On the removal of pores from castings by sintering
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T H E R E i s a c o n s i d e r a b l e body of f o u n d r y l i t e r a t u r e on p o r e f o r m a t i o n d u r i n g s o l i d i f i c a t i o n and on f o u n d r y t e c h n i q u e s f o r i t s r e d u c t i o n o r e l i m i n a t i o n . 1-7 The p o r e s a r e c a u s e d by s h r i n k a g e o c c u r i n g d u r i n g s o l i d i f i c a t i o n , by e x s o l u t i o n of d i s s o l v e d g a s e s , o r by a c o m b i n a t i o n of both. T h u s , the m e t h o d s of m i n i m i z i n g the p o r o s i t y include r e d u c i n g the content of d i s s o l v e d g a s e s , a s s u r i n g an adequate supply of liquid f e e d m e t a l d u r i n g s o l i d i f i c a t i o n , and m a k i n g s o l i d i f i c a t i o n m o r e " d i r e c t i o n a l " . The i m p o r t a n c e of a c h i e v i n g low p o r o s i t y in c a s t m e t a l s i s that even in v e r y s m a l l a m o u n t s it can m a r k e d l y r e d u c e m e c h a n i c a l p r o p e r t i e s . In l a r g e a m o u n t s p o r o s i t y c a u s e s p r e s s u r e c a s t i n g s to l e a k , r e s u l t s in a p o o r s u r f a c e f i n i s h , o r c a u s e s c r a c k i n g d u r ing use. In this p a p e r we d e s c r i b e c u r r e n t w o r k on f o r m a t i o n of m i c r o p o r o s i t y in s o l i d i f i c a t i o n of a l l o y s . We show that g a s i s l i k e l y to be a c o n t r i b u t i n g f a c t o r , if not the m a j o r f a c t o r , l e a d i n g to the m i c r o p o r o s i t y o b s e r v e d in m o s t c a s e s in p r a c t i c e . We s u m m a r i z e o u r u n d e r s t a n d ing of the m o r p h o l o g y of m i c r o p o r o s i t y in t y p i c a l c a s e s , and of i t s r e l a t i o n to g r a i n s i z e and g r a i n b o u n d a r i e s . F i n a l l y , the k i n e t i c s f o r the r e m o v a l of m i c r o p o r o s i t y by s i n t e r i n g a r e c o n s i d e r e d q u a n t i t a t i v e l y .
s o l i d i f i c a t i o n c o n d i t i o n s and the p r o b a b i l i t y of p o r e nuc l e a t i o n with v a r i o u s n e c e s s a r y a s s u m p t i o n s . Howe v e r , the c o m p l e x i t y of the flow f i e l d and the n e c e s s i t y for assumptions regarding pore nucleation make precise prediction difficult. The i m p o r t a n t v a r i a b l e s which m a y a f f e c t p o r e c r e ation are: gas content, solidification shrinkage, surf a c e tensions~ d i s t r i b u t i o n c o e f f i c i e n t , a l l o y c o n t e n t , f r e e z i n g r a t e , c a s t i n g s h a p e , and h e a t e x t r a c t i o n (unid i r e c t i o n a l o r i s o t r o p i c ) . To i l l u s t r a t e the e f f e c t s of t h e s e v a r i a b l e s on p o r e f o r m a t i o n , we c o n s i d e r the f o r m a t i o n of p o r e s in the t h r e e d i f f e r e n t c a s e s i l l u s t r a t e d in F i g . 1. T h e f i r s t is f o r p o r e f o r m a t i o n in single c r y s t a l g r o w t h , when g r o w t h i s c e l l u l a r ; the s e c o n d f o r u n i d i r e c t i o n
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