Commercial Applications of Metal Foams: Their Properties and Production

materials

Review Commercial Applications of Metal Foams: Their Properties and Production

Francisco García-Moreno 1,2

1 Institute of Applied Materials, Helmholtz Centre Berlin, Hahn-Meitner-Platz 1, Berlin 14109, Germany; [email protected] or [email protected]; Tel.: +49-30-8062-42761 2 Institute of Materials Science and Technologies, Technical University Berlin, Hardenbergstr. 60, Berlin 10623, Germany

Academic Editor: Sven De Schampheleire Received: 15 December 2015; Accepted: 26 January 2016; Published: 29 January 2016

Abstract: This work gives an overview of the production, properties and industrial applications of metal foams. First, it classiﬁes the most relevant manufacturing routes and methods. Then, it reviews the most important properties, with special interest in the mechanical and functional aspects, but also taking into account costs and feasibility considerations. These properties are the motivation and basis of related applications. Finally, a summary of the most relevant applications showing a large number of actual examples is presented. Concluding, we can forecast a slow, but continuous growth of this industrial sector.

Keywords: metal foam; metal sponge; production; properties; commercial application

1. Introduction Metal foams are lightweight cellular materials inspired by nature. Wood, bones and sea sponges are some well-known examples of these types of structures. In fact, solid metallic foams are the conserved image of the corresponding liquid metallic foam. Scientific attempts to improve foam quality of any type of foam concentrate on the foam physics, i.e., bubble formation, foam nucleation, growth, stability, development or gas diffusion in the liquid state, where the foam structure is evolving [1–13]. We have to distinguish between closed cell and open cell metal foams, which in fact should be called metal sponges more precisely speaking. The latter possesses also a foam-like structure, and therefore, we will consider the production, properties and applications from both classes. In contrast, other kinds of cellular or porous materials, like lattices, fibers, sintered granulates, honey combs, etc., will not be taken into account, although they represent actually a larger market volume. The engineering and industrial approach is to scale up the processes and provide reliable conditions for serial productions at acceptable quality and cost levels [14–17]. Depending on the production method, the foam structure is more or less homogeneous and comprises different characteristic features that determine its properties and, therefore, the fields of application [18]. Applications based on polymeric foams are well established in the market and part of our daily life. Some applications based on other foamed materials, such as porous concrete or food foams, are also quite popular in society, and several applications of ceramic foams are also well known in industry [19]. However, less common and familiar for us are applications and products based on metallic foams. The reason is that they are still not wide spread, although they have a very high potential, and a large number of applications already exist on the market. Some reviews about the applications of metallic foams are available in the literature [18,20–24]; however, in the last few years, new applications and application fields have emerged, and not all are considered commercially relevant. A large number of companies made a bet on the future and started producing and commercializing metallic foams in the past two decades, although several have already closed, stopped production or

Materials 2016, 9, 85; doi:10.3390/ma9020085 www.mdpi.com/journal/materials Materials 2016, 9, 85 2 of 27 concentrated on their core business, especially during the worldwide economic crisis starting 2007. Their major problem was caused not by the quality or properties of their products, but mainly by two factors: missing effective marketing for the small metallic foam market and the price, i.e., their benefit margin. Some of the most relevant metal foam-producing companies at present are: Alulight (Ecka Granules, Mepura, Ranshofen, Austria), Cymat (Mississauga, ON, Cananda), Aluinvent (Miskolc, Hungary), M-pore (Mayser, Dresden, Germany), Pohltec Metalfoam (Collogne, Germany), Recemat (Dodewaard, the Netherlands), Exxentis (Wettingen, Switzerland), Alantum (Seongnam-City, Korea), Mott Corporation (Farmington, CT, USA), Foamtech (Daegu, Korea), Alveotec (Venissieux, France), ERG (Oakland, CA, USA), etc. A more detailed and regularly updated list can be found on the Internet at www.metalfoam.net [25]. Besides the companies’ names, some trade names have been established, being characteristic of the production method, such as Fominal (Frauenhofer, Bremen, Germany), Alporas (Shinko Wire, now Foamtech, Daegu, Korea), Alusion (Cymat, Mississauga, ON, Cananda), Aluhab (Aluinvent, Miskolc, Hungary), Duocel® (ERG, Oakland, CA, USA), Incofoam (Inco, now Alantum, Seongnam-City, Korea), etc. The main applications of metal foams can be grouped into structural and functional, and are based on several excellent properties of the material [18]. Structural applications take advantage of the light-weight and specific mechanical properties of metal foams; functional applications are based on a special functionality, i.e., a large open area in combination with very good thermal or electrical conductivity for heat dissipation or as electrode for batteries, respectively. We will review the applications of closed, partially open and open cell metal foams. Open structures allow for media to penetrate through and provide interesting and wide-spread properties for functional applications. For serial applications, the production processes should be reliable and reproducible, and the final product should exhibit a reasonable price. In the next paragraphs, we will first describe the commercial production procedures, then consider the most relevant properties of metal foams and discuss some other general properties and cost considerations. Both the production procedures and the unique properties are closely related to the possibility of an application to compete in the market at a reasonable price. Then, we will classify and review the most important fields of application and refer to some prominent examples of products and existing serial applications, but also of promising prototypes trying to explore further fields. In fact, a large number of applications are hard to classify into a certain category, as they are related to several properties. In this case, the classification will rely on the most relevant and characteristic property.

2. Commercial Production Procedures There is a large number of manufacturing processes for foams, sponges and porous materials with macroscopic pores. Here, we will review only the commercially most relevant methods. For closed cell foams, they are divided into two main families, also called manufacturing routes: the melt (ML) and the powder metallurgical (PM) route. They are also referred to as the direct and indirect foaming methods, respectively (see Figure1). Most of these production procedures and their variations are already described in the literature [18,26,27]. Additionally, there are further manufacturing methods, especially applied in the case of metallic sponges, which can obviously not be foamed directly, as the gas will leak [28]. They are not directly connected with foaming in the strict sense of the word, but lead to a foam-like structure. These methods can be also divided into two main families: the ﬁrst is based on a polymeric sponge structure as a pattern or carrier and the second on a placeholder that can be removed [22]. Their applications are usually focused on their functional character, but not only. Materials 2016, 9, 85 3 of 27 Materials 2016, 9, 85 3 of 27

Figure 1. Schematic grouping of the commercially most relevant production methods of metal foams and sponges and some representative tradenames and companies. SAS, Slovak Academy of Sciences; AFS, aluminumaluminum foamfoam sandwich;sandwich; ML,ML, melt;melt; PM,PM, powderpowder metallurgical.metallurgical.

2.1. Melt Metallurgical Route Shinko Wire,Wire, AmagasakiAmagasaki City, City, Japan, Japan, provided provided the the first first commercial commercial production production of of metal metal foams foams in thein the late late 1980s. 1980s. In this In this process, process, calcium calcium is first is added first added to an aluminumto an aluminum melt and melt stirred and in stirred air to producein air to produce oxides and raise its viscosity. Subsequently, powder of the blowing agent (TiH2) is dispersed oxides and raise its viscosity. Subsequently, powder of the blowing agent (TiH2) is dispersed quickly intoquickly the meltinto bythe stirring, melt by decomposing stirring, decomposing into gaseous into hydrogen gaseous and hydrogen titanium and at thetitanium melt temperature. at the melt Thetemperature. melt starts The then melt foaming starts then inside foaming the crucible inside the or insidecrucible a dedicatedor inside a mold.dedicated Finally, mold. it Finally, is cooled it downis cooled to adown big block to a big and block usually and sliced usually into sliced plates into of theplates desired of the thickness. desired thickness. This type This of metallic type of foammetallic and foam the correspondingand the corresponding process is process called Alporas,is called Alporas, which was which patented was patented in the United in the States United of AmericaStates of inAmerica 1987 [29 in]. 1987 The [29]. patent The is patent now expired, is now andexpired, the method and the has method been has used been successfully used successfully by other companiesby other companies and research and centers, research e.g., centers, by Foamtech e.g., by (Daegu,Foamtech Korea) (Daegu, [30 ]Korea) or Shanxi [30] Putai or Shanxi Aluminum Putai FoamAluminum Manufacturing Foam Manufacturing (Linfen, China) (Linfen, [31]. China) [31]. Another approachapproach of of direct direct foaming foaming of of an an aluminum aluminum alloy alloy melt melt was was invented invented in thein the early early 1990s 1990s by Alcanby Alcan International International Limited Limited in Montreal in Montreal (QC, (QC, Canada) Canada) [32], [32], and and Norsk Norsk Hydro Hydro (Oslo, (Oslo, Norway). Norway). In this In process,this process, the melt the needsmelt needs to be prepared to be prepared with ceramic with particles,ceramic particles, such as silicon such carbideas silicon or aluminumcarbide or oxides,aluminum ranging oxides, from ranging 5 to 20 from vol% 5 to increase20 vol% to the increase viscosity the of viscosity the melt of and the to melt stabilize and to the stabilize liquid cellthe walls.liquid cell Subsequently, walls. Subsequently, air bubbles air are bubbles injected are and,injected at the and, same at the time, same dispersed time, dispersed into the into melt the using melt rotatingusing rotating impellers. impellers. The bubbles The bubbles rise to rise the to top the of top the of melt, the wheremelt, where they are they collected are collected in a liquid in a liquid foam andfoam start and solidifying start solidifying after leaving after leaving the furnace, the furnace, where where the foam the canfoam be can continuously be continuously drawn drawn off using off ausing conveyor a conveyor belt. This belt. method This method is used is nowadays used nowadays by Cymat by (Mississauga, Cymat (Mississauga, ON, Canada) ON, Canada) to produce to foamproduce panels foam or panels to fill molds or to fill with molds foams with that foams do not that need do furthernot need processing. further processing. Aluinvent (Miskolc,(Miskolc, Hungary) Hungary) recently recently developed developed a technique a technique to reduce to reduce the pore the size pore and size improve and theimprove pore sizethe pore distribution size distribution of gas-injected of gas‐ foamsinjected using foams ultrasound using ultrasound oscillations oscillations to treat theto treat melt the during melt bubbleduring formation.bubble formation. While the While gas isthe introduced gas is introduced into the melt, into sound the melt, waves sound induce waves earlier induce detachment earlier ofdetachment bubbles during of bubbles their during growing their stage, growing which stage, is not which the caseis not without the case this without external this forceexternal [33 ].force [33].

