Ally Porocity

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    T H E E F F E C T OF ALLZ1I \ I LM C O U T E h T A N D G R A I N R E F I N E M E N TOU P OR OS IT Y F OR MAT IOh I\ Mg -AI AL L O YS

    Paul L. Schaffer, You ng C Lee and Arne K . Dahle

    CRC for Cast Metals Manufacturing (CAST)Department of hlining, Minerals and Materials EngineeringThe Lnikersity of QueenslandBrisbane Qld 4072, Australia

    AbstractPorosity is detrimental to the mechanical properties surfacefinish and pressure tightness of castings and i t is therefoieiiiiportdnt to understand the niechanisnis that contiol porosityformation 4 significant amount of research has been perfoiniedon the relationship betueen composition and porosity formationi n aluniinium alloys, hoMe\er little mark has been performed onmagnesium-based alloys Size and morpho logy of priniar)phase and eutectic, permeability and solidification range areinfluenced b> alloy composition and grain refinement and theirimpact on porosity forination has been studied in the present\\ark Castings of \ar)ing aluniinium content from puremagnes ium to Mg-330/0ut A l dllo) uere produced and thesample density Mas anall sed using Archimedes principle todeteriiiinc the effect of alloyin g content on porosit? formation1he porosity location dnd morphology \\a s then characterised byoptical iiiictoscopy Alloys \s ith high porosit) lel els \+ere thengrain refined by the carbon inoculation method to inhestigate theetfect of grain s i x on porosi ty

    IntroductionThe use of niagiicsiuni in commercial applications has increaseddramatically in recent years, and growth has been projected tobe around 12% per annuni for the next decade [ I ] . he increaseddeiiiand can be attributed to magnesium's excellent s trength to\ \e ight ratio. making i t particularly attractive for automoti1.e~ipplications. lagnesium components are currently mainly

    produced by casting, and high pressure die casting is thedomina nt casting method Other casting methods such aspermanent mould sand and lo\\-pressure di e casting are onl?used to a limited extent but are likely to bec ome m ore importantThe increased use of magnesium com ponents has brought nit h i tthe need for higher integrity castings, defect minimisation andmicrostructure optimisation Porosity is undesirable inmagnes ium alloy castings as i t leads to reduced tensile strength,elongation and fatigue life [2 ,3 ] In order to meet the deniand forhigh quality castings, a detailed understanding of porosityformation in magnesiuni alloys and the contributing parametersinust be realised in order to de\elop prehentice measures andoptimise the performance and properties of cast niagnesiumcomponentsGencrally, t\vo main factors control the tendency of a casting toform porosity upon solidification [4 ] The firs t factor I S th eamount of dissol\ed g as particularly hydrogen, in the melt\shich is segregated to the liquid and subsequently nucleates gaspores upon solidification A considerable aniount of Mark ha sbeen conducted to clarify the nucleation of pores onsolidification o f various aluminiuni alloys I t has been reportedthat heterogeneous nucleation of the pores on inclusions such a soxide films [ 5 ] as \\ell as Larious particles present in the melt [6 ]plays a dominant role as a mechanism for porosity formationThis mechanism is s trongly influenced bq the amount o fturbulence during mould filling, Mhich can also lead to entrainedair pochets in the casting

    Ma g n e siu m T e c h n o lo g 2001Edited b> J Hr!nTM C (The Minerals. Metals Br Materials Societ)). 200187

