Fabrication and characterization of closed-cell
magnesium-basedcomposite foamsXingchuan Xiaa,b,, Junlong Fenga,
Jian Dinga, Kaihong Songa, Xiaowei Chena, Weimin Zhaoa, Bo
Liaoa,Boyoung HurcaSchool of Materials Science and Engineering,
Hebei University of Technology, Tianjin 300130, ChinabKey Lab for
Micro and Nano-Scale Boron Nitride Materials of Hebei Province,
ChinacSchool of Nano Advanced Materials Science and Engineering,
Gyeongsang National University of South Korea, Republic of
Koreaarti cle i nfoArticle history:Received 7 November 2014Revised
26 February 2015Accepted 28 February 2015Available online 3 March
2015Keywords:Porous materialMetallic compositeCompressive
propertyMetal matrix syntactic foamabstractClosed-cell
AZ31magnesiumalloyfoamswithdifferentpercentagesofhollowceramicmicrospheres(CMs)aresynthesizedusingmodiedmeltfoamingmethod.
Thedistributionof CMsisinvestigatedandalsotheeffect of CMs
onthefoamingbehaviors (specicallyfor porosityandporesize)
andquasi-staticcompressivebehaviorsofMg-basedcompositefoamsarecharacterized.
Theresultsshowthat CMs distribute in cell walls homogeneously and
most of them are penetrated by magnesium alloymelt. In addition,
the mean pore size declines with the increase of CMs percentage.
Moreover, the overallporosity of the foams increases rst and then
decreases with the increase of CMs content, and the varia-tion
tendency is more obvious when the foaming temperature is lower
(namely 680 C). Besides,
properpercentageofCMschangesthecompressionfracturemodeofthefoamsfrombrittlenesstoductility.OM/SEM/EDS/XRD
detections and nite element analysis are applied to explain the
reasons. 2015 Elsevier Ltd. All rights reserved.1.
IntroductionMetal matrix syntactic foam (MMSF) is a kind of special
com-posite that consists of a metal matrix and a set of hollow,
sphericalparticles [1]. It has been conrmed that MMSF possesses
excellentmechanical properties compared with traditional metal
foams, e.g.high specic strength and stiffness, good energy
absorption capa-city, etc. [14]. For these reasons, MMSF has been
widely used inautomotive, aerospace, military vehicles and other
industrial elds[5]. Up to now, MMSF is mainly prepared by melt
inltration tech-nique, resulting in conned product dimensions and
much lowerporosity than foams prepared by melt foaming method. It
isbecause CMs are mainly used as pore generation agent or
thicken-ingagentwhensynthesizingMMSF[2]. Generallyspeaking,
itisbelieved that the metal matrix of MMSF can be made of
aluminum,steel, titanium or magnesium alloys. However, to our best
knowl-edge,
mostoftheresearcheshavefocusedonaluminumoralu-minumalloy matrix.
Besides, most of the researches aboutmagnesiummetal foams are about
traditional foams (withoutspherical particles) andit has
beenimprovedthat magnesiummetal foams have the potential to serve
as structural material forregular light-weight applications. Wang
et al. investigated the pro-cessingof
magnesiumfoamsfabricatedbyaninltrationtech-nology. In addition, the
pore structures and mechanicalproperties of space holder particles
as well as the resultant foamwere also characterized. The results
showed that the foams exhib-ited usual stressstrain behaviors and
nearly isotropic properties.Meanwhile, theyieldstrengthof
thefoamsincreasedwiththedecrease of sample porosity and the
relative mechanical propertiesof
foamsweremostlydependentontheirrelativedensities[6].Osorio-Hernndez
et al. prepared open-cell Mg foams by replica-tionprocess
andthemechanical properties of thefoams wereinvestigated.
Theresults showedthat increasingtheporesize,the relative density
decreased, while the porosity increased, regis-tering a minimum
relative density of 0.22. Specimens with
smallerporesizeandlowerpercentporosityshowedhighermechanicalproperties[7].
