influences of super-gravity field on aluminum grain refining

Influences of Super-Gravity Field on Aluminum Grain Refining

LIXIN ZHAO, ZHANCHENG GUO, ZHI WANG, and MINGYONG WANG

The influence of a super-gravity field on metal grain refining has been mainly investigated usingindustrial pure aluminum as an example. The relationship between the super-gravity field(gravity coefficient of less than 1000) and the grain size in the equiaxed crystal zone was studied,and the corresponding mechanism for grain refining was discussed in detail. The effect of thesuper-gravity field on grain refining was remarkable, and the relationship could be well fitted bythe second-decreasing-exponential function. For aluminum, the grain size decreased rapidly asthe gravity coefficient increased from 1 to 250, and then remained nearly unchanged from 250 to1000. Under the experimental conditions, the main mechanism was that the fragments ofdendritic grains—generated by super-gravity—acted as the nucleation sites and resulted in grainrefining. Further, the change of nucleation Gibbs free energy, directly caused by super-gravity,was not remarkable enough to refine the grains, which could be suggested by the stable freezingpoints under different super-gravity fields.

DOI: 10.1007/s11661-009-0130-9� The Minerals, Metals & Materials Society and ASM International 2010

I. INTRODUCTION

IT is well known that grain refining during the castingprocess is one of the effective methods of improving thematerial mechanical properties, such as hardness,toughness, fatigue resistance, etc. Much research hasfocused on grain refining, and the methods used can beclassified into three categories: the addition of grainrefiner, the increase of cooling rate, and the applicationof external force.[1] Some literature also defined thelatter two methods as the change of solidificationconditions. In general, external forces include agitation,mechanical or ultrasonic vibration, mold rotation,rotary magnetic field, and so on. Mold rotation, orrather centrifugal casting, actually is a solidificationprocess under the super-gravity field.

There are many studies regarding grain-refining mech-anisms of vibration and agitation, but only a few focuson the super-gravity field. It is proposed that mechanicalor electromagnetic agitation can cause a turbulent flowin molten metal, which results in the rupture of thedendrite at the growth interface. The fragments from thedendrite become nucleation particles and enhance crystalmultiplication. The mechanical or ultrasonic vibrationonly refines the grains by generating cavitations and

negative pressure, undercooling local liquid, and devel-oping nucleation.[2] Grain refining under the rotarymagnetic field is attributed partially to the super-gravitygenerated by the field.[3] However, for mold rotation, orfor centrifugal casting, its grain-refining mechanism hasnot been discussed in detail, though it has been widelyused in foundry industries, especially for the regularcylindrical as-cast, such as steel tube, wheel hub, andautomobile cylinder, due to the advantages of refinedequiaxed grains and uniform and dense structure.Among the studies about centrifugal casting, most of

them have been focused on the influences of coolingrate, grain refiner, pouring temperature, and otherfactors, but few on the influence of different rotating

speeds (or different super-gravity fields). Yeh and Jong[4]

have investigated the effect of different rotating speedson the grain refining of the aluminum alloy. Theirresults indicated that the grain size in the equiaxed zonedecreased with the increase of rotating speed, which wasattributed to the down movement of upside grains.However, they have not examined the relationship of thesuper-gravity coefficient and grain size in detail. Inanother study, Wu et al.[5] simulated the process ofcentrifugal casting. The results showed that the super-gravity field intensified liquid convection but did notplay a significant role in grain refining. Unfortunately,the rotating speeds were restricted to only 260 rpm intheir study, and the nonmovement of grains was one ofthe hypothetical preconditions in the solidificationsimulation. In recent studies about the centrifugalcasting of alloys, instead of the alloy matrix, it was thesecond phase (or dispersed phase) that was especiallyconcerned.[6,7] While Melgarejo et al.[8] concluded fromthe centrifugal casting of Al-Mg-B alloy that the matrixgrain size decreased slightly under the super-gravityfield, the results were attributed to the increase ofcooling rate caused by super-gravity. Only Chen et al.[9]

have come up with an idea that super-gravity could

LIXIN ZHAO, Doctoral Student, is with the National EngineeringLaboratory for Hydrometallurgical Cleaner Production Technology,Institute of Process Engineering, Chinese Academy of Sciences, andthe China Graduate University of Chinese Academy of Sciences,Beijing, P.R. China, 100049. ZHANCHENG GUO, Professor, is withthe Key Laboratory of Ecologic and Recycle Metallurgy, University ofScience and Technology Beijing, Beijing, P.R. China, 100083. Contacte-mail: [email protected] ZHI WANG, Vice Professor, andMINGYONG WANG, Doctoral Student, are with the NationalEngineering Laboratory for Hydrometallurgical Cleaner ProductionTechnology, Institute of Process Engineering, Chinese Academy ofSciences, Beijing, P.R. China, 100190.

