Materials and Design 40 (2012) 510–515

Contents lists available at SciVerse ScienceDirect

Materials and Design

journal homepage: www.elsevier .com/locate /matdes

Production of graded aluminum foams via powder space holder technique

Amir Hassani ⇑, Ali Habibolahzadeh, Hassan BaftiFaculty of Materials Engineering, Semnan University, Semnan 35195-363, Iran

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 January 2012Accepted 11 April 2012Available online 21 April 2012

Keywords:B. FoamsC. SinteringE. Mechanical properties

0261-3069/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.matdes.2012.04.024

⇑ Corresponding author. Tel./fax: +98 2313354119.E-mail address: ahasani@semnan.ac.ir (A. Hassani)

Aluminum open-cell foams with graded cell size and graded density were successfully fabricated by pow-der-space holder technique using spherical granulated carbamide as space holders. The results revealedthat the technique was practically feasible. Compressive properties of the graded foams were studiedbeing compared with their analogous non-graded foams. In both cases graded foams showed no promi-nence in comparison to their corresponding analogous foams, except for cell size-graded foam (SGF)which demonstrated a wider plateau region in the stress–strain curve, higher densification strain, andhigher energy absorption efficiency in respect to those of analogous non-graded foams. Feasibility studyto produce density graded foam (DGF) by NaCl spacer was also evaluated, using the same technique. Itwas shown that carbamide spacer was more feasible than NaCl spacer did in producing density gradedfoam.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Metallic foams have attracted many researchers from a com-mercial viewpoint, because of their low density, light weight andspecial characteristics [1]. Beside the advantage of practical feasi-bility in production of cellular metallic materials, they have excel-lent impact energy absorption, high specific strength, and goodstiffness [2]. These high properties together with low thermal con-ductivity, recyclability, significant damping and sound absorptionproperties have encouraged many metallurgists to develop newmanufacturing processes to produce these high properties materi-als with relatively low cost [3,4]. Owing to their excellent proper-ties, metallic foams have found numerous applications in differentfields of industry including energy absorption systems for impactprotection, lightweight structural components, heat sinks for elec-tronic devices and acoustic insulation [5–7]. According to the stateof the starting metal, a wide range of methods are currentlyemployed in manufacturing metallic foams including liquid met-als, powder metallurgy (PM), electrochemical deposition, or vapordeposition techniques [3]. The powder metallurgical route is one ofthe important techniques because it allows a wide range of mate-rials and component geometries to be processed. In this respect,production of aluminum metal foams, using space holder methodhas widely been studied. The spacer can be materials such as NaCl,carbamide or calcium carbonate.

Zhao and Sun [8] have developed a technique for manufacturingof low cost open-cell aluminum foams by NaCl particles as spacervia PM route. It is shown that production of aluminum foams by

ll rights reserved.

.

the same route using spherical carbamide as spacer may also leadto the production of appropriate foams [9].

It is currently found that functionally graded materials can opti-mize the performance of components through controlled propertygradients [8]; an approach that could be extended to metal foamshaving graded feature. Changing the cell geometry, density and/ormaterial composition from point to point within the foam can pro-duce functionally graded foams (FGFs) [10].

Zhang et al. [11] studied longitudinal and transverse mechani-cal properties and energy absorption properties of foam-filledsquare tubes under quasi-static loading conditions. The authorsconcluded, ‘‘The results indicated that the plateau region of theload–displacement curve exhibited a marked fluctuant serrationwhich was clearly related to the formation of folds’’ [11].

Zhu et al. [12] also stated that the functionally graded metallicfoams improved the performance of the insulation devices for stea-dy state heat flow or transient heating conditions. Venkataramanet al. reported that ‘‘For open-cell foams, functional grading ofthe foam density can reduce the heat transfer through the foamfor a given thickness’’ [10].