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    Thc second contributing factor to poi-osity formation in a castingis solidification shrinkage which is relatcd to the volumetricchaiigc on transformation froin liquid to solid. This is importantbecause i t initiates 3 hydrostatic pressure gradicnt within thecasting that subsequently drives the fccding of liquid through themushy zone during solidification.On f rew ing , most alloys (except for skin freezing alloys such ascutcctic alloqs) undergo a stage wher e both solid and liquidcoexis t (mush) . A s solidification progresses. the fraction solidreaches a point \ \ he r e equiaxed dendrites start to impinge on oneanother, ie . the dendrite coherency point [ 7 ] .Recent work hassho\\n that the dendrites arc s till free to mo\e relative to eachothcr and first interlock at a point later in the solidificationpi-occss callcd the niaxiiiium packing fraction \\here a rigiddendritic network is formed [S]. After niaximuni packing,fccding becomes difficult, since interdcndritic feeding bcconiesthe dominati ng feeding mechanism and the reduced permeabilityof the niushy Lone restricts efficient flow. The potential forporosity formation is high during this stage because th eiiiterdendritic pernicability decreases with increasing solidfraction as the feeding channels constrict.Campbel l [4 ] suniniarised the fiw characteristic feedingiiicchanisnis that can occur during the solidification of alloys.Figure I . .4tiiOng these mechani sms, liquid and mass feeding arerelati\rly unconiplicated because of the lo\v \ . iscosity and wideactile feeding path. These ar e the major feeding nicchanisnis upto the point of maxiniuiii packing, mass feeding increasingrclatiw to liquid feeding with increasing solid fraction. Beyondpacking, the s i ~ c nd tortuosity of the interdendritic channels,the presence and fraction o f eutectic , thc rigidity of thc dendritenet\iorh and its pcriiicability all affect the feeding efficiency andthe dr i \ i ng force for poros i ty formation .Clelt treatment, particularly grain refincnicnt, is being practicedto improx niechanical properties , not only through productiono f a f in er p a in s i /e , but also through niininiisation of pore sizeand alteration of pore distribution [9]. Despite a considerableamount of research to inLestigate the effect of grain refinementon casting dcfccts [9- I I ] . its effects on the forniation anddistribution of porosity is yet to be clarified. It has becn reportedthat grain refinement in AI-Si alloys produced castings with afiner pore s i x , \vcll dis tributed in the casting [9].It has becn sh o\+ n that grain refinement shifts both coherencyand maxiiiiuni packing to higher solid fractions [7,S].Considering the cffccti\e feeding ability of liquid and massfccding, i t is therefore generally expected that grain refinementshould incrcase the soundness of th e castings by extending theranpe of liquid and mass feeding. Howc\er, the impact of grainrefinement on interdcndritic feeding niay be more critical for theformation ofp oro sity , nhi ch niay be the reason ivhy there is s tilla deba te on the rolc o f ga in re f inement i n porosity forniation.Although the essential mechanisms of porosity formation aresiiiiilar in magnesium alloys as in other alloys, such asa luminium allo)s , there are also significant differences. Thethermal properties of niapncsiuni are significantly different fromaluminium. and magnesium alloqs generally display a niuchf in er gr ai n s i ~ ehan alu minium for identical casting conditions.Hydrogcn is thc main gas contributing to porosity in bothiiictals. but the difference in hqdrogeii solubility between solidand liquid is not as large in magnesium as in aluminium. Feustudies h a x been conducted to characterise porosit) foriiiation

    in magnesium alloys. Gruzleski et al. [12] reported that a smalladdition of s trontium to A291 alloy s ignificantly reduced th etendency of porosity formation and caused a finer grain size, bu ta clear explanation of the results was not g iwn. This lack ofhnowledge initiated the present s tudy to obtain a betterunderstanding of the effect of aluminium content and grain s i xon the formation of porosity in magnesium alloys.

    Figure 1 : Schematic illustration of feeding niechanisnis and thepressure gradient caused by solidification shrinkage. Thepressure drops as the solid fraction increases.

