Liquid metal cooling: A new solidification technique
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D I R E C T I O N A L s o l i d i f i c a t i o n r e q u i r e s that heat be w i t h d r a w n f r o m the c a s t i n g in one p r e f e r r e d d i r e c tion, u s u a l l y to a w a t e r - c o o l e d chill plate, F i g . 1. T h i s technique is known as the p o w e r - d o w n p r o c e s s . 1 As the solidified s e c t i o n grows, the heat conduction path b e c o m e s longer, s l o w i n g down the r a t e of cooling and i n t r o d u c i n g s o m e s t r u c t u r a l v a r i a t i o n o v e r the height of the casting. F o r s i n g l e c r y s t a l c a s t i n g s it is n e c e s s a r y to i n t r o d u c e a c o n s t r i c t i o n of the mold between the ingot and the chill plate. 2 T h i s , of c o u r s e , r e d u c e s the flow of heat to the chill plate in p r o p o r t i o n to the c r o s s - s e c t i o n a l a r e a r a t i o , i n s o m e c a s e s down to 10 pct. The c o n t r i b u t i o n of heat r a d i a t i o n to the s u r r o u n d i n g s is l i m i t e d by the fixed s o l i d i f i c a t i o n t e m p e r a t u r e and the high e n v i r o n m e n t a l t e m p e r a t u r e and this component d r o p s off steeply as the c a s t i n g cools. Since high s o l i d i f i c a t i o n r a t e s at r e l a t i v e l y high t h e r m a l g r a d i e n t s a r e d e s i r a b l e not only for e c o n o m y but a l s o for i m p r o v e m e n t of m e c h a n i c a l p r o p e r t i e s and u n i f o r m i t y of s t r u c t u r e , a s e a r c h for other m e a n s to withdraw heat was w a r r a n t e d .
Note that the T~ t e r m in Eq. [2] is n e g l i g i b l e . Now for the " m u s h y " zone, we have f r o m Eq. [1] and [2]: hc ( z f = 1 cm) = 2450 J / m 2 s K
which f a l l s off l i n e a r l y as the solid height grows and: h r ( z = z f ) = 150 J//m 2 s K
which is i n d e p e n d e n t of height s o l i d i f i e d . Thus h c (z = z f ) has dropped to h c = h r when the s o l i d front position is at z f = 16 cm. Since this v a l u e of h r at the m u s h y zone is the m a x i m u m o b t a i n a b l e ( d i s r e g a r d i n g s u p e r h e a t of the
FELT SUSCEPTOR
CONDUCTIVE AND RADIATIVE HEAT T R A N S F E R Let us c o n s i d e r a c y l i n d r i c a l c a s t i n g in a c o n v e n t i o n a l p o w e r - d o w n setup. We define conductive heat t r a n s f e r coefficient of the solid as h e = kS/Z f
k s = 24.5 J//m s K at an a v e r a g e t e m p e r a t u r e of 900 K, zf = 1 cm, T f = 1590 K, T o = 300 K and s = t~1,2 = 0.5.
"HIMBLE
[1]
COIL
and a r a d i a t i v e coefficient h r = (r(E1T4 - 011,2T4) T- T o "
[2]
:EMENT
The following v a l u e s a r e t y p i c a l : A. F. GIAMEIis Senior Materials Project Engineer, Pratt & Whitney Aircraft, East Hartford, CT 06108. J. G. TSCHINKEL,formerly with Pratt & Whitney Aircraft, now retired, resides at 52 Fairview Terrace, South Glastonbury, CT 06073. Manuscript submitted February 13, 1976. METALLURGICALTRANSACTIONS A
Fig. 1--Arrangement of copper chill, ceramic shell mold, induction coil and susceptor for th
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