In response to these requirements, several approaches havebeen made to study production of density-graded metallic foams,and different processing methods are now available for densitygraded Cu, Ni, Mg, Al [7] as well as stainless steel foams [13].The first to do so was Neubrand [14] who used electrochemicalmethods to create Cu and W foams with continuously graded rel-ative densities in the range of 10–70%. Continuously graded Alfoams were produced by Matsumoto et al. [7] who used gradedchemical dissolution to achieve relative densities of 0.05–0.10.Pollien et al. [15] reported layered aluminum foams, made byreplication of soluble NaCl placeholders, to create sandwiches with

A. Hassani et al. / Materials and Design 40 (2012) 510–515 511

core relative densities of 0.15–0.45. They also showed that density-grading offers little or no advantage for stiffness limited beams, butdoes offer a weight reduction for certain strength-limited applica-tions. Hangai et al. [16] used friction stir processing (FSP) for fabri-cation of functionally graded porous aluminum. Brothers andDunand [6] introduced a different processing method based oninvestment casting. According to Matsumoto et al. ‘‘In this methodgradient open cell metallic foams could be obtained easily, andsuch density-controlled structures are expected to simplify design,reduced production cost and weight, and optimize stiffness andoverall strength’’ [17]. Furthermore, functionally graded structurefor hollow particle (micro-balloon) filled syntactic foams has beenfabricated [18] that is capable of withstanding compression for 60–75% strain without any significant loss in strength. It was con-cluded that, ‘‘The area under the stress–strain curves was foundto be 300–500% higher compared to plain syntactic foams, indicat-ing significantly higher energy absorption in these foams’’ [18].Also, aluminum foams have recently been fabricated by melt-based route using ZrH2 as a foaming agent by Li et al. [19] whoinvestigated the factors which affected the foaming of aluminumfoams during casting process. Some researchers (e.g. [20]) havedeveloped microstructural numerical models of metallic foams.

In order to produce aluminum foams directly from powder forautomotive parts, Shiomi et al. [21] proposed the combined pro-cess of hot powder extrusion and foaming. They extruded theA6061 aluminum powder mixed with TiH2 powder as a foamingagent into the die heated to a temperature higher than the meltingpoint and the aluminum foam was made. The relative density ofthe aluminum foam was 0.22. The molding of the aluminum foaminto the pipe was carried out and the cylindrical aluminum foamwas obtained. The distribution of relative density within the alumi-num foam molded into the pipe was in a range of 0.2–0.3.

The energy absorbing capacity of the pipe increased with thealuminum foam filled by the proposed method.

In present study feasibility of producing aluminum SGF andDGF has been investigated using spherical granulated carbamideas spacer via powder space-holder route. The mechanical proper-ties of the resulted foams were measured and compared to the cor-responding uniform foams. The same technique was adopted toproduce graded foam using NaCl spacer, and the result was com-pared to that of carbamide spacer one.

2. Experimental procedure

Powder space-holder technique [9] was employed to producegraded aluminum foams having cell-size or density gradient.Spherical granulated carbamides were selected as space holdermaterials. Cell-size graded foams (SGFs), with cell size gradationfrom 1 to 5 mm, and relative density-graded foams (DGFs), withdensity gradation from 0.16 to 0.47 mm, were fabricated.

2.1. Materials and instrumentation

To produce the framework of the foams, gas atomized alumi-num powder with the particle sizes ranging between 2 and45 lm and a reported chemical composition of 99.5 Al–0.1 Si–0.27 Fe–0.1 Mg–0.02 Ti–0.02 Zn–0.003 Cu (wt.%), produced byMPKH Ltd. were used.

Spherical granulated carbamides ((NH2)2CO) were supplied asspace holders in the size range of 1–5 mm (Fig. 1) having a densityof 1.33 g cm�3, a melting temperature of 133.3 �C, a thermaldecomposition of �133 �C, and a solubility in water (at 20 �C) equalto 1000 g/L [9].

NaCl powders, produced and supplied by a laboratory, wereused as spacers. They were sieved in the range of 0.3–0.5 mm(Fig. 3), washed by acetone and then dried at 110 �C for 1 h.

