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Scripta Materialia 61 (2009) 552–555

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Improvement of aluminium foaming by powder consolidationunder vacuum

C. Jimenez,a,* F. Garcia-Moreno,b M. Mukherjee,b O. Goerkec and J. Banharta,b

aTechnische Universitat Berlin, Hardenbergstr. 36, 10623 Berlin, GermanybHelmholtz-Zentrum Berlin fur Materialen und Energie, Glienickerstr. 100, 14109 Berlin, Germany

cTechnische Universitat Berlin, Englische Starsse 20, 10587 Berlin, Germany

Received 29 April 2009; accepted 17 May 2009Available online 22 May 2009

Foamable AlSi11 precursors were uniaxially hot-compacted under vacuum and air and then foamed. We studied the influence ofthe compaction atmosphere on the foaming behaviour in situ by X-ray radioscopy. In vacuum-pressed precursors the growth ofelongated bubbles was reduced, resulting in a larger expansion and more reproducible foaming due to improved consolidation.As a consequence, foams made from these precursors show a more regular pore size distribution than foams made from traditionallypressed precursors.� 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Foam; Pore size; Powder consolidation; Foaming; Aluminium

The properties of metal foams are influenced bymorphological features, in particular the pore size distri-bution [1]. Although the exact interrelationship betweenproperties and structure is not fully known, it is assumedthat a uniform distribution of pores free of defects isdesirable [2].

The powder metallurgical (PM) route is one of thecommercially exploited methods to produce aluminiumfoams [3]. Aluminium powder is mixed with alloying ele-ments, and TiH2 is added as a blowing agent in mostcases. The powders are consolidated to yield foamableprecursors, after which the foaming process is initiatedby heating [4].

Al–Si alloys based on pure Al and Si powders are costeffective for this manufacturing route. Studies based onmicroscopy and synchrotron X-ray microtomographyhave shown that in Al–Si precursors pore initiation isspatially correlated with Si particles. In early stagespores are crack-like, leading to irregularities in the finalproducts [5,6]. This observation was attributed to themismatch between the melting of the respective alloyand TiH2 decomposition. Investigations focused onlow-melting aluminium alloys [3,7,8] and pre-treatedTiH2 powders [9–12] have brought significant progress

1359-6462/$ - see front matter � 2009 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2009.05.020

* Corresponding author. E-mail: catalina.jimenez@helmholtz-berlin.de

to the production of more regular porous structures.The observed formation of crack-like pores shows thatnot only does the blowing agent decompose before melt-ing of the alloy, but it also makes deficiencies of powderconsolidation evident.

In traditional powder metallurgy, hot compactionunder vacuum is a practice that is known to improvethe consolidation of aluminium PM parts [13,14]. Hencehot compaction under vacuum could reduce the forma-tion of crack-like elongated pores during foaming.

In order to evaluate this possibility, we compactedAlSi11precursors containing TiH2 both under vacuumand in air. We compared these two types of compactsin terms of properties of the compacted materials, foam-ing behaviour, hydrogen evolution and resulting foamstructure.

We used aluminium (Alpoco Ltd., purity 99.7%), sil-icon (Wacker Chemie GmbH, purity 99.5%) and TiH2

(Chemetall GmbH, purity 98.8%) powders. TiH2 waspre-treated at 480 �C for 180 min in air. We measuredthe oxygen content of the powders by carrier gas hotextraction and the density by helium pycnometry.

The powders were blended to prepare the alloy Al-Si11 + 0.5 wt.% TiH2.

We prepared tablets of 30 g mass and 36 mm diame-ter by uniaxial hot compaction in air and under vacuuminside a chamber. A pre-compaction step was done at200 �C, applying 300 MPa for 60 s. Hot compaction

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Figure 1. Microstructure of compacted precursor material. (a) Lightmicroscopy image and (b) SEM image.

C. Jimenez et al. / Scripta Materialia 61 (2009) 552–555 553

was performed at 400 �C, applying 300 MPa for 300 s inall cases. The gas pressure was kept below 8 � 10�2

mbar during hot compaction under vacuum.A shell of 1 mm thickness was removed from the tab-

lets by machining, after which we determined the densi-ties by Archimedes’ principle. We measured the oxygencontent of both compacted materials, and examinedmicrostructural features by light and scanning electronmicroscopy (SEM). After polishing, the surfaces wereetched for 5 s in an aqueous NaOH solution (10 g ofNaOH in 80 ml of distilled water at 50 �C).

