Effects of applied pressure on microstructure and mechanical propertiesof squeeze cast ductile ironH. Khodaverdizadeh, B. NiroumandDepartment of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iranarti cle i nfoArticle history:Received 3 May 2011Accepted 20 June 2011Available online 26 June 2011Keywords:C. CastingE. MechanicalF. MicrostructureabstractIn this study, the effects of applied pressure during solidication on the microstructure and mechanicalpropertiesofcylindricalshapedductileironcastingswereinvestigated. Magnesiumtreatedcastironmelts were solidied under atmospheric pressure as well as 25, 50 and 75 MPa external pressures. Micro-structure features of the castings were characterized using image analysis, optical microscopy, scanningelectron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) techniques. Tensile proper-ties, toughness and hardness of the castings were also measured. The results showed that average graph-itenodulesize, freegraphitecontentandferritecontentof thecastingsdecreasedandpearliteandeutectic cementite contents increased as the applied pressure was raised from 0 to 75 Mpa. Graphite nod-ule count was rst increased by raising the applied pressure up to 50 MPa and then decreased. The high-est graphite nodule count was obtained at 50 MPa applied pressure. The microstructural changes wereassociated with the improved cooling rate and the expected changes in the corresponding phase diagramof the alloyunder pressure. Theultimatetensilestrength (UTS), yieldpoint strength (0.2%offset)andfracturetoughnessof thecastingswereimprovedwhentheappliedpressurewasraisedfrom0to50 MPa. Further increase of the applied pressure resulted in slight decrease of these properties due tothe formation of more cementite phase in structures as well as reduced graphite nodule count. Hardnessof the castings continuously increased with increasing the applied pressure. 2011 Elsevier Ltd. All rights reserved.1. IntroductionDuctile iron is the most superior member of the cast iron familywith interesting combination of mechanical properties both in theas-cast and heat treated conditions. Althoughdevelopment ofadvanced light and high performance materials such as metal ma-trixcomposites once thought tolimit ductile ironproduction,extensivestudiesovertheyearstofurtherimproveductileironproperties andtodevelopnewcastingpractices andstrategieshaveresulted in slimmer designs which has enabled ductile ironto remain in the competition as a central structural material.Oneofthetechniquesusedinrecentyearsforproductionofadvanced materials with enhanced mechanical properties issqueeze casting process. The method, sometimes known as liquidforging[1], isahybridcastingprocesswhichcombinescastingandforgingadvantages together. Ghomashchi andVikhrov[2]havereviewedsqueezecastingprocessandstatedthat squeezecasting products can have superior mechanical propertiescompared to their conventionally cast counterparts due to sounderinner structure, higher density, ner grain size and more homoge-nousmicrostructure. Thesecharacteristicsareattributedtofourfactorsincluding: (i) improvedheat-transfer betweenthemoldand the casting resulting in higher cooling rates during solidica-tion, (ii) change in the liquidus temperature of the alloy and mod-ication of the corresponding phase diagram, (iii) opportunity forcreation oflargesuddenundercoolingsinthemelt asaresultof(ii), and (iv) reduction of gas and shrinkage porosities formed un-der pressure in the castings [2].Recently, some authors have investigated effect of squeeze cast-ing parameters on the microstructure and mechanical properties ofaluminum alloys [3,4]and magnesiumalloys [7]aswell astheircomposites[5,6,8]. Maleki et al. [3] haveinvestigatedeffectsofsqueezecastingparameterssuchasappliedpressure, melt anddie temperatures on the macrostructure, density and hardness ofLM13alloy. Theyalsohavestudiedeffectsof thesamesqueezecasting parameters on the microstructure of LM13 alloy [4]. Theyshowed that the density of the samples decreased with applicationof a20 MPaexternal pressurebut increasedsteadilyforhigherapplied pressure up to about 106 MPa. As they reported, increasingthe applied pressure resulted in smaller primary a phase grain