A Practical Method for Identifying Inoculants G. S. COLE, K. W. CASEY, AND G. F. BOLLING

The m a c r o s t r u c t u r e of an ingot can be a l t e r e d by con t ro l l ing the f luid flow dur ing so l i d i f i - cat ion. Such con t ro l p r o d u c e s d r a s t i c a l l y d i f fe ren t r e s u l t s depending on whether a so lu te addi t ion a c t s as an inocula t ing agent ( involving a low p r o p o r t i o n of a nuc lea t ing second phase) o r if i t i s m o s t l y so lub le and f o r m s a second phase only as a r e s u l t of m i c r o s e g r e - gat ion. This d i f f e ren t i a t ion , p r e v i o u s l y shown for A1 and some of i ts a l l oys , is r e p e a t e d with m a g n e s i u m conta in ing z i r con ium, which is a known inoculant for MS. Copper is then t e s t e d as an addi t ion to lead . Its ac t ion a l lows the p roduc t ion of v e r y cu r ious m a c r o s c o p i c g r a i n s t r u c t u r e s which wil l be shown to be c h a r a c t e r i s t i c of an inoculant . Subsequent t e s t s suppor t the v a l i d i t y of apply ing f luid flow s t r u c t u r e - c o n t r o l as a method for ident i fy ing inoculan ts .

A r e l a t i v e l y pu re m e t a l or low concen t ra t ion a l loy ca s t i ng which so l i d i f i e s a s a s ingle phase wil l have an in i t i a l a s - c a s t g r a i n s i ze often d e t e r m i n e d by nuc l e a - t ion at the chi l l zone. 1 Subsequent changes in g r a i n shape (such as S t ruc tu r e s r e s u l t i n g f rom the co lumnar to equiaxed t r ans i t i on ) o r g r a i n s i z e depend in many c a s e s upon dend r i t e de tachment and c r y s t a l mu l t i p l i - ca t ion. This mu l t ip l i ca t ion may occu r na tu r a l l y o r be f o r c e d by d i s t u rb ing the l iquid. 2 On the o the r hand the nuc lea t ion c h a r a c t e r i s t i c s of m e l t s t r e a t e d by inocu- lan ts a r e d i f fe ren t f rom those of un t r ea t ed me l t s , s ince they contain second phase s a l m o s t by def ini t ion. It is t he se second p h a s e s which affect nuc lea t ion (or c r y s t a l mul t ip l i ca t ion) and thus g ra in s i ze .

An inoculant is a so lu te addi t ion at a low c o n c e n t r a - t ion which a f fec t s the p r i m a r y m e t a l (or a l loy) by in- c r e a s i n g the number of a s - s o l i d i f i e d g r a i n s . * The

*The word inoculant is also used to describe small additions to alloys con- taining two phases, where the addition affects characteristics of the minor phase (e.g., Na affects the high silicon phase in AI-Si alloys). Such inoculation can be included with our study since a second phase is present from the beginning and does itself affect the primary metal grain size.

" l o w " concen t ra t ion is n e v e r t h e l e s s high enough so that an inso luble second phase i n v a r i a b l y ex i s t s or is f o r m e d in the me l t i m m e d i a t e l y p r i o r to so l id i f i ca t ion . Ju s t how the second phase c o m e s into be ing will not conce rn us h e r e . F o r example , we sha l l not c a r e whe ther Ti addi t ions cause nuc lea t ion of A1 by f o r m - ing A13Ti through a p e r l t e c t i c r e a c t i o n 3-5 or by f o r m - ing TiC through r e a c t i o n with ca rbon i m p u r i t y in the a luminum mel t 6 o r both, 7 as r e c e n t l y r ev iewed . 8

We have p r e v i o u s l y r e p o r t e d a s tudy of a lumin im inocula ted with c o m m e r c i a l add i t ions conta ining Ti and B as the ac t ive e l e m e n t s (Foseco types) . 9 In that p a p e r we s tud ied nuc lea t ion as i t was a f fec ted by changes in the f luid flow env i ronmen t . Obse rva t ion of the c o l u m n a r - t o - e q u t a x e d t r a n s i t i o n was used as a tool which r e v e a l e d the f r equency and e f fec t s of new g r a i n s nuc lea ted by the inoculant .