Luoetal. investigatedtheeffectof
technologicalparametersonpreparationof
Mg-basedfoamsbymeltfoamingmethodusingSiCandMgCO3asthickeningagent
andfoamingagent, respectively. Theresultsshowedthat technological
para-metersmayaffectthepreparationofthefoams,
resultinginthechangesoftheproductsinapparentdensity,
porosityandstruc-tural uniformity. Inaddition,
lightweightMg-basedfoamswithhomogeneousporescouldbeobtainedbysuitablecombinationhttp://dx.doi.org/10.1016/j.matdes.2015.02.0290261-3069/
2015 Elsevier Ltd. All rights
reserved.Correspondingauthorat:SchoolofMaterialsScienceandEngineering,
HebeiUniversity of Technology, Tianjin 300130, China.E-mail
address: [email protected] (X. Xia).Materials and Design 74
(2015) 3643ContentslistsavailableatScienceDirectMaterials and
Designj our nal homepage: www. el sevi er . com/ l ocat e/ mat
desof the technological parameters [8]. Zhang et al. produced a
novelporousMgscaffoldwiththree-dimensional (3D) interconnectedpores
bytheber depositionhot pressingtechnologyandtheeffects of
porosities on the microstructure and mechanical proper-ties of the
porous Mg were investigated. The results showed
thatthemeasuredYoungsmodulusandcompressivestrengthoftheMgscaffoldwererangedin0.100.37
GPa, and11.130.3 MPa,respectively[9]. Chenetal.
investigatedtheinuencesofstrainrate, cell size,
relativedensityandthecontentofSiConenergyabsorption characteristics
of closed-cell Mg alloy foam by dynamiccompression experiments. The
results showed that cell sizehad signicant effect on energy
absorption characteristics.Additionally, the strain rate effect was
more sensitive to the foamswith larger cell size and the inuence of
relative density on energyabsorptioncharacteristics was not
signicant [10]. Inpreviousresearch, the present authors
investigated the corrosion
behaviorandmechanicalpropertiesofclosed-cellMgalloyfoams[11,12].It
can be seen that magnesium matrix has seldom been
involvedinfortheMMSFandfurtherresearchisneeded. Inthispaper, anew
modied melt foaming method is developed to prepare mag-nesium
matrix composite foams with different percentages of CMs.Meanwhile,
theeffectofCMsonthefoamingbehaviorsandthequasi-static compressive
properties are investigated. A simple
3Dporousmodelisestablishedandnumericalsimulationisappliedto
investigate the effect of pore size uniformity on the
deformationmode of closed-cell AZ31 magnesium syntactic foams.2.
Experimental details2.1. Specimens preparationThrough previous
research, it is conrmed that hollow ceramicmicrospheres(CMs)
arehardtobeintroducedintomagnesiumalloy melt homogenously by
traditional adding method, like
add-ingthickeningorfoamingagentstomagnesiumalloymelt.
ItismainlybecauseofthereactionofSiO2, magnesiummelt(Eq. (1)[13])
and O2 (which is brought in along with the addition of CMsdespite
the presence of protective gas and the reasons will be dis-cussed
later), resulting in the burning, coking and reunion of CMs.At
last, CMs adheretomelt surfaceandhardtobeseparatedthrough
mechanical stirring.4Mg SiO2 ! Mg2Si 2MgO 1In this paper the adding
method of CMs and magnesium matrixis modied (as shown in Fig. 1a).
Firstly, AZ31 magnesium ingot iscut into sheets with a thickness of
510 mm by electro-dischargingmachining. Secondly, CMs are divided
into different volumes aver-agely. Thirdly, magnesium sheets and
CMs are stacked layer by lay-erinamildsteel crucible.
InordertoensuretheCMsuniformdistributions oneachmagnesiumsheet, a
homemade woodenshovelisused. Meanwhile,
atubeclampisappliedtoguaranteetheintegrityoftheCMswhenputanothermagnesiumsheetontheCMslayer.
Atlast, thelayeredcompositematerialswiththemildsteel
crucibleareheatedtogether toaxedtemperature.Commercial
AZ31magnesiumalloyis usedas matrix, 1.5 wt.%commercial available
calcium granules (with diameters of12.5 mm) and 2.0 wt.%CaCO3powder
(analytically pure) areselectedasthickeningandfoamingagents,
respectively. SF6andCO2 gas mixture is used to protect the melt
from being ignited oroxidized.
