A mechanism for the columnar to equiaxed transition in castings or ingots
- PDF / 243,347 Bytes
- 2 Pages / 612 x 792 pts (letter) Page_size
- 88 Downloads / 182 Views
1. A. R. Kaufmann,P. Gordon,and D. W. Lillie:Trans. ASM, 1950,vol.42, pp. 785.844. 2. J. F. Peck andD. A. Thomas:Trans. TMS-AIME, 1961,vol. 221, pp. 1240-47. 3. W. F. Hosford: Trans. TMS-AIME, 1964,vol. 230,pp. 12-15. 4. P. L. Tuer and A. R. Kaufmann:in The Metal Beryllium, D. W.Whiteand J. E. Burke,eds.,Chaptervii-b,ASM,Cleveland,1955. 5. R. I. Garber,F. A. Gindin,and Yu. V. Shnbin:Soy. Phys. Solid State, 1963,vol. 5, pp. 315-20. 6. D. Beasleyand A. Moore:in Beryllium Technology, vol. 1, L. M. Schetkyand H. A. Johnson,eds.,pp. 227-37,Gordonand Breach,N.Y., 1966. 7. G. Langfordand M. Cohen:ASM Trans. Quarterly, 1969,vol.62, pp. 623-38. 8. C. Biswas:Ph.D.Thesis,Departmentof Metallurgyand MaterialsScience, MassachusettsInstituteof Technology,t973. 240-VOLUME 6A, JANUARY 1975
A Mechanism for the Columnar to Ecluiaxed Transition in Castings or Ingots M. H. BURDEN AND J. D. HUNT R e c e n t e x p e r i m e n t a l work on d e n d r i t e growth t e m p e r a t u r e s 1'2 has s u g g e s t e d a p o s s i b l e m e c h a n i s m for an a b r u p t c o l u m n a r to equiaxed t r a n s i t i o n in ingots or castings. The c o l u m n a r to equiaxed t r a n s i t i o n is c a u s e d z's e i t h e r by a pile of equiaxed c r y s t a l s b l o c k i n g growth of the c o l u m n a r g r a i n s or by a t t a c h m e n t of equiaxed c r y s t a l s f r o m the liquid to the c o l u m n a r d e n d r i t e front. Both t h e s e m e c h a n i s m s depend on a r a p i d r a t e of growth of equiaxed c r y s t a l s in the liquid. It has b e e n o b s e r v e d z in t r a n s p a r e n t analog c a s t i n g s that s m a l l c r y s t a l s can be p r e s e n t in the liquid ahead of the i n t e r f a c e but that these m u s t grow r a p i d l y to produce a c o l u m n a r to equiaxed t r a n s i t i o n by e i t h e r method. Recent e x p e r i m e n t a l m e a s u r e m e n t s 1 have shown that the growth t e m p e r a t u r e s of c e l l s and d e n d r i t e s v a r y with growth r a t e and t e m p e r a t u r e g r a d i e n t as shown in Fig. 1. F o r a high p o s i t i v e t e m p e r a t u r e g r a d i e n t ahead of the d e n d r i t e s the t e m p e r a t u r e f i r s t i n c r e a s e s then d e c r e a s e s with i n c r e a s i n g v e l o c i t y but for a z e r o or n e g a t i v e g r a d i e n t the t e m p e r a t u r e d e c r e a s e s c o n t i n u o u s l y with i n c r e a s i n g velocity. F r o m the shape of t h e s e c u r v e s a p o s s i b l e m e c h a n i s m can be s u g g e s t e d for a sudden r a p i d i n c r e a s e in the growth r a t e of e q u i axed c r y s t a l s which could p r o d u c e a c o l u m n a r to e q u i axed t r a n s i t i o n . C o n s i d e r a c a s t i n g poured with s o m e i n i t i a l s u p e r heat into a cooled mould, C o l u m n a r c r y s t a l s will s t a r t to grow f a i r l y r a p i d l y f r o m the mould wall into an i n i t i a l l y steep t e m p e r a t u r e g r a d i e n t p r o d u c e d b y the s u p e r h e a t . T h i s c a n b
Data Loading...