We studied hydrogen evolution from compacted pre-cursor materials as a function of time and temperatureby mass spectrometry. For this, we prepared cylinder-shaped samples of 5.7 mm diameter � 4.7 mm height.We heated the samples in flowing synthetic air(50 ml min�1) inside a tube furnace coupled via a skim-mer to a quadrupole mass spectrometer. We heated thesamples at 40 K min�1 from 37 up to 680 �C and heldthat temperature for 30 min before cooling (T(t) inFig. 2).

For foaming, the material was cut to10 � 10 � 3 mm3 in size. The volume-to-surface ratioof these samples is equal to that of the cylinders. Foreach type of precursor, six identical samples werefoamed in air at ambient pressure by heating on a resis-tive heater (�300 W) at 160 K min�1 from 37 to 680 �Cand held at that temperature for 120 s. The heater wasthen turned off and natural cooling took place (T(t) inFig. 3a). Foam temperature was measured by a thermo-couple at the bottom of the sample. The foaming pro-cess was investigated in situ by X-ray radioscopy witha time resolution of 2 s, using the image analysis soft-ware AXIM [15] to determine area expansion as a func-tion of time.

We characterized non-destructively one representa-tive foam from each group by X-ray tomography, per-formed in a setup similar to the one used forradioscopy. A total of 1000 projected radiographs weretaken for 360� rotation and reconstructed using the soft-ware Octopus 8.2. VGStudioMax 1.2.1 was used to ex-tract three-dimensional (3-D) sections and Avizo 5 forquantitative 3-D pore analysis.

The measured oxygen contents of Al, Si and pre-trea-ted TiH2 powders were 0.46 ± 0.07, 0.22 ± 0.01 and4.02 ± 0.08 (in wt.%), their densities 2.73 ± 0.07,2.33 ± 0.05 and 3.76 ± 0.07 (in g cm�3), respectively.From these values, we calculated an oxygen content of0.45 ± 0.07 wt.% and a density of 2.68 ± 0.06 g cm�3

for the powder mixture. The latter was adopted as thetheoretical full density for the compacts.

Oxygen contents and relative densities of the com-pacts are summarized in Table 1. Vacuum-pressed tab-lets achieved higher relative densities than air-pressed

Table 1. Properties of powder compacts.

Compactionatmosphere

Oxygencontent (wt.%)

Relativedensitya (%)

Vacuum 0.48 ± 0.05 99.24 ± 0.07Air 0.67 ± 0.08 98.93 ± 0.03

a Relative to 2.68 g cm�3, the theoretical full density.

tablets. The oxygen content of vacuum-pressed com-pacts was almost the same as that of the powder mixturebefore compaction, whereas pressing in air increased theoxygen level.

The residual porosity in both compacts was mostlylocated near Si particles. Many Si and TiH2 particleswere fractured after compaction (Fig. 1a and b).

The specific hydrogen release for ‘‘mass = 2” per unitsample mass is given in Figure 2. The onset of hydrogenrelease – 343 �C in the air-pressed sample – was shiftedby 212 K up to 555 �C in the vacuum-pressed sample.Below each onset, the release increased almost linearly.The vacuum-pressed material released hydrogen at halfthe rate of the air-pressed material. The first peak ofhydrogen release in both specimens occurred at 579 �Cand the second peak at 663 �C, after which hydrogen re-lease decayed and was almost exhausted after 22 min.

We calculated the maximum area expansion from theexpansion curves in Figure 3a. Vacuum-pressed precur-sors reached 365 ± 21% maximum area expansion,whereas air-pressed precursors reached only288 ± 30%. Up to 555 �C, both groups expanded line-arly. A first range of non-linear expansion occurred be-tween 555 and 647 �C (also indicated in Fig. 2), wherethe deviation from the initial linear expansion was morepronounced for the vacuum group that started to sur-pass the air group. Above 647 �C, the air group ofexpansion curves developed a larger scatter than thevacuum group.

We also compared the foaming behaviour in Figure3b using radioscopic image sequences. Both sampleswere at 37 �C at t = 0, and at 680 ± 5 �C after 245, 280and 336 s. The expansions of these two representative

Figure 2. Specific mass = 2 ion current (hydrogen release) vs. time forprecursor materials either compacted in air or under vacuum. Theexperiments were carried out in flowing synthetic air applying thetemperature profile T(t). The onset and peak temperatures indicatedare discussed in the text; 555 and 647 �C are also denoted in Figure 3a.