Forthedetailsofmeltfoamingmethodpleasereferto[12]
anddetailedparametersabout AZ31magnesiumalloyandthe CMs are shown
in Tables 1 and 2, respectively. For comparison,two types of foams
with foaming temperature of 680 C and 720 Care prepared,
respectively. It should be noted that for each type
offoamsthethickeningtemperatureisidentical
withthefoamingtemperature. In addition, all of the other parameters
remainunchanged(withthestirringspeeds of 500and1000
rpmandstirringdurationof 8 minand40 s for
thickeningandfoamingstages,respectively)
exceptforthepercentageofCMs(0, 2, 4, 8,10and20 Vol.%,
hereinafterrefertoVol.%). Theoverall
porosityismeasuredbyArchimedesprincipleandporesizeisobtainedby a
scanning method [14].2.2. Microstructure observationRepresentative
metallographic preparation process is applied topreparespecimensfor
metallographiccharacterization. Namely,specimens are ground through
successive grades of silicon
carbideabrasivepapersupto2000gritandpolishedusing0.25
lmdia-mondpolishingpaste,
thenultrasoniccleanedusingalcoholanddriedbycoldowingair.
Microstructureandporemorphologyare obtainedbya
scanningelectronmicroscope(SEM, HitachiS4800) equipped with energy
dispersive X-ray spectrometer(EDS).
PhasecompositionisidentiedbyX-raydiffraction(XRD,SmartLab, Rigaku)
with Cu Ka radiation.2.3. Compression testSpecimens for compression
test are cut into 25 25 25 mm(length width
thickness)byelectro-dischargingmachiningtoavoidsizeeffect.
Uni-axialcompressiontests(accordingtoGB/T7314-2005materialscompressiontest
standard) areperformed(b) 1cm(a) 1cmFig. 1. Schematic diagram of
adding CMs and magnesium alloy matrix (a); cross section morphology
(b) of CMs-containing foam under foaming temperature of 680 C
andCMs percentage of 20%.X. Xia et al. / Materials and Design 74
(2015) 3643 37by using SUNS Electron Universal Material Testing
Machine, with amaximum load of 300 kN. All tests are performed
under displace-ment control, with a displacement rate of 1.5 mm/min
(with initialstrainrateof 0.001/s)atroomtemperature.
Vaselineisusedtominimize the friction between specimen and plates.
Load and dis-placement are recorded using a data acquisition unit
and a person-al computer. All engineering stresses and engineering
strains
usedinthispaperarededucedfromtherecordedloaddisplacementdata. For
each parameter two specimens are compressed and theaverage data are
used. Extrapolation method is used to determinethe densication
strain [15].3. Results3.1. Specimen structureFig.
1bshowsthecross-sectionmorphologyofthecompositefoams with 20% CMs
under the foaming temperature of 680 C. Itcan be seen that the pore
structure is homogeneous and the poresarespherical andseparated.
Meanwhile, noburning, cokingorreunion of CMs is observed during the
whole preparation process.SEM observation (as shown in Fig. 2) is
applied on the cell walls toconrm the existence forms of CMs. It is
clear that CMs distributein the cell walls uniformly and most of
them maintain their origi-nal morphology [16], free of being
smashed during the stirring pro-cess. In addition, more than 95%
(numbers) of CMs are penetratedby magnesium melt. The above
mentioned results mean that themodiedmelt
foamingmethodcanproducemagnesiummatrixcomposite foams
successfully.3.2. Foaming behaviors of foams with CMsIn order to
understand the effect of CMs on the foaming behaviorof composite
foams, the variation trends of pore size and porositydistributions
are studied. Figs. 3 and 4 show the variation tenden-cies(here,
680-0 in theblock diagrammeans thefoamingtem-peratureis680
CandtheCMscontentis0%andsoon)ofporesize under the foaming
temperature of 680 and 720 C, respectively.With foaming temperature
of 680 C (as shown in Fig. 3), it is clearthat all of the pores are
distributed between 0.5 and 4 mm whilemainly between 1 and 3 mmfor
the foams without CMs.However, for the CMs-containing foams, the
pores mainly distributebetween 0.5 and 2 mmand the proportion of
these pores(0.52 mm) goes higher along with the increase of CMs
percentage.Meanwhile, pores with the diameter of 34 mm are almost
disap-peared. Similar trendhappenstothefoamsunder
thefoamingtemperatureof720 C(asshowninFig. 4),
thoughtheporesizedistribution range (16 mm) and the main
distribution range(14 mm) are slightly different from the foams
under the foamingtemperatureof 680 C.
