed via peritectic reaction. Finally, the residual molten braze is completely solidified into Cu2 Ti and Cu4 Ti via the eutec-
Fig. 12. Fracture paths after performing the shear test where (a)–(d) are the joints produced at a temperature of 650, 750, 800, and 850 ◦ C, respectively.
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A. Elrefaey, W. Tillmann / Journal of Alloys and Compounds 487 (2009) 639–645
Fig. 13. Fracture morphology for the joint produced at a temperature of 800 ◦ C (a) and joint produced at a temperature of 750 ◦ C (b).
tic reaction. These results are in agreement with those previously reported [17]. The characteristic microstructure of the joint brazed at a temperature of 800 ◦ C is shown in Fig. 8 while the result of EDX chemical analysis is listed in Table 3. On contrast to the joint brazed at a temperature of 750 ◦ C, ␣–Ti phase showed a coarser structure. Moreover, instead of Cu2 Ti, TiCu phase was detected at the titanium/braze alloy interface. This is probably owing to the increase in temperature which led to an increase in titanium diffusion to the braze. It is also recognized that most of the copper solid solution (Cu) in the brazed area is consumed to form Cu–Ti phases. All characteristic feature microstructures of the joint brazed at 850 ◦ C are similar to the joint bonded at 800 ◦ C, except for the enlargement in the areas containing the ␣–Ti and Cu4 Ti3 phases (see Fig. 4b and c). In addition, to confirm the presence of different especially those directly at the fracture surface, XRD from fractured surfaces of the titanium side at brazing temperatures of 750 and 800 ◦ C were analyzed as it is shown in Fig. 9. XRD not only confirmed the presence of Cu–Ti intermetallics such as CuTi, Cu2 Ti, and Cu4 Ti3 , but also suggested the presence of different Cu–In intermetallic phases at the fracture surface. The occurrence of FeTi phase has not been clearly observed perhaps due to its occurrence a way from the fracture surface. 3.2. Mechanical properties Microhardness indentation measurements were performed to evaluate the different phases formed in the joints. The microhardness result of the joint brazed at a temperature of 750 ◦ C is shown in Fig. 10a. Indents across the braze joint are very consistent with
a soft and ductile Ag (∼ =69 HV). There is peak hardness at the interfaces. The steel/braze interface showed the highest level (205 HV) in the FeTi intermetallic phase, whereas the titanium/braze interface indicated lower hardness values (195 HV). In comparison to that, the microhardness of the joint brazed at a temperature of 800 ◦ C is shown in Fig. 10b. Similar to the previous case, the microhardness indicated high values at the interfaces compared to other areas. It is also to note that the joint brazed at 800 ◦ C achieved higher hardness level than its comparable areas in the joint brazed at 750 ◦ C. The high hardness of the intermetallic compounds at the interfaces reduces the ductility and hence the allowable plastic deformations, which can contribute to reducing the tensile shear strength of the joints. The effect of temperature on joint strength was evaluated by the shear strength test as shown in Fig. 11. It can be seen that the shear strength of the joints at first increased with increased brazing temperature to 750 ◦ C and then it decreased at higher temperatures. The voids observed at a brazing temperature of 650 ◦ C lowered the strength at this temperature. At the same time, the relative amount and percentage of CuTi phase at high temperatures (800 and 850 ◦ C) was very high compared to its content at lower temperatures (750 ◦ C) which deteriorates the strength of the joints. Fig. 12 shows that the joints failed mainly at the titanium/incusil interface at all brazing temperatures, except for 750 ◦ C. The fracture path follows the interior of the braze, mainly through the silver solid solution phase, in case of the joint brazed at 750 ◦ C. This implies that the CuTi intermetallic compound is the most harmful phase in the joint despite its low hardness compared to FeTi phase. The corresponding microscope fractography of these fracture surfaces showed basically cleavages in case of fracture path at the interface through the CuTi phase (Fig. 13a). Additionally, a mixed dimplecleavage structure with tearing regions, which is favorable for the strength of the joint, in case of fracture path at the interior of braze (Fig. 13b) was observed. The intermetallic compound formed at the titanium/braze interface reduced the strength of a bonded joint and caused the brittle fracture. 