Manuscript submitted July 9, 2009.Article published online January 13, 2010

670—VOLUME 41A, MARCH 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A

cause the change of nucleation energy and thenaccelerate the nucleation. However, they have not givenmore details about how to calculate the change value. Insummary, how and to what extent the super-gravity fieldaffects the grain size is ambiguous. Further, the solid-ification under super-gravity was also used to preparethe graded materials and to discover the composition ofnew bulk metallic glass,[10] so it makes sense to seriouslyinvestigate the relationship between super-gravity andthe grain size.

To better examine the effect of the super-gravity fieldon grain refining, industrial pure aluminum, which has aclear grain boundary and low freezing point, wasselected as a sample. In addition, to eliminate theinfluences of other factors, especially the cooling rate,the samples were melted directly inside the apparatusand then cooled by the same cooling rate. While thegravity coefficients (symbolized as b, and calculated asthe ratio of super-gravitational acceleration to normal-gravitational acceleration) in most research were lessthan 50, the range of gravity coefficient in this work was1 to 1000 in order to sufficiently investigate theinfluences of super-gravity.

II. EXPERIMENTAL

The super-gravity field was generated by the centrif-ugal apparatus in the experiments, and the furnace wasfixed into the centrifugal rotor. Figure 1 shows thesketch of this apparatus. The industrial pure aluminum(1070*) was used as the sample. The covering slag was a

mixture of 45 pct sodium chloride and 55 pct potassiumchloride, and both of them were of analytical grade.

A. Experimental Procedures

A total of 10 g Al was placed in an alumina crucible(i.d. = 17 mm) and covered with 3 g covering slag. Thetemperature of the furnace was raised to 750 �C and

kept at this temperature for 10 minutes, and then thecentrifugal apparatus was started and adjusted to thespecified angular velocity. The apparatus was keptrotating until the sample was cooled to room temper-ature at 16 �C/min.The sample was cut into halves along the direction of

the super-gravity field, and the cutting face was bur-nished, polished, and then investigated by metallurgicalmicroscope. The grain size was estimated by thefollowing method: using the 100 times visual field ofmicroscope as the benchmark, the total number ofgrains was calculated by the number of the completegrains and the borderline-cutting grains via Eq. [1], andthe mean grain size (showed by area) was measured viaEq. [2]:[11]

n ¼ pþ 0:67q� 1 ½1�

F ¼ S=n ½2�

where n is the total number in 100 times visual field; p isthe number of complete grains; q is the number ofborderline-cutting grains; F is the mean grain size, mm2;and S is the area of the 100 times visual field, mm2.

III. RESULTS AND DISCUSSION

A. Microstructure of the Samples

Figure 2 shows the microstructures of different posi-tions on the longitudinal section of the sample(b = 553). It turns out that the outer portion(Figures 2(a) and (d)) belongs to the columnar crystalzone, and the central portion (Figure 2(b)) is theequiaxed crystal zone. However, the microstructure ofFigure 2(c)—unlike Figures 2(a) and (d) with obviouscolumnar structures or Figure 2(b) with fine equiaxedstructure—is irregular. The growth of grains inFigure 2(c) has the obvious direction—from outside toinside. So its microstructure should be regarded as thetransition state between the two kinds of structures. Incentrifugal casting, the equiaxed crystal was moreappreciated due to its improvement of the cast mechan-ical properties, so the equiaxed crystal zone in themiddle was studied as the object in order to betterinvestigate the influences of super-gravity on grainrefining.The microstructures of Figures 2(a) and (d) were

attributed to the faster rate of heat transfer in the outerlayer, while the structural formation of Figure 2(b)benefited from the lower undercooling degree and theuniform temperature field in the central portion ofthe sample. For Figure 2(c), it was its location—withinthe outer layer and at the bottom of the sample—thatresulted in its microstructure. The synergetic effect of thefaster heat-transfer rate and super-gravity field contrib-uted to the structure of Figure 2(c).Figure 3 shows the microstructures of equiaxed crys-

tal zones under different super-gravity fields. It turnedout that with the increase of gravity coefficient, thegrains became more uniform. In contrast, the grains

1 2 3 4

5

6

7

8910

Fig. 1—Schematic sketch of the experimental centrifugal apparatus.

