Materials and Design 30 (2009) 1878–1885

Contents lists available at ScienceDirect

Materials and Design

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

Influence of processing parameters on aluminium foam producedby space holder technique

R. Surace a, L.A.C. De Filippis b,*, A.D. Ludovico a, G. Boghetich c

a Dipartimento di Ingegneria Meccanica e Gestionale, Politecnico di Bari, 70126 Bari, Italyb Dipartimento di Ingegneria dell’Ambiente e per lo Sviluppo Sostenibile, Politecnico di Bari, 74100 Taranto, Italyc Dipartimento di Ingegneria Civile ed Ambientale, Politecnico di Bari, 70125 Bari, Italy

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

Article history:Received 24 February 2007Accepted 11 September 2008Available online 26 September 2008

Keywords:Aluminium foamPowder metallurgySDPDOE

0261-3069/$ - see front matter � 2008 Published bydoi:10.1016/j.matdes.2008.09.027

* Corresponding author. Tel.: +39 099 473 3264; faE-mail address: l.defilippis@poliba.it (L.A.C. De Fili

The interesting properties of metal foams as light weight, good energy absorption, low thermal conduc-tivity, recyclability have spurred new process developments with the goal to obtain materials with a goodrelation between properties and costs.

The different manufacturing methods are classified according to the starting metal state: liquid, pow-dered or ionized. The aim of this study is to evaluate the morphological and mechanical foam propertiesand to optimise the process parameters of a space holder method starting from powders: the Sinteringand Dissolution Process (SDP).

During the experimental work the samples have been realised following the principles of Design OfExperiment (DOE). The analyzed parameters are: the composition, the pressure and the sintering time.The final evaluations of the most important features (relative density and plateau stress) have been madeby ANOVA statistical analysis to identify the most influencing variables on foam quality.

� 2008 Published by Elsevier Ltd.

1. Introduction

Metal foams are identified as a new class of materials of greatinterest due to their unique combinations of properties derivedfrom their cellular structure and metallic behaviour. The metalfoams (in particular aluminium one) are finding an increasingrange of applications: as structural applications in automotiveindustry (light weight construction, crash energy absorption, noisecontrol), in aerospace, ship, railway, biomedical and buildingindustry, and as functional application for filters, heat exchanger,silencers, flame arresters and for water purification [1].

Generally foams can be obtained by using a leachable material(e.g. salt) together with metal. Following this principle there aretwo different techniques starting from melt metal or powders.

The first method, called Replication Technique, consists of threebasic steps: by packing a soluble salt in a mould to have a pattern,casting melt metal around these granules and finally removing thepattern. Reference technique was first applied in 1966 [2] and sim-ilar processes have been developed in recent years [3–5]. A moreadvanced version of this process uses a hot-wall pressure infiltra-tion process: the salt preform is held under vacuum while a blockof aluminium is melted over it and an inert gas at high pressure isapplied during the subsequent infiltration step.

Elsevier Ltd.

x: +39 099 473 3306.ppis).

In the second method, called Sintering and Dissolution Process(SDP), the powders have been used to produce a dense two-phase precursor where one phase is water soluble. The powders(usually Al and NaCl) are mixed and compacted, forming double-connected structures of both phases. After furnace sintering, bydissolving the leachable phase, a foam of the other phase isproduced.

This process, studied at Liverpool University [6–9], belongs tothe processes defined ‘‘space holder techniques” giving structureof a great uniformity [10]. A recent study compares the sinteringof Al–NaCl compact by traditional electric furnace sintering andby spark plasma sintering that allows the increase of plateau stress[11]. Moreover Zhao developed Lost Carbonate Sintering process(LCS) to manufacture copper foam using potassium carbonate(K2CO3) as leachable salt with the same reference technique ofSDP [12].

The aim of this study is to evaluate the influence ofprocessing conditions on Al foam properties and to optimise,by statistical technique, the parameters. In fact a little numberof studies on SDP are available in the literature and the knowl-edge on correlations between the final properties of the foamedparts and the process parameters is still not complete. In partic-ular, this process consists of four steps: mixing aluminium andsodium chloride powders (with also a little quantity ofmagnesium powder to prevent oxidation problems) to obtain ahomogeneous mixture, compressing them into a mould and

Table 1Specification of powder details

Powder Manufacturer Purity Size Fabrication method Shape

Al Baker n.spec 44 lm Mechanical IrregularAl Riedel-de-Haen 93 % Fine Mechanical IrregularMg Carlo Erba 99.5% n.spec Mechanical FlakeNaCl Culinary salt – 3–4 mm – Irregular

R. Surace et al. / Materials and Design 30 (2009) 1878–1885 1879

sintering the compact in an electrical furnace to obtain enoughstrength. Finally the space holding particles are leached out fromsintered specimen by water or solvent and an open cell foam isrealised.