The aluminum powder was mixed with 1 wt.% of magnesiumpowder (<10 lm) and 1 wt.% of tin powder (<15 lm), as additives,for 2 h in a rotary mixer. These additives promoted liquid phaseformation during sintering stage. Ethanol (2 vol.%) was sprayedon the surface of the granulated carbamides to create a sticky layeron them. The prepared metallic powder mixture was mixed withthe sticky carbamide for two more hours in the rotary mixer. Theprocedure provided an adhesion of metallic powder on the stickysurface of the granulated carbamides and created uniform agglom-erates, having granulated carbamides as the core and metallicpowder as the shell.

The mixture was uniaxially compacted at 330 MPa inside a steeldie of 30 mm diameter. The weights of the compacts were keptconstant and equal to 43 g in all experiments. The carbamide ofthe compacts was leached in a stirring water bath for 2 h at ambi-ent temperature.

The resulted raw foam was washed with ethanol and dried in anelectrical oven at 60 �C for 6 h to ensure removal of their moisturebefore sintering stage.

Liquid phase sintering stage of the raw foams was performed inan electrical furnace at 640 �C for 2 h [9]. The samples were cut bywire cut machine to evaluate macro-porosity. The XL30 scanningelectron microscope (SEM) was used to observe microstructuresof the cell walls. To identify the cell size variation, the linear inter-cept method was used in accordance with ASTM Standard E112-96[22].

Compression testing was performed by Dartec universal testingmachine (model 3500) following the standard test method of com-pressive properties of metal foam presented by Cymat corporation[23] taking into account ASTM E9-09 [24] standard. The cross-headspeed was 1 mm/min. Stress was measured as the load per totalarea of the specimen, including porosity [25]. The elastic moduluswas measured through unloading the testing machine at 1% strainand plateau stress was taken as the average stress in the range of10–50% strain [26]. The densification strain was considered asthe strain at the point of intersection of the horizontal plateaustress line and the backward extended densification line [27].

2.2. Production of graded aluminum foam

Mixture of aluminum and 45 wt.% carbamide was considered tomake aluminum cell-size graded foam (SGF). Ten layers with dif-ferent carbamide sizes, as described in Table 1, were stacked oneach other and compacted (Fig. 2). The mold and the mixture weresmoothly vibrated after stacking of each layer to produce a smoothsurface and to maintain the horizontal uniformity of the layers. Inaddition, the compact of uniform (non-graded) Al-45 wt.% carbam-ide with multi-sizes carbamide, ranging from 1 to 5 mm (as in Ta-ble 1 for SGF), was made as analogous to SGF. Relative densities ofboth graded and non-graded foams were equal to 0.35 (Table 1).Their compressive properties were compared to each other.

Furthermore, a mixture of aluminum powder and granulatedcarbamides with 2 mm diameter was used to make different layerswith different carbamide percentage (as in Table 1 for DGF). Thestacked layers were then compacted, leached and sintered to pro-duce aluminum density graded foam (DGF). Mean weight fractionof carbamide in the produced foam was 52 wt.% with relative den-sity of 0.29. A mixture of aluminum and 52 wt.% carbamide of2 mm diameter was also made and compacted to produce uniformfoam with relative density of 0.3, as analogous to DGF (Table 1).Their compressive properties were also compared to each other.

Density graded foam (as in Table 1 for DFG) was also fabricatedusing NaCl spacer. In this respect, NaCl powder was sieved in therange of 0.3–0.5 mm (Fig. 3), washed by acetone and then driedat 110 �C for 1 h. Afterwards, NaCl powder was heated in an elec-trical furnace for 45 min at 400 �C to eliminate any absorbed water

Fig. 1. (a) Spherical granulated carbamides with different sizes of (I) 2.5–3 mm, (II) 2 mm, (III) 1.5–2 mm, and (IV) 1–1.5 mm, and (b) aluminum powder particles.

Fig. 3. The NaCl particles used as spacer.

Fig. 2. Schematic illustration of layer arrangement in SGF and DGF.

