TECHNICAL OVERVIEW OF ALUMINUM ALLOY FOAM · 2017-04-22 · TECHNICAL OVERVIEW OF ALUMINUM ALLOY...

68 D. K. Rajak, L.A. Kumaraswamidhas and S. Das

© 2017 Advanced Study Center Co. Ltd.

Rev. Adv. Mater. Sci. 48 (2017) 68-86

Corresponding author: Dipen Kumar Rajak, e-mail: [email protected]

TECHNICAL OVERVIEW OF ALUMINUM ALLOY FOAM

Dipen Kumar Rajak1, L.A. Kumaraswamidhas1 and S. Das2

1Department of Mining Machinery Engineering, Indian Institute of Technology (ISM), Dhanbad 826004, JH, India2CSIR-Advanced Materials and Processes Research Institute, Bhopal 462064, MP, India

Received: August 09, 2016

Abstract. In recent years, metal foams have emerged as a popular advanced material for struc-tural purpose. This class of material has novel physical, mechanical and electrical propertiesalong with low density. It is considered to be one of the most suitable materials for structures thatneed to be light weight and at the same time crashworthy, thermally insulate and cheap. Metalfoams normally retain the beneficial physical properties of their base metal. Foams made of non-flammable base metals are non-flammable as well and can be recycled to their base material.Their coefficient of thermal expansion is also similar to that of the base metal while their thermalconductivity gets reduced due to porous internal structure. The present paper highlights themanufacturing process and properties of metal foams as well as their advantages and limita-tions. Till date, nine patents have been obtained on foam manufacturing processes.

1. INTRODUCTION

In recent years, aluminium foams have emerged asa popular advanced material for structural purpose.This class of material has novel physical, mechani-cal and electrical properties along with low density.It is considered to be one of the most suitable ma-terials for structures that need to be light weightand at the same time crashworthy, thermally insu-late and cheap. Aluminium foams normally retainthe beneficial physical properties of their base metal[1-22]. Foams made of non-flammable base metalsare non-flammable as well and can be recycled totheir base material. Their coefficient of thermal ex-pansion is also similar to that of the base metalwhile their thermal conductivity gets reduced due toporous internal structure. Over the years, a numberof patents have been obtained on the productionmethods of metal foams. Even though these meth-ods ensure practicable topological structures ofconstitutive materials and metal foams these foams

are yet to be considered a commodity and as ofnow relatively few commercial producers worldwideuse either closed or open cell foams. One of thesignificant manufacturing approaches involves ultra-light Al-alloy foams. However, its demand in themarket is considerably restricted. One probable rea-son is presumed to be the poor product quality.However, the latest available manufacturing processenables effective control of density by manipulatingprocess parameters. This will facilitate large-scalecommercial acceptance of ultra-light Al-foams inhighly demanding sectors, such as, the automo-bile, defense, aeronautical, etc. The scientific com-munity has long been engaged with resolution ofthe challenges associated with production of Al-foams. Closed-cell Al-alloy foams can be used forlightweight structures, energy absorption and damp-ing structures in different industrial sectors. Particu-larly, it has a huge potential in transportation andstructural engineering. Energy absorption is theability to absorb energy or force by a material under

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69Technical overview of aluminum alloy foam

different mechanical loadings. This ability is mainlyused for determining the energy absorption charac-teristics like peak crushing force mean crushingforce, specific energy absorption and crash forceefficiency. The assessment of energy absorptioncharacteristics of energy absorbing materials isimportant as they are pre-dominantly used in appli-cations of crash energy absorption in high speedvehicle impacts. The reduction in the weight of ob-ject i.e. lightweight vehicle, increased speed of thevehicle resulting high speed crashes and fatalities.These energy absorbing materials have expandedinto the applications of aerospace, ship building,construction and blast industries also. The energyabsorbing materials are highly used for enhancingthe crashworthiness design of the vehicle, as it pas-sively has effects on safety performances. In auto-mobiles, crash box of automobile Body in White(BIW) is used for absorbing the crash energy duringan impact with other vehicle and this crash box iscomprised of thin-wall metal columns mainly of mildsteel and aluminium. The thin-wall metal columnsare the cheapest and conventional energy absorb-ing materials used in trains, ships, high-volume fac-tory products and automobile design and manufac-turing [22-30].

The research from past few decades indicate thatthe energy absorbing capabilities of these metalcolumns can be increased by honeycomb materi-als and metal foams as filler materials. Many re-searchers performed experiments and studies forunderstanding the energy absorption characteris-tics of these metal columns and also designing andimproving their energy absorption characteristics.Metal tubes were widely used as energy absorbingdevices since they are cheap and light in weight.They were used as energy absorbing devices toimprove the safety of passengers in a crash andcrashworthiness of vehicles such as cars, trains andships. In designing of energy absorbing structures,the major goal is to maximize energy absorptioncapacity and minimize mass. The lightweight andgreat energy absorbing capacity of cellular materi-als like foam attracted many researchers to studytheir characteristics. Metallic foams especially oflighter metals emerge as a new range of materialswith great prospective applications due to its excel-lent strength-density ratio which presents advan-tages for the development of components for theautomotive industry. Aluminium foams have very in-teresting combination of characteristics like highcompressive strength and low specific weight. Metalfoams can be classified into open cell and closedcell based on type of pore structure. The Young’s

modulus and plastic strength falls below the ex-pected values of ideal cell model due to imperfec-tions in density distribution and cell wall curvature.A recent improvement in processing techniques ofaluminium foam has significantly improved the me-chanical properties by reducing the defects in cel-lular structure. The four main processing techniquesare liquid state processing, electro deposition pro-cess, vapor deposition process and solid state pro-cessing. The compressive behavior of aluminiumfoam under quasi-static and dynamic conditions areextensively studied in recent years. The results ofcompression test of aluminium foam under dynamicconditions are conflicting on their strain rate sensi-tivity. The scientific community has long been en-gaged with resolution of the challenges associatedwith production of meal foams. Closed-cell Al-alloyfoams can be used for lightweight structures, en-ergy absorption and damping structures in differentindustrial sectors. Particularly, it has a huge poten-tial in transportation and structural engineering. Inthis context, the following two approaches for pro-duction of foams may be examined as well [12-18].(i) Developing new manufacturing processes or

modifying the existing ones in order to obtainfoams with more uniform cellular structures.