The experiments have been carried out basing on the principlesof Design Of Experiment (DOE). The selected parameters for obser-vation have been: mixture composition, compaction pressure andsintering time because they have the high influence on the resultsas clarified during the screening. Then, by visual inspection andsoftware tool, the study of cells shape and morphology, cells distri-bution and cells wall has been realised and also the measure of rel-ative density. Moreover, the mechanical properties of foam samplewere evaluated by quasi-static compression tests. Finally, the out-comes of this investigation (relative density and plateau stress)were evaluated by ANOVA statistical approaches that were ableto identify the correlations and the most influencing variables onfoam quality.

2. Experimental procedure

2.1. Materials

Two types of Al atomised elemental powders from differentsuppliers (Baker and Riedel-de-Haen) have been tested and pow-der morphologies, observed by a scanning electron microscopePhilips XL 20, are shown in Fig. 1a and b. For final experimentationRiedel-de-Haen Al powder has been used.

The NaCl particles have been obtained by commonly culinarysalt particles with different sizes. In order to control the cell sizesof the resulting foam, the commercial salt has been sieved intothree groups with different particle diameters: <3, 3–4, and>4 mm. Large particles (70 wt%) have been mixed with mediumsize particles (30 wt%) in order to increasing porosity and to createan interconnected structures and because Li et al. [4] found thatthe stiffness and the strength of foams increase with multi sizecells with identical relative density. Before mixing, NaCl powderwas dried in two steps: at 110 �C for 1 h in a heater and at400 �C for 45 min in an electrical furnace as suggested in the liter-ature [9].

Magnesium addition in the mixture is due to oxidation prob-lems. In fact the sintering of aluminium has always been consid-ered problematic due to the oxide film present on the surface ofpowder particles. Trace additions of magnesium react with theoxide to form spinel. This breaks up the oxide, which facilitates sin-tering [13]. The tested amount of Mg is 0.15 wt% of Al–NaCl com-pact. Specification of powder details and suppliers are reported inTable 1.

Fig. 1. Morphologies of Al powders: (a) Baker an

2.2. Mixing and compaction

The mixing is the second process step carried out: firstlythrough mixing of the Al with Mg powders to ensure the homoge-nization of metal components, and secondly with NaCl particles.Because of the need to ensure a continuous network of Al, an upperlimit to NaCl fraction is necessary in the compact. Similarly thereshould be a lowest obtainable porosity to avoid trapped NaCl inthe final structure which could lead to corrosion of the foams. Aquantification of the content of NaCl has been carried out in therange 30–70 wt%.

The powder compaction may be performed using different tech-niques (i.e. hot pressing, cold isostatic pressing). In these experi-ments powder compaction has been performed adding about15 g of mixture (depending on composition) in a 25 or 32 mmdiameter steel die followed by uniaxial cold pressing. A compac-tion in the range 100–600 MPa has been realised with a MATESTE157 hydraulic motorised press. This wide range (different fromliterature data) is due to the mentioned problems associated withAl oxide films that have to be disrupted by mechanicaldeformation.

2.3. Sintering and dissolution

The sintering has been carried out in a pre-heated electric fur-nace at different temperatures in the range 650–700 �C (Al meltingpoint is 660 �C). The sintering takes place in liquid state so the me-tal viscosity is enough to fill the spaces between NaCl particles. Ifthe sintering temperature is too low, the sintering can take toolong time causing severe oxidation. If it is excessively high, somemolten Al often oozes out from the surface of the compact to formglobules (Fig. 2). The same considerations involve the sinteringtime. The experiments have been carried out in the range 2–20 h.Moreover NaCl has a much lower thermal conductivity than Al,so the sintering time should be increased in presence of great saltamount.

