Liquid Metal Flow in Horizontal Rods
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FLUID
flow d u r i n g the s o l i d i f i c a t i o n of an a l l o y can be a m a j o r c o n t r i b u t i n g f a c t o r to the s e g r e g a t i o n of the a l l o y i n g e l e m e n t s in the r e s u l t a n t s o l i d . It can a l s o be a c o n t r i b u t i n g f a c t o r to the s t r u c t u r e o b t a i n e d in the s o l i d . The fluid flow m a y r e s u l t f r o m f r e e c o n v e c t i o n due to s o l u t e o r t h e r m a l g r a d i e n t s in the liquid, o r f r o m f o r c e d c o n v e c t i o n due to m e c h a n i c a l o r e l e c t r o m a g n e t i c s t i r r i n g i m p o s e d e x t e r n a l l y on the s y s t e m . A l t e r n a t i v e l y , the f r e e convection can be s u p p r e s s e d by s u i t a b l y i m p o s e d e l e c t r o m a g n e t i c f i e l d s . The effect of fluid flow on c r y s t a l g r o w t h has b e e n r e v i e w e d by H u r l e 1. The flow of l i q u i d s and g a s e s in enclosures resulting from natural convection has been s t u d i e d e x t e n s i v e l y both t h e o r e t i c a l l y and e x p e r i m e n t a l l y a s r e v i e w e d by O s t r a c h . 2 In a c o m p r e h e n s i v e r e view, C a r r u t h e r s s h a s s u m m a r i z e d the n a t u r a l c o n v e c tion e x p e r i m e n t a l and t h e o r e t i c a l r e s u l t s f r o m the point of view of c r y s t a l growth. The r e l a t i o n s h ' i p b e t w e e n m a c r o s e g r e g a t i o n and fluid flow was i n v e s t i g a t e d b y W e i n b e r g 4 in which h o r i z o n t a l r o d s of dilute Sn-Ag a l l o y s w e r e s o l i d i f i e d p r o g r e s s i v e l y . Flow was l i m i t e d b y r e d u c i n g the d i a m e t e r of the r o d . F r o m the m e a s u r e d s o l u t e d i s t r i b u t i o n in the s o l i d r o d s , he c o n c l u d e d that e x t e n s i v e m i x i n g o c c u r r e d in the liquid f o r r o d s having a d i a m e t e r g r e a t e r than 2 mm. Cole and W i n e g a r d s d e m o n s t r a t e d that fluid flow o c c u r r e d in the liquid m e t a l d u r i n g s o l i d i f i c a t i o n in a h o r i z o n t a l boat, b y i n s e r t i n g fine t h e r m o c o u p l e s in the liquid. Above a c r i t i c a l t e m p e r a t u r e g r a d i e n t , t h e y o b s e r v e d that the t e m p e r a t u r e in the liquid a s m e a s u r e d b y the t h e r m o c o u p l e s f l u c t u a t e d , the a m p l i t u d e i n c r e a s Ing with i n c r e a s e d t e m p e r a t u r e g r a d i e n t . The f l u c t u a tions w e r e c o n s i d e r e d to be i n d i c a t i v e of t u r b u l e n t c o n v e c t i o n in the m e l t . F r o m t h e i r a n a l y s i s of the r e s u l t s , they c o n c l u d e d that the t e m p e r a t u r e g r a d i e n t and the m e l t height w e r e the m a j o r v a r i a b l e s c o n t r o l l i n g the L. C. MacAULAY, formerly a Graduate Student, Department of Metallurgy, University of British Columbia is now with the Noranda Research Center, Montreal. F. WE1NBERG is Professor of Metall
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