Solidification in continuous casting of aluminum
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During casting of aluminum ingot by a new process utilizing level pouring, variations in cooling rate within the surface band occur as a result of the discontinuous nature of metal flow. Associated segregation contributes to a marked cyclic pattern of the macrostructure in that region. The kinetics of metal flow into the surface zone are explained graphically using a sequence of macrographs. The proposed mechanism is also supported by results of thermal measurements and Electron Microprobe analyses.
I N p r e s e n t - d a y c o m m e r c i a l c a s t i n g of n o n f e r r o u s m e t a l s by s e m i c o n t i n u o u s m e t h o d s , two p a t t e r n s e x i s t for i n t r o d u c i n g m o l t e n m e t a l into the mold: 1) The m o l t e n m e t a l s u r f a c e i s within the mold and n e a r the l e v e l at which s o l i d i f i c a t i o n takes place. Molten m e t a l is supplied f r o m a trough located at a h i g h e r level. 2) The m o l t e n m e t a l s u r f a c e i s outside the mold and at a higher l e v e l than that at which s o l i d i f i c a t i o n takes place. Molten m e t a l is s u p p l i e d by " l e v e l pouring." In the field of a l u m i n u m ingot casting, the o r i g i n a l d i r e c t c h i l l (or DC) p r o c e s s f a l l s into the f i r s t c a t e gory. Newer a p p r o a c h e s e m p l o y the second method in which the t e r m " l e v e l - p o u r i n g " i n d i c a t e s that no diff e r e n t i a l in m e t a l level e x i s t s between taphole and mold. Let u s r e v i e w b r i e f l y the s e q u e n c e ol m e t a l l u r g i c a l e v e n t s in the m o l d in which the m e t a l s o l i d i f i e s n e a r the l e v e l of the m e n i s c u s of the m o l t e n m e t a l s u r f a c e . When c o n t a c t is made with the w a t e r - c o o l e d mold wall, a solid s h e l l f o r m s to c o n t a i n the m o l t e n core u n t i l d i r e c t w a t e r chilling s o l i d i f i e s the whole ingot. B e c a u s e of the low heat c o n t e n t of the DC s y s t e m , t e m p e r a t u r e s of the m o l t e n m e t a l pool within the m o l d cavity a r e at or n e a r the l i q u i d u s t e m p e r a t u r e . T h i s i m p o s e s l i m i t a t i o n s on the ingot s t r u c t u r e with p o t e n tial disadvantages during subsequent fabrication. L e v e l - p o u r i n g methods, on the other hand, r e p r e s e n t a high heat content s y s t e m , p e r m i t t i n g b e t t e r c o n t r o l o v e r the p r o c e s s of s o l i d i f i c a t i o n and hence o v e r the q u a l i t y of the ingot. S e v e r a l a p p r o a c h e s a r e a v a i l able to attain this goal. C o m m o n to all is a r e s e r v o i r of m o l t e n m e t a l d i r e c t l y above the mold. T h i s p r o v i d e s for m o l t e n m e t a l feed over the e n t i r e ingot c r o s s s e c tion, thus c o n t r i b u t i n g to i n c r e a s e d u n i f o r m i t y of f r e e z i n g c h a r a c t e r i s t i c s . V a r i a t i o n s of p o u r i n g t e m p e r a t u r e and
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