The sintering takes place at normal atmosphere and both endsof the mould are sealed in order to protect the compact from air ef-fects. In the furnace the mould is positioned on a ceramic plate

d (b) Riedel-de-Haen (magnification 200�).

Table 3Experimental factors and levels

Factors Level 1 Level 2 Level 3

Al fraction (fAl) 30 wt% 40 wt% 50 wt%Pressure (p) 550 MPa 600 MPa 650 MPaTime (t) 2 h 3 h 4 h

Fig. 2. Exuded liquid on the surface of an Al sample after sintering at 660 �C for toolong time.

1880 R. Surace et al. / Materials and Design 30 (2009) 1878–1885

guaranteeing no temperature gradient along vertical direction assuggested by Arnold et al. [14]. Careful control of heating condi-tions during foaming is essential to obtain ‘‘quality” foams. Afterrequired time the samples have been cooled by air.

The salt dissolution takes place into two steps. Firstly, the sin-tered specimens have been placed into a running hot water bathto leach out the most of embedded NaCl particles. Secondly, sam-ples have been immerged in an ultrasonic washer to ensure thecomplete removing of salt. The dissolution of the NaCl is a signifi-cant rate-limiting step: in large samples, the tortuosity of the foamstructure means that it takes a long time to dissolve all the salt. Fi-nally the samples have been washed by ethanol and dried.

3. Results and discussion

The first step of the experimentation is the screening, useful toidentify, by qualitatively evaluation, the most influencing parame-ters and their working range.

Variables influencing SDP are associated with NaCl and Mg con-tent, mixing time, compaction tool, pressure, sintering tempera-ture and time, mould features, cooling and dissolutionprocedures. Each variable affects the output foam quality. In thisinitial screening a large number of samples are tested by changingparameters in wide ranges (Table 2). As result, it has been found,that although many control factors affected the process, the outputquality is mainly influenced by three primary control factors: Alfraction (fAl), compaction pressure (P) and sintering time (t).

The authors assumed these three reference main factors andverified their effects on cells area, cells wall thickness, relative den-sity and plateau stress. A standard approach for design of experi-ments is to use the full factorial method that requires 27experimental runs (three factors to be investigated, and each withthree different levels – high, medium and low). Moreover threereplications of each combination are realised. Table 3 shows thelevels of factors in this systematic investigation.

The sintering temperature has been fixed at 660 �C and theamount of Mg at 0.15 wt% in Al–NaCl mixture. Zhao et al. [8]

Table 2Analyzed parameters for screening

Factors Tested range

Al fraction 30–70 wt%Mg fraction 0–1%Mixing time 1–20 minPressure 100–600 MPaTemperature 650–750 �CTime 1–20 hCooling medium Air or water

founded that the addition of elemental Mg powder did not seemto induce benefits to liquid state sintering in SDP, but adding bothSn and Mg the foam exhibited a densified matrix. Differently, inthis investigation, the magnesium addition is of paramount impor-tance for the realisation of the samples. Infact during the screeningstep the most of the sample carried out without Mg are not com-pletely sinterized with presence of unmelted Al powder.

3.1. Cell shape and morphology

Usually metal foams are characterised by the cell topology(open or closed cell), cell size, cell shape and anisotropy. To evalu-ate these parameters, the obtained samples have been longitudi-nally sawed cut with diamond blade (to minimize the celldamage) and the morphological parameters have been measuredin a 2D cross section by an images software tool. Using SDP, the ob-tained foam shows open cells and this feature is necessary to dis-solve all salt particles. The presence of numerous channelsbetween the cells makes SDP foams suitable for damping andsound absorption applications and components.

In cellular material small bubbles, with uniform size and spher-ical shape, guarantee better mechanical properties. In particularBart-Smith et al. [15] demonstrated that the implication of shapeis more important than size in determining the yielding suscepti-bility of the cells. Moreover, improvement of the mechanical prop-erties through control of the cells during manufacturing appears tobe particularly challenging. Myoshi et al. [16] enhanced mechani-cal properties, and, in particular, absorption energy, by an increasein the aspect ratio of cell wall thickness against the cell edge lengthwith the reduction of cell size.

In SDP the pores size is closely controlled by choosing NaCl par-ticles with appropriate grain size. The obtained areas of the cells

Fig. 3. Cells areas distributions (samples 30–40–50 wt% Al, 4 h, 550 MPa).

