carolyn m. joseph
TRANSCRIPT
Detection of Floating Grains in DC Aluminum Casting
by
Carolyn M. Joseph
B.S. Materials Science and EngineeringMassachusetts Institute of Technology, 2015
Submitted to the Department of Materials Science and Engineeringin partial fulfillment of the requirements for the degree of
Master of Sciencein Materials Science and Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
February 2015 \ kjs 'Lo\~I
0 Carolyn M. Joseph
The author hereby grants to Massachusetts Institute of Technology permission toreproduce and to distribute publicly paper and electronic copies of this thesis document in
whole or in part in any medium now known or hereafter created.
Signature of Author: Signature redactedDepartment of Mterials~cience and Engineering
IA Presented December 8, 2016
Certified by:
Accepted by:
Signature redacted____/T' Jl-- Antoine AllanoreThon . King Assistant Professor of Metallurgy
"IIMI RThesis Supervisor
Signature redacted/ %Donald Sadoway
Chair, Departmental ommittee on Graduate Students
MASSACHOsETmIFNSTUTUTEOF TECHNOLOGY
FEB 2 7 2017
LIBRARIESARCHNIES
Detection of Floating Grains in DC Aluminum Castingby
Carolyn M. Joseph
Submitted to the Department of Materials Science and Engineeringon December 8, 2016
in partial fulfillment of therequirements for the degree of
Master of Science
Abstract
Free-moving "floating" grains have been linked to macrosegregation in direct-chill(DC) aluminum castings. The presence of these grains in the sump of a solidifying ingothas been acknowledged based on measurements of cast microstructures and by recentwork using a turbulent jet to suspend solute-poor grains and minimizemacrosegregation." 2 Experiments in this study were designed to sample grains from themushy region of two ingots, one cast by the standard method and another stirred with aturbulent jet. Measurements of floating grain size, concentration, morphology, andchemical composition are reported. The observations from the standard ingot offer apoint of comparison for floating grain theories and casting models. The measurementsfrom the stirred ingot show how the turbulent jet modifies the distribution, concentrationand morphology of the floating grains.
I
Contents1 Literature Review and Background
1.1 DC Casting1.2 Macrosegregation in DC Casting
1.2.1 Free-moving "Floating" Grains ____1.2.2 Macrosegregation Modeling in DC Casting
1.3 Free-Moving Grains in Stirred Melts_ ___1.4 Coulter Counter_1.5 Project Motivation __
2 Experimental Procedures2.1 Molds for Grain Sampling
2.1.1 Surface Quenching (SQ) Mold ____2.1.2 Deep Quenching (DQ) Mold - --- -
2.2 Coulter Counter Grain Measurements_2.3 DC Casting Apparatus and Procedures ___
2.3.1 Sampling Locations and Times ____2.3.2 Samples Obtained During Standard and Stirred Casts __
2.4 Metallographic Procedures ____2.4.1 Sample Preparation2.4.2 SEM_2.4.3 EDS_2.4.4 EBSD_2 .4 .5 O ptical M icroscopy -------------------------------------
3 Results3.1 Qualitative Analysis of Micrographs3.2 Grain Diameters and Shapes3.3 Grain Compositions3.4 Grain Connectivity3.5 Coulter Counter Measurements_
4 Discussion4.1 Floating Grains in Standard DC Cast Ingot o-4.2 Relation to Computational Casting Models4.3 Floating Grains in Turbulent Jet Stirred Ingot4.4 General Discussion_
5 Conclusion5.1 Suggestions for Future Work __
6 ReferencesAppendix A: Mold Cooling RatesAppendix B: Coulter Counter Theory and SensitivityAppendix C: Coulter Counter Crucible Test
4456912151617171717182021232424242424242525263033353636383840414142454749
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Acknowledgment
The author would like to thank Novelis Inc. for providing the support, expertise, andequipment necessary to complete this work. Bob Wagstaff, Rick Bruski, Sam Wagstaff,and many others at Solatens Technology Center have offered invaluable mentorship,guidance, and generosity. The author would also like to thank Professor Antoine Allanorefor his advice, encouragement, and support, as well as members of the Allanore ResearchGroup who serve as examples of kindness, hard work, and excellence in research.
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1 Literature Review and Background
More than half of the 80 million tons of aluminum cast every year are cast by thedirect-chill (DC) method.3 The process makes ingots for rolling and billets for extrusionand forging. The products produced by DC casting support industries ranging fromaerospace, automotive, and construction to packaging (such as beverage cans) andconsumer products. Macrosegregation is a common casting defect that affectsproductivity and the quality of DC cast products. Understanding the main drivers ofmacrosegregation, particularly the influence of free-floating grains, and development ofcomputational casting models are active fields of research. This review includes anintroduction to the DC casting process, a discussion of macrosegregation and currentcomputational casting models, and a description of techniques from the fields ofrheocasting and inclusion detection that offer methods for visualizing floating grains.
1.1 DC Casting
The DC casting process is semi-continuous and consists of a water-cooled mold witha false bottom (called a starter block), which is positioned above a casting pit that isseveral meters deep. Figure 1 shows a schematic of the casting pit and associatedequipment. Molten metal is transported to the mold via a trough and is most commonlydistributed with a fiberglass distribution bag (combo bag). The mold fills with metal untila certain metal level is reached on the tang of the mold. Then, the starter block movesdownward into the pit. Primary cooling of the metal occurs through contact with themold, which is cooled with water circulating inside. As the block is moving down, thesolidifying ingot extends past the bottom of the mold into the casting pit. Secondarycooling occurs as the ingot moves down into the casting pit and the metal surface issprayed with jets of water. Over time, a steady state solidification interface develops, andthe ingot consists of a liquid sump, a transitional mushy zone, and a solid region. Theprocess continues until the desired casting length is reached.
4
Tundish--MoltenAlloy -
Mold
Water 4Film
StarterBlock
/
//
Solid Alloy
PrimaryCooling
WaterManifold
PrimaryCooling
Mushy Secondary-Zone Cooling
TertiaryCooling
+ Casting DirectionFigure 1: DC casting setup showing relevant equipment and regions of solidification.4
1.2 Macrosegregation in DC Casting
Macrosegregation is a common casting defect in DC cast aluminum. It is defined aspartitioning or segregation of alloying elements on the scale of a casting. Whilemicrosegregation, or partitioning of elements on the scale of a grain, can be corrected byhomogenization, the diffusion distances needed to correct macrosegregation are too largeto be achieved with reasonable downstream processing.'- The mechanical properties of arolled product depend on composition, so macrosegregation can result in non-uniformmechanical properties in the final product.6~ 9
In DC cast aluminum ingots of hypoeutectic alloys, the typical macrosegregationpattern along the centerline is a "W" shape, as shown in Figure 2. There is a buildup ofeutectic forming solute elements such as copper, magnesium, and zinc at the outer edgesof the casting and negative centerline segregation these elements. 3,5,1-12
5
I
Liquiu au"
0.06 -A Magnesium
0.04- -- ----- --- Copper -
Center0.02
0
-0.02
-0.04
-0.06- 12
0 20 40 60 80 100 120 140 160 180 200Distance, mm
Figure 2: Typical macrosegregation profiles across a billet diameter for hypoeutecticaluminum alloys. Composition measurements are from a 192mm diameter billet ofaluminum alloy 2024 (Al-Cu-Mg) grain refined with 0.04% Ti. The profiles shownegative centerline segregation of Cu and Mg.
Macrosegregation has been studied extensively and has several contributing causes.Most generally, it is caused by the relative motion of the liquid and solid phases duringsolidificiation.35,13 Several interacting mechanisms are thought to contribute to thisrelative movement. These may include thermal and solutal convection, shrinkage-inducedflow, sedimentation of free-moving "floating" grains, and deformation due to thermal ormechanical stress.
1.2.1 Free-moving "Floating" Grains
Sedimenting grains were proposed as a cause of macrosegregation based on post-solidification observations of a duplex microstructure in the center of some castings.14
The presence of both coarse-celled (large dendrite arm spacing) and fine-celled (smalldendrite arm spacing) dendritic grains suggests that one of the types of grains formedelsewhere and was transported to the center of the ingot. The sedimenting grain theory isbased on the idea that, at low solid fractions, the solid phase is depleted in solute relativeto the alloy composition. Figure 3 shows solidification of a hypoeutectic alloy (XO) on ageneralized phase diagram.
6
T-7 ----- -------
XT- \ ----- XL
Solid compositionassuming equilibrium
conditions
TE-Average solid
compositionassuming no
diffusion in solid
Solute Composition
Figure 3: Generalized phase diagram showing solidification of alloy XO.' 5 XS is the solidcomposition at a given undercooling under equilibrium conditions. Xs shows the averagecomposition of the solid phase assuming no solute diffusion in the solid and completemixing in the liquid.
