© 2017 The Authors. The amount of solutal present in an alloy affects the grain size of the cast metal as solute is rejected at the solidification front. This is normally quantified using the so called growth restriction factor Q. This work presents some considerations about the effect of solutal on the final cast structure with a focus on the nature of the alloy system, the effect of non-equilibrium solidification conditions and the effect of superheating of the molten metal

Bolzoni, L

Hari Babu, N

Materials Letters

Leandro Bolzoni

Nadendla Hari Babu

Crossref

Considerations on the effect of solutal on the grain size of castings from superheated melts

The amount of solutal present in an alloy affects the grain size of the cast metal as solute is rejected at the solidification front. This is normally quantified using the so called growth restriction factor Q. This work presents some considerations about the effect of solutal on the final cast structure with a focus on the nature of the alloy system, the effect of non-equilibrium solidification conditions and the effect of superheating of the molten metal

Bolzoni, Leandro

Babu, Nadendla Hari

Research Commons@Waikato

Materials Letters 201 (2017) 9–12Contents lists available at ScienceDirectMaterials Lettersjournal homepage: www.elsevier .com/locate /mlblueConsiderations on the effect of solutal on the grain size of castings fromsuperheated meltshttp://dx.doi.org/10.1016/j.matlet.2017.04.1170167-577X/ 2017 The Authors. Published by Elsevier B.V.This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).⇑ Corresponding author at: The University of Waikato, Private Bag 3105,Hamilton 3240, New Zealand.E-mail address: bolzoni.leandro@gmail.com (L. Bolzoni).Leandro Bolzoni a,b,⇑, Nadendla Hari Babu ba The University of Waikato, Private Bag 3105, Hamilton 3240, New ZealandbBCAST, Brunel University London, Kingston Lane, Uxbridge, Middlesex UB8 3PH, UKa r t i c l e i n f o a b s t r a c tArticle history:Received 23 March 2017Accepted 23 April 2017Available online 25 April 2017Keywords:CastingSolidificationGrain refiningPhase diagramMetal and alloysThe amount of solutal present in an alloy affects the grain size of the cast metal as solute is rejected at thesolidification front. This is normally quantified using the so called growth restriction factor Q. This workpresents some considerations about the effect of solutal on the final cast structure with a focus on thenature of the alloy system, the effect of non-equilibrium solidification conditions and the effect of super-heating of the molten metal. 2017 The Authors. Published by Elsevier B.V. This is an open access articleunder the CCBY license (http://creativecommons.org/licenses/by/4.0/).1. IntroductionThe achievement of fine equiaxed grains is desirable in metalscasting, independently of wrought or shape-casting alloys. Bothgrades benefit from better feeding of the molten metal in themushy zone resulting in more homogeneously distributed porosityand reduced hot tearing susceptibility [1]. Apart increasing thesoundness of the castings (i.e. lower rejection rate), other benefitsof a fine structure are higher mechanical performances, and easierand more homogeneous subsequent mechanical working forwrought-grade alloys [2]. In every solidification process, metalscasting involves nucleation of a new phase and consequent growthof the solid phase. Easton and StJohn reported that nucleation willnot occur in the bulk of a metal without adequate solute being pre-sent [3]. The concept of growth restriction factor (Q) was first intro-duced by Maxwell and Hellawell [4] as an independent parameterrelevant to the refinement of cast structures, in particular for bin-ary Al alloys. Q is an extension of the early work of Tarshis et al. [5]who proposed that the solute present in the solidification frontaccounts for the degree of growth restriction, a parameter thatwas called constitutional supercooling parameter (P). Q and P arerelated via the equilibrium partition coefficient k [6]:Q ¼ mLðk 1ÞC0 ¼ kP ð1ÞwheremL is the liquid slope, and C0 is the solute content in the alloy.The scientific community focusing on solidification of moltenmetals has widely adopted the