the effect of mold materials on solidification

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THE EFFECT OF MOLD MATERIALS ON SOLIDIFICATION,MICROSTRUCTURE AND FLUIDITY OF A356 ALLOY INLOSTFOAM CASTING

Ramin Ajdar

A thesis submitted in conformity with the requirementsfor the degree of Masterof Applied ScienceGraduate Department of Materiais Science and Engineering

University of Toronto

O Copyrightby RaminAjdar, 2001


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EFFECTOF MOLD MATERIALS ON SOLIDIFICATION,MICROSTRUCTURE AND FLUlDlTY OF A356 ALLOY INLOST FOAM CASTINGBY

Ramin Ajdar

Master of Applied ScienceDepartment of Materials Science and Engineering

University of Toronto2001

ABSTRACTThe total solidification times for plate lost foam castings (LFC) wereexperimentally determined using three mold materials (silica, mullite and steelshot) and five pattern thicknesses. The applicabil ii of Chvorniov's rule to LFCwas confirmed and values deduced for appropriate constants. The influence ofmold materials on the cooling rates was explained using a combination ofendothermic effect of foarn and the mold properties. Taking into account theenergy balance and initial temperatures, an attempt Luas made to predict totalsolidification times. For each of the mold materials the dendrite ami spacingswere related to local solidification times, and an overall correlation established.The rneaured Iower vebcity of liquid metal in vertical patterns was attributed topattern thickness and the effect of gravity. The flow lengths (fluidity) in differentmold materials were related to the effedive superheats and total solidificationtimes.


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AcknowledgmentsEverybody is capable of greatness in Me, and to me this has been the greatestachievement in my life so far. This could not have been possible withoutkindness, inspiration, continuous advice and support from my professors,Professor A.McLean and Professor C-Ravindran. Financial support from NaturalSciences and Engineering Research Council of Canada (NSERC) is greaUyappreciated. I would also like to acknowiedge invaluabfe assistance from myfriends Mr. A.Machin at Ryerson University (Near-Net-Shape Casting Laboratory)and Mr. F.Neub at University of Toronto (Scanning Electron MicroscopyLaboratory).

Behind every success of a mamed man, there is the shadow of a woman. Iwould Iike to thank my wife, Akhtar for providing me with the best care at home,and creating a peaceful and loving environment She and my daughter, Rose,are the most important motivation for completing my thesis.

Finally, I hank God for giving me the ability to perforrn research through hiscompassion and mercy.


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Table of ContentsPage

AcknowledgrnentsList of FiguresList of TablesNomenclatureChapter 1 IntroductionChapter 2- Lierature Review

2.1 Lost Foam Casting (LFC)2.1.1 Preparation of Beads of Polystyrene2.1.2 PatternMaking2.1.3 Coating2.1.4 Sand Filling and Vibration2.1.5 Lost Foam Filling Mechanism

2.2 Solidification Process2.2.1 Nucleation2.2.2 Growth2.2.3 Dendric Solidification2.2.4 Cooling Cunre2.2.5 Cooling Rate2.2.6 HypoeutecticAluminum -Silicon Alloys2.2.7 Silicon Morphology2.3 Solidification Heat Transfer2.3.1 Conduction2.3.2 Energy Equation for Condudon2.3.3 Solidificationof Casting in Semi- Infinite Sand Mold2.3.4 HeatTransfer Coefficient2.3.5 Chvomiov's Rule

2.4 Fluidity (Flow Length)

iiiiiv i iixX1333356799

1O12151717

192121


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Chapter 3 -Experimental Procedure 263.1 Cooling Behaviorof DifterentMold Materials 26

3.1.1 Pattern Making 263.1.2 Pattern Coating and Drying 273.1.3 Temperature Measurement 283.1 -4 Sand and Vibration 293.1 -5 Melting Procedure 29

3.2 Measurement of Secondary DendriteAm Spacing (DAS) 313.2.1 Sample Preparation 313.2.2 Image Analysis 31

3.3 Flow Measurernent in Diffetent Mald Materials 323.3.1 Pattern and Flask Making 323.3.2 Coating and Vibration 323.3.3 Metal Pouring and Vertical Velocity Measurernent 34

Chapter4-Results and Discussion 364.1 Application of Chvorniov's Rule to LFC 36

4.1 .?Cooling Curves in Different Mold Materials 364.1.2 Heat Diffusivity 364.1.3 Cooling Rate 424.1.4 Chvomiov's Plot 424.1.5 The Endothermie Reaction of the Foam Ablation 464.1.6 The Energy Balance between Liquid Metal and Foam 484.1 -7The Effectof Foam Thickness on Melt Temperature 494.1.8 ModifieciChvomiou's Rule for LFC 494.1 -9 Calculated Value of the Total Solidfication Time in LFC 544.1 . IO Applicationd hape Factor 56

4.2 DendriteAm Spacing in LFC 574.2.1 Local Solidification Time of A356 Alloy inSilica Sand

Mold During LFC 574.2.2 DAS of A356 Altoy in Siiica Sand Motd During LFC 57


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List of TablesTable 1. Cooling rates and total solidification times for differentmoduli and rnold materialsTable 2. Comparison of cooling rates and solidification tirnesTable 3. Relevant base data for calculatiun of initial temperaturesTable 4. Corrected initial temperatures for different foam thicknessesTable 5. Thermal properties of mold materialsTable 6. Chvomiov's constant and cp factor for differentmold materialsTable 7. Measured values of dendrite a m spacing and localsolidification time in silica sand moldTable 8. Measured values of dendrite am spacing and localsolidification time in mullite sand moldTable 9. Measured values of dendrite ami spacing and localsolidification time in steel shot moldTable 10. Comparison of DAS constants for solidification of A356 alloy 77Table 11.Velocities of Iiquid A356 alloy for different gate areas 82Table 12. Relevant base data for fluidity caIculation 82Table 13. Cornparision of back pressure data frorn different studies 86Table 14. Calculated data for fluidity (flow iength) of A356 alloy in differentmold materials 89


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NomenclatureAlloy constant in dendrite a m spacing DimensionlessPattern thicknessCross section of gateSurface area of the castingPattern cross sectionSprue cross sectionSurfaceareaof a plateChvorniov's constantSpecific heat of the liquidmetalHeatcapacity of the moldHeat capacity

dTIdX TemperaturegradientAcceleration due to gravityGibbsfreeenergyGibbs volume free energyHeat transfer coefficientEffectivesprue heigMDecomposition energy of thefoam patternLatent heat of the IiquidmetalShape factorThermal conductivity of the plateThermal conductivityof the moldThickness of the plateThe Iength o f characteristicchiII zoneFlow lengthThickness of the solidmetalA constant between1and 2A constant behiveen0.3 and 0.5

JlgrJ/grDimensionlessW/cmaC


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Back pressureHeat fluxHeat f iow rateRadius of nucleusCritical radius of nucleusTotal so lidication timeLocal solidification timeThickness of the patternTime for measuring the velocityArnbient temperatureInitia l temperatureMold temperature

To,TLTemperatureAT Degree of superhaatv Velocity of liquid rnetalV Volume of the castingW The width of the patternX Distance

gr~cm.s2w1cm2WMicronMicronsecsecsecOCOC


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Greek Symbolsa Thermal diffusiviiP A constant in modified Chvorniov's ruleY Liquid isolid surface energyP DensityPL Density of Iiquid metalPm Bulk densrty of the moldPP Densky of foam patternPa Oensdy of solid metalP Raistance to heat transfer in sand mold4 Flow loss coefficicrnt of the metal

AbbreviationsEPS Expandable polystyrenefoamDAS Secondarydendriteam spacinggfn Grain fineness numberLFC Lost foam castingLOI Loss on ignitionPAC PolyalkylenecarbonatePMMA Polymethyl metacrylateSEM Scanningelectron rnicroscopy

Jg/cm3r/cm3

gr/cm3gr/cm3gr/cm3DimensionlessDimensionless


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1 i ntroductionH.F.Shroyer invented the lost foam casting (WC) process in 1958. The patentwas for cavity-less or full mold process of lost foam casting. It was not based onindustrial castings but on artistic castings, Kotu'n [Il.One of the first castingsusing polystyrene foam was the bronze statue "Pegasus" made by A. Duca,sculptor and metallurgist at the Massachusetts lnstitute of Technology, Kotzin [il.Then, T.R. Smith developed the usage of unbonded sand as a mold material forsurrounding a polystyrene pattern during the pouring of molten metal in 1968,Kotzin [Il.LFC has gained prominence as a casting process over the lastdecade. Statistics suggest that LFC can oftera cost swing of 20% to 35% overconventional casting methods, East 121.Lost foam casting is a full mold pmcess, which uses a polystyrene foam pattemin unbonded sand. The foam pattern is prepared by injecthg expandablepolystyrene beads under the combined action of steam and pressure. Then, thepattem assembly is coated with refractory siurry and dried. The coated patternassembly is set in a steel fiask, which is then filled with unbonded sand. Theflask is vibrated io compact the sand. Fnalfy, molten metal is poured directly intothe coated pattem assembly. The poiymer pattem undergoes thermaldegradation and is repiaced by the molten metal.LFC has a number of advantages; 1 allows the caster to design complex shapeswithout wres and parting lines, enables lighter castings with minimum wallthickness, reduces secondaiy machining requirements and provides closedimensional tolerance and good surface finish. The use of unbonded sandprovides a further advantage. The disposai of sand in conventional sand castingis an environmental issue involving high cost; however in LFC sand can bereused easily and the cost for $and reclamation and sand disposal is not high.There is less room for errors in LFC. ln fact, a scrapped casting means replacingnot only the mold but the pattern as well. The pouring is hazardous, pattemcoating and drying are time consuming and pattern handling requires great care.