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2.2. Powder Metallurgical Route In the 1950s, Allen et al. patented a method for foaming metal very similar to the one used nowadays commercially [34]. This route consists of an indirect foaming of solid precursors by heating. The precursor is produced by mixing aluminum powders with the corresponding alloying elements and a blowing agent, typically 0.5 to 1.0 wt% of TiH2. Once the powder mixture is prepared, it is consolidated and sintered by extrusion, uniaxial compaction or rolling to yield a foamable precursor. By the heating of the precursors, the matrix starts melting, and the gas of the blowing agent nucleates. In the course of the temperature increasing, hydrogen production increases, and the gas diffuses to the nucleated pores, letting them grow into big bubbles and expanding the foam. The resident oxides in the metal powders (usually 0.5 to 1%) provide the stability of the foam during the holding time in the liquid state [35]. After several minutes, the foam development is fulﬁlled, and the foamed metal structure can be conserved by temperature reduction, leading to foam solidiﬁcation. In 1991, Baumeister et al. at Fraunhofer-Institute (Bremen, Germany) improved the method [36]. In the 2000s, Seeliger founded a new company, ALM (Saarbrücken, Germany) which become later part of Alulight (Ranshofen, Austria) and is known now as Pohltec Metalfoam (Collogne, Germany). Their product is an aluminum foam sandwich (AFS), which is an optimized lightweight panel of several square meters and thicknesses of 8 to 80 mm composed of a foam core and two bulk face sheets, both based on aluminum alloys. By this process, the metal powders and blowing agent are mixed and introduced inside a welded container. The container is then sealed and hot-rolled to a compact three-layer precursor, the container walls being the sandwich face sheets later. The advantage of this process is the metallurgical bonding between the face sheets and the foam core, obtaining a very stiff product with high temperature resistance that can be fully recycled [15].

2.3. Based on Polymer Sponge Structure Metal sponges can be manufactured by replicating an open porous polymeric structure. Their morphology is based obviously on the structure of the polymeric foam, e.g., a polyurethane foam. This implies certain advantages, as their quality in the sense of pore homogeneity and distribution is superior, as polymeric foam technology is quite advanced and established compared to the metallic foam one. However, it has also several disadvantages concerning the number of production steps, dimensions, costs, etc. An example is given by ERG (Oakland, CA, USA) which produces sponges of different metals, like aluminum or copper, known under the tradename Duocel®, following a foam replication or investment casting method. Here, the polymer sponge is ﬁlled with a slurry of heat-resistant material, e.g., a mixture of mullite, phenolic resin and calcium carbonate [37]. After drying under a certain temperature, the polymer is decomposed and the form stable enough for casting with the corresponding metal. The different grades are given in pores per inch (ppi), and porosities in the range of 80% to 97% are achieved. A similar method, but with an additional wax covering of the polymeric foam for increasing stiffness, is used by Mayser GmbH, formerly M-pore (Dresden, Germany) [38]. The drying step with temperatures up to 350 ˝C allows one to remove the wax and the polymer, and metallic sponges with large and regular pores up to 10 mm in diameter can be produced. Another method consists of a metallic coating of the polymeric sponge with subsequent sintering and removal of the polymer. An example is the deposition onto polyurethane foam by, e.g., chemical vapor deposition (CVD) of nickel tetracarbonyl (Ni(CO)4), which decomposes to elemental nickel and carbon monoxide at a heating temperature of 150 to 200 ˝C[39]. This method was developed and commercialized by Inco (Mississauga, ON, Cananda). Now, this technology is used by Alantum (Seongnam-City, Korea), with a large production plan in China [28,40], where they also produce iron and copper foams. Recemat (Dodewaard, the Netherlands) produces sponges of nickel, stainless steel, titanium, etc., being metallized with the subsequent removal of the polyurethane sponge by pyrolysis. Generally, the initial metallization is done with nickel metal. The resulting nickel metal Materials 2016, 9, 85 5 of 27 foam, after being heat treated, is used in applications where high electrical and thermal conductivity are of importance [41].

2.4. Based on Dissolution of a Placeholder Metal sponges can be produced by infiltration or casting of molds filled with granulates. The latter have to be resistant to the high temperatures of the corresponding molten metal and be able to be removed by leaching or washing, like, e.g., salt or sand. The granules and their contact points correspond later to the pores and pore interconnections, respectively, leading to an open cell structure. The cell structure depends strongly on the granulate morphology. Special attention has to be given to the placeholder removal, to improve porosity and prevent corrosion. Such types of aluminum alloy sponges in any types of forms are produced, e.g., by Exxentis (Wettingen, Switzerland) with pores in the range of 0.14 to 3 mm in diameter [42]. Similarly Alveotec (Venissieux, France), produces metal sponges, but usually with ordered structures [43]. They first produce an ordered core structure with a mix of silica and resin, then a preform together with the core and place them in a mold. Finally, they shed the metal into the grid and replace the preform after solidification. Instead of casting around the placeholder, they can be mixed with metallic powders, compacted and sintered in a traditional PM way. In this case, not necessarily open cell structures are achieved, with different porosity ratios, depending on the nominal volume rate between powders and the placeholder. By the low amount of placeholder, these can stay isolated from each other, making their later removal almost impossible. This method is very interesting for the production of high melting alloys, such as titanium alloys. There, carbamide (urea) or ammonium hydrogen carbonate can be used as placeholders [44,45], but also sodium chloride [46].

3. Properties Applications of metal foams are strongly linked to the properties that such kinds of materials can offer and especially to those that are excellent or even unique. Some of the properties are obviously mainly related to those of the matrix metal itself, e.g., elasticity, temperature or corrosion resistance, etc., while others appear only in combination with the cellular structure, e.g., low density, large surface area or damping. Therefore, the most remarkable types of mechanical, functional and other properties are reviewed here.

3.1. Mechanical Properties The mechanical properties of metal foams are of course correlated to the ones of the corresponding bulk metal, but in a specific manner. The dominating factors here are the density and the structure itself. The foam structure is obviously the characteristic feature of a foam. Mechanical properties depend mainly on the density, but are also influenced by the quality of the cellular structure in the sense of cell connectivity, cell roundness and diameter distribution, fraction of the solid contained in the cell nodes, edges or the cell faces, etc. Gibson and Ashby proposed a simple beam model of a cubic cell for describing the mechanical response of foams [47]. Although this model is based on regular cellular structures, so-called lattices, it provides quite realistic results. Note that foam cells do not have eight neighbor cells, but usually around 14 [48]. Anyhow, this model and the experimental results show quite appropriately the strong dependence of the mechanical properties on the foam’s relative density [47,49–53]. The differences between open and closed cell foams can be comprehended in the sense of material distribution. Only in the case of a high velocity impact, the air captured in a closed cell foam has a notable additional contribution. A certain mechanical property P˚ of the foam should be evaluated in accordance with the weight, i.e., density. Therefore, we should talk about high specific stiffness or moduli, or about mass-related ˚ properties, as P is usually not so good as the one for the bulk samples of the same volume Ps, but there Materials 2016, 9, 85 6 of 27 are exceptions, like damping, energy absorption, etc. Accordingly, the relative property is proposed in ˚ dependence of the relative density ρ /ρs, with the coefficient a and the structural constant k, as:

P˚ ρ˚ a « kp q (1) Ps ρs where a and the structural constant k depend on the property, on the foam’s structure and on the type of deformation. One of the most important mechanical properties for structural applications is the stiffness or Young´s modulus. In our case, as already mentioned, we have to look for the relative modulus according to Equation (1). The Young´s modulus of a foam E˚ corresponds to the slope of the compression strength σ in dependence of the strain ε in the elastic regime, i.e.,: σ E˚ “ (2) ε Ashby [26] and Gibson et al. [47] proposed accordingly Equation (1) for the relative Young´s modulus: E˚ ρ˚ 2 « kp q (3) Es ρs where Es is the modulus of the solid bulk material; and k « (0.1 to 4) in the low-density limit. Based on their model, they further proposed a speciﬁc compressive strength of the foam in the elastic regime and came to the following approximation:

σ˚ ρ˚ 2 el « 0.05p q (4) Es ρs and similarly in the plastic regime: ˚ ˚ 3{2 σpl ρ « 0.3p q (5) σys ρs neglecting the density correction and the contribution of the entrapped gas in the case of closed cell ˚ foams, which plays a role only for high dynamic impact. σ el is the compressive strength in the elastic; ˚ and σ pl in the plastic regime, respectively; σys represents the yield strength. Considering the models and experimental results obtained by Gibson et al. [47], the compressive strength of an open cell foam in the plastic regime can be summarized to:

˚ ˚ 3{2 σpl ρ « kp q (6) σys ρs and of the closed cell foam to: ˚ ˚ 3{2 ˚ σpl ρ ρ « k 0.5p q ` 0.3p q (7) σys « ρs ρs ﬀ with k = (0.1 to 1.0) for both Equations (6) and (7). A more detailed treatment of the models considering also further components in the case of compression, such as a density correction or a gas compression factor, can be found in the literature [47]. ˚ The tensile strength σ ts is a weak property of foams in general, as the resistance against a propagating crack mainly depends on the weakest link of the material, which is given here as ˚ a thin cell wall or Plateau border, which can break easily. σ ts is density independent and follows the relation: σ˚ ts « k1 (8) σys with k’ = (1.1 to 1.4) [26]. Materials 2016, 9, 85 7 of 27

Evans et al. and Banhart showed that the bending stiffness of a ﬂat panel of a given weight is inversely proportional to its density [18,54]. This is a special loading case, where foams can clearly show their potential. A metal foam can suffer fatigue under compression or tension load, but mostly it is a combination of both, e.g., due to bending or torsion. Therefore, fatigue is mostly limited by the weakest property, namely by the tensile strength [55–59]. Many applications, especially in the automotive industry, suffer a large number of cyclic loads, and therefore, it is very important to guarantee no failure during the operative life time. Fatigue can be represented by the number of cycles that a foam can withstand at a certain load without structural degradation. As cracks grow in a catastrophic manner, a structural failure will be the result. Most probably the best and most remarkable mechanical property of metal foams is their energy absorption capability [60,61]. Foams can absorb a maximum of mechanical energy without exceeding a certain stress limit σD due to plastic, irreversible deformation over a large strain range. This property, usually isotropic, makes foams almost ideal crash absorbers. The maximum energy absorption per unit volume Wmax by the plastic deformation of a metal foam is the integral of the stress-strain curve up to the stress limit and given by Gibson and Ashby [47] as:

W σ σ 2{3 max “ D 1 ´ 3.1p D q (9) E E E s s " s * The mechanical properties of metal foams can be improved, similar as for bulk metals, by different hardening mechanisms, such as alloy composition, grain reﬁnements, heat treatments, etc. [57]. Furthermore, structural designs and property optimizations are possible depending on the application and practical aspects [60]. Furthermore, of course, heat or hardening treatments need to be compatible with the foaming procedure or foam stability. Integral foam structures show a density and property gradient, usually with a denser skin and a lighter core, similar to a bone structure, but are difﬁcult to manufacture [62,63]. These structures are bioinspired and can have accordingly better properties, like a homogeneous foam, like, e.g., bending stiffness. A further interesting optimization of a panel is the sandwich structure composed of two bulk face sheets and a lighter metal foam core, as will be presented later [15,64,65]. Concluding, we can say that the most remarkable mechanical properties of foams are their light weight, the relative strength in compression, the bending stiffness and the energy absorption, which are mainly exploited in the applications.