sizeand reduced secondary dendrite arm spacing (SDAS) and thereforeimproved hardness. Italsomodiedtheeutectic siliconparticles[4]. Adecreaseinthemeltordietemperaturerenderedsimilareffectsonthemacrostructureandhardnessofthesamples[3,4].0261-3069/$ - see front matter 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.matdes.2011.06.040Corresponding author. Tel.: +98 935 2131892; fax: +98 311 3912752.E-mail addresses: H.Khodaverdi@gmail.com(H. Khodaverdizadeh), Behzn@cc.iut.ac.ir (B. Niroumand).Materials and Design 32 (2011) 47474755ContentslistsavailableatScienceDirectMaterials and Designj our nal homepage: www. el sevi er . com/ l ocat e/ mat desZhang et al. [5] have investigatedthe productionof Al-basedhybrid composites reinforced with SiC whiskers and SiC nano par-ticlesbysqueezecasting. Asaresultofthisstudy, theoptimumsqueezing casting parameters, especially inltration pressure wereobtained for an Al-based hybrid composites, with 20 vol.%SiC whis-kers and 0, 2, 5, 7 vol.% SiC nano particles.YongandClegg[7,8] haveinvestigatedtheinuenceof keyprocess variables on zirconium-free and zirconium-containingmagnesium alloy and its metal matrix composite reinforced with14 vol.%Safl bers. Theyreportedthat increasingtheappliedpressure from0.1 to 60 MPa, reduced cell size from127 to21 lm. Thehighestultimatetensilestrengthvalueobtainedforthe alloys were approximately 50% higher than those for materialcast under atmospheric pressure. They also indicated that the opti-mumappliedpressureformagnesiumalloyswas80 MPawhereminimumappliedpressureof 60 MPaisnecessarytoeliminateporosityandappliedpressuresgreaterthan100 MPacauseberclustering and breakage [8].Although Squeeze casting has been intensively investigated forcasting of non-ferrous alloys, very little efforts have been devotedto squeeze casting of ferrous alloys because of their high meltingpoints anddifculties associatedwithsqueezecastingof thesealloysand short mold life. Nevertheless, a few studies can be found in theliterature that investigates the effects of applicationof pressure dur-ing solidication on the structure and properties of cast irons.Yun et al. [9] investigated the effect of pressure on the micro-structure and properties of inoculated cast iron and found atendency for transformation from ne ake graphite to compactedgraphite with increasing the applied pressure during squeeze cast-ing. Novruzov [10] also studied the graphitization phenomenon ingraycastironunderpressure. Hereportedthatrosette(twisted)ake and point graphite was formed in gravity castings and pseu-do-eutectic and ne-ake graphite was formed in squeeze castings.Besides, he found that pressure considerably discouraged thegraphite formation and promoted the formation of cementite andneedletypeledeburiteinthestructure. Rajagopal [11] reportedthe results of an experimental program for squeeze casting of mor-tar shells from ductile iron. He stated that very limited noduliza-tion of graphite but considerable formation of platelets ofcementite occurred in the as-squeeze cast microstructures. How-ever after annealing, themicrostructuresshowedlargenumberofgraphitenodulesinamatrixofferriteorferriteandpearlite.MorerecentlyJohansson[12] comparedthemicrostructureandmechanical properties of two austempered ductile iron (ADI) com-ponents, i.e. a ring gear and a suspension fork, produced by gravitysandcasting, diecastingandsqueezecastingprocesses. Here-portedmuchner andbetter distributedgraphitenodules, nomicrostructural defects, superior surface quality andimprovedmachiningandmechanical propertiesintheoptimizedsqueezecast components.Increased cooling rate seems to be one of the most inuentialfactors affecting the squeeze castings properties. Despite the lackof information on the effects of squeeze casting on the microstruc-tureandpropertiesofductileiron, effectsofcoolingrateonthestructure and properties of ductile iron has been extensively stud-ied [13,14]. Studies show that increasing the cooling rate up to acritical valuehasaobviousreningeffect upongrainsizeandgraphite nodule size andnodule count, and, consequently, onimproving mechanical properties as ultimate tensile strength(UTS), impact resistance and hardness [13,14]. On the other hand,if