R. Surace et al. / Materials and Design 30 (2009) 1878–1885 1881

are in the range 2–35 mm2 and cells wall thickness 0.05–0.4 mm.As example, in Fig. 3 are reported the frequency of cells area ofthree samples and it shows that the 70% of cells has uniform sizebelow 6 mm2.

The resulting cell shape differs markedly from that of foamsmade by other methods starting from powders [17]. By visualinspection a lot of samples show pores with extensive elongationperpendicularly to compaction direction due to high pressure level.This effect presents markedly in the samples with 70 wt% of NaCland slightly in the samples with 50 wt% of NaCl maybe for the rea-son that the salt particles are protected by the presence of Al. Theonly exception is the sample reported in Fig. 4 that has 50 wt% Aland elongated pores.

The cells sphericity evaluation has been carried out by assum-ing a ‘‘circularity parameter” C expressed by [18]

C ¼ 4pA=P2; ð1Þ

where A and P are respectively the area and the perimeter of thebubbles; clearly, C = 1 for circular cells. The values of the circularity

Fig. 4. Samples with elongated pores (50 wt% Al, 2 h, 650 MPa).

Table 4Circularity parameter

fAl (wt%) C range C mean value

30 0.13–0.65 0.4040 0.14–0.79 0.5150 0.22–0.85 0.49

Table 5Relative density

fAl (wt%) Relative density RD mean value Theoretical RD

30 0.26–0.36 0.31 0.2640 0.28–0.43 0.37 0.3550 0.32–0.56 0.42 0.44

Fig. 5. SEM images of foam (a) Al cell wall and (b) trapped

parameter are in different range depending from Al fraction asshown in Table 4. The obtained figures are lower than the valuesof foam obtained with other powder metallurgy techniques show-ing values until 0.91 [17]. This result implies that there are manycells with irregular shapes and contours in particular for sampleswith 30 wt% Al confirming the results of qualitative evaluation.

3.2. Relative density

The relative density (RD) and the compressive strength (CS)were identified as the two main functions to evaluate foam quality.The relative density in metal foams, defined as the ratio of foamdensity and base material density (2.7 g/cm3 for pure Al), is animportant parameter indicating foam lightness. Moreover relevantproperties of metal foams can be approximated on the basis of therelative density of the foam and different constant for each prop-erty considered [19]. The foam density can be controlled by adjust-ing the NaCl quantity.

In most cases the NaCl particles in the specimen could not bedissolved completely. Some particles remain in the foam, leadingto a higher relative density, corrosion problems, and differentbehaviour in compression tests. But as reported from Zhao [6],when Al weight fraction in the preform is below 0.4, residualNaCl in the foam is very small. If it is increased to 0.5, NaCl frac-tion in the foam becomes 0.06. So in this experimentationtrapped NaCl can be neglected. The observed relative densityrange is reported in Table 5. The theoretical relative densityhas been defined by Zhao [6] and it has been calculated as theinitially volume fraction of Al, assuming that the density of thepreform after compaction is 100% and that NaCl particles arecompletely removed after dissolution step. The sample withfAl = 30% and fAl = 40% have RD mean value higher than theoreti-cal RD showing more influence of trapped salt; instead for thesample with fAl = 50% the preform density is more influent andthe RD mean value is lower that theoretical. The SEM micro-graphs in Fig. 5 shows a cell wall example: Fig. 5a reveals thepresence of continuos network of melted Al as desired butFig. 5b the presence of entrapped salt granules. This result hasbeen confirmed also with the observation of the other samples.

3.3. Mechanical behaviour

After checking foam density, mechanical tests have been carriedout. Flat surface and cylindrical lateral surface of foam specimenshave been prepared by machining prior to removal salt particlesin order to avoid cell wall ruptures. Sometimes, especially for sam-ples with 70 wt% of NaCl, cracking during machining cannot beavoided. Fig. 6 shows a sample as removed from furnace after sin-tering (a), machining (b) and salt dissolution (c).

salt in sample carried out at 50 wt% Al, 2 h, 550 MPa.

Fig. 6. Sample after sintering (a), machining (b) and leaching (c) (40 wt% Al, 3 h, 650 MPa).

