Effect of grain boundaries on isothermal solidification during transient liquid phase brazing
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INTRODUCTION
T R A N S I E N T liquid phase (TLP) brazing involves a series of steps, namely, melting of the filler metal (at temperature Tin), heating to the final brazing temperature (Tb), base metal dissolution, isothermal solidification, and homogenization. Nakagawa et al.11'2] used finite difference modeling to analyze dissolution and isothermal solidification during TLP brazing of nickel base metal using N i - l l wt pct P and Ni-14 wt pct Cr-10 wt pct P filler metals. Base metal dissolution, isothermal solidification, and homogenization were considered sequentially, and two brazing situations were evaluated, namely, constant liquid width (where liquid was continuously expelled throughout the brazing process) and variable liquid width (where the solid-liquid interface was able to move in an unrestricted manner throughout the brazing process). The finite difference model assumed one-dimensional conditions, a planar solid-liquid interface, the absence of convection, and dissolution and isothermal solidification being wholly controlled by solute diffusion in the liquid and solid. et al.[31 compared experimental and calculated times for isothermal solidification during TLP brazing of nickel using N i - l l wt pct P and Ni-14 wt pct Cr-10 wt pct P filler metals and found that the calculated rate of completion of the isothermal solidification process was slower than in the actual brazing situation. Two explanations were given to explain this difference between calculated and experimental results, namely, that too low a phos-
H. KOKAWA, formerly Visiting Scientist, Department of Metallurgy and Materials Science, University of Toronto, is Associate Professor, Department of Materials Processing, Tohoku University, Sendai 980, Japan. C.H. LEE, formerly Postdoctoral Fellow, Department of Metallurgy and Materials Science, University of Toronto, is Research Engineer, Research Institute of Industrial Science and Technology, Pohang 790-600, South Korea. T.H. NORTH, WIC/NSERC Professor, is with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON M5S 1A4, Canada. Manuscript submitted April 23, 1990. METALLURGICAL TRANSACTIONS A
phorous diffusivity value was employed during calculations and/or that liquid penetration at grain boundary regions had a major effect on the rate of movement of the solid-liquid interface during solidification. It is important to point out that the finite difference model specifically ignored the effect of grain boundary liquid penetration on the advance of the solid-liquid interface. In Lee et al.'s results, the solid-liquid interface only remained planar during the early stages of the isothermal solidification process, and evidence of liquid penetration at grain boundary regions was clearly apparent in test specimens held for long periods at the brazing temperature. It was suggested that liquid penetration at grain boundary regions increased the effective solid-liquid interfacial area and the amount of phosphorous diffusion into the base metal (since the rat
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