4. Conclusions The influence of brazing temperatures on the microstructural changes of the braze metal as well as on the development of the reaction layer was examined. The strength of the steel–titanium joints was evaluated in conjunction with the microstructural changes in the braze metal. The following conclusions were drawn from this study. • The strength of the joint was affected by the brazing temperature. A stronger joint was obtained at a brazing temperature of 750 ◦ C. No harmful intermetallic compounds were formed at this temperature and sound joints were generated. At a temperature of 650 ◦ C, despite absence of the harmful intermetallic compounds, the temperature was not high enough to completely melt the braze alloy and some voids were observed which adversely affected the strength of the joint. Meanwhile, at a temperature above 750 ◦ C, deteriorated TiCu brittle phase was enlarged at the steel/titanium interface, which resulted in a decrease in the strength of the joint. • High shear strength joints were fractured at the interior of the brazed alloy mainly in the silver solid solution and the fracture morphology for these joints presented mixed dimple-cleavage structures with tearing regions. References [1] B. Aleman, I. Gutierrez, J.J. Urcola, Mater. Sci. Technol. 9 (1993) 633–641.
A. Elrefaey, W. Tillmann / Journal of Alloys and Compounds 487 (2009) 639–645 [2] N.F. Kazakov, Diffusion Bonding of Materials, Mir Publishers, Pergamon Press, Moscow, 1985. [3] P. He, J. Zhang, R. Zhou, X. Li, Mater. Charact. 43 (1999) 287–292. [4] O. Smorygo, J.S. Kim, M.D. Kim, T.G. Eom, Mater. Lett. 61 (2007) 305–316. [5] P. He, J.C. Feng, H. Zhou, Mater. Charact. 54 (2005) 338–346. [6] C.C. Liu, C.L. Ou, R.K. Shiue, J. Mater. Sci. 37 (2002) 2225–2235. [7] A. Elrefaey, W. Tillmann, Metall. Mater. Trans. A 38 (2007) 2956–2962. [8] R.M. do Nascimento, A.E. Martinelli, A.J.A. Buschinelli, Ceramica 49 (2003) 178–198. [9] A. Elrefaey, W. Tillmann, J. Mater. Sci. 42 (2007) 9553–9558. [10] J. Harvey, P.G. Partridge, C.L. Snooke, J. Mater. Sci. 20 (1985) 1009–1014. [11] G. Effenberg, Non-Ferrous Metal Systems. Part 3: Selected Soldering and Brazing Systems, Springer, Berlin, Heidelberg, 2007, pp. 63–74.
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[12] G. Petzow, G. Effenberg, Ternary Alloys: A Comprehensive Compendium of Evaluated Constitutional Data and Phase Diagrams, VCH Verlagsgesellschaft, Weinheim, Germany, 1988. [13] L.D. Gulay, J.C. Schuster, J. Alloys Compd. 360 (2003) 137–142. [14] H. Baker, ASM Handbook: Alloy Phase Diagrams, ASM International, Materials Park, OH, 1992. [15] J.A. Van Beek, A.A. Kodentsov, F.J.J. Van Loo, J. Alloys Compd. 217 (1995) 97– 103. [16] G.F. Voort, ASM Handbook, Metallography and Microstructures, ASM International, Materials Park, OH, 2004. [17] R.K. Shiue, S.K. Wu, C.H. Chan, The interfacial reactions of infrared brazing Cu and Ti with two silver-based braze alloys, J. Alloys Compd. 372 (2004) 148– 157.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Correlation between microstructure, mechanical properties, and brazing temperature of steel to titanium joint Ahmed Elrefaey ∗ , Wolfgang Tillmann Institute of Materials Engineering, Faculty of Mechanical Engineering, Dortmund University of Technology, Leonhard-Euler-Str.2. 44227, Dortmund, Germany
a r t i c l e
i n f o
Article history: Received 15 April 2009 Received in revised form 6 August 2009 Accepted 8 August 2009 Available online 15 August 2009 Keywords: Steel Titanium Brazing Microstructure Mechanical properties X-ray diffraction
