GRAIN REFINING IN Al-Li ALLOYS BY ELECTROMAGNETIC STIRRING

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GRAIN REFINING IN Al-Li ALLOYS BY ELECTROMAGNETIC STIRRING

C. Vives, J. Bas, Y. Cans

To cite this version:

C. Vives, J. Bas, Y. Cans. GRAIN REFINING IN Al-Li ALLOYS BY ELECTROMAGNETIC STIR- RING. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-109-C3-115. �10.1051/jphyscol:1987313�.

�jpa-00226543�

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JOURNAL DE PHYSIQUE

C o l l o q u e C 3 , s u p p l 6 m e n t a u n 0 9 , Tome 4 8 , s e p t e m b r e 1987

GRAIN REFINING IN A1-Li ALLOYS BY ELECTROMAGNETIC STIRRING

C . VIVES, J. BAS and Y. CANS'

L a b o r a t o i r e d e Magneto-Hydrodynamique, Universite d l A v i g n o n , F-84000 A v i g n o n , F r a n c e

* ~ ~ g i 2 d u r - ~ ~ c h i n e ~ , Centre d e R e c h e r c h e s e t DcSveloppement, 3.P. 2 7 , F-38340 V o r e p p e , F r a n c e

ABSTRACT

The r o l e of forced convection during s o l i d i f i c a t i o n of A1-Li a l l o y s i n an annu- l a r c r u c i b l e was s t u d i e d . The forced convection was generated by electromagnetic s t i r r i n g . Maps of electromagnetic body f o r c e s and v e l o c i t y f i e l d s were obtained f o r various s t i r r i n g i n t e n s i t i e s and various p o s i t i o n s of t h e s o l i d i f i c a t i o n f r o n t . Tem- p e r a t u r e measurements made i t p o s s i b l e t o follow t h e evolution o f t h e temperature d i s t r i b u t i o n i n s i d e the bulk l i q u i d with time. These experiments were c a r r i e d out i n t h e absence and presence o f electromagnetic s t i r r i n g . A discussion i s presented re- l a t i n g t h e m e t a l l u r g i c a l f i n d i n g s t o t h e heat and f l u i d flow measurements. T t appears t h a t t h e e f f e c t of electromagnetic s t i r r i n g r e s u l t s both i n a s p e c t a c u l a r reduction of t h e diameter of the columnar c r y s t a l and i n a decided refinement of t h e equiaxed g r a i n .

INTRODUCTION

I t i s an e s t a b l i s h e d f a c t t h a t t h e production o f a f i n e grained equiaxed s t r u c - t u r e leads t o a s u b s t a n t i a l improvement of t h e q u a l i t y of t h e metal and allows an i n c r e a s e of t h e ingot drop r a t e and t h e reduction of craking 11-31.

The a d d i t i o n of nucleating agent i s a commonly adopted method i n D . C . c a s t i n g of aluminum a l l o y s 11-31 and i t i s well known t h a t i n o c u l a t i o n of small amounts of g r a i n r e f i n e r s master a l l o y s , r e s u l t s i n a s i g n i f i c a n t reduction of t h e c r y s t a l s i z e . However, t h e a d d i t i o n of grain r e f i n e r s may cause negative secondary e f f e c t s reducing t h e q u a l i t y of t h e metal .

Battery of

thermocouples I

7 I /- ^Argon

Insulation Free surface

Resistance heating

Fig. 1. Schematic diagram of t h e apparatus.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987313

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C3-110 JOURNAL DE PHYSIQUE

An a1 t e r n a t i v e way, c o n s i s t i n g of several dynamic methods producing a vigorous forced convection i n t h e melt during f r e e z i n g , leads t o s u b s t a n t i a l grain refinement.

As an i l l u s t r a t i o n , an e f f i c i e n t electromagnetic method o f grain refinement, t h e CREM process, has been r e c e n t l y developed i n continuous c a s t i n g o f aluminum a l l o y s

14-51.

The present work i s devoted t o the examination of t h e i n f l u e n c e o f a forced convection of electromagnetic o r i g i n during t h e c r y s t a l l y s a t i o n i n mold of t h r e e A1-Li a l l o y s ( C P 271, C P 274 and CP 276).

