Application of electroslag melting
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of the p u b l i s h e d work on the d i r e c t i o n a l f r e e z i n g of eutectic m a t e r i a l s r e l a t e s to s p e c i m e n s which have been g r o w n by conventional m e t h o d s , in p a r t i c u l a r the B r i d g e m a n t e c h n i q u e , although a few e x a m p l e s of zone m e l t i n g have b e e n d e s c r i b e d , z-3 T h e s e t e c h n i q u e s have n e c e s s a r i l y tended to be r e s t r i c t e d in s c a l e and a p p l i c a t i o n , a n d it would s e e m to b e i m p o r t a n t to i n v e s t i g a t e the a p p l i c a t i o n of any p r o c e s s e s which could be u s e d on a l a r g e r s c a l e and for a w i d e r r a n g e of m e t a l l i c m a t e r i a l s . The p r e s e n t note d e s c r i b e s s o m e e x p l o r a t o r y s t u d i e s d e s i g n e d to i n v e s t i gate the u s e of the e s t a b l i s h e d i n d u s t r i a l p r o c e s s of e l e c t r o s l a g m e l t i n g i n the growth of c o m p o s i t e materials. The e s s e n t i a l f e a t u r e s of the e l e c t r o s i a g p r o c e s s a r e i l l u s t r a t e d in F i g . 1, and it can be s e e n that it i s e s s e n t i a l l y a z o n e - m e l t i n g technique in which m a t e r i a l i s supplied to the m o l t e n zone through a l a y e r of fused s a l t , and t h e r e f o r e it has the advantage that long r a n g e s e g r e g a t i o n is avoided. 4 In a t y p i c a l i n d u s t r i a l a p p l i c a tion 5 w h e r e the s p e c i m e n m a y be up to 1 m in d i a m , the l a r g e t h e r m a l capacity of the s y s t e m m a k e s it r e l a t i v e l y stable to t e m p e r a t u r e f l u c t u a t i o n s , but t h i s will be the c a s e for s m a l l s c a l e a p p a r a t u s only if the m e t a l pool depth i s g r e a t e r than v~-t, w h e r e D i s the t h e r m a l diffusion coefficient, and t is the t i m e i n t e r v a l b e t w e e n the a r r i v a l of s u c c e s s i v e d r o p s of m e t a l a t the u p p e r s u r f a c e of the m e t a l pool. If drop size or v e l o c i t y is e x c e s s i v e , long r a n g e d i s t u r b a n c e s of the m e t a l pool may upset this assumption. This aspect is particularly i m p o r t a n t in the growth of s p e c i m e n s of e u t e c t i c a l l o y s w h e r e the continuity of the s t r u c t u r e i s s u s c e p t i b l e to f l u c t u a t i o n s in the growth r a t e . A s u c c e s s f u l low t e m p e r a t u r e model of the p r o c e s s h a s b e e n developed u s i n g a n a p p a r a t u s s i m i l a r to that d e s c r i b e d by C a m p b e l l ~ c o m p r i s i n g a t r a n s p a r e n t s i l i c a m o l d of i n t e r n a l d i a m 2.9 c m with a w a t e r - c o o l e d b a s e plate, and two i n d e p e n d e n t l y m o v a b l e e l e c t r o d e s a s shown in Fig. 2. L e a d / t i n eutectic was chosen b e c a u s e of i t s low m e l t i n g point of 183~ and w e l l - d o c u m e n t e d m i c r o s t r u c t u r e . 7,s A l u m i n u m - b a s e d e u t e c t i
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