Grain Refinement in Alloys: Novel

Approaches

Figure 2A grain refiner rod being injected into a launder during adirect chill casting operation. Image courtesy of J.Grandfield, Grandfield Technology Pty. Ltd.

Introduction

Grain refinement (Fig. 1) is a method of improvingthe quality of castings, either as finished or as semi-finished products, by creating a more homogeneousmicrostructure. A refined grain structure during so-lidification generally, but not always, reduces thepropensity of a casting to defects such as hot tearingand large isolated porosity that leads to reducedpressure tightness. A refined as-cast grain size canalso improve the outcomes from subsequent pro-cessing operations by reducing homogenizationtimes, improving the homogeneity of recrystallizedmicrostructures and mechanical properties.

There has been a great deal of work on the grainrefinement of Al-based alloys over the past century,and the understanding of, and the technology for,grain refinement have improved considerably (Murtyet al. 2002). Today, grain refiners are added as masteralloys directly into a launder (Fig. 2). For mostwrought Al-based alloys, the master alloys are alu-minum based but contain controlled amounts oftitanium and boron, typically from 3% to 5% andfrom 0% to 1%, respectively. Speciality grain refinershave been developed to grain refine Al–Si foundryalloys and alloys that contain alloying elements thatpoison the Ti–B-based grain refiners.

Magnesium alloys (StJohn et al. 2005) are easilygrain refined by Zr additions typically added as anMg–Zr master alloy containing 25–40% Zr. How-ever, the grain refinement action of Zr does not occurin the commercially more important Mg–Al alloys, asAl can readily consume Zr to form Al3Zr

Figure 1Aluminum alloy 6082: not grain refined (left) and grain refinalloy and a 0.02 wt.% addition of Ti solute (right).

intermetallics, which are ineffective for nucleatingmagnesium grains. Grain refining of Mg–Al alloyshas proven to be a scientific and technological chal-lenge with many approaches having been assessed,including melt superheating and inoculation of themelt with carbon-based additions of various formsfrom gases, waxes or ceramic particles; anhydrousiron chloride; and Al–60% Mn master alloy splatter.Although carbon-based grain refiners are currentlyused in the commercial production of wroughtMg–Al alloys, the lack of a potent grain refiner haslimited the application of these alloys by gravity orlow-pressure casting routes.

Copper-based alloys can also be grain refined byinoculant additions, which depend on the alloy.However, grain refinement is often achieved throughcontrol of casting conditions and application of

ed with a 0.005 wt.% addition of an Al–3Ti–1B master

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Table 1Values of m(k� 1) for common elements in Al alloys.

Element m(k� 1)

Ti B220

Zr 6.8

Si 5.9

Cr 3.5

Mg 3.0

Fe 2.9

Cu 2.8

Mn 0.1

Table 2Values of m(k� 1) for common elements in Mg alloys.

Element m(k� 1)

Fe 52.56

Zr 38.29

Ca 11.94

Si 9.25

Zn 5.31

Al 4.32

Grain Refinement in Alloys: Novel Approaches

vibration. Grain refinement of titanium and its alloys,in general, has received lesser attention, as castingis a less important manufacturing process for tita-nium alloys compared with other structural metallicmaterials.

There have been a number of different theoriesrelating to the reasons for effective grain refinement.Two early theories included the particle theory thathighlights the importance of the addition of nucle-ating substrates and the peritectic theory that high-lights the fact that many grain refining systems arerelated to peritectic systems with the Al–Ti andMg–Zr systems being examples.

Today, it is generally agreed from both theoreticaland experimental studies that effective grain refine-ment requires both potent nucleating substratesand segregating solutes (Easton and StJohn 1999).Suitable substrates include mold walls and surfaces,but the most effective way to uniformly grain refinealloy melts is the addition of inoculant particles via amaster alloy. Traditionally, effective substrates havebeen considered to be those that provide a low-energyinterface with the nucleating solid but in practicehave mostly been identified by trial and error. Amaster alloy delivers a fine dispersion of these sub-strates into the melt to provide copious sites for grainnucleation to occur. The addition of these substratesreduces the free energy barrier inhibiting the trans-formation from liquid to solid phase, which is evi-denced by reduced undercooling.