*1070 is a trademark of Aluminum alloy in China NationalStandard.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 41A, MARCH 2010—671

under normal gravity were anomalous and untidy. Thereason may be that the convection inside the liquidbecame stronger with the increase of gravity coeffi-cient,[12] and the concentration and the temperature alsobecame more uniform. Consequently, the grains grewmore uniformly.

B. Influence of Super-Gravity on Grain Refining

Figure 3 also indicates that the grains become smallerwith the increase of gravity coefficient. Figure 4 showsthe relationship between grain size and gravity coeffi-cient. Each point in Figure 4 was the average value ofthree parallel samples. The relationship of grain size (F)and gravity coefficient (1< b< 1000) was fitted by thesecond-decreasing-exponential function (Eq. [3]). Thedata fitted well with the equation, and the correlationcoefficient (R) was 0.97538. The result demonstratedthat the grain size decreased rapidly with the increase of

the gravity coefficient from 1 to 250, but kept steadyfrom 250 to 1000.

F ¼ 0:01467þ 0:24157 exp�b

0:8977

� �

þ 0:07514 exp�b

63:95957

� �

½3�

Grain refining, resulted from super-gravity, should beattributed to the acceleration of the nucleation rate,which can be explained from two aspects. The first wasthe change of the nucleation energy caused by super-gravity, i.e., the increase of relative under-coolingdegree. The other was the increase in the density ofheteronucleation particles.[13] Because samples weremelted directly inside the apparatus and then cooledby the same cooling rate via controlled program in theexperiments, the effect of cooling rate under differentsuper-gravity on nucleation could be neglected.

Fig. 2—Microstructures of different positions on the longitudinal section of the sample, b = 553.


There were a few studies about the effect of super-gravity on the change of the nucleation energy. Chenet al.[9] studied the nucleation under super-gravity, anddetermined that the nucleation energy changed undersuper-gravity field, and its value could be estimated viaEq. [4]:

DG ¼ �ðDGV þ Gg þ GbÞVþ rA ½4�

where DG is the nucleation energy; DGV is the solid-liquiddifference ofGibbs free energy per unit volume;Gg andGb

represent energy per volume exerted on the nucleus pro-vided by normal gravity and super-gravity, respectively;

r is the solid-liquid interfacial tension; V is the volume ofthe solid; andA is the interfacial area. In their study, theyproposed that the values of Gg and Gb were positive and,therefore, the super-gravity accelerated the nucleation.However, there was no detailed explanation about how tocalculate the values of Gg and Gb, so the effect of super-gravity on grain refining was still unclear.Anthony et al.[14] investigated the migration of the

saturated brine droplet inside the salt crystal undersuper-gravity. The study demonstrated that the dropletmigrated in a direction opposite to the direction of thesuper-gravity field. They proposed that the migrationphenomenon resulted from the difference of chemicalpotential between two sides of the droplet, which wasgenerated by the super-gravity field. The difference madethe salt dissolve upside the droplet and crystallize outdownside; therefore, seemingly, the droplet moved in anopposite direction to the super-gravity field. The valueof the difference was calculated through Eq. [5]:

Dl ¼ ðM� qLmÞbgl ½5�

where Dl is the difference of the chemical potentialbetween the two sides of the droplet along the super-gravity field, M is the molar mass of the salt, qL is thedensity of the droplet, m is the partial molar volume ofthe brine, b is the gravity coefficient, g is the normal-gravitational acceleration with the value of 9.81 m/s2,and l is the size of the droplet. The computational methodindicated that Dl actually was the transformation of the

Fig. 3—Microstructures of equiaxed crystal zones under different super-gravity fields.

