The development of macrostructure in ingots of increasing size
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A s e r i e s of A1-Cu ingots r a n g i n g in v o l u m e from a p p r o x i m a t e l y 50 to 500 and to 5000 cc has b e e n c o n v e n t i o n a l l y p r e p a r e d in two ways, by n o r m a l chill c a s t i n g or by using a m a g n e t i c field to c o n t r o l the convective c u r r e n t s . C o m p a r i s o n s allow us to d i f f e r e n t i a t e a m o n g the v a r i o u s m e c h a n i s m s p r o p o s e d for the o r i g i n of equiaxed g r a i n s and for the c o l u m n a r - t o equiaxed t r a n s i t i o n in c a s t i n g s : i) In n o r m a l c a s t i n g , with the n a t u r a l convection a s s o c i a t e d with s t a n d a r d s u p e r h e a t s , the m a j o r i t y of g r a i n s is provided by the m e c h a n i s m suggested by C h a l m e r s (and a l s o known a s the Big Bang), or by c r y s t a l s e t t l i n g f r o m a free s u r f a c e a s suggested by Southin. ii) If new g r a i n s , no m a t t e r how they m a y f o r m , a r e inhibited f r o m m i x ing with the r e s t of the liquid, no s i g n i f i c a n t equiaxed s t r u c t u r e a p p e a r s , iii) U n l e s s s o m e s p e c i a l or p r e s e l e c t e d conditions p r e v a i l , other m e c h a n i s m s such a s those involving c o n s t i t u tional s u p e r c o o l i n g or d e n d r i t e r e m e l t i n g , do not s e e m o p e r a t i v e in the f o r m a t i o n of a c e n t r a l equiaxed zone.
T H E o r i g i n of p a r t i c u l a r c a s t i n g m a c r o s t r u c t u r e s has been of c o n s i d e r a b l e i n t e r e s t as gaged by the p r o p o r t i o n given to this topic in m a j o r r e v i e w s of s o l i d i fication. 1-5 But this i n t e r e s t , which has b e e n p u r s u e d in the l a b o r a t o r y , l e a v e s open the q u e s t i o n s : does a m o r e e a s i l y obtained l a b o r a t o r y - s c a l e s t r u c t u r e r e p r e s e n t the m o r e c o m m o n l a r g e scale product; and if not, what a r e the d i f f e r e n c e s ? Our r e s e a r c h h e r e will not q u e s t i o n the a c c e p t e d e x p l a n a t i o n s for the o r i g i n of the chill zone 6,v and the d e v e l o p m e n t of the c o l u m n a r zone 8 but will c o n c e n t r a t e on the e x p l a n a t i o n s for the c o l u m n a r - t o - e q u i a x e d t r a n s i t i o n . 9-21 T h e s e have often r e s u l t e d f r o m studies on r e l a t i v e l y s m a l l - s i z e ingots (less than 500 cc), even though C h a l m e r s 4 has p r o p e r l y e m p h a s i z e d that it is difficult to accept e x t r a p o l a t i o n to l a r g e r s i z e s without proof. A volume p r o g r e s s i o n of ingot s i z e s is e x a m i n e d : 1, 10, and then 100, in o r d e r to obtain not only the r e s u l t s for the l a r g e s t size ingot (4720 cc), but to e s t a b l i s h whether or not s o m e e x t r a p olation i s allowable. The m a n i p u l a t i o n of fluid flow, u t i l i z i n g applied m a g n e t i c f i e l d s is i m p o r t a n t for this p u r p o s e , and p r o v
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