Study on the effect of cooling rate on the solidication parameters,microstructure, and mechanical properties of LM13 alloy using coolingcurve thermal analysis techniqueV.A. Hosseini, S.G. Shabestari, R. GholizadehCenter of Excellence for High Strength Alloys Technology (CEHSAT), School of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST), Narmak 16846,Tehran, Iranarti cle i nfoArticle history:Received 6 December 2012Accepted 27 February 2013Available online 14 March 2013Keywords:Aluminium alloysMicrostructureShear stressThermal analysisSolidicationabstractIn this study, the effect of cooling rate on the microstructure, solidication parameters, and mechanicalproperty of LM13 alloy has been investigated. To obtain different cooling rates,an air-cooled graphitemold, 3 sand molds with different moisture content, a water-cooled graphite mold, and a water-cooledsteel mold were used. The cooling rates and the solidication parameters were determined by using com-puter-aided thermal analysis method. Results show that with increasing cooling rate from 1.1 to 50 C/s,secondary dendrite arm spacing decreases65%. At higher cooling rates,the nucleation temperature ofreactions shifts to higher temperature, except the nal reaction, but the eutectic recalescence underco-olingiseliminated. Itmodieseutecticmicrostructureanddecreasesparalleleutecticlayersdistancefrom23.24to4 lm. Inaddition, itreducedprimarysiliconparticleappearance(PSPA)from1.17to0.3. Theequivalentporositydiameteralsoreducesfrom72.5 lmto27.8 lm. Shearpunchtest(SPT)shows improvement of ultimate shear stress, yield shear stress, and normalized displacement at highercooling rates. Hardness has also been improved about 30 Vickers as a result of increasing cooling rate. 2013 Elsevier Ltd. All rights reserved.1. IntroductionAl-Si alloys have been developed due to their good castabilityproperties like excellent uidity and easy feed-ability, suitable spe-cicmechanical properties andformability, andhighcorrosionresistance. Amongthesealloys, eutecticandhypereutecticalloysarereallyattractivefortheirwearresistancepropertiesandlowthermal expansion. They are converted to the ideal alloys to man-ufacture the automobile parts because of the high wear resistanceproperties of these alloys. Also the addition of Ni causes the forma-tion of high temperature stable intermetallic compounds toenhance elevated temperature [14]. These properties lead to theapplicationof LM13whichis oneof AlSi eutecticalloys as adecent material for automobile pistons [5].Cooling rate is one of the most important variables whichaffects microstructureandmechanical properties of cast alloys[6]. Highcoolingratedecreases grainsize, shrinkage porosity,and causes more uniform distribution of porosity. Also, it modiesprimary and eutectic silicon and decreases the size of them. Segre-gationbetweendendrites andgrainboundaries decreases withincreasing cooling rate. Therefore, the amount of insolubleelements is reduced [1,79]. Hajjari and Divandari [10] have alsoreported that a higher pressure in squeeze casting of 2024 Al alloyhas brought about a higher cooling rate and lower secondary den-drite arm spacing (SDAS). In addition to heat treatment [11] andalloying elements [12], higher cooling rates have strongly affectedthe mechanical properties andmicrostructure of near eutecticAlSi alloys [5,13].Computer aided cooling curve thermal analysis presents usefulinformationaboutthesolidicationlatentheat, fractionofsolidduring solidication, and the amount of different phases [1416].In addition, it can show dendrite coherency point, degree of eutec-tic silicon modication, phase formation, and transformation tem-perature[7]. Solidicationparameterscanbecalculatedbythiscurveatdifferentcoolingconditions. Themetallurgicalreactionsthat have enough latent heat can be detected by the cooling curve[17]. Although some reactions do not release enough latent heat tobe recognized by the cooling curve, the rst derivative of the cool-ing curve