use of Q to relate it to the grain size(d) of the casting [3,7] because otherwise complex thermodynamicmodels have to be implemented to study grain growth restriction[8,9]:d ¼ aþ b=Q ð2ÞIn Eq. (2), a is a constant related to the number density of activenucleating particles, and b is a constant associated with the effi-ciency of the nucleating particles.Although mainly used to quantify the effect of the presence ofheterogeneous nuclei added to promote nucleation [3] (i.e. chem-ical inoculation), the relationship presented in Eq. (2) can also beused to study the effect of the solutal content on the grain size ofthe casting [3] where nucleation begins heterogeneously in themelt near the wall of the mould. It was shown that of the four con-tributions to the total undercooling (kinetic, curvature, thermaland solutal) the solutal or constitutional supercooling is the con-trolling term under typical Al alloys casting conditions [10]. Highersolutal content results in higher Q and finer grain size (d) for com-mercial wrought Al alloys as shown in Fig. 1a (data from [3]). Nev-ertheless, the same relationship is not valid for shape-casting Alalloys based on the Al-Si system (Fig. 1b) (data from [11,12]).A number of scientists have reported that as Si content initiallyincreases, the Al solid solution grain size decreases. However,beyond a critical value (around 3 wt.% Si for typical Al alloys cast-ing conditions) the grain size increases where this trend is040080012001600200024000 0.1 0.2 0.3 0.4 0.5Grain size,d[µm]1/Q [1/K]1050 2014 3003 50836060 6061 6082 7075(a) 040080012001600200024000 0.1 0.2 0.3 0.4 0.5Grain size,d[µm]1/Q [1/K]Al-xSiSolutalcontentAl-3Si(b) Shape-casting WroughtFig. 1. Average grain size (d) versus growth restriction factor Q. (a) Wrought Al alloys (data from [3]). (b) Shape-casting Al alloys (data from [11,12]).10 L. Bolzoni, N. Hari Babu /Materials Letters 201 (2017) 9–12observed both in grain refined and non-grain refined alloys, withthe non-grain refined alloys exhibiting the coarser grains [12].Although a number of explanations have been proposed such aschange in crystal growth morphology [11] and Si poisoning [13]there is still debate about the real nature of this coarsening innon-refined Al-Si alloys. Moreover, it has been demonstrated viathermodynamic simulations that if Ti is present in excess to theamount needed to form nucleating TiB2 particles, the growth rateof Al dendrites in Al-Si alloys increases leading to grain coarsening[8,9].The purpose of the work is to make some considerations aboutthe effect of solutal content on the grain size of cast structure tak-ing into account three aspects: (i) the nature of the alloy, (ii) theeffect of non-equilibrium solidification conditions and (iii) theamount of superheating.2. Consideration about the nature of the systemArguably, one of the critical parameter influencing the finalgrain size is the total amount of solutal present in the alloy (i.e.smaller d for higher Q) as it is accepted that solute is rejected bythe growing grains upon solidification. However, the first impor-tant difference between the data presented in Fig. 1 is the intrinsicnature of the alloy system. In all the commercial wrought Al alloys,whose grain size is presented in Fig. 1a, the content of each of thesolute elements, although high, it is never above the maximumsolid solubility (CsM) of that specific element in Al (Table 1). Thesealloys are defined as dilute alloy systems in this work and in themthe primary phase to nucleate is a solid solution of Al. For example,the 2014 alloy has an average content of 4.60 wt.% Cu, 0.52 wt.%Mg and 0.98 wt.% Si (these values are normally used in Eq. (1) asTable 1Solute content (C0) and maximum solid solubility (CsM) of different solute elements useconsistency) and CsM values are from the corresponding binary phase diagrams [14].Alloy 2014Cu C0 4.60CsMMg C0 0.52CsMZn C0 –CsMSi C0 0.98CsM* Wrought Al alloys have a maximum Si content of 2 wt.% and shape-casting Al a minC0) and therefore one of the lowest 1/Q and finer grain size inFig. 1a. These solutal contents are lower than 5.65 wt.%, 17.10 wt.% and 1.65 wt.