3.2. Functional Properties There is a wide range of functional properties of metal foams originating from their cellular character. This structure implies a large surface area and number of cells (see Figure2). Very often, the combination of functional properties, such as acoustic [66–71], thermal [72–77], electrical [21] or chemical resistance, with mechanical properties, like strength or stiffness, allows interesting new applications [54]. Foams in general are known for their thermal insulation properties, especially ceramic, glass and polymeric foams. They are the basis for a large number of applications related to thermal control. Thermal ﬂow has to percolate through the matrix network of a foam structure; radiation between cell walls is hindered due to the large number of cells; and gas convection is disabled due to the small gas volume in each separated cell. Metal foams’ thermal conductance is less than that of the corresponding bulk metal, but of course, in absolute values, they cannot compete with ceramic or polymeric foams in terms of thermal insulation. Materials 2016, 9, 85 8 of 27 Materials 2016, 9, 85 8 of 27

Figure 2. ((aa)) Microscopic Microscopic detail detail of the the cellular structure of a NiCrAl sponge from Alantum; and (b) detail of the sponge surface (courtesy(courtesy ofof Alantum).

On the other hand, open cell metal sponges have a large surface area that can be used in On the other hand, open cell metal sponges have a large surface area that can be used in combination with a good conductivity of the matrix metal (e.g., Cu or Al) for a completely different combination with a good conductivity of the matrix metal (e.g., Cu or Al) for a completely different purpose, namely heat dissipation and passive cooling [75]. In fact, Al‐ and Cu‐sponges can conduct purpose, namely heat dissipation and passive cooling [75]. In fact, Al- and Cu-sponges can conduct temperature very well through the matrix, and therefore, such foams are used for heat exchangers. temperature very well through the matrix, and therefore, such foams are used for heat exchangers. The The relative thermal conductivity in metal sponges λ*/λS can be expressed in a simple model as a relative thermal conductivity in metal sponges λ˚/λ can be expressed in a simple model as a function function of the relative density: S of the relative density: ˚ ∗ ˚∗ a λ ρ “ p q (10) λs ρs with a = (1.65 to 1.8) [26]. [26]. Further advanced models are summarized in the literature [[77].77]. Metals are also known to bebe goodgood electricalelectrical conductors.conductors. Again, the combination with a large sponge surfacesurface allows allows their their utilization utilization as electrodes as electrodes in batteries in batteries or for otheror for electro-chemical other electro‐ purposes.chemical Thepurposes. electrical The resistivity electrical isresistivity given by: is given by: ˚ ˚ ´a R ∗ ρ∗ “ p q (11) Rs ρs (11) with a = (1.6 to 1.8) [26]. [26]. Polymeric foams are good for acoustic and vibration damping over a large frequency range due to the cellular structure, and the same property is present in metallic foams [[67,70,78].67,70,78]. The latter can provide additionally electromagnetic shieldingshielding duedue toto theirtheir metallicmetallic naturenature [[15].15]. 3.3. Other Properties 3.3. Other Properties The foam structure and, accordingly, the foam properties are usually described as isotropic. The foam structure and, accordingly, the foam properties are usually described as isotropic. However, obviously, the number of cells has to be large enough to be able to consider the foam as However, obviously, the number of cells has to be large enough to be able to consider the foam as a a continuum. This is why, e.g., for compression tests, the DIN Standard 50134 “compression test of continuum. This is why, e.g., for compression tests, the DIN Standard 50134 “compression test of metallic cellular materials” [79] requests that the number of cells in each direction of the sample tested metallic cellular materials” [79] requests that the number of cells in each direction of the sample tested has to be equal or larger than 10. Other cellular materials, e.g., lattices or honey combs, are obviously has to be equal or larger than 10. Other cellular materials, e.g., lattices or honey combs, are obviously anisotropic in structure and, therefore, also in their properties. Anisotropy is an important advantage anisotropic in structure and, therefore, also in their properties. Anisotropy is an important advantage of metal foams for certain purposes, although some anisotropies in the structure can also be favorable of metal foams for certain purposes, although some anisotropies in the structure can also be favorable for speciﬁc applications, and they also occur depending on the production method [21,80]. for specific applications, and they also occur depending on the production method [21,80]. Recycling is becoming more and more important in our daily lives and is a required property for advanced and new materials. Obviously, the main component in a metallic foam is air, corresponding to 50 to 90 vol%. The rest is the metal matrix, which can be recycled as well as the

8 Materials 2016, 9, 85 9 of 27

Recycling is becoming more and more important in our daily lives and is a required property for advanced and new materials. Obviously, the main component in a metallic foam is air, corresponding to 50 to 90 vol%. The rest is the metal matrix, which can be recycled as well as the metal material itself, among which aluminum alloy foams are the most prominent candidates. Additives, like the blowing agents (TiH2, ZrH2, CaCO3, etc.), are usually hydrides or oxides, which lose the gaseous component during the foaming process. The metal reacts with the matrix material and can be considered part of the alloy due to its metallic character and low amount (0.2 to 1 wt%) [81,82]. Although, there are also exceptions for recycling capability, such as gas-injected Al-foams stabilized by large amounts (5 to 20 vol%) of ceramic particles (usually SiC), commercially known as Cymat or Alusion foams [83]. The possibility to adjust the metallic foam density in a large range from 0.05 to 10 g/cm3 depending on the porosity and matrix metal allows for further properties, like buoyancy, which can be tailored varying the material density. This property can be interesting, e.g., for ships, ﬂoaters or sea markers, etc. It can be also combined with other properties, like high temperatures or corrosion resistance, where standard polymeric foams cannot be used. Metals, like Ti or Mg, are bio-compatible or bio-degradable, respectively. These properties apply also to their foamed structures, being additionally able to adjust the Young´s modulus by adjusting their density and offering a large surface, especially in the case of metal sponges, for osseointegration, making medical applications self-evident.

4. Costs and Feasibility Considerations A large number of successful prototypes have already demonstrated that there is a very wide range of possible applications. Depending on the desired performance and manufacturing feasibility, different production methods are considered. In most claimed applications, metal foams are as good or better than their competitors [18]. However, of course, one key point for industrialization breakthrough is still the price of the product [16]. Furthermore, and thinking about costs and system integration, bi-functional or even multi-functional applications are in favor [18,54]. Therefore, for real applications, cost-effective production methods and multi-functionality need to be combined as the priority goal [84]. A multi-purpose approach could be for example an application as a lightweight, stiff and recyclable material with vibration damping properties and very high volume-effective crash absorption protection for electric cars. In such cases, the balance between costs and benefits can decline to the side of metal foams. However, sometimes, other aspects, like innovation, image, marketing, prestige or strategical aspects, can play an important role. Depending on the application field, the price may then play a less important role, which is the case in the fields of the arts, design, medicine or sports. More important even is to find applications where the properties provided by the metal foam are unique when compared to other materials and the price is only secondary, e.g., the best crash absorption behavior in a minimal space and weight to improve safety. For mobility, i.e., applications for the automotive, railway or aerospace industry, being light weight plays an enormously important role, as in these cases, saving weight leads to a large savings of energy, e.g., due to the continuous acceleration and breaking cycles of the mass. This point seems to be particularly important for actual electric cars and buses, where the batteries still do not provide a sufficient operation range. Applications in aerospace could be the field where multi-functionality offers the most financial advantages. Thinking, e.g., about a plane fuselage, which is made traditionally from Al-sheets with welded or riveted struts, it could be replaced by a foamed, curved sandwich structure. A metallic foam sandwich will provide additionally noise and vibration damping of the turbines and a certain additional temperature insulation compared to the traditional metal sheets. It can be also recycled, and it is non-flammable. Saving 1 kg in a plane fuselage corresponds roughly to one million Euros in savings over the whole life of the plane (e.g., 25 years). Additionally, considering that demand and price for fossil fuels will increase strongly in the future, the advantages of lightweight materials will Materials 2016, 9, 85 10 of 27 increase as well. However, the aeronautic industry seems still to have at present reservations against the application of metallic foams, most probably due to the long and expensive validation procedure of new materials and the actual trend toward fiber-reinforced composites. For small or medium-sized applications, it is difficult to fix an actual price for metal foams, as it depends mostly on the metal, foam density, shape, degree of complexity, number of units of the desired product, etc. In the case of market-ready foams made by following the powder metallurgical route, the price of the raw material is ~3 €/kg for the Al-powder and <1 €/kg for the blowing agent (TiH2) (both related to the weight of the foam). In the last few years, attempts to reduce the price were focused on alternative blowing agents, such as CaCO3, as it almost represents 1/3 to 1/4 of the total price of the raw materials [85–91]. Unfortunately, these advances did not find commercial breakthrough in all types of manufacturing routes until now, due to the lack of properties [92]. However, anyhow, foam prices should not be given in price-per-weight, as this suggests an expensive product compared to the bulk material, but better in price-per-volume or price-per-area. As a rough estimation, we can find actually Al-foam sandwich panels on the market for prices in the range of 60 to 200 €/m2, depending on the type of foam, panel thickness, etc. A diagram of costs related to material density and modulus for different bulk and foamed metals can be found in the literature, although the given prices are no longer up to date [26].