Without inoculants th is t r a n s i t i o n can be de layed by so l id i f i ca t ion in a ro t a t i ng f r a m e (i.e., think of a mold

G. S. COLE, and K. W. CASEY are with the Scientific Research Staff, Ford Motor Company, Dearborn, Michigan. 4812 I. G. F. BOLLING, formerly with the Scientific Research Staff is now with the Product Planning and Research Staff, Ford Motor Company, Dearborn, Michigan. 48121.

Manuscript submitted July 9, 1973.

se t on a s t e a d i l y ro t a t ing tu rn tab le ) . S t e a d y - s t a t e f o r c e s on the ro t a t ing l iquid dampen f luid flow and inhibi t na tu r a l c r y s t a l mul t ip l i ca t ion . On the o ther hand so l id i f i ca t ion of an o s c i l l a t i n g l i q u i d - s o l i d s y s - t em o c c u r s with a much e a r l i e r t r a n s i t i o n to equiaxed g r a i n s b e c a u s e the i n e r t i a of the l iquid i m p o s e s d i s - rup t ions at the s o l i d - l i q u i d in t e r f ace lead ing to ex ten- s ive c r y s t a l mul t ip l i ca t ion . 1~

With inoculants the s i tua t ion jus t out l ined is v iv id ly r e v e r s e d . Sol id i f ica t ion under s t e a d y - s t a t e ro t a t ion s u s t a i n s the inoculant r e a c t i o n and l eads to f ine g r a in s , while so l i d i f i ca t i on under o s c i l l a t i o n inhibi ts the growth of g r a i n s nuc lea t ed by the inoculant and l eads to c o a r s e , often co lumnar g r a in s . These d i f fe ren t events depend on the fact that the nuc lea ted g r a in s mus t have suf f i - c ient t ime to grow. That is , a b lockage of n o r m a l l y o c c u r r i n g c o l u m n a r g r a i n s r e q u i r e s a l a r g e vo lume (s ize and number ) of new g ra ins . If the new g r a i n s a r e swept away f r o m the growing so l id i f i ca t ion f ront , they a r e inhibi ted f rom s topping co lumnar growth and can- not p r o m o t e cont inuous nuclea t ion as the dominant mode of so l id i f i ca t ion , u Osc i l l a t ion p r o v i d e s the s i t u a - t ion for sweeping away the t iny new g r a i n s p roduced by the inoculant . S t e a d y - s t a t e ro ta t ion , on the o the r hand, a l lows the continuous nuc lea t ion p r o c e s s to p r o - t e e d m o r e o r l e s s uninhibi ted, l ike the p r o c e s s tak ing p l ace in a s t a t i c cas t ing .

This d r a m a t i c r e v e r s a l obvious ly p r o v i d e s a tool to d i s c r i m i n a t e the ac t ion of a low concen t ra t ion so lu te addi t ion . Does o sc i l l a t i on re f ine o r c o a r s e n ? We sha l l p r e s e n t new r e s u l t s in a s y s t e m using a second e l e - ment of known inocula t ing power (Zr in Mg) and then p rove that Cu is an inoculant for Pb.

EXPERIMENTS

Five types of ca s t i ngs we re made with Zr in Mg to d e m o n s t r a t e the s t r u c t u r e con t ro l : s t a t i c , ro ta t ed , o s - c i l l a t ed , a p r o g r a m m e d o s c i l l a t i o n - r o t a t i o n - o s c i l l a - t ion, e tc . sequence , and a r o t a t i o n - o s c i l l a t i o n - r o t a - t ion, e tc . sequence . The magnes ium used was of 99.98 pe t p u r i t y obta ined f r o m l a b o r a t o r y me l t ing s tock . Af te r s e v e r a l unsucces s fu l a t t empt s to so lu t ion ize pu re Zr in mol ten Mg it was dec ided to use a 50 pc t Mg-50 pc t Zr m a s t e r a l loy obta ined f rom NL Indus- t r i e s . A Foseco type D-32 flux was used in a l l the me l t ing o p e r a t i o n s . A nomina l compos i t ion of 0.6 p c t by weight Zr in Mg was se l ec t ed . Using RF heat ing,

METALLURGICAL TRANSACTIONS VOLUME 5, FEBRUARY 1974-407

the Mg was me l t ed in a g raph i t e c r u c i b l e and then hea ted to 900~ at which t e m p e r a t u r e the p r o p e r amount of m a s t e r a l loy was added. Main ta in ing the su r f ace of the me l t coa ted with a thin f i lm of flux was suf f ic ien t to e l i m i n a t e oxida t ion and f l a r ing . The m e l t was then cooled to the pour ing t e m p e r a t u r e , in mos t c a s e s to 100~ supe rhea t , and s t i r r e d c o m p l e t e l y in o r d e r to m i n i m i z e Z r - r i c h p r e c i p i t a t e s f rom se t t l ing out dur ing cool ing.

