ISIJ International, Vol. 50 (2010), No. 3, pp. 450–454
The Effect of Substrate Dissolution in Brazing CP-Ti and Ti-15-3 Using Clad Ti–15Cu–15Ni Filler Z.Y. WU,1) T. Y. YEH,1) R. K. SHIUE1) and C. S. CHANG2) 1) Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan. E-mail:
[email protected] 2) Engineered Materials Solutions, 39 Perry Avenue, MS 4-1, Attleboro, MA 02703-2410, USA. (Received on September 17, 2009; accepted on December 16, 2009 )
The effect of substrate dissolution in brazing CP-Ti and Ti-15-3 using Ti–15Cu–15Ni filler metal has been performed in the experiment. Microstructures of infrared brazed joints are strongly related to dissolution of substrate during brazing. For the 1 800 s brazed CP-Ti specimen, the depletion of Cu and Ni from the braze alloy into substrate cause eutectoid transformation of b -Ti into lamellar Ti2Cu and a -Ti. In contrast, dissolution of Ti-15-3 substrate into the molten braze during infrared brazing results in the 1 800 s brazed zone alloyed with V, stabilizing the b -Ti to room temperature. KEY WORDS: infrared brazing; Ti-15-3; CP-Ti; microstructure; TEM.
1.
strength of the joint.18) In general, the average shear strength was increased with increasing infrared brazing temperature and/or time. The average shear strength was further increased for all brazed specimens when a postbrazing annealing was applied. The fracture mode of shear test sample changed from brittle cleavage to quasi-cleavage to ductile dimple as the brazing temperature and time increased. Additionally, the presence of Cu–Ni rich phase corresponded with the low shear strength and brittle fracture of the brazed joint. Because the Ti alloy experiences an a –b phase transformation during the brazing process, transmission electron microscopy (TEM) study of the brazed joint is required to unveil the transformation of the brazed zone. Additionally, effects of substrate dissolution during brazing also complicate the transformation kinetics of the brazed zone. Ti-15-3 is a metastable beta titanium alloy that was developed to reduce the strip processing cost due to its excellent forming characteristics at room temperature.3) The nominal composition of Ti-15-3 alloy, in weight percent, is 15% V, 3% Cr, 3% Al, 3% Sn and the balance being titanium. Ti-15-3 alloy is capable to develop very high tensile strength, on the level of 1 250 MPa with proper thermal-mechanical treatments.3) Two Ti substrates, commercially pure titanium (CP-Ti) and Ti-15-3, were chosen as the base metals in the experiment. The selection of CP-Ti in the experiment was for the purpose of comparison. Infrared heating is a very useful tool in evaluating the microstructural development of brazed joint. The high heating rate (up to 50°C/s) and controlled environment in an infrared furnace are useful to study the effect of process variables, e.g., brazing temperature and time, on the microstructural evolution of the brazed joint. It has been successfully applied in several studies on brazing various alloy
Introduction
Ti and its alloys have become important materials for structural applications due to their high specific strength, oxidation and corrosion resistance.1–4) Joining is always a crucial technique in application of engineering alloys. Welding, diffusion bonding and brazing are common processes in bonding Ti and its alloys.5–11) Welding and brazing are complementary each other in industry. Welding is suitable for bonding the localized joint with the joint strength comparable to base metal.12) In contrast, brazing is applied in bonding huge amount of contact joints at one time. For example, plate heat exchangers can be made by vacuum furnace brazing. Ti alloys post several technical challenges to the conventional brazing process. First, Ti is highly reactive and combines readily with oxygen at elevated temperatures to form an inert scale not wettable by most conventional braze alloys.12) Second, Ti alloy is easily embrittled by the absorption of interstitials such as O, H and N, which are presented in abundance during the joining process. Therefore, brazing of Ti in vacuum or dry inert gas has been thought to be preferred.10,13) Ti brazing has traditionally been carried out using Agand Al-based filler metals, but the service temperatures of such brazed joints are limited to about 300°C.14–16) Joints for elevated temperature service are brazed with Ti-based filler alloys of which Cu and Ni are commonly added as melting point depressants (MPDs). A cold roll-bonding process, which combines Ti, Cu and Ni strips into a layered composite, allows conventional cold rolling process to produce the Ti–Cu–Ni brazing filler in foil form.17) Based on the previous studies, Ti and its alloys were appropriately brazed using the Ti–15Cu–15Ni filler due to excellent shear © 2010 ISIJ
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systems.18–22) The purpose of this investigation is concentrated on TEM study of the infrared brazed CP-Ti and Ti-15-3 alloys using the Ti–15Cu–15Ni braze alloy. TEM analyses of the infrared brazed joints are performed in order to unveil the effect of substrate dissolution on transformation of the brazed zone in greater depth. 2.