512 A. Hassani et al. / Materials and Design 40 (2012) 510–515

[8]. Finally, it was mixed with aluminum powder and compactedas described (Fig. 2) to produce DFG with relative density of 0.3.The compact was sintered at 650 �C for 20 h [28].

The relative density was calculated as qf/qAL while the densityof aluminum foam, qf, was calculated by measuring the weightand the volume of each compact. Density of aluminum, qAl, wasconsidered to be equal to 2.7 g cm�3.

The energy absorption capacity of the metallic foam was calcu-lated from the area under compressive stress–strain curve in therange of 0–50% strain [29], namely:

W ¼Z e

0rde ð1Þ

Energy absorption efficiency was determined using the followingequation [29]:

g ¼Z e

0rde

� ��ðrmaxeÞ ð2Þ

where rmax is the maximum stress in the plateau region. Thenumerator and denominator of Eq. (2) represent the real absorbedenergy and the ideal absorbed energy, respectively.

3. Results and discussion

Aluminum foams were perfectly fabricated by employing pow-der carbamide spacer technique. They showed almost no micro-porosities in their cell walls after liquid phase sintering (Fig. 4),

while macro-pores preserved their shape without any collapsing,cracking or delaminating (Figs. 5 and 6). Although there were sig-nificant differences between particle sizes of the mixtures, or themixtures contained different percentages of aluminum and car-bamide, distribution of resulted macro-pores through the differentlayers of the foams was almost uniform and the changes were al-most gradual and continual in longitudinal direction (Figs. 5a and6a). The proper distribution of different species within the mixturecan be attributed to the well-adhesive surfaces of the carbamidegrains, the low difference between the densities of aluminumand carbamide and finally, employing smooth vibration after lay-ing each layer. The spherical shape of granulated carbamides alsohelped easy movement of the particles related to each other andsubsequently improved uniformity.

The situation also promoted clustering of carbamide spheres inmulti-size cell foam (Fig. 5b) as the small carbamide spheres werearranged around the bigger ones after compaction of multi-sizecarbamide–aluminum mixture. Such an arrangement was also re-ported in the foams produced by infiltration processes [30]. Theobservations revealed that the technique of fabrication of gradedfoams using granulated carbamides as the spacer was practicallyfeasible.

The compressive behaviors of SGF and the multi-size cell foams(Fig. 7) show a standard stress–strain diagram for metallic foams,with three distinct zones [2]. The results which are tabulated in Ta-ble 2 indicate that SGF has lower compressive properties than themulti-size cell foam, especially in terms of its energy absorption,plateau stress and densification stress.

However, SGF offers wider and flat plateau region, and higherdensification strain (Fig. 7) which are desirable for having high en-ergy absorption efficiency [26]. Such differences in properties havebeen resulted from mode of pore distribution in both foams, astheir overall relative densities and pore sizes are identical. Com-

Table 1Specifications of carbamide and relative densities in SGF and DGF.

Al foamtype

Space holder Relativedensity(qf/qAl)

Layers

Carbamidepercentage

Carbamide size(mm)

1 2 3 4 5 6 7 8 9 10

SGFa 45% Variable� 0.35 Dense Al 1 mm 1–1.5

1.5 1.5–2

2 2–2.5

2.5–3

3.5–5

Dense Al

DGFb Variable? Dense Al 35% 40% 45% 50% 55% 60% 65% 70% Dense Al2 mm Variable ? 1 0.47 0.41 0.37 0.32 0.27 0.23 0.2 0.16 1

a Cell-size graded foam.b Relative density graded foam.

Fig. 4. Microstructure of cell wall in aluminum foam after liquid-phase sintering for2 h at 640 �C.

Fig. 5. Macroscopic images of (a) SGF and (b) multi-cell size foam. Cross sections are parallel to the compacting direction. Open circles show aggregation of small size poresaround bigger cells.

Fig. 6. Macroscopic images of (a) DGF and (b) uniform density foams. Cross sections of DGF are parallel to the compacting direction.

Fig. 7. Compressive stress–strain curves of SGF and multi-size cell foam with 1–5 mm cell sizes.