The effect of s t i r r i n g was one of the ex t r aneous v a r - i ab le s examined dur ing th is work. When the me l t was not s t i r r e d i m m e d i a t e l y be fo re pour ing, the r e s u l t i n g ca s t i ng had the a p p e a r a n c e of a pu re Mg ingot. The Z r - r i c h m a t e r i a l had se t t l ed to the bo t tom of the me l t and r e m a i n e d in the hee l lef t within the c r u c i b l e a f t e r the pour . Only in the c a s e of s t a t i c c a s t i n g s was the a i r ; c o o l e d g raph i t e mold not in mot ion when f i l led .

Each ingot was sec t ioned us ing a w a t e r - c o o l e d power saw, ground through 600 g r i t s i l i con c a r b i d e pape r , and f ina l ly c h e m i c a l l y p o l i s h e d and e tched. A c h e m i - cal po l i sh of 50 pc t N i t r i c - 5 0 pc t wa te r and an e tch of 10 pc t Ace t i c -90 pc t wa te r worked v e r y well .

In the Pb-Cu a l loys 99.999 pc t pu r i t y Pb was used with 0.03 in. d iam. Cu wi r e of 99.0 pc t pu r i ty . The lab was a l so me l t ed in a g raph i t e c r u c i b l e using an RF coi l and hea ted to 700~ at which point enough Cu was s t i r r e d into the me l t to obta in a nomina l compos i t ion of 0.25 pc t by weight Cu. Because of the dens i ty d i f - f e r e n c e s be tween the lab and Cu which might l ead to Cu f loa ta t ion , i t was found b e s t to s t i r the me l t f r e - quent ly. A f lux was not used on the me l t su r f ace s ince t h e r e was not e x c e s s i v e oxidat ion . The me l t was cooled to the p r o p e r pour ing t e m p e r a t u r e and cas t into a g raph i t e mold which was cont inuous ly wa te r cooled . Again, only in the ca se of the s t a t i c ingot, was the mold not in mot ion at the t ime of pour ing .

Each cas t i ng was sec t ioned and ground through the 600 g r i t s i l i con c a r b i d e p a p e r using soapy wa te r as a l ub r i c an t - coo l an t . The s u r f a c e s we re then c h e m i c a l l y po l i shed using a so lu t ion of 20 pc t H~O2 (30 pct) and 80 pc t Ace t i c ac id , and f ina l ly e tched in a 10 pct N i t r i c ac id and wa te r so lu t ion .

A s ingle ro t a t ion r a t e of 150 RPM was used on the ca s t i ng tab le . Osc i l l a t ion was a c c o m p l i s h e d by swi tch - ing f rom s t e a d y - s t a t e ro ta t ion in one d i r e c t i o n to r o t a - t ion in the oppos i te s ense . Switching e v e r y 2 s p r e - vents any new s t e a d y - s t a t e and thus deve lops the p a r - t i c u l a r l y d e s i r e d f luid flow p a t t e r n where the thin boundary l a y e r a t the s o l i d - l i q u i d in t e r face , which de - ve lops in seconds , unde rgoes cons tant d i s rup t ion by each subsequent r e v e r s a l . A r a p i d r e t u r n to s t e a d y - s t a t e ro t a t ion i s r e a d i l y a c c o m p l i s h e d by e l i m i n a t i n g the swi tching act ion.

Fig. 1 - -Magnes ium containing 0.6 wt pct Zr c a s t into a s ta t ic mold from 100~ superheat.

Fig. 2- -Mg-0.6 wt pct Zr cas t under the s a m e conditions as in Fig. 1, but into a mold ro ta t ing s teadi ly at 150 rpm.

RESULTS AND DISCUSSION

Mg- Zr

The r e s u l t s obta ined when s t r u c t u r e was con t ro l l ed in the s t a t i c , r o t a t e d and o s c i l l a t e d ingots we re as expec ted ; see F igs . 1, 2 and 3. The inocula t ing effect of the Zr was inhibi ted when the tu rbu lence , p rov ided by the osc i l l a t i on , could sweep away the Z r - r i c h p a r - t i c l e s f r o m the advancing in t e r f ace . Co lumnar growth in the fu l ly o s c i l l a t e d ca s t i ng was t h e r e b y enhanced.

v ~ l ~ L . r ' l j L i . s

Fig. 3--Mg-0.6 wt pct Zr as in F igs . 1 and 2, but c a s t into a mold control led to osc i l la te dur ing the f reez ing .