a b s t r a c t An attempt was made to simultaneously correlate the microstructure, mechanical properties, and brazing temperature of steel to titanium brazed joint. Brazing was conducted by means of silver-based filler (Incusil ABA) at a temperature range of 650–850 ◦ C for 15 min in a high vacuum furnace. It has been observed that the main interaction layer at the steel/incusil interfacial layer was the FeTi phase. Meanwhile, at the titanium/incusil side, ␣–Ti in addition to Ti–Cu phases were observed. The maximum shear strength of the joints was 113 MPa for the specimens brazed at 750 ◦ C. At this temperature, the fracture morphology presented a ductile–brittle fracture through the Ag-rich area, that is contradictory to the joint brazed at higher temperatures, which showed a brittle fracture at the titanium/incusil interfacial area owing to the presence of harmful Ti–Cu intermetalic phases. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The light weight, high strength-to-weight ratio, and excellent corrosion resistance of titanium and its alloys has led to the use of these materials in aerospace, chemical, marine, and medical industries. These properties also can enhance the performance of many weapon systems, such as ships and combat vehicles. The major disadvantages of this metal are its high cost and difficulty in joining it with other materials. Among the methods for overcoming these disadvantages is joining titanium to steel. These joints, if successfully made, will posses all the requirements for high quality joints, and also reduce the amount of materials needed thus making them economically attractive and viable. Titanium–steel combination represent one of the combinations which are difficult to weld, since a number of brittle intermetallics are formed due to the limited solubility of Fe and Ti in the solid state and hence, bond strength deteriorated [1–3]. Brazing is one of the most suitable ways to obtain good welds of titanium to steel because it involves only the melting of the filler material. Thus, problems which typically occur when dissimilar metals are fused are avoided. However, titanium is a highly reactive element, on which a non-wetting scale is readily generated when exposed to elevated temperatures and it reacts readily with many elements to form brittle intermetallics. To avoid the intru-
sion of detrimental impurities such as oxygen and nitrogen, it is mandatory to join titanium alloys in an inert environment. Hence, titanium/steel joints are brazed in a vacuum furnace. It has been reported that pure silver, silver base alloys, titanium base alloys, and copper base alloys have been used to braze titanium and titanium alloys [4–7]. Since the melting point of pure silver greatly decreased by alloying copper into silver matrix and a lower brazing temperature is preferred for most brazing processes, especially for brazing titanium, incusil ABA braze alloy was chosen as filler metal for vacuum brazing of titanium to steel. This filler metal has solidus 605 ◦ C and liquidus 715 ◦ C, which are significantly lower values than those of traditional silver based alloys. It is reported that In and Ti are added in addition to other elements to adjust the alloy behaviour, e.g., to improve the activity of the element, reduce the melting temperature, and increase the alloy fluidity [8]. The objective of this study is to evaluate incusil-ABA filler metal by brazing commercially pure titanium to steel in a vacuum furnace. The relation between brazing temperatures, interfacial microstructure, and the resultant mechanical properties of the brazed joints were extensively studied. 2. Experimental 2.1. Materials
∗ Corresponding author. E-mail address: [email protected] (A. Elrefaey). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.08.029
The materials employed in this investigation were 2 mm thick plates of commercially pure titanium (ASTM grade II) strengthened by small additions of oxygen, nitrogen, carbon, hydrogen, and iron, and low carbon steel. The nominal composi-
640
A. Elrefaey, W. Tillmann / Journal of Alloys and Compounds 487 (2009) 639–645
Fig. 3. Microstructure of brazed joint at 650 ◦ C.
before brazing and then sandwiched between the overlapped areas of the parent metals. 2.2. Brazing condition
Fig. 1. Schematic illustration of the brazed joints; (a) joint for microstructure analysis and (b) joint for shear test.
