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  5Vol.60, No.1 ( 2013 ) RESOURCESPROCESSING Resources Processing 60 : 5–12 (2013) Review Aluminum Metallic Foams Made by Carbonate Foaming Agents Svyatoslav, Vitalievich GNYLOSKURENKO 1,3 , Takuya KOIZUMI 2 , Kazuhiko KITA 2  and Takashi NAKAMURA 3 * 1 Physico-Technological Institute of Metals and Alloys, National Academy of Sciences, Ukraine 2 Machinery and Engineering Group, YKK Corporation, Toyama 938-8610, Japan 3 Institute of Multidisciplinary Research of Advanced Materials, Tohoku University, Miyagi 980-8577, Japan Abstract Recent developments in metal foams, especially aluminum, have produced a new class of light-weight materials at the side of the traditional ones such as polymers, ceramics or glass. Thecombination of a metallic character together with a cellular structure gives an interesting potentialfor a wide application of this material, particularly for high volume markets such as the automotiveindustry.Increased demands concerning cost economy, passenger safety in automobiles and materialsrecycling all bring constructors now to use metal foams. Hereby it provides the additional environ-mental benefits from a potentially improved fuel economy and lower CO2 emissions. Then, shortreview of metallic foam was done in the present paper.The possibility of carbonate and hydroxide as foaming agent for Al-Si-Cu alloy by powder met-allurgy route is studied, after preparation processes of metallic foams were briefly reviewed in the present paper.It was done by measuring thermal decomposition behavior of foaming agents and evaluating cellstructure of those aluminum foams. To obtain fine and homogenous cell structure in powder metal-lurgy route by using safer carbonate as foaming agent, it has made clear that importance of selectingfoaming agent starting decomposition after melting of matrix. It is clearly different from TiH 2 -foamto grow coarse-rounded cell structure. From this point of view, MgCO 3  and CaMg(CO 3 ) 2  is suitablefor matrix of Al-Si-Cu alloy. CaMg(CO 3 ) 2 -foam could expand to 1.19 in specific gravity, and keephomogeneous, fine and spherical cell structure. Key words: Metallic foams, Carbonate, Aluminum alloy, Light materials 1.Introduction Evolution of mankind is going hand in handwith a request for new constructional and toolmaterials. Until recently, the principal evolution-ary forces were those relating to improved perfor-mance and functionality. Although polymers,ceramics, or composites have already been em- ployed in various industrial applications the de-mand for stronger, stiffer and lighter materials isstill growing. However, the production, disposal,and use of materials in products have environmen-tal impacts throughout the whole product lifecycle, and this fact can no longer be ignored.Strong and stiff materials can be found also innature but there they usually do not induce any pollution problems and recycling requirements.Therefore they can be a very good guide for the prospective development of new materials. Thedifference between strong natural and syntheticmaterials has very well been characterized byAshby 1 : “When modern man builds large load-bearing structures, he uses dense solids: steel,concrete, glass. When Nature does the same, shegenerally uses cellular materials: wood, bone,coral.” Really, natural materials are strong enoughto withstand loads in the bones of runningelephant or to carry the weight of a 100m highredwood tree. Cellular structure of these materials Paper presented at the 10 th  Japan/Korea International Sympo-sium on Materials Science and Resources Recycling, 28–30May 2012, Doejeon KoreaAccepted 5 February, 2013*e-mail: ntakashi@tagen.tohoku.ac.jp  6GNYLOSKURENKO, KOIZUMI, KITA and NAKAMURA RESOURCES PROCESSING  provides the tool for the optimal combination of  properties, e.g., realization of highest stiffness atminimum weight.Many of today’s vehicles incorporate deform-able energy absorbing elements within the vehiclestructure 2,3 . These elements, which represent thecrushable zone, manage the collision energy for  protection of the rigid passenger cell. Metallicfoams have been researched recently because of their unique properties, like low density, energy-absorption, low thermal-conductivity.Due to low density and novel physical, mechan-ical, thermal, electrical and acoustic propertiesmetal foams already have a number of establishedand profitable market niches apart from car indus-try. Metal foams are used as heat exchangers,support structures for aerospace applications,electrodes for batteries, gas and fluid filters,acoustic absorbers, electrical applications etc.The viability of metallic foam in a given appli-cation depends on the balance between its perfor-mance and its cost. At present all metal foams are produced in small quantities using time and labor-intensive methods, and all, relative to the solidmaterials from which they derive, are expensive.But it is not the present-day cost which is relevant;it is the cost which would be obtained where the process to be scaled and automated to meet the in-creased.In the present paper, the possibility of carbonateand hydroxide as foaming agent for Al-Si-Cu al-loy by