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23x

PROGRAMMED Osc-Rot-Osc

Fig. 4--Mg-0.6 wt pet Zr as in Figs. 1, 2 and 3 but cast into a mold programmed to oscitlation-rotation-ocillation-rotatton dur- ing the freezing. Inset sections reveal the grain structures of the indicated regions.

A major difficulty was found with the p r o g r a m m e d cas t ings of the r o t a t i o n - o s c i l l a t i o n - r o t a t i o n sequence. We were unable to obtain a cons i s t en t in i t ia l zone at the outside of the cast ing. Many dif ferent t ime and ro ta t ion va r i a t i ons were invest igated, by pour ing into a ro ta t ing or even a s ta t ic mold, with the in i t ia l s e - quence l as t ing up to 12 s, and then proceeding to the ba lance of the sequence o sc i l l a t i on - ro t a t i on and so on. The outside zone was inva r i ab ly of a mixed co lumnar s t r uc tu r e indica t ing that the tu rbu lence of pour ing r e - mained suff icient to r e s t r i c t any inocula t ing effect of the Zr. Solidification d i rec t ly in the mel t ing c ruc ib le was not a t tempted because the s t i r r i n g p rob lem would have confused the r e s u l t s .

In the p r o g r a m m e d cas t ings of the o s c i l l a t i o n - r o t a - t i on -osc i l l a t ion sequence, there was no in t e r f e rence by the in i t ia l pour ing condit ions, which were obviously equivalent to the osc i l l a t ion -p roduced shea r s . A typi- cal r e s u l t is shown in Fig. 4. Since suff icient empha- s i s of the etching detai l is difficult to photograph at low magnif ica t ions , the control of g ra in s ize is shown at higher magnif ica t ion in the inse t s . The Mg ingot here is co lumnar , equiaxed, co lumnar and equiaxed-- obviously not a na tu ra l , but a p r o g r a m m e d sequence.

As for the r e su l t s of the Pb-Cu work, they seem to

METALLURGICAL TRANSACTIONS

Fig. 5--Lead containing ~ 0.25 wt pct Cu solidifiecl through the oscillation-rotation-oscillation-rotation sequence.

ag ree quite well with prev ious work done in " l i g h t " a l loys . Cu is a v e r y eff icient inoculant and p reven t s g ross co lumnar gra ins , but the s t r uc t u r e of the cas t -

VOLUME 5, FEBRUARY 1974-409

700

600

~ 500

400

500 0

L E A D - C O P P E R

/ / '~176

�9 0 20 50 I00 _ ~ iT. % C.

i I i I I I i i I I i

0.2 0.4 0.6 0.8 1.0 Wl.% Cu

Fig. 6--The Pb-Cu phase diagram. 12

1.2

Fig. 7--An incomplete growth form of a Cu par t ic le in Pb characterist ic of a good nucleating agent in a metal-metal system. Magnification about 660 times.

ing was s t i l l con t ro l l ab l e through the p r o g r a m m i n g of o s c i l l a t i o n - r o t a t i o n - o s c i l l a t i o n sequences dur ing f r eez ing .

We sha l l not b e l a b o r the o v e r a l l o b s e r v a t i o n s and will p r e s e n t only one typ i ca l r e s u l t a s in Fig. 5. This is a s t a r t l i n g example of g r a i n s t r u c t u r e cont ro l , not dup l i ca ted e l s e w h e r e to our knowledge. Severa l o ther v e r s i o n s of th is i n v e r t e d s t r u c t u r e could be p roduced depending upon Cu concen t ra t ions , s u p e r h e a t s , and so for th .

Copper , ac t ive inso fa r as nuc lea t ing ab i l i ty , a l so de l inea ted g ra in changes r e s u l t i n g f rom the swi tching sequence dur ing o sc i l l a t i on . The i n t e r r u p t e d c o l u m n a r

Fig. 8--A c ros s sect ion of a 1%-0.25 wt pct Cu alloy showing approximately one Cu part ic le per grain (see the text). Mag- nification about 190 t imes .

growth in Fig . 5 did not however c o r r e s p o n d to r e a d i l y o b s e r v a b l e d i f f e r e n c e s in Cu c o n c e n t r a t i o n s .