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The sand must flow into interna1 cavies, allowing products of pyrolysis toescape. Also, it provides mechanical support to the pattern during pouring andpreventing metal penetration. The ablation of foam causes loss of heat in themolten metal front resulting in misrun and other defects, Shivkumar [3]; therefore,using alternative mold media can facilitate decreased loss of heat and enhancedflow conditions. Total sotidfication time can be predicted by using Chvomiov'srule, a general method which is not well developed in LFC.

The main purpose of this research is to modify Chvomiov's rule suitably for LFCusing dwerent mold media with a view to predicting the total solidification time ofcasting. The mold media investigated in this research are silica sand, syntheticmullite (3Al2O3,2Si02) and steel shot Further, the retationship between localsolidification time and dendrite arm spacing wiU be detemined. Finally, thevelocity of liquid metal during the vertical mold filling will be measured, and theflow lengths in difkrent mold media deriveci ernpirically and conelated with thetotal solidification time of casting.


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In this chapter, the principles of lost foam casting, soticiification process wittiparticular interest in dendritic solidification and silicon morphology in hypoeutecticaluminum-silicon alloys are discussed. Further, heat transfer is discussedfollowed by the Chvomiov's rule to predict total solidification tirne. Finaliy, thereis a brief description of the relation between total solidification and fiuidity.

2.1 Lost Foam Casting (LFC)2.1.1 Preparation of Beads of PolystyreneLFC begins with the molding of the expandable polystyrene pattern. Produciionof expandable polystyrene starts with benzene, a derivative of crude oil, andethylene, a derivative of natural gas. Ethylbenzene is converted to styrene by thechemical rernoval of hydrogen. Initially, styrene is polymerized and thenexpanded into beads of controlled site using pentane. Further, pre-expansion isaccomplished using steam or hot air. This is carried out in an expansionmachine, thus changing the density of the beads from 0.64 grlun) to 0.024-0.028grlcm3 (for aluminum casting produds), Liieton [4J.The final product isexpandable polystyrene (EPS). Qther polyrners such as polymethyl rnethacrylate(PMMA) and polyalkylene carbonate (PAC) have been us& for production offerrous castings-2.15 Pattern MakingThe pre-expanded beads are blown into the cawty of a spli alurninum die, whictiis steam heated to further expand and fuse the beads. The die is coled withwater and the pattern is ejected (Figure 1).Ifcornplex shapes are required, difFerent sections of the pattern may be joinedwith hot melt adhesive. Thus, part consolidation is a major advantage of thisprocess (Figure 2). The foam pattern changes -ts dimension rapidly afterremoval h m he molding machine. First, some polyrner cells in the patternundergo stress relaxation. Thereafter, equalization of gas pressure in the beadscauses both pattern expansion and contraction.


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Figure 1. Steps in molding expandable poiystyrene [ l ) h m E L KotUn. MetalCastins & MoMina Processes,AFS, Des Plaines, Illinois, USA, 1981, p l52)

Figure 2. Part cansolklation: Automotivewater pump EPS pattern[ilfrornMetalCastina & Moldina Processes,AFS, Des Plaines, Illinois, USA, 1981, pl#)The best way to prevent dimensional change is by artificially aging the foampattern in an oven in tfie range, 50-65 O C during the period of stress relaxation.The pattern assembiy operation D compkted by gluing the pattern to &e sprueand the gating system, using hot meIt adhesive to fom a complete eluster(Figure 3).

..',

Figure 3. Cluster assembly


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2.1.3 CoatingThe cluster must be coated to develop a shell between the foam and the sand.The coating material is a thixotropic liquid consisting mostly of refractories,emulsifiers and antibacterial chemicals in a water carrier. The refractories usedare usualfy sitica, alumina or zircon. Coating can beapplied by dipping, sprayingor pouring (Figure 4). Coating provides a physical boundary between thecompacted sand and molten metal, prevents metal penetration and sand erosionduring pouring, and allows decomposed products to escape during degradationof foam. The refractory materials must have controlled permeabilii, insulatingcapacity, abrasion resistance, and a b i i i i to absorb the liquid pyroiysis produ&.The target coating thickness is typically in the range of 0.34.5 mm, Littleton [4].Higher pouring temperatures and higher metal head pressures need thickercoaiings.

Figure4. Application of coating by dipping the clusterThe endothermic decomposition of the foam during the pouring of aluminumresuits in heat lass along the advanced metal front. Therefore, the coatingshould have sufficient insulating properties to prevent freezing in thin-walledcasting before the pattern can be displaced. The permeabiii i of the coating isthe key to control the rate of removal of the products of pymlysis h m he mold.Low pefmeability coating causes liquid polyrner to remain in the metat front,which leads to casting defects such as w o n racks, cold laps and folds. On theother hand, high pemeabi i i i leads to gas defects, because fast metal fillingcauses turbulence and consequently liquid pyrolysis product entrapment After


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Sands can be angular, subangular, round or a combination of these. Coarseround sand enables good p m e a b i l i even after compaction. However, fineangular sand reduces permeability and increase entrapment of gases, becausethe finer particles ne& in the spas behnreen the large sand grains. Figure 6(a)and 6(b) show the coarse round sand and the angular sand.Loss on ignition (LOI) is another important parameter in sand eantrol. It is usedto monitor the build up of organic materials in the sand fmm the plastic residuethat condenses on the surface of the sand grain. Sand does not wmpact or fiowwell m e n LOI exceeds 0.25-0.5%, Monroem.

Figure 6. a) coarse round muIlite x 50, b) angular silica sand x 502.1.5 LmtFoam Filling MechanismIn metal casting, the conventional wisdom is to fil1 the mold cavity unifomly withminimum turbulence and at a constant rate controlled by the gating system.There are niles to predict the filling tirne for conventional casting, Rao [8].However, the conventional tules for empty mold casting do not apply welt to LFC.In ernpty mold casting, the cross section area of sprue, nrnner and gatedetemine the filling time of tbe mold. But in LFC, the fused foam, Iiquid andgases from the styrene exert a cushioning effect on the advancing Muid metal.Figure 7 representsthe schematic shape of a molten metal front


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MD(IUIEoFGAShN0 EPS ( P O L W E ]\

Figure7. Schematic of rnolten rnetal front inCFC.The advancing metal front first rnelts and partially vaporizes the polystyrene. Asexplained, the srnallgap between the molten metal front and EPS is filled with am'Mure of gases (mostly styrene monomer), and Iiquid styrene. The gasesquickly escape through the permeable coating and the Iiquid may be absorbedinto the coating or rnay fom a thin liquid residue between the casting and thecoating. This liquid residue causes the oating to blacken and often leaves alayer of carbon behveen the casting and the coating. The amount of liquid andgases produceci by ablation of EPS depends on the rnolten metal fillingtemperature and the foarn density, Shivkumar [QI. For example, with aluminumcasting at 750 C, the total volume of gases is 40 cm3 er gram of foarn, Monroe1101-It can be observed that the gas layer of the molten metal front is essentiallynegligible when EPS is used during casting and he moiten metal at the movingboundary is atrnost in direct contact with the polymer at most locations. Thus,gases fomed at the molten metal front are diised into the sand alrnostinstantly. In the case of PMMA, the gas layer obsenred in the molten metal frontis repoded to be as Iarge as 2 cm, hivkumar [QJand the amount of gases duringthe degradation of PMMA is greater than that due to EPS.


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2.2 Solidification Proceas2.2.1 NucleaaonThe sodcaton process is important because it controls the structure andhence aie properties of castings. As the liquid metal cools below the freezingpoint, the thermal agitation decreases. In some locations in the liquid metal,small gmups of atoms move ta a crystalline arrangement and form clusters.These clusters then form nuclei. For srnall nuclei, the net energy to form thisnew phase is reduced in proportion to its volume (V) and the free energy per unitvolume (AGv); however, it increases in proportion to the surface area andsolidlliquid surface energy (y). For spherically-shaped nuclei, the free energy iscalculated aius, Porter[Il]:

where AGv is the volume free energy, y the solidAiquid interface free energy, andr the radius of nuclei. Therefore the critical or minimum radius of nuclei (r') isobtained by differentiating Equation (1).

Nuclei that do not reach the criticaI radius and maximum energy shrink anddissolve into lquid (Figure 8). When the temperature is low enough to allownudei above the crical sire, urthergrowth is enabled by a reduction in energy-This i3 called the homogeneous nucleation. When the crystallization begins onimpurities, particles, nucleation agents and mold walls, heterogeneous nucleationtakes place.

Heterogeneous nucleation is common in alloy casting process, in which particlesand mold walls are the basic nucleationsites. In commercialpractice, inwla t ingagents are added to many maften alkys to increase nucleation sites and produce


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fine-grained structure. These elements are titanium and boron for aluminumallay.