Under equilibrium solidification conditions, the composition of the solid phase (Xs) ata given undercooling (T) is depleted in solute relative to the liquid phase (XL). Assumingno diffusion in the solid and complete mixing in the liquid, the average composition ofthe solid phase (Xs) at a given undercooling is more significantly depleted in soluterelative to the liquid phase. 5
If the solute-poor solid grains that form at small undercoolings are free to move in thesump and are denser than the surrounding liquid, they may settle to the center of the ingotand create a region of negative segregation. Electron microprobe measurements on thecoarse- and fine-celled dendritic grains found in the center of castings indicate that thecoarse-celled grains are depleted in solute and could be the free-moving grains suggestedby the theory. 4"6 In addition, the theory would explain why grain refining elements suchas titanium, which exhibit a peritectic reaction with aluminum, show positive centerlinesegregation. Instead of being rejected from the first solid, these elements would be part of
7
the initial solid nuclei and would be transported to the center of the ingot with thesedimenting grains. "
Recent observations suggest that sedimenting grains are the major contributor to theregion of negative segregation at the center of large DC cast aluminum ingots.' 2 ,1 8 Workby Wagstaff and Allanore has shown that stirring with a turbulent jet during casting canresuspend floating grains and minimize the effects of macrosegregation.' In addition, itwas found that the static angle of repose of a coarse-celled dendritic grain matches thesump angle after which negative centerline segregation is observed. 8 A strength of thefloating grain theory over other explanations for macrosegregation is that it can explainboth the negative centerline segregation of eutectic-forming elements and the positivesegregation of peritectic-forming ones.
Existing knowledge on floating grains is conceptual, based on post-solidificationobservations of final cast microstructures.'2, 2 ,14 ,1-18 The size and composition of the
coarse- and fine-celled grains at the center of an ingot have been quantified. 2"6 Line scanmeasurements show that the coarse-celled grains exhibit a uniform plateau of minimumcomposition as compared to the fine-celled grains which show a gradual change incomposition from the center to the grain edges." Figure 4 shows an example of theduplex microstructure as well as line scan measurements across the grains. 12
A B30-
10 to
Distance/ pm Distance/pm
Figure 4: Micrograph and line scans of duplex grain structure observed at the center of a192mm grain refined DC cast billet. 2 (A) Coarse-celled grains and corresponding linescan showing Cu and Mg content. (B) Fine-cell grains and corresponding line scan.
The coarse celled grains found in the center of grain refined billets were found tohave minimum compositions that were 17-25% of the initial composition. The fine cellgrains in these billets had minimum compositions that were 26-56% of the nominal alloycomposition. 6 Despite their differences in dendrite arm spacing, the average diameters of
8
the coarse- and fine-celled grains were not different. It was also observed that the coarse-celled grains seem to be present as clusters. 6
Turbulent jet mixing has been shown to produce ingots with more homogeneouscomposition profiles." In a macrosegregation study by Wagstaff and Allanore, it wasfound that the center of a standard Al-4.5%Cu ingot was depleted by 15% from thefurnace Cu composition.'0 In contrast, an ingot of the same alloy that was stirred with aturbulent jet exhibited minimal depletion in the center region (-5% to +5% deviation fromfurnace composition). 2 Substantial grain refinement was observed with jet processingand it was found that the microstructure became more globular.2 Grain sizes in the centerof the standard ingot ranged from 200-250pm.'0 The jet processed ingot had grains withan average size of 50pim and a maximum size of 150pm. In addition, the measureddendrite arm spacings in the jet processed ingot approached the measured grain sizes,indicating that the globular nature of the grains did not allow for distinguishing betweengrains and dendrite arms.2 A copper quenching mold was used to quench grains from thesurface of the jet processed ingot during casting. Grains captured from the surface wereobserved to be globular and -50pm in size.2
1.2.2 Macrosegregation Modeling in DC Casting
Post-solidification measurements have driven the development of floating grainmacrosegregation theories. Computational DC casting models offer another method fortesting assumptions about grain formation and motion.
Knowledge of floating grain properties is particularly important for casting models,which can combine transport mechanisms in the liquid, solid, and mushy regions of aningot." Composition profiles can be predicted and then compared to experimentalmeasurements on post-solidified castings. Initial models assumed that the solid phasemoved at the casting speed everywhere and thus did not include free-floating grains. 20 Asit became clear that floating grains are a significant contributor to macrosegregation, DCcasting models were modified to include their effect.
Floating grains were first incorporated into a two-dimensional DC casting model byReddy and Beckermann. In this model, the solid phase was assumed to be rigid andmoving at the casting speed at solid volume fractions greater than the packing limit(0.637). At solid fractions lower than the packing limit, the solid was assumed to bespherical particles that could move freely in the surrounding liquid. The model assumedheterogeneous nucleation at a given undercooling and did not consider grainfragmentation, remelting of grains, or grain evolution. Agreement with experimentalmacrosegregation profiles was not consistent, but the model predicted negative centerlinesegregation.
Numerous subsequent macrosegregation modeling studies of DC casting haveincluded floating grains. 9 ,23 -2 7 Vreeman et al. developed a model accounting for free-floating grain transport and fluid flow in the mushy zone, predicted macrosegregation
9
patterns, and compared the model to industrial scale ingots.- 2 Figure 5A shows theingot sump profile predicted by the model for a grain refined ingot during steady statecasting. It includes lines of constant solid fraction, streamlines, and shading to representthe total solute concentration in the solid and liquid phases. Figure 5B shows themacrosegregation profile predicted by the model and the corresponding experimentallymeasured composition profiles.
Predicted Sump Conditions0.00 Streamlines
/u' 0Solid Fraction
00" Macrosegregation Profiles
0
0.15 f =0.36.0 -
00
0.205.
0.25 56 f M Experiments
Predictions
0.30- 5.4
.0 0.05 0.10 0.15 0.20 0.00 0.05 0.10 0.15 0.20
A Position/r BSolute Concentration / wt% Cu
5.7 5.9 6.1 6.3 6.5
Figure 5: Results from macrosegregation model of a grain refined ingot that accounts forfloating grain motion. (A) Predicted sump conditions during steady state casting. (B)Predicted and experimentally measured macrosegregation profiles.
The model includes a mushy region composed of liquid and free-floating dendrites atsolid fractions less than the packing fraction (0.3). At larger solid fractions, a rigid solidphase moves at the casting speed. The values for grain diameter and packing fractionwere assumed to be constant. The model was found to be highly dependent on these twokey parameters, as the grain diameter determines the velocity difference between theliquid and solid phases, and packing fraction separates the region of free-floatingdendrites from the rigid solid structure. The grain diameter and packing fraction areunknown quantities, however, and are difficult to predict since grain coalescencemechanisms and grain growth with convection are not well understood and are heavily
10
dependent on grain morphology. The grain size is likely a distribution of diameters, andthe packing fraction is not necessarily constant in the ingot.
This model was compared to experimental macrosegregation profiles from casts withand without grain refiner." Since floating grain formation was assumed to occur throughheterogeneous nucleation on grain refiner, the model's grain refined case had floatinggrains while the non-grain refined case did not. The model underpredicted the magnitudeof centerline segregation in the grain refined cast (Figure 5B) and did not predict thenegative centerline segregation that was observed in the non-grain refined cast. Whilemany model parameters may affect the final composition profile, the results suggest thatgrains may form by fragmentation in addition to heterogeneous nucleation on grainrefiner.
In order to rigorously determine the dendrite diameter distribution, volume fraction,and path of motion in an ingot, models must include a nucleation or fragmentation modelas well as a transport equation that accounts for grain growth. Multiscale and multiphasemodeling studies of equiaxed solidification have worked to develop macroscopictransport equations and connect them to microscopic relations for nucleation, crystalfragmentation, and growth.2S32
One example of a DC casting model that couples grain nucleation and growth withmacroscopic transport equations is presented by Zaloznik et al. This model assumes aporous solid phase at volume fractions greater than the packing limit (0.3), and free-floating spherical grains at smaller solid volume fractions. Grain refiner particles weremodeled to move at the liquid velocity, and nucleation was assumed to occur at a criticalundercooling, dependent on grain refiner particle diameter. In this model, grain motionduring casting could be tracked. Figure 6 displays the predicted sump conditions,including lines of constant solid fraction, streamlines of solid motion, and shading torepresent the floating grain concentration.
11
Predicted Sump Conditions0.0 -- -
Solid Fraction
Grain f 0.0Concentration/ m- -
> 2Solid Streamlines
1.15x 10"1.10x10"1.05X 1012a1.00xIO"
0.2
0.15 0.10 0.05 0Position / m
Figure 6: Predicted sump conditions for a grain refined cast showing grain concentration(number per m3 ), solid streamlines, and lines of constant solid fraction.
The model predicts that grains may circulate in three different thermal regions. If theyremain close to the inclined solidification front, solidification would be fast. If they enterthe core of the mushy region with minimal undercooling, grains would experience slowgrowth before settling to the bottom. Closer to the liquidus front, remelting would occur.This led to the conclusion that some of the grains found at the center of the ingot are fromthe fast-growing zone close to the packing front, while others settled from the slow-growing zone.