% which are the CsM values for Cu, Mg and Si, respec-tively (Table 1). Although it was shown that the simultaneous pres-ence of two or more solutes can change the primary forming phase[10], the principle of using CsM seems to be still applicable in highlyalloyed wrought Al alloys. In the case of shape-casting Al alloys(whose minimum Si content is approximately 5 wt.%) at equilib-rium, off eutectic compositions solidify with a two-phase structureand are defined in this work as two-phase alloy systems. The factthat in shape-casting alloys C0 is greater than CsM has implicationwhich are discussed in the latter section jointly with the effect ofnon-equilibrium solidification conditions.3. Consideration about non-equilibrium solidificationconditionsCasting in practical instances, independently whether at indus-trial scale to fabricate products or at lab scale to study the nucle-ation and growth of crystals, does not happen under equilibriumconditions. This has direct implications on the equilibrium phasediagrams and on the cast structure. For example, it is well knownthat solidified grains have a cored structure, and thus microsegre-gation because the amount of solutal inside each grain increases assolidification progresses [15]. This justifies the eutectic phasebeing present in the cast structure of alloys whose solute contentis much lower than the eutectic composition (i.e. CE). The non-equilibrium conditions should then be addressed using metastablephase diagram, an idealisation of which is presented in Fig. 2a. Thismeans that under non-equilibrium solidification conditions theeutectic point is shifted to a higher solutal content (i.e. CE) and tod in wrought and shape-casting Al alloys. Note: average C0 values are from [3] (for5083 7075 Binary Al-xSi– 1.69 –5.654.19 2.33 –17.1– 5.22 –16.50.35 0.28 1.0–11.0*1.65imum Si content of 5 wt.%; Si  3 wt.% is commonly used to discern them.TmTLCsM CEDilute alloy systems Two-phase alloy systemsC0Metastable eutectic*sMC *EC*ETSolute contentTemperatureTEΔT0ΔC0C2C1C3C4*LC*SC(a) TemperatureΔTNSi contentΔTSΔT (b) 030050070010 20 30Fig. 2. Binary phase diagrams. (a) Sketch of an idealised equilibrium and metastable eutectic binary phase diagram. (b) Al-rich part of the eutectic binary Al-Si phase diagram[14].L. Bolzoni, N. Hari Babu /Materials Letters 201 (2017) 9–12 11lower temperature (i.e. TE). The maximum solubility point (CsM) isalso virtually shifted to higher concentrations (CsM) and lower tem-perature. On the base of the data of Fig 1b, in the case of shape-casting Al-Si alloys CsM ¼ 1:8  CsM .Considering Fig. 2a, DC0 is the composition range and DT0 is thetemperature range for each solutal content. The differencebetween the solutal content in the solid (CS) and in the liquid(CL) is DC0 and they are used to define the equilibrium partitioncoefficient k of Eq. (1) (i.e. k ¼ CS=CL). The DT0/DC0 ratio then rep-resents the liquidus slope mL of Eq. (1). From Fig. 2a, it can be seenthat two scenarios arises as solutal content increases depending onthe intrinsic nature of the alloy system. On the one side, CL and CSincrease proportionally and thus k remains constant for dilute alloysystems, such as compositions C1 and C2, that are equivalent towrought alloys. Moreover, DC0 and DT0 increase proportionallyand therefore mL is also approximately constant. On the other side,k remains constant but mL continuously decreases, as DC0increases but DT0 decreases with the solutal content, for two-phase alloy systems like compositions C3 and C4 (that correspondto shape-casting alloys). This is relevant because the parametersand theories proposed (i.e. Eqs. (1) and (2)) are based on the inher-ent assumptions that k and mL are independent from the composi-tion, and thus constant, as equilibrium is assumed to bemaintained at the solid-liquid interface during solidification.040080012001600200024000 2 4 6 8 10Grain size,d[µm]Si content [wt.