Nature’s porous materials, 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 separated into two distinct categories. Those with closed, gas-filled pores (usually referred to as foams, since a foaming process, like putting bubbles into beer, is required) have good strength and are mainly used for structural applications (for example weight-saving and impact-absorbing structures in vehicles). Those with open, connected cells (like a sponge, usually called porous metals) are mainly used in applications where the continuous nature of the porosity is exploited (for example, vibration and sound absorption, filtration and catalysis at high temperatures, for heat exchange and in medical devices).
There are many different ways to produce porous metals and metal foams. Porous metals can be made by the replication of open cell polymer foam templates (for example by PVD or electroplating), by compacting and sintering metal powders into which a sacrificial space filler (also called a porogen) has been incorporated, and which is then removed by dissolution or thermal decomposition, or built layer upon layer using processes such as selective laser sintering.
Whilst mastering these processes is far from trivial, the structure of the end product (pore size and volume fraction) is predictable, defined by the geometry of the polymer template, size and addition level of porogen, or by a CAD model. Figure 1 shows examples of porous metal structures made by replication processes using NaCl beads as porogens in Al and a polymer foam template coated in stainless steel. The ability to produce or copy uniform structures, part after part, leads to products with reproducible and predictable physical and mechanical properties and, therefore, reliable in-service performance. With most porous metals being used for thermal, acoustic, catalysis, filtration and biomedical applications, good mechanical performance at light weight is usually of secondary importance. Research mainly focusses on optimising the geometry of the foam and coating methods to enhance the 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
The manufacture of metal foams is quite different. By definition, a foaming process needs to take place. The preferred routes involve either bubbling gas through a molten metal or using compounds (hydrides or carbonates) that decompose to form gas bubbles when heated in a liquid metal or semi-solid pellet. These processes are stochastic producing bubbles (or pores) that can change size and shape throughout the foaming period. The resulting foam, just like the head on a freshly poured beer or a soufflé straight from the oven, is unstable and the pores rupture and coalesce as the liquid in the cell walls drains out under the influence of gravity. These processes lead to irregular pore structures and, inevitably, to collapse of the foam. Cross sections of two aluminium foams, one with coarse, irregular closed pores and the other with fine, spherical pores are shown in figure 2. These examples are for foams made by the decomposition of titanium hydride powder which was incorporated, by stirring, into molten aluminium.
Figure 2. Cross sections of two aluminium foams with contrasting closed pore structures (left) showing coarse and irregular pores and (right) showing fine and spherical pores
For established foaming processes, research conducted within academia and industry has a strong emphasis on eliminating problems which would otherwise limit the wider use of these materials. The main focus is improving the uniformity and reproducibility of the foam structures, aiming to achieve, what is readily attained in porous metals, uniform pore sizes and densities throughout the component and similar foam structures from part to part. Producing foams that have stable structures (their rate of change in foam density and structure with time is small) is vital to achieving this and requires an understanding of the mechanisms of foam formation and the processes that drive collapse.
Our understanding of the formation, life and decay of aqueous foams is very good, helped by the transparent nature of these materials and the resulting ease with which they can be studied. The effect of varying the physical properties of the fluid (viscosity and surface tension for example), the use of surfactants, or the introduction of fine, solid particles to improve stability is well established and this practise has been translated to metal foam systems, with varying degrees of success.
The desire to be able to study metal foams in the same way as their aqueous counterparts has led to the use of X-ray radioscopy and 3D tomography which, in addition to being powerful tools to characterise foam structures and the size and location of defects and solid stabilising phases in a non-destructive fashion, can be employed to monitor, in real time, the structural evolution of these materials often (with the use of synchrotron facilities) at high resolution and even under microgravity. These techniques have given insights into; the decomposition behaviour of the foaming agent, the nucleation of gas bubbles, the evolution of the foam structure, the location and frequency of pore rupture events and the role of gravity in coarsening processes. Figure 3 shows selected images taken from a series of 2D X-ray radioscopic images collected during the foaming of two different aluminium alloy pellets containing titanium hydride gas-forming particles. In this example it shows the benefit (in the lower series of images) of alloying to improve the foam stability and reduce collapse.
Figure 3. 2D X-ray radioscopic images for foaming aluminium, showing the benefit (in the lower series of images) of alloying to improve foam stability.
Porous metal and metal foam research is challenging and multi-disciplinary, necessitating the development of new and innovative measurement, characterisation, modelling and testing methods. This will remain the situation for many years as there is still much to learn about the processing-structure-property relationships for existing systems, let alone those in which interest is growing (for example, magnesium alloys, high temperature intermetallics, shape memory alloys, bulk metallic glasses and superplastic alloys). As research progresses to development, the challenge is to ensure that production routes are able to produce parts with the precision and size required to match the application, part after part, in the quantities required and as economically as possible.
Although there are numerous examples of commercially-successful porous metal and metal foam components in service, for these materials to become as firmly established as their polymeric counterparts, their profile needs to be increased in the wider engineering community. Central to this is exploiting the multi-functionality of these materials, not just designing to save weight in structural parts, replacing like-for-like, but taking advantage of the other benefits that porous metal structures offer, such as impact, vibration and sound absorption and electro-magnetic shielding, where it is possible to use a single porous metal or metal foam structure to replace several materials in an assembly. The benefits of this approach need to be affirmed through compelling case studies based on innovative design, simulation and testing to demonstrate to end users that despite the higher prices for porous metals and metal foams compared to the monolithic metal (it does cost money to add holes!), that these costs can be more than offset by the increases in performance, reductions in weight and savings in energy offered by these novel materials.
The author would like to acknowledge the contributions from current and past members of the Metal Foam Group at Nottingham. He would also like to thank the foam group at the TU Berlin for the radioscopy of samples used in figures 2 and 3.
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