thecoolingrateisincreasedbeyondacritical value, graphitenucleationtendstobesuppressedandcarbideformationispro-motedevenwithouttheassistanceofcarbideformingelements.Ceccarelli et al. [13] have reported that although the presence ofcarbidesinthemicrostructurecanbeadvantageouswhenwearresistance is of the main concern; it can be detrimental for proper-ties such as toughness. Therefore, the main challenge for develop-ment of squeeze cast ductile irons withenhancedmechanicalproperties seems to be the ability to balance the structural rene-ment and carbide formation.The lack of information on squeeze casting of ductile iron pro-moted the authors to investigate the effects of solidication underpressure on the microstructure and mechanical properties of thisimportant structural material. This paper reports the relationshipsbetween the applied pressure during solidication and the micro-structural characteristics and mechanical properties of cylindricalshaped ductile iron castings.Table 1Chemical compositionof thealloyusedinthisstudybeforeandafternodulizingtreatment.Element (wt.%) C Si Mn Mg P S FeBefore nodulizing 3.65 2.24 0.46 0.005 0.01 0.03 BalanceAfter nodulizing 3.62 2.83 0.46 0.05 0.01 0.01 BalanceFig. 1. Schematic illustrations of Metallic die designated for squeeze casting process.4748 H. Khodaverdizadeh, B. Niroumand/ Materials and Design 32 (2011) 474747552. Experimental procedure2.1. Casting procedureTable 1 shows the chemical composition of the alloy used in thisstudy before and after nodulizing treatment. The carbon equivalentof about 4.4 was chosen for the starting alloy after some prelimin-ary trails to avoid excessive formation of carbides on the surface ofthecastings. Inthesetrialsthesiliconcontentof thealloywasgradually increased by ferrosilicon (75%Si) addition to the melt be-fore magnesium treatment and the ease of cutting and machiningof the castings after squeeze casting were taken note of.Afterward in each experiment the required quantity (about 5 kg)of the chosen alloy was melted in a laboratory scale gas red cruci-blefurnaceandspheroidizedbyadditionof 3 wt.%ferro-siliconmagnesium alloy (FeSi5 wt.%Mg) at 1400 C using a closed lid la-dle method. The melt temperature was checked by a S-type ther-mocoupleandapyrometer. Subsequently, themeltwasquicklyskimmed and poured into a preheated cylindrical die made of heattreated H13 steel schematic illustration of which is shown in Fig. 1.The die and the punch were coated with an Al2O3-basedceramic coating and preheated to 400 C to increase their life anddiscourage rapid solidication of the melt. Samples were cast un-deratmosphericpressureaswellas25, 50and75 MPaexternalpressures applied by a preheated punch attached to the upper pla-ten of a 100 tonnes hydraulic press. The applied pressure was keptuntil theendof solidication. Thecastingswerethenremovedfrom the die and cut to pieces as shown in Fig. 2a for investigationof their microstructure and mechanical properties.2.2. Microstructural examinationCharacterization of graphite nodules in the matrix were carriedout on polished surfaces of the slices cut across the diameter of thecastings using standard metallographic techniques and Clemex im-age analysis software [15]. Samples were then etched with 2%Nitalsolution(10 s toreveal ferrite andpearliteand60 s torevealcementite) and microstructures of their matrices were character-ized. For characterization of graphite nodules, a few images includ-ing at least 2700 nodules, taken randomly at given distances fromthecastings wall (10, 25and40 mm), wereanalyzedandthegraphitecontent, graphitenodulecountandaverageequivalentsphere diameter of graphite nodules as well as their size distribu-tion were measured. The nodule count was dened as the numberof graphite nodules per a specied unit of area (square millimeterinpresent research). Equivalent spherediameter (DEq) of eachgraphite nodule was calculated according to Eq. (1) [15].DEq 1:2247 4Apr1where A is graphite nodule area in lm2. Energy dispersive X-rayspectroscopy (EDS) technique was also used to identify small par-ticles observed in the microstructures.Fig. 2. (a) Locations of metallographic and tensile and impact test specimens on the castings, (b) dimensions