(ii) Understanding and quantifying the thermo-physico-chemical mechanisms during the foamformation in order to control the process and toavoid structural imperfections and defects. Thenewly developed manufacturing processes formetal foams may be classified into two groups:-(I) Direct foaming method and (II)Indirect foam-ing method.

In the direct foaming method, molten metal,added with ceramic particles, is uniformly dispersedinto which gas bubbles are directly injected orchemically generated by decomposition of a foam-ing agent (titanium hydride or calcium). It can alsobe done by precipitation of gas dissolved in the meltby controlling temperature and pressure.

Indirect foaming method first requires prepara-tion of formable precursors that are subsequentlyfoamed by heating. The formable precursors con-sist of a dense compacted powder with the foamingagent particles which are uniformly distributed inthe metallic matrix [18-36].

The present study offers an overview of the state-of-the-art manufacturing methods, properties andapplication of aluminium foams.


2. MANUFACTURING METHODS

In the last few years, many technologies haveevolved for the production of metallic foams [1-9].However, it is important to stress that only a few ofthese processes are appropriate for the manufac-ture of metal foams on a commercial scale, keep-ing environmental factors in mind. Following are themost viable technologies:

2.1. Liquid metal (Melt Route Process)

Most of the commercially accessible metal foamsare based on alloys containing aluminium, nickel,magnesium, lead, titanium, steel, copper, and evengold [5]. Among the metal foams, Al-alloys are com-mercially the most preferred ones because of theirlow specific weight, high ductility, high thermal con-ductivity and competitive cost.. Among the produc-tion methods, it is the direct foaming method that isbeing commercially exploited on a large-scale. TheCymat Aluminum Corporation (Canada) is globallythe leading manufacturer of aluminium foams, whichbrands this product as “stabilized aluminum foam”.It is produced by gas injected directly into the mol-ten metal [5]. Added ceramic particles (silicon car-bide, aluminum oxide and magnesium oxide) areused to control viscosity of the melt, enhance porequality and adjust its foaming properties. Hence,foam panels with 1m in width and thickness rang-ing from 25 - 150 mm can be produced at a rate of900 kg/hour. The relative density of these foams iswithin the range of 0.05 - 0.55 gcm-3. The averagepore size is 2.5 - 30 mm. This process is the mostcost effective of all and allows for manufacturing inlarge volumes.

2.1.1. Cymat process

The Cymat process for manufacturing metal foamsis the first of its kind i.e. first foam manufacturingprocess and as mentioned above, the globally lead-ing manufacturer of aluminium foams, Cymat Alu-minium Corporation (Canada), invented this process(Fig. 1). In this process, ceramic particles like Sili-con carbide, aluminium oxide or magnesium oxideis added to the melt for its stabilization and thenthe foaming action is started by injecting gases likeair, nitrogen or argon into the melt. Injection of thesegases is performed by vibrating nozzles or rotatingimpellers which are specially designed in such away that the foam cavities or bubbles formed aredistributed evenly throughout the melt. This foamcavity or bubble formation leads to formation of aviscous liquid melt and bubbles mixture which floatson the molten melt surface and it later becomes dryliquefied foam after the liquid melt is completelydrained. As mentioned above the foams producedby this process are called as ‘Stabilized foams’ asthey contain the above mentioned ceramic particlesand the volume fraction of these ceramic particleslies between 10 to 20% of average particle size vary-ing from 5 to 20 mm [6]. Due to the close exteriorsurface of the foam, it can be cut into any requiredshape and also due to the presence of huge amountof ceramic particles, the machining of these foamsare a bit difficult.

2.1.2. Alcan process (Melt GasInjection)

In Alcan process, silicon carbide, aluminum oxideor magnesium oxide particles are used to enhanceviscosity of the molten metal (its volume fraction

Fig. 1. Cymat process.


Fig. 2. The Alcan process.

Fig. 3. Foam filled tube showing the mechanical bonding (front and lateral view (a,b-c) and foam part with3D shape (d)) [AMPRI, India].

ranges from 10% - 20% and the mean size from 5 –20 mm ). The ceramic particles trap gas bubblesand stabilize the cell walls and delay their union.Velocities of the rising bubbles also get reducedbecause of the enhanced viscosity of the melt [7-14]. In this process, firstly, molten solution contain-ing proportionate substances has to be prepared. Avariety of aluminum alloys can be used for makingthe molten solution, e.g. the casting alloy A359 orwrought alloys, such as, 6061, 3003, 6016, or 1060.In the second step, liquid Metal Matrix Composites(MMC) is foamed by injecting gas (air, nitrogen orargon) into it. Gas injection is done by using spe-cially designed rotating propellers or vibratingnozzles. This specially designed injecting setup canrelease fine bubbles. This is important because onlyafter sufficiently fine bubbles are created, foams ofsatisfactory quality will be produced. The floatingfoams are then continuously taken out from thesurface of the melt by conveyor belt. Over the years,various mechanisms have been developed to bettercontrol the cell properties and quality. Silicon Car-bide (SiCp) or Alumina (Al

2O

3) is added to the melt

(12 to 15 wt.%) for enhancing its viscosity. Foamslabs of considerable size (0.1×1×10 m) can beproduced by this method. The foam sheets producedwith this technology are porous. Porosities rangefrom 85% to 95%. Fig. 2 shows the line diagram ofAlcan process manufacturing technique.