    Expmincnta l Procedure

    Casting and Grain RefinementEach alloy \\as prepaied in a mild s teel crucible coated nithboron nitride The al1oSs \bere produced i n an elcctiicalresis tance furnace using commercial punt) ingots ( 9 9 7 \ i t o " ) o fmagnesi um and aluniinium Clclting and casting \\as conductedunder a protectiLe atmosphere o f 0 5% SF , in dry air in order topre\ent sebere oxidation Each alloy M as melted and heated to atemperature of approximately 720C fo i 20 minutes and ea t ingw as conducted at a temperature 70C a b o \ e th e equilibriumIiquidus teniperaturc of the alloys The mclt Mas poured into asteel mould. preheated to 200"C, Nith a cylindrical cabity \\ithdiameter of 50 mm , he igh t o f 50 mm and a ibdll-thickness of 2inni The niould wa s placed on an insulating felt and allo\+cd ocool freely n i t h no thermal insulation of thc top surfaceGrain refinement \\as conducted for alloys n i t h 9 \ i t n o 1 I \ \ t o oan d 13 u t % a lu i i iin ium us ing a grain refiner consisting o f I\ \ t% paraffin Max, 76 \ \ t % CaF,, an d 5 \\t% carbon Aftci thegrain refiner addition (addition of 0 3 \ito/o of th e grain refiner),the melt \\as held at 750C and stirred for 70 minutes to ensurcsufficient dissolution of the grain refinerRans leq moulds ner c also cast froni all the m e l t s to detetniinet h e theoretical base-line density of the alloy for both sciics ofca i t ings , ie unrefined and grain refined The grain si /c of thesamples \\as measuied by the linear intercept method describedin A S T M E l 12-88, and more than 50 intercepts Mere countedfor each grain w e determination

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    Dc n s i v Mca su re men s Results and DiscussionDens i tq measurements Mere performed by bch imedesprinciple, according to the ASTM standard [13] in order todetermine thc lebel of porosity i n each sample The theoreticaldensity \\as obtained using the same technique on the Ransleyniould castings Th e lebel of porosity in the samp le LLas thencdiculated using these trio densities

    The sainplcs \rer c sectioned lcngthn ise through the centre of th ecklinder On e hdlf \ \as polished to a I p m surface finish andetched ~ i t hhospho-picral etchant ( 0 7 ml H I P O I , 4-6 g picricacid. I00 nil ethanol (9 5% )) to rebeal the as-cdst niicrostructurefor metallogrdphic analysis These samples \re re used tocharacterise the phases present and the location and morphologyof the porosityThe rcnidining half of the samples \\ere solution treated at413C fo r 2 hours The I O U aluminium content castings \\erethen ctchcd using acetic-picro acid (1 0 ml acetic. 4 2 ml picric,10 ml H 2 0 and 10nil ethyl alcohol), Mhile citric acid ( 5 g citricac id , 100 ml distilled \\ater) Mas used for the medium and highaluiminiut~i ontent castings The etched samples se r e thcn usedfo r niedsurcnients of grain s ize

    The porosity of each casting obtained from the dcnsirymeasurements i s shonn as a function of aluminiutn content inFigure 2 I t can be readily observed that porosity content isstrongly dependent on the aluminium content h o porosity isfound in pure magnesium or in the eutectic alloy (3 3 \Vt% Al)Porosity increases M ith increasing aluminium content from puremagnesium, and a peak in porosity is found at about 1 1 \ \ t %aluminium Porosity then decreases gradually until a pore-freecasting is produced v+ith the eutectic allo) To Lcrify the trendsob se ne d in Figure 2, t\\ o additional samples Mere cast for 9 an d13 u t % a l um i ni u m Th e results from thesc castings arc alboincluded in the Figure, confirming the obserbed trendThe equilibrium freezing range, as defined bq the temperaturedifference bemeen thc liquidus (T,) dnd the solidus (Ts ) i n th eequilibrium binary Mg-AI phase diagram, hale been plottcdalong nith the measured porosity content in Figurc 2 T h ere la t ionsh ip beheen the equi l ib r ium frce r ing range and theporosity content is readily obserbed, and the peaks in porositvand maximum freerin g range are Lery close An incrcasedequilibrium freezing range generally correlates M i t h increascdporosity, and Lice Lersa A similar result m as dlso reported bq\\ hittcnbergcr and Rhines [ I41 M here porosity increased linearly\\ith aluminium content up to the maximum equilibriumfreezing range (ie max imu m porosity le\el in magnesium with13 \\t% aluminium alloy) for then to decrease The maximum inporosity correlates M ith the first occurrence of eutectic in themicrostructure according to the equilibriuin phase diagram

    ,.