gives more details about these reactions [18].Kumruoglu[19]haveshownthatacomplexproductsuchascamshaft shows widerangeof microstructureduetodifferentcooling rates. Consequently, obtaining the solidication path of dif-ferent cooling conditions using computer aided cooling curve ther-mal analysistechniquehelpstopredict themicrostructureandprevent the formation ofcasting defects beforepouring themeltinto the mold. This is a signicant step to improve the quality ofthe casting. Near eutectic AlSi alloy has four characteristics which0261-3069/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.02.088Corresponding author. Tel./fax: +98 21 77240371.E-mail address: Shabestari@iust.ac.ir (S.G. Shabestari).Materials and Design 50 (2013) 714ContentslistsavailableatSciVerseScienceDirectMaterials and Designj our nal homepage: www. el sevi er . com/ l ocat e/ mat desimpact on its microstructure and mechanical properties. These aredendrite arms, eutectic phases, primary silicon particles, and mi-cro-porosity. Wear resistance of this alloy strongly depends on pri-mary silicon particles, and strength of the alloy has a closerelationshipwiththeeutectic microstructure.Inthiswork, com-puter-aided cooling curve thermal analysis has been used to studythe effect of solidication parameters on the microstructural fea-tures and mechanical properties of LM13 piston alloy.2. Experimental procedures2.1. Materials and thermal analysisThe commercial LM13 alloy was used for this study. The compo-sition of this alloy is given in Table 1.An electrical resistance furnace was used to prepare the melt.The melt was held in the 3 kg-capacity graphite crucible at750 5 C. Toprotect themelt fromoxidation, coverall ux11(preheated at 100 C) was used during the melting procedure.DifferenttypesofmoldsandcoolingconditionswereusedtoobtaindifferentcoolingrateswhicharepresentedinTable2. AK-type thermocouple covered with stainless steel sheath was usedforthermal analysis. Thisthermocouplewasattachedtoadataacquisition system and a computer. To record the timetempera-ture data, ADAM-4000 Utility software was installed on the com-puter. Ineachtest, datawererecordedwiththefrequencyof 5readingsper secondandthentransferredtoMATLABsoftware.Adjacent averaging method over 30 points was applied to smooththe thermal analysis curves.Fig. 1 shows the thermocouple position and the thermal analy-sis cup which was used to take the melt sample from the crucible.It should be noted that the thermocouple was calibrated with purealuminum (99.99%) before the tests.The solidication parameters were calculated based on the dataobtained from Fig. 2a and Table 3. Three cooling curves were plot-ted for each mold and the average of the cooling rates and solidi-cation parameters were reported.2.2. Microstructural evaluationAfter solidication and removing the thermocouple, each sam-plewassectionedatthetipof thestainlesssteel thermocouplesheath for metallographic procedure. The samples were mechani-callypolishedandetchedwith0.5%HFfor15 s(ASTM:E003-11and ASTM: E407-7e1). The microscopic observation was performedusing Unimet optical microscope (union 819 model) and TescanVega II scanning electron microscope. Also, Quantitative metallog-raphy was carried out by means of Clemex image analysissoftware.SDAS was calculated for at least 20 measurements for each cool-ing rate. The same approach was used to calculate the parallel eu-tectic silicon spacing. To study the primary silicon particle,roundness, aspect ratio, distance of a silicon particle from the near-est one, and also the diameter of about 50 primary silicon particleswere measured for each cooling rate.2.3. Mechanical testsShearpunchtest(SPT)andVickersmicro-hardnesstesthavebeen carried out to investigate the mechanical properties of LM13.SPThasbeenemployedwhenasmall amountof material isavailableformechanicaltests. Thedetailedprocedureandeffectof different parameters on SPT have been reported in several stud-ies [2022]. In this work, because the mechanical