tion of the commercially pure titanium, in weight percent, was 0.08% C, 0.20% Fe, 0.03% N, 0.18% O, 0.02% H, and balance Ti. On the other hand, the composition of steel, in weight percent, was 0.12% C, 0.05% P, 0.05% S, 0.05% Mn, and balance Fe. The plates were cut into 30 mm × 25 mm × 2 mm chips for shear strength testing and 14mm × 10 mm × 2 mm chips for microstructure analysis. The schematic diagram of the specimens is shown in Fig. 1. The brazing was carried out using 200 m thick foil of incusil ABA (Ag–27.25Cu–12.5In–1.25Ti, wt.%). The parent metals were first polished with SiC papers up to 1000 grit and subsequently cleaned by an ultrasonic bath using acetone as the solvent prior to furnace brazing. The brazing foils were cleaned in acetone
Titanium to steel overlap width was kept at 6 mm since it is recommended to use lap width of no more than 3 times the thickness of the base metals to achieve high strength for the joint [9]. The joints were fixed with stainless steel clamp, and then carefully placed into a vacuum furnace (SCHMETZ GmbH). Initially, the samples were heated up to a temperature 50 K below the solidus temperature of the filler alloy for a dwell time of 5 min. This step aimed at achieving the thermal equilibrium of the couple. The sample was then heated up to the brazing temperature, as schematically illustrated in Fig. 2. Brazing experiments were carried out at 650, 750, 800, and 850 ◦ C to study the effect of the brazing temperature on the metallurgical and mechanical properties of the joint. At all brazing temperatures, the dwell time was 15 min. 2.3. Microstructure observation and mechanical testing The polished microstructures of the joints were examined with a light optical microscope and a scanning electron microscope (SEM), equipped with an energy dispersive X-ray (EDX) for chemical analysis. The hardness measurement was performed with the help of a Vickers hardness testing machine with 25 s impressing time. Tensile shear specimens were machined from brazed lap joints as suggested by Harvey et al. [10]. The test was carried out at room temperature and the displacement rate was 0.5 mm/s. Three samples were used to calculate the average shear strength of the joint. X-ray diffraction (XRD) analysis of the fracture surfaces was used to detect and analyse the reaction products. The X-ray scan rate was set at 4◦ /min, and its range was between 20◦ and 150◦ .
3. Results and discussion 3.1. Microstructure and phase analysis
Fig. 2. Schematic illustration of the brazing temperature and time profile.
Preliminary studies to braze at 650 ◦ C led to a higher tendency to form big pores or voids in the brazed seam, especially at steel/incusil interface. Although not all joints brazed at a temperature of 650 ◦ C have showed such tendency, the pores were clearly observed in some samples. A typical form of this defect is shown in Fig. 3. The brazing filler metal could not melt completely at a braz-
Fig. 4. Microstructure of brazed joint at 750 ◦ C (a), 800 ◦ C (b), and 850 ◦ C (c).
A. Elrefaey, W. Tillmann / Journal of Alloys and Compounds 487 (2009) 639–645
641
Table 2 Chemical analysis of areas shown in Fig. 6, at.%.
Table 1 Chemical analysis of areas shown in Fig. 5, at.%. Average chemical analysis
Suggested phase
Ag
Ti
Cu
In
Fe
73.13 3.20 1.04
– 4.33 23.35
19.47 90.12 72.45
6.22 2.35 3.16
1.17 – –
Ag solid solution Cu solid solution Cu4 Ti
ing temperature of 650 ◦ C since this temperature lies in between the solidus and liquidus temperature of the filler metal. A representative cross section microstructure of the brazed joints at brazing temperatures of 750, 800, and 850 ◦ C is shown
Symbol
A B C D E
A B C D E
Fig. 6. SEM microstructure of titanium/steel joint brazed at 750 ◦ C.
