Pipe formation in Pb-Sn alloys
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AND
F. WEINBERG
It has been shown that upward flowing pipes can be f o r m e d in the s o l i d - l i q u i d region of a Pb-Sn alloy when a density inversion e x i s t s in the liquid. P i p e s do not form when t h e r e is no density i n v e r s i o n . These findings support proposed m e c h a n i s m s for freckling and A s e g r e g a t e formation, b a s e d on o b s e r v a t i o n s of t r a n s p a r e n t ammonium c h l o r i d e - w a t e r models. R ECENTLY Giamei e t a l . 1'2 r e p o r t e d the p r e s e n c e of f r e c k l e s ( t r a i l s of equiaxed grains) on the outer surface of u n i d i r e c t i o n a l l y cast nickel based s u p e r a l l o y s . They p r o p o s e d that the f r e c k l e s w e r e caused by the f o r m a tion of lower density liquid close to the bottom of the casting during solidification. This density inversion r e s u l t e d in an upward flow of the lighter liquid through the s o l i d - l i q u i d r e g i o n along d i s c r e t e p i p e s . Examination of a t r a n s p a r e n t ammonium c h l o r i d e - w a t e r model in which a density inversion o c c u r r e d during s o l i d i f i c a tion, showed that upward flowing liquid pipes did form through the s o l i d - l i q u i d regions, supporting their hypothesis. McDonald and Hunt s examined an ammonium chlor i d e - w a t e r model of a conventional casting, and obs e r v e d that fluid flow o c c u r r e d through the s o l i d - l i q u i d r e g i o n s in the form of r i s i n g p i p e s , which they a t t r i b uted to the formation of lower density liquid in the int e r d e n d r i t i c r e g i o n s during solidification. They cons i d e r e d these pipes to be analogous to A s e g r e g a t e s in l a r g e steel castings. However, the use of ammonium c h l o r i d e - w a t e r s y s t e m s to s i m u l a t e the solidification of castings has been questioned by Standish. 4 It has a l s o been shown by Stewart and Weinberg, 5 using r a d i o a c t i v e t r a c e r techniques, that the convective flow velocity and flow path in w a t e r b a s e d s y s t e m s can be m a r k e d l y different to that which o c c u r s in liquid m e t a l s . This is due to the l a r g e difference in the Prandtl n u m b e r s , a d i m e n s i o n l e s s p a r a m e t e r which c h a r a c t e r i z e s convective flow. In spite of the advantages of t r a c e r techniques, the fact that m e t a l s a r e opaque makes it e x t r e m e l y difficult to r e p r o d u c e the ammonium c h l o r i d e - w a t e r e x p e r i m e n t s in an a l l - m e t a l s y s t e m . F o r example, to e s t a b l i s h that pipes flow upwards during solidification, it would be n e c e s s a r y to introduce t r a c e r close to the bottom of the advancing s o l i d - l i q u i d region. E x p e r i m e n t a l l y this would be v e r y difficult. F o r this r e a s o n e x p e r i m e n t s were designed to e s t a b l i s h the following: a) whether a density inversion can cause liquid to flow upward through the mushy zone;
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