Thementionedresultsabovemeanthatthe pore sizes of the composite
foams tend to be smaller and morehomogeneous with the addition of
CMs.Fig. 5a and b shows the effect of CMs on the overall porosity
ofthe composite foams with foaming temperatures of 680 and720 C,
respectively. TheredsolidlineshowsthemeanvalueofTable 1Composition
of AZ31 (wt.%).AL Zn Mn Si Fe Cu Ni Mg2.7852 0.7925 0.5635 0.0032
0.0002 0.0003 0.0004 BalTable 2Parameters of CMs.Stacking density
(g/cm3) Size range (lm) Wall thickness (lm)0.42 45150 7.5
0.8PoreFig. 2. Existence forms of CMs in cell walls.Fig.3.
Poresizesdistributionofcompositefoamsunderfoamingtemperatureof680 C
with CMs percentages of 0%, 2%, 4%, 8%, 10% and 20%.Fig.4.
Poresizesdistributionofcompositefoamsunderfoamingtemperatureof720 C
with CMs percentages of 0%, 2%, 4%, 8%, 10% and 20%.38 X. Xia et
al. / Materials and Design 74 (2015)
3643theporositiesobtainedfromtheindividual measurements. It isclear
that in both cases the mean porosity increases rst and
thendecreases with the increase of CMs percentage. On the whole,
theporosity is higher and the variation tendency is more moderate
forfoams with foaming temperature of 720 C. In addition, the
poros-ity attains high level with CMs percentage of 28% under
foamingtemperature of 680 C. While it requires the CMs percentage
to bebetween 8% and 15% under foaming temperature of 720 C.3.3.
Engineering stressengineering strain curvesUp to now, metal foams
are mainly used in energy absorptionelds and in these elds most of
the components experience com-pressive deformation process. Thus,
in the present paper,quasi-static compression test is applied to
investigate the deforma-tion process of the CMs-containing foams.
Fig. 6 shows the engi-neering stressengineering strain curves of
the foams withdifferent percentages of CMs and different porosities
(mainlybetween53%and67%), foreachpercentageoffoamstwospeci-mens are
compressed and showed. Here, 8-0-59% in the block dia-gram means
the specimen with the foaming temperature is 680 Cand the CMs
content is 0% and porosity of 59% and so on. It is clearthat in all
cases the curves show typical three deformation stagesas most of
metal foams: First, a linear stage where stress increasesalmost
linearly with the increase of strain until the rst peak
stress(denedas yieldstrength). Then, a plateaudeformationstagewhere
stress maintains within a certain level with the increase
ofstrainandsomeuctuationorworkhardeningoccursonsomecurves. At last,
adensicationstagewherethestressincreasessharply with the strain
increasing slightly. It should be noted thatin the plateau
deformation stage serrations appear on some of thecurves,
meaningbrittlefracturebehaviors. While,
fortheothersthecurvesaresmooth, meaningductilefracturebehaviors.
Itisclearthatthat foamswithoutCMsshowtypical
brittlefracturebehaviors. Meanwhile,
theuctuationrangeforthefoamswithCMsof
2%ismuchsmallercomparedwithfoamswithoutCMs.ContinuetoincreasethecontentofCMs,
namely4%and8%, thecurves inthe plateaudeformationstage become very
smooth(meaning ductile fracture behavior). While, if more CMs (10%
and20%) present in the composite foams, the serrations appear
again,implying the brittle fracture behaviors. As shown in Fig. 7,
similarresultscanbeobservedwhenthethickeningandfoamingtem-perature
is720 C. All oftheabove mentioned results meanthatwithproper
additionof CMs the deformationbehavior of theclosed-cell Mg-base
foams changed from brittle fracture to ductilefracture behaviors
and if excessive CMs are contained the deforma-tion mode returns to
brittle fracture again and the reasons will bediscussed later.4.
DiscussionMelt
foamingmethodwithsimpleprepareprocessandhigheconomical efciency is
the most popular way to produceclosed-cell metal foams with large
dimensions and uniformFig. 5. Effect of CMs on the porosity of
foams with foaming temperatures of 680 C (a) and 720 C (b), it
should be noted that the lines are only used to describe the
trend.Fig. 6. Quasi-static compressive engineering
stressengineering strain curves of thefoams with different
percentages of CMs under the foaming temperature of 680 C.Fig. 7.
