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Abstract
Metal foams possess remarkable properties, such as lightweight, high compressive strength, lower specific weight, high stiffness, and high energy absorption. These properties make them highly desirable for many engineering applications, including lightweight materials, energy-absorption devices for aerospace and automotive industries, etc. For such potential applications, it is essential to understand the mechanical behaviour of these foams. Producing metal foams is a highly challenging task due to the coexistence of solid, liquid, and gaseous phases at different temperatures. Although numerous techniques are available for producing metal foams, fabricating foamed metal still suffers from imperfections and inconsistencies. Thus, a good understanding of various processing techniques and properties of the resulting foams is essential to improve the foam quality. This review discussed the types of metal foams available in the market and their properties, providing an overview of the production techniques involved and the contribution of metal foams to various applications. This review also discussed the challenges in foam fabrications and proposed several solutions to address these problems.
Keywords:
compressive properties, Gibson and Ashby model, mechanical properties, metal foams, melt foaming, powder metallurgy
1. Introduction
Metal foams are cellular structures comprising solid materials with a large portion of gas-filled pores by volume. Due to their cellular structure, metal foams possess a set of unique mechanical and physical properties. These properties allow them to become highly efficient in several engineering applications, notably in components for blast resistance, fire resistance, thermal insulation, foam core sandwich panels, and sound and vibration damping [1,2]. In addition, they are recyclable, with no disposal issues [3]. Consequently, these materials have attracted immense attention in recent years. One of the exceptional features of foams is that their mechanical properties are flexible, and their pore size, geometry, density, and choice of foaming material can be controlled. When used as energy absorption materials, these foams could go through substantial deformations under nearly constant stress [4]. With the rapid advancements in defence, aerospace, and automotives, there is an increasing demand for lightweight materials with high specific strength, better fuel efficiency, and high energy absorption capacity to withstand impact forces [5,6]. Thus, their good mechanical, acoustic, electrical, thermal, and chemical properties make them ideal for structural and functional applications [6,7]. Metal foams generally consist of aluminium (Al), nickel (Ni), magnesium (Mg), copper (Cu), zinc (Zn), and steel. In particular, Al and their alloys are widely used as non-flammable materials for thermal and sound insulation, sandwich cores, mechanical damping, lightweight panels and impact resistance in transportation, strain insulators, and vibration control [8,9,10].
These foams are mostly developed by the addition of foaming agents or space holders in the matrix metal. Several methods have been used for foam development. These methods include adding foaming agent in liquid melt [11], compaction of metal powder and blowing agent [12], blowing gas [13], etc. This review discusses the microstructure of various metal foams, their mechanical properties, manufacturing methods, and industrial applications. Evaluating the properties of metal foams developed through different techniques is essential to find the optimum manufacturing strategies and the effects of various parameters on their microstructure and strength. After introducing different types of metal foams, we discuss the microstructure and properties of the metal foams. The fabrication techniques such as melt foaming and powder metallurgy techniques are discussed in the next section, Section 6 and Section 7 discusses the industrial applications of metal foams and the way forward respectively.
2. Types of Metal Foams
Foam structures are divisible into two types, i.e., the open- cell foams and closed-cell foams, as listed in . In open-cell foams, pores are connected to allow matters to pass through them. By contrast, pores are isolated in closed-cell foams. In general, open-cell foams are preferred for functional applications, such as in filters, catalyst supports, heat exchangers, etc., and closed-cell foams find applications in silencers, automobiles, bearings, sound and energy absorbers, etc. [14].
Table 1
Manufacturing TechniquesMaterialFoaming Agent/Space HoldersMicrostructureReferenceMelt foamingAl matrix,
graphene NaCl, KCl and PMMA
-
Closed pores due to incomplete dissolution of NaCl primarily due to hinderance offered by the high dense samples.
-
Good interfacial bonding strength between the Al matrix and the NaCl interface.
-
Lesser micro-porosities due to high dense material manufactured through the MI process.
[49]Al-Si13-MgX (X = 2.5–15 wt %) alloyMg
[50]AlMg50, Ca TiH2
-
Uniformly distributed Mg in the matrix.
-
Due to the restriction effect of cell walls, the grain morphology of primary α-Al in cell walls of Al foams is irregular.
-
Cell-wall grains are much smaller than those in the pore-free layer.
[48]A356 foamsCaCO3
-
The stabilization was achieved due to foaming gas (CO)/melt reaction during foaming producing CaO, Al2O3 and MgO.
-
The porosity increased with holding time.
-
The cell size increased with increase in CaCO3 content.
