Microstructural Evolution in the Transient-Liquid-Phase Bonding Area of IN-738LC/BNi-3/IN-738LC
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TRANSIENT-LIQUID-PHASE (TLP) bonding is a hybrid joining process that combines the benefits of brazing and diffusion joining techniques together. Unique possibilities of the TLP bonding process (such as tolerance of stable surface oxides, precise thermal cycle, high strength of joint, avoidance of hot and aging cracking, etc.) introduce this bonding method as a reliable technique for joining nonweldable materials.[1,2] IN-738LC is one of these nonweldable materials. IN-738 is an investment casting, polycrystalline Nibase superalloy that was patented in 1969 by Bieber et al.[3] This superalloy is produced in two different versions, low carbon and high carbon, which are designated as IN-738LC and IN-738C, respectively. The low carbon version of IN-738 has a better castability than the other one. The mechanical properties of IN-738 are not significantly affected by carbon contents.[4] The high alloying nature of IN-738LC, especially considerable contents of c¢ promoting elements (Al + Ti > 6 wt pct), substantially reduces the weldability of this superalloy.[5] Haafkens and Matthey[6] showed large shrinkage stresses due to the rapid c¢ precipitation during weld cooling caused heat affected zone cracking in welded IN-738LC. Difficulties and problems associated with diffusion bonding and brazing techniques restrict the industrial usage of these bonding processes M. MOSALLAEE, Postdoctoral Student, and A. EKRAMI, Professor, are with the Materials Science and Engineering Faculty, Sharif University of Technology, 11365-9466 Tehran, Iran. Contact e-mail: [email protected] K. OHSASA, Associate Professor, and K. MATSUURA, Professor, are with the Graduate School of Engineering, Hokkaido University, 060-8628 Sapporo, Japan. Manuscript submitted November 15, 2007. Article published online July 15, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
for joining of IN-738LC.[7,8] Therefore, TLP bonding of IN-738LC was investigated by some researchers.[9–12] Most of these researches were about kinetic and mechanism of isothermal solidification stage. To achieve a sound and reliable joint in the precipitation-hardened superalloys, not only the microstructure of TLP bonding centerline but also that of adjacent base alloy to the TLP zone should be free from deleterious constituents. Furthermore, the base alloy should not be affected by the TLP bonding thermal cycle. Microstructural evolution in the adjacent base alloy of the TLP zone, in the multicomponent systems, was not investigated comprehensively. In this regard, the effect of bonding temperature on the microstructural evolution in the different zones of TLP bonded samples (bonding centerline, adjacent base alloy, and base alloy) was studied in the current research.
II.
EXPERIMENTAL PROCEDURE
Polycrystalline nickel-base IN-738LC superalloy and 25.4-lm-thick amorphous Ni-Si-B foil (American Welding Society designation BNi-3) were used as the base alloy and interlayer, respectively. The chemical composition and melting temperature range of these materials were presented in Table I. Both B
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