The current method for validating casting models is by comparison to post-solidification measurements. If there are discrepancies between the predicted andmeasured macrosegregation profiles, it is difficult to know which assumptions about thesolidifying ingot must be modified. Measurements of floating grains in the mushy regionduring solidification can serve as an intermediate point of comparison for modeldevelopment and validation.
1.3 Free-Moving Grains in Stirred Melts
Jet processing of DC cast aluminum shares many similarities with semi-solid metalprocessing techniques such as rheocasting and thixocasting. Rheocasting is a metalforming process by which metal is partially solidified under vigorous agitation before it iscast into its final shape. The semi-solid mixture is composed of solid spheroids, whoseproperties depend on the shear rate and cooling rate.3 In both rheocasting and jetprocessed DC casting, solidification occurs under cooling with vigorous convection and
12
the resulting microstructures are equiaxed.' 4 Because of these similarities, therheocasting literature offers valuable insight into the theory of grain formation andevolution during stirring as well as useful techniques for characterizing free-movingsolids during solidification.
Several methods have been used to characterize stirred melts during solidificationincluding viscosity measurements and quenched samples. A common sampling method inrheocasting is rapid quenching of the melt with a copper mold to preserve themicrostructure. This method was developed to study chill zone formation.36 A similarmold was subsequently used to quench semi-solid melts and cool the liquid phase quicklyenough that it would be distinguishable from the existing solid.' The study found thatsamples obtained by the mold showed a fine, equiaxed microstructure combined withlarger grains, which were either dendritic or globular depending on the solidificationconditions, and are often found in clusters. The larger grains were presumed to be solid atthe time of quenching, while the fine grains were part of the liquid phase. Whenexamined with an optical microscope, the large grains showed a darker region aroundtheir outer edge, adding -40pm to the total diameter. This area correlated to a region ofhigh Cu concentration when a line scan was done across the grain with SEM-EDS. Thisregion was assumed to be growth of the grain during quenching with the copper mold.
The Cu mold quenching method has been used in several rheocasting studies.3Quantitative metallography is difficult on quenched samples of semi-solid melts and onfinal rheocast microstructures.4 ' This is because of the complex structure of the particles,which can be dendritic, "cramped" dendrites, or agglomerates of individual particles. 4
It is difficult to determine particle connectivity from two-dimensional cross-sectionalmeasurements since particles interconnected as dendrites or as agglomerates ofindependent particles may appear to be separate when cut at certain orientations. Serialpolishing is one way to determine the connectivity. Several serial polishing studies ofquenched grains have revealed that seemingly separate grains are in factinterconnected.' 4 3 There is still discussion as to whether the particles are connected aslarge dendritic grains or as agglomerates of individual particles. Another metallographictechnique that can assist in understanding the connectivity of grains is electronUakAttN Le1 UeLLtIon (EBSD) U1 eCLLhi1g Le111qUCs LldL lreveal the crystaliUgrapmicorientation of the particles. 4' These methods can help distinguish between grains that areindependent particles that have agglomerated into a single mass, which would exhibitdifferent crystallographic orientations and large dendritic masses, which would show thesame orientation.
In semi-solid metal processing, spherical grains form under conditions of vigorousconvection and slow cooling. Leading theories suggest that that grains are initiallyequiaxed dendrites, which then ripen and evolve to form "rosettes" and eventuallyspheres.34 Figure 7 is a general schematic of grain structural evolution during rheocasting.
13
'~' ~ 4-Initial dendrite
B C Dendritic growth
Increasing shear rate
C Roet Increasing time+-- Rosette Decreasing cooling rate
D4- Ripened rosette
Entrapped eutectic
Spherical grain
Figure 7: Grain structural evolution during solidification with forced convection.34 Theextent of evolution depends on the shear rate, the amount of solidification, and thecooling rate. (A) Initial dendrite fragment. (B) Dendritic growth. (C) Rosette. (D)Ripened rosette. (E) Sphere.
Spherical grains often contain entrapped eutectic, which is evidence of theirpreviously dendritic structure. In one study, spherical grains were found at very lowfractions solid (less than 0.05) after only 5 seconds of solidification. No entrappedeutectic was observed in these grains 3 7 This indicates that in some cases, spherical grainsform directly and dendritic growth can be avoided.
The primary and secondary dendrite arm spacings of solidified structures are relatedto local solidification time. Studies of semi-solid melts indicate that the particle diameterdepends strongly on cooling rate, and the particle diameter for a fixed cooling rate isapproximately equal to what the primary dendrite arm spacing would be in an unstirredmelt of the same cooling rate. Figure 8 shows experimental data for the of particle sizeat varying cooling rates for A14.5%Cu. Points labeled "Final diameter" are graindiameters measured on samples quenched from the semi-solid melt. "Initial diameter"subtracts out the expected grain growth during quenching. The remaining points are thefinal diameters from various studies obtained after isothermal ripening of small grains.
14
The upper and lower bound lines are based on final dendrite arm spacing vs solidificationtime measurements for various Al-Cu solidification studies.
1000
100
:1Dendrite arm spacing 0
2 Upper bound
. 10
0Final diameterDendrite arm spacing 0 Initial diameter
Lower bound 0 Diameter after ripeningA Annavarapu and Doherty (ripening)+ Manson-Whitton (ripening)
0.1 1 10 100 1000 10000Local Solidification Time / s
Figure 8: Grain diameter in semi-solid melt and solidification time.
1.4 Coulter Counter
A coulter counter device is commonly used in aluminum casting for detection ofnonmetallic inclusions (particles of oxide, flux, or refractory). The principles are basedon the electrical sensing zone method of aqueous coulter-counters, which can measurethe concentration and size of particles in a conducting liquid. In aluminum casting, themost widely used coulter counter device is the LiMCATM , which was developed in the
1980s by Doutre and Guthrie." It is inserted into the trough either before or after metalfiltration, and usually measures particle concentrations in units of thousand inclusions(greater than a given size) per kilogram. Size distributions measured by the LiMCA andsimilar coulter counter devices are used to monitor metal cleanliness and filterefficiencies.
In principle, the coulter counter device can also be used to detect conductive particlesin molten metal. There are several examples in the literature of coulter counters beingused to measure conductive particles. During the development of the LiMCATM , thisprinciple was tested by use of the device to measure particles of copper in liquid
15
gallium.4 5 The device has also been used to measure conductive grain refiner particles,particularly in studying grain refiner fading.47 There are also examples of a coultercounter device being used to measure the size and concentration of aluminum grains in asolidifying crucible.4 8'49 In these studies, a crucible of metal was allowed to cool belowthe liquidus and a LiMCATM was used to measure particle sizes and concentrations untilthe orifice became blocked with solid metal. Both the particle concentration and averageparticle size increased as a function of decreasing temperature. Rapidly solidified samplestaken at various temperatures showed the presence of particles, and an increase in particleconcentration and size with decreasing temperatures. The coulter counter measurementswere not validated quantitatively with the metallographic samples.
1.5 Project Motivation
The floating grain macrosegregation theory has progressed significantly in recentyears. Evidence for the presence of grains is based on macrosegregation profiles,modifications by jet processing, and analysis of critical sump angles. Computationalmodels that include floating grains can predict the negative centerline segregationobserved experimentally in grain refined ingots in two-dimensions. Casting models areevaluated by comparison to post-solidification measurements, which offer information onthe final morphology and composition of grains. There has not, however, been directobservation of the grains in standard DC casts, nor is there information on how themorphology and composition of grains evolves during casting. Measurement of floatinggrain properties can help advance computational macrosegregation models and can offeran intermediate point of comparison for model validation. Techniques from the field ofrheocasting as well as technology currently used for inclusion detection may be useful incharacterizing the grains.
We hypothesize that direct observation of these floating grains will be possible withmolds designed to rapidly quench metal from the sump of DC cast ingots. This projectseeks to sample floating grains from ingots during casting in order to quantify theirproperties. These quantitative measurements can help assess the validity of our currentunderstanding of floating grains in the sump of DC cast ingots.
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2 Experimental Procedures
Two sampling techniques were chosen to probe grains from the sump of DC castaluminum ingots during casting. Copper quenching molds, similar to those developed forrheocasting studies, were used to rapidly solidify samples from the molten metal sump. Amolten metal coulter counter, developed for measurement of inclusions, was used todetect grains in the molten sump during casting.
2.1 Molds for Grain Sampling
2.1.1 Surface Quenching (SQ) Mold
Copper (Cu) quenching molds were used to rapidly cool the molten alloy sampledduring casting. The surface quenching (SQ) mold was based on the design of Martinezand Flemings. Figure 9A shows the surface quenching mold dimensions. The mold wasmade of two 13mm thick Cu plates separated by 1mm graphite spacers, which created anopening with dimensions 127mm x 25mm x 1mm. The bottom of the mold was sealedwith plastic wrap and the top was connected to vacuum. Samples were obtained bytouching the bottom of the mold to the liquid metal surface. The plastic wrap melted anda thin sample of metal was pulled into the mold's opening. The sample was removed withpliers and the plastic wrap was replaced so that the mold could be used again.