[51]
Powder metallurgy
Using foaming agentAlSi10
alloyTiH2
-
Alloy and the reinforcements are bonded metallurgically strong.
-
As the temperature rises to 150 C, the matrix softens and undergoes plastic deformation of the cell walls.
[52]Mg, Al, Cu, and Zn, yttrium TiH2
[53]AlMg4Si8 alloy and multi-walled carbon nanotubes (MWCNT)TiH2
[54]Space holder techniqueTi-based Cu alloy Acrawax
[55]Steel
(iron, graphite phosphorous)Urea granules
[56]Aluminium, Graphene NaCl, KCl, and
PMMA
[49]316L austenitic stainless steel Urea particles
-
Cell size was comparable to that of space-holder particles.
-
Cells are mostly interconnected, open, and spherical in shape.
-
Cell walls are larger in size.
-
Thinner cell walls with microporosities as a result of higher evaporation rate of the space holder.
-
Strong cell wall with low microporosity.
[57]Al matrix and MWCNTUrea
particles
-
Uniformly distributed pores formed in the foam structure with shapes similar to spherical urea granules.
-
Large number of pores formed across the cross-section of the foams with increase in urea content.
[58]Open in a separate window
2.1. Open-Cell Foams
In open-cell foams, no film occurs between adjacent cells in the matrix material. A larger effective surface area (10–1000 times) is exposed to the surroundings compared to the dense material. They have sponge-like interconnected pores, as shown in a [15]. Metal foams have a high specific surface area. These features make them suitable materials for heat exchangers, sound absorbers, catalysts, hydrogen storage, filter elements, etc. [16,17]. Meanwhile, high-porosity metal foams are commonly used in several real-world devices, including heat exchangers [18,19,20,21,22], fuel stacks [23], solar collectors [24,25], heat storage [26,27], etc. These foams possess excellent thermal properties, high permeability, high conductivity, and volume-to-area ratio.
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2.2. Closed-Cell Foams
Cells in the closed-cell foams are separated by a thin film of matrix material ( b) [28,29]. Due to their remarkable energy absorption capabilities with high specific stiffness and high damping capacity [30,31,32], these foams are commonly used in foam-filled tubes, blast resistance, sound and noise insulation, foam core sandwich, shock absorbers, etc. [33]. Owing to their structure, closed-cell foams usually have a higher compressive strength and are denser, requiring more materials to manufacture. Since they are denser, they thus exhibit higher strength and need a special gas (inert gases) to fill the pores for better insulation and low thermal conductivity [34,35,36].
7. Challenges and Way Forward
The technology of metal foaming is developing at an accelerated pace with considerable progress. This research area involves interdisciplinary collaborations among physics, chemistry, and materials engineering to produce the required quality economically with reproducible foamed materials that possessed a unique range of properties. These diverse applications allow the construction of functionally new parts or devices. Since foams have many competitors that are mostly less expensive, it becomes crucial to improve fabrication technologies for large-scale foam production, generating a greater variety of low-cost metal foam products. Despite the successful development, more basic research remains essential to understanding the correlation between the composition of metal alloy and foamability. Producing foams of consistent quality, morphology, and control of structure is challenging. However, the foam quality and properties can be controlled by optimizing the factors and parameters. In addition, the foam’s porosities can be controlled by heat treatment or coating the blowing agent to stop the decomposition of the gas-blowing agent in the melt. Overall, to produce uniformly structured metal foams, it is necessary to optimize the processing parameters in the existing fabrication techniques. In addition, further studies are required to compare the available processes to produce metal foams from raw material (powders) or Al scrap. The development of alloy foams for the core with improved properties may contribute to their successful usage in various applications.
The academic and industrial research plays a crucial role in eradicating problems that otherwise limit the wider applications of metal foams. Improvement is required to ensure the reproducibility and uniformity of the foam’s cell structures for attaining even densities and distribution of pore sizes in the entire part or component, resulting in uniform foam structures. Additionally, new processing routes, cheaper raw materials, and less wastage may reduce the processing and material costs of metal foams. Case studies based on simulation, innovative design, and testing could show end-users that despite the higher costs of some foams or parts, there are more benefits associated with these new structures and materials, such as savings in weight and energy.
