Porous Metals and Metal Foams - In Search of the 'Holey' ...

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Nature’s porous materials, such as bone, coral, and cork, are synonymous with strong and lightweight structures. Driven by the prospect of producing a family of materials with unique combinations of properties, materials scientists and engineers have followed nature’s lead and spent the last 20 years developing porous metals and metal foams from laboratory curiosities to commercial components.

Metal structures containing large fractions (typically 75-95%) of porosity are normally divided into two distinct categories. The first category consists of structures with closed, gas-filled pores, commonly referred to as foams. The production of these foams requires a foaming process, similar to introducing bubbles into beer. They exhibit good strength and are mainly used for structural applications, like weight-saving and impact-absorbing components in vehicles. The second category includes structures with open, connected cells, akin to a sponge, typically called porous metals. These are primarily used in applications that leverage the continuous nature of porosity, such as vibration and sound absorption, filtration and catalysis at high temperatures, heat exchange, and in medical devices.

There are numerous methods to produce porous metals and metal foams. Porous metals can be created by replicating open-cell polymer foam templates, utilizing techniques such as PVD or electroplating, compacting and sintering metal powders with a sacrificial space filler (also known as a porogen), which is subsequently removed via dissolution or thermal decomposition. Alternatively, they can be built layer by layer using processes such as selective laser sintering.

While mastering these processes is challenging, the structure of the final product—specifically pore size and volume fraction—is predictable. This is defined by the geometry of the polymer template, the size and amount of porogen added, or by a CAD model. Figure 1 illustrates examples of porous metal structures made using replication processes with NaCl beads as porogens in aluminum, along with a polymer foam template coated in stainless steel. The capacity to produce uniform structures consistently results in products with reproducible and predictable physical and mechanical properties, ensuring reliable performance during use. Most porous metals are applied in thermal, acoustic, catalytic, filtration, and biomedical fields, where good mechanical performance at light weight is usually secondary. Research primarily focuses on enhancing the geometry of the foam and the coating methods to improve electrical, thermal, biological, or catalytic responses.

Figure 1. Examples of porous (open cell) metal structures made by replication processes using (left) NaCl beads as porogens in Al and (right) a polymer foam template coated in stainless steel

Manufacturing metal foams differs significantly. By definition, a foaming process must occur. The preferred methods include bubbling gas through molten metal or utilizing compounds (like hydrides or carbonates) that release gas bubbles upon heating in a liquid metal or semi-solid pellet. These stochastic processes produce bubbles (or pores) that may vary in size and shape throughout the foaming duration. Resulting foams, similar to the head on freshly poured beer or a soufflé, are unstable; pores may rupture and coalesce as the liquid within the cell walls drains due to gravity. Consequently, this leads to irregular pore structures and foams prone to collapse. Figure 2 shows cross-sections of two aluminum foams—one with coarse, irregular closed pores and another with fine, spherical pores—creating through the decomposition of titanium hydride powder, mixed by stirring into molten aluminum.

Figure 2. Cross sections of two aluminum foams highlighting contrasting closed pore structures (left) with coarse and irregular pores and (right) with fine and spherical pores

For established foaming processes, research in both academia and industry is heavily focused on overcoming issues that would otherwise hinder broader use of these materials. The primary goal is to enhance the uniformity and reproducibility of foam structures, striving to achieve uniform pore sizes and densities throughout components, akin to what is typically achieved with porous metals. Creating stable foam structures, characterized by minimal changes in foam density and structure over time, is essential for this goal and requires a thorough understanding of the mechanisms behind foam formation and the collapse processes.

Our comprehension of the formation, life, and decay of aqueous foams is robust, supported by their transparent nature which allows for easy study. The effects of altering the physical properties of the fluid, such as viscosity and surface tension, employing surfactants, or introducing fine, solid particles to enhance stability, are well documented and have been adapted to metal foam systems, albeit with varying degrees of success.

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The aspiration to study metal foams similarly to their aqueous counterparts has led to the adoption of X-ray radioscopy and 3D tomography. These methods, in addition to being powerful tools for characterizing foam structures and identifying defects and solid stabilizing phases non-destructively, can also monitor structural evolution in real-time, often at high resolutions and even under microgravity conditions using synchrotron facilities. These techniques have provided valuable insights into the decomposition behavior of foaming agents, gas bubble nucleation, foam structure evolution, pore rupture events, and the influence of gravity in coarsening processes. Figure 3 features selected images from a series of 2D X-ray radioscopic studies conducted during the foaming of two different aluminum alloy pellets containing gas-forming titanium hydride particles, highlighting the benefits of alloying to enhance foam stability and reduce collapse in the lower series of images.

Figure 3. 2D X-ray radioscopic images during the foaming of aluminum, demonstrating the advantages of alloying for improved foam stability in the lower series of images.

Research into porous metals and metal foams is multifaceted and challenging, requiring the development of innovative measurement, characterization, modeling, and testing methods. This situation is likely to persist for many years as there is still much to learn about processing-structure-property relationships for existing systems. The challenge is to ensure that production routes can create parts with the precision and size required for specific applications consistently and economically.

Despite numerous examples of commercially successful porous metal and metal foam components currently in use, for these materials to become as established as their polymeric counterparts, their visibility within the broader engineering community must increase. This requires leveraging the multifunctionality of these materials—not only in designing lightweight structural components but also by capitalizing on the additional benefits that porous metal structures offer, such as impact absorption, vibration, sound dampening, and electromagnetic shielding. This approach could allow a single porous metal or metal foam structure to replace multiple materials within an assembly. The advantages of this strategy must be demonstrated through compelling case studies grounded in innovative design, simulation, and testing, showcasing to end users that despite higher costs for porous metals and metal foams compared to monolithic alternatives (due to added "holes"), the benefits in performance, weight reduction, and energy efficiency can outweigh these costs.

The author acknowledges contributions from both current and past members of the Metal Foam Group at Nottingham. Special thanks are given to the foam group at TU Berlin for their assistance with the radioscopy of samples used in Figures 2 and 3.

Further Reading

• Ashby MF, Evans A, Fleck NA, Gibson LJ, Hutchinson JW, Wadley HNG. , Metal Foams: A Design Guide. Butterworth Heinemann, UK.
• Banhart, J. Manufacture, Characterisation and Application of Cellular Metals and Metal Foams. Progress in Materials Science, 46(6), 559-632.
• Kennedy A.R. The Manufacture of Porous and Cellular Metals by Powder Metallurgy, in Powder Metallurgy, Ed. Dr. Katsuyoshi Kondoh, Intech Publications. ISBN 978-953-308-46-2, (open access).
• Francisco Garcia-Moreno, Manas Mukherjee, Catalina Jiménez, Alexander Rack, John Banhart. Metal Foaming Investigated by X-ray Radioscopy, Metals, 2, 10-21 (open access).

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