2.1.2 Deep Quenching (DQ) Mold
A second Cu mold was designed to obtain samples from up to 0.6m deep in the sumpduring casting. Figure 9B shows the deep quenching mold dimensions and geometry. Itconsisted two 19mm thick Cu plates spaced Imm apart by a cut made into one of theplate halves. The opening created was 121mm x 19mm x 1mm. The Cu plates wereinserted into two 51mm diameter graphite rings with rope seals. The mold and rings werefit snugly into the bottom of a 91.4cm long silica matrix composite tube (76mm OD,64mm ID). A refractory cap with a rope seal was inserted into the tube below the moldand a refractory plug attached to a wire was inserted into an opening at the bottom of thecap i s51ual tIh mu. I U Lax a saimpie, L1h MIUM was 1o wereU InL Me metal to the desired
depth. The plug was removed to allow metal to pass into the mold's opening. The totalsampling process took fewer than 5 seconds.
17
tTo
Vac
I
Graphite .Spacers
E
13m
~IMoldOpening
1mm
FJiL
25mm
Figure 9: Schematic of molds used for grainmold. (B) Deep quenching (DQ) mold.
+-*-Thermocouple
Al-SiTube
64mm
Mol Mo
RefractoryCap
Refractory
B ,---pug
sampling. (A) Surface quenching (SQ)
Appendix A includes mold temperature profiles during sampling, which weremeasured with K-type thermocouples inserted into the end of the Cu mold, as shown inFigure 9B. Appendix A also shows calculations for cooling rates, which were estimatedbased on measurements of secondary dendrite arm spacing in the quenched samples. Theaverage cooling rate for the deep quenching mold starting at room temperature was~300*C/s. The average cooling rate for the surface quenching mold starting at roomtemperature was ~500*C/s. Both of these cooling rates created a microstructure of finecrystals.
2.2 Coulter Counter Grain Measurements
Figure 10 shows a diagram of a molten metal coulter counter. It consists of twotungsten electrodes, one of which is encased in a glass tube with an orifice. For thisstudy, a tube with a standard orifice was used (located 15mm from the bottom of the tubeand 3 0 0 pm in diameter). The device was lowered into the molten metal (70mm below thesurface), and a constant current was maintained between the electrodes (60-65Amps).
18
CuPlates
GraphiteSpacers
A
I
I RCoulter-Counter
Apparatus
Argon
AVac Tif
I 41ii
The
Figure 10: Diagram of coulter counter used in molten metal.
The coulter counter uses a vacuum to draw metal into the glass tube and monitors thevoltage between the two electrodes as metal flows in. When a particle passes through theorifice, changes in voltage are proportional to the volume of the particle passing through.Appendix B explains how the measured voltage is related to particle volume and why thesign of the voltage change depends on particle conductivity. Particles that are lessconductive than the surrounding liquid (inclusions) will cause positive voltage changes,while particles that are more conductive than the liquid will cause negative change.Appendix B also discusses how the voltage response is sensitive to orifice diameter andshape.
The number of voltage changes (peaks) that occur when a fixed volume of fluid ispulled into the tube (3mL) can be used to determine the concentration. Metal is expelledwith argon when it reaches a certain fill level, which is monitored by a thermocoupleinside the glass tube.
Generally, the resistivity value used in coulter counter inclusion measurements is thatof pure liquid aluminum (2.5x10-7Qm), and in literature examples of grain detection, theresistivity of pure solid aluminum at 700*C is used.48'4 9 Appendix B discusses thesensitivity of particle size measurements to the liquid resistivity values used in the sizecalculation. It shows that variation in measured particle diameter based on the liquidresistivity value (A14.5%Cu compared to pure Al) can range from 5-40prm depending onparticle diameter.
19
-mocoupl
ElectrodesGlassTube
OrfI LiquidMetal
The coulter counter's ability to detect conductive aluminum grains was tested bysampling continuously during solidification of a crucible of metal. Appendix C describesthe test procedures and presents the resulting particle measurements. The coulter counterwas able to detect solid particles, which increased in number as the metal was cooledbelow the liquidus. Samples from the surface quenching mold confirm the presence offloating solid particles in the liquid.
2.3 DC Casting Apparatus and Procedures
Two ingots of A14.5%Cu were cast using a commercial scale DC casting apparatus.The first ingot was cast using a standard distribution method (hereafter called standard),while the second accelerated the metal with a turbulent jet as used by Wagstaff andAllanore to introduce stirring in the sump of the ingot (hereafter called stirred).' Figure11 shows the Al-Cu phase diagram, and Table 1 shows relevant properties of this alloy.
Q0
700
600
500
400
300
200
1000 4.5wt
CU0.1 0.2 0.3
Weight Fraction Cu0.4 0.5
Figure 11: Al-Cu phase diagram.marked on the diagram.
The alloy cast in these experiments (A14.5%Cu) is
560.5
Al 548.2CA,- -2
20
Table 1: Relevant Al-Cu system properties.4 21
Property Unit ValueLiquid Density (A14.5%Cu) kg m-3 2460Solid Density (A14.5%Cu) kg m 3 2750
Solid Density (Al) kg m- 2700Melting Point Pure Al 0C 660.5
Al-Cu Eutectic Temperature 0C 548.2Al-Cu Eutectic Composition wt% Cu 33
Liquid Diffusivity (A14.5%Cu) m 2 s1 5x10-9
Viscosity (A14.5%Cu) kg m-' s-1 0.0013Partition Coefficient - 0.171
An A14.5%Cu alloy was melted in a natural gas fired furnace. From the furnace, themetal moved through alumina-silica troughs and 1 Oppm of boron grain refiner was addedin the form of 3:1 TiB rod. Hydrogen was removed with a run-through degassingmachine that pumped chlorine and argon through spinning rotors in the metal. Inclusionsand impurities were removed with a 30ppi ceramic foam filter.
During the standard cast, the metal moved through a ceramic spout with a pin thatallowed for control of metal flow and casting speed. The metal was distributed through astandard fiberglass combo bag into an open-top 1750x600mm WAGSTAFF LHCTMmold. The casting temperature was maintained at 690'C (35'C superheat). The castingspeed ramped to a steady state speed of 60mm/min, and the metal level during steadystate casting was maintained at 60mm above the tang of the mold.
During the stirred cast, the metal was accelerated downward into the mold through aspout with a 2cm opening. No pin was used for metal flow control, and no combo bagwas used for metal distribution. The casting temperature was 690'C, the speed was60mm/min, and the metal level was maintained at 60mm.
2.3.1 Sampling Locations and Times
Figures 12A and 12B show the sampling locations for the coulter counter and the twoquenching molds. The coulter counter was mounted above the head of the ingot 38cmfrom the short face. The surface quenching mold samples were obtained next to thecoulter counter tube insertion point. The deep quenching mold was lowered into thebottom center of the sump diagonally from a position next to the spout.
21
A
By
Figure 12: Sampling locations for coulter counter and quenching molds in DC cast ingot.(A) Schematic of ingot sump and sampling locations. (B) z-axis view of head of ingotwith marked sampling locations.
When the steady state sump was developed, the sump depth was measured from thedeep quench sampling location. For each sample, the mold was lowered in the samelocation to a depth 5cm above the measured sump depth. Four deep quenched samples
22
Surface CoulterQuench Counter Deep
Quench 1
1750
x Rolling Face (1.750mm)
0
Coulter Counter
380mm) Deep Quench Mold
Surface Quench Mold
x
were obtained from the sump of the standard ingot at 870, 1230, 1560, and 1945mm incast length. Five samples were obtained from the surface of the standard cast at 915,1318, 1520, 1929, and 2553mm in length. The cast lengths for the surface quenchingmold correspond to samples obtained by the coulter counter.
The sampling depth for the deep quenching mold in the stirred cast was determinedby the same method as the standard cast. Six deep quenched samples were obtained fromthe sump of the stirred ingot at 915, 1090, 1250, 1510, 1769, and 1973mm in length, andthree samples were obtained from the surface at 1670,2190, and 2330mm in length.
The coulter counter was inserted into the ingot during both the stirred and standardcasts. The device was preheated over the surface of the metal during cast startup, wasinserted at 500mm in cast length to 70mm below the surface, and drew in its first sampleat 800mm. The coulter counter sampled continuously until the end of the standard cast.During the stirred cast, the orifice became blocked before sampling could occur.
2.3.2 Samples Obtained During Standard and Stirred Casts
Figure 13 shows an example of a quenched sample and a representative image ofsample microstructure. The microstructure of the samples obtained with the quenchingmolds during DC casts exhibited large grains (presumably those present in the mushyregion) that were surrounded by fine crystals (formed during quenching).
Figure 13: (A) Sample obtained with DQ mold. (B) Representative SEM image ofquenched microstructure.