of tensile test specimens, and (c) dimensions of impact testspecimens.H. Khodaverdizadeh, B. Niroumand/ Materials and Design 32 (2011) 47474755 47492.3. Mechanical testsTensile test specimens (Fig. 2b) were prepared in conformancewithASTME8standard[16] andtensiontest werecarriedoutusing a Hounseld tensile tester machine. Impact test specimens(Fig. 2c) were prepared according to ASTM E23 standard [17] andimpact tests were done using of a 150 J Wolpert pendulum appara-tus. Fractured surfaces of the impact test specimens were studiedusing a PHILIPS XL-30 scanning electron microscope (SEM). Vickershardness was also measured at given distances from the wall of thecastings.3. Results and discussion3.1. Microstructural characterization3.1.1. Microstructural characteristics of graphite nodulesFig. 3 shows micrographs of non-etched surfaces of the castingssolidiedunder different appliedpressures at 25 mmdistancefromthecastingswall. Sizedistributionsof graphitenodulesatthese locations are also shown in the images. As it can be seen fromthe gure, almost all the graphite particles present in the castingshavespherical shape, andwithincreasingtheappliedpressurefrom 0 to 75 MPa, the average size of graphite nodules decreases.Riposan et al. [18] have recently dened a three-stage model fornucleation of graphite in grey cast iron which involves nucleationof graphiteoncomplexsuldeswithdiameterslessthat 5 lmwhich themselves had nucleated on small oxides with diametersless than 2 lm. Observation of a large number of graphite-like par-ticles with diameters as small as 1 lm in the microstructures of thecastings, promptedtheauthorsthatsomeoftheseparticlesmaynot be indeed graphite particles and might be just some surface de-fectsformedduringsamplepreparation. Thiscangreatlydistortthe image analysis results. Therefore, in order to clear the identityof these small particles, Energy dispersive X-ray spectroscopy(EDS) analysis was performed on some of the particles, examplesof whichare shown in Fig. 4. EDS results showed that particle A(DEq = 4 lm)andparticleB(DEq = 2.8 lm)wereindeedgraphite,but particle C (DEq = 3.6 lm) and particle D (DEq = 0.7 lm), whichwould appear as graphite in optical microscopy examination, weresurface defects or other kinds of imperfections.Based on the results EDS analysis, it was noticed that many ofthe graphite-like particles with diameters of less than about3 lmwerenot indeedgraphite. Therefore, inorder toincreasethe certainty of the measurements, it was decided to leave the par-ticles with DEq of equal or less than 3 lm out of the analysis. Thequantitative results presented in Fig. 3 and other gures are basedon this perception.Fig. 5a shows the variation of the average graphite nodule diam-eter with the applied pressure at three different distances from thecastings wall. It reveals that the average nodule diameter has de-creasedcontinuouslyinallofthreedistances(forexamplefrom11.7 lmto9.3 lmat25 mmdistancefromcastingswall)whenthe applied pressure was increased from 0 to 75 MPa. The percent-age of ne graphite nodules which have average diameter less than10 lm is also varied with applied pressure. Fig. 5b shows variationof these small nodules percentage versus appliedpressures atthree different distances fromcastings wall. As can be seen,increasingof appliedpressureresultsinaconsiderableincreaseinthe percentageof nenodules (for examplefrom43.1%to61.1%at 25 mmdistancefromcastings wall).This is inconfor-mance with size distribution histograms in Fig. 3. Fig. 5c demon-strates the effect of applied pressure on the free graphite contentofthemicrostructure. Asshown, byincreasingtheappliedpres-sure, the free graphite content of the microstructure has decreasedat all three distances (for example from 9.3% to 5.9% at 25 mm dis-tance from castings wall). Fig. 5d shows the effect of the appliedpressure on the graphite nodule count at three different distancesFig. 