0 200 400 600 800 10000.00

0.04

0.08

0.12

0.16

0.20

Gra

in S

ize,m

m2

Gravity coefficient

Fig. 4—Relationship between the grain size and gravity coefficient.


potential energy difference in the super-gravity field. Inother words, the solubility difference between the twosides of the droplet was caused by the static pressuredifference, whichwas generated by the super-gravity field.In fact, the variations of physical properties, related to thestatic pressure, have been considered widely in chemicalengineering (for example, the loss of temperature differ-ence in evaporation,[15] which was considered only in thelarge-scale equipment for the intensity of normal gravity).However, in Anthony’s study,[14] the intensity of thesuper-gravity field was very large, more than 50,000, sothe static pressure difference was big enough to create theremarkable solubility difference between two sides of thedroplet.

Based on these studies, we proposed that the changeof nucleation Gibbs free energy caused by super-gravityshould be attributed to the rise of static pressure.According to thermodynamics, the change was calcu-lated via Eq. [6]:

DGb ¼ DV�DP ¼ DVðqbghÞ ½6�

where DGb is the change of the nucleation Gibbs freeenergy caused by super-gravity, DV is the volumechange from liquid to solid, DP is the rise of staticpressure caused by super-gravity, q is the density of theliquid, b is the gravity coefficient, and h is the depthunder the liquid. For most metals, including Al, DV isnegative, so the nucleation rate could be accelerated bythe super-gravity field. In the experiments, the depth ofthe equiaxed crystal zone was 10 mm below the surface,where the largest static pressure under the experimentalconditions (b< 1000) was less than 2.5 atm, and theabsolute value of DGb was less than 0.1865 J/mol.

To estimate the effect of DGb on grain refining, thevalue of DGb was compared with the value of DGV, thesolid-liquid difference of Gibbs free energy per unitvolume. Based on the standard entropy change (DS*,11.55 J/molÆK) at 933.5 K,[16] when the degree ofundercooling was only 1 K, the absolute value of DGV

was about 11.55 J/molÆK, which was much larger thanDGb; thereby, the change of the nucleation rate, causedby DGb, was not significant enough to achieve thestructure, as shown in Figure 4.

If the change of the nucleation energy was not themain reason, the increase of heteronucleation particledensity would play an important role in grain refiningunder super-gravity. In this system, instead of beingimported from other resources, the heteronucleationparticles were the fragments from other portions of thesample. It is known that violent convection can makethe dendritic crystal rupture or burn out on the growthsurface, and the fragments would become the nucleationparticles in liquid.[2,3] Here, super-gravity intensified theconvection and generated the crystal fragments. Becauseof the density difference between the fragments and theliquid, the fragments were dragged down by the super-gravity field and presented as the nucleation particles inthe liquid. Further, the free grains also played the samerole as the fragments.

For aluminum, with the increase of the gravity coeffi-cient, the convection was intensified gradually, and more

and more fragments were generated and presented as thenucleation particles. Therefore, the grain size becamesmaller. However, when the coefficient was larger than250, the number of fragments and free grainmight reach amaximum value, and the further increase of the super-gravity intensity did not lead to the increase of the totalfragment number, so the grain size became stable.

C. Change of Freezing Points under DifferentSuper-Gravity

The degree of undercooling would change if thesuper-gravity could cause the change of nucleationenergy. Therefore, the freezing points under differentsuper-gravity were measured in order to further inves-tigate the influence of the super-gravity field. Themeasurement was implemented by the thermocouplelocated inside the equiaxed crystal zone. Figure 5presents the freezing points under different super-gravityfields, which show that there is no significant differenceamong the freezing points, except for the low freezingpoint in the case of normal gravity. The result suggestedthat the change of nucleation energy with super-gravitywas not appreciable. Each point in the figure has beenmeasured 3 times and the average value reported.We have proposed that the increase of ruptured

fragments should be the main reason for grain refiningunder super-gravity. Because the fragments and the freegrains came from the other part of the sample, and themain ingredient was aluminum, its crystal texture (forexample, the interatomic distance and atomic array) wasalmost the same. It was expected that the wettabilitybetween the fragments and liquid aluminum was excel-lent, and the wetting angle (h) should be close to 0. Thus,the heterogeneous nucleation energy (DG*), calculatedvia Eq. [7],[13] was 0, which suggested that nucleationcould happen without undercooling:

DG� ¼16pr3LS3DG2

V

2� 3 cos hþ cos2 h

4

� �

½7�

This might be the reason why the freezing point did notchange with grain size, and also served as evidence toprove that the introduction of the fragments and the freegrains should be the main reason for grain refining.