8. Summary
This review presents an overview of the metal foams developed by various techniques to enhance their properties and performance, summarising the efforts to improve the foaming behaviour and characteristics. Depending on their applications, foam structures could be tailored to generate open- or closed-cell foams. Microstructural studies evaluated the effect of reinforcements and fabrication techniques on the foams’ microstructure, and it defines their mechanical behaviour. The foams’ properties, particularly the compressive strengths and energy absorption, could be improved by adding different reinforcements, such as metal or ceramic particles, to stabilise the foam structure. The porosities in these foams are dependent on foaming agents and space-holder particles. The processing techniques that fabricate metal foams play a vital role in deciding their properties and foaming behaviour. Their applications as structural components in automotive parts, such as ships and aerospace transportation, and as sound dampers, filters, electrodes, etc. can be effectively attained. However, improvement would be possible by identifying the downsides and feasible solutions for better performances under various working conditions and production techniques. Although metal foams find applications in many sectors due to their lightweight and high strength, the high cost of fabrication techniques hinders large-scale production. The challenge of future process development should therefore focus on reducing the production cost as well.
Acknowledgments
This research is supported by the Structures and Materials (S&M) Research Lab of Prince Sultan University, and the authors acknowledge the Prince Sultan university for paying the article processing charges (APC).
Funding Statement
This research was supported by the Ministry of Higher Education (MOHE) of Malaysia and International Islamic University Malaysia (IIUM) (FRGS/1/2019/TK08/UIAM/02/5).
Author Contributions
Writing—original draft preparation, B.P.; conceptualization, N.A.J.; validation, H.A.; resources, Y.A.; review and editing, A.A.; evaluation, M.B. All authors have read and agreed to the published version of the manuscript.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Conflicts of Interest
The authors declare no conflict of interest.
Footnotes
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Ceramic foams made from a wide range of ceramic materials, both oxide and nonoxide, are being considered for a whole range of potential applications. These include hot gas filters, interpenetrating composites and biomedical applications as well as thermal insulation, kiln furniture and catalyst supports amongst others. Three of these are described briefly below as well as a new processing route based on gel casting.
The process for fabricating the ceramic foams, which has now been taken to full commercial status by Hi-Por Ceramics, a new company created specifically for the purpose, is given by the flowchart in figure 1. A stable, well-dispersed, high solids content, aqueous ceramic suspension is prepared which also incorporates an acrylate monomer together with an initiator and catalyst. The latter is used to provide in-situ polymerisation. After the further addition of a foaming agent, a high shear mixer is used to provide simple mechanical agitation that results in the formation of a wet ceramic foam that can be dried and then fired.
Figure 1. Process flow chart for production of ceramic foams.
One of the advantages of the in-situ polymerisation method is that it is common to observe a period of inactivity between the addition of reagents and the actual beginning of the polymerisation reaction. This is known as the induction period or idle time (ti). The induction period is beneficial since it allows the casting of the fluid foam into a mould prior to polymerisation and control over the pore size.
A wide range of ceramic materials have been produced as foams using the new process. Although the majority of work has focused on the engineering ceramic oxides, materials such as alumina, cordierite, mullite and zirconia, a large number of other ceramics can be foamed. These include the bioceramic hydroxyapatite, the electroceramic lead zirconate titanate (PZT), low thermal expansion sodium zirconium phosphate (NZP) and, more recently, non-oxides such as silicon carbide and aluminium nitride. In general, foams in the range 5% to 40% of theoretical density can be produced; a typical micrograph showing the structure can be seen in figure 2. Note how the pore walls and struts are solid and fully dense; this provides high strength and chemical resistance.
Figure 2. Typical micrograph of a 30% dense alumina foam.
Although the lower the density of the foam the larger the cell or pore size, recent research has allowed a far greater degree of control to be achieved. Foams can now be produced with cells as large as 1 mm and densities as high as 30% of theoretical, whilst 20% dense foams have been produced with cells as small as 20-50 μm.
Foams made of the engineering ceramics such as alumina offer comparatively high strengths, up to 80 MPa crush strength and 25 MPa modulus of rupture. Thermal insulation is almost as good as fibre-based products whilst also offering a totally fibre- and dust-free working environment. With zirconia the service temperature can be as high as 2000°C. A wide range of component shapes is also available. The production route itself is intrinsically a casting process and hence tiles, tubes and a range of other custom shapes can all be produced very easily. In addition, both the green and fired foam may be readily machined, drilled, turned and slit opening up the possibility of producing some very complex shapes indeed. In addition, it is a simple process to apply a dense coating to one or more surfaces, either to eliminate permeability or to increase the mechanical properties of the surface layers. One application that could utilise this is the production of ultra-low mass ceramic crucibles. These can significantly reduce the thermal mass to be heated during processing of their contents, thus improving energy efficiency.
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