Natural convection transients and their effects in unidirectional solidification
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r s n e c eA ssarilyL in the m a N A convection T Uo c c u R j o r i t y of solidification p r o c e s s e s , where a portion of the m e l t is above the f r e e z i n g t e m p e r a t u r e . The e x i s t e n c e of n a t u r a l c o n v e c t i o n in the e a r l y s t a g e s of ingot solidification has been r e c o g n i z e d 1-3 and it is g e n e r a l l y thought that convection in the m e l t m a y affect the p r o c e s s in the following ways: i) bulk motion in the m e l t m a y modify the rate of heat t r a n s f e r at the m e l t - s o l i d i n t e r f a c e and hence affect the a c t u a l , g r o s s solidification r a t e , ii) convective motion m a y influence the s t r u c t u r e of the solid formed, in p a r t i c u l a r the t r a n s i t i o n from c o l u m n a r to equiaxed o r i e n t a t i o n , 4 iii) c o n v e c t i o n in the m e l t would affect the d i s t r i b u tion of solutes in m u l t i c o m p o n e n t s y s t e m s , and finally iv) v e l o c i t i e s in the m e l t would affect the rate at which n o n m e t a l l i c i n c l u s i o n s can float to the surface, s Of these effects mentioned~ the role of convection in m o d i f y i n g solid s t r u c t u r e r e c e i v e d by far the m o s t a t tention. In an elegant e x p e r i m e n t a l study Cole and Bolling 4 d e m o n s t r a t e d that in c a s t i n g s the c o l u m n a r equiaxed t r a n s i t i o n may be s i g n i f i c a n t l y h i n d e r e d by a r r a n g e m e n t s that e l i m i n a t e n a t u r a l convection. Chalm e r s 8 and F l e m i n g s 7 and t h e i r c o w o r k e r s showed that c o n v e c t i o n may be r e a d i l y s u p p r e s s e d by the a p p l i c a tion of an e l e c t r o m a g n e t i c field. The actual convective heat t r a n s f e r p r o c e s s occ u r r i n g ahead of a s o l i d i f i c a t i o n front has also b e e n studied and some i n t e r e s t i n g qualitative o b s e r v a t i o n s have been r e p o r t e d in the l i t e r a t u r e , e-l~ F i n a l l y , in r e c e n t work it has been shown that in u n i d i r e c t i o n a l , c o n t r o l l e d solidification both the shape of the solidification front and its rate of a d v a n c e m e n t a r e c o n s i s tent with e m p i r i c a l e x p r e s s i o n s r e p o r t e d for n a t u r a l convection. 11 While all this work d i s c u s s e d above r e p r e s e n t s the effects of fluid motion in the m e l t , the exact n a t u r e of this fluid motion is yet to be investigated. A b e t t e r u n d e r s t a n d i n g of the (transient) v e l o c i t y d i s t r i b u t i o n in the fluid, o c c u r r i n g ahead of a m o v i n g J. SZEKELY is Professor of Chemical Engineering,State University of New York at Buffalo, Buffalo, N.Y.V. STANEK,formerly PostDoctoral Research Associate, State Unwersity of New York at Buffalo, is at the Institute of ChemicalProcess Fundamentals, Czechoslovak Academy of Science, Prague, Czechoslovakia. Manuscript submitted November 17, 1969. METALLURGICALTRANSACTIONS
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