Quasi-static compressive engineering stressengineering strain
curves of thefoams with different percentages of CMs under the
foaming temperature of 720 C.X. Xia et al. / Materials and Design
74 (2015) 3643 39structure, specially for traditional aluminumand
magnesiumfoams. In previous research CMs was introduced into
theclosed-cell aluminum foam with melt foaming method to
producealuminumcompositefoams[16,17]. CMswereaddedwithalu-minumfoil
coated and it was conrmed that by this wayCMs-containing aluminum
foam can be successfully obtained withthe CMs homogeneously
distributed in the foams. As
forCMs-containingmagnesiumcompositefoams, duringtheinitialstageof
theexperimentssimilaraddingmethodisused, unlikethe
aluminumcomposite foams, serious burning, coking andreunion of
CMsis observed. Thisismainly due toexcess
oxygenisinvolvedontothemelt surfaceandchemical
reactionoccursbetweenMg, SiO2andO2. Thisisowingtothat
whenCMsareadded with aluminum foil coated, a lot of space will
exist amongtheCMswhenonlyonekindof CMsisused[18],
resultingininvolving air (O2) to the crucible. Meanwhile, unlike
the thickeningagent (Ca) and foaming agent (CaCO3), the stacking
density of CMsis very low (as shown in Table 2). When the aluminum
foil coatedCMs are added longer time is need to bring the CMs into
the slurry,comparedwiththefoamingagent. Thus,
duetotheprotectiveeffectofaluminum
foilO2willhaveenoughtimetogodowntothemelt surfaceandchemical
reactionwill occur, resultinginthe burning, coking and reunion of
CMs on the melt surface. CMswithout aluminumfoil
protectarealsointroduceintothemeltandmoreseriousburningappearedduetotheinvolvement
ofgas ow. It should be noted that all the experiments are
conductedunder the protection of SF6 and CO2 gas mixture. In this
paper, theadding method of AZ31 Mg alloy and CMs is modied as
describedin Section 2.1 and in this way the O2 involved in the CMs
can bedrove away before magnesium alloy melting due to the rising
tem-perature and the protection gas mixture. Meanwhile, with the
tem-perature increasing the reaction (Eq. (1)) will occur (which
will beconrmed later) in situ, resulting in the permeation of the
CMs andhigh bonding strength between magnesium alloy and CMs. In
addi-tion, with the temperature increasing CMs can be heated evenly
toavoid being broken due to the unevenly local heating. Besides,
thereaction will restrict the oatation of the CMs, resulting
inhomogenous distribution of CMs in the composite foams.According
to the binary phase diagram of MgSi system [19], inthe present
experiment the phase composition of the as-cast foamconsists of
primary Mg2Si and eutectic Mg2Si + halphai-Mg phases.XRD detections
are applied and the results are shown in Fig. 8.
ThemaindifferencebetweentheCMs-containingspecimensandthespecimenwithoutCMsistheappearanceofMg2Siphaseontheformer.
Meanwhile, theintensityof theMgOisincreasingwiththe increase of CMs
percentage. The above mentioned results meanthe reaction between Mg
and SiO2 (which is the main compositionof CMs) occurred during the
preparation process [20]. It is knownthat
theintermetalliccompoundof Mg2Si exhibitsanexcellentcombinationof
superior properties, suchas highmeltingtem-perature (1085 C), low
density (1.99 103kg/m3), high hardness,lowthermal
expansioncoefcient andreasonably highelasticmodulus[21]. All
thesepropertiesmeanthatunderthepresentconditionsMg2Si
phasecanstablyexistinthecompositefoamswhich is benecial to the
macro structures and mechanical proper-ties of foams. Furthermore,
the Mg2Si phase is exceptionally
stableandthereforecouldeffectivelyimpedegrainboundaryslidingatelevated
temperatures, which is benecial to mechanical proper-ties of the
composite foams [21]. It has been conrmed
inSection3.2thatwiththeadditionofCMstheporesizebecomessmallerandtheuniformityof
theporesincreasedasshowninFigs. 3 and 4. This is mainly due to the
existence of Mg2Si phase.As ne Mg2Si particles canact as
nucleationparticles of thebubblesjustascalciumparticles[22].