0 200 400 600 800640

660

680

Fre

ezin

g P

oin

t,o

C

Gravity coefficient

Fig. 5—The freezing points under different super-gravity fields.


From the preceding analysis, the lower freezing pointunder normal-gravity condition could also be rational-ized as follows: under normal gravity, the convectionwas weak, and the number of fragments was less, so thenucleation process was mainly spontaneous and thedegree of under-cooling was large.

IV. CONCLUSIONS

Using industrial aluminumas an example, the influenceof the super-gravity field on metallic grain refining hasbeen carefully investigated in a broad gravity coefficientrange (1< b< 1000). The grain size in the equiaxedcrystal zone decreased rapidly with an increase in thegravity coefficient from 1 to 250, but remained steadyfrom 250 to 1000, and the relationship between grain sizeand gravity coefficient could be well fitted by the second-decreasing-exponential function. Freezing points underdifferent super-gravity fields were almost invariant exceptfor the low freezing point in the case of normal gravity.

The main grain-refining mechanism was proposed tobe that the super-gravity contributed to the rupture ofdendritic grains, and then the fragments became theheterogeneous particles and resulted in grain refining.Further, the change of nucleation Gibbs free energy,directly caused by super-gravity, did not contribute tograin refining significantly, which was proved by thestable freezing points under different super-gravity.

ACKNOWLEDGMENTS

This work is supported by the Natural ScienceFoundation of China (Grant Nos. 50804043 and

50674011) and Major Programs on EquipmentDevelopment of the Chinese Academy of Science(Grant No. YZ0618).

REFERENCES

1. C.N. Wei, H.Y. Bor, C.Y. Ma, and T.S. Lee: Mater. Chem. Phys.,2003, vol. 80, pp. 89–93.

2. J.W. Yeh, S.H. Jong, and W.P. Liu:Metall. Mater. Trans. A, 1996,vol. 27A, pp. 1933–43.

3. X.T. Li, Z.X. Guo, X.W. Zhao, B. Wei, F.B. Chen, and T.J. Li:Mater. Sci. Eng. A, 2007, vols. 460–461, pp. 648–51.

4. J.W. Yeh and S.H. Jong: Metall. Mater. Trans. A, 1994, vol. 25A,pp. 643–50.

5. S.P. Wu, D.R. Liu, J.J. Guo, C.Y. Li, Y.Q. Su, and H.Z. Fu:Mater. Sci. Eng. A, 2006, vol. 426, pp. 240–49.

6. Y.W. Sui, B.S. Li, A.H. Liu, H. Nan, J.J. Guo, and H.Z. Fu:Trans. Nonferrous Met. Soc. China, 2008, vol. 18, pp. 291–96.

7. K.B. Kim, J. Das, W. Loser, M.H. Lee, D.H. Kim, S.K. Roy,and J. Eckert: Mater. Sci. Eng. A, 2007, vols. 449–451, pp. 747–51.

8. Z.H. Melgarejo, O.M. Suarez, and K. Sridharan: Compos. Part A,2008, vol. 39, pp. 1150–58.

9. K.Y. Chen, Z.Q. Hu, and B.Z. Ding: J. Mater. Sci. Technol., 1994,vol. 10, pp. 307–09.

10. J.F. Loffler and W.L. Johnson: Intermetallics, 2002, vol. 10,pp. 1167–75.

11. ‘‘Aluminium and Aluminium Alloys Inspection Method ofMicrostructure for Wrought Products,’’ China National StandardGB 3247-82, 1982.

12. G. Muller: J. Cryst. Growth, 1990, vol. 99, pp. 1242–57.13. Y.H. Zhou: Solidification Technique, China Machine Press,

Beijing, 1998, pp. 21–6, 436–41.14. T.R. Anthony and H.E. Cline: J. Cryst. Growth, 1972, vols. 13–14,

pp. 790–94.15. M.H. Chen, D.Z. Cong, and T.N. Fang: Principles of Chemical

Industry, Chemical Industry Press, Beijing, 1995, p. 305.16. L.F. Mondolfo: Aluminum Alloys Structure and Properties,

Butterworth and Co., London, 1976, p. 56.


influences of super-gravity field on aluminum grain refining

Documents