Figure 8. The variation of free energy with radius of nucleus. r' is the criticalradius for those nuclei which will grow (111 (From DA-Porter and K.E.Easterling,Phase Transformations in Metals and Allovs, Van Nostrand Reinhold Co.Ltd,United Kingdom, 1981, p266)

When the molten metal is poured into a cold mold, the metal at the surface coolsrapidly and heterogeneous nucleation starts on the mold wall. After nucleation,solidification process will only occur if heat is extracted through the solid, coolingthe advancing front below the equilibrium freezing point. As the rate of extractionof heat increases, the temperature of the solidification front falls and the growthrate (G) increases, where G is defined as the velocity of the solid growing into theliquid. For pure metal, as the driving force for solidification increases, the series


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of transitions at the solidification front are planar. At a higher growth rate, cellularsolidification is seen to develop. At a very high cooling rate, the cells growrapidly and the advancing projections have cornplex treelike geometry. This typeof growth is called dendritic solidification. (Figure9 a-c)

Fmni Sd e Ternocrature rea~rne

Figure 9. The transition of growth morphology from a) planar,b) cellular and c)dendritic, as compositionally induced undercooling increases [12]. (RomJ.Campbell, Castinas. Butterworth-Heinemann Ltd, Oxford, England,1991, p143).

The process of nucleation and growth is illustrated in Figure 10. The ratio ofnucleation rate (N) to growth rate (G) determines the grain size. If the NJG ratiois high a fine grain size is expected and if it is low a large grain size will result.


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NJG high

Figure 10. Effect of NIG ratio on the final grain size (131. (from Brooks,Treatment. Structure and Properties of Non-ferrous ~llovs, SM, Metals Park,Ohio 44073,1982, p79).2.2.3 Dendritic SolidificationIn fact, dendritic solidification is the most common fonn of solidification in casting.The growth rate into the Iiquid depends on the crystal orientation, and only thosethat have a rapid growth direction no n a l o the mold wall will survive the growthprocess and these grains are columnar in shape. In general, the columnar grainsgrow from the surface to the interior, with equiaxed grains at the center. In thisprocess, small pieces of the dendrite amis are separated from the dendrite andswept into the center. This happens when, the heat released at the meidendritea m nterface is sufficient to raise the temperature locally, aius causing the sideamis to melt. These srnaIl crystals move into the center of the casting to serveas the nuclei for the formation of equiaxed grains (Figure 11).During the growth, dendrite amis will knit together fom ing a single crystal Iatticeknown as a grain. The grain may include thousands of dendrites. Themechanical properties of most cast alloys are strongly dependant on secondary


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a m spacing. A decrease in dendriteam spacing (DAS) is accompanied by anincrease in ultimate tensile strength and du c ti li , Campbell [i4].

FigureII.chematic of the side am remelting of a dendrite and movement tothe center tu serve as a nucleus for an equiaxed grain [13]. (from Brooks,mtTreatment. Structure and Pmerties of Non-ferrous Allovs, ASM, MetaIs Park,Ohio 44073,1982, p82).Furthemore, the DAS is directly proportional to the local solidification tirne, whichis the t h e interval (Figure 18) be-n Iiquidus and solidus temperatures in thecooling cunre, Gnrzleski [15]. Typically, DAS is the average spacing be-n thesecondary dendrite amis and is detemined by the linear intercept method(Figure?2).

Figure 12 A rnethodobgy for measurement of DAS [15]. (frorn J.E.Gnizleski,Microstructure evelo~mentDurina Metal Casbiq, AFS, Des plaines, Illinois,USA, 2000, p 113.


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For many alloys, an empirical relationship exists between DAS and localsolidificatication tirne, Gruzleski [15j:

DAS = atf"' (31Where, a is a constant for an atloy, tf the local solidification time (See Figure 16),and ni a constant with the value between 0.3 to O.S. The mechanism forwarsening of dendrite is as follows: Dendriteamis fom at the very smalt spacingnear the tip of the main body or the pnmary a m . After some time, the dendritesattempt to reduce their surface energy by decreasing their surface area. As aresuit, the srnall amis go back into the solution and the large arms grow. Thus,the average spacing between amis increases (Figurel3).The rate of dendrite warsening appean to be Iirnited by the rate of diffusion ofsolute in the liquid, as the solute transfers from the dissolving arms to thegrowing arms. As the cooling rate is decreased and DAS grows, the uttirnatestrength decreases until it reaches the yield strengai. At this moment, the alloy isbecome brittfe resulting in fracture (Figurel4).

Figura 13. Scanningelectron miemgtaph (SEM) of dendrie marsenhg inA356 (AI-7%Si) alloy.!4


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Figure14. a) An Al-7Si-0.4Mg alloy casting, b) mechanical properties, showinggood strength and toughness near the chill [i6]. (tom J.Campbell, Castinas,Butterworth-Heinemann Ltd, Oxford, England,1991, p. 265)

2.2.4 Cooling CuweThe cooling cunie is the temperaturethe relation obtained during thesolidification of a metal or an alloy in the mold. It is a valuable tool to study thechanges occumng during the solidification process.

An ideal cooling curve for a hypo or hypereutectic alloy is shown in Figure 15. Inthis figure, solidification of the primary phase starts at point (1). The eutecticsolidification starts at point (2). After the release of the latent heat at this point,the cooling continues at a constant temperature (eutectic plateau). Finally, thesolidification is completedat point (3).


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TirneFigure 15. An idealized example of simple cooling cunre for a hypo orhypereutectic alloy, Gruzleski [IA.ln an actual foundry, the cooling curve does not represent the ideal situationdepicted by above figure. Thus, the salidification rate is extrernely high, andreactions do not begin exactly at the equilibriurn freezing temperature; therefore,it is necessary that the temperature dmps below the equilibriurn value to impartenough time for the nucleation process. This phenomenon is known asundercooling, and is demonstrated by Figure 16, This figure shows a typicalcooiing curve for a hypoeutectic alurninum-silicon A356 alloy.

Figure16. Typical cooiing curve for theA356 AI-Si h y p ~ e u t e ~ clloy showinglocaland total solidification time.


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2.2.5 CoolingRateThe cooling rate shows the rate of change of temperature over time duringsolidification. The cooling rate may be obtained at any temperature from thewoling curve by drawing a tangent to the curve at that temperature anddetermining the slope of the tangent, Avner[18].2.2.6 Hypoeutectic AlurninumSilicon AlloysAluminum ailoys with silicon are widely used in commercial castings. The mainfeature distinguishing various alloys in this group is the eutectic reactionoccurring at 12.5% silicon (Figure 17). Hypoeutectic alloys contain iess thanf 2.5% silicon. Depending on the level of purity, they alsocontain srnaIl amountsof iron, manganese, magnesium, copper and zinc. Copper and magnesima das alloying elements to increase the strength and hardenabilityof castings.The impurities and alloying elements can go into the solid solution in the matrix orproduce an interrnetallic compound during the solidification process. Inhypoeutectic alloys, the following phenomena are known to occur:

Formation of dendrite network of a-aluminum.An alurninum-silicon eutectic reactionPrecipitation of secondary eutectic phases suchas Mg2Silntermetallic compounds containing iron and manganese are also known toform.The most common of these compounds are A15FeSi and AIl&inFe)&. Themorphology of AISFeSi s plate-like and brittle and appears as a needle with anextension of up to several mm; Al15(MnFe)& has a shape resembling Chinesescript The forna tion of these two compounds is responsible for a decrease inmechanical properties in AI-Si alloys, Gruzleski[19].


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Silicon content. massp e m iFigure 17. Equilibrium binary AI-Si phase diagram (201. ( h m 1 C. KammerAtuminum Handbook, Aluminum-Verlag, Marketing & Komrnunikation Gmbh,Dusseldorf, Gennany, 1999, p86).The Al-Si alloy 356 contains 7%Si,


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1- The ornation of aluminum dendrite begins at 610 O C hen latent heat evolvesto cause an increase (approximately 10 O C ) in temperature.

2- The development and growth of the aluminum dendrite network, and theevolution of intermetallic cornpouds containing iron and manganese.

3- The eutectic reactionwith silicon occurs at approximately 577 O C .4- The growth of Mg2Si and continued growth of aluminum and silicon phases

at 568 O C .

2.2.7 Silicon MorphologyLu and Hellawell [21] have dernonstrated the evolution of silicon morphology.The effects of silicon in aluminum alIoys are well known: increased fluidity,decreased casting shrinkage and improved wear resistance. Silicon has a fan-shaped morphology in metallographic sections (Figure 19). This kind of eutecticstructure, also known as lamellar structure, imparts puor mechanical propertiescompared with the modified structure of the castings.t-d Iamellar--. I

Figure 19. The lamellar and fibrous structuresof silicon in Al S i alloys x 800 [22].(from J.E.GruzIeski, B.M.CIosset, The Treatment of L i~ u id luminum-siliconAllovs, AFS, Des Plaines, Illinois, U.SA, 1990,p40,41).Small additions (up to 0.02%) of elements such as sodium, strontium, calcium,antimony and cerium, campletely change the morphology of the silicon eutecticphase from large flakes to a fibrous structure (Figure 19).

Several attempts have been made ta exptain the mechanisrn of modification.Hellawellet al [21]explain the formation of fibrous silicon by assuming that atoms


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of the rnodmng element adsorb on and distur the growth step of the siliconcrjstals. This causes repeated twinning and h e m the fibrous structure of thesiticon phase. Furthemore, these elements have a strong affinity for silicon tohm chernical cornpounds that remain on the surface of the silicon crystal,preventing a normal growth step, and act as nucleatim sites to produce a fibrousstructure. Adding these elernents is crucial, and the amount should not be morethan 0.02%. Emadi et at [231 have shom that adding these ekments causes adecrease in the surface tension of aluminum, thus increasing micro-porosityduring the wlidiication of castings.


