MECHANICAL PROPERTIES AND MICROSTRUCTURE OF Al-Li INVESTMENT CASTINGS

T. G. HAYNES, M. WYTE, D. WEBSTER
1987 Le Journal de Physique Colloques  
A b s t r a c t The o b j e c t o f t h i s p r o g r a m is t o d e v e l o p A1-Li i n v e s t m e n t c a s t i n g s w i t h m e c h a n i c a l p r o p e r t i e s a t l e a s t e q u i v a l e n t t o A356 (0.2% t e n s i l e y i e l d s t r e n g t h 260-310 mPa a n d t e n s i l e e l o n g a t i o n 3-5%) b u t w i t h a 5-10% r e d u c t i o n i n d e n s i t y . T h i n w a l l c a s t i n g s w i t h complex s h a p e s have b e e n s u c c e s s f u l l y c a s t a n d 25 c o m p o
more » ... a n d 25 c o m p o s i t i o n s have b e e n examined w i t h l i t h i u m c o n t e n t s up t o 3.6%. M e c h a n i c a l p r o p e r t i e s w e l l a b o v e t h e minimum r e q u i r e m e n t s f o r A356 have b e e n o b t a i n e d w i t h some c o m p o s i t i o n s . D e n s i t i e s a s low a s 2.44 g/ml were r e a c h e d w i t h t h e h i g h e r l i t h i u m a l l o y s . The m i c r o s t r u c t u r e , f r a c t u r e b e h a v i o u r a n d e f f e c t o f l i t h i u m o n t h e m e c h a n i c a l p r o p e r t i e s o f v a r i o u s a l l o y m a t r i c e s i s d e s c r i b e d . F u t u r e work w i l l b e aimed a t i n c r e a s i n g t h e s t r e n g t h o f t h e a l l o y s t o t h e l e v e l s o f A201. I n t r o d u c t i o n I n c o n t r a s t t o A1-Li w r o u g h t a l l o y s t h e r e i s v e r y l i t t l e a c t i v i t y a t p r e s e n t i n t h e f i e l d o f A1-Li c a s t i n g s . The o n l y p u b l i s h e d work i n t h i s a r e a was by Haynes e t a 1 o f Rex P r e c i s i o n P r o d u c t s i n 1986 < r e f . 1> w h i c h d e s c r i b e d t h e p r o p e r t i e s o f A 1 -L i i n v e s t m e n t c a s t i n g s o f t h r e e d i f f e r e n t c o m p o s i t i o n s . Because o f t h e r e a c t i v i t y o f m o l t e n A1-Li a l l o y s s p e c i a l a t t e n t i o n m u s t b e p a i d t o t h e r e f r a c t o r i e s a n d a t m o s p h e r e s u s e d d u r i n g m e l t i n g a n d c a s t i n g . C o n t a c t w i t h oxygen a n d m o i s t u r e w i l l r e s u 1 . t i n l o s s o f l i t h i u m a n d h y d r o g e n l e v e l s u p t o f i f t y t i m e s t h o s e found i n o t h e r aluminum c a s t i n g s . R e s u l t s a n d Di s c u s s i o n M i c r o s t r u c t u r e . Many o f t h e a l l o y s d e v e l o p e d o n t h i s program h a d few p h a s e s t h a t were i n s o l u b l e a t t h e s o l u t i o n t r e a t m e n t t e m p e r a t u r e . However a l l o y s w i t h h i g h s i l i c o n c o n t e n t s showed a u n i f o r m d i s t r i b u t i o n o f a n u n u s u a l i n t e r m e t a l l i c p h a s e . The f r a c t u r e b e h a v i o u r o f t h i s a l l o y was a l s o u n u s u a l i n t h a t u n l i k e t h e n o r m a l d u c t i l e r u p t u r e o r i n t e r g r a n u l a r f a i l u r e o b s e r v e d i n m o s t A l -L i a l l o y s , t h i s a l l o y f a i l e d m a i n l y by c l e a v a g e . A more n o r m a l f r a c t u r e b e h a v i o u r w a s o b s e r v e d f o r a 2.6% L i a l l o y w i t h m a t r i x C, which i n t h e peak a g e d c o n d i t i o n f r a c t u r e d p r e d o m i n a n t l y i n a n i n t e r g r a n u l a r manner < F i g . 