10th April 2017 by Marc Saunders
One of the great attributes of additive manufacturing (AM) is its ability to produce detailed, efficient and complex components. Lattices are perhaps the ultimate expression of this, comprising intricate structures that use the minimum of material to fill a volume whilst still providing structural rigidity. They can also be good at absorbing impact energy and may be designed to provide insulation from vibration and noise. Their low density makes them effective thermal insulators. Alternatively, their high surface area to volume ratio can be exploited to maximise heat transfer with fluids. Finally, lattices can also provide porous surfaces with tailored properties for effective joining.
So lattices can be extremely useful. This post explores some of the applications for lattices in AM parts and also looks at the challenges that they pose to design and to manufacture.
Lattices can be a major help with light-weighting, eliminating mass in areas of a component where it is not needed, whilst also providing support for the more solid areas of the part during an additive build. The small scale of lattice elements means that they can be self-supporting, minimising the need for sacrificial supports and therefore reducing waste.
Lightweight design for AM can create a virtuous circle in which weight reductions also reduce component costs, both in terms of the material used and as a result of shorter build times. The volume of material to be melted in the component (as well as in any support structures) is the major driver of the build time. So, generally speaking, if we can make our design lightweight and self-supporting, then we will minimise the time that we will need to build it.
Lattice structures can also have functional roles beyond merely filling a void. Their meso-scale properties can provide desirable macro-scale performance.
One promising application area is in energy absorption, in which lattice structures are used to cushion the loads imparted in rapid impacts, reducing the peak impact stress. The deformation characteristics of a lattice structure will depend on both its geometry (stretch- or bending-dominated) and the material from which it is made (especially its ductility). Recent research on this can be found here: Energy absorption in lattice structures in dynamics: Experiments, Ozdemir et al at the University of Sheffield. I will return to the different types of lattice geometry later.
The volume fraction of a lattice is the percentage of the total volume that is occupied by metal, the rest being gaps that are filled with air. This porosity can be useful for limiting conduction of heat through the material. Heat is conducted across the lattice primarily through the metal struts, only one third of which are aligned with each axis.
In small-scale lattices, convection flows do not get established and so the gas trapped in between the lattice struts remains still. Relatively little heat will be conducted through the gas as its thermal conductivity is much lower than that of the metal. The thermal conductivity of the lattice is therefore reduced significantly relative to the solid metal. At low volume fractions, this can be approximated to:
More details about the mechanical and thermal properties of various lattice structures can be found here: The properties of foams and lattices, M.F. Ashby
The role of an intercooler is to remove heat from an induction gas after it has been compressed to boost the efficiency of a combustion process. With their high surface area to volume ratio, lattices can be used to extract heat from a hot gas flow and transfer it into the surrounding atmosphere. The opposite effect is exploited in heat sinks, where heat is transferred from the hot metal lattice into a cool gas flow.
In the example of the helicopter exhaust nozzle that we looked at earlier (see image left), a lattice structure is used for heat transfer. The hot exhaust gas flows up the central passage whilst cooler gas from the rotor downdraft passes through the surrounding lattice in the opposite direction. The effect of this is to reduce the temperature of the helicopter exhaust.
In orthopaedic implants it is common to create porous surfaces designed to promote ‘osseointegration’ – where a patient’s bone is encouraged to fuse with the metal implant to secure it firmly in place. The implant’s surface is covered with a carefully designed layer of lattice, which has the optimum spacing and strut thickness to enable living bone to grow into it and form a strong, load-bearing bond with the metal device.
So lattices have a number of attractive qualities, although their complexity and delicacy can make them challenging both to design and to manufacture.
Most lattice materials are made up of arrays of slender members that resemble familiar lightweight super-structures such as bridges and building frames, but obviously on a much smaller scale. These complex meso-structures can be produced additively in length scales that vary from microns to millimetres.
Lattice structures can be regular (such as the cubic lattices shown above) or irregular, and designed to provide either homogeneous or heterogeneous properties. The length scale can vary throughout the lattice volume to tailor its properties – particularly its density and stiffness – in different locations.
