Brazing and Interfacial Reaction of Commercially Pure Titanium with Ti-Zr-Based Filler Metals

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1 Materials Transactions, Vol. 8, No. (7). to 9 #7 The Jaan Institute of Metals Brazing and Interfacial Reaction of Commercially Pure Titanium with Ti-Zr-d Filler Metals Kotaro Matsu, Yasuyuki Miyazawa, Yoshitake Nishi and Tadashi Ariga Engineering Deartment, Tokyo Braze Co., Ltd., --, Tokyo 7-6, Jaan Deartment of Metallurgical Engineering, School of Engineering, Tokai University, Hiratsuka 9-9, Jaan In the last years, brazing of titanium and/or titanium alloys has been studied and a number of filler s have been develoed. Commercially ure titanium was brazed with commercially roduced atomized brazing filler owder in an industrial vacuum furnace. Ti- Zr-based alloy filler s were utilized in this study. We investigated the joint strength by shear strength tests and by observing the microstructure at the brazed joint. The shear strength tests revealed that the joint strength of Ti-Zr-based alloy filler s for a brazing time of 6 min was aroximately MPa, which is almost the same as the base strength. Zirconium diffused into the base, thereby transforming from -Ti to -Ti for a temerature below the - transformation temerature of the base, and the diffusion of nickel and coer into the base was accelerated. The brazing filler remained at the brazed joint for a brazing time of min; however, for an increased brazing time, it disaeared because of isothermal solidification. It is understood that isothermal solidification was mainly controlled by the diffusion of zirconium. [doi:./matertrans.8.] (Received Setember, 6; Acceted February, 7; Published Aril, 7) Keywords: titanium, brazing, titanium brazing, titanium-zirconium filler s, transient liquid hase bonding. Introduction In the last years, brazing of titanium and/or titanium alloys has been studied and a number of filler s have been develoed. The filler s used for titanium brazing can be divided into Ag-, Al-, and Ti-based filler s. ) In articular, Ti-based filler s exhibit excellent mechanical roerties such as high corrosion resistance and joint strength.,) However, the manufacture of Ti-based filler s is not as easy as that of other filler s because of their characteristics; therefore, they are manufactured in the form of mechanically alloyed or clad ribbons, which are well known as Ti-Cu-Ni. Recently, Ti-Zr-based filler s have been develoed in the form of alloy owder by the gas atomization rocess and have been manufactured successfully ever since. On the other hand, many studies have reorted titanium brazing with Ti-based filler s, where an amorhous foil roduced by a raid solidification or a clad form foil were used. ) However, very few studies have focused on brazing using titanium in the form of an alloy owder by the gas atomization rocess. Moreover, the brazing rocesses reorted were mostly carried out by induction heating as a raid heating rocess. Therefore, not many studies based on the alication of furnace brazing to mass roduction rocess have been conducted. In this study, commercially ure titanium was brazed with commercially roduced atomized brazing filler owder in an industrial vacuum furnace. We investigated the joint strength by shear strength tests and by observing the microstructure at the brazed joint. This investigation has rovided further insight into the interfacial reaction related to the dissolutive reaction between solid titanium and liquid brazing filler during brazing. Metal. Fig... Exerimental Procedure D8.. Secimens and filler s Commercially ure titanium (grade ) was used as the base. The schematic diagram of the secimen is shown in Fig.. The secimens were olished to a articular roughness and then washed ultrasonically in acetone. The shae of this secimen is simle as comared to that of the convectional shear strength test secimen. ) Therefore, this is a much better design for materials such as titanium, which are difficult to machine. In addition, the mechanical roerties of this secimen have already been evaluated. It was confirmed that identical results are obtained for this secimen and the convectional shear strength test secimen. ) Ti-Zr-based alloy filler s roduced by gas atomization were utilized in this study. A tyical secondary electron image (SEI) of the brazing filler owder is shown in Fig.. A tyical sherical atomized owder with no residues or slugs was observed on the surface of each article... Schematic diagram of the test secimen. Brazing Filler Metal

