Effect of superheating on structure of some aluminum alloys
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AND
J. G. BARLOCK
The e f f e c t of i n c r e a s i n g pouring t e m p e r a t u r e s was i n v e s t i g a t e d on s e v e r a l a l u m i n u m a l loys, which w e r e f r o z e n at v a r i o u s r a t e s r a n g i n g f r o m l e s s than 0.01~ (furnace cooled) to m o r e than 2 0 ~ (thin m e t a l mold). It was found that the higher the s u p e r h e a t , the f a s t e r the s o l i d i f i c a t i o n r a t e . I n t e r d e n d r i t i c spacing, s i z e of p r i m a r y and e u t e c t i c c o n s t i t u en t s and g r a i n s i z e d e c r e a s e with the i n c r e a s i n g s o l i d i f i c a t i o n r a t e that r e s u l t s f r o m i n c r e a s e d s u p e r h e a t i n g , in spite of the fact that the actual f r e e z i n g t i m e i n c r e a s e s . I n d i cations a r e that a l u m i n u m alloys contain a nucleant that acts at z e r o u n d e r c o o k i n g and that s u p e r h e a t i n g up to 500~ above the f r e e z i n g point r e d u c e d the s i z e and p o s s i b l y the n u m b e r of n u c l e a t i n g p a r t i c l e s , but does not e l i m i n a t e them c o m p l e t e l y .
PREVIOUS
w o r k 1 had shown that when a l u m i n u m a l loys w e r e s u p e r h e a t e d a f t e r fluxing, the s i z e of the c o n s t i t u e n t s and the i n t e r d e n d r i t i c s p a c in g w e r e s m a l l e r than in c a s t i n g s not s u p e r h e a t e d . This was a t t r i b u t e d to " d e n u c l e a t i o n , " that is r e m o v a l of nuc l e a n t s , which f o r c e d f r e e z i n g at h i g h e r u n d e r c o o l ings and t h e r e f o r e at a f a s t e r r a t e . The work p r e s e n t e d h e r e was undertaken in an a t t e m p t to e s t a b l i s h the e x a c t m e c h a n i s m and o th e r c h a r a c t e r i s t i c s of s t r u c t u r e r e f i n e m e n t by s u p e r h e a t ing. It was found that s u p e r h e a t i n g r e d u c e d the n u m b e r and s i z e of n u c l e a n t s , but the e v i d e n c e i n d i c a t e s that in a l u m i n u m a l l o y s c o m p l e t e r e m o v a l is s e l d o m , if e v e r , a c h i e v e d . TECHNIQUE The alloys used in the experiment are shown in Table I. Alloy 13 was used for all the experiments, the other ones were used mainly as confirmation of the results obtained on 3003. Five different types of cooling were used to represent the range found in commercial practice: the fastest cooling was achieved by pouring into a cold steel mold, 20 • 40 x 240 ram; intermediate rates were obtained by pouring into molds with a 35 m m diam, 40 m m deep cavity, m a d e of clay, marinite or insulating brick; the slowest by cooling in the furnace. In the first series of experiments all of the alloys were cast in the metal and brick molds and furnace cooled at three or four temperatures ranging from 25~ to m o r e than 500~ above their freezing temperature. 300 g were melted in an alund u m crucible, heated to 700~ fluxed with D o w C h e m ical Flux 230, heated or cooled to pouring temperature and skimmed. Sample
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