The experimental device consisted of an annular c r u c i b l e holding Al-Li a l l o y , which was melted and s o l i d i f i e d i n t h e presence of an argon s h i e l d i n g . The i n n e r c y l i n d e r could be water cooled and t h e o u t e r c y l i n d e r r e s i s t a n c e h e a t e d . In addi- t i o n , t h e pool could be i n d u c t i v e l y s t i r r e d using a c y l i n d r i c a l c o i l , supplied with a 50 Hz a l t e r n a t i n g c u r r e n t .

The inductor generates i n t h e melt a time-varying magnetic f i e l d which, i n t u r n , gives r i s e t o an induced c u r r e n t . T h u s , t h e melt i s s u b j e c t t o electromagnetic body f o r c e s ~ a u s e d by t h e i n t e r a c t i o n of t h e eddy c u r r e n t of d e n s i t y 3 and t h e magnetic f i e l d B .

The time mean electromagnetic f o r c e ( J x B) may be resolved i n t o a p o t e n t i a l p a r t balanced by a pressure g r a d i e n t , which r e s u l t s i n t h e deformation of t h e f r e e s u r f a c e , and a r o t a t i o n a l p a r t , due only t o t h e end e f f e c t s and which i s r e s p o n s i b l e f o r an electromagnetic s t i r r i n g equivalent t o a forced convection. The i n t e n s i t y of t h e forced convection depends on t h e magnitude of t h e magnetizing f o r c e , t h e frequen- cy o f t h e c u r r e n t , t h e geometries of t h e c r u c i b l e and c o i l and, a l s o , of t h e e l e c t r o - magnetic boundary conditions .

EXPERIYENTAL APPARATUS AND P R O C E D U R E

The p r i n c i p a l f e a t u r e s of t h e apparatus a r e schematically presented i n Fig. 1.

The mold was made of s t a i n l e s s s t e e l and consisted of an inner c y l i n d e r of 24 , o r 72 mm , O . D . and of an o u t e r c y l i n d e r o f 174 mm I.D. . The bottom wall c o n s t i t u t e d a d i s k 8 mm t h i c k , except f o r a crown 16 mm t h i c k and 125 and 183 mm i n t e r i o r and e x t e r i o r r a d i i .

The inner wall was i n t e r n a l l y cooled by water a t 20°C, w i t h a flow r a t e l i k e - l y t o be included between 50 - 1500 d m 3 . h - l . All t h e e x t e r i o r w a l l s of t h e furnace were thermally i n s u l a t e d .

The induction electromagnetic f i e l d was generated by a c o i l made up of 140 j o i n t e d t u r n s and t h e magnetizing f o r c e was modulated a t w i l l , from 0 t o 10500 A t , by an autotransformer.

I t should be noted t h a t i n t h e s e phenomena, t h e s u p e r h e a t , t h e melt volume, t h e boundary conditions and consequently t h e convective flow a r e time dependent. So, i n o r d e r t o follow both t h e e v o l u t i o n o f t h e s o l i d i f i c a t i o n f r o n t and o f t h e temperatu- r e d i s t r i b u t i o n i n s i d e t h e bulk l i q u i d with time, s i x thermocouples were placed ho- r i z o n t a l l y a t prescribed h e i g h t s within t h e melt ( F i g . I ) , t h e i r outputs being fed t o a six-channel recorder. Each experiment was performed by meltinn t h e metal and heating i't t o 10°C above t h e required superheat temperature, a t t h i s moment t h e heating was c u t o f f so t h a t t h e l i q u i d metal could cool slowly t o t h e required superheat temperature T, . Then, t h e i n g o t s were s o l i d i f i e d by d e l i v e r i n g cooling water from an i n i t i a l superheat of AT°C above t h e l i q u i d u s temperature (included hetween 642 - 645°C). By another way, t h e measurement?techniques f o r v e l o c i t y c u r r e n t d e n s i t y , magnetic f i e l d and phase s h i f t have been already described 161 .

Electromagnetic parameters measurements

Methodical measurements of t h e r.m.s. of t h e c u r r e n t d e n s i t y J and of t h e r a - d i a l component of t h e magnetic f i e l d Br a s well a s t h e phase angle q ( ~ , B r ) allow determination o f t h e time mean a x i a l component o f t h e electromagnetic body f o r c e s given by :

- Fz = J.Br cos (3, i t r ) .

