302 Chiang Mai J. Sci. 2009; 36(3)

Chiang Mai J. Sci. 2009; 36(3) : 302-311www.science.cmu.ac.th/journal-science/josci.htmlContributed Paper

1. INTRODUCTIONProduction of aluminium (Al) foams

through powder metallurgy route is attractivedue to the ease of fabrication and the possibilityof manufacture of near-net-shape productswith a relatively homogeneous cellularstructure [1]. There are several importantfactors on the production of Al foams usingthis technique. These factors involve basematerials, foaming agents and processparameters such as precursor density andgeometry, mould size and geometry, foamingtemperature, heating rate, atmosphere andpressure, etc., that can affect the final foamproduct with different density distributionalong foaming direction and the appearanceof the foam. Based on Taguchi methodology,

On the Production of Aluminium Foams StabilisedUsing Particles of Rice Husk AshSeksak Asavavisithchai* and Rath TantisiriphaiboonDepartment of Metallurgical Engineering, Faculty of Engineering, Chulalongkorn University,Bangkok 10330, Thailand.*Author for correspondence; e-mail: fmtsas@eng.chula.ac.th

Received: 22 May 2009Accepted: 18 June 2009

ABSTRACTAluminium-titanium hydride precursors added with rice husk ash particles have been

successfully foamed with improved pore structure. Initially, the powder materials were blendedand compacted to produce a high density precursor. The precursor was heated to a temperatureslightly above the melting point of aluminium, resulting in hydrogen gas release of hydrideparticles, leading to foam formation. The particles significantly influence the foam structure byincreasing the viscosity of aluminium melt, modifying foam microstructure and increasing thecell wall strength, resulting in an improvement in the structure and an increase in compressivestrength and energy absorbed.

Keywords: aluminium foam, rice husk ash, powder metallurgy, metal-matrix composite,porosity, mechanical properties, energy absorption.

different melt viscosity and different contentof foaming agent are shown to play a crucialrole to variability in foam structure andproperties [2]. Al foams fabricated by thisapproach exhibit a closed-cell structure withhigher mechanical strength than open-cellfoams. It is also possible to manufacturefoams of various metals and alloys. Theclosed-cell foam is particularly attractive forapplications requiring reduced weight andenergy absorption capabilities.

The technique requires a high-densityconsolidation of Al and titanium hydride(TiH2) powder mixture, followed by heatingto a temperature slightly above the meltingpoint of Al. The TiH2 then decomposes and

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releases the hydrogen gas while Al starts tomelt, resulting in the formation of highlyporous Al specimens. During foaming, foamstructure dramatically and continuouslychanges with time as a result of cellcoarsening; drainage and cell coalescence[3, 4]. Drainage is movement of liquid throughthe foam structure, driven by gravitationalforce and surface tension gradients from cellwalls to Plateau borders. Cell coalescence is aphenomenon where at least two cells mergecreating a bigger cell, driven by gas diffusionthrough cell walls until the critical cell wallthickness is reached. Cell coarsening is a majorcause to foam collapse. Extensive cellcoarsening destabilises cell structure and resultsin a non-uniform foam structure.

In order to improve foam expansion andstability, ceramic particles, such as Al2O3, SiCand TiB2, are commonly added to powdercompacts before foaming [5-9]. It is believedthat the particles can retard the flow of moltenAl through cell structure, resulting in anincrease in bulk liquid density, leading to areduction in the rate of cell coalescence anddrainage [10]. Only for good wetting particleswith the liquid metal, the attachment ofparticles to the gas/metal interface mightmodify the curvature of the cell walls, leadingto a decrease in the rate of liquid flow fromthe cell walls to the Plateau borders [11]. Arecent study based on a lattice Boltzmannmodel showed that the particles in cell wallscan induce a repulsive disjoining pressurewhich results in high foam stability duringfoam formation [12].

Rice is one of major agricultural productsthat are abundantly produced in Thailand. Ricehusk is a waste by-product of rice industryand usually eliminated by combustion. Afterburning at temperature between 1046-1146oC, white rice husk ash (RHA) is obtainedand typically found to contain 87-97 wt.%amorphous silica (SiO2) [13]. The silica

crystallises in the forms of cristobalite andtrydimite are detected at higher temperaturesof > 1073 K and > 1423 K, respectively [14].In last decades, utilisation of the RHA hadbeen made mainly for producing concrete[15-18] A variety of other RHA applicationsare also reported [13, 19-22].