Therefore, whentherearemorenucleationparticlesinthemelt,
thenascentbubbleshavemore choices to attach and more pores will
generate,which willimprove the homogeneity of the pores. Meanwhile,
the totalvolume of the gas is assumed to be constant as the foaming
agentpercentageremainsunchanged(2 wt.%) forall foams. Thus,
theCMs-containingfoamspossessmuchsmallerporesizesandthepore size
decreases with the increase of CMs percentage. In addi-tion, as the
existence of Mg2Si phases, the viscosity of the magne-sium melt
increases further besides the effect of calcium
particles(thickening agent). As it is known, viscosity is signicant
for metalfoamspreparation[23]. Thus,
whenthefoamingtemperatureislower(680 C)theoriginal viscosityof
themeltishigherandasmall quantityof CMs(Mg2Si particles)
canmaketheviscosityappropriate to produce higher porosity foams.
When the foamingtemperatureis higher (720 C) moreCMs (Mg2Si
particles) areneeded. While, excessive CMs (Mg2Si particles) will
make the meltFig. 8. Phase compositions of foams with different
percentages of CMs under thefoaming temperature of 680 C.(a)(b)
Fig. 9. Blocky Mg2Si phase in synthetic foams with CMs percentages
of 10% (a) and 20% (b) under foaming temperature of 680 C.40 X. Xia
et al. / Materials and Design 74 (2015) 3643viscosity too high and
the bubbles need more driving force to growup [22]. However, during
the preparation process, the parametersremain unchanged as
described above and no extra driving forceisavailable,
resultinginsmallerporesize. Meanwhile, theporesarehardtogrowup,
leadingtothedecreaseof entireporosity(as shown in Fig. 5).As shown
in Figs. 6 and 7,the addition of CMs has importanteffect on the
compressive deformation behavior of the compositefoams, namely
proper percentages of CMs change the deformationmode
formbrittleness to ductility. In previous research Mukai et
al.investigated the dynamic compressive behaviors of open-cell
AZ91magnesium alloy foam and the results showed that foams
underdifferent dynamic strain rate present typical brittle fracture
behav-ior [24]. Yang etal. and Xuet al. studied compressive
propertiesthe closed-cell commercial pure Mg foams with different
porositiesandtheclosed-cell
AZ91magnesiumalloyfoamswithdifferentpore sizes, respectively. Both
of the results showed typical brittlefracturebehaviors[25,26].
Meanwhile, inourpreviousresearchclosed-cell AZ31 magnesium alloy
foams (prepared by the
identicalmethoddescribedabove)bothunderas-castandheattreatmentconditions
showed apparent brittleness [12,27,28]. In general,
thedeformationmodedependsonthepropertiesof
basematerialsandtheporestructuresof foams[22]. Thus,
inthisexperimentproper addition of CMs should have changed the
properties of basicmaterials or the pore structure of the composite
foams. It can befoundfromFig.
2thattheCMsdistributehomogeneouslyintheMgalloymatrixandarealmostcompletelylledbyMgalloy.
Asmall part of the CMs are fractured at the thinnest regions or
con-centratingporositiesontheirwallsbecauseof
thedifferenceofthermal expansion coefcients between CMs particles
and matrixalloy and the reaction between Mg and SiO2 (Eq. (1)). It
is accessi-ble that the content of Si element (or Mg2Si phase)
increases withthe increase of CMs percentage. During SEM
observation the distri-bution of Mg2Si phase in the foams with CMs
of 2%, 4% and 8% ishomogeneous ontheentirecross-section. While, for
theothertwo specimens (especially for the CMs percentage of 20%),
the seg-regationofSi elementisobservedandtheresultsareshowninFig.
9. Comparedwiththeotherfoamssomedarker andbulkyregions (as the
black arrows indicated) embed in the matrix espe-cially around CMs,
which has been conrmed to be coarse Mg2SiCompression
direction(b)1mmD1 D2 (a)Fig. 10. Schematic model (a) and meshing
result (b) of a unit cell of closed-cell foam.Fig. 11. Simulation
results of models with local pore diameters of 2 mm (a), 3 mm (b)
and 4 mm (c).X. Xia et al. / Materials and Design 74 (2015) 3643
41[29]. AccordingtothestatisticalresultsitisconrmedthatCMshas an
important effect on the pore size distributions of the com-posite
foams (as shown in Figs. 3 and 4), which should also havea
signicant effect on the compressive behaviors of the foams.