A. Hassani et al. / Materials and Design 40 (2012) 510–515 513

Table 2Results of compressive properties for cell size graded and multi-size cell aluminum foams.

Al foamtype

Relative density(qf/qAl)

Plateau stress r�Pl

(MPa)Elastic modulus E�

(GPa)Densificationstrain ed

Energy absorption(MJ/m3)

Densification stressrd (MPa)

Energy absorptionefficiency g (%)

SGF 0.35 5.13 1.53 0.47 2.56 5.50 93.20Multi-cell

size0.35 7.07 2.195 0.37 3.53 8.18 86.40

Fig. 8. Compressive stress–strain curves of DGF and uniform density foam, with2 mm cell sizes.

Fig. 9. Density graded aluminum foam produced using NaCl spacer.

514 A. Hassani et al. / Materials and Design 40 (2012) 510–515

pressive deformation and failure process of closed-cell aluminumfoams at the cell/membrane level at the plastic collapse stagewas investigated by Yongliang et al. [31] who identified the mainenergy absorption mechanisms of the materials. In their study,they found that cell shape and non-homogeneity of the foam struc-ture were the primary factors affecting the deformation modes. Ithas also been demonstrated in the compressive stress–strain curveof TD and LD specimens with the same relative density [32] that incontrast to the LD samples, the TD samples showed not only a low-er initial peak but lower stress drop ratio and also, TD specimenpresented a smoother plateau region indicating a stable compres-sive process. The influence of various cell shape anisotropy ratioson the compressive behavior of closed-cell Al alloy foams has alsobeen studied [32,33] in which it was revealed that the cell shapeanisotropy ratio increased firstly and decreased afterward withincreasing relative density. It was also found that during the meltfoaming process, especially in the cooling stage, the pores wereelongated in rise direction and that anisotropic cell morphologywas the prime cause. It could be summarized that the energyabsorption efficiency depends mostly on pore distribution. Poredistribution affects cell wall thickness and its distribution, whichsubsequently affects stress distribution. Arrangement of fine parti-cles around the larger ones in the multi-size cell foam (Fig. 5b)would strengthen the foam due to the change in stress state ex-posed on the cell walls. It was found that the central parts of the

Table 3Results of compressive properties for density graded and uniform aluminum foams.

Al foam type Relative density(qf/qAl)

Plateau stress r�Pl

(MPa)Densificationstrain ed

DGF 0.29 4.104 0.38Uniform density

foam0.30 3.37 0.41

nodes have a marginal share in the mechanical properties of thefoam. In fact, the addition of a small amount of fine spacer besidethe larger spacers leads to creation of a spatial distribution bywhich fine pores arrange in the vicinity of larger pores and causesthe transmission of solid part (matrix material) from the centerpart of the nodes to the intercellular regions [30]. The modelingof loading on such a cellular distribution has suggested that plasticstrain distribution occurs in the weak intercellular region for aspecimen having uniform cell size, however, by adding a smallamount of fine particles in foams, plastic strain transmits to largeintercellular regions where the material resists against bendingand yielding due to stress balance [30].

Compressive behaviors of DGF and multi-size cell foam (analo-gous to DGF) are shown in Fig. 8 and the extracted data are pre-sented in Table 3. The multi-size cell foam shows compressivebehavior which is characteristic of conventional uniform metallicfoams with a near constant plateau stress, while DGF has a limitedelastic region and exhibits a smoothly ascending plateau stress.The slope in plateau region might be due to presence of differentlayers which sequentially yield at different stresses. This is consis-tent with the suggestion of Brothers et al. [6] who proposed that

Energy absorption(MJ/m3)

Densification stress rd

(MPa)Energy absorptionefficiency g (%)

2.052 8.026 51.11.38 4.4 62.7

A. Hassani et al. / Materials and Design 40 (2012) 510–515 515

deformation began in the lowest density region of the foam andprogressed towards the stronger and higher density regions [8].

