A fluid mechanics model of the planar flow melt spinning process under low reynolds number conditions
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INTRODUCTION
BECAUSEof the superior mechanical and corrosion resistance properties of microcrystalline metal, rapid solidification processes are of considerable interest in the aluminum industry. One of the commonly used industrial processes is the melt spinning process. Molten metal is ejected through a nozzle onto a chilled rotating wheel in the process and solidifies into a thin ribbon. There are two variations in the melt spinning process: the chilled block process and the planar flow process. 5 In the chilled block process, the nozzle is placed at some distance away from the rotating wheel and molten metal flow is controlled by the applied pressure drop across the nozzle. In the planar flow process, on the other hand, the clearance between the nozzle and the rotating wheel is small and the dominant force in controlling molten metal flow is the viscous drag generated by the rotating wheel. What is discussed is a mathematical model that describes the fluid flow behavior in the planar flow melt spinning process. The constraints imposed on the model are that the Reynolds numbers for fluid flow under the nozzle and the liquid puddle beyond the nozzle are small. Trend predictions calculated by the model agreed well with experimental observations and have provided some insights to the process.
~ Molten metal
PROCESS D E S C R I P T I O N
A schematic diagram of the planar flow melt spinning process is shown in Figure 1. Metal is melted by an induction heater and kept at a predetermined degree of superheat in the reservoir. The nozzle is placed at a small distance from the chilled wheel, rotating at a linear velocity, U. Typical clearance between the nozzle and the wheel is less than 0.001 m. Molten metal is forced through the nozzle by an applied pressure, Ps, and onto the chilled rotating wheel. The molten metal puddle under the nozzle is contained by the upstream meniscus and the downstream meniscus. Solidified ribbon is formed and detached from the wheel downstream of the nozzle.
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THEORETICAL DEVELOPMENT
An idealized geometry of the planar flow melt spinning process is shown in Figure 2. The radius of the chilled wheel is normally large in a commercial unit. Its surface can be considered to be flat and centrifugal force acting on the molten metal puddle is ignored. Chu, Giron, and Granger ] have shown that the solidification front velocity in a melt spun ribbon can be approximated to be constant throughout
P = Pg Reservoir J-
I[
P "~ Patm
g
--
p=Patm U-stream
t-
meniscus
~-~ H I~l~ ~ ~
INozzle I ~ ~1 ] \\| --. \ t,uome
~ Induction heater ~ / Downstream meniscus ~//Solidification ~ / front . Solidified ..... metallic r i b b o n
Chilled rotating wheel
Fig. 1 --Schematic diagram of the planar flow melt-spinning process.
H. YU is Senior Technical Specialist, Alcoa Laboratories, Aluminum Company of America, Alcoa Center, PA 15069. Manuscript submitted November 24, 1986.
METALLURGICAL TRANSACTIONS B
the thickness of the ribbon if no undercooling is involved. They have measured solidif
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