Toclearly show the inuence, a simple nite element analysis
(FEM)model is built to investigate the inuence of local larger pore
onthe compressive deformationbehavior of the synthetic foams.Fig.
10ashowstheplansketchoftheunitmodelwiththelocallarger pore diameter
of D1 = 4 mm. To simplify the calculation pro-cess all the other
surrounding pores diameters are set as D2 = 2 mmand the cell wall
thickness (distance between the larger pore andthe surrounding
pore) is assumed as 1 mm as shown in Fig.
10a.UGandDeform3Dsoftwareareappliedtobuildupthemodelsand simulate
the quasi-static compression test at roomtem-perature (20 C)
respectively, using AZ31 Mg alloy as raw
materialandwithcompressionspeedof1.5 mm/min. Fig.
10bshowsthelinegraphoftheunitmodel afterbeingmeshed.
Inthepresentpaper, the evolution of overall stress distribution on
the unit modelunder the same amount of deformation is used to
discuss the effectof local larger poreonthedeformationbehavior of
thefoams.Fig 11ac shows the overall stress distribution under the
identicalamount of deformation for the local larger pore sizes of
D2 = 2, 3and 4 mm, respectively. It is clear that the effective
stress on thefoams with larger local pore size is more concentrated
andpropagated outward. Meanwhile, stress concentration appears
onthe larger pore (Fig. 11c), meaning under the identical amount
ofdeformationthelocationwithlargerporesizewillrstfracture.Thus, the
foams with larger pore size distribution ranges (as showninFigs.
3and4) will
possessreducedfractureconsistencyandobviousuctuationontheengineeringstressengineeringstraincurves
(as shown in Figs. 6 and 7). Also, it has been conrmed thatthe
magnesium matrix composite with small particle size of
Mg2Sidispersoid possesses optimal combination of ultimate
tensilestrengthandelongationdue to the pinning effect of the
neMg2Si [30]. Under the present conditions the distribution ofMg2Si
is homogenous as described above and the overall amountofSi
elementisrestricted when CMscontent islow,resultinginthe small
dimensions of the Mg2Si phase and this is benecial tothe
improvement of the magnesium alloy composite foams elonga-tion (or
ductility). However, when more content of CMs is
added,largeparticlesizeof Mg2Si phaseappears(asshowninFig.
9)andtheductilityofthecompositefoamswilldecreaseseriously[21,29],
resulting in the brittle fracture of the foams.As described above,
the deformation mode of the foams is main-ly determined by two
factors, the morphology of Mg2Si phase andthe uniformity of the
pore size. When the content of CMs is low thedimension of Mg2Si
phase is small and the uniformity of the poresize is increased,
which are benecial to the ductile fracture behav-iorofthefoams.
However, whenthecontentofCMsishighthedimension of Mg2Si phase
changes to from small pieces to blockyones and the increase of the
pore size uniformity are not so obviousas the initial stage,
leading to the brittle fracture behavior of thefoams.5.
ConclusionsA modied melt foaming method is used to
produceCMs-containing magnesium matrix composite foams. The CMs
dis-tributes in the cell walls homogeneouslyand most of them
keeptheoriginal shapes, freeof beingsmashedbystirring. Most ofCMs
are permeated due to the reaction between the CMs and mag-nesium
alloy melt. Meanwhile, due to the addition of CMs (Mg2Siparticles),
the number of nucleation particles of bubbles
increasesandtheviscosityof magnesiummelt is improved,
resultinginsmaller pore size, more homogeneous pore structure
andcontrollable porosity of the composite foams. Besides, the
properaddition of CMs changes the foams deformation mode from
brittle-ness to ductility due to the homogeneous distribution of
Mg2Si andthe improved uniformity of thepore size. However, when
exces-sive CMs is added the Mg2Si segregates to blocky ones,
leading tothe brittle fracture behavior of the
foams.AcknowledgementsThe present authors wish to thank the nancial
support provid-ed by Hebei Province School Cooperation Fund
Projects, Programfor Changjiang Scholars and Innovative Research
Team inUniversity (No. IRT13060), Science and Technology Project
ofHebei Province (13211008D).References[1] K. Mjlinger, I.N.
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