Figure 3. (a) Area expansion vs. time for 12 foams. Legends indicatethe compaction atmosphere. All the samples were foamed applying thetemperature profile T(t). The temperatures 555 and 647 �C (alsomarked in Fig. 2) denote the onset of non-linear expansion in the solidstate and the beginning of continuous expansion in the semi-moltenstate for the vacuum-pressed powder compacts. (b) Radioscopic imagesequences of representative foams at times 0, 245, 280 and 336 slabelled in (a) with filled circles and squares.

554 C. Jimenez et al. / Scripta Materialia 61 (2009) 552–555

foams at those times are marked with filled circles andfilled squares in Figure 3a. Elongated bubbles perpendic-ular to the compaction direction formed in both kinds ofprecursor. They initially grew perpendicular to the com-paction direction. In air-pressed precursors this growthwas more localized, and after 245 s the large elongatedbubbles became roundish but evolved to an inhomoge-neous structure. In contrast, in the vacuum-pressed pre-cursor more and smaller elongated bubbles were formed.Their growth perpendicular to the compaction directioneventually stopped and the foam expanded homoge-neously after 245 s.

X-rays tomographic sections and 3-D pore size distri-butions of the representative foams are compared inFigure 4. The sections correspond to the central planeof the foams perpendicular to the compaction direction.The foam made from air-compacted precursor had alarge population of small pores and a large solid fractionconcentrated in the outer region. In comparison, thefoam made from vacuum-pressed powder had a smallerpopulation of small pores in the outer region and a more

Figure 4. 3-D pore size distributions of foams made from air- andvacuum-pressed precursors. Frequency � pore volume fractions asfunction of diameter were fitted by a double-peak Gaussian model(dotted curves). Values indicate mean diameters Di and standarddeviations ri for every peak. Top-right, tomographic reconstructionsof central sections perpendicular to the compaction direction.

uniform distribution of the solid fraction throughout thesection. Structural defects such as missing cell walls andinterconnections between pores were present in bothfoams.

Three-dimensional pore analysis was performed andthe volume of spherical pores of equivalent diameter Dcalculated, applying a lower threshold of 100 lm. Bothpore size distributions were bimodal and fitted with adouble-peak Gaussian model. The resulting parametersare included in Figure 4. The total pore volume of thefoam made from the air-pressed precursor was855 mm3, while that of the foam made from the vac-uum-pressed precursor was 1172 mm3. The ratio ofthese two volumes, 0.73, expresses the improvement inexpansion. The ratio between standard deviations ofthe second peak was r2;air=r2;vacuum ffi 1:8: Hence thelarge pores of the foam made from vacuum-pressed pre-cursor were not only centred at a smaller mean diameter,2.39 mm, but were much more equally distributed thanin the foam made from air-pressed precursor.

Besides the spatial correlation between early crack-like pores and Si particles [5,6], Helfen et al. concludedthat the bonding strength between Al and Si is lowerthan between Al and TiH2 or between Al particles [6].Si and TiH2 are both brittle. They fracture and hostresidual porosity inside or nearby them (Fig. 1).Strengthening the Al/Si or Al/TiH2 bonds would notimprove the resistance of the compacted material tothe growth of crack-like pores. The alternative is tostrengthening the bonding between Al particles. Al pow-der is ductile but the powder particles are covered by astable oxide layer, which can also react with moistureto form hydroxides. During hot compaction, Al parti-cles are squeezed and sheared to become metallicallybonded. Compaction under vacuum helps remove en-trapped gases, dehydrating oxides and preventing fur-ther oxidation [13,14]. Thus the oxide layers break intosmaller pieces and the metallic bonding between Al par-ticles is achieved in larger regions compared to compac-tion in air. In this manner, we produce a moreconsolidated Al matrix able to reduce cracking by con-trolling the growth of crack-like pores formed in the so-lid state.

The increase in density due to vacuum pressing as gi-ven in Table 1 indicates a tendency, but does not revealto what extent the Al particles are metallically bonded.The oxygen content gives complementary informationas it is localized on unbound surfaces. Al PM partsare degassed typically above 500 �C [13,14]. Therefore,one possibility to ensure good consolidation of foamableprecursors would be to hot-press at temperatures higherthan 400 �C. However, untreated TiH2 starts to decom-pose at around 400 �C [11]. Even though TiH2 is pre-treated in air to retard its decomposition, the particlesbreak after compaction, as shown in Figure 1b, creatingnon-oxidised surfaces, thus facilitating decomposition.The release of H2 in the presence of oxygen can leadto the formation of water [16,17], which could be thereason for the increase in oxygen content of the air-pressed compacts.

