Isothermal solidification kinetics of diffusion brazing
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I.
INTRODUCTION
DIFFUSION brazing (DB) or transient liquid phase (TLP) bonding produces a strong and interface-free joint with no remnant of the bonding phase. The process differs from diffusion bonding in that the formation of a thin liquid interlayer eliminates the need for a high bonding or clamping force. The interlayer can be provided by foils, electroplate, sputter coats, or any other process that deposits a thin film on the faying surfaces. Diffusion brazing has been applied to many metallic systems throughout the ages and yet it still holds promise as a technique for joining in aerospace and semiconductor applications. The general characteristics of the process as described elsewhere[1] are as follows. A foil of an alloying metal containing a melting point depressant (MPD) is placed between the faying surfaces of the joint metal (JM). Upon heating, a thin liquid interlayer forms because the melting point of the interlayer has been exceeded or because a reaction with the parent metal results in a low melting liquid alloy. The liquid then fills voids formed by the unevenness of the mating surfaces and can sometimes dissolve residual surface contamination. The MPD diffuses into the parent metal, resulting in isothermal solidification. Upon cooling, there remains no trace of the liquid phase, and ideally, the joint becomes indistinguishable from other grain boundaries, aside from its relatively planar shape. The DB process is not limited to binary eutectics, but rather can be applied in any system where the parent metal W.D. MacDONALD, formerly Senior R&D Professional with Sherritt, Inc., Edmonton, AB, Canada T8L 3W4, is Manager, Process Research, Timminco Metals, Haley, ON, Canada K0J 1Y0. T.W. EAGAR, Head, Department of Materials Science and Engineering, and POSCO Professor of Materials Engineering, is with the Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted September 18, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
or alloy will form a relatively low melting phase, which has solubility for the MPD. The concept can also be applied to other systems, whereby a chemical or other driving force inherently leads to solid-state equilibrium. In essence, any system wherein a liquid phase disappears by diffusion, amalgamation, volatilization, or other process is a candidate for DB. The kinetics of these processes are controlled by various transport mechanisms, although in this article, only bulk diffusion is considered. One of the more important processing conditions to control is the time required at temperature needed to isothermally solidify the joint. This time, tIS, is a function of the diffusivity and the phase relationships between the MPD and the JM, both of which are dependant on the bonding temperature. In Tuah-Poku et al.,[2] the experimentally determined solidification rate was an order of magnitude greater than that predicted by an analytical model. Their results indicate a bonding time of 141.2 hours as compared to their predicted time of 1200 hours. This discrepanc
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