Experimental Procedures
CP-Ti and Ti-15-3 were infrared vacuum brazed at 970°C for 300 s and 1 800 s, respectively. The braze alloy was Ti–15Cu–15Ni foil in wt% with the thickness of 50 m m. The heating rate was set at 10°C/s, and all samples were preheated at 800°C for 300 s before heating up to the brazing temperature. The average cooling rate between 970°C and 600°C during infrared brazing was 1.5°C/s. Cross sections of infrared brazed joint were examined using a JEOL JXA 8600SX electron probe microanalyzer (EPMA) equipped with the wavelength dispersive spectroscopy (WDS). The acceleration voltage was 15 kV, and its minimum spot size was 1 m m. For detailed microstructural observations, TEM specimens were sectioned in thin slices from various brazed zones of the joint. Thin foil specimens were prepared by a standard jet-polisher using an electrolyte of 6% HClO4, 30% C2H5OH and 64% CH3COOH at room temperature. The operation voltage was 30 V, and the current was 40–50 mA. Thin foil specimens were examined using a Philips TECNAI G2 TEM operated at 200 kV. It was equipped with an energy dispersive spectroscopy (EDS) for chemical analysis of the specific area in the brazed zone. 3.
Fig. 1. EPMA SEIs and WDS chemical analysis results of CP-Ti joint using Ti–15Cu–15Ni filler infrared brazed at 970°C for (a, b) 300 s, (c, d) 1 800 s. The observation areas of 1(b) and 1(d) are marked in 1(a) and 1(c), respectively.
Results and Discussion
3.1.
Infrared Brazing CP-Ti Using the Ti–15Cu–15Ni Filler Figure 1 displays EPMA secondary electron images (SEIs) and wavelength dispersive spectroscopy (WDS) chemical analysis results of infrared brazed CP-Ti using clad filler Ti–15Cu–15Ni at 970°C for 300 s and 1 800 s, respectively. Microstructures of brazed joints are similar except for the blocky phase in the middle of joint. The blocky phase as marked by A in Fig. 1(b) has been identified as Ti2Ni, and the blocky Ti2Ni is vanished at the interface between the CP-Ti and the braze alloy due to the depletion of Ni from the braze alloy into the CP-Ti substrate. Additionally, the Ti-rich phase is also observed in Fig. 1(a) as marked by B. The maximum solubilities of Cu and Ni in b Ti are 13.5 at% and 10 at%, respectively.23) They are much higher than those in a -Ti, 1.6 at% and 0.5 at%, so the Tirich matrix in the brazed zone may contain more than one phase.23) Figure 1(c) illustrates EPMA BEIs and WDS chemical analysis results of infrared brazed CP-Ti at 970°C for 1 800 s. The central blocky Ti2Ni is disappeared from the joint due to depletion of Cu from the brazed joint into CP-Ti substrate. Accordingly, the Ti-rich matrix dominates the entire brazed joint. TEM analysis is required in order to unveil the Ti-rich phase in greater depth. Figure 2 displays TEM micrographs, bright field (BF), dark field (DF) and selected area diffraction pattern
Fig. 2. TEM micrographs and EDS chemical analysis results of region I in Fig. 1(c) for the CP-Ti joint using Ti–15Cu–15Ni filler metal infrared brazed at 970°C for 1 800 s: (a) BF image, (b) DF image of Ti2Cu using (1¯10) diffraction spot, (c) SADP of Ti2Cu at position B with the zone axes of [001], (d) SADP analysis.