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F i g . 2 . D i s t r i b u t i o n o f v e r t i c a l component o f t h e e l e c t r o m a g n e t i c body f o r c e s .

f c m

-

¹⁰

F i g . 3 . Yeasured v e l o c i t y f i e l d s i n f o r c e d c o n v e c t i o n ( a ) Ri = 1 . 2 cm, ( b ) Ri = 3 cm, ( c ) Ri = 5 cm.

The v e r t i c a l components Fz a r e p r i m a r i l y r o t a t i o n a l and s o r e s p o n s i b l e f o r t h e - s t i r r i n g o f t h e bath ; c o n s e q u e n t l y , knowledge o f t h e Fz d i s t r i b u t i o n w i l l a l l o w e x p l a n a t i o n o f t h e f l u i d flow p a t t e r n s .

D i s t r i b u t i o n s o f Fz ( F i g . 2 ) were p l o t t e d w i t h o u t w a t e r c o o l i n g , w i t h a prac- t i c a l l y uniform t e m p e r a t u r e o f 680°C. I t may be s e e n t h a t , because o f t h e e l e c t r o - magnetic s k i n d e p t h , Fz d e c r e a s e s a l o n g a r a d i u s , from t h e o u t e r wall towards t h e c e n t r e o f t h e p o o l . The v e r t i c a l f o r c e s a r e i m p o r t a n t w i t h i n t h e h a l f upper p a r t of t h e c r u c i b l e and i n s i g n i f i c a n t i n s i d e t h e lower p a r t ; moreover, Fz i s always d i - r e c t e d downward, e x c e p t f o r a small zone l o c a t e d i n t h e v i c i n i t y of t h e v e r t i c a l po- s i t i o n d e f i n e d by z = 1 cm.

The d i s t r i b u t i o n o f Fz i s here d i f f e r e n t from t h a t observed i n t h e c o r e l e s s i n d u c t i o n f u r n a c e , where t h e v e r t i c a l component of t h e e l e c t r o m a g n e t i c f o r c e s a r e , a p p r o x i m a t e l y , symrnetri c a l l y d i s t r i b u t e d w i t h r e s p e c t t o t h e h o r i z o n t a l p l a n e o f

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JOURNAL DE PHYSIQUE

Fig. 4. Temperatures recorded a t mid-height of the m e l t , i n natural and forced convections.

F i g . 5. Temperature f i e l d s i n n a t u r a l convection : ( a ) t = 15 s , ( b ) t = 30 s, and i n forced convection : ( c ) t = 15 s , ( d ) = 30 s .

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Fig. 6 . Superheat drop a s a function of t h e thickness o f t h e s o l i d i f y i n g c r u s t , i n n a t u r a l and forced convections.

symmetry of t h e c r u c i b l e . In our furnace, t h e horizontal lower wall and e s p e c i a l l y t h e crown, a l r e a d y mentioned and shown i n Fig. 1 , a r e conducting and behave as an electromagnetic screen _161. The eddy c u r r e n t s , which c i r c u l a t e through t h e s h i e l d , a r e p r a c t i c a l l y opposite i n phase t o those o f t h e i'nductor ; t h i s r e s u l t s i n addi- t i o n a l body f o r c e s a r i s i n g t h a t a r e opposite t o t h e r o t a t i o n a l f o r c e s c r e a t e d by t h e c o i l . Consequently, the r e s u l t a n t electromagnetic f o r c e f i e l d i s considerably a t t e - nuated i n t h e lower p a r t of t h e mold.

Velocity measurements

The v e l o c i t y maps, presented i n Fig. 3 , were p l o t t e d wi'thout c o o l i n g , i n order t o avoid t h e s o l i d i f i c a t i o n o f t h e hath. The progression of t h e f r o n t , which r e s u l t s i n a modification i n time of t h e geometry o f t h e annular pool and of t h e boundary c o n d i t i o n s , has been simulated by the adjunction of two c y l i n d e r s made of s o l i d s t a i n l e s s s t e e l , t h e e x t e r i o r r a d i i R i of which were 3 and 5 cm, r e s p e c t i v e l y .