5. Applications

5.1. Structural Applications Lightweight materials with high stiffness are often desired for various applications. Standard products on the market, like honeycomb panels, use a cellular structure as the core and brazed or glued face sheets to provide the desired properties. They are usually inexpensive, but have some disadvantages, as they cannot be curved, resist high temperatures due to the glue nor be recycled. Aluminum foam sandwiches (AFS) and steel aluminum foam sandwiches (SAS) are promising products for structural applications and are already on the market [15,93–95]. AFS panels are used as support frames, e.g., for solar panels, mirrors, etc., and everywhere where light and rigid metallic panels are needed. Most of the customers prefer to provide the material as panels and manufacture their own products themselves. Often they even keep their innovative field of application in secret to assure a competitive advantage for their products on the market. A good example is industrial machines, where foam-filled beams and columns are stiff, but light. With reduced inertia, they can be moved quickly and positioned precisely. Examples are drilling, milling, textile, cutting, printing, pressing or blanking industrial machines. Additionally, damping of the system and of, e.g., an additional vibrating tool can improve the performance in the precision of positioning and wear, reduce fatigue problems and increase the operational lifetime. An example of an application for a high-speed milling machine of Niles-Simmons (Chemnitz, Germany) in cooperation with the Fraunhofer-Institut für Werkzeugmaschinen und Umformtechnik (IWU, Chemnitz, Germany) is given in Figure3a. The sliding bed is made of 11 welded AFS parts, and the construction is 28% lighter than the cast part with the same stiffness, but improving vibration damping. Around 15 parts per year are manufactured. Figure3b shows a beam of a textile machine filled with Alporas foam produced by the Au Metallgießerei in Sprockhövel, Germany. This part is 1590 mm ˆ 280 mm ˆ 160 mm and provides a 60% reduction in the amplitude at the resonance frequency. The production is ~1000 pieces per year. A similar hybrid material concept was applied to a tool column prototype of the Technical University Prague for a cutting machine (Model Prisma S) from TOS Varnsdorf s.a. (Varnsdorf, Czech Republic) in which an Alporas foam core is integrated. Materials 2016, 9, 85 11 of 27 Materials 2016, 9, 85 11 of 27

Materials 2016, 9, 85 11 of 27

Figure 3. (a) Sliding bed of a milling machine made of welded aluminum foam sandwich (courtesy of Figure 3. (a) Sliding bed of a milling machine made of welded aluminum foam sandwich (courtesy of Thomas Hipke, IWU, Chemnitz, Germany); and (b) a beam of a textile machine filled with Alporas Thomas Hipke, IWU, Chemnitz, Germany); and (b) a beam of a textile machine filled with Alporas foam (courtesy of the Au Metallgießerei, Sprockhövel, Germany). foamFigure (courtesy 3. (a) of Sliding the Au bed Metallgießerei, of a milling machine Sprockhövel, made of welded Germany). aluminum foam sandwich (courtesy of ThomasThe automotive Hipke, IWU, industry Chemnitz, wants Germany); to benefit and (alsob) a beamfrom ofthe a textiledevelopments machine filled of new with lightAlporas‐weight Thematerials.foam automotive (courtesyFor large of industry serial the Au productions, Metallgießerei, wants to not benefit Sprockhövel, only material also Germany). from properties the developments and costs are important, of new light-weight but also materials.other Forengineering large serial and productions,strategical aspects not onlyplay materialan important properties role, for and example costs the are necessity important, of butthe also The automotive industry wants to benefit also from the developments of new light‐weight otherredesign engineering of other and components, strategical the aspects system play integration an important capability role, or for the example number theof available necessity of materials.suppliers [96].For largeBesides serial a large productions, number of not prototypes, only material several properties large series and costswent areinto important, production. but As also an the redesign of other components, the system integration capability or the number of available otherexample engineering (Figure 4a), and the strategical door sill was aspects filled playwith anAl ‐importantfoam to reinforce role, for the example frame and the increase necessity stiffness, of the suppliers [96]. Besides a large number of prototypes, several large series went into production. redesignas well as ofthe other behavior components, in the case the of asystem side crash integration in high premium capability cars, or likethe the number Ferrari of360 available and 430 As ansuppliersSpider. example The [96]. (Figurecompany Besides4a), Alulight a large the door number (Ranshofen, sill of was prototypes, Austria) filled with holds several Al-foam a production large series to reinforce from went 1999 into the toproduction. frame 2009 with and As~6000 increase an stiffness,examplepieces as wellper (Figure asyear. the 4a), behavior theThe door same sill in was thecompany filled case with of manufactured a sideAl‐foam crash to reinforce infrom high premium 2006the frame another and cars, increase likepiece the stiffness,for Ferrari the 360 and 430asAudi well Spider. Q7 as SUV the The behaviorvehicle company in in a thecomplete Alulight case of automated a side (Ranshofen, crash series, in high Austria) namely premium a holds small cars, 7 a g productionlike crash the absorber Ferrari from 360 (Figure 1999and 4304b) to 2009 with ~6000Spider.placed pieces atThe the company support per year. Alulightframe The of same (Ranshofen,the safety company net Austria) [14]. manufactured With holds over a production 120,000 from 2006 parts from another per 1999 year, to piece this2009 is for with until the ~6000 now Audi Q7 pieces per year. The same company manufactured from 2006 another piece for the SUVthe vehicle largest in serial a complete production automated of metallic series, foams namely in the automotive a small 7 g industry. crash absorber (Figure4b) placed at Audi Q7 SUV vehicle in a complete automated series, namely a small 7 g crash absorber (Figure 4b) the support frame of the safety net [14]. With over 120,000 parts per year, this is until now the largest placed at the support frame of the safety net [14]. With over 120,000 parts per year, this is until now serialthe production largest serial of metallicproduction foams of metallic in the foams automotive in the automotive industry. industry.

(a) (b)

Figure 4. (a) Al‐foam part for the Ferrari 360 and 430 spider; and (b) small crash‐absorbing element for the Audi Q7 (courtesy(a) of Alulight). (b) 11 Figure 4. (a) Al‐foam part for the Ferrari 360 and 430 spider; and (b) small crash‐absorbing element Figure 4. (a) Al-foam part for the Ferrari 360 and 430 spider; and (b) small crash-absorbing element for for the Audi Q7 (courtesy of Alulight). the Audi Q7 (courtesy of Alulight). 11 Materials 2016, 9, 85 12 of 27 Materials 2016, 9, 85 12 of 27

However, newnew opportunities opportunities for applicationsfor applications in the automotivein the automotive industry offer industry new developments offer new developmentsin the electric car in segment,the electric as there,car segment, lightweight as there, construction lightweight becomes construction mandatory. becomes Furthermore, mandatory. new Furthermore,car designs are new necessary car designs due toare the necessary rearrangement due to ofthe components, rearrangement making of components, it possible to making consider it possiblecellular solutionsto consider from cellular the beginning.solutions from Additionally, the beginning. safety Additionally, is another importantsafety is another factor, important where in factor,a reduced where volume, in a reduced a light, but volume, very effective a light, but crash very protection effective system crash protection is needed. system An example is needed. is shown An examplein Figure 5is. shown Metal foam in Figure parts 5. developed Metal foam by parts the Technical developed University by the Technical Berlin and University Pohltec Metalfoam Berlin and Pohltecare foreseen Metalfoam in the prototype are foreseen of an in ultra-light the prototype electric of an vehicle ultra developed‐light electric in the vehicle frame developed of the European in the projectframe of Evolution the European [97]. project Evolution [97].

(a) (b)

Figure 5. ((aa)) Crash absorber box of the electric car prototype in the frame of the European project Evolution made with rectangular Al Al-proﬁles‐profiles filled ﬁlled with Al Al-foam;‐foam; and ( b) CAD design of the body in white (courtesy of Cidaut, Valladolid, Spain, Pinningfarina, Cambiano, Italy and Pohltec metal metal foam, foam, Collogne, Germany).

Exploiting also the energy absorption capability of metallic foams, Foamtech (Daegu, Korea) Exploiting also the energy absorption capability of metallic foams, Foamtech (Daegu, Korea) provided crash elements for a guardrail at the Massan‐Chanwon Bridge in Korea. Further, the provided crash elements for a guardrail at the Massan-Chanwon Bridge in Korea. Further, the suitability of Al‐foam as a protection element in A‐pillars was also analyzed in the literature [98]. An suitability of Al-foam as a protection element in A-pillars was also analyzed in the literature [98]. A‐pillar of a Ford passenger car filled with Al‐foam reinforcement provides an improvement of 30% An A-pillar of a Ford passenger car filled with Al-foam reinforcement provides an improvement of 30% in crash energy absorption at only a 3% weight increase [99]. A redesign of the components instead in crash energy absorption at only a 3% weight increase [99]. A redesign of the components instead of of a simple filling could even improve theses values. a simple filling could even improve theses values. Since 2003, Pohltec Metalfoam (Collogne, Germany), has been producing a support of a working Since 2003, Pohltec Metalfoam (Collogne, Germany), has been producing a support of a working platform made of welded AFS parts for mobile cranes for Teupen (Gronau, Germany) (Figure 6). This platform made of welded AFS parts for mobile cranes for Teupen (Gronau, Germany) (Figure6). This is a serial production of ~100 pieces/year, where the metallic foam component is saving around 95 kg is a serial production of ~100 pieces/year, where the metallic foam component is saving around 95 kg compared to the original steel counterpart. This fact gives the company and the end user an important compared to the original steel counterpart. This fact gives the company and the end user an important strategical advantage against competitors on the market, namely it is the only crane having a range strategical advantage against competitors on the market, namely it is the only crane having a range of of 25 meters and fulfilling the 3.5 ton weight limit to be driven with a normal driving license. AFS 25 meters and fulfilling the 3.5 ton weight limit to be driven with a normal driving license. AFS panels panels are also tested as lightweight support and crash protection of heavy battery modules at the are also tested as lightweight support and crash protection of heavy battery modules at the bottom of bottom of electric car or bus bodies. electric car or bus bodies. In the frame of mobility, we have to consider also the railway industry. Promising prototypes In the frame of mobility, we have to consider also the railway industry. Promising prototypes have have evolved in the past years as possible future serial application. AFS foam panels delivered by the evolved in the past years as possible future serial application. AFS foam panels delivered by the IWU IWU (Chemnitz, Germany) are used in the floor of a wagon of the metro in Peking in continuous (Chemnitz, Germany) are used in the floor of a wagon of the metro in Peking in continuous operation operation without issue for several years. A train front structure was welded from curved AFS plates without issue for several years. A train front structure was welded from curved AFS plates by by the Wilhelm Schmidt GmbH (Groß‐Kienitz, Germany) in cooperation with the Brandenburgische the Wilhelm Schmidt GmbH (Groß-Kienitz, Germany) in cooperation with the Brandenburgische Technische Universität Cottbus (BTU, Cottbus, Germany) [100]. A more prominent and recent Technische Universität Cottbus (BTU, Cottbus, Germany) [100]. A more prominent and recent prototype is the power head cover of the Intercity‐Express‐Train (ICE) train fabricated by Voith prototype is the power head cover of the Intercity-Express-Train (ICE) train fabricated by Voith Engineering (Ludwigsfelde, Germany) and IWU (Chemnitz, Germany). It is made of welded AFS plates and carbon fibers in the front, with a total length of around 6 m (Figure 7). A weight reduction of 18% was achieved with the same stiffness, improving vibration damping and reducing

12 Materials 2016, 9, 85 13 of 27

Engineering (Ludwigsfelde, Germany) and IWU (Chemnitz, Germany). It is made of welded AFS plates and carbon ﬁbers in the front, with a total length of around 6 m (Figure7). A weight reduction of 18%Materials was achieved2016, 9, 85 with the same stiffness, improving vibration damping and reducing additionally13 of 27 the manufacturingMaterials 2016, 9, 85 steps compared to the traditional construction procedure. These examples13 of 27 show additionally the manufacturing steps compared to the traditional construction procedure. These clearly the advantages of metallic foams or AFS panels against honey comb panels, namely when additionallyexamples show the clearlymanufacturing the advantages steps comparedof metallic tofoams the traditionalor AFS panels construction against honey procedure. comb panels, These curvedexamplesnamely sections when show are curved clearly required. sections the advantages are required. of metallic foams or AFS panels against honey comb panels, namely when curved sections are required.