Suggested phase
Ag
Ti
Cu
In
1.86 5.21 5.48 2.77 1.38
1.74 26.24 77.24 40.50 32.61
2.67 25.11 13.08 56.43 65.72
– 3.71 4.20 0.30 0.29
Fe 93.73 39.73 – – –
␣Fe + (Cu) FeTi + Cu ␣–Ti Cu4 Ti3 Cu2 Ti
Table 3 Chemical analysis of areas shown in Fig. 8, at.%. Symbol
Fig. 5. Microstructure of the braze material in the joint brazed at 750 ◦ C.
Average chemical analysis
Average chemical analysis
Suggested phase
Ag
Ti
Cu
In
1.23 2.03 0.42 2.76 3.28
2.16 38.56 92.38 45.57 38.27
3.23 18.87 7.02 51.67 54.60
0.95 – 0.17 – 3.85
Fe 92.43 40.54 – – –
␣Fe + (Cu) FeTi + Cu ␣–Ti CuTi Cu4 Ti3
in Fig. 4. It is clear that a continuous reaction layer between the brazed seam and the titanium substrate was formed. Besides, the reaction layer has increased in thickness after the brazing temperature had been prolonged. At the same time, the steel/braze interface showed tiny interaction layers at all brazing temperatures. Additionally, coarse grain structure was formed at the steel boundary to the silver brazed foil. This coarse grain structure results from diffusion growth accompanied by recrystallization of the steel substrate at high temperatures. It is also important to note that the thickness of the brazed area gradually decreased by increasing the temperature due to the increase in overflow of the Ag-rich liquid during brazing. Brazing is usually performed through melting of the braze alloy, dissolution of parent metals, and solidification of the braze metal. In respect to our study, the main elements in concern are silver and copper, which are derived from the braze alloy and titanium, derived from the titanium substrate. Iron seems to be of no relevance in this process, owing to its very low solubility in silver and copper. It may only react with titanium to form Fe–Ti phases. Due to a wide immiscibility region in the Ag–Cu–Ti ternary system [11],
Fig. 7. The Cu–Ti binary alloy phase diagram.
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Fig. 9. XRD from fractured surfaces of the titanium side at brazing temperatures of 750 ◦ C (a) and 800 ◦ C (b).
Fig. 8. SEM microstructure of titanium/steel joint brazed at 800 ◦ C.
transformations are located close to the boundary of the binary Cu–Ti system and in a narrow Ag-rich region. Fig. 5 illustrates the microstructure of the braze material in the joint brazed at 750 ◦ C. Based on the EDX analysis results, see Table 1, the joints primarily consist of a matrix of Ag solid solution (Ag) alloyed with Cu and In. The ternary Ag–Cu–In phase diagram [12] indicates the presence
of this phase under equilibrium condition. Additionally, a mixed structure containing copper solid solution phase (Cu) and Cu4 Ti was clearly formed in the Ag matrix. Tiny precipitates are randomly distributed in the Ag solid solution matrix. This phase is too fine to be accurately analyzed by EDS. However, its chemical analysis suggested that it could be Ti3 In since the incorporation of titanium in braze metal increases at high temperatures and the solubility of In in Ti is quite high at high temperatures and decreases at lower temperatures according to Ti–In binary phase diagram [13]. In order to characterize the reaction layers of the joint brazed at 750 ◦ C, an SEM image was performed at the steel/incusil and tita-
Fig. 10. Hardness distributions along the joint produced at a temperature of 750 ◦ C (a) and 800 ◦ C (b).