Many studies exist on the application ofwaste materials containing high concentrationof silica, such as fly ash or RHA, in order toreplace ceramic particles in compositematerials [16, 20, 21, 23-25]. The motivationbehind the use of the waste materials is toreduce the cost of the composite materialsby decreasing the cost of reinforcements orby using low cost reinforcements. A fewstudies are known for the use of thesematerials in the production of metal foam[26, 27]. Owing to their chemical and physicalstability in the Al melt, it is conceived that suchwaste materials could be used as thickeningagent to increase the viscosity. However, theuse of RHA as stabilising material in metalfoams has never been investigated before.

The objective of the present study is toinvestigate the use of RHA waste, insteadof commercial ceramic particles, for animprovement in the stability and mechanicalstrength of Al foams fabricated throughpowder metallurgical approach.

2. MATERIALS AND METHODSAir-atomised Al powder (Ecka Granules),

was mixed with 0.6 wt.% TiH2 powders(Sigma Aldrich), using a rocking mill for 60min. As-received RHA particles, obtainedfrom Wang Noi CCGT Power Plant inAyuthaya, Thailand, were thoroughly washedwith water to remove the dust and dried at50oC in the oven. The cleaned RHA was thenadded to the powder mixture up to a levelof 3 wt.%. The mixed powders wereuniaxially consolidated in a lubricated 22-mmtool steel die to a pressure of 700 MPa.

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Compacts were foamed in a 22-mm stainlesssteel mould and placed in a pre-heated furnaceat 800oC. The heating times were varied from285 to 360 s, with an interval of 15 s increasedfrom the previous holding time of foamedcompact, in order to produce foam specimenswith different expansions. After heating,foams were removed from the furnace andallowed to cool in air. Densities of foamspecimens were measured using Archimedes’densitometry technique. The specimens wereweighed in air and then after covering thesurface with a thin layer of petroleum jelly,weighted in distilled water. The petroleum jellywas applied to prevent inflow of water intoany surface pores. Equation 1 was used tocalculate the foam density (ρ foam) and thepercentage volume expansion (Δvol) wascalculated according to equation 2. It is notedthat the weight of petroleum jelly can beignored and is not included in the equation 1for when a large specimen is used and a thinfilm of jelly is applied.

(1)

(2)

Where ma and mw are masses of foam in airand in water, respectively, andρprecursor and ρware the densities of compact and of water,respectively.

To examine foam macrostructure, thefoam specimens were longitudinally sectioned,along the mid-plane, using electro dischargemachine (EDM). The sectioned specimenswere painted with matt black spray, grindedusing SiC papers and scanned using a HP PSC1402 at resolution of 200 dpi. Microstructuralexamination was performed using JSM-5800LV scanning electron microscope (SEM).Characterisation of RHA size and distributionwas carried out using Mastersizers particle size

laser analyzer. Chemical compounds of RHAwere determined using x-ray fluorescence(XRF). X-ray diffraction (XRD), using JEOLJDX-8030 diffractometer, was also performedon the RHA in order to identify the phasespresent.

Specimens for compression test wereprepared by sectioning the foams in the mid-sections with the length of 30 mm. The bulkdensity of the specimens was determined bymeasuring dimension and weight of thespecimens, and only the foams with the densityof 0.6 0.05 g/cm3 were selected for thetest. The specimens were deformed to 60%strain at a constant cross head speed of 50mm/min using an Instron 5567 universalmechanical testing machine.

3. RESULTS AND DISCUSSION3.1 Powder characteristics

Figure 1 presents the morphology ofRHA particles. The particles are brittle and havea natural fibrous shape with two differentsurfaces. The outer surface is comprised oflarge protuberances, while the inner surfaceshows a shallow and overlapping wavy skin.No agglomeration of RHA particles wasobserved. The particle size distribution andchemical compounds of RHA particles areshown in Table 1. It can be seen that theparticle contains approximately 95 wt.% silica

Figure 1. Morphology of the rice husk ash.

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with small amounts of other oxides. The XRDpattern, as shown in Figure 2, shows that thesilica in RHA is crystalline, consisting ofcristobalite which is reported to form atsintering temperature > 1346oC [14]. The

RHA in the present study was the waste fromthe incineration of rice husk in the powerstation which was typically subject to the hightemperature of >1500oC.

Table 1. Particle size distribution and chemical composition of the rice husk ash.