23
2.4 Metallographic Techniques
2.4.1 Sample Preparation
Thin samples from quenching molds were mounted in KonductoMet phenolicmounting compound, ground, and polished to a mirror finish (ending with 0.05pmcolloidal silica). A14.5%Cu samples were etched for optical microscopy with Keller'sreagent (2.5ml HNO 3, 1.5mL HCl, 1.0mL HF, 95mL H20) for 30 seconds. A coloretchant, Wecks reagent (4g KMnO 4, 1 g NaOH, IOOmL H20) was used on severalsamples to highlight variation in composition (microsegregation) around the grains.4
2.4.2 SEM
A Hitachi SU1510 scanning electron microscope was used for imaging grains,making energy dispersive x-ray spectroscopy measurements, and gathering electronbackscatter diffraction data. A 20kV accelerating voltage was used throughout this study.
2.4.3 EDS
Energy dispersive x-ray spectroscopy measurements were made with an Oxfordinstruments x-act EDS detector SEM attachment and analyzed with AZtec® analysissoftware.
2.4.4 EBSD
Samples were analyzed with an Oxford Nordlys electron backscatter diffractiondetector and AZtec software.
2.4.5 Optical Microscopy
Optical micrographs were obtained with a Nikon Eclipse LV 150 microscope with al0x magnification eyepiece and lOx-100x objective lenses. The microscope was used inboth the bright and dark field settings. A rear polarizer and tinted lens were used tohighlight the Cu-rich regions on samples etched with Weck's reagent. The Cu-richregions were highlighted in the green spectrum, and the Al-rich regions were black. ANikon Digital Sight D5-2Mv camera and Image Pro 6.3 software were used to obtainimages. Optical micrographs were analyzed with open source ImageJ software.
24
- - - - - I
3 Results
Floating grains were sampled with the copper quenching molds from both thestandard and stirred ingots. The metallographic samples obtained with these molds wereanalyzed with optical and scanning electron microscopy to determine the floating grainsizes and shapes, and the resulting distributions are presented. Compositionmeasurements made with energy dispersive x-ray spectroscopy are reported. Particlemeasurements made with the molten metal coulter counter are also presented.
3.1 Qualitative Analysis of Micrographs
Figure 14 shows representative SEM images of the samples obtained with the copperquenching molds. No large grains were present in the samples quenched from the surfaceof the standard DC cast (Figure 14A). Floating grains were present in the samplesobtained with the deep quenching mold from the standard ingot (Figure 14B) as well asin the surface and deep quenched samples from the stirred ingot (Figures 14C and 14D).The standard cast contained both globular grains and equiaxed dendrites, as labeled inFigure 14B, while grains from the stirred cast were generally more globular in shape.
Figure 14: Representative SEM images of quenched samples. (A) Standard surfacequench. (B) Standard deep quench with examples of equiaxed "dendritic" and "globular"morphologies. (C) Stirred surface quench. (D) Stirred deep quench. Floating grains arevisible in all samples except those from the surface of the standard cast.
25
3.2 Grain Diameters and Shapes
Dark field optical micrographs at 100x magnification of two-dimensional polishedand etched planes were used to make grain size and shape measurements. Figure 15Ashows an example unprocessed micrograph, and Figure 15B shows the outlined grainsafter the image analysis software was used to identify floating grains, separate grains thattouch, and record the area and perimeter of each grain. The grain measurements weremade based on observable grain boundaries, so grains that appeared to be weldedtogether were counted separately.
00~I B
Figure 15: Example of grains identified and outlined with image analysis software. Theresulting outlines were used to obtain the area and perimeter of each grain. (A) Dark fieldoptical micrograph of floating grains obtained with the deep quenching mold from thestirred ingot. (B) Corresponding outlines of grains identified with image analysissoftware.
Grains that had been improperly identified by the software were removed from thedata set. The total numbers of grains measured were 303, 406, and 209 for the standarddeep quench, the stirred deep quench, and the stirred surface quench, respectively.
The measured grain areas were converted to equivalent circular diameters. Figure 16shows the grain size distribution histograms for floating grains sampled from the standardand stirred casts.
26
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
Standard Deep Quench A
10 30 50 70 90 110 130 150 170 190Particle Diameter /pm
Stirred Deep Quench
1 30 50 70 90 110 130 150 170 190Particle Diameter /pm
Stirred Surface Quench
10 30 50 70 90 110 130 150 170 190Particle Diameter /pm
Figure 16: Grain size distributions. (A) Standard deep quench. (B) Stirred deep quench.(C) Stirred surface quench.
27
1111111212 1 1 a11
Floating grain diameters range from 15-130pm in both the standard and stirred casts.In the stirred cast, the grain size distributions are shifted toward larger diameters forgrains sampled with both the deep and surface quenching molds. The average graindiameters in the stirred cast were 47ym and 52plm for the deep and surface quenchingmolds, respectively. The grain size distribution from the standard cast is shifted towardssmaller diameters relative to the stirred cast. The average measured grain size in thestandard cast was 39pm.
The quenched samples were also used to estimate the fraction of solid grains in themeasured locations. Area fractions were determined by adding the grain areas for a givenmicrograph and dividing by the total micrograph area. A 20mm 2 region was analyzed oneach sample. The grain area fraction in samples obtained with the deep quenching moldin the standard cast was found to be 0.06. The grain area fractions for the samples fromthe stirred cast were found to be 0.12 and 0.09 for the deep quenching mold and surfacequenching mold, respectively. These area fractions serve as estimates for the volumefraction at each location.
The shape of the grains was analyzed using the two-dimensional form factor proposedby Saltykov.'" This parameter compares the perimeter of a circle of equal area as thegrain to the measured grain perimeter. The parameter ranges from 0 to 1, with 1 being acircle. Figure 17 shows the grain shape distribution histograms for each cast.
28
0.350.3 Standard Deep Quench
0.25
0.2
0.15
0.1
0.05
0 0.4 0.8Shape Factor
0.35
0 Stirred Deep Quench B
0.25
0.2
0.15
0.1
0.05
00 0.4 0.8
Shape Factor
0.350 Stirred Surface Quench
S0.25
0.2
0.150.1
0.05
00 0.4 0.8
Shape Factor
Figure 17: Grain shape factor distributions. (A) Standard deep quench. (B) Stirred deepquench. (C) Stirred surface quench.
29
The grain shape distributions do not vary significantly for grains sampled with thedeep and surface quenching molds from the stirred cast (Figures 17B and 17C). Theaverage form factors were 0.74 and 0.73 for the deep and surface quenching molds,respectively. The grain shape distribution for the standard cast is shifted towards lowerform factors (less circular grains) relative to the stirred cast (Figure 17A), with theaverage form factor being 0.67.
Table 2 summarizes the grain size, shape, and area fraction results. It shows theaverage grain diameters and grain area fractions as well as the form factors for the entiregrain populations and the subset of grains with diameters greater than 40pm. Grains withdiameters greater than 40pm made up 35% of the grains captured with the deepquenching mold in the standard cast, 56% of the grains captured with the deep quenchingmold in the stirred cast, and 67% of the grains captured with the surface quenching moldin the stirred cast.
Table 2: Floating grain diameters, form factors, and grain area fraction. Reported errorrepresents standard deviation from the mean.
Average Grain Grain Saltykov 2D Saltykov 2DCast Diameter Area Form Factor Form Factor
(Pm) Fraction (all grains) (grain diameter >40jm)
Standard Cast 39 22 0.06 0.02 0.67 0.18 0.50 0.13Deep QuenchStirred CastDed~en Cast47 22 0.12 0.04 0.74 0.13 0.70 0.14
Deep QuenchIStirred Cast 52 19 0.09 0.04 0.73 0.13 0.70 0.13
Surface Quench
As shown by the form factors for grains with diameters greater than 40ptm, largegrains are generally less spherical than small grains, particularly in the standard cast. Theaverage form factor for grains from the stirred cast decreased to 0.70 for both samplinglocations in the stirred cast, and to 0.50 for grains in the standard cast.
3.3 Grain Compositions
Line scan measurements of copper composition were made across several grainsobtained from each cast. Figure 18 shows an example line scan for each cast type and animage of the corresponding grain.
30
-. ~-=~-~==.~- - --- ~---=~------~ -.-- I
0
9
8
7
6
.54
3
0
10
9
8
7
6
5
4
3
0
Standard Deep Quench A
0
50 -40 -30 -20 -10 0 10 20 30 40 5(Position /pm
Stirred Deep Quench B
50 -40 -30 -20 -10 0 10 20 30 40 5(Position / pm
-40 -30 -20 -10 0Position / pm
10 20 30
Figure 18: Line scans showing copper composition as it varies with position in grain. (A)Standard deep quench. (B) Stirred deep quench. (C) Stirred surface quench.
The line scans show that, in both the standard and stirred casts, the floating grains areuniformly depleted in solute. The solute content does not increase gradually movingoutward along the radius, and there is an abrupt buildup of solute in the 3-5pm near thegrain edges. The line scans have similar profiles regardless of whether the grains areglobular (Figures 18A and 18C) or equiaxed dendrites (Figure 18B).