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2.3 SolidificationHeat Transfer2.3.1 ConductionConduction is the mechanism in which heat is transferred internally within ?hesolidifying metal and the mold, Poirier [24). Figure 20-a illustrates the steaciystate temperature distribution ina planewall and the heat conducted from lefi toright. The surface at the x = O is ho ttei than the surface at x = L; therefore onecan write:

Q = heat fiow rate, WA = area of the plate, cm2L= thickness of the plate, cm

TOITL temperature, OCK = thermal conductivity of the plate, W c~- 'oc* '

A betterway to write Equation4 is,

q = heat fiux ~ . m "dTfdx = temperature gradiant. O C cm"

Figure20. Temperaturedimiution ina plane wall: a) Iineartransition, b) non-Iinear transition.


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The temperature distribution is linear only when a steady state predominates.However, when the situation is transient the temperature distributionis nonlinear(Figure 20-b) and the equation for this situation is:

p = Density, gr/cm3Cp = Heat Capacity, Jlgr O C

2.3.2 Energy Equation for ConductionCombining Equations 5 and 6, the energy equation for conduction heat transferis,

where a is thermal difhsivity (cm2/s) and is defined as

2.3.3 Solidification of Casting in Semi-infinite Sand MoldWhen a pure liquid metal at its melting temperature, TMis poured into the sandrnold, a temperature gradient develops in the rnold wall, and a transfer of heatfcom the metal to the mold causes freezing of the metal and henevolution oflatent heat of solidification. Evolution of latent heat determines the rate ofadvance of the solid-liquid interface into the liquid metal. If the mold cm beconsidered to be semi-infinite in thickness, the thickness of the solidified metallayer (M) at time (t), as s h o w in Figure 21, can be written as follows, Gaskell(251:


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M = Thickness of the solid metal, cmTM= Melting temperature of the pure metal, O CTV= Sand mold temperature, O Ch= Density of solid metal, grlcm3H = Latent heat of themetal, JlgrK,= Thermal conductivity of the mold, Wfcm O Cpm= Bulkdensity of the mold, grtan'Cm = Heat capacity of the mold, Jlgr OCt = total solidification time, s

Equation 8 was first derived by Flemings [26], and the term K m ~ m C ms known asthe heat difhsivity of the mold and measures the ability of the mold to absorbheat As a resuk the amount of solidified metal depends on the properties of themetal and the heat difFusivity of the rnold.

TMLiquid Solid Liquidmetal metal ; maai

0 x O xFigure21: Temperature profiles during solidification of liquid metal contained in asand mold; a) at t = O and b) at t > O [25]. (fiom D,.R.Gaskell, An Introduction toTrans~ort henornena in Material Enaineerinq, Macmillan Publishing Company,Canada, Toronto, 1992, p402)2.3.4 b a t Tnnsfer CoefficientAfter pouring the liquid metal into the mold, a gap develops between the metaland the mold. Poirier [271 improved Equation 8 by considering the general heattransfer coefficient (h,), which accounts for the heat resistance in a gap behnreenmetal and mold. Then he developed a modified factor (q) to account forresistance to heat transfr due to rnetallmold gap in sand molds.


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2 1where, hg= Heat transfer coefficient, Wcm' C't = Total solidification time, sec

Now Equation 8 can be writen as follows:

2.3.5 Chvorniov's RuleIf (V) is the volume of the casting and (A) is the surface area of the mold, (M) canbe replaced by (VIA), which is the modulus of the casting, Chvomiov [28] so thatthe total solidification time of the casting (t) is:

This is Chvorniov's rule, C is Chvorniov's constant and n, the constant between 1and 2. Under an ideal condition, n= 2; this assumes (i) isotropy and homogeneityof semi-infinite casting, (ii) absence of superheat, (iii) pure or eutectic alloy, and(iv) constant moldlmetal interface temperature. In actual conditions, n assumesa value between 1 and 2 Tiryakioglu [29]. Okorafor (301attempted to appiyChvomiov's rule in predicting the total solidification time for foam casting usingbonded sand, and met with limited success.


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2.4 Fluidity (FlowLength)The tenn fluidity is defined as the ability of the liquid metal to fil1 the mold cavity,Rao (31). Fluidity is a complex propefty and isaffected by a number of variables.The casting material's properties that affect the fluidity are viscosity of the melt,latent heat of the melt, surface tension, fteezing range and density of the liquidmetal. The lower the viscosity of the moiten metal, the higher the fluidity, sincethe melt will be able to fiow freely. Since an increase in the latent heat andsuperheat of liquid metal decreases the viscosity, itwill also be responsible for anincrease in fluidity. Similariy, lower surface tension, which promotes wetting ofthe mold by the meit, would cause the mold to be quickly filled, particularly thenarrow sections. In geneml, the alloys that have a short freezing range have ahigher fluidity than those that have a long freezing range. During the process ofsolidification of alloys with long freezing range, the dendrites are spread over amuch large?part of the mold, thus reducing the flow of the metal, decreasing thefluidiy.For long freezing range alloys, the fiuidity (or flow length) is given by Campbell[32]:

Where v is velocity and t, total solidification time. The mold properties that affectthe fluidrty are the thermal propertks, permeability and the mold surface quality.LFC is an endothermic process involving extraction of heat from the liquid metalto fuse and eventually vaporize the foam. Thus, fluidity is significantly affeded bythe foam barrer. Measurement of fluidity during the LFC process is necessary toinvestigate the fillabiiii of the mold.


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The experirnentai procedure consists of three parts. The first section (3.1)describes pattern making and casting for an assembly of patterns of differentthicknesses in three different mold materials. The second section (3.2) explainsthe procedure for secondary dendrite arm spacing (DAS) measurement. Finallythe pattern making and casting for flow Iength and velocity measurement aredescn'bed in the third section (3.3).3.1 Cooling behavior of different mold materials3.1.1 Pattern makingRectangular plate patterns of foam (EPS), 160 mm long and 80 mm wide werecut to various thicknesseses 10,15, 20, 25 and 30 mm (Figure 22). They werecut using a hot wire to an accuracy of i .5 mm.

Figure22: Foam pattern dimensions


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The gating system consisted of a sprue (W30xT30xL180 mm) as well as aninner (W30xT15xL60 mm) and a pouring cup (100 mm diameter) al1of whichwere cut from the sarne foam block The density of the foam block was 0.028gr/cm3 (supplier's specifcation). Five patterns were attached to the gatingsystem with a hot melt adhesive to form the final cluster with bottorn gating(Figure 23). Nine such clusters were made for experiments (i.e.,three clustersper mold material). The pattern assemblies were then coated with a ceramicmaterial and dried.

4 8 0 mthemiocouples \LRunnerFigure 23: Cluster assernbly.3.1.2 Pattern Coating and DryingThe refractory coating used for theseexpen'rnents is Thiem Pink Styrokote 145.3.It is an alumina-water base refradory. The insulating characteristic of the coating


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reduces heat transfer fmrn the liquid metal to the sand mold, thereby increasingthe flow Iength of the Iiquid aluminum alloy and improving wstability. Therefractory sluny should have the appropriate viscosity to cover the surface of aiefoam duster. The foam cluster was dipped in the refractary slurry and wasshaken to drip dry. The coated duster was placed in a temperature controlleddrying oven (Figure 24). Two variable speed fans enabled circulation of air in theoven. The temperature in the oven was set at 60 O C (* 1 OC). The duster wasleft to dry for almost 8 iiours.

Figure 24. An air circulation oven with temperature controller.3.1.3 Temperature Measuremeiitlndiiidual thennocouple wires (type k, 26 gauge) were inserted into the ceramicinsulator tube. The positive and negative ends of two such wires were welded tofonn the couple and the themiocouple tip was inspected for proper wefd. Theywere tested at 100 O C in boiling water to detemine their temperature


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measurement accuracy. Finaliy they were inserted into the center of the dried,coated patterns at the location show in Figure 23. Five sets of thermocoupIeswere then connected to a data acquisition system. This system made it possibleto record temperatures to an accuracy of I 2 O C at the fwe locations of thepattern assernbly at 0.1second intervals. The pattern assembly was then placedin a steel flask partially filled with mold materials.3.1.4 Sandand Vibration

Three mold rnaterials (media) of standard AFS-grain fineness number (gfn) wereused: silica sand (SiO2) of gfn 35, synthetic mullite (3AI2O3,2Si02) of gfh 40 andsteel shot of gfn 30. These three media were chosen because they widely diferin their insulating properties.The unbonded sand was filled into the flask from the sand silo by gravity tosurround the pattern assembly. Uniforni vibration of the fiask was facilitated by 3-point clamping of the flask to the vibration table. Actual vibration was carried outby two off-center motors with counter weights (Figure 25). The flask was thussubjected to a horizontal vibration of 1.2 g for 30-45 seconds during the sandfilling process. Because the orientation of the pattern assembly was vertical, thevibrating was horizontal rather than vertical to prevent moki wllapse. The flaskwas ready for melt pouring.

3.1.5 Melting ProcedureThe melting was camed out in a Speedy Melt Model 8-300 gas fired furnace witha graphite crucible (Figure 26). A total of 3.5 kg of pre-cut slabs of the A356 alloywere placed in the crucible. A portable pyrometer with an adequatemeasurement range (0-1100 OC) nd suitable accuracy (k0.5 C )was used tomonitor the melt temperature. Upon reaching a liquid metal temperature of 785CIhe furna was shut off and the surface of the crucible covered by cover fluxpowder. Then, a degassing tablet (35 grams of hexachloroethane) was


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submerged into the melt at 780 OC using a preheated plunger until there was noevoluon of aas.

Figure 25. A steelflask ai he vibration table with two off center moton.