%]Al-XSi (Tp = 700ºC)Al-XSi ( TS = 40ºC)Shape-castingWroughtSolutalcontent(a) Fig. 3. Average grain size of binary Al-xSi alloy. (a) Grain size (d) versus Si content fo(DTS = 40 C). (b) Logarithm of the difference in grain size (Dd) vs 1/DT. The effect of thebecause it is a diffusion-controlled process.4. Consideration about superheatingA survey of the literature about the effect of solutal on the grainsize of cast structure of dilute alloy systems and two-phase alloy sys-tems, that it is commonly studied jointly with the effect of chemicalinoculation, indicates that different alloys are generally cast fromconstant temperature. This is done for the sake of simplicity andcomparison. It is worth mentioning that in the case of Al alloys,chemical inoculation is normally done by means of the additionof Al-Ti-B master alloys. When the same alloy system is inoculatedfor grain refinement, the addition of free Ti is generally low (i.e.0.005–0.02 wt.%) and thus it does not affect significantly the totalsuperheating (DTN) of the alloy (Fig 2b). Nonetheless, when alloyswith different composition are compared, the total solutal amountdetermines DTN. This is especially relevant for scenarios like theone found in binary Al-xSi alloys where the total amount of Si isused to quantify the effect of solutal on the grain size (Fig. 1b)becauseDTN proportionally increases with the increment of Si con-tent (Fig. 2b). To prove the importance of the effect of the totalsuperheating, binary Al-XSi alloys, where X = 4–10 wt.%, were pro-duced by mixing commercially pure Al (purity > 99.7%) with an Al-50Si master alloy. The alloys were melted at 800 C during 2 h priorcasting to check their composition; found to be within ±0.2 wt.%.Pure Al and binary Al-XSi alloys were melted and cast either fromd = 137.15e-988.2(1/ T)R² = 0.944.555.566.577.580.0028 0.003 0.0032 0.0034 0.0036 0.0038Ln d [ µm]1/ T [1/K](b) r alloys cast from constant temperature (Tp = 700) or with constant superheatingsuperheating on solidification can be represented with an Arrhenius-like equation12 L. Bolzoni, N. Hari Babu /Materials Letters 201 (2017) 9–12a constant pouring temperature Tp of 700 ± 3 C or with a constantsuperheating DTS of 40 ± 3 C at a constant cooling rate of 3.5 C/simposed by a water flow (i.e. TP-1 test [16]). Ground and polishedsamples were anodised in a HBF4 solution using a current of10V/1A to reveal the grains. Grain size measurements were doneaccordingly to ASTM E112 (intercept method) [17].The data shown in Fig. 3a confirm the trend found by other sci-entists [11,12] when casting binary Al-XSi alloys from a constanttemperature; the grain size initially decreases and then increasesagain. The decrement of the grain size happens for compositionslower than CsM , and thus for wrought-equivalent composition,whereas the increment is for compositions corresponding toshape-casting alloys. The casting of the binary Al-XSi alloy withconstant superheating leads to a rather constant grain size inde-pendently of the Si content. It is worth noticing that even thoughAl-1Si and Al-2Si alloys where only cast from constant tempera-ture, their DTS (50 C) is comparable to that used to cast alloyswith much higher Si content from a constant superheating. Compa-rable final grain size are then roughly obtained as shown by thelinear relationship in Fig. 3a.Solidification of undercooled melts where equilibrium is kept atthe solid-liquid interface is normally studied using Eq. (3) [18]:kS  T 0S  kL  T 0L ¼ m  Lv ð3Þwhere kS and kL are the thermal conductivities of the solid and liq-uid phase respectively, T 0S and T0L the temperature gradients, m is thegrowth velocity and Lv is the latent heat. Considering the casewhere the left-hand term of Eq. (3), which is related to the thermalgradient, it is supposed to remain constant because imposed by theconstant cooling rate and the thermal conductivity, the grain size isdependent on the pouring temperature. For constant superheating,the amount of heat to be extracted from the molten metal is con-stant, and so it is the mLv product. Conversely, for a constant pour-ing temperature the total amount of heat to be extracted uponsolidification is proportionally higher for higher Si contents as themolten metal acts as a heat reservoir allowing more time for thenucleated grains to growth. Conduction and convection areexpected in this scenario, and a basic thermodynamics analysis ofheat exchange indicates that for both mechanisms the heat trans-ferred per unit time is proportional to DTN. The variation of the dif-ference in grain size