The solidification of inoculated aluminum ingots
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The m a c r o s t r u c t u r e s of a l u m i n u m a n d its a l l o y s solidified with i n o c u l a n t s a r e r e e x a m i n e d here by c o m p a r i n g s t r u c t u r e s produced through n o r m a l s o l i d i f i c a t i o n with those p r o d u c e d by the e x t e r n a l force fields of s t e a d y - s t a t e mold o s c i l l a t i o n . The i n o c u l a t i o n r e a c t i o n of one specific c o m m e r c i a l product shows how f o r e i g n additions used for n u c l e a t i o n i n t e r a c t with the t h e r m a l and fluid flow fields to modify s t r u c t u r e . A s p e c i a l c o m b i n a t i o n of r o t a t i o n and o s c i l lation within the s e l f s a m e ingot allows d i s c r i m i n a t i o n between the g r a i n r e f i n e m e n t produced by a n inoculant and that produced by any u s u a l alloying e l e m e n t .
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e x p l a n a t i o n s which have b e e n a d v a n c e d for the t r a n s i t i o n from c o l u m n a r to equiaxed growth r e q u i r e that equiaxed g r a i n s n u c l e a t e in the liquid when the t e m p e r a t u r e g r a d i e n t in the neighborhood of the solidliquid i n t e r f a c e b e c o m e s s u f f i c i e n t l y low. 1-4 U n d e r t h e s e conditions when the n u m b e r and size of the e q u i axed g r a i n s become l a r g e enough~ no f u r t h e r e x t e n s i o n of c o l u m n a r g r a i n s is p o s s i b l e s and c o l u m n a r growth i s s u p e r s e d e d by equiaxed growth. The t r a n s i t i o n f r o m c o l u m n a r to equiaxed g r a i n s m a y thus occur n a t u r a l l y when the t e m p e r a t u r e g r a d i e n t d e c r e a s e s to a c r i t i c a l v a l u e , or it may also be induced by e x t e r n a l f o r c e s of c h e m i c a l or physical o r i g i n . No r e v i e w is a t t e m p t e d h e r e , but we intend to show how e x t e r n a l l y a p p l i e d or p h y s i c a l f o r c e s can a l t e r the efficacy of c h e m i c a l ones. Only one type of e x t e r n a l force field will be d i s c u s s e d . A r o t a t i o n , which p r o d u c e s a q u i e s c e n t liquid, o r a r o t a t i o n - r e v e r s a l (oscillation), which p r o d u c e s a d i s r u p t i v e force at the s o l i d - l i q u i d i n t e r f a c e ; p a r t i c l e s or bits of the s o l i d - l i q u i d i n t e r f a c e a r e t h e r e b y r e m o v e d from their u s u a l p o s i t i o n and a r e r e d i s t r i b u t e d throughout the liquid, s T h e s e b i t s s u r v i v e since the m e l t also cools m o r e r a p i d l y to the f r e e z i n g t e m p e r a t u r e of the alloy under the r a p i d heat t r a n s f e r c a u s e d by the o s c i l l a t i o n a c t i o n . A s t e a d y - s t a t e r o t a t i o n , on the other hand, damps out n a t u r a l fluid m o t i o n , thus r e d u c i n g the n u m b e r of any free solid b i t s ; at the s a m e t i m e a r e d u c t i o n in n a t u r a l flow allows the t e m p e r a t u r e g r a d i e n t s in the m e l t to r e m a i n higher wh
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