Inspection of Fig. 3 ( a ) connected w i t h t h e s t a r t o f t h e s o l i d i f i c a t i o n , i . e . (Ri = 1 . 2 cm) r e v e a l s t h e presence o f a s i n g l e loop ; on t h e o t h e r hand, f o r an e x c i -

t a t i o n of 4200 A t , the v e r t i c a l components of t h e v e l o c i t y a r e o f t h e o r d e r of 20 cm s - l , i n t h e v i c i n i t y of t h e w a l l s of t h e i n n e r and o u t e r c y l i n d e r s . I t may be s e e n , i n Fig. 3 ( b ) (Ri = 3 cm t h a t t h e v e l o c i t i e s decrease, e s p e c i a l l y i n s i d e t h e lower p a r t of t h e mold, where a new r e c i r c u l a t i n g vortex occurs, but a vortex i n which t h e f l u i d flows t h e opposite way r e l a t i v e t o t h e upper one. In t h e case depic- t e d i n Fig. 3 ( c ) (Ri = 5 cm), t h e volume of t h e upper c e l l decreases i n favor of t h e lower loop.

In summary, t h e i n i t i a l s i n g l e c e l l i s replaced by t w o c o n t r a - r o t a t i n g v o r t i c e s which a r e s i m i l a r , only from t h e point of view of t h e a s p e c t , with t h e t y p i c a l f l u i d flows w i t h i n a c o r e l e s s induction furnace. This flow s t r u c t u r e can be e a s i l y under- stood by examination of t h e Fz map ( F i g . 2) : apparently t h e upper loop i s p r i n c i - p a l l y driven by t h e v e r t i c a l component of t h e electromagnetic body f o r c e s , whereas t h e lower c e l l seems t o be mainly induced by the upper one ( v i s c o u s e f f e c t ) . Thermal measurements

Examination of t h e two recordings, presented i n Fig. 4 , shows c l e a r l y t h a t forced convection leads t o rapid d i s s i p a t i o n of s u p e r h e a t . Temperature f i e l d s , p l o t - t e d f o r t = 15 s and t = 30 s , i n t h e ahsence and presence of s t i r r i n g , a r e sketched i n Fig. 5 . Inspection of Fig. 5 ( c ) y i e l d s support t o t h e presence o f t h e two v o r t i c e s , a1 ready revealed by means of v e l o c i t y measurements. Furthermore, t h e h o t t e r zone i s l o c a t e d i n t h e near v i c i n i t y o f t h e eye of t h e lower loop, while i n t h e case of n a t u r a l convection, t h e hot zone was s i t u a t e d i n t h e upper-ri'ght-hand s i d e corner ( F i g s . 4 ( a ) and ( b ) . Fig. 5 ( d ) shows again t h a t forced convection promotes t h e d i s s i p a t i o n of superheat ; moreover, it may be seen t h a t t h e hulk l i q u i d

i s isothermal and t h a t t h e l i q u i d u s temperature i s reached. The evolution of super- h e a t with t h e average r a d i u s o f t h e i n t e r f a c e is i l l u s t r a t e d i n Fig. 6 , where i t i s seen t h a t , i n t h e presence of s t i r r i n g , superheat compl-etely vanishes when t h e quar- t e r of t h e m e t a l l i c mass i s s o l i d i f i e d .

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C3-114 JOURNAL DE PHYSIQUE

Fig. 7 . Hacrostructures of A1-Li a l l o y s ( v e r t i c a l s l i c e s of about 100 mm h e i g h t ) , AT = 30°C, s o l i d i f i e d i n natural convection : (A) CP 271, ( C ) CP 276 and i n t h e presence o f electromagnetic s t i r r i n g : (6) C P 271, ( D ) C P 276.

F i g . 3. Dlacrostructures of A1-Li a l l o y s ( h o r i z o n t a l s l i c e s of 1 7 0 mm diameter) AT = 30°C, s o l i d i f i e d i n natural convection : ( A ) C P 271, (C) CP 274, ( E ) C P 276 and i n t h e presence o f electromagnetic s t i r r i n g : ( B ) C P 271, ( D ) CP 274, ( F ) CP 276.

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YETALLOGRAPHIC STUDY

For each s l i c e c u t a l o n g whole r a d i a l and v e r t i c a l c r o s s - s e c t i o n o f t h e a n n u l a r i n g o t s , t h e l e f t and r i g h t - h a n d s i d e s of t h e samples p r e s e n t e d h e r e a r e r e s p e c t i v e l y connected w i t h t h e c o l d and h o t wall o f t h e mold.