    Unrefined Alloys A Grain Refined Alloys Equilibrium Solidification Range

    F i p r c 2 Porosit) \crsus aluniinium content for samples poured at J constant superheat of 70C Thc equi l ibnuni f rcwing range 2s dfunction of alumin ium content is also gi\e n Note that the niauimuni porosity ial ue occurs at 1 1 \ 4 t o i O ,Mhich is thc maxinium equilibriumaolidification rdngc

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    T h e low porosity content obtained for pure magnesium and theeutectic alloy ( 3 3 u t % Al ) can be readily understood as thesealloys would be expected to freeze near isothermally andtherefore be so-called skin-freezing. enabling efficient feedingthroughout the solidification process .The result that maximum porosity correlates with the maximurnin freezing range and the appearance of the first eutectic withincreased aluminium content is not surpris ing. However, thedirect relationship between equilibrium freezing range andporosity content is unexpected s in ce the alloys are not expectedto solidify under equilibrium conditions. In other alloy systemsthe peak in porosity is normally correlated with the appearanceof the firs t riori-eqiciiibrricr,2eutectic. This is the compositionwhere the maximum hydrostatic pressure is developed andtherefore causing a maxi mum in porosity. Hot tearing is anothercasting defect that is related to the maximum freering range andthe presence of eutectic liquid. Measurements of hot-tearingsusceptibility as a function of aluminium content in Mg-AIalloys sho\v a peak at about 1 u.t% aluminium [ 151.The Schcil equation can be used to predict non-equilibriumsolidification; CL = Co(l - J s ) k . where CL is th econcentration of the solute i n the liquid, C0 is the originalconcentration of the solute elcment and k is the partitioncoefficient. This equation proposes that eutectic forms at solutelevels much lower than those predicted by the equilibrium phasediagram , Th e non-equilibrium freezing range and the volumefraction of eutectic, calculated by the Scheil equation for the,Mg-.Al system, are sho\vn in Figure 3 .

    400& 350

    The maximum non-equi l ib r ium freenng range is obseri edaround 1 u t % Al, which correlates cer y \bell \\ith themeasurements of hot tearing, but this clearly does not correlate\vith the peak in porosity Accordi ng to the model for calculatinghydrostatic pressure proposed by Campbell [4], the maxin iumhydrostatic tension develope d by the flow of feed metal throughthe dendrite network should occur at about I a t % Al, but theporosity results indicate that this is not the case The equilibriumfreezing range is a much better correlation to porosity contentThe maximum porosity occurred at 1 I \\t% Al \ \he re thefreezing range calculated by the Scheil equation is about 50Cles s than the ma um um a t about 1 ut% AII t is not entirely clear \\hy a maximum in porosity shouldcorrelate \+ith the appearance of the firs t non-equilibriumeutectic liquid [4 ] First, the holunie fraction \ \ i l l initially beextreme11 I O R and the eutectic may therefore be located insmall, isolated, pockets , xhich are not contributing to th eoperating hydrostatic tension across the mushy zone Openchannels where flou is possible only form uhen a criticalLolume fraction of eutectic is exceeded Thi s means that beforethe critical composition is reached, the part of the mushy zonewhere actiLe feeding occurs is closer to the equilibriumsolidification range uh er e most o f the non-equilibriumsolidification \\ill also occur Furthermore, according to th eScheil calculation the eutectic colume fraction for a 2 w t 9 o Alalloy, for example is dbout I 2 % , u h i c h \\auld not contributecery much to ocerall lecel of porosity if i t is left unfed I t istherefore not impossible that the \olume fraction of eutecticcorrelating \\ith a peak in porosity is that \$hich is linked \ + i t hsufficiently large interdendritic chann els for f l o ~o be possibleand a lso of suff ic ien t io l um e to