The line scans were used to determine the average and minimum grain compositionsfor each cast. The average solute concentration includes the buildup of solute observed atthe edges of each line scan, while the minimum solute concentration averages only theminimum plateau points. The results are reported in Table 3.
31
9
8
7
6
.54
3
2
0 --L-50
Stirred Surface Quench Ce
40 50
0
0
Table 3: Average and minimum solute compositions in the A14.5%Cu standard andstirred ingots. Reported error is the standard deviation from the mean.
Average MinimumCast Composition Composition
(wt% Cu) (wt% Cu)Standard Cast 1.40 0.20 1.16 0.09Deep QuenchStirred CastDtree Que h 1.40 0.37 1.04 0.12Deep QuenchStirred Cast 1.21 0.13 1.09 0.05
Surface Quench
The minimum grain composition in the standard cast ranged from 1.07-1.25wt%Cu,which is a 72-76% depletion from the alloy composition (4.5wt%Cu). The floating grainsfrom the stirred cast do not vary in minimum composition from the grains in the standardcast. The minimum compositions were 0.92-1.16wt%Cu and 1.04-1.14wt%Cu for thegrains obtained with the deep and surface quenching molds, respectively, which is a 74-80% depletion from the alloy composition.
Copper microsegregation in the grain can be observed qualitatively in samples etchedwith Weck's reagent. Figure 19A and 19B show color etched grains from a samplequenched from the surface of the stirred cast, and Figure 19C shows color etched grainssampled with the deep quenching mold from the standard cast. A green ring ~3-5pmthick is visible around the edge of the large grains, which is consistent with the region ofcopper buildup in the line scans. The thickness or intensity of the region of copperbuildup does not vary between samples from the stirred or standard cast.
32
-A--t
Figure 19: Micrographs of samples etched with Weck's reagent and viewed underpolarized light. (A, B) Stirred surface quench. (C) Standard deep quench.
3.4 Grain Connectivity
The floating grains obtained from both the standard and stirred casts are found inclusters in the metallographic samples. The scale of agglomerates measured on two-dimensional cross sections was -300pm for the standard cast and -500pm for the stirredcast.
The samples were analyzed using electron backscatter diffraction (EBSD) todifferentiate between agglomerated particles and single dendrites based on theircrystallographic orientations. In principle, grains that are connected because they aredifferent arms of the same dendrite will have the same crystallographic orientation (andthus the same color in the EBSD map), while individual grains that agglomerated orwelded together will not likely have the same crystallographic orientation. Figure 20shows several EBSD maps on the floating grains quenched from DC cast ingots andcorresponding optical micrographs taken at the same location.
33
;
S 100 p
Figure 20: EBSD maps of grain clusters. (A) Standard deep quench. (B) Stirred deepquench. (C) Stirred surface quench.
The EBSD maps show that in some cases, adjacent grains have the same orientation,but it is rare to find more than two or three grains with the same orientation in a givencluster.
34
3.5 Coulter Counter Measurements
The coulter counter was inserted into the surface of both the standard and stirredcasts. During the stirred cast, no samples were obtained with the coulter counter becausethe orifice became blocked with solid shortly after insertion of the glass tube. The coultercounter detected particles at the surface of the standard cast. Peak voltage values rangedfrom 16.7-24.9pV over the background. Assuming solid pure aluminum particles in pureliquid aluminum, these peaks correspond to particles ranging in size from 22-25p.m. Themetallographic samples obtained with the surface quenching mold during the standardcast did not show the presence of grains ~25pm in size distinguishable from the finecrystals in the quenched microstructure (Figure 14A). If solid aluminum is nucleating onTiB2 particles, the total resistivity of the particle would increase (PTiB2-9.OX1O 8 m).47
TiB2 particles range from 0.2-6pm, with most particles being less than 2pm in size.3
Assuming that TiB2 grain refiner particles are 2pm and that the resistivity increasesaccording to the volume fraction TiB2, the total particle diameters would range from 12-13m. Figure 21 shows the average particle size (assuming nucleation on 2pm diameterTiB2 particles) and particle number as a function of cast length.
14
12 - **0e@@@e100
S10 '80
8 -60
6-
40
* Particle Diameter2 -2
*Number of Particles
0 V110500 1000 1500 2000 2500 3000
Cast Length / cm
Figure 21: Diameter and number of particles measured by coulter counter in standardcast. Particle diameters were calculated assuming that pure aluminum nucleates on TiB2grain refiner, and particle number reports the total number of particle per samplemeasured by the coulter counter.
35
4 Discussion
4.1 Floating Grains in Standard DC Cast Ingot
Floating grains were observed in samples obtained with the deep quenching moldfrom the sump of the standard ingot (Figure 14B). This confirms that free-floating grainsexist in the mushy region, and allows for direct measurements of their size, morphology,fraction, and composition at this particular location.
There were no large grains visible in the samples obtained with the surface quenchingmold (Figure 14A). Insertion of a thermocouple at the surface sampling location (380mmfrom the short face as shown in Figure 12B) indicates that the melt temperature is abovethe liquidus (660-675'C). This is in agreement with ingot temperature measurementsfrom the literature, which report that the melt temperature at this location would not dropbelow the liquidus until 575-600mm below the metal surface.4 The lack of sampledgrains also indicates that there are no convective flows bringing grains to the surface, aswas observed in the stirred ingot. These observations validate the expected grainconcentration of 0 at this location in casting models.
Despite no detection of large floating grains with the surface quenching mold, thecoulter counter measured low concentrations of conductive particles when samplingmetal from 70mm below the surface. Assuming these are nuclei of aluminum on TiB2grain refiner particles, the particles would be -12pm in diameter and would be difficult todifferentiate from the quenched microstructure.
As expected from the literature, there is a distribution of particle sizes in the ingot.2327The constant particle diameters used in macrosegregation models in the literature rangefrom 5 to 100pm. 24 ,26 The grains measured in the samples obtained from the standard castwith the deep quenching mold have a distribution of diameters ranging from 15-13Opmand have average diameters of 20-60ym (Figure 16A and Table 2).
Studies of grain size in semi-solid melts show that the particle diameter depends oncooling rate by the same relation as primary dendrite arm spacing.34 The graindiameters measured from the standard cast can be compared to literature data to estimatethe solidification time. For a 20pm particle, the corresponding solidification time isbetween 10 and 40 seconds. A 60pm particle corresponds to 100-1000 seconds ofsolidification.
Comparison of the floating grain sizes to the grain sizes measured in the solidifiedcross section of an ingot cast under the same conditions shows that the grains obtainedfrom the standard ingot with the deep quenching mold are 8-30% of their final grain size.Significant grain growth occurs after the point of sampling, likely once the grains havejoined or formed a rigid cohesive network.
Floating grains being solute poor is the basis of the floating grain macrosegregationtheory. Measurement of solute (Cu) content on these floating grains confirms that theyare solute poor relative to the alloy composition. The leading assumption in
36
computational macrosegregation models in the literature is that the floating grains areuniformly depleted in solute. Line scans of solute composition (Figure 18A) confirm thisassumption. The shape of the line scans resembles line scans in the literature made on thecoarse-celled grains from the duplex microstructures in billet cross sections." Instead of agradual buildup of solute that one might expect during Scheil-type solidification, there isa uniformly depleted region (~75% depleted from alloy composition) followed by a sharpbuildup of solute.
The region of sharp solute buildup occurs on the outer 3-5pm of each grain, and it canbe observed both in the line scan measurements (Figure 18A) as well as the color etchedsamples (Figure 19). This thin layer of enriched liquid may be a liquid boundary layerthat moves with the grain, or it may be an artifact of the quenching process in the coppermold.
The final composition measured at the centerline of a standard ingot cast under thesame conditions is significantly depleted in Cu (15%) relative to the alloy composition." 0
A mass balance allows for evaluation of whether the solid grains measured with thequenching molds could cause the 15% variation in composition at the centerline.Assuming that all the measured grains sediment to the centerline (6% solid), that theyhave a uniform composition of 1%Cu, and that that the liquid composition is not depletedin solute, then the number of measured grains is not sufficient to cause a 15% depletion atthe centerline. This suggests that in the standard cast, there are regions with a higherfraction of solid grains (up to 20%) than was measured with the quenching mold in thepresent work, indicating that the distribution of grains is not uniform throughout the ingotsump.
There is a distribution of particle morphologies in the grains captured from thestandard ingot. Some grains are equiaxed dendrites, while others are more globular inshape, as can be seen in the SEM image and shape distribution in Figures 14B and 17A.
If globular particles evolve from equiaxed dendrites according to the theory inrheocasting, the shape distribution could indicate that particles are captured at differentstages of the solidification process. For example, equiaxed dendrites might roll down thesolidification interface and then ripen to form globules. Grains at both stages might becaptured at the sampling location. The shape distribution could also indicate that grainstraveled to the sampling location by different paths. As suggested by computationalcasting models, grains might experience slower or faster growth depending on whetherthey remain close to the solidification interface or get swept further towards the liquidregion. 27 However, globular particles captured in the standard ingot do not show evidenceof a previously dendritic structure. There is no evidence of entrapped eutectic betweenthe dendrite arms before ripening. In the literature, direct globular grain formation hasbeen observed, and it was found to occur at a large grain density, caused by a largenumber of grain fragments. This would suggest that grain fragmentation is a significantmechanism for grain formation and affects final grain morphology in standard DC ingots.