Hence, a fine grain size is promoted by particleswith a high potency (commonly defined as beingproportional to 1/DTn, where DTn is the undercoolingrequired for nucleation), but alloy chemistry alsoplays a major part. The role of solute (alloy consti-tution) is characterized by what is known as the‘‘growth restriction factor,’’ now commonly referredto as Q. A larger value of Q means that constitutionalundercooling is generated at a faster rate achievingDTn and consequently the next nucleation eventsooner resulting in a finer grain size. The value of Qfor a particular alloy can be estimated from phasediagram features by

Q ¼X

i

ml;ic0;iðki � 1Þ

where ml is the liquidus gradient, c0 the composition,and k the partition coefficient for each element i inthe alloy (Johnsson and Backerud 1996).

Different alloying elements have different relativeeffects, which can be evaluated by comparing theirm(k� 1) values for different elements. Values forsome typical alloying elements in Al and Mg alloysare given in Tables 1 and 2, respectively. It is notedthat Ti has a very high m(k� 1) value in Al as does Zrin Mg, both of which are the most effective elementaladditions for grain refinement in each alloy system.

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The other important feature of Q is that each in-cremental increase in the value of Q for a particularalloy system has a relatively smaller effect on grainsize of the cast alloy. Thus, a unit increase in Q causesa greater decrease in grain size for lean alloys than foralloys rich in alloying elements. For this reason,solute elements with high m(k� 1) values grain refinewrought alloys very effectively, whereas in foundrycasting alloys, which generally have much highersolute contents, there is little grain refinement re-sponse to the addition of these extra alloying elem-ents (Easton and StJohn 2001). Thus, for efficientrefinement, low solute wrought alloys require bothnucleant particles and additional solute (e.g., Ti),whereas foundry alloys require only nucleant par-ticles. Hence, grain refiner master alloys should betailored for different classes of alloys.

There are a number of other factors that limit theamount of grain refinement that can be achieved. Themost common factor is elements that poison nucleantparticles, usually due to a chemical reaction thatrenders the substrate less effective as a refiner. In Al-based alloys, it is well recognized that Zr additionspoison Al–Ti–B grain refiners. Another problem thathas been highlighted in the literature is that in Al–Sialloys at additions higher than 2–3%, the grainsize increases even though Q continues to increase(Johnsson and Backerud 1996). A feature of grainrefinement observed in Al alloys is that only a smallpercentage of the nucleant particles added actually

Grain Refinement in Alloys: Novel Approaches

nucleate grains and this also limits the amount ofgrain refinement that can be achieved.

Grain refinement fades with time in most cases.This can occur for two reasons. One is that the par-ticles drop out of the melt due to density differencesbetween the particles and the molten alloy. This isusually recoverable by stirring the melt. The other isdue to the agglomeration of particles, which is dif-ficult to recover from on restirring. In grain refiners,where the nucleant particles are not thermo-dynamically stable such as Al–Ti–C refiners in Alalloys, grain size fade can occur due to reactions inthe melt. This is also not recoverable. Such grainrefiners can only be used in applications where shortcontact times with the melt occur such as in thecontinuous grain refinement during direct chillcasting.

1. Novel Approaches to the Analysis of GrainRefinement

Recently, there have been substantial advances in themodeling of grain refinement using both physics-based and empirically based models. Pleasingly, thereis a large degree of convergence in the understandingof grain refinement including the importance of the

Commercialrefiner

050

100

150

200

250

Gra

in s

ize

(µm

)

1 2 3 4

Mean particle di

Figure 3The predicted effect of particle size (diameter) for a constanparticles are added, a greater proportion of particles are activadded. Small particles mean that there is a high density of pointermediate particle diameter at approximately 1 mm, wherachieved to obtain the finest grain size. Reproduced with peSchneider W, Schumacher P, Spittle J A, Tronche A 2003 GEng. Mater. 5, 81–91. Copyright Wiley.

interplay between the roles of solute and nucleantparticles.