property in theexact tip point of thermocouple was very important, SPT was usedin order to investigate the samples with precise measured coolingrates. In SPT, small specimens with the diameter of 6 mm and thethicknessof 1 mmwereprepared. Thediediameterof SPTwas3.1 mmandthepunchdiameterwas2.9 mmandthespecimenwas sheared at a constant strain rate of 0.1 mm/s. Two shear punchtestswerecarriedout for eachcoolingconditionwhichalmostshowed the same results.Micro-hardness test was also carried out using 100 g load(ASTM: E384-11e1). Holding time was15 s,atleast threemicro-hardness tests were performed for each cooling rate and the aver-age of them was reported.3. Results and discussion3.1. Solidication parametersFig. 2a shows different regions of the cooling curve and its rstderivative. Table3alsopresentssolidicationreactionsthataredetected in Fig 2a. The phase formation sequences during solidi-cation were determined based on the Backeruds work [23].To calculate the cooling rate (CR), slope of the curve at the tem-perature range of 590630 C was measured. Because of the nar-rowmushyzoneinthisalloy, thenearesttemperaturerangetothis zone was considered to calculate the cooling rate.As it is shown in Fig. 2b, all of the curves are drawn from 630 C,except the curve having CR = 50 C/s. This curve is started from eu-tectic temperature. Since the cooling rate is too high and the heat isreleased extremely fast, the initial step of the solidication occurstoo fast to be recorded. In addition, the rst derivative curve cannotTable 1chemical composition of LM13 ingot.Si Cu Ni Mg Fe Mn Al11.20 0.93 0.91 0.97 0.1 0.02 BalancedTable 2Cooling rates for different cooling conditions.No. Mold and cooling condition Cooling rate (C/s)1 Thin air-cooled graphite molda1.12 Low moisture sand mold 2.13 Medium moisture sand mold 3.14 High moisture sand mold 4.25 Thin water-cooled graphite mold 5.66 Thin water-cooled steel mold 50baMerged in the melt to reach the same temperature.bThis cooling rate was calculated by SDAS vs. cooling rate equation.Fig. 1. Mold dimensions and thermocouple position, tsteel mold = 0.9 mm,tgraphitemold = 3 mm.8 V.A. Hosseini et al. / Materials and Design 50 (2013) 714arrestthelatentheatreleasedfromthephaseformationathighcooling rates. Consequently, it can be concluded that cooling curvethermal analysis is not an accurate method for high cooling rates.Therefore, to estimate the cooling rate in this condition, SDAS vs.CR equation is established which will be discussed later.Table 4 illustrates the effect of the cooling rate on solidicationparameters. It is shownthat increasing cooling rate increasesnucleation temperature. Shabestari and Malekan reported the sim-ilarresultfor319alloy[7]. Thesameresultshavealsobeenre-portedforamagnesiumalloy[24] andACAlSi7Cu2alloy[25];whileBackerudet al. reportedaconictingresult [23]. Afewresearchershavediscussedaboutthecauseofthisphenomenon[7,25]. While the cooling rate increases, more nuclei particles arecreated on the wall of the mold. These particles are transferred intothe bulk of the melt because of the convection. At high tempera-ture gradient, the convection is enhanced and more solidied par-ticles are distributed in the melt. It is also reported [26] that theclustering of silicon atoms occurs at a higher temperature beforethenucleationof theprimarysiliconparticlesstarts. Whenthecooling rates and as a result undercooling increase, critical nucleisize decreases which changes embryos to nuclei.AsshowninTable4, eutecticgrowthtemperatureincreaseswithincreasingcoolingrate. Eutecticnucleationhas twomainmechanisms: (1) nucleation on a-dendrites, (2) nucleation on theactive substrates. In addition, in hypereutectic alloys, primary sili-con can also be a suitable substrate. Fig. 3 shows that primary sil-icon particles act as the nucleation site for eutectic silicon. Fig. 4shows the distribution of silicon particles at the different coolingrates. Verynemicrostructurehasbeenformedathighcoolingrates. In