0

2

4

6

8

10

12

14

16

18

0.25 0.30 0.35 0.40 0.45 0.50

Relative density

Co

mp

ress

ive

stre

ng

th (

MP

a)

1882 R. Surace et al. / Materials and Design 30 (2009) 1878–1885

Compressive strength is the second selected evaluation crite-rion because it is relevant in the context of energy absorption offoam. The compressive tests, in quasi-static condition, have beencarried out using a machine model 5869 from INSTRON followingthe statements of the standard test method for compressive prop-erties of metal foam [20] prepared by Cimat Company taking intoaccount ASTM and DIN standards. Crosshead speed has been fixedat 1 mm/min. Stress is evaluated as the load per total area of thespecimen, including porosity [21]. Likewise, strain, defined as anominal value for the foam structure, is not the real strain experi-enced by the cell walls. As an example compressive deformationcurves at different Al fraction for three samples are shown in Fig. 7.

The curves are characterised by three zones: an initial stressthat appears to be elastic, even if the strain is only partially revers-ible. The second zone, showing strong plastic deformation, is char-acterised by a small positive slope and in some cases the curve ishorizontal. Finally there is a densification zone where the cellstouch each other. Literature data report different protocols forcompression strength determination [22]. In this experimentationall the curves show the presence of an upper yield point and thus

0 10 20 30 40 50 60 70 800

10

20

30

40

50

60

70

80

90

100

Strain, %

Stre

ss, M

Pa

30 wt% Al40 wt% Al50 wt% Al

Fig. 7. Stress–strain diagram (samples carried out at 30–40–50 wt% Al, 4 h,600 MPa; relative density 0.35–0.41–0.43).

Table 6Compressive strength

fAl (wt%) Compressive strength (MPa) CS mean value (MPa)

30 0.73–10.14 2.5340 2.86–9.07 5.4950 3.90–34.50 11.83

the strength value of these points was selected for measurement.The values are in the range from 0.73 to 34.50 MPa (Table 6).Observing the curves, it is evident that increasing the relative den-sity of the foam increases compressive strength, raises the plateau

Fig. 8. Stress-relative density law.

Table 7Process parameters and results

fAl (wt%) p (MPa) t (h) RD CS (MPa)

30 550 2 0.26 2.603 0.36 10.144 0.26 1.44

600 2 0.35 2.343 0.33 2.124 0.35 1.33

650 2 0.27 1.363 0.36 0.734 0.26 0.73

40 550 2 0.43 8.803 0.33 3.504 0.28 3.27

600 2 0.37 4.803 0.39 5.834 0.41 9.07

650 2 0.28 2.863 0.42 6.114 0.40 5.14

50 550 2 0.49 16.103 0.40 6.894 0.43 6.87

600 2 0.32 4.043 0.32 3.904 0.43 11.34

650 2 0.56 34.503 0.42 10.824 0.47 12.02

Table 8ANOVA tables for (a) relative density (b) stress

Source DF SS MS F P-value

Pressure 2 0.002295 0.001147 0.25 0.780Time 2 0.000030 0.000019 0.00 0.996Al fraction 2 0.060787 0.030394 6.66 0.006Error 20 0.091285 0.004564Total 26 0.154406

Pressure 2 49.00 24.50 0.68 0.528Time 2 53.00 26.90 0.75 0.407Al fraction 2 460.29 204.14 5.66 0.011Error 20 720.75 36.04Total 26 1283.04

R. Surace et al. / Materials and Design 30 (2009) 1878–1885 1883

stress and reduces the strain at which densification starts. More-over the stress plateau for foam is slightly serrated. Finally increas-ing fAl increase the compressive strength.

550 600 650

0.42

0.39

0.36

0.33

0.30

0.42

0.39

0.36

0.33

0.30

Pressure (MPa)

Rel

ativ

e de

nsit

y

30 40 50

Al fraction (wt%)

Fig. 9. Main effect plot for re

2 3

Pressure (MPa)

550

600

650

Time (h)

2

3

4

Time (h)

Pressure (MPa)

550

600

650

Pressure (MPa)

550

600

650

Time (h)

2

3

4

Time (h)

2

3

4

Fig. 10. Interaction plots

Correlations between density and compressive strength offoams (respectively q and r) were studied by Gibson and Ashby[19] that found a power law, where N and n are constant:

rrAl¼ N

qqAl

� �n

: ð2Þ

Differences from the model can be associated with heterogene-ities related to uneven density distribution, imperfections in themicrostructure and trapped NaCl. The experimental relationshipis reported in Fig. 8.