D10

D50

D90

Mean Chemical composition (wt. %)

Particle (μμμμμm) (μμμμμm) (μμμμμm) diameter(μμμμμm) SiO

2K

2O P

2O

5CaO Others

RHA 0.10 131.93 424.14 179.56 94.876 1.952 1.077 0.744 1.351

3.2 Foam expansion and macrostructureFigure 3 presents the expansion plots

and macrostructural images of cross-sectionalfoams with relative densities, correspondingto the plots, of Al-RHA foams at themaximum expansion, compared with pure Alfoam. The pure Al foam shows a typicalexpansion for which the powder mixtureprecursor expands to a maximum (382 vol.%)and followed by a rapid collapse. It shouldbe noted that the compaction density of all

precursors used in the experiments was at99%. Over 3 wt.% RHA particle addition, theprecursor density reduces rapidly, resulting inpoor and unacceptable foam expansion, dueto gas escape through interconnected porosityin the precursor. For the production of metalfoams through powder metallurgicalapproach, it is generally recommended tomaintain the compaction density at 99% orcloser to theoretical density to ensure a largefoam expansion [28]. Using hot compaction

Figure 2. XRD pattern of the as-received rice husk ash particles.

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approaches, it is likely that more amount ofRHA can be added while still be able tomaintain high density of the precursor.However, special attention must be givenwhen hot compaction is applied. Too highcompaction temperature above the decom-position temperature of TiH2 results in thegas being lost during powder consolidation.Conversely, using lower compactiontemperatures means that lower densities areachieved, resulting in weaker precursors withinterconnected porosity which enables gas to

escape on heating to the melting point of Al.In addition, a thick oxide film on precursorsurface formed during compaction canrestrain foam expansion. To avoid thedrawbacks of hot compaction, it is possibleto apply the double pressing action in coldcompaction method to the precursor andreceive similar high density [29].

The content of RHA particles plays animportant role to foam expansion andstability. It is clear that the addition of 1 wt.%RHA particles results in an increase in foam

Figure 3. Expansion and pore structure at maximum expansionof Al foams added with rice husk ash at various contents.

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expansion (393 vol.%), and beyond thisamount, the expansion decreases. Theexpansion also decreases more with increasingRHA content. A possible explanation is that,with the addition of 2 and 3 wt.% RHA, theviscosity of Al melt was too high for evolvedhydrogen gas in pores to enlarge the foamseffectively, resulting in lower expansion.Owing to the irregular morphology of theRHA particles, the melt viscosity can be readilyincreased. Nevertheless, the pore structure ofAl-RHA foams, in all cases, is more uniform,indicated by less density gradient and lessirregular pore shape, compared with thepure Al foam. Typically, pore growth andcoalescence during foam expansion occur toform a few very large pores. Cell wall ruptureis a sudden instability in a liquid film, leadingto its disappearance. It is thought that the RHAparticles attached to a liquid/gas interface canreduce the surface tension of the film, resultingin a reduction in the critical cell wall thicknesswhich enables larger foam expansion.However, too much particle can also rupturethe film due to its own weight which mightbe the case of 2 and 3 wt.% RHA additionsthat had many large pores throughout the

foam structure.Drainage of the foams is also reduced,

as shown that no dense layer at the bottomof the foams is observed. The improvementin cell structure is attributed to the uniformdispersion of RHA particles throughout thefoam body. The presence of the RHA particlesin the Plateau borders and cell walls can hinderthe melt flow, resulting in an increase in bulkviscosity. It is thought that the RHA particlescan strengthen the cell walls in much the sameway as ceramic particles [7, 12], and then canprevent the premature rupture of the cellwalls.

3.3 Foam microstructureFigure 4 shows the local sites of the RHA

particles in a cell wall. It can be seen thatparticles mainly reside at the gas/metalinterface of the pore. The rupture of someparticles was observed. The RHA particles arevery fragile and some particles tend to breakdown during consolidation process, owing tohigh shear force. The rupture of the particlesin this case might have a benefit to foamstabilisation. It is reported that large particlesmight lead to the premature rupture of the

Figure 4. Rice husk ash particles embedded in a cell wall.