Moreover, with this method, 3D-shaped parts withcomplicated configurations can be produced withthe minimum modification of the original process(Fig. 3) [10-18]. These 3D-shaped parts can be uti-lized as filling materials and also for enclosing com-ponents without machining. Casting aluminumaround Al-foams can produce lightweight castingcomponents with more or less the same strength.Potential applications for this kind of foam core cast-ings are in space frame nodes, machine bed,knuckle, control arms, cross members and stiffness-providing structural components.

2.1.3. Alporas process

Another way of foaming the melts is to directly adda foaming agent to the molten metal. The foamingagent releases gas by decomposition under the in-


fluence of heat which then carries out the foamingprocess. Shinko Wire Company, Japan has beenmanufacturing foamed aluminum under the regis-tered name ‘Alporas’, as shown in Fig. 4. This pro-cess initially involves addition of 1.5% of calcium tothe molten aluminum for adjusting the viscosity andsecondly addition of molten aluminum at 670 °C to690 °C, stirred for 6 to10 minutes in an ambientatmosphere.

It thickens the aluminum melt which is thenpoured into a casting mould and stirred with addi-tion of powdered TiH

2 (foaming agent) by using a

rotating impeller. Addition of a sufficient amount ofhydride (usually 1.6%) decomposes the melt underthe influence of heat and it releases hydrogen gas.As a result, the foam expands and fills up the mouldwithin 15 to 20 minutes [36-48]. It is cooled by air(at room temperature) in the mould and solidifies asa block with porosity between 85% and 95%. Theproduction rate of Alporas is reported to be 1000kg/day by application of the batch casting process[11-33]. This process is capable of producing largeblocks of good quality foams. Blocks of 450 mm

Fig. 4. The Alporas process.

Fig. 5. GASCAR process.

width, 2050 mm length, and 650 mm height can beproduced [1,2] with this technique. These foamshave uniform pore structure and they are better thanthe foams which are produced with addition of ce-ramic particles [34]. The latter process makes foamsbrittle. However, addition of hydride, which requirescomplex processing equipment, makes the processmore expensive than the one in which foaming ofmelts is carried out by gas injection. The densityrange of these foams is 0.18 - 0.24 gcm-3 and themean pore size is about 4.5 mm. Addition of cal-cium powder in aluminum melt enhances the vis-cosity of the melt. With increased viscosity, flow ofaluminum melt slows down, resulting in its reduceddrainage before solidification.

2.1.4. GASCAR process (Solubilityvariation)

The Foam manufactured by this process, i.e.GASCAR process, is obtained by the pressure de-pendent solubility variation of hydrogen (Fig. 5). Ini-tially, the metal is melted in an autoclave beyond


its boiling point and later this high temperature meltis brought into contact with high pressure for solv-ing the huge amounts of hydrogen in the melt. Afterthe solving process, the saturated melt is then trans-ferred to a mould, present within the autoclave andsolidification of the saturated melt is performed un-der low pressure conditions. Due to this solidifica-tion process under low pressure conditions, hydro-gen present in the saturated melt gets precipitatedat the solidification area. The porosity of the foamsproduced by this process is of 5-75%.

2.2. Metal powder (Powder MetallurgyMethods)

Powder Metallurgy (PM) method can be used tomanufacture foams of different metals and their al-loys [4,5,7,8], such as, aluminium, zinc, tin, steeland gold. Among all the metal foams, the Al-foamsare the one that has received the most favorableresponse from both the research community andthe structural industry. Al-foams have enormouspotential, mainly, in terms of specific weight, spe-cific energy absorption, vibration damping and cor-rosion resistance. The most preferred Al-alloys forfoaming are pure aluminum or casting aluminum(LM25). The good quality of aluminium foams de-pends on a proper blowing agent which ensuresuniform physical properties. In addition, the manu-facturing parameters at different stages require ap-propriate adjustments. The first step is the prepara-tion of a compacted, dense and solid semi-finishedproduct, called foamable precursor. This is attainedby using a conventional technique which compactsmetal powder mixture containing a blowing agentand the metal. The second step involves production

of the metal foam by heating this foamable precur-sor at a temperature above its melting point. ThePM method can be applied by two techniques [10].

2.2.1. Expansion with gas released bya foaming agent (IFAM-Technology)

Fig. 6 shows the MEPURA process (Alulight byMepura, Ranshofen, Austria) uses an uninterruptedextrusion technology for the compaction of the mix-ture. Then it is heat-treated up to the melting pointof the compacted metal and above the decomposi-tion temperature of the foaming agent. At this tem-perature, the foaming agent decomposes and re-leases hydrogen gas. This gas helps the materialto evolve into a highly porous structure with closedcells. The foaming process stops while cooling un-der the melting point. The porosities range from 65%to 85%. Compaction (technology, degree of defor-mation, temperature, pressure, time), powder qual-ity (particle type, particle size, alloy, mixing condi-tions), foaming parameters (temperature, coolingperiod, air), and the geometry of the semi-finishedproduct together control the final quality of the foam.The density of the foam can be controlled by thecontent of the foaming agent, by the temperatureand by the heating rate [2,47-50].