(SADP) of region I in Fig. 1(c) infrared brazed at 970°C for 1 800 s. Based on the Fig. 2, fine lamellar eutectoid Ti2Cu and a -Ti transformed from b -Ti are widely observed from the figure. The lamellar spacing of the eutectoid is approximately 100 nm, so it cannot be distinguished from the EPMA analysis. The hexagonal a -Ti has very limited solubilities of Cu and Ni as marked by A in Fig. 2(a). There is no Ti2Ni in the eutectoid since the Ni content in the brazed 451
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Fig. 3. EPMA SEIs and WDS chemical analysis results of Ti15-3 joint using Ti–15Cu–15Ni filler infrared brazed at 970°C for 300 s.
joint is primarily dissolved in the eutectoid Ti2Cu. According to the isothermal section of Cu–Ni–Ti phase diagram at 800°C, Ti2Cu dissolves Ni up to 15 at%, but the maximum solubility of Cu in Ti2Ni is below 8 at%.24,25) Based on the TEM structural analysis, the Ni content in the brazed joint is prone to be dissolved in Ti2Cu instead of forming eutectoid Ti2Ni. Therefore, the Ti-rich phase displayed in Fig. 1 is a lamellar eutectoid containing HCP a -Ti and tetragonal Ti2Cu. 3.2.
Infrared Brazing Ti-15-3 Using the Ti–15Cu–15Ni Filler Figure 3 shows EPMA SEIs and WDS chemical analysis results of Ti-15-3 joint using Ti–15Cu–15Ni filler infrared brazed at 970°C for 300 s. The microstructure of the brazed zone in Fig. 3 is similar to that of Fig. 1(a). The blocky phase shown in region II of Fig. 3 is Ti2Ni as marked by A. Additionally, the Ti-rich phase is also observed as marked by B and C in Fig. 3. The Ti-rich phase in position B is alloyed with higher contents of Cu and Ni than those in position C due to depletion of Cu and Ni from the brazed zone into Ti-15-3 substrate during infrared brazing. Both Cu and Ni contents in the Ti-rich phase exceed the solubility of a Ti at room temperature, so TEM analysis of the joint is required in order to identify the phase(s) in the joint. Figure 4 shows TEM micrographs and EDS chemical analysis results of region II in Fig. 3 for the Ti-15-3 joint using Ti–15Cu–15Ni filler metal infrared brazed at 970°C for 300 s. Based on EDS and SADP analysis results, microstructures of region II consists of blocky Ti2Cu (marked by A), Ti2Ni (marked by C) and b -Ti (marked by B). The b -Ti is alloyed with 9.2 at% V, which is a strong b stabilizer. In contrast, trivial contents of V are found in Ti2Cu and Ti2Ni. The dissolution of Ti-15-3 substrate into the molten braze during brazing results in high content of V dissolved into the brazed zone. Because the V is a strong b stabilizer for the Ti alloys, huge amount of b -Ti is stabilized to room temperature. Figure 5 illustrates TEM micrographs and EDS chemical analysis results of region III in Fig. 3 for the Ti-15-3 joint using Ti–15Cu–15Ni filler metal infrared brazed at 970°C for 300 s. The blocky Ti2Cu is disappeared from the region, and needle-like Ti2Cu is precipitated from the b -Ti matrix as demonstrated by DF and SADP analysis (Figs. 5(b)– © 2010 ISIJ
Fig. 4. TEM micrographs and EDS chemical analysis results of region II in Fig. 3 for the Ti-15-3 joint using Ti–15Cu–15Ni filler metal infrared brazed at 970°C for 300 s: (a) BF image, (b, c) SADP analysis of Ti2Ni at position C with the zone axes of [11¯1], (d, e) SADP analysis of Ti2Cu at position A with the zone axes of [001], (f, g) SADP analysis of b -Ti at position B with the zone axes of [11¯1].
5(d)). Ti2Cu is alloyed with 11.9 at% Ni and trivial Al, Cr, Sn as well as V. The V content of b -Ti in region III, 8.4 at% in Fig. 5, is slightly lower than that in region II, 9.2 at% in Fig. 4. The b -Ti is stabilized by alloying V. It is deduced that the Ni dissolved in both Ti2Cu and b -Ti results in the brazed joint free of Ti2Ni. Figure 6 displays EPMA BEI and WDS chemical analysis results across the Ti-15-3 joint using Ti–15Cu–15Ni filler infrared brazed at 970°C for 1 800 s. The Ti-rich phase dominates the entire brazed zone for the 1 800 s brazed specimen. According to the EPMA chemical analysis results, the V content is increased from center of the joint towards Ti-15-3 substrate as illustrated in Fig. 6(b). Because the V content of the brazed zone is above 9.5 at%, the b -Ti is stabilized after infrared brazing. Both Cu and Ni contents are decreased with increasing the distance from center of the braze into Ti-15-3 substrate as displayed in Fig. 6(c). As 452
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Fig. 5. TEM micrographs and EDS chemical analysis results of region III in Fig. 3 for the Ti-15-3 joint using Ti–15Cu– 15Ni filler metal infrared brazed at 970°C for 300 s: (a) BF image, (b) DF image using (2¯00) spot, (c) SADP of Ti2Cu at position A with the zone axes of [001], (d) SADP analysis.