Table 4, the number of primary silicon per square millime-ter is presented. The number of primary silicon particles increasesconsiderably as a result of higher cooling rates and it facilitates theeutecticgrowth. Inaddition, ratiooftheareatovolumeofden-drites increases with increasing cooling rate [7]. All of these placesplay as nucleation sites for eutectic reaction. Therefore, similar tograinreneradditions[8], increasingthecoolingrateincreasesenormously the nucleation sites and eutectic temperature.Increasing cooling rate not only shifts the primary phases andmain eutectic temperature to a higher temperature, but also affectson the post eutectic reaction temperatures. Table 4 shows the for-mation temperature of these phases as a function of cooling rate. Itis observed that the temperature of reactions (3) and (4) increaseswith increasing cooling rate, but it has no signicant effect on thenalreaction(reaction(5)). Atthehighercoolingrate, therearemore suitable sites for the nucleation of intermetallic compoundssuch as eutectic silicon particles. Fig. 5 shows intermetallic com-pounds whichhave grownonthe eutectic siliconparticles. Itshouldbenotedthat whenthereactionshappenat theendofsolidication, the effect of cooling rate decreases. This is becauseenoughsuitablenucleationsitesareavailableatthenal stagesof solidication. Table 4 shows the eutectic recalescence underco-oling (TGrowth TMin) vs. cooling rate. The reduction of recalescenceundercooling at the higher cooling rates has also been reported byother researchers [7,14]. Recalescence undercooling is createdwhen the nuclei need driving force to grow. The minimum temper-ature occurs when the released latent heat for nucleation is equalFig. 2. (a) Cooling curve and solidication parameters for LM13 alloy. (b) Cooling curves at different cooling conditions.Table 3Reactions detected from thermal analysis curves [23].No. Reactions1 Solidication of primary a-aluminium dendrite and primary silicon(nucleation temperature)2 Liq. ?Al + Si + Al15(Mn, Fe)3Si2Liq. ?Al + Si + Al9FeSi3 Liq. ?Al + Si + Mg2Si + Al8Mg3FeSi64 Liq. ?Al + Al3NiLiq. + Al3Ni ?Al + A3NiCu5 Solidication of intermetallic h-phase Al2Cu and other phases6 Solidication endTable 4Solidication parameters.CR (C/s)TNucleation(C)TEutectic(C)TReaction1(C)TReaction2(C)TReaction3(C)Solidication range(C)Recalescence undercooling(C)Solidication time(s)1.1 575.1 570.7 536.4 519.5 512.3 76 3.9 3802.1 576.2 570.5 535.9 520 511.2 73 4.3 3953.1 582.3 570.7 537.2 521 515 82 4.2 3464.2 586 571.6 541 522 511 93 0.2 2435.6 588 572.3 NA NA NA 101 0 99V.A. Hosseini et al. / Materials and Design 50 (2013) 714 9to the heat extraction. After minimum temperature, higher amountof heat is releasedcomparedtotheheat extractedduetothegrowth initiation. Thus, the latent heat overcomes the heat extrac-tion and therefore, the temperature increases. At high cooling rate,therearemoreactivenucleationsites. Therefore, theminimumtemperaturedoes not occur intheseconditions, becausethesenucleation sites need lower activation energy for the growth. Con-sequently, the recalescence undercooling is eliminated.Table 4 also shows the effect of cooling rate on the solidicationrange (Tnuc Tend) and the total solidication time. The nucleationtemperature increases with increasing cooling rate. Also, the solid-ication is completed at the lower temperatures at the higher cool-ing rates [23]. These phenomena increase the temperature range ofsolidication. Furthermore, the solidication time is shortened atthe high cooling rates.3.2. Microstructure3.2.1. Secondary dendrite arm spacing (SDAS)Effect of cooling rate on the SDAS is shown in Fig. 6. SDAS de-creases with increasing the cooling rate because of the followingreasons:(1) Increasing the cooling rate increases the constitutionalundercooling. This condition causes the formation of more second-ary dendrite arms. (2) Increasing the cooling rate causes the inter-face of the liquid and solid to move faster [7]. Consequently, ratioof