For attaining the preferred shape of the foam,the precursor material has to be inserted into a hol-low mould. Otherwise, the result will be a block ofmetal foam with an undefined shape. With foaminginside closed moulds, almost any shape can bemade. Foam and metal parts are fused during thefoaming process. For example, tubes can be filledwith aluminum foam in various ways which is shown

Fig. 6. The MEPURA process (Alulight by Mepura, Ranshofen, Austria).


Fig. 7. Aluminium alloy foam filled tubes (AMPRI).

in Fig. 7. Fig. 6 shows the last method to produce ametallic bonding between all layers of the sandwich.The resulting ‘precursor sandwich’ can be deformedprior to foaming by deep drawing.

Considerably complicated foam parts can be ob-tained by injecting liquid expanding foam into suit-able moulds and allowing final expansion there. Thefinished parts have densities between 0.5 gcm-3 and1.0 gcm-3. The first step in this technology is simi-lar to the MEPURA process, i.e., mixing of alumi-num powder with a foaming agent and continuousextrusion into a compact foamable precursor. How-ever, in this process, the precursor then is heatedin a foaming cavity up to the melting point of thealloy. This leads to the creation of liquid foams, whichare injected in a controlled manner into the mouldthat can be made of sand or metal. This allows cost-effective large and small-scale production andprototyping.

2.2.2. Process involving entrapped gas

In this process, a hermetic lockable container isfilled with aluminum powder. Then this gas is pressedinto powder, e.g., Argon. The gas fills all the avail-able space between the powder particles. If thismixture is heated, the powder particles melt togetherand entrap the gas. If the so produced metal blockis rolled and heated, the entrapped gas expandsand forms metal foams (McDonell Douglas).

2.3. Precursors (Foaming compactblowing agents)

This method of manufacturing foams is another ofclass of foam manufacturing processes which ac-tually consists of an additional stage of preparing aprecursor of the molten melt which consists of evenlydistributed blowing agents in it and later melting theprecursor for evolving the blowing agents and form-ing bubbles. Because of these precursors, the manu-facturing of foams of desired and intricate shapescan be made very easily, as in case of Cymat pro-cess. The important aspect or work of this typemanufacturing process lies in the formation of theprecursor required for our purpose. This preparationof precursors is basically achieved either by dis-solving the powders of blowing agents into the meltor by solid state densification of powder mixtures orby shaping the solid state densified powder mix-tures using thixo-casting process. The following arethe three types of manufacturing processes thatutilizes the previously mentioned precursor prepa-ration techniques for preparing precursors for theirfurther foaming process.

2.3.1. Foaming the densified powdermixtures

In this manufacturing process [51], the main stepfor preparing precursor starts with the mixing of metalpowders and blowing agent powders for preparing aunfinished dense product for compaction process.The basic compaction processes used for prepar-


ing these precursors are uniaxial compression, ex-trusion, and powder rolling. Any one of these pro-cesses can be used for compaction for better mix-ing of blowing agent powders into the metal pow-ders without any air gaps and porosities. After thepreparation of these precursors with compactionprocess, they are heated for melting the metal pow-der present in the precursors so that the blowingagents get decomposed. This decomposition ofblowing agents results in gas evolvement whichcauses the precursor to expand and form the po-rosities in the precursor. The expansion process ofthe precursor takes a lot of time as it is the impor-tant step in the manufacturing process as the po-rosity is created by it and also highly influenced bytemperature and size of the precursor material, seeFig. 8.

2.3.2. Foaming the thixo-castedprecursor

This process is similar to the previous densifiedpowder mixture process instead where the densifi-cation is achieved by the semi-solid state thixo cast-ing process rather than the compaction process. Inthis process, first the powder mixtures are pre-den-sified by cold uniaxial pressing process to achievedensity of approximately 80% and billets are pre-pared from them. These billets are later heated totemperature required to achieve the semi-solid stateof the metal powder present in the billets and areprepared to required shapes by die-casting machine.

Fig. 8. Foaming the densified powder mixtures.

The precursors obtained from the die-casting ma-chine are then foamed by exact foaming process ofheating the precursor which is used in the foamingof densified powder mixtures. The major advantageof this process is that the porosity of the precursormaterial will be uniform and complex and intricateshapes can be easily produced without any furtherprocessing after the foaming process, see Fig. 9.

2.3.3. Foaming the melts withdissolved blowing agents

In this process, the metal powders are not used forthe preparation of precursor instead titanium hydrideis added to the molten metal which is later solidi-fied for preparing the precursors. The Titanium hy-dride to the molten metal can be added by injectingit into the die simultaneously with the melt [52] orby slow stirring addition to the molten metal pro-vided that annealing is performed after a requirednumber of heat treatments [53]. These precursorsare then subjected to foaming process which ismentioned in the above sections for the preparationof foams. In this process, hydrogen gas may getevolved and to avoid such action the solidification ofthe molten metal should be rapid. The foams manu-factured using this process is named as Form grip,which is the short version of Foaming of Reinforcedmetals by gas release in precursors, see Fig. 10.


Fig. 9. Foaming the thixo-casted precursor.

Fig. 10. Foaming the melts with dissolved blowing agents.

3. PROPERTIES OF ALUMINIUMFOAM

Aluminium foams are basically complex in nature,both in micro and macro-structure and these havevarious strange properties due their porous struc-ture because of which they diversified in variousapplications of automobile, construction and otherindustries. These foams even float in water becauseof their low densities ranging from 0.3-0.8 gcc-1

(Closed-cell foams) and also their thermal and elec-trical conduction properties are less compared totheir metal or alloy. The strength of foam is lowerthan that of the metal or alloy used for its manufac-turing and the strength of foam is directly propor-tional to the density of foam i.e. it increases with

increase in density and vice versa and also thesefoams are non-toxic and non-combustible and canwithstand temperatures up to their melting points.The properties of foam are highly influenced bychanging the production parameters and heat treat-ment process conditions as in case of 6XXX alloyfoams, the compressive strength can be enhancedby simply varying the heat treatment process con-ditions [54].

