Isothermal solidification kinetics of diffusion brazing |
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Authors: | W D MacDonald T W Eagar |
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Affiliation: | (1) Present address: Sherritt, Inc., T8L 3W4 Edmonton, AB, Canada;(2) Process Research, Timminco Metals, K0J 1Y0 Haley, ON, Canada;(3) Department of Materials Science and Engineering, USA;(4) the Massachusetts Institute of Technology, 02139 Cambridge, MA |
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Abstract: | Diffusion brazing (DB) is a process that produces interface-free joints that approach the bulk properties of the material
that is to be joined. Solid-state diffusion of the melting point depressant (MPD) element into the joint metal causes the
solid/liquid (S/L) interface to advance until the joint is solidified. The time required to complete this isothermal solidification
stage was modeled using a moving boundary analysis. Precision measurements of the interlayer width as a function of time were
made on the copper-silver system. The bonding apparatus consisted of a suspended load feedback system that prevented leakage
of joint metal due to extrusion or surface wetting, thus preserving mass balance. The interlayer width vs bonding time measurements obtained were found to be within a factor of 4 of the theoretical model. The difference was ascribed
to the difficulty in accurately measuring the actual liquid width and loss of silver due to vaporization. Large spherical
protrusions grow during bonding, which further roughens the interface. However, experimental and predicted concentration profiles
of silver were found to be in complete agreement. As the liquid remaining in the grain boundary grooves is diffused away,
porosity can develop due to volume changes. By holding the joint at temperature for extended periods, complete solidification
followed by homogenization will occur. Selecting the appropriate bonding temperature to achieve a specified maximum concentration
of braze metal at the joint is dependent on both the isothermal solidification and homogenization kinetics. |
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