area to volume of dendrite arms should increase in order to facil-itate the heat extraction. (3) Increase of dendrite thickness is be-cause of ripening and coalescence which needs diffusionandtime. But, whencoolingrateincreasesthereisnotenoughtimefor these phenomena.Flemings [27] presented Eq. (1) related to the cooling rate andSDAS. In Eq. (2), a and b are characteristic constants of each alloy.SDAS aCRb1b = 0.30.5Eq. (2) is obtained by tting a power curve to the results of thisstudy.SDAS 50:7CR0:262In Eq. (2), b is not in the same range which has been reported byFlemings. This difference is possibly because of the different meth-odsofcalculationofcoolingrate. AsLM13hasanarrowmushyzone, calculationof thecoolingrateinthisareaisimpractical.Therefore, the slope of the cooling curve above the liquidus (linearFig. 3. Primary silicon particles act as the eutectic nucleation sites.Fig. 4. Microstructure of the primary and eutectic silicon particles at different cooling rates.10 V.A. Hosseini et al. / Materials and Design 50 (2013) 714partofthecoolingcurvebeforenucleation)iscalculatedforthecooling rate. In this case, b is less than that of Flemings equation.The cooling rate reported for the water-cooled steel mold was esti-mated by Eq. (3). SDAS in this sample is equal to 18.7 lm, and thecooling rate is estimated about 50 C/s.3.2.2. Primary silicon particlesTo investigate the suitable eutectic silicon particles distribution,SPA(siliconparticleappearanceindex) wasofferedbyBoostaniand Tahamtan [28]. In this study, a similar approach is taken intoconsideration for primary silicon particles which is named primarysilicon particle appearance index (PSPA). This factor is based on theaspect ratio, equivalent diameter, distance between primary siliconparticles, and roundness of primary silicon particles. Eq. (3) showsthe calculation method of this factor.PSPA a da=L Ra 3whereaistheaspectratioofprimarysiliconparticles, daistheaverageof theirequivalentdiameter(de = (4A/p)0.5), Listhedis-tance between two adjacent primary Si particles, and Ra is the aver-ageof theroundness(R)of siliconparticles(R = 4Ap/p2, pistheperimeter of the particle).As shown in Table 5, increasing the cooling rate decreases theaspectratio ofprimarysiliconparticles. Highaspectratio causesfracture to occur easier [29]. The cause of this phenomenon is thatthetransparencyof themicrostructurefor dislocationreduces;consequently, the slip of dislocation decreases. Thus, the probabil-ity of separation and cracking of second phases increases [28]. Inotherword,thinnerandlonger primarysilicon particlesmayactascracksduetotheincoherentbondoftheirinterfaceswitha-Al. Through theinitial stagesofprimary silicon particles growth,their morphology remains spherical. But it changes to octahedralwhen the diameter becomes more than 15 lm [30]. According tothe simulation done by Peng and Fu-guo [31], the edges of harderparticles in asoft matrix actas stressconcentratorsandfractureoccursfromthosesites. Intherangeof coolingrates between3.1 C/s and50 C/s, aspect ratiodoes not changesignicantly,whileroundnessincreasescontinuouslyandplaysanimportantrole to reduce the deteriorating effects of silicon particles.Another important factor in the PSPA equation is the equivalentdiameter showninTable 5 andshowthat dadecreases withincreasingthecoolingrate. Whilethecoolingrateincreases, thenumber of silicon particles increases, because the nucleation rateincreases. Furthermore, diffusiondecreasesandfeweratomsat-tach to nuclei. Consequently, da decreases due to lack of diffusionand higher amount of silicon particles.Next parameter that affects the PSPA is the distance from a sil-icon particle to its nearest silicon particle (Table 5). When the cool-ing rate increases, the number of siliconparticles per squaremillimeter increases andconsequentlytheaveragedistancetothe nearest silicon particle should decrease.ThevariationofPSPAwithcoolingrateispresentedinFig. 