Fast Isothermal Solidification During Transient Liquid Phase Bonding of a Nickel Alloy Using Pure Copper Filler Metal: S
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bonding is a nearly ideal bonding process in which a filler metal containing a melting point depressant (MPD), usually boron (B), silicon (Si), and phosphorous (P), is melted to join base materials, including single-crystal and polycrystalline superalloys, together.[1–5] It is generally accepted that the process is composed of three different stages: the dissolution of the base material, isothermal solidification, and homogenization.[6–10] The isothermal solidification stage during TLP bonding that plays a key role in achieving an intermetallic-free joint[11–14] is controlled
ALI GHASEMI and MAJID POURANVARI are with the Department of Materials Science and Engineering, Sharif University of Technology, 11365-9466 Tehran, Iran. Contact email: [email protected] Manuscript submitted October 11, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
by the solid-state diffusion of the MPD element into the base material.[15–17] Since the isothermal solidification is controlled by solid-state diffusion of the MPD element into the base material, the process is slow especially when the gap size is large.[18,19] The time required for isothermal solidification completion is, therefore, of primary interest when deciding whether any particular system is suitable for diffusion brazing.[20] In order to reduce the bonding time, it is important to use filler metals enabling archiving faster isothermal solidification. That is why filler metals containing B as the MPD are extensively used in the TLP bonding process of the nickel-base alloys.[20–25] Boron, as an interstitial atom, possesses high diffusion kinetics in Ni-base alloys.[22] Therefore, depletion of B from the liquid phase seems to occur quickly and solidification seems to be over in a relatively short period of time. Not only diffusion kinetics but also thermodynamics of diffusion is essential to determine the completion time of isothermal solidification.[14,26,27] This means that isothermal solidification during bonding using a filler metal containing a lower diffusivity MPD but a higher
driving force of diffusion can be considered to be completed in a shorter time than that of a B containing filler metal.[27] Therefore, there is a competition between kinetics and thermodynamics of diffusion to determine the rate of isothermal solidification during TLP bonding. In this article, this competition is investigated for bonding of a Ni-based alloy, Monel 400, using a pure copper (Cu) filler metal and a Ni-Si-B (MBF-30) filler metal. Pure Cu filler metal is representative of a low diffusivity element with a high thermodynamic driving force for diffusion in Ni-based alloys, and B in Ni-Si-B filler metal is representative of a high diffusivity element with a low thermodynamic driving force for diffusion in Ni-based alloys. Therefore, TLP bonding behaviors of two systems, including Monel 400/MBF-30/Monel 400 and Monel 400/Cu/Monel 400, are investigated in this work. Before presenting and discussing the experimental results, the fundamental basis for isothermal solidification is reviewed and the roles of both kin
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