3. Micrographs of non-etched surfaces of the castings solidied under different applied pressures of: (a) 0, (b) 25, (c) 50, and (d) 75 MPa at 25 mm distance from thecastings wall.4750 H. Khodaverdizadeh, B. Niroumand/ Materials and Design 32 (2011) 47474755fromcastings wall. As shown, Increasing applied pressure from0 to50 MPa, the nodule count increases. But it was followed by a de-crease when the applied pressure was further increased to 75 MPa.Some important factors must be considered when studying theeffect of the applied pressure on the microstructural characteristicsofgraphitenodulesformedinthemicrostructureofductileiron.Different studieshaveshownthat applicationofpressureonthemelt duringsolidicationreduces themetal-moldair gapandtherefore increases the heat transfer coefcient between castingsandmold[2,19]. Increasedcoolingrateduringsolidicationandthe ensuing higher undercooling of the melt is expected to resultin higher nucleation rate at the initial stages of solidication andconsequently ner but more abundant graphite nodules in accor-dancewithFig. 5aandb. Thisisinagreementwiththendingsof Ceccarelli et al. [13] on chill casting of austempered ductile ironwho reported that nucleation conditions were completely altereddue to high undercooling and that more eutectic cell appeared inthe structure.Another effect of the applied pressure is the changes it bringsaboutintheliquidustemperatureandthecorrespondingphasediagram of the alloy. The nal carbon equivalent of the alloy usedin this study is about 4.4% which indicates a nearly eutectic alloy.FeC phase diagram shows an equilibrium (austenite plus graph-ite)andanon-equilibrium(austeniteplusironcarbide)eutectictransformation at temperatures of 1154 and 1148 C, respectively[20]. ClausiusClapeyron equation [2] reveals that during solidi-cation under pressure,the system favors earlier formation of thephasesthathavelowerspecicvolumes(higherdensities)thanthe melt and hinders the formation of the phases that have higherspecic volumes (lower densities) than the melt. In other words,the melting point of the former increases while that of the latterdecreases. From this account one would expect that, upon applica-tion of pressure on an eutectic cast iron alloy, the non-equilibriumeutectictransformationtostartatahighertemperatureandtheequilibrium eutectic transformation to occur at a lower tempera-ture. This wouldresult indecreasingthetemperatureintervalbetweentheequilibriumandnon-equilibriumeutectictransfor-mations. Decreasing the temperature interval between the equilib-riumand non-equilibriumeutectic transformation has beenevidentlyshowntofacilitatecarbideformationanddiscouragegraphite formation during solidication [20]. Based on the aboveaccounts, therefore, decreaseinthefreegraphitecontentof themicrostructure with increase in the applied pressure, as shown inFig. 5c, is expected.The behavior presented in Fig. 5d is the results of two opposingeffects induced by increasing the cooling rate as the applied pres-sure increases. For given free graphite content, the graphite nodulecount is expected to increase when the nodule diameter decreasesat higher applied pressures (Fig. 5a). However, as Fig. 5c shows, thefree graphite content does not remain constant and falls when theapplied pressure is increased. The delicate balance between thesetwoeffectswill determineifthegraphitenodulecountisgoingtoincreaseordecreasewiththechangeintheappliedpressure.It seems that at applied pressureshigher than 50 MPa, the effectof reduction in the free graphite content has overcome the effectofreductionintheaveragegraphitenodulediameter. Thereforethe highest graphite nodule count of 1582 mm2was achieved atabout 50 MPa applied pressure.3.1.2. Microstructural characteristics of the matrixFig. 6 shows the etched microstructures of the castings solidi-edunder different appliedpressures at 25 mmdistancefromthe castings wall. The microstructural features include ferrite,pearlite and cementite whose corresponding percentages in differ-ent castings are shown in Fig. 7. Since the cementite phase is neg-ligibleatpercentage(between0%and2.5%dependsonappliedpressure) therefore, it is not clearly distinguishable at lower mag-nication. Fig. 8showscementitephaseandeutecticledeburitestructure in higher magnication.As shown, the percentage of ferrite phase decreases from about67% for the sample cast under atmospheric pressure to about 53%for the sample cast under 75 MPa applied pressure. Conversely, thepearliteandcementitecontent of themicrostructures increasefromabout24%and0%toabout38%and2.8%, respectively, astheappliedpressureisincreased from0to75 MPa. Thechangesinthemicrostructuresarealsotheresultsofhighercoolingrateandfacilitationof non-equilibriumeutectictransformationdueto thedecrease in the temperature interval between theequilib-rium and non-equilibrium eutectic transformations when the ap-plied pressure is increased, as explained before.3.2. Characterization of mechanical properties3.2.1. Tensile properties and toughnessFig. 