An approach developed to consider the potency ofTiB2 particles in aluminum alloys relates the potencyto the size of the nucleant particle

DTmin ¼4gSLDSvl

ð1Þ

where DTmin is the undercooling required for freegrowth, gSL the solid–liquid (S–L) interfacial energy,DSv the entropy of fusion per unit volume of thesolid, and l the particle diameter (Greer et al. 2003).That is, larger particles have a higher potency or aremore efficient as nucleants. The model suggests thatthe barrier to growth of a new grain is at least asimportant as the barrier to nucleation in the for-mation of new grains. Although conceptually useful,the distribution of particles in a master alloy is notusually known. The effect of particle size on nucleantefficiencies and grain size is shown in Fig. 3.

The lattice matching between the substrate and thesolid has long been recognized as important foreffective nucleation. Recently, a new method called‘‘edge-to-edge matching’’ (Zhang et al. 2005), acrystallographic model originally developed to de-scribe and predict crystallographic features of

No.

and

vol

ume

effic

ienc

ies

(%)

ameter (µm)

5 6 7 80

20

40

60

80

100

t fraction added to a melt. It is seen that when largere, but the grain size is still large because few particles aretential sites but few of them are active. Hence, there is ane the optimum combination of density and efficiency isrmission from Greer A L, Cooper P, Meredith M W,rain refinement of aluminium alloys by inoculation. Adv.

3

0

200

400

600

800

1000

0 0.1 0.2 0.3 0.4

Gra

in s

ize

(µm

)

0%TiB2

0.005%TiB2

0.02%TiB2

0.01%TiB2

1/Q (K−1)

Figure 4Lines fitted to experimental data showing the influenceof nucleant particle type, nucleant density, and solutecontent (Q) on grain size. The data are based onresults from seven different Al-based alloys with avariety of Ti solute additions. Reproduced withpermission from Easton M A, StJohn D H 2005 Ananalysis of the relationship between grain size, solutecontent and the potency and content of nucleantparticles. Metall. Mater. Trans. A 36A, 1911–20.Copyright Springer.

Figure 5Transmission electron micrograph of a TiB2 particle(bottom) embedded in a glassy matrix (top).Reproduced with permission from Schumacher P, GreerA, Worth J, Evans P, Kearns M, Fisher P, Green A 1998New studies of nucleation mechanisms in aluminiumalloys: implications for grain refinement practice.Mater.Sci. Technol. 14, 394–404.

Grain Refinement in Alloys: Novel Approaches

diffusion-controlled phase transformations in solids,has been used to explain the relative effectiveness ofnucleants in both Al- and Mg-based alloys and toidentify new nucleants in Mg–Al alloys. The techni-que assumes that interfacial energy is minimized byminimizing the lattice strain. This is assumed to occurwhere there is matching of rows of atoms in the twophases, and that these matching or coincident atomrows consist of directions in the two crystals that areclose packed or nearly close packed. Crystallographicdata of phases are used to determine the matchingplanes between substrates and the solid, and fromthis, interatomic and interplanar mismatches arecalculated. Typically it is assumed that mismatcheslower than 10% and 6%, respectively, mean that asubstrate is a candidate as an effective nucleantparticle.

A new analytical method has been developed inwhich experimentally measured trials have shownthat grain refinement data can be easily described bya simple equation (Easton and StJohn 2005 (also seefurther development by Qian et al. 2010)) (Fig. 4):

d ¼ 1ffiffiffiffiffiffiffiffifNv

3p þ b DTn

Qð2Þ

where d is the grain size, Q as previously defined, Nv

the number of nucleant particles in the alloy, f theproportion of particles that are active, DTn the nu-cleation undercooling, and b the fitting factor that isknown to decrease with increased cooling rate. Thisequation has been observed to describe grain sizedata in Al-, Mg-, and Ti-based alloys for a variety ofsolidification conditions from slowly cooled in

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preheated molds, to castings in chill molds and evenin high-pressure die casting.