PM route is studied with a brief review onmetallic foam. 2.Various processes for preparation of metal-lic foams Solid metallic foams are cellular materials thatare made up from a framework of solid materialenclosing and surrounding gas-filled voids (bub-bles). In order to make a metallic foam it is firstlynecessary to create a gaseous phase within themetal, then to rearrange the two-phase mediuminto a foam and finally to cool it beneath the melt-ing point of the respective metal and obtain a solidcellular structure. A variety of different methodsfor the production of metallic foams are availableand can be classified in accordance with the initialstate of the metals to be foamed—Liquid, Powder,Ionized.Only the most widespread methods will be de-scribed in detail. The main processing techniqueswith their basic materials specification are listed inTable1 and 2.Many processes of manufacturing metallicfoam have been proposed. Most popular processuses titanium hydride (TiH 2 ) as foaming agent.The processes using TiH 2  have two routes, melt processing and powder metallurgy (PM) routes.Typical melt processing route is “Alporas”method 4  by Sinko Wire, melt foams with calciumas the viscosity-enhancing additive and TiH 2  asfoaming agent. In other side, typical PM route is“Fraunhofer” method 5  by IFAM, aluminum ma-trix powder and TiH 2  is hot-extruded, it is called precursor, then precursor is heated in closed die tofoam for near-net shaping. In comparison withAlporas and Fraunhofer method, Alporas methodis lower cost, but Fraunhofer method is better atcontrolling cell structure and near-net shaping. Table 1 Processing techniques for production of metallic foam and porous structures, together with associated basicmaterial specificationsMelt-based routes  Processing techniqueSolid phase composition Porosity (%)Cell size and typeTradename or type Melt-based routesGas injectionAl alloys +  ceramic particles80–983–25mm closedALCAN (also CYMAT and HYDRO)In situ prior oxidation of the meltAl +  (oxide particles)89–934.8mm (mean) closedALPORAS ® Delayed release gas generationAl alloys +  ceramic particles50–960.8–3mm closedFORMGRIP ® Gas-eutectic reactionNi, Cu, Mg, Al and others5–755m–10mm closed/openGASAR, LotusInfiltration and replication6101 and A35688–920.5–4.3mm openDUOCEL ®  Aluminum Metallic Foams Made by Carbonate Foaming Agents7Vol.60, No.1 ( 2013 ) 2.1Gas foaming in liquids Early attempts concentrated on vaporizationdifferent foaming agents in liquid metal with thegas serving as a blowing agent 6 . These ideas weredeveloped and applied and foamed aluminum wasfirstly produced in 1951 7 . According to this pro-cess, metal hydrides such as TiH2 or ZrH2 where put into the melt. Next, the hydrides were decom- posed under heating and the evolved gases causedthe molten metal to foam. After foaming the re-sultant body was cooled forming a cellular solidstructure.The techniques mentioned above meet some problems as follows:(1)a non-uniform cellular structure arises, be-cause the foaming process is rather difficult tocontrol;(2)a relatively short time interval between addinga foaming agent to the molten metal and foamformation which rendered metal casting diffi-cult;These problems have been treated as follows:Particles of the foaming agent were rapidly dis- persed through the molten metal mass by highspeed mixing 8 , which bubbles growth as was pre-vented by increasing the melt viscosity utilizingalloys with a wide difference between the alloysolidus and liquidus temperatures or by introduc-ing viscosity-increasing agents 9 . Numerous attempts have been also made toovercome the structural weakness of foamedmetals 10 . All these methods improved the foamingbut could not sufficiently optimize yield foams of a satisfactory quality and cost.Currently there are two ways for direct foamingof metallic melts.Perhaps the simplest method of melt foaming isto inject gas into a melt and then arrange for it so-lidification in such a way that the bubbles becomeentrapped in the material matrix. This approachwas developed by Alcan International Ltd.,Canada 11 , Hydro Aluminum, Norway 12  and li-censed to Cymat Aluminum Corp., Ontario 13 . The process scheme is shown in Figure1. The startingmaterial is aluminum containing solid substances(silicon carbide, aluminum oxide, etc.) to increasethe melt viscosity and stabilize gas bubbles. Theliquid melt is then foamed by blowing gas, usuallyair, using rotating impeller. The bubble size can becontrolled by adjusting the gas flow rate, the im- peller design and the speed of impeller rotation.The bubbles float up to the surface of the liquidwhere foam can be pulled off e.g. by conveyor belt. The collected foam is cut into the requiredshape after cooling. Porosity ranges from 80 to97%. The advantage of the process is its ability of  producing large volumes at rather low cost com- pared to other metallic foams. It is not also neces-sary to add an expensive gas-generating com- pound or to conduct the foaming in a restrictedmelt temperature range and processing time.While the process is simple and cheap, the largeand variable cell sizes constitutes a significant problem for many potential applications. The pos-sible