Figure 6. (a) Support of a working platform made of welded AFS manufactured by Pohltec Figure 6. (a) Support of a working platform made of welded AFS manufactured by Pohltec Metalfoam; FigureMetalfoam; 6. ( aand) Support (b) mobile of a craneworking vehicle platform from Teupenmade of (Gronau, welded Germany)AFS manufactured where the bysupport Pohltec is and (b) mobile crane vehicle from Teupen (Gronau, Germany) where the support is applied (courtesy Metalfoam;applied (courtesy and ( bof) Pohltecmobile Metalfoam).crane vehicle from Teupen (Gronau, Germany) where the support is of Pohltec Metalfoam). applied (courtesy of Pohltec Metalfoam).

(a) (b)

Figure 7. (a) Prototype of German(a) high velocity train ICE made of welded aluminum(b) foam sandwich; Figureand (b) 7. view (a) Prototype of the interior of German with the high low velocity number train of components ICE made of required welded (courtesy aluminum of foam Thomas sandwich; Hipke, Figure 7. (a) Prototype of German high velocity train ICE made of welded aluminum foam sandwich; andIWU, (b Chemnitz,) view of the Germany interior andwith Voith the low Engineering, number of Chemnitz,components Germany). required (courtesy of Thomas Hipke, and (b) view of the interior with the low number of components required (courtesy of Thomas Hipke, IWU, Chemnitz, Germany and Voith Engineering, Chemnitz, Germany). IWU,Further Chemnitz, railway Germany applications and Voith can Engineering, be mentioned, Chemnitz, e.g., for Germany).the Combino tram in Budapest, where AlulightFurther (Ranshofen, railway applicationsAustria) delivers can be a mentioned,foam block placede.g., for behind the Combino the bumper tram into Budapest,absorb energy where in FurtherAlulightcase of collisions railway(Ranshofen, applications with Austria) cars, deliverstrying can be to a mentioned, foamavoid block expensive placed e.g., for damagebehind the Combino theto bumperthe structure tram to absorb in Budapest,or writeenergy‐offs in where Alulightcase(Figure (Ranshofen,of 8a).collisions A similar Austria)with crash cars, absorption delivers trying ato box foam avoid can blockbe expensive found placed in thedamage behind Sprinter to the Lightthe bumper structureTrain (SLT) to absorbor in write Holland, energy‐offs in (Figure 8a). A similar crash absorption box can be found in the Sprinter Light Train (SLT) in Holland, 13 13 Materials 2016, 9, 85 14 of 27

where ~1000 pieces per year were delivered since 2008 by (Gleich, Kaltenkirchen, Germany) and the Slovak Academy of Sciences (SAS, Bratislava, Slovakia) (Figure 8b). MaterialsCrash2016 ,absorption9, 85 capability is of special interest for military applications related to armored14 of 27 protection of vehicles or blast mitigation. These types of applications are obviously mostly confidential, and it is difficult to gain detailed information about them. Metallic foams for protection caseand ofcrash collisions absorption, with cars, e.g., trying at the to bottom avoid expensiveof helicopters damage and to of the blast structure mitigation or write-offs systems (Figure are good8a). Acandidates. similar crash Figure absorption 9 shows a box foam can block be found from inAluinvent the Sprinter with Lightexplosives Train to (SLT) test inits Holland,blast mitigation where ~1000Materialscapability. pieces 2016 ,Duocel 9, per 85 year® aluminum were delivered foam provides since 2008 critical by (Gleich, blast energy Kaltenkirchen, absorption Germany) for crew and protection the Slovak14 of in27 Academythe cabin retrofit of Sciences in the (SAS, family Bratislava, of military Slovakia) medium (Figure tactical8b). vehicles and in cabin seat mounts [37]. where ~1000 pieces per year were delivered since 2008 by (Gleich, Kaltenkirchen, Germany) and the Slovak Academy of Sciences (SAS, Bratislava, Slovakia) (Figure 8b). Crash absorption capability is of special interest for military applications related to armored protection of vehicles or blast mitigation. These types of applications are obviously mostly confidential, and it is difficult to gain detailed information about them. Metallic foams for protection and crash absorption, e.g., at the bottom of helicopters and of blast mitigation systems are good candidates. Figure 9 shows a foam block from Aluinvent with explosives to test its blast mitigation capability. Duocel® aluminum foam provides critical blast energy absorption for crew protection in the cabin retrofit in the family of military medium tactical vehicles and in cabin seat mounts [37].

(a) (b)

Figure 8. Crash absorbers (a) of the Combino tram in Budapest produced by Alulight; and (b) of the Sprinter Light Train inin HollandHolland produced by Gleich andand SAS Bratislava (courtesy of Alulight, Gleich and SAS Bratislava).

Crash absorption capability is of special interest for military applications related to armored protection of vehicles or blast mitigation. These types of applications are obviously mostly confidential, and it is difficult to gain detailed information about them. Metallic foams for protection and crash absorption, e.g., at the bottom(a) of helicopters and of blast mitigation systems(b) are good candidates. FigureFigure9 shows 8. Crash a foam absorbers block (a from) of the Aluinvent Combino tram with in explosives Budapest produced to test its by blast Alulight; mitigation and (b) capability. of the ® DuocelSprinteraluminum Light Train foam in Holland provides produced critical by blast Gleich energy and SAS absorption Bratislava for (courtesy crew protection of Alulight, in Gleich the cabin retrofitand in SAS the Bratislava). family of military medium tactical vehicles and in cabin seat mounts [37].

Figure 9. Block of Aluinvent continuous casted aluminum foam with an 85 mm thickness and 15 mm sized bubbles for a blast mitigation test with explosives shortly before detonation (courtesy of Norbert Babcsán, Aluinvent).

The ship building industry is considering metal foam applications concerning weight reduction and distribution, i.e., upper structures should be as light as possible, to increase ship stability and, as a consequence, to increase ship load capacity. In 2009, a project concerning the development of a new type of inland area cargo vessels for weight reduction in ships started [101]. Furthermore, prototypes of sea markers made of metallic foam showed that in the case of damage due to ship contact or ice Figure 9. Block of Aluinvent continuous casted aluminum foam with an 85 mm thickness and 15 mm pressure,Figure they 9. Block still ofcontinue Aluinvent floating, continuous being casted an important aluminum advantage foam with an for 85 security mm thickness reasons. and 15 mm sized bubbles for a blast mitigation test with explosives shortly before detonation (courtesy of sizedThe bubblesaeronautic for a industry blast mitigation considered test with metallic explosives foams shortly for before protection detonation against (courtesy bird of Norbert strikes of Norbert Babcsán, Aluinvent). planesBabcsán, [102]. Aluinvent).Protection against micrometeoroids for satellites or in the space station are also of great interest, especially due to their isotropic properties [103,104]. Pohltec Metalfoam manufactured a prototypeThe ship of a building conical adapter industry (>4 is m considering diameter) formetal the foam Ariane applications rocket with concerning welded, curved weight AFS reduction panels, and distribution,The ship building i.e., upper industry structures is considering should be metal as light foam as applicationspossible, to increase concerning ship weight stability reduction and, as and distribution, i.e., upper structures should be as light as possible, to increase ship stability and, a consequence, to increase ship load capacity. In 2009,14 a project concerning the development of a new typeas a consequence, of inland area to cargo increase vessels ship for load weight capacity. reduction In 2009, in a ships project started concerning [101]. theFurthermore, development prototypes of a new oftype sea of markers inland area made cargo of metallic vessels forfoam weight showed reduction that in in the ships case started of damage [101]. due Furthermore, to ship contact prototypes or ice pressure, they still continue floating, being an important advantage for security reasons. The aeronautic industry considered metallic foams for protection against bird strikes of planes [102]. Protection against micrometeoroids for satellites or in the space station are also of great interest, especially due to their isotropic properties [103,104]. Pohltec Metalfoam manufactured a prototype of a conical adapter (>4 m diameter) for the Ariane rocket with welded, curved AFS panels,

14 Materials 2016, 9, 85 15 of 27 of sea markers made of metallic foam showed that in the case of damage due to ship contact or ice pressure, they still continue ﬂoating, being an important advantage for security reasons. The aeronautic industry considered metallic foams for protection against bird strikes of planes [102]. Protection against micrometeoroids for satellites or in the space station are also of great interest, especially due to their isotropic properties [103,104]. Pohltec Metalfoam manufactured a prototype of a conical adapter (>4 m diameter) for the Ariane rocket with welded, curved AFS panels, performing better than standard materials due to additional vibration damping and demonstrating Materials 2016, 9, 85 15 of 27 again the feasibility of curved and 3D structures. performingFor some sportbetter equipment,than standard the materials unique due properties to additional of metallic vibration foams damping become and demonstrating more relevant, as priceagain usually the feasibility plays here of a curved secondary and 3D role. structures. For example, some golf putters use the damping properties of the metalFor some foam sport structure equipment, to improve the unique the strikeproperties control of metallic [21]. Some foams areas become like more shinbone relevant, protectors as for footballprice usually players plays or here helmets a secondary could role. also For be example, exploited. some For golf example, putters use Alcarbon the damping (Bremen, properties Germany) combinesof the metal the properties foam structure of aluminum to improve foams the strike with control carbon [21]. ﬁbers Some for areas sports like articlesshinbone [ 105protectors]. for football players or helmets could also be exploited. For example, Alcarbon (Bremen, Germany) 5.2. Functionalcombines the Applications properties of aluminum foams with carbon fibers for sports articles [105].