37
Grains are observed as clusters in the quenched samples. This supports observationsin the literature that the coarse-celled dendrites at the center of billet cross sections formclusters. 6 EBSD analysis indicates that these clusters are made of several interconnecteddendrites. This connectivity suggests that at the sampling location, grains have alreadybegun to form an interconnected mass.
4.2 Relation to Computational Casting Models
Composition assumptions made in casting models match the measurements made onthe floating grains. The solid grains are uniformly depleted in solute regardless of theirsize or shape. A key aspect of casting models is how to model the transport of grains.This depends heavily on the grain size and morphology. It has been shown that there is adistribution of grain sizes, which can replace the single diameter used in many models.Grain movement depends on drag assumptions, which are dependent on grain shape.There is a distribution of grain shapes that varies from globular particles and equiaxeddendrites to agglomerated grains.
In addition to affecting transport, grain morphologies influence both the mechanismby which grains form a cohesive, rigid network and the solid fraction at which cohesionoccurs. The particle clustering observed in these samples suggests that grainagglomeration has already occurred to some extent in the sampling location, as severalgrains are connected. This sheds light on the cohesion mechanism and also indicates thatmodels might need to consider transport of agglomerates instead of individual grains.
Grain formation is generally modeled as heterogeneous nucleation on grain refiner,and coulter counter measurements suggest that there are grain refiner particles in theliquid region of the sump. The large number of globular grains observed in samplesquenched with the deep quenching mold suggests that fragmentation (either ofheterogeneously formed dendrites or of the solidification interface) plays an importantrole in grain formation in standard DC cast ingots.
4.3 Floating Grains in Turbulent Jet Stirred Ingot
In the Jet processed ingot, grains were observed in samples from both the deep andsurface quenching molds, as shown in Figures 14C and 14D. This indicates that the jetbrings grains to the surface of the ingot from deeper in the sump, as was observed inprevious studies.2 It is known that stirring with a turbulent jet affects that the size,composition, and morphology of grains measured at the center of ingot cross sections.1,2The results from the turbulent jet stirred ingot help understand the effect of stirring on thefloating grains at an intermediate step as they circulate in the ingot.
The grains brought to the surface of the ingot by the turbulent jet (sampled with thesurface quenching mold) do not differ in size, morphology, or composition from thegrains sampled with the deep quenching mold in the same ingot. It is not known how fargrains travel to reach the surface of the stirred cast. If they originate at the bottom of the
38
sump, where the deep quenching mold samples were obtained, then the grains do notchange significantly as they travel to the surface. It is possible, however, that the grainsobtained with the surface quenching mold originate much closer to the surface.
Comparison of grains from the stirred cast to those from the standard cast shows thatgrain size distributions are shifted towards larger grain sizes in the stirred casts. Overall,however, the grains in both cases range in diameter from 15-130pm and have averagediameters from 30-70pm. The floating grains in the stirred cast are much closer indiameter to the final grain diameters measured in ingots cast with the same techniques.Despite the grains being refined in the final ingot microstructure, the floating grain sizesare not decreased in the melt. It is possible that the grains measured with the deepquenching mold end up in the final ingot microstructure and do not grow much furtherbecause the grain density is very high. It more likely that the turbulent jet prevents theirsettling altogether and propels them into a liquid region where they might remelt.
As reported in Table 2, the fraction of grains in the stirred cast was estimated to be0.12 at the deep quenching location and 0.09 at the surface quenching location. Thesefractions are higher relative to the deep quenching location in the standard cast (0.06).Stirring with a turbulent jet increases the grain density in a given location. Increasedfragmentation caused by the shear from the turbulent jet is likely a key player incontrolling grain density. Stirring also makes the distribution of grains more uniformthroughout the mushy region.
The shape of grains in the stirred cast is different from grains in the standard cast.Grains tend to be more globular in the stirred cast, and there are fewer equiaxeddendrites. The turbulent jet alters the solidification conditions to promote formation ofglobular grains. As discussed in the rheocasitng literature, forced convection duringsolidification can accelerate the transition of grains from equiaxed dendrites to globules.As in the standard cast, the grains in the stirred cast do not show entrapped eutectic orsigns of a previously dendritic structure. This suggests direct formation of globules fromfragmentation of heterogeneously nucleated dendrites or of the solidification interface.
The compositions of grains sampled from the turbulent jet stirred ingot are notdifferent relative to the standard ingot. Floating grains are uniformly depleted in solute.
The meHanism, by Vwhh% gain morphology changes wit stirring does not a thebuildup of solute in the grain.
The final composition measured at the centerline of a jet processed ingot does notshow the same depletion observed in the standard ingot.' A mass balance was done todetermine the centerline segregation that would result if floating grains measured with thedeep quenching molds (12% solid) settled to the centerline. Assuming that the liquidcomposition is the alloy composition (4.5%Cu), the settling of these grains would createa region with greater solute depletion than has been experimentally measured. Thissuggests that only a fraction of the grains measured with the quenching molds end up in
39
the final ingot cross section because the turbulent jet prevents them from attaching to thesolidification front.
The plugging of the coulter counter with solid metal prevented it from making sizeand volume fraction measurements at the surface of the stirred ingot. The temperature atthe sampling location was significantly reduced (635-640'C instead of 660-675*C in thestirred ingot). The high fraction of solid, agglomerated grains means that sampling at thislocation is not feasible with the current configuration of the coulter counter.
4.4 General Discussion
The results from this work offer an intermediate "snapshot" of the ingot sump. DCcasting modelers can use the grain properties to validate and improve models. In addition,the results shed light on the interplay between heterogeneous nucleation andfragmentation, and the evolution of grains from equiaxed dendrites to globules. Finally,the capabilities of two different grain measurement methods, the coulter counter and thequenching molds, have been tested.
DC casting models do not currently account for grain fragmentation and insteadassume heterogeneous nucleation on grain refiner. The coulter counter measured grainrefiner nuclei moving freely in the ingot sump, and the presence of equiaxed dendritessuggests that heterogeneous nucleation is occurring. However, the large number ofglobular grains without evidence of a previously dendritic structure suggests that dendritefragmentation is also a significant mechanism.
Increasing the shear rate from the standard ingot to the turbulent jet stirred ingot hasbeen found to increase the fraction of grains at a given location. It also increases thenumber of globular grains relative to equiaxed dendrites. This offers further evidence thatgrain formation by fragmentation is a significant mechanism whose contribution isincreased by the turbulent jet.
It is not known whether the globular grains found in both the standard and stirredingots evolve from equiaxed grains or form directly. The lack of entrapped eutecticsuggests that globular grains form directly as fragments.
The coulter counter method and quenching molds were evaluated in this study as twopotential grain measurement techniques. The coulter counter method is effective atdetection of conductive particles in low volume fraction melts (such as grain refiner).However, it was not able to continuously sample grains at the surface of the stirred cast,which was estimated to be 9% solid (Table 2). The method would be particularly difficultto implement in the mushy solidification zone because of the high fraction solid andtendency for grains to agglomerate. The quenching method cannot measure continuouslyduring casting and it is difficult to sample precisely from specific locations. However,quenching is effective at detecting grains at higher volume fractions and is advantageousbecause it preserves grains in metallographic samples that can be observed and measuredin great detail under a microscope.
40
5 Conclusion
Floating grains have been observed in the sump of DC cast aluminum ingots. The sizedistribution, fraction, morphology, and composition of grains in the standard DC castingot can be compared to assumptions used in computational macrosegregation models.The grain characteristics that offer the most potential for model improvement areobservations of grain size, morphology, and interconnectivity, as transport equationsdepend heavily on these characteristics. Comparison of the floating grains captured fromthe standard and stirred ingots shows that stirring with a turbulent jet increases thefraction of grains in a given location and increases the proportion of globular relative todendritic grains. The findings suggest that grain fragmentation is a significant grainformation mechanism in standard ingots and its contribution is increased with jetprocessing.
5.1 Suggestions for Future Work
This work has made experimental observations about floating grain properties andformation mechanisms. One area for further study is to quantify the interplay betweenheterogeneous nucleation and fragmentation. This may be done by varying the amountgrain refiner used in standard casts and comparing the numbers of globular and dendriticgrains. Observations of grains in ingots stirred with jets of varying power may also offerinformation about fragmentation mechanisms.
A more complete picture of the ingot sump could be obtained with moremeasurements along in the inclined solidification front and toward central regions in theingot. These measurements could improve theories of grain motion down thesolidification front and of grain remelting in the liquid regions of the sump.
The coulter counter has not been proven to detect grains at high volume fractions,such as those present in the mushy region. The quenching molds would be able to capturegrains from many regions in the solidifying ingot.