The hydrogen evolution from compacted materialshown in Figure 2 was used to evaluate the effect ofthe compaction atmosphere on consolidation applying

C. Jimenez et al. / Scripta Materialia 61 (2009) 552–555 555

a temperature profile similar to the one used for foam-ing. The difference of 212 K (555–343 K) between onsettemperatures confirmed that compaction under vacuumsignificantly improved consolidation.

According to the binary phase diagram, the first li-quid should be formed at the eutectic temperature of577 �C [18]. We observed that at 579 �C hydrogen lossesdecay (first peak), as if the formation of liquid partiallysealed the paths through which hydrogen escaped. Thesame melting sequence applies for both compacts, whichis why the two peaks were observed at the same temper-atures. Matijasevic-Lux et al. [11] measured hydrogenevolution from AlSi6Cu4 and von Zeppelin et al. [19]from AlSi7, both containing TiH2. Both groups ofauthors reported single peak structures for hydrogen re-lease rather than the double peak structure we observe.We attribute this difference to the lower heating rates (20and 5 K min�1, respectively) and the smaller samplesizes (3 � 3 � 3 mm3) used by them. After the first peakhas levelled off, hydrogen release approaches a secondmaximum, as if TiH2 decomposition becomes dominantin the semi-molten state of the alloy. The second peak at663 �C is close to the melting point of Al (660 �C).

The porous structure is initiated in the solid state andcarries a history that cannot be erased by the melting se-quence during foaming, as shown in Figure 3. The betterconsolidated vacuum-pressed precursors reached largerexpansions than air-pressed ones due to the significantreduction of hydrogen losses, especially below the eutec-tic temperature. The expansion of vacuum-pressed pre-cursors deviated from the initial linear increase ataround 555 �C, the temperature at which these precur-sors release their first hydrogen (Fig. 2). The moreabrupt change in hydrogen release and expansion thatoccurs at 555 �C for the vacuum-pressed material pointsat the relationship between onset of hydrogen releaseand non-linear expansion.

Helfen et al. [20] studied the evolution of the porousstructure of AlSi7 using computed tomography. Theyobserved that with the appearance of the liquid thecrack-like interconnected pore morphology graduallyrounded off, thereby reducing surface tension. This isconsistent with our observations that above 647 �Cexpansion could develop steadily sustained by thedecomposition of TiH2 and a large enough fraction ofliquid. The system tended to have more roundish bub-bles, but for the definite decay of hydrogen release themelting sequence needed to be completed, i.e. to reachthe melting point of Al (the second peak in Fig. 2).

Shorter and more numerous elongated bubbles in thevacuum-pressed precursors were further evidence of thebetter consolidation. These compacts permitted higherpressures to build up in the smaller pores. The initialgrowth perpendicular to the compaction direction wasarrested, leading to more efficient, homogeneous andreproducible expansion. The more regular porous struc-ture obtained after solidification from vacuum-pressedmaterial (shown in Fig. 4) was consistent with the lim-ited growth of elongated pores in the early stages ofexpansion.

In the case of air-compacted precursors, the lower on-set temperature and the larger quantity of hydrogen re-leased made expansion more sluggish over the entire

foaming course. After melting, big elongated bubblesformed and maintained their anisotropic shape, eventu-ally leading to non-uniform expansion. As hydrogen es-caped through the surface, the build-up of pressure inthe outer pores was reduced, as was their ability to growafter melting. Thus the larger hydrogen losses promotedthe accumulation of small pores in a denser outer regionof the resulting solid porous structure.

In conclusion, hot compaction under vacuum led tobetter consolidation of foamable AlSi11 precursors byimproving the degree of metallic bonding between alu-minium particles. The better consolidation retardedhydrogen evolution and reduced losses especially beforemelting. The vacuum-pressed metallic matrix was morecapable of arresting the growth of elongated bubbles.The result was a larger and more homogeneous expan-sion and a more regular pore size. The foam made fromvacuum-pressed powders had a 37% larger volume thanthe foam made from air-pressed powders and a nar-rower pore size distribution.

We thank Christian Abromeit for helpful discus-sions and Andreas Benz for assistance with thetomography.

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