7.PSPAathighcoolingratesislessthanthatoflowcoolingrates.Thus, increasing the cooling rate causes the better distribution ofprimarysiliconparticles. AccordingtoBoostanisresearch[28],improvement of morphological featuresof harderparticlesinaductile matrix enhances mechanical properties.3.2.3. Eutectic microstructureFig. 8showseutecticlayersspacevariationwithcoolingrate.Increasingcoolingrate has a signicant effect onthe eutecticFig. 5. Intermetallic formation on the eutectic silicon (a) optical microscope (b) SEM micrograph.Fig. 6. Effect of the cooling rate on the SDAS.Table 5Shape features of primary silicon particles.CR(C/s)Number (1/mm2)AspectratioDiameter(lm)RoundnessAreafraction (%)Distance(lm)1.1 12 2.8 44.9 0.63 1.5 1712.1 14 1.7 35.6 0.65 1.1 1203.1 20 1.4 31.5 0.74 1.3 1094.2 21 1.3 29.3 0.76 1.2 1035.6 47 1.4 24.5 0.8 0.74 10650 98 1.1 11.3 0.82 0.74 50V.A. Hosseini et al. / Materials and Design 50 (2013) 714 11structure. For instance, eutectic siliconbecomes morelamellar[32]. To investigate the eutectic feature quantitatively, the eutecticlayers spacing was measured using two methods. One of them isthecalculationof thedistancebetweenparallel eutecticsiliconparticles. The second approach is based on following equation:kA A=N0:54In Eq. (4), A is the examined area and N is the number of eutecticsilicon particles present in this area [33]. kll is the parallel eutecticlayers spacing. Fig. 8 shows the distances between the eutectic lay-ers calculated by using these two methods. It is clear that the eu-tecticlayersdistancedecreaseswithincreasingthecoolingrate.Eqs. (5) and(6) showtherelationshipbetweenkAandkllwiththe cooling rate.kA lm 50:4CR0:475R2= 0.94kll lm 24:45CR0:486R2= 0.93Some researchers [3335] haveofferedpower relations be-tweeneutecticlayersspacingandgrowthrateandtemperaturegradient. Powerof thegrowth rate (V) and temperature gradient(G) have been reported 0.5 and 0.33, respectively [33]. These re-sults show a good agreement with the results of this work consid-ering the fact that V G = CR.3.2.4. PorosityThe content of shrinkage porosity between the secondary den-drite arms depends on how these zones are fed. Additionally, thehydrogenatomscontent inthemelt andtheir releasethroughsolidicationareimportantfactorsforformationofgasporosity.If themeltisabletorunintheinterdendriticregions, thesizeand content of porosity decrease. In addition, at high cooling rate,there is not enough time for hydrogen atoms to escape from thesolidlatticeandbecomemolecular. SincetheformationoftheseFig. 7. Effect of the cooling rate on the PSPA factor.Fig. 8. Effect of the cooling rate on the eutectic silicon particles distance.Fig. 9. Shape and size of porosities at different cooling rates.12 V.A. Hosseini et al. / Materials and Design 50 (2013) 714defects occurs at the end of solidication, smaller microstructureimproves their distribution.Fig. 9showsthat thecontent andsizeof theinterdendriticporosities decrease with increasing the cooling rate. With increas-ing the cooling rate, dendrite coherency point (DCP) is transferredto the higher solid fraction in AlSi cast alloys [36]. In other word,at higher cooling rate, more dendrites with smaller size are formed.Smaller dendrites with lower coherency point and extended massfeedingshouldresultinlowershrinkageporositycontentofthecast product. Furthermore, at higher cooling rates, the wall of themold is more decent place for start of solidication, thus, the direc-tional solidication occurs from the wall to the center of the mold.Thissituation enhances feedabilityofinterdendriticregionsanddecreases the porosity percentage. Additionally, interdendritic re-gionsaresmallerathighercoolingratesbecauseof thesmallerSDAS. Fig. 10 shows variation of porosity size and percentage withcooling rate.3.3. Mechanical propertiesTo transform load vs. displacement diagram to shear stress vs.normalized displacement curve, following formulas are used [37].s F=pt Davr7Normalized displacement Displacement=t 8Davr D0 D1=2 9where F is the applied load