    I2 300.$ 250m0: 00.- 1500L5 100D-

    K050

    = o0 5 10 15 20Aluminium Content (wt%)

    0.60.5 50.4

    .-.+s0

    0.0

    * I w Non-Equilibrium Freezing Range- utectic Volume FractionFigure 3 . Yon-equilibrium frecring range and cutcctic \,oluiiie fraction for h l g - A l allo>.scalculated using thc Scheil equation.90

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    Figure 4 . Micrographs of typical pores observed in the Mg-AI castings. (a) 2 \ v tO /o Al, (b) 5 wt% Al, (c ) 9 u?% Al, (d) 1 1 wt% Al, ( e ) 13\vt% 41, (0 7 wtO%Al , (9) 4 \vW' Al. An increase in volume fraction of eutectic and size of pores with increased aluminium content isclearly vis ible.

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    (a ) Mg-2 wt% Al (b) M g- 9 \Vt% AlFigurc 5 . Typical porosity dis tributions obsened in the castings. The castings with low and very high Al contents had porosityconcentrated in a small region close to the top ( a ) ; while the castings \vith intermediate alumi nium contents (ie . 9, 1 I, 13 \\to6 A l ) ha dporosity in a iiiuch larger region (b) .

    ;i110\\~ largc pores to gro\v.. This a lso foi-ms a link to thefoi-iiiation of gas porosity through hydrostatic pressure.Incrusing the aluminiuni content produced a progressi1.eincrease in thc amount of eutectic phase surrounding the pores,as can be observed in Figure 4 a) - g) . In the l o iv aluminiumcoiitcnt saniplcs, ie. 2 an d 5 \vt% A l . the pores are surroundedcntircly b y primary m agnesium and no cutcctic can be observed.Thc first cutectic is obscrved in the 9 mt?o Al alloy, and the\.oluiiic fraction of eutcctic increases progressively u.ithincreasing aluminium content. As the aluminium contcnt isincreascd to 13. 17 and 74 \ I t % Al, the dendrites arc almostcompletely surrounded by eutectic and the s ire of the poresincreases, which is also related to the size of interdendriticfeeding channels . The decrease in porosity with increasedaluniiniuni contcnt beyond the peak in porosity at 1 I \bt% Al isthcrcfore explained by an increasing volume fraction ofisotherinally frcexing cutectic and large interdendritic channels i x a l lon ing for re la t iwly unrcs tric ted feeding , and a lso adecreasing fc u in g range with increascd aluminiu m content.I t is worth noting that the cutectic morphology in Mg-Al alloysdisplays significant changes Mith increascd aluniinium content.S a \ c ct i l l. [ 16,171 sho\vcd rccent ly that a ran ge of differentiiiorphologics for-ni in Mg-XI alloys. depending on the alloycontcnt and cooling rate. A fully di\,orccd eutectic is observed atIo n aluminiuni contents and high cooling rates. \\hereas thefull) eutcctic composition tends to solidify with a fully laniellaror fibrous s ti-ucturc. The m icrographs sh own in Figure 4reproduce sonic of thesc obser\.ations, shouing divorced,partially di\,or ced and fibrous morphologies , and also somesolid-state discontinuous precipitation. These changes in eutecticmorphology arc bvorth noting as it is not entirely clear how theymay affcct interdendritic feeding. hydrostatic pressure andporosity formation. Shearhouse and Mikucki [ IS] havesuggcstcd that rejcction of hydrogen froin the eutectic Mgi iAIphasc during cutcctic solidification is an essential mechanismfor iiiicroporosity formation in alloy A Z 9 l . The intermetallichas an extrcniely l o w solubility of hydrogen. If this phase doesplay a critical role in the initiation of porosity in Mg-.A1 alloys, i tis possible that the mo rpholog y of the eutectic growth interphase