9 shows the effects of applied pressure on the mechanicalproperties of different castings. The gure reveals that as the ap-pliedpressureincreasesfrom0to50 MPa, theultimatetensilestrength, toughness andelongationof thesqueezecastings in-creasefrom559 MPa, 3.9 J and1.6%to620 MPa, 5.4 J and4.6%,respectively, buttheyieldstrength(0.2%offset)remainsalmostunchanged at about 475 to 480 MPa. Further increase in theappliedpressureto75 MParesultsinadecreaseintheultimatetensile strength, toughness and elongation of the castings to589 MPa, 4 Jand0.7%, respectively. Italsoresultsinanincreasein the yield strength (0.2% offset) to 500 MPa.Fig. 4. Results of EDS analysis on some of the particles resembling free graphite inoptical microscopy.H. Khodaverdizadeh, B. Niroumand/ Materials and Design 32 (2011) 47474755 4751Fig. 5. Effect of applied pressure on (a) average graphite nodule diameter, (b) percentage of ne graphite nodules (DEq < 10 lm), (c) free graphite content, and (d) graphitenodule count.Fig. 6. Micrographs of etched surfaces of the castings solidied under different applied pressures of: (a) 0, (b) 25, (c) 50 and (d) 75 MPa at at 25 mm distance from the castingswall.4752 H. Khodaverdizadeh, B. Niroumand/ Materials and Design 32 (2011) 47474755Since the load bearing properties of graphite particles are verypoor compared to those of the matrix alloy, they are normally re-garded as defects or porosity frommechanical point of view. Gleiter[21] has suggested an equation (Eq. (2)) that denes nal strengthchange of materials based on matrix and particles strength.DrK5:2G1=2b2rprm 2where rp and rm are respectively strength of particle and matrix, Gisshear modulusof thematrixinMPa, bisdislocationsburgervectorandKisaconstantcoefcient. KnowingthatrGrislowerthan rm, it can be seen from Eq. (2) that the graphite particles nor-mallyhave a decreasing effect onmaterial strength. Therefore,properties improvement is anticipated when the size and the quan-tityof thegraphitenodulesaredecreased. Althoughtheconfor-manceoftheobtainedexperimentaldatawiththestatedtheory,theydeviatefromthistrendat75 MPaappliedpressure(Fig. 9).The ultimate tensile strength, toughness and elongation of the cast-ings were dropped at 75 MPa applied pressure. This behavior can beattributed to fracture micromechanismof ductile iron. Someauthorshaveinvestigatedductileironfracturemicromechanismsand have shown that graphitematrix interfaces play an importantrole in ductile iron fracture [22,23]. Therefore, calculation of graph-itematrix interfaces area can be useful for explaining the proper-tiesvariationwithappliedpressure. Assuminganideal sphericalgraphitenodule, nodulessurfacearea(NSA) per unit volumeofthe material is derived from the following equation:AGr 3VGrr3AGrVt3VGrrVt3VGrVtr3frwhere Vt is total volume of the material, AGr and VGr respectively aresurface area and volume of a spherical graphite nodule, r is radius ofgraphite nodule and f is volume fraction of graphite. Fig. 10 showsnodulessurfacearea(NSA)perunitvolumevariationsversusap-plied pressure at 25 mm distance from castings wall. As can be seen,increasing applied pressure from 0 to 50 MPa, NSA per unit volumedecreases from51 103to 42 103lm1(equals to 17.6%Fig. 7. Effect of applied pressure on ferrite, pearlite and cementite contents of themicrostructures at 25 mm distance from castings wall.Fig. 8. Enlarged optical microstructure of castings solidied under 75 MPa pressureat 25 mm distance from castings wall: (a) graphite, pearlite and cementite phase,and (b) ledeburite structure.Fig. 9. Effects of applied pressure on mechanical properties of the castings.Fig. 10. NSA per unit volume variation versus applied pressure at 25 mm distancefrom the castings