A. Elrefaey, W. Tillmann / Journal of Alloys and Compounds 487 (2009) 639–645
nium/incusil interfacial layers as it is shown in Fig. 6. Moreover, a quantitative overview of the chemical analyses for the different areas is listed in Table 2. The microstructure of the steel/incusil alloy revealed two distinct phases. The diffusion of copper into the steel substrate produces a solid solution of limited solubility [14]. It is assumed that eutectoid transformation ␥ → ␣Fe + (Cu) took place with a high content of ␣Fe with respect to (Cu). The narrow eutectoid structure (∼ =5 m in width) is clearly shown in Fig. 6a. This is indicated by area A which presented high percentages of Fe in respect to Cu. Besides, a very thin interaction layer (area B) was formed close to area A. Based on the chemical analysis of this area, it is expected to contain FeTi + Cu intermetallic phase. According to Van Beek et al. [15], the Fe–Ti–Cu ternary alloy phase diagram suggests that nearly 38 at.% Cu could be dissolved in FeTi. At the same time, titanium/braze alloy revealed three phases as it is shown in Fig. 6b. It is worth noting that the migration of Cu in the titanium substrate lowers the eutectoid transformation temperature of Ti and a thin ␣–Ti phase forms by the decomposition of -Ti during cooling, as reported by Voort [16]. The ␣–Ti phase is shown by region C in Fig. 6b. The ratios between Ti and Cu in layer D and E are close to the Cu4 Ti3 and Cu2 Ti phases which formed at the interface. Based on the Cu–Ti binary alloy phase diagram shown in Fig. 7 [14], a series of invariant reactions upon cooling of the liquid are listed below: CuTi + L ↔ Cu4 Ti3 (peritectic, 925 ± 10 ◦ C).
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Fig. 11. Relation between average shear strength of the joint and brazing temperature.
Cu4 Ti3 + L ↔ Cu2 Ti (peritectic, 890 ± 10 ◦ C). L ↔ Cu2 Ti + Cu4 Ti (eutectic, 875 ± 10 ◦ C). The formation of interfacial Cu4 Ti3 phase is caused by the peritectic reaction between the molten braze and the CuTi phase. Accordingly, the interfacial Cu4 Ti3 phase subsequently reacts with the residual molten braze upon the cooling cycle, and the Cu2 Ti phase is formed via peritectic reaction. Finally, the residual molten braze is completely solidified into Cu2 Ti and Cu4 Ti via the eutec-
Fig. 12. Fracture paths after performing the shear test where (a)–(d) are the joints produced at a temperature of 650, 750, 800, and 850 ◦ C, respectively.
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Fig. 13. Fracture morphology for the joint produced at a temperature of 800 ◦ C (a) and joint produced at a temperature of 750 ◦ C (b).
tic reaction. These results are in agreement with those previously reported [17]. The characteristic microstructure of the joint brazed at a temperature of 800 ◦ C is shown in Fig. 8 while the result of EDX chemical analysis is listed in Table 3. On contrast to the joint brazed at a temperature of 750 ◦ C, ␣–Ti phase showed a coarser structure. Moreover, instead of Cu2 Ti, TiCu phase was detected at the titanium/braze alloy interface. This is probably owing to the increase in temperature which led to an increase in titanium diffusion to the braze. It is also recognized that most of the copper solid solution (Cu) in the brazed area is consumed to form Cu–Ti phases. All characteristic feature microstructures of the joint brazed at 850 ◦ C are similar to the joint bonded at 800 ◦ C, except for the enlargement in the areas containing the ␣–Ti and Cu4 Ti3 phases (see Fig. 4b and c). In addition, to confirm the presence of different especially those directly at the fracture surface, XRD from fractured surfaces of the titanium side at brazing temperatures of 750 and 800 ◦ C were analyzed as it is shown in Fig. 9. XRD not only confirmed the presence of Cu–Ti intermetallics such as CuTi, Cu2 Ti, and Cu4 Ti3 , but also suggested the presence of different Cu–In intermetallic phases at the fracture surface. The occurrence of FeTi phase has not been clearly observed perhaps due to its occurrence a way from the fracture surface. 3.2. Mechanical properties Microhardness indentation measurements were performed to evaluate the different phases formed in the joints. The microhardness result of the joint brazed at a temperature of 750 ◦ C is shown in Fig. 10a. Indents across the braze joint are very consistent with