4. Statistical optimisation

This study has been carried out adopting experimental designtechnique because the effective planning of experiments, usingstatistical methodology, increases reliability and contributes toimprove performances [23]. The statistical approach is also nec-essary to draw meaningful conclusions from the data. In fact,

2 3 4Time (h)

Stre

ss (

MP

a)

12

9

6

3

12

9

6

3

Relative densityStress

Relative densityStress

lative density and stress.

Al fraction (wt%)4 30 40 50

0.5

0.4

0.3

0.5

0.4

0.3

Rel

ativ

e de

nsit

y

for relative density.

2 3 4 30 40 50

20

10

0

20

10

0

Pressure (MPa)

550

600

650

Time (h)

2

3

4

Al fraction (wt%)Time (h)

Stre

ss (

MP

a)

550

600

650

Time (h)

2

3

4

Time (h)

2

3

4

Fig. 11. Interaction plots for stress.

1884 R. Surace et al. / Materials and Design 30 (2009) 1878–1885

when a problem, as foam manufacturing, involves data affectedby experimental errors, a statistical methodology is the onlyobjective approach to analysis. Thus, there are two related as-pects about experimental work: the design of experiment andthe analysis of data and the latter depends directly on the designemployed [24].

The ANOVA method (ANalysis Of VAriance) has been used tocorrelate the significance of parameters on the relative densityand the compressive stress after the verification of its applicationhypothesis. All the results and process parameters are shown in Ta-ble 7. It is interesting to note that the samples compacted atp = 550 MPa show a good behaviour for RD values while the sam-ples compacted at p = 650 MPa for CS values. This result could bea consequence of hydrogen in the precursor material; it has a bet-ter development if pressure is not so high. Moreover the strongoxidation could be the possible cause for the low CS values inthe samples sintered for t = 4 h. The only parameter affecting boththe relative density and compressive stress is Al fraction (95% con-fidence interval) as reported in Table 8. The factors influence hasbeen plotted using MINITAB software (Fig. 9) evidencing the effectsof various factors on output parameters. Morphological andmechanical features of foams are affected by pressure and sinter-ing time for a fixed Al/NaCl ratio as clarified during screening,but in the selected operative windows pressure and sintering timedo not show any large effect on relative density and stress (theirlines present a little slope compared to mean value). As alreadymentioned relative density and stress increase with Al fraction.

The optimised parameters can be easily recognized. Realizingthat low relative density and high compressive strength is the bestcompromise, best setup parameters are the following:

p ¼ 550 MPa; t ¼ 2 h and f Al ¼ 40 wt%:

Figs. 10 and 11 show interaction plots. By analyzing the matrixof interaction plots it is possible to evaluate the synergy of two fac-tors on relative density and stress. An interaction is present whenthe response at a factor level depends upon the levels of other fac-tors. Parallel lines indicate no interaction. The greater is the depar-ture of the lines from the parallel state, the higher the degree ofinteraction. The obtained plots show non-parallel lines indicatingthe significance of interactions among parameters. It is evidentthat increasing Al fraction, stress and relative density increase withdifferent slope depending from time and pressure.

5. Conclusions

A statistical approach was applied to investigate the influenceof processing conditions on the relative density and compressivestrength of aluminium foam. The main disadvantages of powdermetallurgy techniques are their relatively high cost, due mainlyto the handle fine metal powders. But differently from others pow-der techniques, the Sintering and Dissolution Process does not re-quire an expensive powder foaming agent but only a culinary saltwith saving of money and precautions.

Using DOE techniques and ANOVA analysis, the authors havebeen able to identify the most significant factors affecting thefoams (pressure, sintering time and Al fraction). As founded, Alfraction is the most important parameter affecting relative densityand compressive stress. Morphological and mechanical features offoams are affected also by pressure and sintering time for a fixedAl/NaCl ratio as clarified during screening, but in the selected oper-ative windows pressure and sintering time do not show any largeeffect on relative density and stress. The optimal manufacturingparameters have been set as follows: compaction pressurep = 550 MPa, sintering time t = 2 h and Al fraction fAl = 40 wt%. Asclarified, Al fraction is the most important parameter affecting rel-ative density and compressive stress.

It seems that experimental indetermination (data scatter) is tobe associated to heterogeneities in the regions at variable densitydistribution and to the trapped salt that cannot be avoided.