In the present study, the results of several attempts indicatedthat producing DGF through this technique using NaCl spacerwas restricted as it cracked and delaminated during sintering stage(Fig. 9). The produced DGF simply cracked along interface of differ-ent layers after sintering stage. The reason should be related to thedifference between thermal conductivity of NaCl particles and alu-minum framework during sintering stage [34]. It is reported thatappropriate sintering time and temperature for these foams mainlydepend on volume fraction of the NaCl spacer in the mixture [34].Hence, proper sintering time and temperature would vary for dif-ferent layers in the produced DGF, as volume fraction of the NaClspacer changes longitudinally in each layer. It seems that the pro-duced DGFs also suffer from presence of non-uniform residualstresses which are intensified by employing angular NaCl particlesin the mixture (Fig. 3). The angularity of those particles raisesresidual stresses within matrix during the foam fabrication andfacilitates cracking and delaminating of the graded foam in sinter-ing stage (Fig. 9). Foams which have been made by granulated car-bamide spacer are not involved with such a problem, as the usedcarbamide is almost spherical with a quite low stress concentra-tion factor. The carbamide is also leached from the aluminumframework before sintering stage. Hence, the difference betweenthermal conductivity of carbamide particles and aluminum frame-work has no effect on its productivity during sintering stage.

4. Conclusions

Aluminum powder-carbamide spacer technique is a practicallyfeasible technique for manufacturing aluminum SDF and DGF. Suchpractical feasibility seems to have rather restriction by using NaClspacer to fabricate density graded aluminum foam. Smooth andspherical shape of the carbamide spacer provides uniform distribu-tion of different species in the uniform and graded foams. Multi-size cell foam demonstrates higher compressive properties in com-parison to that of cell-size graded foam; however, SGF offers widerplateau region, higher densification strain and more energyabsorption efficiency. Compression stress–strain graph of the den-sity graded aluminum foam exhibits inferior properties in compar-ison to its analogous uniform foam.

References

[1] Banhart J. Manufacture characterization and application of cellular metals andmetal foams. Prog Mater Sci 2001;46:559.

[2] Ozan S, Bilhan S. Effect of fabrication parameters on the pore concentration ofthe aluminum metal foam manufactured by powder metallurgy process. Int JAdv Manuf Technol 2008;39:257–60.

[3] Ashby MF, Evans A, Fleck NA, Gibson LJ, Hutchinson JW, Wadley HNG. Metalfoams: a design guide. Boston: Butterworth-Heinemann; 2000. p. 1–54.

[4] Surace R, De Filippis LAC, Ludovico AD, Boghetich G. Influence of processingparameters on aluminum foam produced by space holder technique. MaterDes 2009;30:1878.

[5] Gibson LJ, Ashby MF. Cellular solids: structures and properties. 2nded. Cambridge: Cambridge Uni. Press; 1997. p. 2–11.

[6] Brothers AH, Dunand DC. Mechanical properties of a density-graded replicatedaluminum foam. Mater Sci Eng A 2008;489:439.

[7] Matsumoto Y, Brothers AH, Stock SR, Dunand DC. Uniform and gradedchemical milling of aluminum foams. Mater Sci Eng A 2007;447:150.

[8] Zhao YY, Sun DX. A novel sintering-dissolution process for manufacturing Alfoams. Scr Mater 2001;44:105.

[9] Bafti H, Habibolahzadeh A. Production of aluminum foam by sphericalcarbamide space holder technique-processing parameters. Mater Des2010;31:4122.

[10] Venkataraman S, Haftka RT, Sankar BV, Zhu H, Blosser ML. Optimalfunctionally graded metallic foam thermal insulation. AIAA J 2004;42:2355.

[11] Zhang CJ, Feng Y, Zhang XB. Mechanical properties and energy absorptionproperties of aluminum foam-filled square tubes. Trans Nonfer Met Soc China2010;20:1380–6.