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cell wall in which they reside [30]. The smallersolid particles stabilise the foam structurebetter than larger ones since for the samevolume fraction of particles in the liquid, theliquid/gas interface is increasingly beingcovered by smaller particles, resulting inincreasing strength of the cell wall andblockage of drainage. The reaction betweenthe particles and Al matrix was not observedsince the SiO2 in the RHA particles is quitestable with liquid Al. The protrusion of theparticles from the cell wall indicates the non-wetting behavior of the particles. The poorwetting of SiO2 by pure molten Al iscommonly known as the contact angles aregenerally larger than 90o [31]. Since the RHAparticle contains a high concentration of SiO2,it is expected that the wettability of RHAparticle by Al melt is similar to that of theSiO2. As a result, the particles are ejected fromthe bulk of molten Al to the cell wall surface.It should be noted that although the additionof RHA particles can improve the foamstructure, it may have an adverse effect on thefoam expansion and compressive behaviour,particularly at increasing particle concen-trations. In the study, it was proved difficultto produce the precursors with the minimumdensity of 99% by cold compaction methodwhen the particle content increases, due to hardfibrous nature of the particles. Using hotcompaction method, such as hot pressing,HIPing or extrusion, would be a better wayto achieve the required precursor density toassure large foam expansion. In addition, asAl-RHA foam becomes more brittle whenthe particle content increases, the stressconcentration either at the particle/metalinterface or in the regions of particlesegregation might lead to foam collapse andnon-uniform structure. However, even withnon-wetting behavior in the Al-RHA systemand a problem with increasing particleconcentrations, the present study has shown

that the addition of RHA particles in a smallamount resulted in an improvement in thestability of foam structure.

3.4 Mechanical propertiesFigure 5 presents the compressive

strength and energy absorption of pure Aland Al-RHA foams with various particlecontents. The summary of mechanicalproperties of the foams is shown in Table 2.The yield stress of the foams is taken at 0.02offset strain. The energy absorbed per unitvolume (E) can be determined by the areaunder the stress-strain plots as follows

(3)

where σ is compressive stress, l is the limitof strain concerned and ε is compressivestrain. The energy absorption efficiency (η),which is commonly used to evaluate theperformance of absorbing materials forengineering design purpose, can be definedby

(4)

where σmax is the maximum stress applied.The compressive plots, in all cases, show

a plastic semi-plateau region which is similarto Al foams containing conventional ceramicparticles [5-8]. The compressive strength ofthe composite foams strongly depends on theparticle content. It is clear from the figure that,of equivalent density to the pure Al foam, theAl-RHA foams have higher yield stresses andlarger energy absorption, for a given strain.The strength and energy absorbed of thefoams also increases with increasing RHAparticle contents.

An increase in the strength of the Al-RHA foams is thought to be resulted fromthe modification of the microstructure of the

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foams and the increasing strength in the Plateauborders and cell walls at which the RHAparticles reside. The investigation of porestructure shows that the composite foam tendsto have more spherical cell morphology andless cell defect density than the pure Al foam.Moreover, a decrease in the fraction of metalin the Plateau borders and cell walls increasesthe hardness and strength of foamed metal.

The structure of the resulting foamsmight be modified by varying the particlecontent and the foaming conditions. It can beseen from the foam expansion and structureshown in Figure 3 that in order to obtain thebest use of the RHA particles, a compromisemust be made to optimise the expansion,structure and mechanical properties of thefoams for a specific application. The structureof Al foam and its mechanical properties areinterdependent. The mechanical propertiesof Al foams are highly sensitive to cellmorphology and density distribution in thecellular structure [32]. As the foam structurestrongly depends on the RHA particle contentand characteristics, it is thereby possible totailor mechanical properties of Al foams by

controlling the amount of particle addition,particle size and morphology.

4. CONCLUSIONSThe conclusions can be drawn from the

present work as follows.1. Al foams reinforced with RHA

particles have been successfully produced.2. The addition of RHA particles in the

foams resulted in a significant improvementin the foam structure.

3. A larger maximum expansion ofcomposite foams was found at 1 wt.% RHAparticle addition, compared with pure Alfoams, and beyond this amount, the foamexpansion decreases.

4. The composite foams also show anincrease in the compressive strength andenergy absorption, for a given strain.

5. The improvement in foam structureand mechanical properties of the foams areresulted from the increasing viscosity of Almelt, the modification of foam microstructureand the increasing strength in the cell walls atwhich the particles reside, respectively.

Figure 5. Compressive behavior of Al foams addedwith rice husk ash at various contents.

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ACKNOWLEDGEMENTThe authors gratefully acknowledge the

Thailand Research Fund (TRF) for thefinancial support.

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