References

[1] Banhart J. Manufacture, characterisation and applications of cellular metalsand metal foams. Prog Mater Sci 2001;46:559–632.

[2] H.A. Kuchek, Method of making porous metallic article. US Patent 3,236,706,1966.

[3] San Marchi C, Mortensen A. Deformation of open-cell aluminium foam. ActaMater 2001;49:3959–69.

[4] Li JR, Cheng HF, Yu JL, Han FS. Effect of dual-size cell mix on the stiffnessand strength of open-cell aluminum foams. Mater Sci Eng A 2003;362:240–8.

[5] Gaillard C, Despois JF, Mortensen A. Processing of NaCl powders of controlledsize and shape for the microstructural tailoring of aluminium foams. Mater SciEng A 2004;374:250–62.

[6] Zhao YY, Sun DX. A novel sintering-dissolution process for manufacturing Alfoams. Scripta Mater 2001;44:105–10.

[7] Sun DX, Zhao YY. Static and dynamic absorption of Al foams produced bysintering and dissolution process. Metall Mater Trans B 2002;34:69.

R. Surace et al. / Materials and Design 30 (2009) 1878–1885 1885

[8] Zhao YY, Han F, Fung T. Optimisation of compaction and liquid-state sinteringin sintering and dissolution process for manufacturing Al foams. Mater Sci EngA 2004;364:117–25.

[9] Sun DX, Zhao YY. Phase changes in sintering of Al/Mg/NaCl compacts formanufacturing Al foams by the sintering and dissolution process. Mater Lett2005;59:6–10.

[10] Ashby MF, Evans AG, Fleck NA, Gibson LJ, Hutchinson JW, Wadley HNG. Metalfoams: a design guide. USA: Butterworth-Heinemann; 2000.

[11] Wen CE, Mabuchi M, Yamada Y, Shimojima K, Chino Y, Hosokawa H, et al.Processing of fine-grained aluminum foam by spark plasma sintering. J MaterSci Lett 2003;22:1407–9.

[12] Zhao YY, Fung T, Zhang LP, Zhang FL. Lost carbonate sintering process formanufacturing metal foams. Scripta Mater 2005;52:295–8.

[13] Schaffer GB, Sercombe TB, Lumley RN. Liquid phase sintering of aluminiumalloys. Mater Chem Phys 2001;67:85–91.

[14] Arnold M, Korner C, Singer F. PM aluminium foams stabilizing mechanism andoptimisation. In: Banhart J, Fleck NA, Mortensen A, editors. Cellular metals:manufacturepropertiesapplication.Berlin:VerlagMITPublishing;2003.p.71–6.

[15] Bart-Smith H, Bastawros AF, Mumm DR, Evans AG, Speck DJ, Wadley NG.Compressive deformation and yielding mechanism in cellular Al alloysdetermined using X-ray tomography and surface strain mapping. Acta Mater1998;46:3583–92.

[16] Myoshi T, Itoh M, Mukai T, Kanahasci H, Kohzu H, Tanabe S, et al. Enhancementof energy absorption in a closed-cell aluminum by the modification of cellularstructures. Scripta Mater 1999;41:1055–60.

[17] Surace R, De Filippis LAC, Ludovico AD, Boghetich G. Experimental andstatistical approach to quality in manufacturing aluminium foam, In: KyttnerR, editor. Proceedings of 5th international DAAAM Baltic conf industrialengineering – adding innovation capacity of labour force and entrepreneurs,Tallinn Estonia; 2006. p. 301–6.

[18] Costanza G, Montanari R, Tata ME. Ottimizzazione del contenuto di TiH2 e SiCnelle schiume di Al. Metall Ital 2005;6:41–7.

[19] Gibson LJ, Ashby MF. Cellular solids–structures and properties. Cambridge:Cambridge University Press; 1997.

[20] Cymat. Standard test method for compressive properties of metal foams. 2002.Available from: <http://www.cymat.com/cymat-technical.htm>.

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

[22] Banhart J, Baumeister J. Deformation characteristic of metal foams. J Mater Sci1998;33:1431–40.

[23] Barker TB. Quality by experimental design. New York: Marcel Dekker; 1994.[24] Montgomery DG. Design and analysis of experiments. New York: John Wiley &

Sons; 1991.