2 6 K. Matsu, Y. Miyazawa, Y. Nishi and T. Ariga µm Maximum Load ( kgf ) Shear Strength ( MPa ) = Brazed Area ( mm ) Secimen Load. 7. R Fig. SEI of the atomized brazing filler owder. Fig. Schematic diagrams of the shear strength test jig. Table Tye No. No. Chemical comosition and melting oint of the filler s. Chemical Comosition at% (mass%) Melting Point Ti Zr Cu Ni ( C) (7.) (7.) (.) (.) (.) (.) (.) (.) designed to enable a erfect fit for the secimen so that it stays in osition during testing, thereby enabling the shear stress to be loaded recisely on the joint. The crosshead seed was set to mm/min, and the test was erformed at room temerature. The shear strength was determined as given in the equation in Fig.. The number of secimens was n ¼ 8, and the average strengths were defined as the joint strength. Two tyes of brazing filler s, No. and No., were used in this study; the chemical comositions are shown in Table. These atomized filler owders were made into a aste by mixing them with a articular amount of organic binder. A fixed amount of brazing filler,. g, was alied at the joint, and the secimens were set using a fixture.. Brazing condition Brazing was erformed in an industrial vacuum furnace using the high-urity shield gas rocess (carrier gas rocess ) ). In this rocess, a articular amount of high urity argon gas (99.999%) is assed into the vacuum furnace chamber during vacuuming in order to revent the vaorization of the filler and to exel the gas from the chamber, fixture, or secimen. Considering the transformation temerature of the base and the melting oint of the brazing filler s, the brazing temerature was fixed at 88 C. It was reorted that it is ossible to obtain a sound joint with better joint strength in order to romote the interfacial reaction between solid base and liquid brazing filler in the case of formation of a brittle interlic comound at the brazed joint. ) Therefore, the brazing time was varied to,, and 6 min, in order to facilitate the interfacial reaction between the solid base and the liquid brazing filler.. Shear strength test The joint strength was evaluated by the shear strength test. The shear strength test jig is shown in Fig.. This jig is. Microstructure observation and analysis The cross section of the brazed joint was observed using an otical microscoe and the SEI of the field emission scanning electron microscoe (FE-SEM). The elemental distributions at the brazed joints were analyzed from the energy-disersed X-ray (EDX) examinations.. Results and Discussions. Shear strength test result The result of the shear strength tests revealed that the joint strength increased slightly with the brazing time. In the case of a brazing time of min, the joint strength with filler No. was MPa and that with No. was MPa. When the brazing time was 6 min, the joint strength of both No. and No. was MPa.. Microstructure at the brazed joint The tyical SEI of the cross-sectional microstructure at the brazed joint with filler No. for min and its EDX results are shown in Fig.. The brazed joint comrises a brazing filler layer, an alloy hase, an aciculate microstructure, and the base. The aciculate microstructure was formed with thin needle-like black and white hases. The result of the EDX analysis on area in Fig. revealed that identical comositions of the filler remained at the center of the brazed joint. Thus, it is understood that brazing filler artially remained. Besides the brazing filler layer, the alloy hase formed by the reaction between the base and the brazing filler was observed. The result of the EDX

3 Brazing and Interfacial Reaction of Commercially Pure Titanium 7 µm Chemical Comosition (at%) Ti Zr..6 Cu. 6.9 Ni c b a b c a. Brazing filler b. Reaction layer c. Aciculate microstructure Fig. SEI of cross-sectional microstructure at the joint brazed with filler No. for min and EDX results. µm Chemical Comosition (at%) Ti Zr Cu Ni Aciculate microstructure and reaction layer Fig. SEI of cross-sectional microstructure at the joint brazed with filler No. for 6 min and EDX results. analysis of area in Fig. showed that, in this alloy hase, the main comonent of the base was titanium from the base and zirconium from the brazing filer. The Ti-Zr binary hase system is a comlete solid solution; thus, it was assumed that this alloy hase was formed by the diffusion of zirconium from the brazing filler into titanium in the base. In addition, the Ti-Zr binary hase diagram shows that the - transformation temerature decreases with increasing zirconium content. This occurs until the comosition of Ti-67 mass%zr is obtained. Thus, this alloy hase was transformed to (-Ti, -Zr) at an elevated temerature in the brazing rocess. Consequently, nickel and coer, which were used as brazing filler s, raidly diffused into (-Ti, -Zr) in comarison to their diffusion into (-Ti, -Zr). An aciculate microstructure was thus formed during the brazing rocess. It is inferred that this alloy hase comrises a solid solution of (-Ti, -Zr) containing nickel and coer. An aciculate microstructure, which was assumed to be a eutectoid microstructure, was formed during the cooling rocess after brazing since the mixed microstructure comrised the (-Ti, -Zr) solid solution as a black hase, (Ti, Zr) Ni, (Ti, Zr) Cu, and the solid solution of (-Ti, -Zr) containing nickel and coer.

