Effect of solidification microstructure on corrosion behavior of the chill zone in aluminum binary alloys
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n a t u r e and v a r i a b l e s affecting the c h i l l z o n e in i n g o t s and c a s t i n g s have b e e n s t u d i e d to s o m e e x t e n t . 1-3 B i l o n i e t al 2-3 s t u d i e d the s u b s t r u c t u r e s o r i g i n a t e d at the c a s t s u r f a c e in r e l a t i o n s h i p with d i f f e r e n t v a l u e s of the heat t r a n s f e r c o e f f i c i e n t of the m e t a l / / m o l d i n t e r f a c e , hi, r a n g i n g f r o m e x p o s e d m o l d s u r f a c e s to t h o s e p a i n t e d with s e v e r a l r e f r a c t o r y p r o d u c t s . In a l l c a s e s d e n d r i t i c growth i s p r e c e d e d by p r e d e n d r i t i c r e g i o n s . 4 In o r d e r to s u m m a r i z e the r e s u l t s of t h o s e a u t h o r s it i s n e c e s s a r y to t a k e into account the s t a t e m e n t of P r a t e s and B i l o n i about the n a t u r e of the h i c o e f f i c i e n t : a " T h i s c o e f f i c i e n t , m e a s u r e d by a l i n e a r f l u i d i t y t e s t , i s an a v e r a g e v a l u e of the h e a t t r a n s f e r c o e f f i c i e n t at the m e t a ~ m o l d i n t e r f a c e , a f a c t t h a t m u s t be kept in m i n d b e c a u s e the n a t u r e of c o n t a c t b e t w e e n the liquid and the m o l d w a l l s m a y v a r y f r o m p o i n t to point and, a s a c o n s e q u e n c e , the heat e x t r a c t i o n m a y change in the s a m e way at the m o l d s u r f a c e . " i) F o r a given m i c r o g e o m e t r y the v a l u e of t h e h e a t t r a n s f e r coefficient at t h e m e t a l / m o l d i n t e r f a c e , hi, a p p e a r s to c o n t r o l the n u m b e r of n u c l e i f o r m e d in c o n t a c t with the m o l d s u r f a c e . The n u c l e i can be d e t e c t e d t h r o u g h m e t a l l o g r a p h i c o b s e r v a t i o n of p r e d e n d r i t i c r e g i o n s .3 ii) F o r a given h i and b a r e m o l d s , a r e l a t i o n s h i p b e t w e e n s u r f a c e r u g o s i t y of t h e m o l d and d e n s i t y and d i s t r i b u t i o n of p r e d e n d r i t i c r e g i o n s h a s b e e n found, t a k i n g into account how the liquid c o n t a c t s the a s p e r i t i e s of the m o l d s u r f a c e . At t h e s e a s p e r i t i e s a high r a t e of heat e x t r a c t i o n o c c u r s , and the liquid s u f f e r s t h e r m a l s u p e r c o o l i n g ; p r e d e n d r i t i c nuclei which f o r m a s a r e s u l t can be s e e n by m i c r o s c o p i c i n s p e c t i o n . P r a t e s and Biloni s r e p o r t e d one to one r e l a t i o n s h i p e x i s t i n g b e t w e e n the c u s p s of t h e s u r f a c e m i c r o p r o f i l e and the p r e d e n d r i t i c n u c l e i , s e e F i g s . 7(a) and (b) f r o m R e f . 3. F r o m t h e s e r e s u l t s and t h o s e r e p o r t e d by B i l o n i and C h a l m e r s , F i g s . 3 and 4 f r o m Ref. 4, i t i s p o s s i b l e to p r e d i c t t h a t t h r o u g h a s u i t a b l e a b r a s i o n of
A. M. ZOSSI is with Universidad Nacional d
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