and t is the thickness of specimen. D0and D1 are die and punch diameters, respectively. Fig. 11 shows typ-ical shear stress curve of SPT. Shear punch test has been widely usedto investigate mechanical properties of an especial place for minia-ture test [37]. In this research, because the mechanical properties ofLM13 close to the tip of the thermocouple are important and pre-paring the tensile test samples from that place was impossible, soSPT was carried out.As shown in Fig 12a, increasing the cooling rate increases yieldshear strength (YSS) and ultimate shear strength (USS). It is causedbyseveral microstructuralfeatures. Firstofall, amorehomoge-nized microstructure is formed by decreasing SDAS and a homog-enousdeformationoccurs. Inaddition, primarysiliconparticleshave better distribution at higher cooling rates. Lower PSPA whichisformedathigher coolingratesaffectsUSSandthenormalizeddisplacement (ND), therefore, the more deformationoccurs athigher cooling rates. Also, increase of cooling rate and lower PSPAimproves the yield shear stress.In LM13 alloy, the eutectic phases have the main constituents ofthe microstructure. Therefore, modication of the eutectic siliconparticles caused by higher cooling rates could be the most impor-tant factor toimprove themechanical properties of the alloy.Decreasingthespacingofthesiliconeutecticlayersleadstotheformation of more barrier against yielding of a-Al. Also, decreasingthe size of eutectic silicon particles decreases the detrimental ef-fect of silicon particle on the initiation of the cracks. Consequently,modicationofthisregioncanimproveYSS, USS, andtheNDatUSS point.One of the important factors that affects the mechanical proper-tiesistheporositycontent. Decreasingtheporositypercentagecauses the higher surface to be sheared in SPT. It means that thepunchshear stressimprovesat higher coolingratesandlowerporosity contents.Micro-hardness is also affected by cooling rate, which shown inFig. 12b. Increasing cooling rate increases the micro-hardness. Be-cause of the reduction in both primary silicon particle spacing andeutectic silicon particle spacing, modication of the microstructurecauses thehigher micro-hardnessofthealloy. Inaddition,it hasbeen reported that at higher cooling rate, more silicon atoms areFig. 10. Effect of the cooling rate on the porosity diameter and area percentage.Fig. 11. Shear stress vs. normalized displacement for LM13 alloy.Fig. 12. (a) Ultimate shear stress, yield shear stress, and normalized displacement at USS point vs. cooling rate. (b) Vickers micro-hardness vs. cooling rate by 100 g force.V.A. Hosseini et al. / Materials and Design 50 (2013) 714 13dissolvedinthealuminum lattice[38]. Consequently, becauseofsolutionhardeningeffect, movementofthedislocationbecomesdifcult andit leadstothehighermicro-hardnessof thealloy.Meanwhile, the renement of the structural features such as pri-marysilicon, a-Al dendrite, andeutecticstructureimprovethehardness values.4. ConclusionsThe effects of the cooling rate on the solidication parameters,microstructure, andmechanical properties of LM13alloywereinvestigatedthroughthermalanalysistechnique. Theresultsaresummarized as follows:1. Increasingthecoolingratefrom1.1 C/sto5.6 C/s, increasesthenucleationtemperatureofLM13alloywithin12.9 Candeliminatestherecalescenceundercooling. Solidicationrangeincreases from74 C to 101.7 C while solidication timedecreasesfrom375 sto99.4 s. Effect of thecoolingrateonthe intermetallic formation temperatures decreases by the pro-gress of solidication. These results show the intensive effect ofcooling conditions on the solidication path of LM13 alloy.2. Increasingthecoolingratefrom1.1 C/sto50 C/s, decreasesSDAS from 50 lm to 18.7 lm, and it follows from the equationSDAS = 50.7CR0.26.3. LowerPSPAcorrespondstothemoresuitabledistributionofprimary silicon particles. Increasing the cooling rate from1.1 C/s to 50 C/s, decreases PSPA from 1.17 to 0.30.4. 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