5.2.A wide Functional palette Applications of functional applications based on metallic foams can be found on the market. Again, a multi-purpose approach has the best chances to offer a competitive or unique product. A wide palette of functional applications based on metallic foams can be found on the market. Again,Ceilings a multi in auditoriums‐purpose approach or large has the rooms best chances are very to oftenoffer a planked competitive with or unique perforated product. metal sheets for soundCeilings control. in auditoriums As an alternative or large rooms to thisare very traditional often planked construction with perforated material, metal applications sheets for of metallicsound foam control. panels As foran alternative sound absorption to this traditional are already construction available material, on the market,applications offered of metallic by different companiesfoam panels[30,83 for,94 sound]. At the absorption open foam are already surface, available the sound on the waves market, are offered guided by and different redirected companies to the foam interior,[30,83,94]. where At they the open are caught foam surface, and damped the sound after waves several are guided reﬂections. and redirected The pore to the size foam distribution interior, and the differentwhere they orientations are caught allow and damped for a very after effective several dampingreflections. over The a pore broad size frequency distribution spectrum. and the These applicationsdifferent couldorientations be considered allow for alsoa very as architectural,effective damping but weover include a broad them frequency here as spectrum. their main These function applications could be considered also as architectural, but we include them here as their main is sound absorption. They combine the advantage of the lightweight, self-supporting capability of function is sound absorption. They combine the advantage of the lightweight, self‐supporting large metallic foam panels made of open cell or just sliced closed cell foams, with a design component. capability of large metallic foam panels made of open cell or just sliced closed cell foams, with a Figuredesign 10 shows component. the ceilings Figure of10 anshows audience the ceilings hall and of an a restaurant audience hall covered and a byrestaurant Alusion covered foam provided by by CymatAlusion (Mississauga, foam provided ON, by Cymat Cananda). (Mississauga, ON, Cananda).

(a) (b)

FigureFigure 10. 10.Ceiling Ceiling of of (a )(a audience) audience hall; hall; andand ( (bb)) restaurant restaurant covered covered by byAlusion Alusion foam foam for sound for sound control control (courtesy of Cymat). (courtesy of Cymat). Further sound absorption applications made by Alporas foams provided by Shinko Wire (AmagasakiFurther sound City, Japan) absorption can be found applications in train rails, made metro by tunnels, Alporas elevators, foams providedon the under by‐side Shinko of an Wire (Amagasakielevated highway, City, Japan) etc. [106]. can be Newer found products in train based rails, also metro on Alporas tunnels, foams elevators, were ondeveloped the under-side by of anFoamtech elevated (Daegu, highway, Korea)etc.[ and106 applied]. Newer to concert products halls, based conference also on rooms, Alporas auditoriums, foams were sport developed centers by Foamtechand walls (Daegu, and ceilings Korea) of and the machinery applied to and concert operating halls, rooms conference of industrial rooms, plants auditoriums, as non‐flammable, sport centers andacoustic walls and absorbers ceilings [30]. of Further the machinery applications and can operating be found rooms in the ship of industrial industry, where, plants e.g., as non-flammable,prevention of noise from the engine room, the combustive exhaust pipe and the air cleaning system are provided acoustic absorbers [30]. Further applications can be found in the ship industry, where, e.g., prevention by foamed inside walls between cabins [30]. Metallic foams are also used by Foamtech for sound of noise from the engine room, the combustive exhaust pipe and the air cleaning system are provided absorption in metro tunnels, in the railway and on the tunnel and station walls, where they have to support high air pressure changes and vibration, being at the same time non‐flammable [30]. We can find aerodynamic noise reduction prototypes in the railway industry, e.g., in pantographs for the Shinkansen train in Japan [107,108]. There, open cell Al‐foam is used for shape smoothing of the panhead and its support and covering. Applications for aerodynamic noise reduction of jet turbines of airplanes are also under discussion [109]. 15 Materials 2016, 9, 85 16 of 27 by foamed inside walls between cabins [30]. Metallic foams are also used by Foamtech for sound absorption in metro tunnels, in the railway and on the tunnel and station walls, where they have to support high air pressure changes and vibration, being at the same time non-flammable [30]. We can find aerodynamic noise reduction prototypes in the railway industry, e.g., in pantographs for the Shinkansen train in Japan [107,108]. There, open cell Al-foam is used for shape smoothing of the panhead and its support and covering. Applications for aerodynamic noise reduction of jet turbines of airplanes are also under discussion [109]. Due to the high temperature and chemical resistance of metal foams, combined with sound absorbingMaterials properties, 2016, 9, 85 open cell foams from Alantum have been applied as silencers, mufflers,16 of 27 diesel particulateMaterials filters, 2016, selective 9, 85 catalysis reduction, etc. [28,40] (see Figure 11). Exxentis16 of (Wettingen,27 Due to the high temperature and chemical resistance of metal foams, combined with sound Switzerland)absorbing and properties, Mott Corporation open cell foams (Farmington, from Alantum CT, have USA) been applied offer as filters silencers, for mufflers, the filtration diesel of gases Due to the high temperature and chemical resistance of metal foams, combined with sound ® and liquids,absorbingparticulate flow control,properties, filters, selective diffusion, open cell catalysis foams sparging, fromreduction, Alantum fluidizing, etc. have[28,40] been (see venting applied Figure as and 11). silencers, wickingExxentis mufflers, (Wettingen, [42, 110diesel]. Duocel , Recemat,particulateSwitzerland) Inco and filters, Alantum and Mott selective Corporation have catalysis open (Farmington, reduction, cell metal CT,etc. foamsUSA)[28,40] offer (see as filters aFigure catalyst for 11). the filtrationExxentis carrier, of(Wettingen, for gases filtration and fluid ® control orliquids,Switzerland) anti-sloshing flow control,and purposes Mott diffusion, Corporation in sparging, their (Farmington, portfolios fluidizing, CT, [venting28 USA),37, offer41and,111 wicking filters]. Even for [42,110]. the nanoporous filtration Duocel of ,gases Recemat, gold and foams find Inco and Alantum have open cell metal foams as a catalyst carrier, for filtration fluid® control or applicationliquids, as catalysts flow control, [112 diffusion,]. sparging, fluidizing, venting and wicking [42,110]. Duocel , Recemat, Incoanti‐ sloshingand Alantum purposes have inopen their cell portfoliosmetal foams [28,37,41,111]. as a catalyst Evencarrier, nanoporous for filtration gold fluid foams control find or A strongantiapplication‐sloshing market as catalystspurposes for metallic [112]. in their sponges, portfolios with [28,37,41,111]. a large number Even ofnanoporous different gold products, foams isfind represented by heat exchangers.applicationA strong as The marketcatalysts very for [112]. good metallic thermal sponges, conductivity with a large number of metals of different and the products, large areais represented of metal sponges provide veryby heat effectiveA strong exchangers. market passive The for very cooling metallic good sponges, [thermal37,41,54 conductivitywith,75 a]. large As an numberof metals example, of and different the Figure large products, area11 shows of metalis represented a sponges large number of provide very effective passive cooling [37,41,54,75]. As an example, Figure 11 shows a large number filters fromby Alantum, heat exchangers. Korea, The and very Figuregood thermal 12 of conductivity different heatof metals exchangers and the large from area M-pore,of metal sponges Germany [38]. provideof filters veryfrom effectiveAlantum, passive Korea, coolingand Figure [37,41,54,75]. 12 of different As an heat example, exchangers Figure from 11 Mshows‐pore, a Germanylarge number [38]. of filters from Alantum, Korea, and Figure 12 of different heat exchangers from M‐pore, Germany [38].

Figure 11. Array of products (filters) for functional applications (courtesy of Alantum). Figure 11. Array of products (ﬁlters) for functional applications (courtesy of Alantum). Figure 11. Array of products (filters) for functional applications (courtesy of Alantum).

Figure 12. Diverse types of heat exchangers based on Al‐ and Cu‐foams (courtesy of M‐pore and FigureMayser 12.GmbH). Diverse types of heat exchangers based on Al‐ and Cu‐foams (courtesy of M‐pore and Figure 12.MayserDiverse GmbH). types of heat exchangers based on Al- and Cu-foams (courtesy of M-pore and Mayser GmbH).ERG produces also heat exchanger from Duocel® (Al‐ and Cu‐based open cell foams) [37]. They haveERG built produces different alsodevices, heat exchangere.g., a small from heat Duocel exchanger® (Al‐ toand thermally Cu‐based stabilize open cell the foams) lens of [37]. electron They scanninghave built microscopes different devices, or thermal e.g., energya small absorbersheat exchanger for medical to thermally laser applications. stabilize the Furthermore, lens of electron the scanning microscopes or thermal energy absorbers16 for medical laser applications. Furthermore, the 16 Materials 2016, 9, 85 17 of 27

ERG produces also heat exchanger from Duocel® (Al- and Cu-based open cell foams) [37]. They have built different devices, e.g., a small heat exchanger to thermally stabilize the lens of electron scanning microscopes or thermal energy absorbers for medical laser applications. Furthermore, the breatherMaterials 2016 plug, 9, 85 used in the Lockheed Martin F-22 fighter aircraft for pressure releases during17 of 27 rapid altitude changes, electromagnetic shielding protection and moisture wicking is made of Duocel® [37]. breather plug used in the Lockheed Martin F‐22 fighter aircraft for pressure releases during rapid An ERG heat exchanger made of Al-foam was used as heat exchange media and support matrix of altitude changes, electromagnetic shielding protection and moisture wicking is made of Duocel® [37]. granulatedAn ERG chemicals heat exchanger consisting made of of multiple Al‐foam layerswas used of amine-based as heat exchange filter media beds and to remove support carbon matrix dioxideof and moisturegranulated in chemicals the space consisting shuttle, andof multiple it is used layers now of alsoamine in‐based the International filter beds to space remove station. carbon After reachingdioxide its and full moisture absorption in the capacity, space shuttle, the assembly and it is used rotates now to also emit in the the COInternational2 and moisture space station. into space, afterAfter which reaching it can rotateits full back absorption towards capacity, the interior the assembly and continue rotates to filtration, emit the CO facilitating2 and moisture uninterrupted into CO2 space,removal after [113 which]. it can rotate back towards the interior and continue filtration, facilitating Auninterrupted special application CO2 removal of heat [113]. exchangers is passive thermal cooling, a field where the demand of very effectiveA special heat application sinks is of increasing heat exchangers from is day passive to day thermal due to cooling, the rapidly a field growingwhere the performance demand of of very effective heat sinks is increasing from day to day due to the rapidly growing performance of computers and mobile electronic devices. Nowadays, standard bulbs are being replaced by modern, computers and mobile electronic devices. Nowadays, standard bulbs are being replaced by modern, powerful LED lamps (see Figure 13). This power has to be cooled down to assure LED efficiency and powerful LED lamps (see Figure 13). This power has to be cooled down to assure LED efficiency and protectprotect the electronics. the electronics. Here, Here, where where a a “noisy” “noisy” fanfan cannot be be installed, installed, foamed foamed passive passive cooling cooling devices devices havehave an opportunity. an opportunity. These These applications applications can can combine combine their their functionality functionality with with an aninnovative innovative design. design.