F u r t h e r s tudy was focus sed on d i r e c t subs t an t i a t ion of the inoculant r e a c t i o n supoosed lv p r o v e d by this in- d i r e c t t e s t ing . Cons ide r the phase d i a g r a m por t i on shown in Fig . 6. TM T h e r e is no doubt that a t a concen- t r a t i on of 0.25 wt pct , a second phase wil l be p r e s e n t in the me l t when l ead beg ins to f r e e z e . This is a f i r s t condi t ion for an inoculant .

Sect ions of equiaxed g r a i n s ob ta ined dur ing s t a t i c f r e e z i n g with even h igher concen t r a t i ons of Cu in Pb a lways r e v e a l e d p o l y h e d r a l l y deve loped f o r m s of the C u - r i c h phase . These growth f o r m s a r e c h a r a c t e r i s - t ic of good nuc lea t ing agents i f their surfaces are in- complete; such incomple t e s u r f a c e s a r e a l m o s t a lways found in m e t a l - m e t a l s y s t e m s as seen h e r e in Fig . 7. F ina l ly , we o b s e r v e d a c o r r e l a t i o n be tween the num- b e r of C u - r i c h p a r t i c l e s and the number of g r a i n s . This was not exac t nor was i t p u r s u e d v e r y fa r b e - cause the r e s u l t was so s i m p l e , one Cu p a r t i c l e : one lab g ra in . A good example is shown in Fig . 8. Seve ra l Cu p a r t i c l e s a r e seen s l i gh t ly out of focus while t h e r e a r e a l so s e v e r a l g r a i n s which r e v e a l no p a r t i c l e s s ince the m e t a l l o g r a p h i c s ec t ion cannot t r a v e r s e a l l g r a i n c e n t e r s . The o n e - t o - o n e r e s u l t is n e v e r t h e l e s s a p p a r e n t and the cen t e r i ng of g r a i n s by C u - r i c h p a r - t i c l e s i s c l e a r . A s m a l l amount of Cu-lab eu tec t ic can be seen in the r e s i d u a l l y f reeziz :g g r a i n b o u n d a r i e s .

We conclude that Cu n u c l e a t e s lab in a cas t ing , that i t ac t s as an inoculant , and that the method of inocu- lant d i s c r i m i n a t i o n works e f f ec t ive ly a s a p r a c t i c a l tool .

410-VOLUME 5, FEBRUARY 1974 METALLURGICAL TRANSACTIONS

ACKNOWLEDGMENT

We are Indebted to J. Clss~ for his advice and par- ticipation in e a r l i e r u n p u b l i s h e d p r o p r i e t a r y w o r k u s -

i n g P b - C u a l l o y s . T h e s e e x p e r i m e n t s w o u l d h a v e b e e n

i n c o m p l e t e w i t h o u t h i s g u i d a n c e .

REFERENCES

1. H. Biloni and B. Chalmers: J. Mater. Sci., 1968, vol. 3, p. 139. 2. G. S. Cole and G. F. Boiling: Trans. TMS-AIME, 1967, vol. 239, p. 1824.

3. M. D. Eborall: s Inst. Metals, 1949-50, vol. 76, p. 295. 4. F. A. Crossley and L. F. Mondolfo: Trans. AIME, 195 l, vol. 3, p. 1143. 5. G. W. Delamore and R. W. Smith: Met. Trans., 1971, vol. 2, p. 1733. 6. A. Cibula: J. Inst. Metals, 1949-50, vol. 76, p. 321. 7. J. Ciss4, G. F. Boiling, and H. W. Kerr: J. CrystalGrowth, 1972, vol. 13/14,

p. 777. 8. G. S. Cole, J. Ciss4, H. W. Kerr, and G. F. Boiling: AFS Trans., 1972, vol. 80,

p. 211. 9. G. S. Cole, K. W. Casey, and G. F. Boiling: Met. Trans., 1970, vol. l, p. 1413.

10. G. S. Cole and G. F. Boiling: The Solidification ofMetals, ISI Pub. No. 110, p. 323, 1968.

II. G. S. Cole and G. F. Boiling: Trans. TMS.AIME, 1969, vol. 245, p. 725. 12. M. Hansen and K. Anderko: Constitution of Binary Alloys, McGraw-Hill,

New York, 1958.

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