Figure 26. A gas furnace with a graphite crucible30


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The liquid metal was skimmed to remove any excess dross and slag. The dataacquisition system for the pattern assembly was activated at this point, and themetal (at a temperature of 760 OC)n the cnicible was poured into the mold at asteady rate by rnanually controlling the metallostatic head to ensure a constanthead pressure.

3.2 Measurement of DendriteA m Spacing (DAS)3.2.1 SarnpIe PreparationTwo sarnples were cut from each casting near the thennocouple locations asshown in Figure 27. The samples were cut at 70 mm from the bottom and 70mm from the top of the casting. Each group of castings, as shown in Figure 23,consisted of five castings with different thicknesses; therefore, a total of tensamples were extracteci from each group of castings, which were solidified in aunique mold material.

Once a total of 30 samples were collected from three difFerent mold materials, theinner surfaces of the sarnples were prepared using the following steps: first,coarse grinding with a belt sander using 80 grit paper; second, removal of themetal particles using an ultrasonic cleaner; third, fine grinding with 240,320 and600 grit papers; fourth, polishing with onmicron diamond paste; and finally,etctiing of the samples using Keller's reagent (HF, HCL, HN03and Hz0 in equalparts) for 15 seconds.

3.2.2 Image AnalysisThe samples were studied using optical microscopy at a magnification of x 100.DAS was then measured using the Buehler image analyzing systern and linearintercept method, Radhakrishna [33]. Several different regions on the innersurface of the samples (paraIIel to the thennocouple) were selected to determinethe average of DAS.


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se for metallography

Figure 27. Samples for DAS rneasurement and their locations in the casting.

3.3 Flow Measurement in Different Mold Materials3.3.1 Pattern and Flask MakingThe foam pattern was cut using a hot wire cutter device from a foam block withthe following dimensions: Hl85 x W80 xTlS mm. The gating systern included asprue (H230x W 30 XT 30 mm), a runner (LI0 x W30 xT15 mm) and a pouringcup I# 100 mm (Figure 28). A hot melt adhesive was used to glue the foampattern to the gating system with bottorn pouring condition. A special castingflask was fabricated with a heat resistant glass window in a vertical position(Figure 29).

3.3.2 Coating and VibrationThe same wating and drying procedure in part 1was applied for this experiment,but the side view of the pattern assembly remained without coating. The non-wated side view of the foarn pattern assernbiy was then attached to the heatresistant glass.


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The glass was ptaced inside thewindow, the unboundedsilica sand was pouredinto the fiask,and vibration was applied. Prior to sand filling, the patternassernbiy was taped to the glas to prevent rotation of the pattern duringvibration. The schmatic apparatus is show in Figure 30.

Figure 28. Three views of the foam pattern assernbiy.


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Figure 29. Video photograph of the foam pattern.

Foam Pattern

FlaskSand

GlassWindowf Gas Fumace

Figure 30. Schematic of apparatus used to measurethe velocity of Iiquid metal (top view).

3.3.3 Metal Pouring and Vertical Velocity MeasurementPrior to metal pounng, with the possible shattering of the flask and consequentsafety hazard in mind, a mirror was positioned at a 45" angle in front of the glass


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window. This allowed video photography of the fusion of foam and the metal flowin a vertical position. Video photography of foam casting in a vertical positionwith lowest metallostatic pressure and a negative effect of gravity force on theliquid metal is a novel idea, and such a study has b e n imited to a few instancesin horizontal measurement, Tseng [34]andShivkumar [35].The same melting procedure was cari& out as in section (3.1.5). Precut slabsof the A356 alloy (1.5 Kg) were placed in the crucible. After degassing andfiuxing, the metal (at a temperature of 760C)was poured into the mold while thevideo camera with a chronometer started recording the pouring of the verticalmold.


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4.1 Application of Chvorniov's Rule to LFCIn this section, the ooling curves obtained for A356 alloy in difFerent moldmaterials are explained using the heat ditfusivities. Then, the total solidificationtimes and cooling rates obtained from cooling cuwes are related to the moduli ofthe castings, and the deviations from expected behavior are discussed. TheChvorniov correlations for the three mold materials are then developed, andsuitably madified to take into acount the unique energy balance in LFC.4.1.l ooling Curves inDifferentMold MaterialsThe cooling cuwes for the A356 alloy in three diierent mold materials, silicasand, mulliie and steel shot, are shown in Figures 31,32 and 33. In each figure,the curves for difFerent plate thicknesses (or rnoduti) are presented. Platecastingswith a #in section and a low modulus have relatively short solidificationtimes in al1 mold media. The solidification time increases with an increase inmodulus of the casting. Furthemiore, for the castings with similar thickness ormodulus, the solidification time is maximum for mullite and minimum for steelshot. This is as expected, considering the relative thermal wnductivities of themold materials Poirier [2q,Kubo [36] and Woolfoik C37J.4.1.2 Heat Difb iv i tyThe ability of the mold to absorb heat from the casting (during solidification) isrepresented by heat difiusivii, Campbell [38]. l t is the product of the thermalconductivity (Km), density (hi) and heat capacity (Cm) of the mold material(KmpmCm). lt is inversely proportional to the solidification time of the casting(Equation 11) in Chvorniov's rule [28]. The heat diffusivity of mullite is known tobe less than that of siiica sand, Poirier [27j. Therefore a casting takes a longertime to solidify in mullite Sand than in silica sand. This is clearly demonstrated inal1 the Cumes presented in Figures 31 to 33. However, there is a distinctdeviation frorn the general trend in that relatively longer solidification time is seenwith the Iawest modutus sample in silica sand (Figure 34).


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600 800 1000 1200 1400 1600Time (s)

Figure 32.Cooling curves for A356 alloy in mullite sand mold


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The values for to tal solidification time (differenee in time between the liquidus andthe end of the eutedic plateau) are presented in Table1.4.1.3 CoolingRateTable 1 presents the cooling rates of the A356 alloy in different mold materials,obtained by drawing tangent lines to the cooling curves between the liquidus at610 C and the eutecticplateau at 57 C.The graphical representation of cooling rates and moduli Fable 1) is shown inFigure 35. It shows that for al1 the mold materials, the cooling rates decreasewith increasing casting moduli. The mold with steel shot provides the highestcooling rates for the castings. However, the insulating nature of the mullite andsilica sand cause siower cooling rates in al1 the castings, Poirier[27).Moreover,for al1 the mold media, as the casting modulus decreases from0.98 to 0.42, thecooling rates increase at teast threefold. This will be explained through theeffect of the foam thickness on the cooling rate in section4.1.6.

4.1.4 Chvorniov's Plot(ExperimentalDeterminationof Total SolidificationTime)The classic Chvomiov plot is total solidification time versus the modulus of thecasting, Chvomiov [28]. The total solidification times of the castings with thesame geometries but difFerent sizes (modulus) foliow a general rule with a leastsquare correlation andR' value close to unity. The total solidification times of thecastings, which are tabulated in Table 1, are obtained from cooling curves insilica sand, mullite and steel shot. Figure 36 is a Chvorniov's plot for the LFC indifferent mold materials. The slope of the line for silica mold is parallel to thatobtained with the conventional casting processes. Theseare illustratecl in Figure37, Campbell [39]. However the slope of the mullite line presented in Figure 36 ishighef than those for conventional casting processes. This means with lowercasting modulus, shorter solidification times are obtained in LFC compared toconventional processes.


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Madulus {mm)figure 37. Application of Chvorniov's rule in conventional casting processes [39]( h m J-Campbell, Castin~s,Butterworth-Heinemann Ltd, Oxford, England,l991,P 37).


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The least square correlations of Chvorniov's rule, obtained from Figure 36 indfierent mold materials during LFC, are given below

The above equations are valid for plate shape casting in LFC. They confirm thatLFC follows Chvorniov's rule. Okorafor [30] stated that Chvorniov's rule relatingsolidification time and square of casting modulus is not valid for foarn polystyrenepattern rnolds. However, the data obtained with this research prove thatChvorniov's rule can be applied precisely to predict solidification time in LFC, andthe values of power (n) are between 1 and 2. This is reinforced by theobservations of Tiryakioglu et al [29].The total solidification times in silica sand mold as measured during this studyand Okorafots data [40] are compared in Table 2. It ndicates that an increase inbarn thickness causes a decrease in the cooling rate and an increase in the totalsolidification time. While there are significant differences between coolhg ratesin previous and cumnt data, the total solidification times are almost the same.The data obtained with the cumnt research are considered more reliable,possibly due to an accurate data acquisition system.4.1.5 The Endotherrnic Reaction of the Foam Ablationit is generally accepted, [Shivkumar, 411 that the endothermic reaction assaciatedwith the ablation of foam and the extraction of heat from the advancing Iiquidfront resuits in a decrease of superheat of the liquid metal. Pan and Liao [42]have shown experimentally for the A356 alloy and vertical plate patterns that the


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Table 2. Cornparison of cooling rates and solidification tirnes insilica sand rnoldsFoamThtckness(cm)SolidificationTime (s)[Ohmfor and

~oper)"'

SolidificationTime (s)Pre~ent ataCooling Rate

("Cls)[Obn fo r and~ o p e r ) ~

CoolingRate("Ch)PresentDa&


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loss of superheat is 8.33 C h . This is in agreement with the semi-empiricalprediction of Sun e t al [43].

In the LFC of aluminum alloys, the toss of temperature increases with a decreasein the thickness of the plate pattern and an increase in the distance of flow,Shivkumar [41]. In addition, it is known to increase with the velocity of flow,Tseng [34]. The fluidity of the A356 alloy in LFC is decreased by increasing thecoating thickness and the foam patterndensity, Pan and Liao (421.

