The effect of natural convection on the shape and movement of the melt-solid interface in the controlled solidification
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E x p e r i m e n t s a r e r e p o r t e d on the study of the effect of n a t u r a l convection on the c o n t r o l l e d solidification of lead under conditions of n e a r l y u n i d i r e c t i o n a l heat flow within the s y s t e m . The e x p e r i m e n t a l l y found shape and position of the s o l i d - m e l t i n t e r f a c e was r e a s o n a b l y well p r e d i c t e d by the c o n s i d e r a t i o n of heat t r a n s f e r by n a t u r a l convection from the melt to the solid s u r f a c e . T r a n s i e n t r u n s were also c a r r i e d out and here the r a t e of m o v e m e n t of the m e l t solid i n t e r f a c e was r e a d i l y p r e d i c t e d f r o m the ( n u m e r i c a l ) s o l u t i o n of the a p p r o p r i a t e unsteady state conduction equation.
ALTHOUGH
n a t u r a l c o n v e c t i o n heat t r a n s f e r and s o l i d i f i c a t i o n have b e e n e x t e n s i v e l y studied i n d i v i d u a l l y , their c o m b i n e d o c c u r r e n c e r e c e i v e d only little a t t e n t i o n until r e c e n t t i m e s . N a t u r a l convection will n e c e s s a r i l y occur in m a n y s o l i d i f i c a t i o n p r o c e s s e s , where a p o r t i o n of the m e l t i s at a higher t e m p e r a t u r e than the m e l t i n g point of the s o l i d phase. Under these c o n d i t i o n s , if there e x i s t s a t e m p e r a t u r e g r a d i e n t , and hence a g r a d i e n t in d e n s i t y with a n o r m a l c o m p o n e n t to the d i r e c t i o n of g r a v i t y , fluid motion will take place.* *Firedmotionwillalsooccurif there existsgradientm densityin a direction opposite to the gravitationalforce field,but thisproblemwillnot be discussed here. Many i n v e s t i g a t o r s have r e c o g n i z e d the e x i s t e n c e of convective fluid m o t i o n in the e a r l y stages of ingot s o l i d i f i c a t i o n 1-3 and it i s g e n e r a l l y thought that conv e c t i o n in the m e l t m a y affect the p r o c e s s in the following m a n n e r : i) Movement of the m e l t may modify the r a t e of heat t r a n s f e r at the m e l t solid i n t e r f a c e and hence the actual g r o s s solidification r a t e . ii) Convective m o t i o n may influence the s t r u c t u r e of the solid f o r m e d , 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 s t r u c t u r e , and finally iii) Convection in the m e l t would a l s o affect the solute d i s t r i b u t i o n in m u l t i c o m p o n e n t s y s t e m s . Of these effects m e n t i o n e d the role of c o n v e c t i v e m o tion in modifying 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 attention. In an e l e g a n t e x p e r i m e n t a l study Cole a n d 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 m a y be s i g n i f i c a n t l y h i n d e r e d by e x p e r i m e n t a l 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. C h a l m e r s and c o - w o
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