Detailed investigations of the nucleation eventhave also been undertaken. A novel approach thathas been used is the introduction of grain refiners intoglass-forming Al-based alloys (Fig. 5) (Schumacheret al. 1998). These alloys are then annealed to allowcrystallization of the glass, nucleated by the inoculantparticles. A more recent novel approach is the use ofsynchrotron radiation and advanced techniques suchas three-dimensional X-ray diffraction, where phasetransformations during solidification can be observedin situ (Iqbal et al. 2005). Both of these approacheshave been able to illuminate the initial stages of nu-cleation. These kinds of advanced observation tech-niques will no doubt be used extensively in futureresearch of grain refinement.

2. Novel Approaches to Achieving GrainRefinement

Apart from the use of chemical additions to alloys,grain refinement can be achieved by controllingprocessing conditions. Different casting techniqueslead to very different grain sizes being achieved. Forexample, high-pressure die casting with cooling ratesbetween 100 and 1000 1C s�1 typically achieves grainsizes of the order of 10 mm, whereas in direct chill

Grain Refinement in Alloys: Novel Approaches

casting even with the addition of a considerableamount of grain refiner, it is difficult to obtain grainsizes of less than 100 mm for wrought Al alloys.Gravity or low-pressure casting of Al–Si alloystypically leads to grain sizes in the hundreds ofmicrometers range.

Owing to the limits in achievable grain size affordedby these casting processes and chemical additions,other methods have been developed to reduce thegrain size further. All of these methods modify thecasting process to deliberately generate a higher rate ofnucleation early in the solidification process close tothe start of solidification or in the semisolid zone be-tween the times when solidification starts and finishes.

2.1 Methods of Generating Semisolid Grain Sizes

Semisolid forming has held much promise for anumber of years but to date has not significantlypenetrated commercial production of castings. Thislack of take-up is mainly driven by the extra cost ofadding a processing step to establish a semisolidslurry of the appropriate solid phase morphology andsize. However, semisolid processing has found a nichefor castings requiring consistent quality with a verylow defect structure and it remains an effectivemethod of reducing grain size with or without theaddition of grain refiner particles.

(a) (b)

Figure 6Schematic diagrams of methods of producing nondendritic(c) electromagnetic stirring with continuous casting. ReprodBehaviour of metal alloys in the semisolid state. Metall. Tra

There are a number of innovative ways to usetemperature control and melt processing to grainrefine melts, which have typically been used toprepare the feed material for semisolid casting(Kaufmann and Uggowitzer 2007) (Fig. 6). Theseinclude the following:

• the use of chilled surfaces and melt temperaturecontrol, e.g., lower casting temperatures;

• shearing of melts while the melt is semisolid;

• conditioning of melts before solidification.When a melt that is near the solidification tem-

perature (i.e., low superheat) is poured onto a rela-tively cold surface, i.e., one that is many degreesbelow the liquidus temperature, a large amount ofthermal undercooling can be generated in the melt,allowing for a large number of nucleation events.This process is as effective as, if not more effectivethan, the addition of inoculant grain refiners. How-ever, there is a compromise with melt fluidity, i.e.,melts with little superheat have limited flow distances.Hence, the process is, in general, not suited tostandard liquid casting situations but is an effectiveroute for obtaining a fine globular structure forsemisolid billet or the even more desired semisolidslugs for rheocasting (Fig. 7).

Sheering the melt in the semisolid state produces afine globular microstructure. This is because shearingleads to the fragmentation of dendrites and con-sequently a refined grain size. There have been a

(c)

structures: (a) batch, (b) continuous, anduced with permission from Flemings M C 1991ns. A 22, 957–81.

5

Melting

New rheocasting

PouringNew rheocasting machine

Cooling Heating Ladling Casting Runnerseparation

Figure 7A schematic diagram of the innovative new rheocasting process where a semisolid slurry is produced by controlledpouring and cooling of a melt to form a semisolid slug of appropriate microstructure, which is then cast into amachine. Reproduced with permission from Kaufmann H, Uggowitzer P J 2007 Metallurgy and Processing of High-Integrity Light Metal Castings. Fachverlag Schiele & Schon GmbH, Berlin.

Grain Refinement in Alloys: Novel Approaches

number of approaches that have been taken alongthese lines.