3> a l t h o u g h o c c a s i o n a l g r a i n s were o b s e r v e d t o f r a c t u r e by c l e a v a g e w i t h f r a c t u r e i n i t i a t i n g a t a p a r t i c l e w i t h i n t h e g r a i n . C l e a v a g e c r a c k s i n w r o u g h t A1-Li a l l o y s h a v e b e e n o b s e r v e d by F a g e r e t a 1 whose work s u g g e s t e d t h a t a l k a l i m e t a l i m p u r i t i e s which were l i q u i d a t room t e m p e r a t u r e , s u c h a s t h e Na-K e u t e c t i c , a c c e n t u a t e d t h e t e n d e n c y t o c l e a v a g e f a i l u r e . (')consultant at 20565 Verde Vista Lane. Saratoga. CA 95070, USA Article published online by EDP Sciences and available at http://dx.s e a t a g r a i n b o u n d a r y l o c a t i o n w i l l r e s u l t i n i n t e r g r a n u l a r f r a c t u r e . Na a n d K have b e e n o b s e r v e d t o s e g r e g a t e t o s o l i d p a r t i c l e s i n g r a i n b o u n d a r i e s i n A1-Li a l l o y s . I n c r e a s i n g t h e l i t h i u m c o n t e n t w h i l e m a i n t a i n i n g t h e m a t r i x c o m p o s i t i o n c o n s t a n t p r o d u c e s s e c o n d a r y h a r d e n i n g i n c a s t i n g s when t h e l i t h i u m c o n t e n t is above 1.4% . The y i e l d s t r e n g t h shows a s i m i l a r i n c r e a s e d u r i n g a g i n g < F i g . 5> w h i l e t h e d u c t i l i t y d e c r e a s e s a s t h e l i t h i u m c o n t e n t is i n c r e a s e d . S i m i l a r b e h a v i o u r i s o b s e r v e d f o r o t h e r a l l o y m a t r i c e s a n d t h e y i e l d s t r e n g t h -e l o n g a t i o n r e l a t i o n s h i p f o r a l l t h e a l l o y s i n v e s t i g a t e d a r e shown i n F i g u r e 6. The stress s t r a i n c u r v e s f o r t h r e e of t h e a l l o y s a r e shown i n F i g u r e 7 and t h e s e e m p h a s i z e a g a i n t h a t h i g h e r l i t h i u m c o n t e n t s which mean l o w e r d e n s i t i e s r e s u l t i n e i t h e r a l o s s of d u c t i l i t y o r a l o s s o f s t r e n g t h . The d e g r e e t o which t h i s o c c u r s i s shown i n F i g u r e 8 where t h e two d e s i g n p a r a m e t e r s , d e n s i t y a n d y i e l d s t r e n g t h a r e p l o t t e d t o show t h e t e n s i l e e l o n g a t i o n which c a n b e o b t a i n e d w i t h e a c h c o m b i n a t i o n . C o n c l u s i o n s . 1. A1-Li i n v e s t m e n t c a s t i n g s c a n b e made which e a s i l y meet t h e minimum r e q u i r e m e n t s f o r A356 c a s t i n g s and g i v e 5-8% r e d u c t i o n i n d e n s i t y . 2. A s t h e d e n s i t y o f A1-Li i n v e s t m e n t c a s t i n g s i s r e d u c e d a s a c r i f i c e h a s t o b e made i n e i t h e r s t r e n g t h -o r d u c t i l i t y , . C l e a v a g e f r a c t u r e was o b s e r v e d i n some A1-Li i n v e s t m e n t c a s t i n g s a f t
doi:10.1051/jphyscol:1987315 fatcat:dt3kkh76cjeflkrhbmtvhsxj4y