wall.H. Khodaverdizadeh, B. Niroumand/ Materials and Design 32 (2011) 47474755 4753reduction). Further increasing applied pressure to 75 MPa, the NSAper unit volume increases from 42 103to 44 103lm1. Thismeans that increasing the applied pressure up to 50 MPa, reducesgraphitematrix interfaces area about 17.6%. As a result, mechanicalproperties improve up to 50 MPa due to retardation of micro cracksinitiationandpropagation. Furtherincreasingofappliedpressureresultsingraphitematrixinterfacesincrementandsubsequentlyeasier micro crack initiation and propagation; therefore, mechanicalproperties were dropped.Comparison of Figs. 5 and 9 suggests that the collective effectsof graphite nodule diameter as well as those of free graphite con-tentonthemechanicalpropertiesofsqueezecastductileironisbest presented by graphite nodule count. Although graphite nodulecount rst increases with the applied pressure, it falls above a crit-ical pressure, where a signicant amount of eutectic cementite isformedduetotheincreasedcoolingrate. Resultsof thisstudyshowed that the maximum mechanical properties were obtainedattheapplied pressurewhere thehighest graphitenodule countoccurred.3.2.2. SEM fractographyFig. 11 shows the SEM fractographs of the impact test samplesfor different applied pressures. The cleavage type and brittle natureof the fracture in the ferrite phase surrounding the graphite nod-ulesareclearlyseeninFig. 11aforthecastingssolidiedunderatmospheric pressure. With increasing the applied pressure, somedimples appear in the fractured surfaces pointing to improved duc-tility of the castings. This is related to a decrease in the graphitenodulediameter andfreegraphitecontent andresultant lowerNSA per unit volume of alloy. The highest contribution of ductilefracture was observedinthe castings solidiedunder 50 MPa(Fig. 11c). At 75 MPa applied pressure (Fig. 11d) the cleavage frac-ture appearstobethedominant fracture mode again becauseofformation of higher amount of graphitematrix surface as well ashigher amount of cementite phase.3.2.3. HardnessFig. 12showstheVickershardnessvaluesof thecastingsat25 mm distance from the walls. As shown, the hardness increaseswiththeappliedpressure. Hardness is mainlyaffectedbythemicrostructure of the matrix of the castings. The effect of appliedpressureonthemicrostructureof thematrices of thecastingswas shown in Fig. 7. Decreased ferrite content and increased pearl-ite andeutectic cementite contents of the matrices, at higherapplied pressures, would result in higher hardness of the castings.4. ConclusionsIn this study, effects of applied pressure during solidication onthemicrostructureandmechanical propertiesof asqueezecastductile iron were investigated. The results showed that the averagegraphite nodule size, the free graphite content and the ferrite con-tent of the microstructures of the castings decreased as the appliedpressurewasincreased. Ontheotherhand, thepearliteandtheFig. 11. SEM fractographs of the impact test samples for different applied pressures of: (a) 0, (b) 25, (c) 50, and (d) 75 MPa.Fig. 12. Effects of applied pressure on Vickers hardness values of the castings.4754 H. Khodaverdizadeh, B. Niroumand/ Materials and Design 32 (2011) 47474755eutectic cementite content of the microstructures increased as theapplied pressure was increased. The highest graphite nodule countwas obtainedat 50 MPa appliedpressure. The microstructuralchanges were associated with theimproved cooling rate and theexpected changes in the corresponding phase diagram of the alloyunder pressure. The best combination of the mechanical propertieswas achieved at 50 MPa applied pressure. Further increase in theapplied pressure, resulted in increased cementite content and de-creasedgraphitenodulecountwhichresultedinlowerultimatetensile strength, fracture toughness and elongation of the castings.Mechanical properties trend was explained graphitematrix inter-face debounding micromechanism. Hardness of the castings con-tinuously increased with increasing the applied pressure.References[1] Murali S, YongMS. Liquidforgingof thinAlSi structures. J MaterProcessTechnol 2010;210:127681.[2] Ghomashchi MR, VikhrovA. 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