    is also important. The present work does not shed any furtherlight on this hypothesis . 0vrelid et al. [I91 have shomn that thehydrogen solubility of the liquid decreases with increasedaluminium content, but magnesium melts are claimed to rarelybe saturated with hydrogen and hydrogen content is thereforenot expected to affcct the trends observed in the present study.Grain size is another important parameter that is likely to affcctthe feeding efficiency and the level of porosity in castingsthrough its impact on coherency and permeability of the mushyzone. The variation of grain s ize observed in this s tudy is s imilarto the results reported by Lee et al. [20]. A s for most soluteadditions to a pure metal, a dramatic decrease in grain sizefoIlo\vs the initial addition of aluniiniuni to magnesium Thedecrcase of grain s ize continues with increascd aluminiumcontent until a saturation of grain density occurs around about 9\+t% A l . Further increases in aluminium content rcsults in onlymarginal reductions of grain size. As can be observed in Table1 , the grain s ize of the non-grain refined alloys is relati\ .c lysmall , particularly when the aluminium content reaches 9 \vto/b.These variations in grain s ize with aluminium content in theunrefined alloys are very small compared to those observed in,for example, aluminium alloys. I t is not clear whether thcscdifferences in grain size are sufficient to cause significantvariations of the rheological properties of the mushy zone, ic .dendrite cohcrency and maximum packing point. a lthough somevariation is likely.A study by Easton and StJohn [ I I ] has shown that there is anoptimu m range of grain s ize which reduces, or at least balances.the tendency for porosity formation (external shrinkage of aparticular casting) and hot tearing in AI-Si alloy castings. This isa very interesting concept and me ans that thc optim um grain s i xwill depend on the casting conditions and geometry as well asalloy composition. Since the initial grain s ize of the samplesused in the present study is relatively small, the niarginalchanges in grain s ize with alumin ium additions niay ha w aniininial effect on the porosity compared to other paranictcrssuch as \ ,olum e fraction o f eutectic .

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    l 'hc effcct of adding a carbonaceous grain refiner to the alloyscontaining 9. I 1 an d 13 \\to/o \\.as investigated to test the effectof grain refinement on grain size and porosity formation.The measured grain si /es of these alloys ar e giwn in Table I ,and the porosity contents ha w been included in Figure 3 . Fromthe results i t can be observed that only the 9 \vt% A l alloy had as ign i fi c an t de c re as e of p i n s i x (Table I ) , and this decrease ingrain s i x \\as accompanied by an increase in porosity (Figure2 ) . One possible explanation for thc increased porosity levelnith grain refinement is that the reduction in grain size, until theonset of the maximum porosity level. ad\.erscly affectsintei-dcndritic feeding by reduci ng perm eability. Grainrefinement of the 1 I an d 13 \\t?,6 A1 alloys resulted in aninsigniticant alteration o f grain sizc and the change in porosity\ \ a s also insisnificant. The increase in porosity with grainreiinenicnt of the h lg-9 \ \ to% A1 alloy is probably not sufficientto draw conclusions about the effect of grain refinementalthough the result does show a s imilarity to that often found i nother nietal alloy systems. Reduction o f permeability caused bymorc tortuous interdendritie channels and also a reduction in thejl/ c and number o f preferential flow paths is one likelyc \p lana t ion

    IA 111min um Con ent

    ( n t 0 )I

    3-

    1 iblc I Gr'iin s i x nicdsurements of unrefined and giain refined~ l l o s s\ ith different alumin ium contentsGrain S i x(Wl ) Ii h o n grain refined Grain refineddlloys al lo)s