41
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40. Canyook, R., Petsut, S., Wisutmethangoon, S., Flemings, M. C. & Wannasin, J. Evolutionof Microstructure in Semi-Solid Slurries of Rheocast Aluminum Alloy. Trans. NonferrousMet. Soc. China (English Ed. 20, 1649-1655 (2010).
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42. Nafisi, S. & Ghomashchi, R. Combined Grain Refining and Modification of Conventionaland Rheo-cast A356 Al-Si Alloy. Mater. Charact. 57, 371-385 (2006).
43. Niroumand, B. & Xia, K. 3D Study of the Structure of Primary Crystals in a Rheocast Al-Cu Alloy. Mater. Sci. Eng. A 283,70-75 (2000).
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45. Dupuis, C. & Doutre, D. Calibration and Evaluation of Some of the Factors Influencingthe Response of the LiMCA Technique. (1984).
46. Guthrie, R. & Doutre, D. No Title. in Proceedings of the International Seminar onRefining and Alloying of Liquid Aluminum and Ferro-Alloys 146 (1986).
47. Mohanty, P. S., Guthrie, R. I. L. & Gruzleski, J. E. Studies on the Fading Behaviour of Al-Ti-B Master Alloys and Grain Refinement Mechanism Using LiMCA. Light Met. 859-868(1995). doi: 10.1080/10643389.2012.728825
48. Pekguleryuz, M. 0. & Pedneau, N. In-Situ Method for the Investigation of EquiaxedGrain Growth in Hypoeutectic and Hypereutectic Al-Si Alloys. Scr. Mater. 38, 1533-1539(1998).
49. Pedneau, N. & Pekguleryuz, M. 0. Equiaxed-Grain Size Analysis in the Mushy ZoneDuring Solidification via an In-Situ Method Based on the Electrical Sensing ZonePrinciple. Scr. Mater. 37,903-909 (1997).
50. Heyen, M. Effect of Electromagnetic Stirring on Grain Refinement of Al-4.5%Cu Alloy.(The University of Alabama, 2013).
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Submicron Particles and Pores by the Resistive Pulse Technique. J. Colloid Interface Sci.61, 323-335 (1977).
53. DeBlois, R. W. & Bean, C. P. Counting and Sizing of Submicron Particles by theResistive Pulse Technique. Rev. Sci. Instrum. 41, 909-916 (1970).
54. Maxwell, J. C. A Treatise on Electricity and Magnetism. (Clarendon, 1904).55. Li, M. & Guthrie, R. I. L. Liquid Metal Cleanliness Analyzer (LiMCA) in Molten
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Appendix A: Mold Cooling Rates
In order to monitor the Cu temperature during sampling with the deep quenchingmold, a K-type thermocouple was inserted into the Cu at the end of the mold, as shown inFigure 9B. The resulting temperature profiles are shown in Figure 22.
100
90
80
0
70
60
50
40
30
20
10
00 10 20 30 40 50 60
Time / sFigure 22: Temperature profile during sampling with deep quenching mold. Twoquenching experiments are shown, with initial mold temperatures of 52*C (Trial 1) and330 C (Trial 2).
The cooling rate (dT/dt) for each mold type was estimated by secondary dendrite armspacing measurements. 13 ,38 Four optical micrographs were taken at 200x magnification forthe surface quenching and deep quenching molds, and secondary dendrite arms weremeasured as the perpendicular distances between secondary dendrite branches. Thesecondary dendrite arm spacing (DAS) in microns, is related to solidification time, t, by:
DAS = 7.59 to.4
The solidification range, dT, for A14.5%Cu is the difference between the liquidustemperature and the eutectic temperature.
645*C-550 0C = 95*C = dTThe cooling rate was determined by dividing the solidification temperature range, dT,
by the calculated solidification time, dt.
45
Trial 2
Tri al I
The average DAS for the deep quenching molds starting at room temperature was
5pm, corresponding to a cooling rate of ~300C/s. The average DAS for the surfacemold starting at room temperature was 4pm, corresponding to a cooling rate of ~500C/s.
46
Appendix B: Coulter Counter Theory and Sensitivity
The relationship between the resistance change (AV/I) measured with a coultercounter and the particle volume is as follows:
AR = 4 Pid 1 - 0.8 (d 2-2k) k = Pliq
rD 4 ( D) \2+k psol'
where AR is the change in resistivity, Pliq and psoL are the liquid and solid resistivityvalues, d is the particle diameter, and D is the orifice diameter.
This relationship was developed by Deblois and Bean based on Maxwell'sapproximate solution for the effective resistivity of a dilute solution of insulatingspherical particles.52-54 This solution assumes infinitely resistive particles and that thediameter of the particle is far less than the diameter of the orifice (d<<D). DeBlois andBean derived correction factors for when d<<D doesn't hold and for spheres of finiteresistance.
Spheres of finite resistance require the factor k, which is the resistivity of the liquiddivided by the resistivity of the solid. This factor determines the sign of the voltagechange. If the particle is much less conductive than the liquid (like an inclusion), thefactor including k is ~1, and the voltage change will be positive. The factor including kwill be negative if the particles are more conductive than the liquid, meaning that thechange in voltage will be negative.
The particle size resolution of the coulter counter depends on the diameter of theorifice. For a 300pm orifice, particles between 20 and 100pm can be measured.45 Smallerparticles do not have peaks large enough to be distinguished from noise, and largerparticles tend to block the orifice and prevent further metal flow. A smaller orifice allowsfor detection of particles less than 20ym in diameter, while a larger orifice would detectparticle with larger diameters and not block as easily with large solid fractions.Calculation of particle size is based on spherical particles, though it has been shown thatparticle shape and position in the orifice affects the magnitude and shape of the voltageresponse.
Figure 23 shows the sensitivity of particle size measurements to the liquid resistivityvalues used in size calculation. The resistivity values used are for nure linuid aluminum
at 700*C and A14.5%Cu liquid (extrapolated linearly from AlI%Cu data at 700'C). Thesolid is assumed to be pure aluminum in both cases, since free-moving grains in a lowsolid fraction melt would be solute poor in non-equilibrium solidification. For a 5 0 pmparticle, use of the resistivity of pure aluminum over that of the A14.5%Cu alloy results in<5pm variation in particle diameter. The discrepancy is greater for larger particles, with a100tm particle having variation ~10pm and a 200pm particle having variation - 4 0pm.Experimental measurements found that for A17%Si melts, resistive controlled sizeparticles were measured to be ~3pm larger than they were in pure liquid Al. This
47
variation corresponded to literature values of the increased resistivity of the melt due tothe presence of Si.
300
250
2 200
150
100-
50.
00 0.01 0.02 0.03
Measured Voltage / V0.04 0.05
Figure 23: Calculated particle sizes corresponding to measured voltage values for alloysof different resistivity. The resistivity values used are for pure liquid aluminum at 700*C,A14.5%Cu liquid (extrapolated linearly from AlI%Cu data at 700'C). The solid isassumed to be pure aluminum in both cases.
48
I-.I
IV
.IVV
- 0 Pure Al
A14.5%Cu
Appendix C: Coulter Counter Crucible Test
The coulter counter's ability to detect solid particles was tested by using the device tosample continuously during solidification of a crucible of metal. An alloy of Al-3.5%Mgwas melted in a 1001b gas fired furnace. The metal was cleaned with a salt flux mixtureof KCl, NaCl, and Cryolite followed by bubbling argon through the melt for 5 minutes.The temperature was brought down to 700'C. The coulter counter was inserted into metaland began sampling continuously. At 30'C superheat, a graphite impeller was insertedinto the melt. At 10 C superheat, the impeller was turned on. The coulter countersampled continuously until the orifice was blocked and the tube could no longer fill withmetal. As the coulter counter sampled metal, the surface quenching mold was used toobtain samples of metal near the point of insertion of the glass tube.
The coulter counter was able to detect particles during the crucible test. Changes involtage were distinguishable from the background as particles passed through the orifice,and an increase in the total number of measured particles occurred as the metal wascooled below its liquidus. The orifice of the glass tube became plugged and metal couldnot be sampled soon after the liquidus temperature was reached. Figure 24A shows thetotal number of solid particles measured by the coulter counter during the crucible test ofAl-3.5%Mg. Figure 24B shows an example peak from one particle detected during thetest, and Figure 24C shows a micrograph from a sample taken from the surface of thecrucible with the surface quenching mold.
49
00 450
A 400
350680
300 6
670 250
S66() .- 200
650 150
100
640 50650-50
630 00 200 400 600 800 1000 1200 14MX 1600
Time / s4(X)
B ,, 60 m
300
250
-2(X)
20015040pm
100
50
0
-500 2 4 6 8 10
Time /ps
Figure 24: (A) Total number of measured particles and melt temperature during crucibletest of AI-3.5%Mg. (B) Example voltage changes from solid particles measured incrucible. (C) Optical micrograph of semi-solid melt captured with the surface quenchingmold.
50