disadvantage is the high content of ceramic particles that cause difficulties during foam ma- Table 2 Processing techniques for production of metallic foam and porous structures, together with associated basicmaterial specificationsPowder-based routes  Processing techniqueSolid phase composition Porosity (%)Cell size and typeTradename or type Powder-based routesBaking of powder-blended consolidated precursorAl alloys +  residual oxide particles63–891–8mm closedALULIGHT FOAMINAL(IFAM) ALUFOAMBaking of entrapped gas  precursorTi–6Al–4V20–4010–100m closed“LDC sandwich”Sintering Steel, Ti–, and  Ni–based hollow spheres65–870.5–6mm closedHollow sphere structures Fig. 1 A schematic depiction of the Alcan process  8GNYLOSKURENKO, KOIZUMI, KITA and NAKAMURA RESOURCES PROCESSING chining. Also the problem of wetting the particlesby the melt and their nonuniform distributionwithin the melt should be solved. But the refracto-ry particles play a critical role. The objective is toensure that they adhere to the gas-liquid interfacewithin the foam, so as to stabilize the bubbles andinhibit their movement and further coalescence inthe melt.Addition of a foaming agent (TiH2, ZrH2) intothe melt instead of blowing gas is the second wayfor foaming melts 4 . A novel technique (tradenameALPORAS) (Fig.2), developed by Shinko WireCompany Ltd., Japan differs from the previousone 7  by using calcium additives and stirring themixture in an ambient atmosphere. These steps in-crease melt viscosity, prevent the occurrence of bubble flotation and ensure a relatively uniformdistribution. Foam slabs are obtained after coolingand can be cut into sheets of the required thick-ness. Typical foam porosity is in the range of about 84–93%, with a mean cell diameter of about4.8mm.The concept of foaming agent (TiH2) pre-treatment was developed by Gergely and Clyne 14 .The FORMGRIP process provides relatively easycontrol over the kinetics of hydrogen evolution bymanipulating parameters such as the foamingagent pre-treatment, thermal histories during bak-ing and composite melt viscosities (ceramic parti-cle content and size) 15 .Porous materials (metal, alloy, or ceramic),termed the GASAR (an acronym from the Russianfor “gas reinforced”) are produced by utilizing aeutectic reaction in melts which are already super-saturated with hydrogen 16 . Such a melt is cooledbelow the eutectic point, resulting in concurrentformation of both a gaseous and a solid phasefrom the liquid. Nakajima et. al also successfully produced lotus-structured porous metals withelongated pores 17 .The advantage of this method is the possibilityof making metal foams with medium and highmelting point (copper, nickel, iron) and with elon-gated pores. The maximum porosity achieved is75%. 2.2Powder-based rotes The second group includes methods to makehighly porous metallic structures using metal powders instead of the liquid metal. As a startingmaterial in some of these processes the powdersare processed into a compact precursor material prior to the actual foaming step (Fraunhofer and“Alulight” processes, Gas entrapment technique).Early methods used powders or slurry, polymer mixtures and filler materials directly.According to IFAM process, commercial alu-minum powder is mixed with foaming agent powder  5 . The mixture then compacted to a semi-finished product and worked into sheets, profilesetc. in order to improve the flow conditions duringits eventual foaming inside moulds.Heat treatment at temperatures near the melting point of the matrix metal causes the foaming agentto decompose and precursor material to expandyielding a highly porous structure. This processcan be used to manufacture different productssuch as near-net shaped parts, sandwich panels,and tubes. The density of aluminum foam typical-ly ranges from 0,5 to 1g/cm 3 . Distribution of cellsizes and shapes is at random.“Alulight” is another novel melt processingtechnique, combining melt and powder routes inmetallic foam production 18 . At first a pre-treated powder (TiH2) is distributed into an aluminum al-loy. Allowing this melt to solidify produces a pre-cursor material, having a relatively low porositylevel (20–25%). At the second stage, the precursor material is heated into a semi-liquid state, when progressive evolution of hydrogen converts it intoa cellular structure. To obtain complex 3D-shapedfoam the precursor can be injected in a controlledmanner into the cavity with the desired shape.Foamed metal can also be made by compressing powders to a precursor material and allowing gasto be entrapped in the metal structure 19 . Heatingthe precursor leads to an expansion of the metaldue to the internal pressure created by the en-trapped gas.TiH 2  is most popular foaming agent because itsdecomposition temperature is closed to meltingtemperature of aluminum alloys. But TiH 2  has a problem which is high cost and dangerous of hy-dride. So, lower cost and safer foaming agent isexpected.One of the authors reported the method to usecalcium carbonate (CaCO 3 ) as foaming agent bymelt processing route 20 . CaCO 3  is low cost and Fig. 2 Schematic depiction of the Alporas process forfoam production
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