Fig. 7. TEM micrographs and EDS chemical analysis results of region IV in Fig. 6(a) for the Ti-15-3 joint using Ti–15Cu–15Ni filler metal infrared brazed at 970°C in 1 800 s: (a) BF image, (b) SADP of b -Ti at position A with the zone axes of [001], (c) SADP analysis.
described earlier, the maximum solubilities of Cu and Ni in b -Ti are 13.5 at% and 10 at%, respectively. There is no Ti2Cu and Ti2Ni in the brazed zone due to low contents of Cu and Ni. Figure 7 shows TEM micrographs and EDS chemical analysis results of region IV in Fig. 6(a) for the Ti-15-3 joint using Ti–15Cu–15Ni filler metal infrared brazed at 970°C in 1 800 s. b -Ti is the only phase identified in the region, and it is consistent with the EPMA chemical analysis result. 3.3.
Discrepancy between Infrared Brazing CP-Ti and Ti-15-3 The microstructural evolution of the infrared brazed joint strongly depends upon substrate dissolution during infrared brazing. Cu and Ni are b stabilizers, and they belong to b eutectoid alloying elements. The b -Ti can transform to a plus another phase of compound.4) In contrast, V is completely miscible in the b -Ti, and it belongs to the b isomorphous system. The b transus temperature of Ti is significantly decreased as the amount of V content increased.4) For the 300 s brazed CP-Ti specimen, blocky Ti2Ni and eutectoid a -TiTi2Cu were observed in the brazed zone. In contrast, the dissolution of Ti-15-3 substrate during infrared brazing results in higher V content in the brazed zone even for the 300 s brazed specimen. Blocky Ti2Cu, Ti2Ni and b Ti are observed in the central region of brazed zone. The decomposition of b -Ti upon cooling cycle of brazing is retarded due to higher V content of the joint. The infrared
Fig. 6. EPMA (a) BEI and area of WDS analysis, (b, c) WDS chemical analysis results of Ti-15-3 joint using Ti–15Cu– 15Ni filler infrared brazed at 970°C for 1 800 s.
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brazed joint is free of eutectoid reaction, and the retained b Ti is widely observed in the joint. Discrepancy between infrared brazing CP-Ti and Ti-15-3 using Ti–15Cu–15Ni filler has been observed in the experiment. Beta stability of brazed area of CP-Ti decreases during brazing, whereas beta stability of brazed area of Ti-15-3 alloy increases during brazing. For the infrared brazing CPTi substrate, the depletion of Cu and Ni from the braze into substrate causes eutectoid transformation of b -Ti into lamellar eutectoid Ti2Cu and a -Ti for the 1 800 s brazed specimen. For the infrared brazing Ti-15-3 substrate, dissolution of Ti-15-3 substrate into the molten braze results in stabilizing the b -Ti to room temperature, and Cu and Ni are also dissolved in the b -Ti. The 1 800 s brazed joint primarily consists of b -Ti. Therefore, transformation of b -Ti is strongly affected by substrate dissolution during infrared brazing. 4.