The s o l i d i f i c a t i o n i n t h e p r e s e n c e o f n a t u r a l c o n v e c t i o n i s c h a r a c t e r i z e d by t h e p r e s e n c e o f an u n i c e l l u l a r l i q u i d metal f l o w , with f l u i d r i s i n g n e a r t h e h o t wall and descending a l o n g t h e s o l i d i f i c a t i o n f r o n t . I t i s s e e n i n F i g s . 7 ( a ) and ( c ) t h a t t h e columnar c r y s t a l s a r e s l o p e d i n t h e upstream d i r e c t i o n t o t h e i n t e r f a c e .

In f o r c e d c o n v e c t i o n , i t a p p e a r s t h a t t h e columns a r e i n c l i n e d downward i n t h e upper p a r t o f t h e s o l i d i f i ' e d i n g o t and upward i n t h e lower p a r t ( F i g s 7 ( b ) and ( d ) ) . The h e i g h t o f t h e s e zones may be e a s i l y connected w i t h t h e s i z e and t h e d i - r e c t i o n of c i r c u l a t i o n o f t h e two v o r t i c e s s e e n i n F i g s 3 ( b ) and ( c ) . Moreover, t h e e f f e c t s o f t h e e l e c t r o m a g n e t i c s t i ' r r i n g r e s u l t s i h a marked r e d u c t i o n o f t h e ave- r a g e d i a m e t e r o f t h e columnar c r y s t a l s . On t h e o t h e r hand, a f t e r t h e e a r l y s t a g e s o f c o o l i n g , a f i n e g r a i n e d equiaxed s t r u c t u r e , o c c u r s ( F i g s . 8 ( b ) , ( d ) and ( c ) ) , i n - s i d e an a r e a c o r r e s p o n d i n g t o t h e t o t a l e e v a c u a t i o n o f s u p e r h e a t ( F i g . 6 ) . In t h e s e r e g i o n s t h e c r y s t a l s i z e i s o f t h e o r d e r o f 200 m i c r o n s .

CONCLUSIONS

This e x p e r i m e n t a l s t u d y shows t h a t a v i g o r o u s f o r c e d c o n v e c t i o n promotes both t h e homogenization o f t h e t e m p e r a t u r e o f t h e hulk l i q u i d and t h e withdrawal of s u p e r - h e a t .

From t h e s t a n d p o i n t o f t h e c r y s t a l s t r u c t u r e , t h e e f f e c t o f e l e c t r o m a g n e t i c s t i r r i n g r e s u l t s i n a s p e c t a c u l a r r e d u c t i o n o f t h e d i a m e t e r o f t h e columnar c r y s t a l s and i n a d e c i d e d r e f i n e m e n t o f t h e equiaxed g r a i n s . I t should be emphasiZed t h a t t h e a p p l i c a t i o n o f a t i m e - v a r y i n g magnetic f i e l d i s of p a r t i c u l a r i n t e r e s t , because f o r - ced c o n v e c t i o n i s g e n e r a t e d i n a r e a d i l y c o n t r o l a b l e manner, w h i l e a v o i d i n g d i r e c t c o n t a c t between t h e s t i r r e r and t h e m e l t , hence w i t h o u t r i s k o f p o l l u t i o n .

REFERENCES

1. B . Chalmers, P r i n c i p l e o f S o l i d i f i c a t i o n ( R . K r i e g e r P u b l i s h . Comp,, 1 9 8 2 ) . 2 . P I . Flemings, S o l i d i f i c a t i o n P r o c e s s i n g (Mc Graw-Hill , 1 9 7 4 ) .

3 . G . Abbaschian and S . David, Grain Refihement i n C a s t i n g and Welds, (Met. Soc.

of A.I.M.E., 1 9 3 2 ) .

4 . Ch. Vives and B . F o r e s t , "CREH : A New C a s t i n g Process - P a r t I : Fundamental Aspect". L i g h t P e t a l s 1 9 8 7 , pp. 769-778.

5 . J.P. Riquet and J.L. Meyer, "CREPI : A New C a s t i n g P r o c e s s - P a r t I 1 : Indus- t r i a l s A s p e c t s " . L i g h t Metals 1987, pp. 779-784.

6 . Ch. Vives and R . Ricou, "Experimental Study o f Continuous E l e c t r o m a g n e t i c Cas- t i n g o f Aluminum A l l o y s " . Flet. T r a n s . B, 16 , pp. 377-384, 1985.