    I2 0

    9 108 82

    I 7 S9, 24 ~ 71 I I

    I t i a i i be assumed that the combination of the pxamete rsout l ined abolc , freezing range, solu me fraction of eutectic , sizeof interdendritic feeding channels and permeability combine toproduce the obsened peak in porosity at approximately 9 to 1 Iu t " o 21 This tendency niay b e a function of the cooling rate ofthe casting. i e the peak in porosity may shift depending on thecooling rate , most likely appearing at loner A l contents as thedcgrec of d e\ iation f r o m equilibrium increasesT h e distribution of porosit) in the casting also Larieds ig i i f i c a n t l )~s i t h alumini um content hlost of the porosity \\asconcentrated in a region close to th e top of the casting for her)IOU and \e r > h igh d umi niu i n conten ts, F igure 5 a) In alloysM ith intermediate aluminiuni contents . the porosity \\ as muchmore dis tributed as shoun in F igure 5 b) Although the generalticnd is that the po l e size in the high A l content allobs wdsla ge r than for the ION l contents, the pore size for intermediatealuminium contents , ie 9, I I and 13\\t% Al. L C ~ S nl y 93

    marginally larger compare d to those obserked at lo \ \ aluniinitinicontents (Figure 4) The pores \ \e re , hoxe \c r . much n io iedistributed for the intermediate alu minium contents

    Conc lus ionsThe porosity characteris tics of cdst 'LIg-41 alloys as a function ofaluminium content up to the eutectic composition (3 3 \ \ too A l )and grain refinement ha \e been inbestigated The results shoMthat

    T he l a e l of porosity increases \{ith aluniiniuni contentup to 1 1 \ \ to$ \ \he re a peak in porosity is o b s c r \ c dPorosity decreases \\ith increased alu minium contentfrom 1 1 nt% to the eutectic composition The peakporosity represents the \\or st feeding condition s bq thecombined effects froni pdrameters such as a \\ idc niuyh)zone. I O M permeability and a criticdl \olunie fi;iction ofunfed eutecticThe a lumin ium conten t corre la t ing with the tm\imumporosity le\el obserbed in this study docs not corresponddirectl) \+ith the prediction o f the firs t non-equilibriumeutectic liquid around I \ i t% .4I \+ h e x th e ma x imumhkdrostati c tension should appear Porosit) insteadsho\ \s qu i te a good correlation \\ith the cxtcnt ofequilibrium freezing rangeThe pore size increases L\ith increasing rlluniiniunicontentThe porositl dis tribution changes as the ,Iluiiiinitiiiicontent increases and two dis tinctl) differentd is t r ibu t ions \ \e re obsened .4t both IOU and highaluminium contents the porosity is concentrated in asmall region close to the top of the sample, L ih i le th eporosity is much more dis tributed at intermediateal u m n uni contentsGrain refinement of a LIZ-9 \\too 41 a1101 resulted in anincreased l e b e l of porosit)

    Ackno\\ IcdqmentsThe aut hors \could like to acknoLtledge financial support fr omthe Cooperati\ e Research Centre for Cast Metals hfanufacturing(CAST) C.4ST \ \as established and is funded in part by theAustralian GoLernnient 's Cooperatibe Research CentresProgram

    References1 D hlagers and J Burssels , "Global Outlook on theL se of Magnes ium Diecas t ing in Automoti \eApplication", Magnesium Allo\s and TheirAml ic a t io n s , B L Mordike and K U Kainer eds ,Wolfsburg, Germany,( 1998), 105-1 122 F A Fox, "The Propert ies o f Some Magnes ium-Aluminium-Zinc Casting Alloys and the Incidence ofMicroporosity", Journal of the Institute of Cletals ,(1945), 415-338

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    3 .

    7 .

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    9 .

    10.

    I .

    12 .

    13 .

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    1 5 .

    16.

    17

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