Observations of ingot macrosegregation on model systems
- PDF / 2,075,892 Bytes
- 8 Pages / 612 x 783.28 pts Page_size
- 29 Downloads / 214 Views
IN
t h i s p a p e r m a c r o s e g r e g a t i o n is u s e d to d e s c r i b e the g r a d u a l change in c o m p o s i t i o n f r o m one p a r t of the c a s t i n g to a n o t h e r . T h i s i s in c o n t r a s t to the m i c r o s e g r e a t i o n b e t w e e n d e n d r i t e s and the l o c a l o r c h a n n e l t y p e s e g r e g a t i o n , r e s u l t i n g f r o m l a r g e p o c k e t s of liquid b e i n g f o r m e d 1-4 o r left behind the g r o w t h i n t e r face. M a c r o s e g r e g a t i o n is c o n s i d e r e d to be the r e s u l t of a n u m b e r of s e p a r a t e p r o c e s s e s : 1. The m i x i n g s o r flow of the diffusion l a y e r a h e a d of the g r o w t h i n t e r f a c e into the bulk liquid. 2. The p r e c i p i t a t i o n of s o l i d (or a n o t h e r liquid) p h a s e in the bulk liquid and i t s s u b s e q u e n t d e p o s i t i o n . 3. The flow of liquid in the s e m i - s o l i d r e g i o n r e s u l t i n g f r o m a v o l u m e change on f r e e z i n g . 6'7 4. The c o n v e c t i v e flow of liquid in the s e m i - s o l i d r e g i o n r e s u l t i n g f r o m liquid d e n s i t y c h a n g e s within the f r e e z i n g m a t e r i a l . 1'4 The f i r s t of t h e s e p r o c e s s e s o c c u r s when the diffus i o n l a y e r a h e a d of the i n t e r f a c e flows o r is m i x e d into the bulk liquid. T h i s m e c h a n i s m is l i k e l y to be i m p o r t a n t when a m a t e r i a l g r o w s with a p l a n a r o r a l m o s t p l a n a r i n t e r f a c e . It will b e l e s s i m p o r t a n t in d e n d r i t i c g r o w t h b e c a u s e of the much s m a l l e r diffusion l a y e r a h e a d of the d e n d r i t i c i n t e r f a c e . B e c a u s e flow o r m i x i n g is a slow p r o c e s s , a p p r e c i a b l e m a c r o s e g r e g a t i o n only o c c u r s when long f r e e z i n g t i m e s a r e involved. The s e c o n d m e c h a n i s m o c c u r s when e q u i a x e d c r y s tals or non-metallic inclusions are preferentially dep o s i t e d in one p a r t ' o f the ingot. I m p o r t a n t f a c t o r s in c o n t r o l l i n g the s e g r e g a t i o n will be the n u c l e a t i o n and growth of the new p h a s e s . M a c r o s e g r e g a t i o n will howe v e r only o c c u r when the new p h a s e s e t t l e s out, due to g r a v i t y o r s o m e o t h e r p r o c e s s , Again a p p r e c i a b l e s e g r e g a t i o n w i l l only be o b t a i n e d when s u f f i c i e n t t i m e is available during freezing. M a c r o s e g r e g a t i o n o c c u r s b y the t h i r d m e c h a n i s m when fluid flows b e t w e e n the d e n d r i t e s to t a k e up the v o l u m e change on f r e e z i n g . A p p r e c i a b l e s e g r e g a t i o n o c c u r s when the g r o w t h c o n d i t i o n s , the v e l o c i t y and t e m p e r a t u r e g r a d i e n t s , a r e changing r a p i d l y (no m a c D. J. HEBDITCH, formerly Student, Department of Metallurgy, Oxford University, is now with the Marchwood
Data Loading...