3.1. Mechanical properties

The Mechanical properties of foams are different fromthat of the dense metals or alloys used for theirmanufacturing due to their cellular structure andporous nature. Conventional testing do not acquire


any useful results for studying their properties andare meaningless, except for compression tests, dueto which energy absorption and characteristics andstress-strain behavior of foams are well known toresearchers and made them useful in many appli-cations of automobile, lightweight structures andother sophisticated technologies.

3.1.1. Stress-strain behavior undercompression

The stress-strain behaviors of all foams are almostsimilar and exhibit mostly similar deformation andfailure mechanisms. The stress-strain curve of foamtypically consists of three regions, namely plasticregion, plateau region and densification region atstrain rate 0.01s-1 shown in Fig. 11. The plastic re-

Fig. 11. Stress-strain curve of Al foam.

Fig. 12. Stress-strain curve of (a) LM30 Al-alloy foams, (b) LM25 Al-alloy foam.

gion of foam is similar to the elastic deformation ofdense metals or alloys of the foam and the stressincreases linearly with strain but the deformation offoam is rather plastic. The plateau region of foam isthe region where the stress remains almost con-stant with strain and occurs due to homogeneousplastic deformation and the nearly constant stressis known as plateau stress, which plays a majorrole in understanding the energy absorption proper-ties of the particular foam. The last region of thestress-strain characteristics of foam is densifica-tion region where the stress increases with a steepdue to the total collapse of the pore cell walls.

Because these stress-strain characteristics,these foams are capable of absorbing more energyat lower stress levels and also was found that thestrength of metal foams are about 30 times that ofthe PU foams with similar percentage of porosity.The stress-strain characteristics of foams are highlyinfluenced by the density and the material i.e. metalor alloy, selected shown in Figs. 12a and 12b. Itwas found that the plateau region of the aluminiumfoam is larger for the lower density aluminium metalor alloy and the plateau stress is lower for low den-sity aluminium metal or alloy foam.

The compression behavior of the foams aretested for two different strain rates, namely low andhigh strain rates, otherwise static and dynamic con-ditions. Many researchers performed their investi-gations and studies for understanding the compres-sive behavior of foams under static and dynamicloading conditions. Paul and Ramamurty [55] per-formed experiments for studying the strain rate sen-sitivity on ALPORAS Al foam characteristics com-pared to dense Al. It was found that during deforma-tion of the foam there is an increase in energy ab-


sorbed with increase in strain rate. Zhihua et al.[56] assessed the compressive behaviour of open-cell aluminium foams, fabricated by infiltrationmethod, of different morphologies and densitiesunder quasi-static and dynamic loading. The resultsshowed that the yield strength and energy absorp-tion capacity are dependent on strain rate and themechanical properties are independent of the cellsize or morphology. Edwin Raj et al. [57] preparedaluminium foam by liquid melt route using titaniumhydride and studied for its energy absorption andplateau stress for quasi-static and dynamic loadingand found that the energy absorption was high forhigher strain rate and dynamic loading, making itapplicable for lightweight crash energy absorbingapplications. The effects of strain rate and alloying

Fig. 13. Energy absorption characteristics of Al al-loy (LM25) foam at diverse density.

Fig. 14. (a) FE-SEM micrograph of LM30 Al alloy 10 wt.% SiCp composite foam cells and cell wall & (b), (c)and (d) magnification micrograph also showing the cell and cell wall at diverse point respectively.

of foams on the compressive behaviour of closed-cell aluminium foams were studied by Hamada etal. 2009 [58]. He inferred that the pure Al foamsshown an increase in plateau stress with increasein density in static compression test and with in-crease in volume of test specimen also there is anincrease in the plateau stress and also the dynamicplateau stress also increases with density. The Alalloy foam also show increase in plateau stress withincrease in density for both static compression anddynamic compression tests. Yi Sun et al. [59] ex-perimented on aluminium/copper hybrid foam, fab-ricated from Al 6101-T6 alloy and copper depositedon the foam using electro-deposition method, andstudied its behaviour during compression at highstrain rate loading. He found that the energy ab-sorption characteristics of the foam are enhancedby the copper coating on the foam and also with theincrease in thickness of the copper coating, theenhancement of the energy absorption characteris-tics can be increased.

3.1.2. Energy absorptioncharacteristics

The energy absorption characteristics of aluminiumfoams are found to be more compared to the densealuminium but the energy absorbed was found tobe enhanced by the collaboration of the foams withtubes or columns (Fig. 13), which are used in en-ergy absorption applications. Many studies wereconducted experimentally and theoretically for un-


derstanding and enhancing the energy absorptioncharacteristics and compression behavior of alu-minium foams filled in different cross-sectionedtubes or columns. Pinto et al. [60] conducted re-search on the compressive properties and energyabsorption for aluminium foams for different cellulargeometry. They used two different foams, namely,uniform cell structure and dual-size cell structurefoams for the research and found that the dual-sizecell structure foam has enhanced stiffness andcrash-energy absorption characteristics than theuniform cell structure foam. Jeenager and Pancholi[61] prepared aluminium foam by decomposition ofTiH

2 in a stabilized melt with different microstruc-

tures of cell walls, which is obtained by solutionizingand quenching prior to thermal ageing treatment.He performed investigations for energy absorptioncharacteristics and found that there is a significantenhancement in plateau stress and energy absorp-tion characteristics due to the solutionizing andquenching prior to thermal ageing treatment and alsoextended thermal ageing treatment does not improvethe energy absorbing capabilities of the foam. Guoet al. [62] prepared an aluminium foam reinforcedwith in-situ generated MgAl

2O

4 spinel whiskers and

studied its energy absorption and compressivebehaviour. He found that the MgAl

2O

4 whiskers are

embedded in cell walls and partially out of the cellwalls and micro-pores and this orientation of thewhiskers have significant effect on the micro-struc-ture and mechanical properties of the foam and wasobserved that the plateau stress is almost near to

2.5 times of that of the original Al foams and higherenergy absorption than the original Al foams.