The choice of lattice geometry is critical to effective light-weighting, enabling porous materials that approach the strength of their solid counterparts, yet at far lower densities. Some porous structures such as foams are light, but not all that strong. Their structures are said to be ‘bending-dominated’, so that loads applied to the macro-structure are resisted by bending of the struts in the meso-structure. This makes them compliant and good for energy absorption.
By contrast, lattices that are ‘stretch-dominated’ carry their loads axially along the struts, either in tension or compression. Such structures are characterised by high levels of node connectivity, providing cross-bracing to prevent relative motion of the nodes. These ‘space frame’ meso-structures, like their large-scale architectural equivalents, provide the best strength-to-weight ratio.
Not all lattices comprise simple strut-and-node arrangements. Gyroids are triply periodic structures, built up of small cells of curved ‘minimal’ surfaces that repeat in all directions to form a regular structure. In certain circumstances, these structures can have higher specific strengths than regular diamond lattices.
Gyroids are also being considered for heat exchange applications by HiETA Technologies and the University of Exeter. They have used computational fluid dynamics (CFD) to model the variation in pressure drop and heat transfer of the flow through a gyroid cell, as a function of the Reynolds number. Experimental testing was done on a lattice with a 4 mm cell size and 30% volume fraction at three different Reynolds numbers, showing good correlation with the CFD model. For more details, see http://www.openfoamworkshop.org/assets/uploaded/files/slides/OFWP055.pdf
Thanks to the flexibility of additive manufacturing, we can now design and build ‘architectured materials’ in which the meso-structure has been tailored to provide specific mechanical properties.
Hybrid lattice structures
As we have seen above, lattices can be integrated neatly into product designs. They can also be combined with other weight-saving techniques such as topological optimisation. The super-structure of the part can be shaped using generative design, with further weight-saving gains achieved by applying a lattice meso-structure onto some of the super-structure.
Lattice build preparation
A key computational challenge for the additive manufacturing of lattices is how these complex structures are represented and converted into a build file. Most additive components are designed in 3D CAD and then converted into a triangulated surface format (STL). These STL models are then sliced into thin layers from which we compute the laser paths needed to build up the part.
If we take this approach with lattices, especially those with a small length scale, then we quickly run into enormous models and interminable build file preparation. Specialists in lattice design and additive manufacture are looking at ways to simplify the generation and manufacture of such complex geometries through novel representations and custom laser exposure strategies.
Manufacturing of fine details requires precise control of the laser energy, as the melting on each layer of the lattice build often comprises thousands of sparsely distributed exposures. Renishaw AM systems feature a modulated laser focussed down to 70 microns spot size, enabling production of struts and walls as thin as 140 microns.
Intricate lattices can also be challenging to manufacture, especially in materials such as titanium that can exhibit significant residual stress. With so many small features that might distort, it is very easy to catch delicate components on the re-coating mechanism. A flexible wiper is helpful here – Renishaw AM machines uses a silicon rubber blade to spread powder across the build plate.
For all their attractions, there remain some barriers to deploying lattices in production parts. A key challenge is to substantiate the suitability of the design for stressed applications, particularly where fatigue is the critical performance attribute. By necessity, lattices contain lots of ‘as built’ surfaces and sharp intersections, which create stress raisers. To balance this out, lattices have a lot of built-in redundancy, and so may not fail catastrophically.
These characteristics make structural parts with load-bearing lattices best suited to applications where the loads are either constant or single, dynamic events. Cyclical load bearing applications look likely to be a little further off.
A related concern is how to validate lattice quality in manufacturing. The complexity and inaccessibility of lattice features makes them hard to inspect. CT scanning offers a solution, albeit a somewhat time-consuming one. The best answer here is likely to be in-process monitoring, so that we can demonstrate that each element of the lattice material has been built correctly.
Complex lattice structures can deliver exceptional product performance – both in efficiency and functional terms. They are key tools in component light-weighting, and can also boost heat transfer, energy absorption, insulation and joining performance. Careful lattice design can introduce precisely tailored properties into efficient components.
Additive manufacturing is often the only practical way to produce such intricate materials. If you plan to make lattice components, investigate the latest software tools and choose an AM machine that is optimised for fine detail work.