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Together with the rapid development of transporta-tion industry the avid demand for new, light, yetdurable materials which hold desired performancestandards grows. Material engineers answer thisneed with solutions like modern composites, innova-tive alloys, shield plates and cellular cores for sand-wich panels, etc. One of the most investigated mate-rial groups in the automotive and aviation industry aremetal foams, due to their light weight and still goodstructural properties. Technologists and scientistscontinue to improve the existing or find alternativeways for manufacturing these cellular metals so thatthe production would be cheaper and/or wouldenable tailoring material properties to the desiredlevels. In the recent years vast and far-reaching ad-vances in types of metal foams and their productiontechniques have been made. It seems valuable then toprovide an up-to-date and possibly complete over-view of the state of the art in production methods ofporous metals, including specific requirements formetallic foams. The present article aims at this goal,giving a brief classification of porous metallic media,their manufacture routes with describing metal foamswith their production technologies in detail.

Classification of metal foamsBefore the proper discussion of metal foams will

be started, it seems valuable to make an importantinitial remark on adequate nomenclature, with theaccount of the comments stated in [1]. The widestterm referring to the class of materials discussed inthe paper is multiphase materials, which is media con-

sisting of a matrix and a dispersed phase. Dependingon the state of matter of each of the initial phases, theresulting mixed material is classified as shown intab. I.

The term foam defines a material which is producedby dispersing a gas in a liquid. In foams the gas in-clusions are fully surrounded with (enclosed in)the matrix phase and do not connect. Yet, if one lets

Methods of production of metallic foams

Wspó∏czesne metody wytwarzaniapian metalicznych

ANNA M. STR¢K

Abstract: This paper discusses manufacturing methods of metal foams. The introduction briefly presents a divisionof porous metallic materials and an overall classification of methods for their production, with particular emphasis onmethods leading to produce foam structures. The main part of the paper describes production techniques in detail.The paper also includes information on difficulties and benefits of particular methods, as well as on the developmentopportunities for manufacturing technologies of metallic foams.

Keywords: metallic foams, production of metallic foams, liquid phase metallurgy

Streszczenie: W artykule zaprezentowano wspó∏czeÊnie stosowane metody wytwarzania pian metalicznych. Przedstawionopodzia∏ porowatych materia∏ów metalicznych i ogólnà klasyfikacj´ metod ich produkcji, ze szczególnym uwzgl´dnieniemmetod prowadzàcych do wytworzenia pian. Omówiono tak˝e trudnoÊci i korzyÊci zwiàzane ze stosowaniem poszczególnychmetod, a tak˝e kierunki rozwoju technologii wytwarzania pian metalicznych.

S∏owa kluczowe: piany metaliczne, produkcja pian metalicznych, metody ciek∏o-fazowe

TABLE I. Mixture type due to the state of aggregation of itscomponents (adapted from [1])

Dispersed in:gas liquid solid

cellular solid⊃ solid foam

Gas gas mixture foam⊃ sponge⊃ other

Liquid fog emulsion gel

Solid smokesuspensionslurry embedded particles

Mgr in˝. Anna M. Str´k – Katedra Wytrzyma∏oÊci, Zm´-czenia Materia∏ów i Konstrukcji, Wydzia∏ In˝ynierii Mecha-nicznej i Robotyki Akademii Górniczo-Hutniczej, Al. Mickie-wicza 30, 30-059 Kraków, e-mail: strek@agh.edu.pl.

the foamed matrix solidify and still preserve its mor-phology, we speak of a solid foam. The latter class isa special case of materials called cellular solids.Cellular solids are gases dispersed in a solid phaseand – in contrast to the structure of foams, which isconditioned by the minimum surface energy of thedispersing liquid – they can have almost any morphol-ogy, depending on the production mode. If the poresof a cellular solid interconnect and form an opennetwork, the material is classified as a sponge. Hence,what is commonly called ‘a closed-cell metal foam’ isactually a solid metal foam, while the often used term‘an open-cell metal foam’ refers to a metal sponge.In the present work the customary names are alsoused, but with keeping in mind the actual multiphasematerials they relate to.

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General classificationof production methods

The focus of the article is on metal foams (andtheir manufacturing methods) in the tight definitionalmeaning, that is on materials commonly called”closed-cell metal foams”. However, before moving tothe description of this specific section, it is worth tooutline how porous metals are produced in generalto indicate which techniques suit creation of (solid)foams best.

Methods of production of cellular metals can bedivided in various ways; the most popular classifica-tion is done according to the state of matter in whichthe metal is before the processing begins and isreferred here after [1]. Detailed list of methods, whichcan be accounted to particular groups due to the stateof aggregation, is given in tab. II.

reinforcing particles would be difficult to mix andtoo small would set too severely. Also, if the volumefraction of the particles exceeded a certain value,viscosity would be too high, while on the otherhand, not enough of them would cause the foammorphology to remain unstable [5]. Typical ranges ofparticle size and quantity of the additions can befound in tab. III (as well as other characteristics due tothe foaming method).