3.2. Physical properties

The conductivity of foams i.e. both thermal and elec-trical, are generally lower than that of aluminiummetal, as the most volume of the foam is occupiedby pores and the volume proportion of foam walls isvery less. The density of foam also plays an impor-tant role in the conductivity i.e. thermal and electri-cal, of the foam and also the conductivity is directlyproportional to the density of the foam, which isclearly indicated from Table 1. The thermal conduc-tivity of foam is nearly approximated to 0.1 timesthat of the aluminium metal and with oxidation ef-fect, the thermal conductivity of the foam decreasesmuch more. Foams are highly temperature resis-tant, like aluminium metal because the coefficientof thermal expansion for both foam and aluminiummetal are same and due to this high temperatureresistance and low thermal conductivity of foams,they are now highly used in high temperature appli-cations.

Due to the porous nature and high temperatureresistance, foams exhibit a unique property anddesired characteristic of vibration and sound absorp-tion. It was found that due to the uneven porousstructure of the foam, the vibrations which strikethe foam material gets damped and their energy isconverted into thermal energy, to which the foamsare resistant. This damping of vibrations cause a

Table 1. Mechanical properties of some commercially popular closed cell aluminium and its alloy foams.

Property Units Symbol Cymat Alporas Alulight

Material - - Al-SiCAl AlDensity Mg/m3 * 0.07 - 0.56 0.3 - 1.0 0.2 - 0.25Relative Density - */ 0.02 - 0.2 0.1 - 0.35 0.08 - 0.1Young’s Modulus GPa E 0.02 – 2.0 1.7 – 12 0.4 – 1.0Shear Modulus GPa G 0.001 – 1.0 0.6 – 5.2 0.3 – 0.35Bulk Modulus GPa K 0.02 – 3.2 1.8 – 13.0 0.9 – 1.2Poisson’s ratio - 0.31 – 0.34 0.31 – 0.34 0.31 – 0.34Compressive strength MPa

C0.04 – 7.0 1.9 – 14.0 1.3 – 1.7

Endurance limit MPa E

0.02 – 3.6 0.95 – 13 0 9 – 1.0Densification strain -

D0.6 – 0.9 0.4 – 0.8 0.7 – 0.82

Thermal Conductivity W/m-K 0.3 – 10 3.0 - 35 3.5 – 4.5Thermal expansion 10-6 /K 19 - 21 19 - 23 21 - 23Resistivity 10-8 W-m R 90 - 3000 20 - 200 210 - 250


very little deflection in the pore structure of the foamand made them helpful in reducing the intensity ofthe vibrations and also applicable in damping appli-cations. Research works were conducted on study-ing the vibration absorption and damping propertiesof foams and found that the damping and vibrationabsorption properties can be amplified by fine anduniform porous structure and also it was found thatthe open cell foams have higher vibration absorbingcapabilities than the closed cell foams.

3.3. Chemical Properties

Foams are generally non-toxic and non-combustiblein nature. These are non-reactive to heat and do notproduce any kinds of toxic gases. Similar to alu-minium metal, these are also susceptible to gal-vanic reactions and highly resistant to corrosion.The oxide layer around the aluminium is a toughprotection against corrosion and due to this the foamremains stable in a pH range of 4.5 – 8.5 [56]. Thepermeability of foam is highly influenced by the po-rosity and pore structure of the foam and it is di-rectly proportional to the pore size for a particularamount of porosity [57]. The above remaining prop-erties are also highly influenced by the pore sizeand porosity of the foam, like permeability and alsodesired properties of foams can be accomplishedby incorporating suitable manufacturing technolo-gies and material.

4. MECHANICAL TESTINGTECHNIQUES FOR STUDYING THEPROPERTIES OF FOAM

The following tests are performed on foam samplesfor studying its micro-structure, chemical composi-tion, percentage of different phases of constituentmaterials and other important properties like com-pression behavior, energy absorption characteris-tics and etc., which are helpful in understanding andanalyzing the foams and also for conducting inves-tigations and researches for enhancing and improv-ing the properties of the foams for desired applica-tions.

4.1. Micro-structure characterization

Aluminium foam samples (LM30 Al alloy 10 wt.%SiCp) were studied under Scanning Electron Micro-scope (SEM) for characterizing the pore structure,size and its thickness. The foam samples were cutfrom top to bottom of a rectangular bar of foam andthe samples were polished by means of standard

metallography with Keller’s reagent with gold sput-tering and were forwarded to SEM test. The charac-terization of pores and its geometry is achieved withhelp of SEM (Model: ZEISS, SPURA 55, Germany)at ISM, using Mean Intercept Length Method (MILM).Figs. 14a-14d demonstrates the pore structure andits geometry with pore (P) and cell wall (W) indi-cated in it. These studies of foam under SEM wereperformed for around 30 mm of the sample and it isobserved that from point to point, pore size and itsthickness vary slightly. This investigation also in-ferred that the pore thickness or cell wall thicknessof the foam increases with presence of SiC particlesand clearly visible in Fig. 14a.