[12] Zhu H, Sankar BV, Haftka RT, Venkataraman S, Optimization of a functionallygraded metallic foam insulation under transient heat transfer conditions. In:44th AIAA/ASME/ASCE/AHS/ASC structures, Structural Dynamics andMaterials Conference. Norfolk: AIAA; 2003. p. 1196–206.

[13] Nishiyabu K, Matsuzaki S, Okubo K. Porous graded materials by stacked metalpowder hot-press moulding. Mater Sci Forum 2005;493:765.

[14] Neubrand A. Electrochemical processing of porosity gradients for theproduction of functionally graded materials. Appl Electrochem 1998;28:1179.

[15] Pollien A, Conde Y, Pambaguian L, Mortensen A. Graded open-cell aluminumfoam core sandwich beams. Mater Sci Eng A 2005;404:9.

[16] Hangai Y, Koyama S, Utsunomiya T, Hasegawa M. Manufacturing offunctionally graded porous aluminum of dissimilar aluminum alloy. JapanInst Metals 2010;74:285.

[17] Matsomato Y, Brothers AH, Dunand DC. Density-graded aluminum foams bythe corrosion method. Mater Sci Forum 2007;539–543:1903.

[18] Gupta N. A functionally graded syntactic foam material for high energyabsorption under compression. Mater Lett 2007;61:979.

[19] Li DW, Li J, Li T, Sun T, Zhang XM, Yao GC. Preparation and characterization ofaluminum foams with ZrH2 as foaming agent. Trans Nonfer Met Soc China2011;21:346–52.

[20] De Giorgi M, Carofalo A, Dattoma V, Nobile R, Palano F. Aluminum foamsstructural modeling. Comput Struct 2010;88:25–35.

[21] Shiomi M, Imagama S, Osakada K, Matsumoto R. Fabrication of aluminiumfoams from powder by hot extrusion and foaming. J Mater Proc Technol2010;210:1203–8.

[22] ASTM Standard E112-96. Standard test method for determining average grainsize. Philadelphia, USA: ASTM International; 2004.

[23] Cymat Corp., Standard test method for compressive properties of metal foams,In: Technical manual for stabilized aluminum foam, Ontario; 2002. p. A5–1.

[24] ASTM Standard E9-09. Standard test method for compression testing ofmetallic materials at room temperature. Philadelphia, USA: ASTMInternational; 2002.

[25] Fusheng H, Zhengang Z. The mechanical behavior of foamed aluminum. JMater Sci 1999;34:291–9.

[26] Kim A, Hasan MA, Nahm SH, Cho SS. Evaluation of compressive mechanicalproperties of Al-foams using electrical conductivity. Compos Struct2005;71:191.

[27] Ramamurty U, Paul A. Variability in mechanical properties of a metal foam.Acta Mater 2004;52:869.

[28] Yang YJ, Han FS, Yang DK, Zeng K. Compressive behaviour of open cell Al–Al2O3

composite foams fabricated by sintering and dissolution process. Mater SciTechnol 2007;23:502.

[29] Sirong Y, Jiaan L, Ming W, Yanru L, Xianyong Z, Yaohui L. Compressive propertyand energy absorption characteristic of open-cell ZA22 foams. Mater Des2009;30:87.

[30] Li JR, Cheng HF, Yu JL, Han FS. Effect of dual-size cell mix on the stiffness andstrength of open-cell aluminum foams. Mater Sci Eng A 2003;362:240.

[31] Yongliang M, Guangchun Y, Lisi L, Hongjie L, Guoyin Z. Deformationmechanisms of closed-cell aluminum foam in compression. Scr Mater2010;63:629–32.

[32] Yongliang M, Guangchun Y, Hongjie L. Effect of cell shape anisotropy on thecompressive behavior of closed-cell aluminum foams. Mater Des2010;31:1567–9.

[33] Yongliang M, Guangchun Y. Anisotropic compressive behavior of closed-cellAl–Si alloy foams. Mater Sci Eng A 2010;527:1117–9.

[34] Sun DX, Zhao YY. Static and dynamic energy absorption of Al foams producedby the sintering and dissolution process. Metall Mater Trans B 2003;34:69.