4 8 K. Matsu, Y. Miyazawa, Y. Nishi and T. Ariga Tyical SEI of the cross-sectional microstructure at the joint brazed with No. for 6 min and its EDX results are shown in Fig.. As the brazing time increased, the brazing filler layer disaeared comletely; in addition, an alloy hase and an aciculate microstructure were observed at the brazed joint. The growth of the aciculate microstructure was significant as comared to that obtained for a brazing time of min. The EDX analysis revealed that each comosition in area in Fig. was the same as those in area in Fig. ; thus, the brazing filler layer disaeared because the growth of the (-Ti, -Zr) solid solution containing nickel and coer caused isothermal solidification that led to the diffusion of zirconium, coer, and nickel into the base during brazing. For the secimen with filler No., similar results for brazed joint observation and EDX analysis were obtained under identical brazing conditions. In conclusion, it is understood that for brazing that occurs below the - transformation temerature of the base, zirconium lowers the melting oint of the filler, diffuses into the base as -Ti, and changes the hase of the base to -Ti. Diffusion of nickel and coer into the base was accelerated. Therefore, brazing was erformed with sufficient diffusion as Transient Liquid Phase bonding because isothermal solidification was driven by raid diffusion desite the short furnace brazing rocess. In addition, the ossibility of - transformation at the brazing temerature was checked by microstructure observation of the base where there was no effect of diffusion of the brazing filler elements. It was found that grain growth was not driven by re-crystallization because of overheating in that area. Thus, it is confirmed that the brazing temerature was below the - transformation temerature in this study. The result of the shear strength test for a brazing time of min was significantly low because the brazing filler artially remained and isothermal solidification was not yet comleted. On the other hand, the results of the shear strength test for a brazing time of 6 min were almost the same as the base strength because isothermal solidification occurred comletely and comlete Transient Liquid Phase bonding was achieved.. Mathematical modeling of the isothermal solidification rocess The isothermal solidification rocess was discussed based on the following mathematical modeling of the results of the EDX analysis and the diffusion equation. It was reorted that the isothermal solidification rocess can be analyzed as a concentration rofile Cðx; tþ of the joint area with the solidliquid moving interface W during the isothermal solidification, as illustrated in Fig. 6. ) The diffusion coefficient in the solid hase is denoted ¼ C ðx > WÞ ðfick s II lawþ J ¼ ðc L C S x¼w¼ dw ðmass bal.þ ðþ dt where C L is the concentration of the liquidus, and C S is the concentration of the solidus. C (x, t) = C B C (, t) = C B C (x, t) = C S C (x, t) = C L h α Liquid (x W) (x = W + ) (x W - ) Solid Moving Boundary Since ¼ W= ffiffiffiffiffi Dt, using the condition given in Fig. 6, the solution for eq. () can be obtained as follows: Cðx; tþ ¼C B þ C S C B erfc erfc x ffiffiffiffiffi ðþ Dt where C B is the concentration of base. When equation () is substituted in eq. (), we obtain the following equation. ðc S C B Þ=ðC L C S Þ¼ ffiffiffi ex erfc ðþ is calculated from eq. (). When the thickness of the liquidus is h, the time necessary for isothermal solidification becomes t F ¼ h =ðd Þ ðþ where C and h are the initial concentration and initial thickness of the liquidus, resectively. If the base is dissolved by the initial liquidus until equilibrium is attained at the liquidus/solidus interface, h is given as