Printing metals with crystal formations

Materials World magazine
,
27 Feb 2019

Simulating crystal structures could improve strength of 3D-printed lattices. James Fernandes reports.  

The robust crystal structures of metals can be mimicked in 3D-printed lattices for added strength, says a team of researchers. Through imitating the hardening mechanism of crystals, scientists at Imperial College London and the University of Sheffield, UK, have found they can significantly boost the strength and reduce the weight of lattice materials. This advance could accelerate the use of 3D-printed metal parts in engineering projects.

The fabrication method produces lattices in grid-like formations. Researchers believe that varying the patterns in the lattice micro-scale structures and building the materials in layers could substantially add to its durability. This would help reduce the frailties found in current 2D lattices, which tend to follow the same structural pattern and are prone to fracturing. Due to the atomic alignment of the 3D-printed lattices resembling that of crystalline materials, any crack should either stop or ease when coming into contact with a ‘crystal’ that follows a different structural pattern.

According to Imperial, ‘When loaded with weight, the new material — dubbed “meta-crystal” — is far stronger and more damage-tolerant than conventional lattice materials.’ They also found that the strength of the meta-crystals can be increased by reducing the size of each grain-like lattice region within the structure, therefore, these engineering materials have the potential to benefit any number of industries, including construction, automotive, aviation and medical devices.

‘We bring what we learn in metal science to guide our design in architectured materials,’ explains Dr Minh-Son Pham, from Imperial’s Department of Materials. ‘Everything is inspired by nature with what we found in crystal. In that understanding of the science of nature, we guide our design and solve the problem while maintaining the low weight and high specific strength.’

Reinforced strength, reduced weight

The technique, developed for 3D printing, allows lattice materials to be designed with specific structural properties on an atomic level to meet the exact demands of their intended application. 3D-printed lattices are designed to reinforce material strength during fabrication, removing any potentially defective areas. This technique can ensure that any structural weaknesses are lessened so that weight is distributed more evenly, prolonging the material’s life. The long-term plan is to be able to achieve this on an industrial scale.

So far, the majority of data from 3D printed lattices has been gathered using polymers, as they are cheaper to produce and experiment with. Yet it is applying this approach to metals that offers the greatest industrial potential. Pham explains how the team is investigating different combinations of materials to assess their structural performance. Provided funding is secured to continue the research, Pham aims to apply the 3D-printed lattice method to metal alloys. This would
not only make the materials tougher, but also increase their resilience
to high temperatures.

‘It’s very exciting if you can combine this approach with shape memory alloys, because you can not only make them robust, but also make them smart,’ he says. ‘It can open up a wide space of opportunities to design smarter and higher performing metals. The benefit of metals is not only about strength, but also resilience to damage and they can also bear the load of very high temperatures.’

Applications aplenty

3D printing is currently being used by a number of industries, but the technology is still relatively new and unrefined in certain applications. Nevertheless, the growth of additive manufacturing is evident and, according to Pham, ‘its potential is almost limitless and will only
become more widespread’. While it is unlikely that an entire aeroplane will be made from 3D-printed architectured lattices any time soon, these materials could be used to create individual segments and components onboard, to reduce the overall weight.

Beyond the aero sector, these high-strength lattices could have applications in protective equipment, being used to produce outerwear for the military, or even sports gear, such as American football helmets. 3D lattices also present a wide range of possibilities for medical devices and orthopaedics such as hip replacements, given their lightweight structure and durability.

‘A lot of people want to use 3D printing to fabricate an artificial hip,’ Pham says. ‘Because with this, we can increase the integration between the hip and the tissue in the human body, and also reduce the weight as well, to make it more compliant. One problem with orthopaedic application is stress shearing. That means the artificial hip becomes stiffer than nature would want. That carries the load and the bone doesn’t carry the load anymore. So, the bone will degrade over time.

‘If you can make the artificial hip have the same stiffness as natural bone and become lighter, that means we have much more benefit, increasing the integration between the artificial hip and the bones enables the re-healing of the bones and make the patient feel more comfortable. There’s quite a range of opportunities and applications
for this approach.’

Read the paper, Damage-tolerant architected materials inspired by crystal microstructure, published in Nature, here: go.nature.com/2DC5PNX