Magnesium alloys are often thixomolded. This iswhere alloy chips are melted into the semisolid stateand then sheared in a barrel by a screw mechanism toproduce a fine grained spherical semisolid micro-structure. This material is then injected into a mold.

To overcome the need for incorporating an add-itional process into a casting machine, the ATMprocess was developed (O’Donnell et al. 2009). Thetechnology incorporates a shear device into the de-sign of the in-gate of a high-pressure die castingcavity. The device shears the incoming partly solidi-fied mush into a fine grain-sized mush that enters thecavity with a uniform flow front producing better-quality castings that can be heat treated.

Grain refinement has also been achieved byshearing the melt before solidification in magnesiumalloys (Fan et al. 2009). This has been termed ‘‘meltconditioning.’’ MgO forms readily on any surface incontact with air and shearing breaks up the MgOparticles or films and disperses them throughout themelt. MgO appears to be a useful nucleant for mag-nesium grains when dispersed in the melt.

2.2 Ultrasonic Treatment

Ultrasonic irradiation during solidification modifiesconventionally obtained grain structures for bothmetallic and nonmetallic materials (Qian et al. 2009).The effect of ultrasonic grain refinement was dem-onstrated on billet materials up to 1200mm indiameter. The process, in general, can be divided intodirect and indirect approaches depending on themelting point of the liquid and its chemical reactivitywith the sonotrode material. For direct ultrasonicirradiation, the sonotrode material is required to havehigh efficiency in transmitting the ultrasonic signalsat the melt temperature, good chemical inertness tothe melt, and superior dimensional stability andmechanical performance. Ultrasonic irradiation forsolidification control is typically applied at a

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frequency of 20 kHz in the amplitude range of10–50 mm. Experimental evidence has been producedto show that fully developed cavitation is essential toeffective ultrasonic grain refinement. This requiresthe applied ultrasonic intensity to exceed the thresh-old for cavitation, which is alloy dependent. Bothcavitation-induced heterogeneous nucleation andfluid flow phenomena appear to contribute to ultra-sonic grain refinement. Ultrasonic irradiation at-tenuates in liquid metals and therefore affects thedegree of refinement. Recent work has shown that thealloy chemistry plays an appreciable role in deter-mining the grain size by ultrasonic refinement ac-cording to the aþ b/Q model (Eqn. (2)) and inalleviating the attenuation. This provides an alter-native avenue of alloy development for effectiveultrasonic refinement.

2.3 Electromagnetic Stirring

Electromagnetic stirring is a technique commonlyused industrially in the grain refinement of steel andcopper alloys. It has also been found to be effective inthe grain refinement of Al- and Mg-based alloys.Recent work on electrostatic solidification (free fromstirring) and electromagnetic solidification (withstirring) has revealed that melt flow is vital for theattendant equiaxed grain formation due to dendritefragmentation (Sugiura and Iwai 2005). Limited evi-dence suggests that intense electromagnetic stirringbeyond a threshold induces nucleation, possiblythrough the effects of cavitation. An improvedunderstanding of the electromagnetic solidificationmechanisms may significantly extend the applicationof electromagnetic stirring.

3. Summary

Grain refinement theory and our predictive capabil-ities continue to develop due to greater understandingof the relationships between grain size and nucleantparticle size and potency, nucleant density, and the

Grain Refinement in Alloys: Novel Approaches

alloy’s constitution. This has been achieved by the useof modeling and novel experimentation, including theuse of advanced experimental methods such as crys-tallization of metallic glasses and synchrotron radi-ation to view nucleation processes in real time.

Industrially, the recent innovations in grain re-finement have been related to semisolid processingwhere control of melt temperatures and the use ofmelt shearing technologies have been able to improvethe properties of castings. Ultrasonic treatment andelectromagnetic stirring are attracting renewedinterest and may provide a way to grain refine alloyswithout the use of inoculant grain refiners in thefuture.

See also: Aluminum Alloys: Thermal Treatments;Metal Working: Extrusion; Primary Recrystalliza-tion in Two-phase Alloys; Semisolid Processing; So-lidification of Light Metals (Non-ferrous)

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M. A. Easton, M. Qian, and D. H. StJohn

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