A Kinetic and Thermal Study of the Superalloy Melt Spinning Process
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A KINETIC AND THERMAL STUDY OF THE SUPERALLOY MELT SPINNING PROCESS
S. C. HUANG AND R. P. LAFORCE General Electric Corporate Research and Development, P.O. Box 8, Schenectady, NY 12301
ABSTRACT The correlation between ribbon thickness and the length of the melt puddle residing on the surface of a meltspinning wheel was established for a Ni-base superalloy. Since the melt puddle length defines the solidification time in which a ribbon with a certain thickness is formed, the above correlation allowed a direct derivation of the propagation velocity of the solid-liquid interface. The solidification rate V (mm/s) so obttined as a function of ribbon thickness S (mm) is V = 3.54S . Further, the above solidification correlation was analyzed using heat transfer considerations to yield information about the ribbon-wheel interfacial thermal conductance, the solid-liquid interfacial temperature, and the local cooling rate through the ribbon thickness. These thermal results are compared to those deduced from the secondary dendrite arm spacing measurements. Finally, there is a discussion on the ribbon microstructure based on our rapid solidification kinetic result.
INTRODUCTION Dendritic, cellular and planar solidification, in that order, produce microstructures of increasing compositional homogeneity. The solidification morphology is the manifestation of the solid-liquid interface stability (with the planar interface being the most stable mode), and is generally controllable by processing parameter adjustments according to several stability guidelines [1-31. At ordinary solidification rates where the constitutional undercooling principle [1,2] is applicable, the interface increases its stability at an increasing ratio between the liquid thermal gradient and solidification velocity, G /V. At high solidification rates where the absolute stability theory [2,1] prevails, the interfacial stability increases with the rate of solidification, V. Note that the solidification velocity exhibits reverse effects on the interface stability in the two solidification regimes. Note also that in the rapid solidification regime, the solidification velocity becomes the sole process parameter controlling the interface stability and morphology (at a fixed alloy composition). An adequate knowledge of the rapid solidification kinetics is therefore critically demanded for an understanding of the rapid solidification microstructure. In this paper, we report on the measurement of solidification velocity in the process of melt spinning a Ni-base superalloy. The measurement technique involves the correlation of ribbon thickness to melt puddle residence time, which is determined by high speed photography [4]. This kinetic result is used with the absolute stability theory to interpret the cellular-to-dendritic transition observed in superalloy ribbons as reported previously [5,6]. The above correlation also yields information about the ribbon-wheel interfacial heat transfer coefficient as well as the ribbon
Hat. Res. Soc. Symp.Proc. Vol.
28 (1984) Publi
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