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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.
Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of AZoM.com Limited (T/A) AZoNetwork, the owner and operator of this website. This disclaimer forms part of the Terms and Conditions of use of this website.
Metal additive manufacturing (AM) is a manufacturing technique for making metal parts in a layer-by-layer fashion. Different forms of AM exist to print metal materials, the most prominent of which is powder bed fusion (PBF), which includes processes like direct metal laser sintering (DMLS) and selective laser melting (SLM).
An overview of the main metal AM technologies and their strengths, from a BCG study.One of the principal advantages of metal AM is design freedom. Because parts are built up layer by layer (and not formed in a mold), they can have virtually any shape, with features like fine details, cavities, interlocking sections, and thick or thin walls.
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This means that components can potentially undergo part consolidation ' several connecting components are replaced by a single, functionally equivalent part ' leading to part count reduction and reduced assembly time.
General Motors and Autodesk collaborated to redesign a seat bracket, resulting in a 40% lighter and 20% stronger part. Source: WiredAnother advantage is that metal AM has fewer manufacturing steps than MIM. Molding entails the making of the mold and the forming of the moldings within it. Metal AM does not require tooling (fabricating parts directly instead), making it significantly faster for one-off parts or small batches. Both processes also require post-processing steps like sintering.
The absence of tooling also makes it easy to modify the design of metal AM parts. Digital designs can be adjusted right up until the point of production, whereas tooling cannot easily be adjusted once it is made. Similarly, multiple iterations of a part can be printed at no added expense.
Metal injection molding (MIM) is a form of injection molding that uses metal as a feedstock (though MIM pellets are actually a combination of metal powder and thermoplastic binder). Pellets are heated and forced into a metal mold, and the solid parts are removed once cool. They are then sintered to remove the binder and increase density.
The video below offers a comprehensive overview of how MIM works:
The principal advantages of MIM are high throughput and economy of scale. Once a mold has been produced, it can be used to make thousands of duplicate parts, with the cost-per-part reducing as the quantity increases. With metal AM, there is no such economy of scale: printing thousands of identical parts is inconvenient and does not result in a lower cost-per-part.
Mass production with MIM is not just affordable; it is also fast. Although mold production and material preparation can take several weeks, each injection or 'shot' takes a matter of seconds. The speed is somewhat counteracted by sintering times, but several parts can be sintered in the same furnace.
Additionally, MIM produces a slightly superior surface finish compared to AM, with less noticeable roughness. That being said, neither process can match the finish of a CNC machined part, and machining is often required as a post-processing step.
There are certain situations where either AM or MIM could be used to make a metal part. However, each process requires its own unique design considerations; it can be difficult to turn a part designed according to the principles of DfAM (design for additive manufacturing) into a moldable part, and it can also be difficult ' albeit less so ' to turn a MIM design into a 3D printable part.
On the whole, metal AM has fewer design constraints than MIM, and PBF specifically has fewer constraints than many other forms of AM, since unsintered powder helps support parts during printing. Wall thicknesses should generally exceed 0.5 mm, unsupported overhangs should be more than 45° from the horizontal to prevent collapse, and, if using a PBF process, escape holes should be implemented so that unsintered powder can be removed.
The model geometry of an overhang being L-PBF processed. Source: Research GateDesign considerations for MIM are different, because the need to eject parts from a mold limits possible shapes. Internal features and channels, for example, must be ruled out, while overhanging sections ' while possible ' necessitate expensive multi-component molds with sliding sections.
Other rules to follow during MIM design include maintaining even wall thickness and eliminating sharp corners with fillets and radii. (Conversely, squared-off corners may print more easily than rounded ones when using metal AM.) Engineers must also consider factors like gate placement, parting lines, and feature orientation.
In many cases, the choice between AM and MIM comes down to production volumes. Put simply, AM is cost-effective ' and therefore the better choice ' for prototyping and low-volume production, while MIM is more cost-effective in higher volumes. Identifying the point at which MIM becomes better value is, of course, easier said than done; it's generally comprised between 20,000 and 50,000 units.
If quantity is not a deciding factor, there are other reasons why one might choose AM over MIM.
For instance, complex designs tend to favor metal AM, since the geometries of MIM parts are restricted by part ejection. And design freedom can in turn lead to desirable material properties such as a good strength-to-weight ratio: parts can be printed with topologically optimized interiors (lattice structures, for example) that maximize strength and eliminate surplus material.
In practical terms, the ability to 3D print complex designs makes metal AM suitable for applications in medicine (e.g. patient-specific titanium implants with porous internal geometries), aerospace (strong and lightweight jet engine components, which can even be certified), and energy (turbine blades).
Metal AM also trumps MIM in terms of part refinement. Digital designs can be tinkered with, at no cost, immediately prior to printing, and multiple iterations of a part can be printed simultaneously or in sequence. This is useful for prototyping during R&D, and also acts as a financial safety net during production, since costly tooling will not need modifying or replacing.
Besides the question of AM vs MIM, companies may wonder whether to go with in-house manufacturing (which requires upfront investment) or use an external supplier (where the cost-per-part is higher).
Metal AM and MIM systems are both expensive ($100,000+), so the decision again comes down to production volumes. The infrequent production of small batches will likely not justify investment in in-house equipment, regardless of the technology. That being said, companies can be more flexible when working with external AM part suppliers, since they rarely specify a minimum order quantity (MOQ).
Metal AM and MIM need not be considered rival processes. In fact, metal AM can be used to complement MIM, either as a prototyping tool for parts that will ultimately be molded, or when leveraged as a mold-making technique.
A Stainless Steel 3D printed mold. Source: SculpteoTraditional molds are typically made from tool steel feedstock, which is not especially amenable to metal AM. However, titanium 3D printed molds ' much cheaper than traditional molds ' can also perform to a high standard.
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