4.1.6 The Energy Balance between Liquid Metaland FoarnThe loss of heat in the melt occurs in synchronization with the thermaldegradation of the foam and the mold materials. The thermal degradation of thefoam extracts energy from the liquid metal front. In addition, it has k e nreported, Tseng [Ml, that the heat extraction happens in the narrow zone(characteristic chill zone) located in the liquid metal front. Pan and Liao [42] statethat

Heat loss of melt in the charaderistic chill zone = Themai degradation energy of barn pattern

L c W t , p t c ~ A T = t y l p p HE (17)Where,

Lc = The length of characteristic chill zone, cmW = The width of the pattern, cmt,, = Thickness of the pattern, cmh= ensity of rn o k n metal. gr cmJCL=Specific heat of moken metal, J g 6 OCAT= Degree of superheat, OCLf= Flow length, cmpP= Density of pattem, gr cm'HE=The decornposition energy of EPS pattern, Jgr"


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Equation 17 represents the energy balance for a Iocalized cross-section of foampattem affected by the processof ablation. It has been reported that the lengthof the characteristic chill zone for a vertical pattem is almost equal to 2.2 timesthe pattem thickness, Pan and li a o [42], Replacing Lcby 2.2 tp and rearrangingthe above equation,

Figure 38 is a schematic illustration of a localized cross-section between Iiquidmetal and the foam. While Equation 18 is specific for LFC, it confirms that bydecreasing the thickness of foam pattern, the decrease in melt temperature perunit length (ATRI) is increased.

4.1.7 The Effectof Foam Thickness on Melt TemperatureThe melt temperatures at the themocouple locations are plotted againstthefoam thicknesses in Figure 39. This s h ~ what there is a significant increase inthe cooling rate as foam pattem thickness decreases from 3 cm to 1 cm, asexplained earlier through the energy balance equation.Decrease in melt temperature per unit length was calculated using the energybalance equation for each pattern thickness (Equation 18) and the data in Table3. The relations between decrease in melt temperature andthe foam thicknessis shown in Figure 40. Here, aie corrected initial temperature (Tm) atthemocouple locations was calculated foreach pattem with due consideration tothe loss of melt temperature throughthe gating and the pattern (see Appendix A).The summary of these calculated temperatures is presented in Table4.Modified Chvorniov's Rule for LFCIn this work, Chvomiov's nrle was modified to include the corrected initialtemperature (Tm)n Chvorniov's constant (C). Poirier [271 introduced Chvomiov's


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Figure 38. A schematic of localized zone between liquid metal andthe foam


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1 TotaI SolidificationT i e (s) SeeTable 1 1

Table 4. Corrected Initial temperatures for different foam thicknessesFoam Pattern

No.1No.2No.3No.4N0.5

Foam Thickness(cm)1

1.522.53

lnitial Temperature atThemocoupk location ('C)610630640645650


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constantwith due consideration of the heat transfer coefficient, as in the followingequation:

Here, ps = Density of solid metal, gr cmJHL= Latent heat of moiten metal,J gr-'Tm= Initial temperature, O CTi = Arnbient mold temperature, O C

= Thermal conductiwty of the mold,w ~ " o c - 'pm= Density of the mold, gr cmJCm = Specific heat of the mold, gr""^-'

Utilizing the above equation and equation(9) which is explained in section 2.3.4,with in itial temperatures (Table4) and mold materials data (Table5), parametersC and cp were determined (See Appendix B), and their values are tabulated inTable 6. The correlation between cp and total soIidification time is shown inAppendix B. The values of rp for high rnoduli castings in mullite sand areessentially the same or slightly higher than those with silica sand. However inlow rnoduli castings, cp values for silica sand are higher than mullite. This canbeexplained through the effect of foam in thin sections on total solidification timeand the fact that the heatdiffusivity of m u li i i is less #an that of silica.4.1.9 Calculated Value of the Total Solidification Time inLFCThe calculated values of C based on Poirier's expression, which are presented inTable 6, produce significant differences betvveenthe calculated and experimentalvalues for the total solidification tintes. For exarnple, consider foam pattern No.5and mullite sand. According to the experimental data, the total solidification timeis 790 seconds.


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Table 5. Thermal properties of rnotd materials 27,36

I I IMullite 40 1 0.0050 1 2.047 1 1.104@7OO0C

Materials

Silica Sand

Table6. Chvamiov's constant, Q factor and total solidification tirne for different

*Note: See sedion 2.3.4 (page 23)

Specific HeatCm

J gr - 10~ - 11.138 @700C

ThemalConductivityKm(w m - l O ~ l )

0.0062

- .mold materials

DensityPm

gr cmJ1.539

ToalSolidificationTirne (sec)(Calculated)ToialSolidificationTime (sec)(Expedmental)

MoldMaterials

SiIicaSand

MulliteSand

FoamPattern240 1 125o.1No.2

~ 0 . 3No.4No.5No.1No.2~ 0 . 3

, N0.4N0.5

340445524

C (Chvomiov'sConstant)( s d mFoamThickness(cm)

165204240

6375329304296275387318294281264

11.522.5311.522.53

740 1 267

0.7770.8000.8190.8280.8490.7480.7970.8180.8300.848

170338 84122480580790169211254


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On the other hand, the calculated value of the total solidification time is 254seconds. This means that the method described by Poirier for predicting the totalsolidification time yields values that are about one third to one half of theexperimental values. The reason for this significant variation may be explainedthrough the assumption that the casting has a semi-infinite shape and a constantvalue was assurned for the heat transfer coefficient.

4.1.10 Application of Shape FactorTiryakioglu et al [44] have suggested a new pararneter, the shape factor with aview to improving the correlation between expenmental and calculated values oftotal solidification tirne. They define the shape factor (k) as a ratio of the surfaceof a sphere (which has the same volume as the casting) to the surface area ofthe casting.

k= XD~ IA~ (20)

Where D is the diameter of the sphere with the same volume as the casting, andA, is the surface area of the casting.

t=ckSV (21Here O/) epresents the volume of the casting.

Although shape factor has been successfully applied in conventional casting,firyakioglu [44], there has not been any attempt to apply it in LFC. Parametersthat are defined for the modified Chvomiov's rute are (n] and (P). ln theconventional casting process (n) s presented as 0.67, iryakioglu [Ml;however,there is no published value of (n) or the LFC process.


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4.2 Dendrite Ami Spacing in LFCIn this section, the dendrite a m spacing (DAS) of the A356 alloy, which isrneasurd in different mold rnaterials, is empirically correlated with the localsolidification time during LFC. Further, the DAS is related to the cooling rate indifferent rnold media.4.2.1 Local Solidification Tirne of A356 Alloy in Silica Sand Mold During LFCTypical cooling curves for the A356 alloy for various thicknesses of foarn patterns(i.e., modulus ratios) are shown in Figure41. They show that with an increase inpattern thickness, the cooling rate decreases with an attendant increase in localsolidification time. Further, the local solidification tinte is inversely proportional tothe average caoling rate at a given location (between 610 C to 577 C) duringsolidification, Flemings [45].

4.2.2 DAS of A356 Alloy in Silica Sand Mold During LFCDAS rneasurements were camed out near the location of the thennocouplesusing the linear intercept method, Radhakrishna (331. Typical optical rnicrographsof the alloy cooled in silica sand mold for different casting thicknesses are shownin Figure42. The results of DAS masurementare presented in Table 7.The variation of dendrite am spacing as a fundion of local solidification time fordifferent foarn thicknesses in a silica mold is shown in Figure43. From this plot itcan be obsenred that the value of DAS decreases with a decrease of the foamthickness. Moreover, the value of DAS is decreased from the center of thecasting toward the surface, which is expased ta the mold (Figure 44). This canbe explained through the higher heat extraction on the surface of the casting thatis strongly dependent on the heat diffusivi i of the mold rnaterials. The empiricalcorrelation between DAS and the [ocal solidification time ofA356 can be derivedfor silica sand mold during LFC as follows: -


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Table 7. Measured values of dendrite ami spacing and local solidificationtime insilica sand rnoldMoldMaterial DendriteAmi Spacing(DAS) (Microns)Patterns AverageDAS(Microns)

Sarnples LocalsolidificationTime (s)


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4.2.4 DAS of A356 AlIoy in MuIliteSand Mold During LFCThe microstructures of the A356 alloy solidified in a rnullite mold for differentcasting thicknesses are shown in Figure 46.

They show that the average DAS in a mullite mold is slightly larger than that in asilica mold. Moreover, DAS size increases from the surface of the casting towardthe center. Table 8 dernonstrates the results of DAS measurernent in a mullitemold by the linear intercept method in various locations.

The variation of dendrite a m spacing as a function of locat solidification time fordifferent foam thickness in a mullite mold is shown in Figure 47. The combinationof the foam thickness effect and a slightly low heat difbsivity of the mullite mold,specifically in Pattern No.1, provide a significantly higher cooling rate comparedto the silica mold. Therefore, in Pattern No.1 the average DAS size of the A356alloy in a mullite mold is smaller than in a silica mold; however, in a thick foampattern (No.5) the effect of foam thickness on increasing the cooling ratesignificantly decreases and the casting is solidified more slowly, causing a largeraverage DAS size. The least square correlation between DAS and the localsolidification time ofA356 derived for a mullite mold during LFC isgiven below:

This is the first correlation for a mullite mold during LFC. Moreover, the exponentvalue (n) is in the acceptabk range, and compares well with other processes indifferent mold materials, Gruzleski [15].

ln order to study theMect of mold materials with a high thermal conductivity onsolidification of the A356 alloy during LFC, steel shots (plain carbon steelparticles) with an average diameter of 0.5 mm were used.