follows: h ¼ C h =C L ð6þ The calculation results of the isothermal solidification time t F at 88 C using eq. () are shown in Table. These results are based on isothermal solidification, which rogresses with the diffusion of coer, nickel, and zirconium. C L and C S were obtained from the EDX results in Fig.. h was calculated, for which C was the same as the comositions listed in Table, and h was mm. D was calculated from the activation energy Q and vibrational frequency factor D of each element in -Ti, which were obtained from the data book. 6) d on these results of the calculations, it takes aroximately min to comlete isothermal solidification with only the diffusion of zirconium, while it takes aroximately to min with the diffusion of coer or nickel. In fact, isothermal solidification was comleted in 6 min when it was controlled by the diffusion of zirconium, as shown in Fig. 6. C C L C S W X (Initial Condition) Fig. 6 Concentration rofile of the joint area with a solid-liquid moving interface during isothermal solidification. C B

5 Brazing and Interfacial Reaction of Commercially Pure Titanium 9 Table Isothermal solidification time t F calculated for each element. Element 6Þ D Q 6Þ D C L C S h t F (m /s) (kj/mol) (m /s) (at%) (at%) (mm) (s) Cu : 7 6: :6 :6 Ni :7 6 : : :9 Zr : : :68 :7 D ; Q: Literature values for hase of CP-Ti C L ; C S : Measured values by EDX analysis. Conclusions In this study, titanium was brazed with Ti-Zr-based alloy filler s roduced by gas atomization in an industrial vacuum furnace using a high-urity shield gas rocess, and the following results were obtained. () The shear strength tests revealed that the joint strength of both filler s No. and No. for a brazing time of 6 min was aroximately MPa, which is almost the same as the base strength. () Zirconium diffused into the base, thereby transforming the hase from -Ti to -Ti for a temerature below the - transformation temerature of the base, and the diffusion of nickel and coer into the base was accelerated. () The brazing filler remained at the brazed joint for a brazing time of min; however, for an increased brazing time, it disaeared because of isothermal solidification. It is understood that isothermal solidification was mainly controlled by the diffusion of zirconium. REFERENCES ) T. Onzawa: Welding Technique 7 (989) 6 6 (in Jaanese). ) T. Watanabe: Welding Technique 6 (998) 76 8 (in Jaanese). ) H. Yamamoto, D. Ito, K. Uenishi and K. Kobayashi: Prerints of The National Meeting of J.W.S. 6 (999) 6 7 (in Jaanese). ) D. Ito, M. Nojiri, A. Hirose and K. Kobayashi: Prerints of The National Meeting of J.W.S. 6 (997) (in Jaanese). 6) M. Nojiri, D. Ito, A. Hirose and K. Kobayashi: Prerints of The National Meeting of J.W.S. 9 (996) 6 7 (in Jaanese). 7) K. Okabe and K. Hamamura: Prerints of The National Meeting of J.W.S. (99) (in Jaanese). 8) H. Yamada, T. Kono, T. Shimizu and T. Shibata: DENKI-SEIKO 9 (988) 8 87 (in Jaanese). 9) T. Onzawa, A. Suzumura and M. Ko: Quarterly Journal of The Jaan Welding Society (987) (in Jaanese). ) C.-T. Chang and R.-K. Shiue: J. Mater. Sci. (6). ) Y. Miyazawa and T. Ariga: Weld J. 7 (99) 9s s. ) Y. Miyazawa: Joining of Advanced and Secialty Materials V, Conference Proceedings from Materials Solutions, (ASM International, ). 8. ) Jaan Industrial Furnace Manufacturers Association: Kougyouro Handbook, (The Energy Conservation Center, Jaan, 997) 6 66 (in Jaanese). ) H. Ikawa, Y. Nakao and T. Isai: Journal of High Temerature Society (976) (in Jaanese). ) T. Onzawa, A. Suzumura and M. Ko: Quarterly Journal of The Jaan Welding Society 7 (989) 6 67 (in Jaanese). 6) The Jaan Institute of Metals: Kinzoku Data Book, Rev. ; (Maruzen, Tokyo, 99) (in Jaanese). ) T. Onzawa: Welding Technique (986) (in Jaanese).