These microstructure studies were conductedby many researchers for understanding the effect ofmicrostructure on the properties of foam. Caty etal. [67] studied the microstructure of closed-cell foamof relative density 0.11 for understanding the micro-structure of the foam and model a FEM model forfurther analytical investigations. The microstructurewas scanned using a standard tomography and SEMand using the results of these tests, they prepareda FEM model for analytical investigations and stud-ied the methodology of preparing a FEM model andmodifications that can be made for enhancing theproperties of foam. Jeenager and Pancholi [61] stud-ied the effects of microstructure on energy absorp-tion characteristics of foam samples which aresolutionized and aged. The microstructures of thesefoam samples is a soluitionized foam and aged foamsample.

4.2. Energy Dispersive X-Ray (EDX)studies

The Energy Dispersive X-Ray (EDX) studies wereconducted to study the chemical composition of thefoam. Aluminium foam samples (LM30 Al alloy 15wt.% SiCp) were analyzed using EDX tests anddetermined that the chemical composition of thefoam sample is 4.56 wt.% Cu, 0.57 wt.% Mg, 0.67wt.% Fe, 0.4 wt.% Mn, 17.05 wt.% Si, and the re-maining wt.% is Al as shown in Fig. 15.

Fig. 16 shows the energy dispersive X-ray analy-sis (EDXA) at various locations. This figure indicatesthat the existence of oxides on the cell wall of alu-minium alloy (LM30) foam [62-68].

4.3. Impact loading test

Impact tests are useful in determining the energyabsorption characteristics of foams in a very shortinterval of time or simply impact energy absorption


Fig. 15. Energy Dispersive X-ray (EDX) report of LM30 Al alloy 15 wt.% SiCp.

Fig. 16. Microstructure of AlSi17 alloy foam cell wall with EDAX analysis at different point.

characteristics of the foams. Liu et al. [69] performedimpact tests on two different types of aluminiumfoams for determining their impact energy absorp-tion characteristics using INSTRON 9250HV impactsystem. The results inferred that the impact energyabsorption capacity is highly influenced by the foamdensity and micro-inertia effects in their compres-sive behavior and found that the impact energy ab-sorption capacity can be enhanced by increasingthe foam density the variation of specific energyabsorption of foam with their density. Merrett et al.[70] performed blast and impact tests on aluminiumfoams for studying their energy absorption charac-teristics. Impact tests performed on the foam aredirect forward impact test and Taylor’s test (directreverse impact test) using a SHPB apparatus withhollow aluminium tubes as Hopkinson bar and the

blast tests were conducted by subjecting the foamto the blast energy produced by the detonation ofPE4 explosive in a cuboidal chamber of 100 x 100 x200 mm dimensions. The results were compared toquasi-static compression results and it was foundthat the strength of the foam is enhanced in boththe direct forward and reverse impact tests and theblast test results clearly showed that the foam failedwith a uniaxial ductile crushing mechanism. Peroniet al. [71] performed impact experiments on twodifferent Al foams with different densities using aSHPB apparatus and studied the mechanical char-acteristics of the two foams. The results inferredthat the mechanical characteristic has exhibited verygreat density sensitivity.


4.4. Compression loading studies

The compressive test studies are helpful in under-standing the compression behavior and energy ab-sorption characteristics at lower and higher strainrates of the foams. These lower and higher strainrate compressive loading can also be referred asstatic/quasi-static and dynamic loading conditions,as mentioned in one of the above previous section.Koza et al. [72] performed these compression testson aluminium foam samples of 30 and 60 mmlengths using MTS 810 testing machine at 1 mm/sram speed for studying the influence of relative den-sity of foam on the compressive strength. The re-sults inferred that the compressive strength of 60mm specimens are lower than that of 30 mm speci-mens and this highly prevalent with the specimenswhose relative densities are higher than 0.22 andthis is clearly indicated. Wang et al. [73] performedexperimental investigations on compressivebehaviour of foams subjected to medium strain rateranging from 10-3 to 450 s-1. The tests were per-formed using a High rate Instron test system, whichis capable of applying loads at a constant rate. Thestress-strain characteristics of the foam and theresults indicate that there is no particular variationin the stress-strain characteristics and show a nor-mal dependence on strain rate by plateau stressand energy absorption characteristics. Castro et al.[74] examined the compression and low-velocityimpact characteristics of an aluminium syntacticfoams, produced by aluminium (1100 and 6061 alu-minium alloy) and hollow alumina spheres and foundthat the aluminium syntactic foam exhibits greaterenergy absorption and strength than the ordinary

Fig. 17. (a) XRD phase patterns of an Al alloy foam (b) Expanded view of XRD phase pattern at the peakpoint.

aluminium foams, under quasi-static loading and the1100 Al (Ductile) exhibited greater resistance forperforation than the 6061 Al (Hardened) in low-ve-locity impact testing.

4.5. X-Ray Diffraction (XRD) studies

Aluminium foam samples (Al-Si7Mg 5 wt.% SiCp)were analyzed for studying the phases of the sur-face of the foam under a X-Ray Diffractometer (XRD,Bruker D8 FOCUS, UK) at ISM and the results in-ferred that the surface of the foam were comprisedof three different phases of aluminium as Al, Al

3Ti

and Al4Ca as shown in Fig. 17a. Fig. 17b is an ex-

panded view of XRD report of the foam sample show-ing the variation of phase content near to the peakpoint, where the Al is directly present as metal athighest concentration. These foams also containoxide layer on the surface of the foam but were notdetected due to their low amount of presence.