between the alloys cast at constanttemperature and those cast from constant superheating (Dd) vs 1/DT has an Arrhenius-like behaviour (Fig. 3b) as solidification is adiffusive process:Dd ¼ A  e EaRDT ð4Þwhere the pre-exponential constant A is equal to 137.15 and theactivation energy Ea is 50.18 kJ/mol for binary Al-Si alloys.5. ConclusionsAn overarching and simple parameter able to fully quantify theeffect of solutal on the grain size, independently of the nature ofthe alloy (i.e. wrought and shape-casting), is not available in thesolidification community. As demonstrated, the growth restrictionfactor Q is valid for dilute alloy systems but cannot properly quan-tify the solutal effect on two-phase alloy systems because theparameter mL tends to zero as the eutectic point is reached. Undernon-equilibrium solidification conditions, the maximum solubilityof a particular solutal (CsM) is shifted to higher concentration (CsM).In the case of cast Al-Si alloys, this is experimentally found to beCsM ¼ 1:8  CsM . Caution must be taken when comparing experi-mental data as the actual superheating, that is intrinsically depen-dent on the total amount of solutal, has also a significant influenceon the final cast structure.AcknowledgmentsThe authors want to acknowledge the financial support fromthe Engineering and Physical Sciences Research Council (EPSRC)through the EP/J013749/1 and EP/K031422/1 Projects and fromthe Technology Strategy Board (TSB) through the TSB/101177Project.References[1] E.L. Rooy, Aluminum and Aluminum Alloys, ASM International, Ohio, USA,1988.[2] B.S. Murty, S.A. Kori, M. Chakraborty, Grain refinement of aluminium andalloys by heterogeneous nucleation and alloying, Int. Mater. Rev. 47 (1) (2002)3–29.[3] M. Easton, D. St. John, An analysis of the relationship between grain size, solutecontent, and the potency and number density of nucleant particles, Metall.Trans. A 36 (7) (2005) 1911–1920.[4] I. Maxwell, A. Hellawell, A simple model for refinement during solidification,Acta Metall. 23 (1975) 229–237.[5] L.A. Tarshis, J.L. Walker, J.W. Rutter, Experiments on the solidification structureof alloy castings, Metall. Trans. 2 (9) (1971) 2589–2597.[6] A.L. Greer, A.M. Bunn, A. Tronche, P.V. Evans, D.J. Bristow, Modelling ofinoculation of metallic melts: application to grain refinement of aluminium byAl-Ti-B, Acta Mater. 48 (11) (2000) 2823–2835.[7] M. Johnsson, Influence of Si and Fe on the grain refinement of aluminium,Zeitschrift für Metallkunde 85 (11) (1994) 781–785.[8] T.E. Quested, A.T. Dinsdale, A.L. Greer, Thermodynamic modelling of growth-restriction effects in aluminium alloys, Acta Mater. 53 (5) (2005) 1323–1334.[9] R. Schmid-Fetzer, A. Kozlov, Thermodynamic aspects of grain growthrestriction in multicomponent alloy solidification, Acta Mater. 59 (15) (2011)6133–6144.[10] M. Johnsson, L. Bäckerud, The influence of composition on equiaxed crystalgrowth mechanisms and grain size in Al alloys, Zeitschrift für Metallkunde 87(3) (1996) 216–220.[11] L. Bäckerud, M. Johnsson, Relative importance of nucleation and growthmechanisms to control grain size in various aluminium alloys, Light Met.(1996) 679–685.[12] J.A. Spittle, Grain refinement in shape casting of aluminium alloys – Part I,Foundry Trade J. (2008) 308–314.[13] J.A. Spittle, J.M. Keeble, A.L. Meshhedani, The grain refinement of Al-Si foundryalloys, Light Met. (1997) 795–800.[14] T.B. Massalski, J.L. Murray, L.H. Bennett, H. Baker, Binary Alloy Phase Diagrams,1st ed., ASM International, OH, USA, 1986.[15] M.C. Flemings, Solidification Processing, McGraw-Hill, NY, USA, 1974.[16] The Aluminum Association, TP-1 – Standard Test Procedure for AluminumAlloy Grain Refiners, Washington DC, USA, 2002.[17] M. Nowak, Development of Niobium-Boron Grain Refiner for Aluminium-Silicon Alloys, Brunel University, U.K., 2011, available at: http://bura.brunel.ac.uk/handle/2438/8321.[18] D.A. Porter, K.E. Easterling, M. Sherif, Phase Transformations in Metals andAlloys, 3rd ed., CRC Press, FL, USA, 2009.

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Considerations on the effect of solutal on the grain size of castings from superheated melts

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