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Table 8. Measured values of dendrite a m spacing and local solidification time inmullite sand mold

MulliteroldMaerial AverageDAS(Microns)

Patterns LocalsolidificationTime (s)Samples Dendrite Arm Spacing(DAS) (Microns)


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DAS= 16.215 P.3m1R* =0.9651

FoamThicknesses----.---.---A- No.l= ? cmNo.2 =1.5 cm. - -- A--- No.3 = 2 cmNo.4 = 2 .5 cm

No.5 = 3 cm1 r I I I

60 I I 0 160 210 260 31 0 360Local Solidification Tirne tf (s)

Figure 47. Secondary dendrite a m spacing Vs local solidification time in mullite sand mold


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4.2.5 Local Solidification fim e of A356 Alloy in Steel Shot Mold During LFCTypical cooling curves, which show the local solidification times during thesolidification of the A356 alloy in a steel shot mold, are illustrated in Figure48. Itshows the fastest cooling rate in this study.It has a high thermal conductivity and a high cooling rate, and shows that fromthe thin foam pattern (No.1) to the thick one (No.5) the local solidification timesare increased. Furthemore, the Iowest local solidification time in al1 the moldmaterials that are utilized in this study belongs to Pattern No.1 in the steel shotmold.

4.2.6 DAS of A356 Alloy in Steel Shot Mold During LFCThe microstructures of the A356 alloy, solidified in steel shot mold, are shown inFigure 49. The results of DAS measurement in steel shot rnold by the lineintercept method in various locations are shown in Table9. The average DASsize close to the thermocouple location (center) is larger than that on thesurfaceof casting; however, it is smaller than any measured DAS size in other moldmaterials. The variation of DAS as a function of local solidification time fordifferent foam thicknesses in a steel shot mold is shown in Figur550. The leastsquare correlation behiveen DAS and the local solidification time of A356 canbederived for a steel shot mold during LFC as follows:

DAS = 17.4 t 3'This unique correlation indicates that the steel shot can be a good alternative tosilica sand in order to result in a srnaIl DAS size and improve mechanicalproperties. However, a high dense material like steel shot (five times heavier thansilica sand) may pose a transfemng problern in production lines, and there is nosuction system that can evacuateith m he flask after casting. Although a steelshot mold provides fine DAS s h , t may create misrun in th in sections duringthe casting.


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40 90 140 190 240 290Time (s)

Figure 4%. Cooling curves showing local solidification tirne in steel shotmold


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A Silica sand rnoldMullite sand Mold

rn SteelShot Moid .-

0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65Cooling Rate (Cfs)

Figum 51. Dendrite a mspacing Vs cooling rate indifferent rnold materials


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50 1O0 150 200 ,250 300 350Local SolidificationTime (sec)

Figure 52. Overall correlation between DAS and local solidification time in this study


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Figure54. DAS of A356 alky in verticai orientafion (scanning electronmicmscopy, secandafy elecfrons,20W, x 200)


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4.3 Velocity of Liquid Metal During LFCIn this section, the effect of the foam thickness on the velocity of Iiquid A356 alloyis explained. Further, the back pressure on the liquid rnetal front and the flowlength are ernpin'cally calculated for specified conditions.4.3.1 Empirical Calculation of the Velocity of Liquid Metal in the VerticalMoldTypical photographs illustrating mold filling (Pattern No.2) are shown in Figure55. Inorder to calculate the velocity of the liquid metal, itwas assumed that: 1)During the pouring of the liquid metal, for most part a constant metallostaticpressure is ensured by rnaintaining a constant distance between the crucibk andthe pouring basin. 2) The filling velocity of the liquid rnetal in the pattern iseonsbntThus,

Here,v = The velocity of the liquid metal, cmlsX = Distance (The height of the foam pattern), cmt = Time, s

The calculated velocity for pattern No.2 (1.5cm thickness, vertical filling) is inreasonable agreement with the values calculated under similar conditions byTseng et al [34]. Typically, the calculated velocity for a pattern thickness of 1.5cm (pattern No.2, vertical filling) was 2.46 crnlsec. It has been reported by Tsenget al [34] that the observed vefocity was 4 cmlsec for 1.3 cm pattern thiclcness(horizontal filling). The lower velocity calculated in this research is attributable tothe thickness of the pattern and the effect of gravity. The fiow rate equationbased on the law of continuity, Rao (311c m be written as:


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Table 11.Velocities of liquid A356 alloy fordifferent gate areas

Table 12. Relevantbase data for fluidii~alculation~~

VeIocity (cmls)3.692.461.851.481.23

PatternNo.1, No.2No.3No.4NOS

Density of Liquid metal (pt)Degradation Energy of Foam(HE)Heat Transfer Coefficient (h)Flow Loss Coefficient (6)Melting Point (TM)Ambient Temperature

1 PatternsCross SectionArea 1 1 ~ 8 , 1 . 5 ~ 8 , 2 ~ 8 , 2 . 5 ~ 8 , 3 ~ 8m2 1

Gate area (cm)'34.567.59

2.37 gr/cm31684 J/gr0.042 w/crnZC28.4610 C

20 CVI)Pattern Density (pp)Acceleration of aravitv 0.028 grlun3980m i s L(Ap)Latent Heat of Liquid Meta1(H)Cross Section Area of Sprue 389 Jfgr9 cm


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The velocity increases with effective sprue height and sprue cross section. Themethod of determination of effective sprue height is described elsewhere, Rao(81. The back pressure was calculated using Equation 27 and data in Table 11and 12 (See Appendix D).An increase in the foam pattern thickness results in a decrease in the backpressure (Figure 57). This can be explained through an increased area of crosssection in thicker pattern. However, factors such as coating thickness andmetallostatic pressure influence the back pressure.

The back pressure calculated for 1.5 cm pattern thickness was 2.28 kpa for avertical pattern with bottom filling and coating thickness of 0.5 mm. This value isindeed high relative to published data (Table 13). This can be attributed tothicker coating and lower metallostatic pressure (bottorn gating). Thus, it iscritical to enhance removing the liquid decomposion by-products through thecoating to increase the flow length in the LFC process.4.3.3 Calculation of the Flow Length During LFCFlemings [56j derived a classic fiow length equation for liquid meta! as follows:

PL = Density of liquid metal, gr/cm3v = Velocity of liquid rnetal, crnlsa = Pattern thickness, cmhg = Heat transfer coefficient, w/cm2 OCTM=MeRing temperature of metal, OCTi = Ambient temperature, OC


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Table 13. Comparisoii of back pressure data from different studies

Cunent StudyPan and ~ i a o ~ ~

Yang et al5'

BackPressure

W P ~ )2.281.5

0.2-0.5

PatternThickness

(cm)1.50.73

CoatingThickness

(mm)0.50.40.2

PatternOrientation

VerticalVertical

Horizontal


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CL= Heat capacity of liquid metal, JlgPCAT = Superheat, OCHL= Latent heat of Iiquid metal, Jlgr

The above equation shows that the flow length of liquid metal increases with anincrease in the superheat and velocity; however, it is negatively affected by thernelting temperature and the heat transfer coefficient. In LFC, the decompositionenergy of the polystyrene foam eliminates a part of the heat energy from theliquid metal front. Pan and Liao (551 determined the heat of foam ablation, andthen rnodified Equation 28 as follows:

HE= Decomposition energy o f the foam pattern, Jigrpp= Density of foam pattern. grlcm3

The effective superheat (the temperature of the liquid metal over and above themeiting point) was taken from the maximum points in the cooling curves. Usingthe velocity data in Table I I , base data in Table 12, the efecve superheat inTable 14and Equation 29, the flow length for A356 alloy in LFC was empiricallydetermined; The flow lengths are in Table 14 (See Appendix 1.It is of interest to note that superheat (Table 14) increases from a low value forpattern No.1 in stee l shot mold to a high value for pattern No.5 in mullite sand.This is possibly related to the relative thermal conductivities of the moldmate ials.FIow length increases with an increase in effective superheat (Figure 58). Inmold materials with a high cooling effect (steel shoi), a lower effective superheat


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Table 14. Calculated data for ffuidity (flow length) of A356 alloy indifFerentmoldmaterialsSilica Sand MoldPatternNo.1No.2No.3No.4

Superheat (" C)6090100105

Velocity (cmls)3.692.481.851.48

Flow Length (cm)42.247.949.850.8NOS 110 1.23 51.7Mullite Sand MoldNo.1No.2No.3No.4

508595110130

3.692.461.851.48

40.2946.948.851,71.23 55.5Steel Shot Mold3552607088

3.692.461.851.481.23

37.440.642.244.i47.5


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300 400 500 600Total solidification time (s)

Figure 59.Relationship between flow length and the total so lidification timein different mold materials


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Conclusions1. Chvomiov's rule was successfully adapted for application to LFC of the A356

atloy. The least square correlation between total solidification time andmodulus of casting in silica sand mold followed the conventional castingprosses. However, in case of mullite as a mold material the dope washigher than conventional casting, possibly due to lower value of thermaldiffusivity of the muIlite mold.

2. The influence of mold media on cooling rates was effectively explainedthrough the endothemiceffect offoam ablation and mold properties. Further,the effective superheat was calculated through the energy balance betweenfoam and Iiquid metal. Finally, foam ablation significantly increased thecooling rate particularly in casting with 10 mm thickness in al1 the moldmaterials.