of this research by the National Science Council (NSC), Taiwan, Republic of China, under the grant number 982221-E-002-052. REFERENCES 1) J. L. Walter, M. R. Jackson and C. T. Sims: Titanium and Its Alloys: Principles of Alloying Titanium, ASM Int., Materials Park, OH, (1988), 1. 2) J. R. Davis: ASM Handbook Volume 2: Properties and Selection: Nonferrous Alloys and Special Purpose Materials, ASM Int., Materials Park, OH, (1990), 586. 3) R. Boyer, G. Welsch and E. W. Collings: Materials Properties Handbook: Titanium Alloys, ASM Int., Materials Park, OH, (1993), 1. 4) W. F. Smith: Structure and Properties of Engineering Alloys, McGraw-Hill Inc., New York, (1993), 443. 5) W. C. Chung, L. W. Tsay and C. Chen: Mater. Trans., 50 (2009), No. 3, 544. 6) L. W. Tsay, C. X. Lee and C. Chen: Mater. Trans., 50 (2009), No. 7, 1785. 7) F. Karimzadeh, M. Heidarbeigy and A. Saatchi: J. Mater. Process. Technol., 206 (2008), Nos. 1–3, 388. 8) Y. G. Song, W. S. Li, L. Li and Y. F. Zheng: Mater. Lett., 62 (2008), No. 15, 2325. 9) L. W. Tsay, Y. S. Ding, W. C. Chung and C. Chen: Mater. Lett., 62 (2008), Nos. 6–7, 1114. 10) A. Shapiro and A. Rabinkin: Weld. J., 82 (2003), No. 10, 36. 11) D. W. Liaw, Z. Y. Wu, R. K. Shiue and C. S. Chang: ISIJ Int., 47 (2007), 869. 12) D. L. Olson, T. A. Siewert, S. Liu and G. R. Edwards: ASM Handbook Volume 6: Welding, Brazing and Soldering, ASM Int., Materials Park, OH, (1993), 943. 13) M. Schwartz: Brazing: for the Engineering Technologist, ASM Int., Materials Park, OH, (1995), 87. 14) M. Schwartz: Brazing, ASM Int., Materials Park, OH, (1987), 80. 15) G. Humpston and D. M. Jacobson: Principles of Soldering and Brazing, ASM Int., Materials Park, OH, (1993), 31. 16) H. W. Chuang, D. W. Liaw, Y. C. Du and R. K. Shiue: Mater. Sci. Eng., A390 (2005), 350. 17) C. S. Chang and B. Jha: Weld. J., 82 (2003), 28. 18) C. T. Chang, Y. C. Du, R. K. Shiue and C. S. Chang: Mater. Sci. Eng., 420A (2006), 155. 19) C. T. Chang, R. K. Shiue and C. S. Chang: Scr. Mater., 54 (2006), No. 5, 853. 20) R. K. Shiue, S. K. Wu, Y. T. Chen and C. Y. Shiue, Intermetallics, 16 (2008), No. 9, 1083. 21) R. K. Shiue, S. K. Wu and S. Y. Chen: Acta Mater., 51 (2003), No. 7, 1991. 22) D. W. Liaw, Z. Y. Wu, R. K. Shiue and C. S. Chang: ISIJ Int., 47 (2007), No. 6, 869. 23) T. B. Massalski: Binary Alloy Phase Diagrams, ASM Int., Materials Park, OH, (1990), 194. 24) K. P. Gupta: Phase Diagrams of Ternary Nickel Alloys, Indian Institute of Metals, Calcutta, India, (1990), 228. 25) P. Villars, A. Prince and H. Okamoto: Handbook of Ternary Alloy Phase Diagrams, ASM Int., Materials Park, OH, (1995), 9846.
Conclusions
The effect of substrate dissolution in brazing CP-Ti and Ti-15-3 using Ti–15Cu–15Ni filler metal has been performed in the experiment. Important conclusions are listed below: (1) For the 300 s brazed CP-Ti specimen, blocky Ti2Ni and eutectoid a -TiTi2Cu are observed in the brazed zone. With increasing the brazing time to 1 800 s, the joint mainly consists of eutectoid a -TiTi2Cu. The presence of blocky Ti2Ni intermetallic compound can be removed via extended brazing cycle. (2) For the 300 s brazed Ti-15-3 specimen, blocky Ti2Cu, Ti2Ni and b -Ti are observed in the central region of brazed zone. Both blocky Ti2Cu and Ti2Ni are disappeared from the region next to center of brazed zone, and needlelike Ti2Cu precipitates are formed in the b -Ti matrix. b -Ti is the only phase identified from the 1 800 s brazed specimen. The V content of the brazed zone is above 9.5 at%, so the b -Ti is stabilized after infrared brazing. (3) The microstructural evolution of the joint is attributed to the substrate dissolution during infrared brazing. For CP-Ti brazing, the depletion of Cu and Ni from the braze into substrate cause eutectoid transformation of b -Ti. In contrast, dissolution of the V content in the Ti-15-3 substrate into the molten braze during infrared brazing results in stabilizing the b -Ti to room temperature. Acknowledgements The authors gratefully acknowledge the financial support
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