(a) (b)

Figure 13. Passive thermal cooling of LED lamps: (a) Al open cell foam from M‐pore (courtesy of M‐pore); Figure 13. Passive thermal cooling of LED lamps: (a) Al open cell foam from M-pore (courtesy of and (b) cut AFS from the Technical University Berlin. M-pore); and (b) cut AFS from the Technical University Berlin. Due to the high melting temperature and thermal conductivity of metals, they can be used for Dueheat and to the fire high resistance. melting Alulight temperature provided andAl‐alloy thermal foam conductivitypanels for the protection of metals, of they concrete can against be used for heat andflames fire in resistance. fire houses, Alulightdissipating provided the heat over Al-alloy large foam surfaces. panels Similarly for the heat protection dissipation of was concrete used by against flamesPohltec in fire Metalfoam houses, dissipatingon AFS panels the by heat hot over forming large and surfaces. covered Similarly by a ceramic heat plasma dissipation coating was as used by Pohltecbarbecue Metalfoam and cooking on plates, AFS panels providing by hota convenient forming homogeneous and covered temperature by a ceramic distribution plasma coating [15]. as ® barbecueDuocel and aluminum cooking foam plates, was providing used as the a primary convenient component homogeneous in satellite temperature cryogenic tanks distribution to provide [15]. uniform heating and cooling. Space‐based infrared optics using solid cryogenic coolers utilize Duocel® aluminum foam was used as the primary component in satellite cryogenic tanks to provide Duocel® aluminum foam also as an isothermalizer and baffle structure. Keeping a solid cryogen at a uniform heating and cooling. Space-based infrared optics using solid cryogenic coolers utilize Duocel® uniform temperature gives the infrared optics a longer useful lifetime [37]. aluminumAl foam‐, Zn‐ alsoand asNi‐ ansponges isothermalizer are used for and electro baffle‐chemical structure. applications, Keeping a where solid cryogenthe large atsurface a uniform temperatureoffers good gives advantages the infrared during optics operation, a longer e.g., useful in water lifetime purification [37]. where ions react with the matrix Al-,material Zn- or and as electrodes Ni-sponges for Zn are‐ and used Ni‐based for electro-chemical batteries [28,39,41,111,114–117]. applications, The where application the large of Inco surface offersNi good‐foams advantages as the anode during in NiCd operation, and NiMH e.g., batteries in water comprehended purification a where production ions reactvolume with of about the matrix material3,000,000 or as square electrodes meters for of Zn- foam and per Ni-based year, being batteries the most [28 successful,39,41,111 commercial,114–117]. application The application of metal of Inco Ni-foamssponges as in the the anode market. in NiCd and NiMH batteries comprehended a production volume of about 3,000,000Bio square‐medical meters applications of foam perare another year, being field the of mostinterest. successful Here, high commercial quality products application based ofon metal Ti‐foams provide excellent bio‐compatibility properties. The trend goes into the direction of porous sponges in the market. structures to improve osseointegration, as is shown in Figure 14, where a tomography of a Ti‐based

17 Materials 2016, 9, 85 18 of 27

Bio-medical applications are another field of interest. Here, high quality products based on Ti-foamsMaterials 2016 provide, 9, 85 excellent bio-compatibility properties. The trend goes into the direction of porous18 of 27 structures to improve osseointegration, as is shown in Figure 14, where a tomography of a Ti-based dental implant shows its porous structure. Although Ti is quite difficult difficult to foam, foam foam-like‐like structures can be created through different productionproduction methods.methods. This fieldfield should be exploited more deeply in the future.Materials 2016, 9, 85 18 of 27

dental implant shows its porous structure. Although Ti is quite difficult to foam, foam‐like structures can be created through different production methods. This field should be exploited more deeply in the future.

(a) (b)

Figure 14.14. ((aa)) TomographyTomography of of a a Ti-based Ti‐based porous porous dental dental implant implant for for osseointegration osseointegration improvement; improvement; and (a) (b) (andb) detail (b) detail of the of inner the inner structure structure and porousand porous surface surface (courtesy (courtesy of Louis-Philippe of Louis‐Philippe Lefebvre). Lefebvre). Figure 14. (a) Tomography of a Ti‐based porous dental implant for osseointegration improvement; 5.3. Architectural and (b) detail Applications of the inner structure and porous surface (courtesy of Louis‐Philippe Lefebvre). The organic cellular structure of metallic foams makes their surface unique and very attractive The5.3. Architectural organic cellular Applications structure of metallic foams makes their surface unique and very attractive for designfor design purposes. purposes. Closed Closed cell cell foams foams with with a bubbly a bubbly surface, surface, cut closedcut closed cell cell foams foams with with a first a first open open cell cell layerThe or openorganic cell cellular foams, structure which of are metallic more foams or less makes translucent their surface depending unique and on verythe poreattractive size and layerfor or design open cell purposes. foams, Closed which cell are foams more with or less a bubbly translucent surface, depending cut closed oncell the foams pore with size a first and open thickness thickness of the material, have become in the last decade very interesting for architectural of thecell material, layer or have open becomecell foams, in thewhich last are decade more veryor less interesting translucent for depending architectural on the applications. pore size and These applications. These types of applications, especially facades, are very appealing for metallic foam typesthickness of applications, of the material, especially have facades, become are in very the appealing last decade for very metallic interesting foam producers,for architectural as usually largeproducers,applications. areas areas usually required, These large types promising areas of applications, are good required, benefits. especially promising facades, good are benefits. very appealing for metallic foam Inproducers, 2003, the as architectusually large Slawomir areas are Kochanowicz required, promising covered good his benefits. office office building in in Bochum, Germany, Germany, with closedclosedIn 2003, cellcell AlporastheAlporas architect Al-foam.Al ‐Slawomirfoam. Further Further Kochanowicz facades facades covered came came hisin in officethe last building few years, in Bochum, like the Germany, one of the conferencewith closed center cell inin Alporas Mallorca,Mallorca, Al‐ Spain,foam.Spain, Further mademade ofoffacades ~20,000~20,000 came mm²² in ofof the Cymat/AlusionCymat/Alusion last few years, Al Al-foam,like‐foam, the one a $2.2 of the million conference center in Mallorca, Spain, made of ~20,000 m² of Cymat/Alusion Al‐foam, a $2.2 million contract. Cymat also provided cladding of the exterior facade of a 12,000 mm²² building in downtowndowntown Tbilisi,contract. Georgia, Cymat and also recently provided of a cladding Protestant of the church exterior in Terrasa,facade of Spaina 12,000 (see m² Figurebuilding 15). in downtown Tbilisi,Tbilisi, Georgia, Georgia, and and recently recently of of a Protestanta Protestant church church in Terrasa, Spain Spain (see (see Figure Figure 15). 15 ).

Figure 15. Protestant church in Terrasa, Spain, clad with Alusion foam. The inset shows a detailed Figure 15. Protestant church in Terrasa, Spain, clad with Alusion foam. The inset shows a detailed Figureview 15. of Protestant the foam structure church (courtesyin Terrasa, of John Spain, Banhart). clad with Alusion foam. The inset shows a detailed view of the foam structure (courtesy of John Banhart). view of the foam structure (courtesy of John Banhart). In 2015, Pohltec Metalfoam clad a cottage located in Switzerland at high altitude, with additional functionality, namely to protect the facade from deterioration due to high temperature variations and In 2015, Pohltec Metalfoam clad a cottage located in Switzerland at high altitude, with additional strong weather conditions (see Figure 16). The individual and organic surface of the tiles is reached functionality,by remelting namely the face to sheetprotect of anthe AFS facade panel. from deterioration due to high temperature variations and strong weather conditions (see Figure 16). The individual and organic surface of the tiles is reached by remelting the face sheet of an AFS panel. 18

18 Materials 2016, 9, 85 19 of 27

In 2015, Pohltec Metalfoam clad a cottage located in Switzerland at high altitude, with additional functionality, namely to protect the facade from deterioration due to high temperature variations and strong weather conditions (see Figure 16). The individual and organic surface of the tiles is reached by Materials 2016, 9, 85 19 of 27 remelting the face sheet of an AFS panel.

Materials 2016, 9, 85 19 of 27

(a) (b)

Figure 16. (a) Cottage “Anenhütte” (Lötschental, Switzerland) with AFS cladding; and (b) detail of Figure 16. (a) Cottage “Anenhütte”(a) (Lötschental, Switzerland) with AFS cladding;(b and) (b) detail of the the facade tiles (courtesy of Pohltec Metalfoam). facadeFigure tiles 16. (courtesy (a) Cottage of Pohltec “Anenhütte” Metalfoam). (Lötschental, Switzerland) with AFS cladding; and (b) detail of theNot facade only tilesfacades, (courtesy but ofother Pohltec architectural Metalfoam). applications, where realized with Alusion foam, for Notexample only the facades, entrance but of othera souterrain architectural or the support applications, structure where of a clarion realized as withpart of Alusion a bell tower foam, for examplemonument,Not the entranceonly as facades,shown of in abut Figure souterrain other 17a,b, architectural orrespectively. the support applications, Furthermore, structure where the of realized memorial a clarion with of as ServiceAlusion part ofEmployees foam, a bell for tower example the entrance of a souterrain or the support structure of a clarion as part of a bell tower monument,International as shown Union, in dedicated Figure 17 toa,b, its respectively.members lost in Furthermore, the World Trade the Centre memorial attack of of Service 11 September Employees monument, as shown in Figure 17a,b, respectively. Furthermore, the memorial of Service Employees International2001, designed Union, by dedicated the architects to itsFurnstahl members & Simon lost in (Montclair, the World CA, Trade USA) Centre is made attack of Alusion of 11 foam. September International Union, dedicated to its members lost in the World Trade Centre attack of 11 September 2001,2001, designed designed by theby the architects architects Furnstahl Furnstahl & & Simon Simon (Montclair, CA, CA, USA) USA) is ismade made of Alusion of Alusion foam. foam.

