Microstructural Evolution and Bonding Behavior during Transient Liquid-Phase Bonding of a Duplex Stainless Steel using t
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TRANSIENT liquid-phase (TLP) bonding has been considered an attractive alternate joining process for duplex stainless steels (DSSs).[1–5] The TLP bonding developed by Duvall et al.[6] is a hybrid joining process, which combines the beneficial features of liquid phase joining and diffusion bonding techniques.[7] In the TLP bonding process, an insert alloy containing melting point depressant elements is sandwiched between the base alloy surfaces, and then the assembly is heated to a temperature between the liquidus of the insert metal and that of the base metal. Generally, it is considered that four distinct stages are present during the TLP bonding process, namely dissolution of base metal, widening of interlayer, isothermal solidification of liquid, and homogenization of bond region. If sufficient time for complete isothermal solidification is not allowed at the bonding temperature, then the formation of eutectic microconstituents could occur along the bond centerline. It is normally the goal of the TLP bonding process to prevent the formation of these microconstituents as they have been deleterious to the mechanical properties of bonded metals.[8,9] Therefore, to achieve reliable TLP bonded joints, isothermal solidification has been recognized as the most important stage and the time required for isothermal solidification completion is a fundamentally important parameter during the TLP bonding. Several models of TLP bonding, most of which are XINJIAN YUAN, Doctoral Candidate, MYUNG BOK KIM, Graduate Student, and CHUNG YUN KANG, Professor, are with the Department of Materials Science and Engineering, Pusan National University, Busan 609735, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted November 10, 2009. Article published online November 17, 2010 1310—VOLUME 42A, MAY 2011
based on binary phase relationships between the base and filler alloy, have been proposed to explain the microstructural development and model the isothermal solidification process.[10–13] Recently, Sinclair et al.[14] studied numerical modeling of isothermal solidification process in ternary systems and reported that the actual phase relationships encountered during TLP bonding of their multicomponent systems could cause a significant modification of isothermal solidification behavior in contrast to conventional expectations. Littke work has been done regarding this discrepancy in duplex stainless steel. Therefore, the objective of the present research is to investigate and interpret the notable variations sufficiently in the microstructure evolution and bonding behavior during TLP bonding of a DSS using two different Ni-B-based filler metals. II.
EXPERIMENTAL PROCEDURE
A nitrogen-containing DSS and four Ni-B-based amorphous foils with a thickness of 25.4 lm, MBF-30, MBF-50, MBF-80, and MBF-35 (Hitachi Metals Ltd., Tokyo, Japan), were used as the base alloy and the filler metals, respectively. The use of MBF-80 and MBF-35 was to clarify and explain the effects of Cr and Si on joint microstructure and isothermal solidification. The chem
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