The use of gas-generating mold coatings for the grain refinement of aluminum
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N. L. CUPINI,
M. PRATES,
AND
G. J. DAVIES
E x p e r i m e n t s a r e d e s c r i b e d in which an e f f e r v e s c e n t mold coating c o n t a i n i n g h e x a c h l o r o ethane has b e e n u s e d to p r o m o t e the d y n a m i c g r a i n r e f i n e m e n t of c o m m e r c i a l p u r i t y a l u m i n u m ingots. The i n f l u e n c e of the a m o u n t of h e x a c h l o r o e t h a n e in the mold coating, the d e g r e e of s u p e r h e a t of the cast m e t a l and the n a t u r e of the mold m a t e r i a l was a s s e s s e d . C o m p a r i s o n has b e e n made with u n r e f i n e d ingots and with ingots g r a i n r e f i n e d u s i n g conv e n t i o n a l i n o c u l a t i o n p r o c e d u r e s . It is concluded that low l e v e l s of h e x a c h l o r o e t h a n e (10 to 20 wt pct in the mold coating) can p r o d u c e effective r e f i n e m e n t p r o v i d e d conditions e x i s t f a v o r a b l e for the s u r v i v a l of d e n d r i t e f r a g m e n t s o r n u c l e a t i n g p a r t i c l e s g e n e r a t e d by the r e m e l t i n g a s s o c i a t e d with t u r b u l e n c e in the liquid or c h e m i c a l a n d / o r m e c h a n i c a l i n t e r a c t i o n s b e t w e e n the e f f e r v e s c i n g gas and the mold coating. The e f f e c t i v e n e s s of r e f i n e m e n t is e n h a n c e d by high r a t e s of heat t r a n s f e r and low p o u r i n g s u p e r h e a t s .
F I N E - G R A I N E D equiaxed s t r u c t u r e s a r e r e q u i r e d in c a s t i n g s and ingots for all but a few s p e c i a l i z e d a p p l i c a t i o n s . Such s t r u c t u r e s a r e i s o t r o p i c and t h e i r p r o p e r t i e s a r e m a r k e d l y i m p r o v e d as r e g a r d s both m e c h a n i c a l b e h a v i o r , s u r f a c e f i n i s h and r e s p o n s e to h o m o g e n i z a t i o n t r e a t m e n t s .1,2 In the l a t t e r case the d e n d r i t e a r m s p a c i n g is of p r i n c i p a l i m p o r t a n c e while the g r a i n shape and size i n f l u e n c e the s c a l e and r a n g e of m a c r o s e g r e g a t i o n . ~'4 To develop equiaxed s t r u c t u r e s r e q u i r e s the supp r e s s i o n of c o l u m n a r growth and this is a c h i e v e d by e n c o u r a g i n g conditions f a v o r a b l e for the f o r m a t i o n of equlaxed n u c l e i . A n e c e s s a r y p r e r e q u i s i t e is the d i s r u p t i o n of c o l u m n a r c r y s t a l s and the f o r m a t i o n of a c r y s t a l n e t w o r k which will act a s an effective b a r r i e r to f u r t h e r c o l u m n a r growth. The methods adopted can be c l a s s i f i e d into t h r e e g r o u p s . F i r s t l y n u c l e a t i o n can be c o n t r o l l e d by c o n t r o l l i n g the c a s t i n g conditions so as to p r o m o t e copious n u c l e a t i o n and c r y s t a l m u l t i p l i cation in the i n i t i a l l y chilled liquid and on the mold wall. S-s Secondly, i n o c u l a n t s can be i n t r o d u c e d into the liquid to e n h a n
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