The second step in direct foaming procedure is toblow gas – it can be air, nitrogen or argon – into theliquid; this can be obtained by rotating propellers orvibrating nozzles. Both the rotating speed and vibra-tion frequency (but not only them) influence the re-sulting foam structural characteristics - cell size, den-sity, wall thickness. Next, the gas-liquid mixture ispulled out of the vessel, where it initially had beenmixed, by a conveying belt. The foam, thanks to theadded carbides or oxides, is stable enough to keepits morphology during and after being pulled outand next it is left to cool down and solidify. Theprocessing allows for receiving foam sheets of anylength, width dependent on the initial vessel widthand thickness of about 10 cm.

The major advantage of this processing techniqueis its lower cost compared to other methods and thefact that the resulting product can have relatively largedimensions. Also, low foam densities are counted tothe value of the method. On the other hand, disadvan-tages comprise irregular structure and high fracture ofreinforcing particles. Non-uniform foam morphologyis attributed to gravitation, causing gradient density,and shearing forces produced during belt conveyingwhich in turn cause anisotropy. Also, machiningcauses an adverse effect, leaving cells on the foamsurface open after cutting.

� Direct foaming with blowing agentsGeneral scheme of the processing for direct

foaming with blowing agents is given in Fig. 2. Thefirst step is similar as in the previous method, it con-sists in adding typically 1,5 wt.% calcium metal asa substance that would secure enough viscosity [6].The addition, due to several minutes long stirring,forms calcium oxide, calcium-aluminium oxide andeven aluminium-calcium intermetallics. There aremany ways to improve stability of the melt, apart fromthe addition of calcium. They include e.g.: creation ofalumina by bubbling gas mixtures (air, oxygen)

TABLE II. Methods of production of cellular metals due to theinitial state of aggregation of the metal (based on [1 – 4])

Metal form Methods of production of cellular Resultingmetals material

– direct foaming with gas– direct foaming with blowing agents

Liquid – gasars solid foam– powder compact melting– ospray process

– casting sponge

– slurry foamingsolid foamor sponge

– gas entrapmentPowdered – sintering of powders or fibres

spongesolid – processing around fillers– sintering of hollow spheres

– extrusion of polymer/metal honey- mixtures combs

Vapour – vapour decompositionhollowsponge

Ions – electrochemical deposition

Liquid phase methodsThere are a couple of ways to produce gas bubbles

in the molten metal and receive foam in this way,they include e.g. direct injection of gas into the meltor adding a blowing agent [1 – 8]. The major problem,common for all liquid phase methods, is that afterinserting or generating gas bubbles in the moltenmetal, it is difficult to keep them in the volume.Instead, due to gravitation, they tend to rise to themelt surface or to join together to form gas spheres ofgreater dimensions. What can keep them inside,prevent from connecting and stabilise the mixture isthe increase of the molten metal viscosity. It can beachieved by adding fine ceramic powders or alloyingelements, for example. The system consisting of themolten metal and reinforcing particles is called ametal-matrix composite (MMC).

� Direct foaming with gasA process scheme for this foaming method is

shown in fig. 1. First, viscosity is improved by additionof silicon-carbide, aluminium-oxide or magnesium-oxide or other substances. The quality and quantityof the additions is to be determined experimentally inorder to provide optimum mixing conditions. Too big

Fig. 1. Direct foaming with gas

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through the melt or by adding powdered alumina.Viscosity influences future cell diameter and foamdensity in a way that has to be determined empirically.

In the second process stage, titanium hydride (TiH2)is introduced to the melt as a blowing agent (1,6 wt.%[6]). Due to hot temperature the hydride decomposesand hydrogen gas is created initiating the desiredfoaming process. Lastly, the vessel with the foamedmelt is cooled, the mixture solidifies and a large block(e.g. 2000 mm × 650 mm × 450 mm) of solid foam isobtained.

The great advantage of foaming with blowingagents is that the resulting foams have a veryhomogenous structure. Again, typical characteristicsof foams obtained by this route are summarised intab. III.

� GasarsGasars are obtained on the route of temperature

control of an eutectic system [7]. First, a metal ismelted under high pressure in hydrogen atmosphere,next, by cooling the mixture the system undergoeseutectic transition. As the temperature is continuouslylowered, the metal gradually solidifies and hydrogennear solidification plane forms bubbles. It is crucial to

choose processing parameters (the most important iscooling rate) so that hydrogen does not escape fromthe solidifying zone to the remaining liquid part of themelt, but stays trapped to produce pores. The processof pore growth can be seen in Fig. 3, along with typi-cal pore characteristics in tab. III.