3. The correlation between dendrite am spacing (DAS) and local solidificationtirne has been successfulty derived for theA356 alloy in different mold media.The smallest DAS size was obtained insteel shot motd while the largest wasin rnullite moId. Further, the DAS size dose to the edge of the sarnples isfiner compared to those in ttie center of the casting. Finally, the e f k t of thecooling rate on dendrite ami spacing of the A356 alIoy in different moldmatenats and thicknesses of casng was demonstrated.

4. The measured lower velocity of liquid metal in vertical pattern in mis researchwas atbibuted to pattern tfiickness and the effectof gravity.

5. An increase in foam pattern thickness resulted in a decrease in the backpressure h i l e this can be explained as being due to an increased area ofcross section, factors such as coating thickness and metallostac pressurehave to be considered.


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6. The fluidity (flow length) of the A356 alloy was calculated and mrrelated toaie superheat and total solidification time in dRerent mold materials. Thefluidity of A356 alloy in mullite mold had the maximum value possibly due tohigh effective superheat and total solidification time compared to other moldmaterials.


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Suggestions for future workI. hvorniov's rule can be modified for different shapes of casting (eg., cylinder

and sphere shape) to investigate two dimensional heat transfer condition.2. DifFerent mold materials such as zircon and chromite sand, and different

alloys (hypo or hypereutectic) with various freezing ranges can be goodcandidates for further work.

3. The effects of different foam densities and coating thicknesses on the fluidityof liquid metal can be investigated.

4, The combined effect of chills and mold materials on fluidity, microstructureand DAS for various alloys in LFC can be investigated.


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1. E L Kotzin, Metal Castinci & Moldina Processes, AFS, Des Plaines, lllinois,USA, 1981, pp.149-153.2. W.R.East, " New Option and Opporkinities with Lost Foam CastingnMaterialsEnaineen'nq, ASM, Cleveland, Ohio, USA,Vol. 105, May 1998, pp.63-65.3. S.Shivkumar, "Casting Characteristics of Aluminum Alloy in the EPC ProcessnAFS Transactions, Vol.101,1993, pp.513-5 t 8.4. H.E.Liteton, J.Griffin, D.R.Askeland, B.A. Miller, C.E.Bates and D.S.Shedon'Advanced Lost Foam Casting Technology* Universitv of Alabama,Birmingham, U S A , May 1996, pp.24-25.5. RW -Monroe, Expandable Pattern Casting, AFS, Des Plaines, Illinois, U.S.A,

1992, pp.13-15.6. A.T.Spada, "Core Competency lncludes Lost Foam", Modem Casting, Vol.91, May 2001, pp. 27-30.7. R.W.Monroe, Expandable Pattern Casting AFS, Des Plaines, Illinois, U.S.A,

1992, pp.85-90.8. P.N.Rao, "Manufacturina Technotoav. Foundw, Forniinri and Welding*, TataMcGraw- Hill Publishing Company, New Delhi, 1992, pp.137-14I.9. S.Shivkumar, L.Wang and D.Apelian, ' he Lost Foam Casting Of Aluminurn

Alloy Components"JOM, VoI.42, Nov 1990, pp.38-40.I O . R.W.Monroe, bandable Pattern Casting, AFS, Des Plaines, !Ilinois, U.S.A,1992, pp.96-97.Il..A.Porter and tCE.Esteriing, Phase Transformations in Metals and Allovs,Van Nostrand Reinhold Co-Ltd, United Kingdom, 1981, p.266.12. J Campbell Castinas, Butferworth-Heinemann LM, Oxford, England,1 91, p.143.13.C.R.Brooks, Heat Treatrnent. Structureand Pro~e rties f Non-Femus Allovs,ASM, Met& Park, Ohio 44073,1982, pp.79-82.14.J .Campbell, Casnas, Buttervuorth-Heinemann Ltd, Oxford England,1991,

p. 146


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15. J.E.Gruzleski, Microstructure evek~m ent urinci Metal Castinq, AFS, DesPlaines, Illinois, USA, 2000, pp.112- 113.16.J.Campbell, Castinas, Butterworth-Heinemann Ltd, Oxford, England, 1991

p.265.17.J.E.Gruzleski, B.M.Closset, H. Mulazimoglu and N.Tenekedjiev, ThermalAnalvsis of Strontium-Treated Aluminum Allovs AFS, Des Plaines, Illinois,U.S.A,1995, pp. 41-45.18. S.Avner, Introduction to Phvsical Metallumy, McGraw Hill, Toronto, Canada,1974, p.283.19. J.E.Gruzleski, B.M.Closset, The Treatment of Liquid Aluminum-Silicon Alloys,

AFS, Des Plaines, Illinois, U.S.A, 1990, p.14.20. I.C. Kammer, Aluminum Handbook, Atuminum-Verlag, Marketing &Kommunikation Gmbh, Dusseldorf, Germany, 1999, p 86.21. S.Z.Lu and A.Hellawell, "The Changing Mechanism of the Silicon Morphologyin AI-Si Modified Alloys" Metalluroical Trans A, VoI.l8A, Oct 1987, pp.1721-1733.22.J.E.Gruzleski, B.M.Closset, The Treatment of Liquid Aluminum-Silicon Alloys,

AFS, Des Plaines, Illinois, U.S.A, 1990, pp. 40-41.23.O.Emadi, J.M.Toguri, J.E.GNzleski "Effect of Na and Sr Modification onSurface Tension and Volumetric Shhnkage of A356 Alloy and Their Influenceon Porosity Formation " MetaIluraical Trans A, Vol. 248, Dec 1993, pp.1055-1063.24. D.R.Poirier and E.J.Poirier, Heat Transfer Fundamentals for Metal Casting,TMS, Warrendale, Pennsylvania, USA, 1994, pp.1-6.25.D.R.GaskeIll An Introduction o Transport Phenomena in MaterialEnaineenna, Mcrnillan Publishing Company, Canada, Toronto, 1992, p.402.26.M.C.Flemings, Solidification Processing, McGraw Hill, Toronto, Canada,1974,pp. 10-12.27,D.R.Poirier and E.J.Poirier, HeatTransfer Fundamentals for Metal Castinq,TMS, arrendale, Pennsylvania, USA, 1994, pp.41-45.28.N.Chvorniov, Yheory of Casting SolidificationmGiesserei,Vol. 27, 1040, pp.201208.


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29. M.Tiryakioglu, D. R.Askeland, C.A.Butler, D.A.Gentges and E.Tiryakioglu,"Effectof Superheat on Solidfication Times of Aluminum CastingsnMaterialsSolutions ConferenceB98 n Aluminum Castina Technolo_ciy,Materials Park,Ohio, ASM International 1998,pp. 297-300.30.0.E.0koraforl "Thermal Aspects of Gasifiable Pattern Moulds andSolidification of AI-4.25%Cu-1.03%Si Alloy CastingsnAluminium, No. 62,1986, pp. 110-112.31. P.N.Rao, "Manufaciurina Technoloav. Foundrv, Fonnina and Weldingn, TataMcGraw- Hill Publishing Company, New Delhi, 1992, p.106.32. J.Carnpbeli, Castinas, Butterworth-Heinemann Ltd, Oxford, England,1991, p.76.33. K-Radhakrishna, S.Seshan and M.R.Seshadri, " Dendrite A m Spacing inAluminum Alloyn, AFS Transactions, Vol 88, 1980, pp. 695-702.34. C.H.E.Tseng and D.R.Askeland "Study of the EPC Mold Filling ProcessUsing Metal Velocity and Mass and Energy BalancenAFS Transactions, Vol.100,1992, pp. 519-527.35. S.Shivkumar "Casting Characteristics of Aluminum Alloy in the EPC Process"AFS Transactions, Vol. 101, 1993, pp. 513-51 8.36. K.Kubo and R.D.Pehlke, " Thermal Properties of Molding Sands"Transactions, Vol. 93,1985, pp. 405-414.37. S.Woolfolk," merm al Properties of Mullite 30 and 40" Orton RefractoriesTestina 8 Research Center' Westenrille, OH, 43082, U.S.A, 1999.38. J-Campbell, Castinas, Butterworth-Heinemann Ltd, Oxford, England.1991, p.130.39. JCampbell, Cas tin~s , utterworth-Heinemann Ltd, Oxford, England,l991

p.3740.O.E.Okorafor, C.R.Loper, Jr, "Metallurgical Factors Affecting theMicrostructureof Expendable Polystyrene Pattern Cast AI-4.25Cu-1.03SiAlloy", AFS Trans, Des Plaines, Illinois, U.S,A, Vol 90, 1982, pp.285-295.41. S-Shivkumar, " Modeling of Temperature Losses in Liquid Metal DuringCasting Formation in Expendable Pattern Casting Process "MaterialsScience and Technolociv Vol. 10,1994, pp.986-992.42. E.N.Pan and K.Y.Liao "Study on The Filling Behavior of The EvaporativePatternCasting (EPC) A356 Alloy" CASTEXPO 99, AFS, St Louis, Missouri,

USAMarch 1999, Preprint No: 99-031.


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43. Y.Sun, H.L.Tsai and D.RAsksfand,"lnfluence of Pattern Geometry and OtherProcess Parameters on Mold Filling inthe Aluminum EPC ProssnAFSTransactions, Vol. 103, 1995, pp. 651 662.44. M.Tiryakioglu, D.R.Askeland and E.Tiryakioglu, 'Statistical Investigation ofthe Effects of Shape, Size and Superheat on Solidification Times of Castings"AF

the effect of mold materials on solidification

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