a soft and ductile Ag (∼ =69 HV). There is peak hardness at the interfaces. The steel/braze interface showed the highest level (205 HV) in the FeTi intermetallic phase, whereas the titanium/braze interface indicated lower hardness values (195 HV). In comparison to that, the microhardness of the joint brazed at a temperature of 800 ◦ C is shown in Fig. 10b. Similar to the previous case, the microhardness indicated high values at the interfaces compared to other areas. It is also to note that the joint brazed at 800 ◦ C achieved higher hardness level than its comparable areas in the joint brazed at 750 ◦ C. The high hardness of the intermetallic compounds at the interfaces reduces the ductility and hence the allowable plastic deformations, which can contribute to reducing the tensile shear strength of the joints. The effect of temperature on joint strength was evaluated by the shear strength test as shown in Fig. 11. It can be seen that the shear strength of the joints at first increased with increased brazing temperature to 750 ◦ C and then it decreased at higher temperatures. The voids observed at a brazing temperature of 650 ◦ C lowered the strength at this temperature. At the same time, the relative amount and percentage of CuTi phase at high temperatures (800 and 850 ◦ C) was very high compared to its content at lower temperatures (750 ◦ C) which deteriorates the strength of the joints. Fig. 12 shows that the joints failed mainly at the titanium/incusil interface at all brazing temperatures, except for 750 ◦ C. The fracture path follows the interior of the braze, mainly through the silver solid solution phase, in case of the joint brazed at 750 ◦ C. This implies that the CuTi intermetallic compound is the most harmful phase in the joint despite its low hardness compared to FeTi phase. The corresponding microscope fractography of these fracture surfaces showed basically cleavages in case of fracture path at the interface through the CuTi phase (Fig. 13a). Additionally, a mixed dimplecleavage structure with tearing regions, which is favorable for the strength of the joint, in case of fracture path at the interior of braze (Fig. 13b) was observed. The intermetallic compound formed at the titanium/braze interface reduced the strength of a bonded joint and caused the brittle fracture. 4. Conclusions The influence of brazing temperatures on the microstructural changes of the braze metal as well as on the development of the reaction layer was examined. The strength of the steel–titanium joints was evaluated in conjunction with the microstructural changes in the braze metal. The following conclusions were drawn from this study. • The strength of the joint was affected by the brazing temperature. A stronger joint was obtained at a brazing temperature of 750 ◦ C. No harmful intermetallic compounds were formed at this temperature and sound joints were generated. At a temperature of 650 ◦ C, despite absence of the harmful intermetallic compounds, the temperature was not high enough to completely melt the braze alloy and some voids were observed which adversely affected the strength of the joint. Meanwhile, at a temperature above 750 ◦ C, deteriorated TiCu brittle phase was enlarged at the steel/titanium interface, which resulted in a decrease in the strength of the joint. • High shear strength joints were fractured at the interior of the brazed alloy mainly in the silver solid solution and the fracture morphology for these joints presented mixed dimple-cleavage structures with tearing regions. References [1] B. Aleman, I. Gutierrez, J.J. Urcola, Mater. Sci. Technol. 9 (1993) 633–641.
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[12] G. Petzow, G. Effenberg, Ternary Alloys: A Comprehensive Compendium of Evaluated Constitutional Data and Phase Diagrams, VCH Verlagsgesellschaft, Weinheim, Germany, 1988. [13] L.D. Gulay, J.C. Schuster, J. Alloys Compd. 360 (2003) 137–142. [14] H. Baker, ASM Handbook: Alloy Phase Diagrams, ASM International, Materials Park, OH, 1992. [15] J.A. Van Beek, A.A. Kodentsov, F.J.J. Van Loo, J. Alloys Compd. 217 (1995) 97– 103. [16] G.F. Voort, ASM Handbook, Metallography and Microstructures, ASM International, Materials Park, OH, 2004. [17] R.K. Shiue, S.K. Wu, C.H. Chan, The interfacial reactions of infrared brazing Cu and Ti with two silver-based braze alloys, J. Alloys Compd. 372 (2004) 148– 157.