TABLE III. Foam properties according to production methods (based on detail info from [1 – 13])

Foaming with Foaming with Gasars Powder compact Spray Slurry foaminggas blowing agents melting processing

Examples Hydro, Cymat, Alporas Foam-in-Al,Alusion Alulight (Mepura)

aluminium alloys aluminium alloys nickel, copper, alloys of variety of aluminium,(e.g. AlSi10Mg, are most common magnesium, aluminium (e.g. metals and beryllium,

Typical matrix AlSiC) (e.g. AlCa(Ti), aluminium, AlSi10, AlMg1Si06), alloys (e.g. nickel, copper,material AlCa5(Ti)) steels, cobalt, tin, copper carbon steel, iron powders

chromium and (e.g. AlCu4), zinc CuSn6)others

Density g/cm3

0,069 – 0,54 g/cm3 0,18 – 0,24 g/cm3 5% – 75%0,2 – 1,1 g/cm3

up to 60%relative density

or porosity % 70 – 90% down to 7%

diameter:10 µm –

Pore size 3 – 25 mm 2 – 10 mm10 mm,length:100 µm –300 mm

10 – 20% vol., 1.5 wt.%, calcium, silicon orReinforcing silicon carbide, other: powdered managaneseparticles aluminium or alumina oxides, BaCO3

magnesiumoxides

wall thickness: reinforcing pore metal powder grain blowing agents:Other 50 – 85 µm particle size: asymmetry size: 60 – 200 µm e.g. aluminiumproperties reinforcing 5 – 20 µm [length to hydroxide,

particle size: diameter]: hydrochloric5 – 20 µm 1 – 300 acid

Gasars can be made of nickel, copper, magnesium,aluminium, steels, cobalt, chromium and others. Thetechnique of production of this type of cellular solidshas to be developed still, because pores are unevenand the effecting material is often not homogenous.

� Powder compact meltingAt the beginning of this production method, whose

scheme is in Fig. 4, metal powder has to be mixedwith a blowing agent (usually TiH2 or ZrH2) of corn sizeof about 5 µm [8]. The next step is to compact bothcomponents to a semi-product; this can be achievedfor example by uniaxial or isostatic compression orrod extrusion. At this point it is very important to usea compacting method that would not leave anyresidual porosity in the mixture. The following stageof the process is to heat the system to a temperaturenear the melting point of the matrix material, so thatthe blowing agent decomposes creating gas bubblesin the molten matrix.

The heating takes place in a furnace; the startingmixture can be put there without any vessel and thenthe produced foam is of an uneven, undefined shape.

Fig. 2. Foaming with a blowing agent

Fig. 3. Pore growth in gasars

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Alternatively, the powder mixture may be put in aform, and when the expansion process due to gasformation has started, it can be relocated to a formof a specific, even complicated shape in order toobtain foam of this shape. Yet, as a consequence, thematerial lacks homogeneity, the most inner part is themost porous one, while the outer layer is a closed“skin”. However, the great advantage is that the foammay be formed to a desired shape. Another favour ofthis method is that the foam can be attributed to ashield plate in sandwich panels directly at the stage ofheating: one has just to apply the starting mixture ona metal sheet of higher melting point and the heat thesystem up; metallic bonds between the shield and thecore will be created.

Metals that can be foamed in this way comprisealuminium and its alloys, tin, zinc, brass, lead, gold,and other.

� Ospray processMetallic melt is atomised and continuously sprayed

onto a substrate, where the droplets gather and growto form a deposit. Powdered carbides or oxides maybe added to the sprayed metal and embed togetherwith the metallic drops creating in this way an MMCdeposit. The setting mixture temperature allows forreactions which free gas from additions and thereforepores are created. The expanding foam solidifies alayer by layer, as consequent portions are beingsprayed, see Fig. 5. Even though a considerableadvantage of this method is that the deposit may haveoptional cross-section shape (cylinder, tube, etc.)and that its properties may be calibrated graduallyamong the whole procedure of spraying (more/less ordifferent type of additions), the big disadvantage isthat the created foams are non-homogenous becausepores are highly irregular.

Powder metallurgy – slurry foamingThis is the only method for solid metal substrate

which may lead to produce solid metal foams. A slurryconsisting of metal powders, blowing agents and re-active additives is prepared and undergoes mixing.Next, elevated temperature and additives turn themixture viscous and gas is formed inside the slurrycausing it to expand. Stabilisation and drying of theexpanded pre-product prepares it for sintering, whichsolidifies the foam. Some more specific informationcan be found in tab. III.

SummaryMethods of foam production listed above involve

a lot of field for investigation and development,starting from the material itself – new metals or alloyscan be used as the matrix. Moreover, viscosity en-hancing mechanisms are not yet fully understood.The quantity and type of reinforcing particles in-fluences structural characteristics (cell size, wallthickness, etc.) of the resulting foam, as well as thepresence of the additions in the foam skeleton makessolid metal foams rather composite foams than puremetal porous materials. This fact in turn also leavespotential for study and analysis. Moreover, processingconditions like temperature control and its changerate influence the final product characteristics. Allthese factors make it a great and promising challengefor technologists to calibrate them to obtain a con-stant-parameter product or be able to predict theoutcoming characteristics in detail.

REFERENCES

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14. Szeptyƒski P., Jakliƒski B.: private communication 2012.

Fig. 4. Method scheme for foaming of compacts Fig. 5. Spray foaming process (based on [1])