(a) (b)

Figure 17. Translucent entrance of(a a) souterrain and bell tower monument made of Alusion(b) foam (courtesy of Alusion). Figure 17. Translucent entrance of a souterrain and bell tower monument made of Alusion foam Figure 17. Translucent entrance of a souterrain and bell tower monument made of Alusion foam In(courtesy 2003, Kauffmann,of Alusion). Theilig & Partner and Markgraph created for Daimler Chrysler a booth at (courtesy of Alusion). the international auto show in Geneva, Switzerland, from Alporas foam blocks to represent their advancedIn 2003, technologies. Kauffmann, Ferrari, Theilig Audi & Partner and others and Markgraphpresented also created booths for with Daimler metal Chrysler foam components a booth at the(Figure international 18). The company auto show Aluinvent in Geneva, has Switzerland,manufactured from a reception Alporas desk foam for blocks Market to Zrt.represent (Budapest, their advanced technologies. Ferrari, Audi and others presented also booths with metal foam components (Figure 18). The company Aluinvent has manufactured19 a reception desk for Market Zrt. (Budapest,

19 Materials 2016, 9, 85 20 of 27

In 2003, Kauffmann, Theilig & Partner and Markgraph created for Daimler Chrysler a booth at the international auto show in Geneva, Switzerland, from Alporas foam blocks to represent their advanced technologies. Ferrari, Audi and others presented also booths with metal foam components (Figure 18). Materials 2016, 9, 85 20 of 27 TheMaterials company 2016, 9, 85 Aluinvent has manufactured a reception desk for Market Zrt. (Budapest, Hungary)20 of 27 (FigureHungary) 19 ).(Figure Pohltec 19). Metalfoam Pohltec Metalfoam has design has tiles design made oftiles a molten made of AFS a molten surface AFS in their surface portfolio in their for Hungary) (Figure 19). Pohltec Metalfoam has design tiles made of a molten AFS surface in their portfoliointerior architecture for interior purposesarchitecture (Figure purposes 20). (Figure 20). portfolio for interior architecture purposes (Figure 20).

Figure 18. Booth for the convention center with an Alusion foam structure (courtesy of Alusion). Figure 18. Booth for the convention center with an Alusion foam structure (courtesy of Alusion).

(a) (b) (a) (b) Figure 19. (a) Reception desk from Aluinvent made of Aluhab foam with a bubble size of 3 mm built Figure 19. (a) Reception desk from Aluinvent made of Aluhab foam with a bubble size of 3 mm built forFigure Market 19. ( aZrt.) Reception Budapest, desk Hungary; from Aluinvent and (b made) detail of Aluhabof the foamfoam withsurface a bubble structure size of(courtesy 3 mm built of for Market Zrt. Budapest, Hungary; and (b) detail of the foam surface structure (courtesy of Norbertfor Market Babcsán, Zrt. Budapest, Aluinvent). Hungary; and (b) detail of the foam surface structure (courtesy of Norbert Babcsán,Norbert Babcsán, Aluinvent). Aluinvent). 5.4. Design, Art and Decoration 5.4. Design, Art and Decoration The classification of an application as architectural or design, art and decoration is very difficult. The classification of an application as architectural or design, art and decoration is very difficult. The transitionThe classification is smooth of anand application seamless, asand architectural it is often not or design,possible art to and distinguish decoration between is very them. difficult. In The transition is smooth and seamless, and it is often not possible to distinguish between them. In Themost transition cases, a is combination smooth and seamless,of purposes and itis is present; often not we possible can tospeak distinguish then about between multi them.‐functional In most most cases, a combination of purposes is present; we can speak then about multi‐functional applications.cases, a combination In this field, of purposes an immense is present; variety we of can individual speak then artworks about exists, multi-functional making it impossible applications. to applications. In this field, an immense variety of individual artworks exists, making it impossible to reviewIn this field,all of an them; immense therefore, variety only of individual a few examples artworks exists,will be making mentioned it impossible here. The to reviewpeculiar, all review all of them; therefore, only a few examples will be mentioned here. The peculiar, modernof them;‐looking therefore, and only sophisticated a few examples organic will surface be mentioned of the cellular here. metal The peculiar, foam structure modern-looking is here always and modern‐looking and sophisticated organic surface of the cellular metal foam structure is here always insophisticated the foreground, organic while surface the other of the properties cellular metalstay almost foam structurein the background. is here always in the foreground, whilein the theforeground, other properties while the stay other almost properties in the background. stay almost in the background.

20 20 Materials 2016, 9, 85 21 of 27 Materials 2016, 9, 85 21 of 27

Materials 2016, 9, 85 21 of 27

Figure 20.20. DifferentDifferent AFS, AFS, cut cut AFS AFS and and tiles madetiles made of AFS of for AFS internal for internal architectural architectural applications applications (Courtesy of(Courtesy PohltecFigure of Metalfoam). 20. Pohltec Different Metalfoam). AFS, cut AFS and tiles made of AFS for internal architectural applications (Courtesy of Pohltec Metalfoam). Several artists artists and and designers designers use use metallic metallic foams foams for fordecorations decorations and andartistic artistic compositions. compositions. Several artists and designers use metallic foams for decorations and artistic compositions. The designer Gerd Kaden produced several several compositions compositions of of wood wood and and metallic metallic foam foam to to show show the The designer Gerd Kaden produced several compositions of wood and metallic foam to show the contrast between natural and man‐made cellular materials (Figure 21). contrastthe contrast between between natural natural and man-made and man‐made cellular cellular materials materials (Figure (Figure 21 21).).

Figure 21. Artistic compositions of the designer Gerd Kaden created from wood and metallic foams. Figure 21. Artistic compositions of the designer Gerd Kaden created from wood and metallic foams. 21 Figure 21. Artistic compositions of the designer Gerd Kaden created from wood and metallic foams.

21 Materials 2016, 9, 85 22 of 27 Materials 2016, 9, 85 22 of 27 Materials 2016, 9, 85 22 of 27 Figure 2222aa shows the support of a statue made of Alusion foam and Figure 2222bb the head of Figure 22a shows the support of a statue made of Alusion foam and Figure 22b the head of Pythagoras foamed in a mold with Alulight aluminum foam by Fero SimanˇcíkfromSimančík from the Slovak Pythagoras foamed in a mold with Alulight aluminum foam by Fero Simančík from the Slovak Academy of ScienceScience inin Bratislava.Bratislava. A design chair from welded Aluhub foam was produced at the Academy of Science in Bratislava. A design chair from welded Aluhub foam was produced at the Imperial College London by Andor Ivan (Figure 2323a).a). An artistic light scattering effect of a designer Imperial College London by Andor Ivan (Figure 23a). An artistic light scattering effect of a designer lamplamp manufactured with M-poreM‐pore open cell foam can be observedobserved inin FigureFigure 2323b.b. Additionally, eveneven lamp manufactured with M‐pore open cell foam can be observed in Figure 23b. Additionally, even gold and silver foams are used for jewelry, demonstrating the diversiﬁeddiversified ﬁeldsfields of applications based gold and silver foams are used for jewelry, demonstrating the diversified fields of applications based on metallic foams existing onon thethe market.market. on metallic foams existing on the market.

(a) (b) (a) (b) Figure 22. (a) Statue support made of Alusion foam (courtesy of Alusion); and (b) head of Pythagoras Figure 22. (a) Statue support made of Alusion foam (courtesy of Alusion); and (b) head of Pythagoras made of Alulight metal foam byby thethe SlovakSlovak AcademyAcademy ofof SciencesSciences (courtesy(courtesy ofof FeroFero Simanˇcík,SAS).Simančík, SAS). made of Alulight metal foam by the Slovak Academy of Sciences (courtesy of Fero Simančík, SAS).

(a) (b) (a) (b) Figure 23. (a) Welded design chair made of Aluhab (courtesy of Norbert Babcsán, Aluinvent); and Figure 23. (a) Welded design chair made of Aluhab (courtesy of Norbert Babcsán, Aluinvent); and Figure(b) art composition 23. (a) Welded with design metal chair swarm made and of light Aluhab scattering (courtesy effect of from Norbert M‐pore Babcsán, (courtesy Aluinvent); of Mayser). and ((b)) art compositioncomposition withwith metalmetal swarmswarm andand lightlight scatteringscattering effecteffect fromfrom M-poreM‐pore (courtesy (courtesy of of Mayser). Mayser). 6. Conclusions 6. Conclusions 6. Conclusions The technology of foamed or spongy metals is finding its place in the market; the number and The technology of foamed or spongy metals is finding its place in the market; the number and volumeThe of technology commercial of applications foamed or spongyis growing metals slowly, is finding but step its by place step. in Scientific the market; research the number should volume of commercial applications is growing slowly, but step by step. Scientific research should andcontribute volume by of gaining commercial knowledge applications about isthese growing complex slowly, structures but step and by supporting step. Scientific research research and contribute by gaining knowledge about these complex structures and supporting research and shoulddevelopment contribute activities, by gaining e.g., by knowledge developing about new these alloys, complex improving structures foam and homogeneity supporting researchand the development activities, e.g., by developing new alloys, improving foam homogeneity and the andproduction development methods. activities, Although e.g., the by developingactual developments new alloys, do improving not forecast foam a revolution homogeneity in the and field, the production methods. Although the actual developments do not forecast a revolution in the field, productionalmost new methods.fields and types Although of applications the actual emerge. developments This industrial do not forecastsector, thanks a revolution to the superior in the field, and almost new fields and types of applications emerge. This industrial sector, thanks to the superior and almostunique newproperties fields andin certain types applications, of applications will emerge. continue This prospering industrial as sector, in the thanks past years. to the A superiorcombination and unique properties in certain applications, will continue prospering as in the past years. A combination uniqueof cost‐effective properties production in certain processes applications, and will quality continue and reproducibility prospering as in improvements the past years. could A combination accelerate of cost‐effective production processes and quality and reproducibility improvements could accelerate ofthe cost-effective process. production processes and quality and reproducibility improvements could accelerate the process. theAcknowledgments: process. The author would like to thank René Poss, Alantum, Thomas Hipke, IWU Chemnitz, Acknowledgments:Voigth Engineering, AuThe Metallgießerei, author would Alulight, like to thank Fero Siman René čPoss,ík, Slovak Alantum, Academy Thomas of Sciences, Hipke, NorbertIWU Chemnitz, Babcsan, VoigthAluinvent, Engineering, Alusion, Au Metallgießerei,Cymat, M‐pore, Alulight, Mayser Fero SimanGmbH,čík, SlovakWolfgang Academy Seeliger, of Sciences, Pohltec Norbert Metalfoam, Babcsan, Aluinvent, Alusion, Cymat, M‐pore, Mayser GmbH, Wolfgang Seeliger, Pohltec Metalfoam, 22 22 Materials 2016, 9, 85 23 of 27

Acknowledgments: The author would like to thank René Poss, Alantum, Thomas Hipke, IWU Chemnitz, Voigth Engineering, Au Metallgießerei, Alulight, Fero Simanˇcík,Slovak Academy of Sciences, Norbert Babcsan, Aluinvent, Alusion, Cymat, M-pore, Mayser GmbH, Wolfgang Seeliger, Pohltec Metalfoam, Louis-Philippe Lefebvre, John Banhart, Jörg Weise, Cidaut and Pinningfarina for support and providing the pictures used in this review. The ﬁnancial support from the European FP7 project Evolution is also acknowledged. Conﬂicts of Interest: The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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