4.6. Fourier Transform InfraredSpectroscopy (FTIR) studies

The Fourier Transform Infrared Spectroscopy (FTIR)is an effective method to study the chemical bondspresent in the foam. In this method, the foam sampleis grinded into powder form and later 100-200 mg ofKBr is added to the foam powder and made into apaste with the help of ceramic pestle and mortar.This paste is transferred to the pallet holder and ahigh pressure is applied by means of a hydraulicpressure. As a result of this pallets are formed andare collected after the disassembly of the holder.These pallets are then studied using FTIR spec-trum by means of a Perkin-Elmer spectrometer and


the results are as shown in Fig. 18. The FTIR spec-trum is prepared for a wave number range of 400-4000 cm-1 and indicates that O-H stretch, X-Hstretch, C=O stretch, -C=C- stretch and =C-H bondare present at 3440 cm-1, 2924 cm-1, 1734 cm-1,1628 cm-1, and 1102 cm-1 wave numbers, respec-tively It is also clear that the presence of =C-H bondindicates the presence of impurities.

4.7. Hydrostatic compression studies

The hydrostatic compression test on aluminiumfoam samples were conducted by Peroni et al. [75]using a Hydraulic testing machine designed andmade for applying hydrostatic pressure force on thefoam without the direct intervention of the opera-tional fluid and foam by covering the foam with latexsheath and applying pressure force by decreasingthe volume of the chamber where the foam is placedand parallelly increasing the pressure of the opera-tional fluid inside the chamber where the foam isplaced. The stress-strain curves are similar to thecompressive stress-strain curves of the foams.

5. AREAS OF APPLICATION

Now-a-days aluminium foams are used in varies di-versified applications from heat, vibrations and en-ergy absorption to light weight structures. Theseapplications led to its implementation in various fieldsof industries and researches according to the re-quirements. The energy absorption, light weightapplication and thermal and vibrational insulation

Fig. 18. FTIR spectrum of Al foam sample.

properties of the foams made them useful in theAutomobile industry and is now a good replacementfor many unnecessary weighing components. Inautomobiles, foams are first implemented for crashbox applications for absorbing impact energy pro-duced during front and side impacts of the automo-bile body. Due to the light weight nature, in automo-bile, some weighing structural parts and larger ar-eas like hood, top and body cover, are incorporatedwith thin foam sandwiched layers and vibration andthermal absorbers in the engine compartment forreducing the vibrations and heat for creating goodand peaceful environment for the passengers. Thislight weight application of foam sandwiched sheetsled to its use in aerospace industry also, by replac-ing the honeycomb structures. The implementationof foam sandwiched sheets is extended to ship build-ing industries as weight is the important character-istic of ship that has to be reduced for its efficientworking and these foams also found their applica-tion in anti-corrosion agents, as they are good anti-corrosive materials. The thermal insulation and firepenetration resistant properties of foams led to theiruse as foam sandwiched panels in the interior andexterior of the buildings, beneath the highways androad and railway tunnels and these panels also hasan extra advantage of vibration and sound damping,which results in reducing the intensity of soundsand induced vibrations produced by traffic, movingtrains and other external noises. In engineering ap-plications, foams are used in the walls and roofs ofthe rooms containing power electronic equipment


as electromagnetic shielding. These foams can alsobe incorporated as heat exchangers, heat shieldsin nuclear plants and catalyst carriers and also aslight weight rollers in many industries, especially inpaper industry. These foams can be used as a re-placement for sand cores which used for prepara-tion of stiffeners, which incorporated for weight sav-ing applications. The replacement of sand cores withfoams made the manufacturing of stiffeners easierby implementing thixocasting, die casting, squeezecasting and other modern casting methods whichdo not sand cores and even smaller cross-sectionsare manufactured with foams fillings, which are lightin weight. These light weight characteristic of foamsmade them as extra reinforcement for polymer struc-tures that are used as crash absorbers in rail, tramsand other heavy duty crash applications.

6. SUMMARY AND CONCLUSIONS

Despite several manufacturing approaches availablefor ultra-light Al-alloy foams, their demand in themarket seems to be restricted. Poor quality of pro-duction seems to be the main reason for its limitedacceptance. Inconsistencies in density, for instance,significantly affect the quality of foams. In recentyears, however, a couple of new manufacturing pro-cesses have been developed which enable effectivecontrol of density by regulating the process param-eters. This has enhanced commercial acceptabilityof ultra-light metal foams, particularly in automo-bile, defence and aeronautical sectors. The two pri-mary production processes, i.e., Melt Route andPowder Metallurgy have their own advantages aswell as shortcomings. The first method is better thanthe second in terms of quality. The second method,on the other hand, is superior in terms of capitalrequirement. Again, the Melt Route process is pre-ferred when it comes to bulk shapes with easy op-eration and simple equipment. 3D-complex shapeswith external finished surface, on the other hand,are possible by applying Powder Metallurgy tech-nique. Metal joining and machining can be donemore easily in foams produced by the Powder Met-allurgy process as compared to the Melt Route pro-cess. The Melt Route process is cost-effective ascompared to Powder Metallurgy process becausea powder metallurgy plant needs costly equipment.The focus of research nowadays is on the manufac-turing process of metal foams in order to solve prac-tical problems that crop up during their production.Metal foams have high potential for various engi-neering and non-engineering applications. However,so far their use has been limited mainly due to high

cost and lack of homogeneity in properties. None-theless, one expects that cost of foams will decreaseas their bulk production increases. Recent techno-logical advances have led to the development of anassortment of processing systems for open cell andclosed cell morphologies. The processing techniquewill eventually be decided on the basis of manufac-turing cost, material properties and proposed appli-cations of the final product.

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