Titanium metamaterial unlocks strength beyond nature.

Pushing beyond what is found in nature, scientists from the Royal Melbourne Institute of Technology have engineered a lightweight and high-strength titanium material that holds promise for applications in medical, transportation, military, energy, and everything else that could benefit from an already widespread metal.

Using a common titanium alloy, Ti-6Al-4V (titanium-aluminum-vanadium), the research team constructed something called a "metamaterial." Used to describe artificial or manmade materials that possess unique properties not observed in nature – meta means "beyond" in Greek.

Contrary to its name, however, many such intricate and surprisingly strong structures actually do exist in nature. For example, the Victoria water lily, native to South America, is a gigantic floating plant strong enough to support the weight of an average adult due to its unique lattice structure of veins.

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A Victoria water lily boasts a structure capable of holding a person on its surface without sinking into the water.

Inspiration from nature is what typically gives humanity a goal line through imitation. Spider's silk, abalone shells, bamboo, and porcupine quills – there are quite a few things that evolved incredible functional properties that are still not perfectly replicable.

The structures of manmade materials are often designed to mimic in some form or fashion these plants and other natural porous materials like coral, with different lattices that range from simple cubes to complex dodecahedrons.

The pores in these lattice structures interconnect, forming channels that operate as a kind of scaffolding to evenly distribute various properties such as durability, strength, resiliency, flexibility, etc. Referred to as "cellular" materials, manmade variants often encounter strength tradeoffs if not designed optimally, according to the RMIT scientists.

"However, metal 3D printing is a game changer, allowing researchers to design and fabricate highly innovative light and strong cellular metals," said Jordan Noronha, a Ph.D. candidate who worked on the project at RMIT.

In cellular materials, lattices are connected in three dimensions by thin, solid rods or beams called struts. By using hollow struts instead, the researchers aimed to make a low-density cellular material just as strong as a solid metallic alloy with a similar density to high-strength magnesium alloys.

Printing unlocks the impossible

Led by Professor Ma Qian at RMIT's Center for Additive Manufacturing, the team used one of the most common 3D printing methods, laser powder bed fusion, to fabricate its titanium metamaterial.

Fraunhofer Institute for Laser Technology

Laser powder bed fusion technique in the process of melting a fine layer of metal powder on a flat surface.

By aiming a concentrated beam of heat onto a surface covered in metal powder, the prepared design is then slowly constructed layer by layer and is used to make complex parts of less than a millimeter up to nearly two meters in size.

Due to the intricacy and accuracy with which the printing can target, this has given near infinite creativity for engineers and researchers alike to discover the limits of metal 3D printing and reach as close as possible to designs found in nature, pushing even beyond that.

"First, the entire lattice metamaterial sample is designed as a digital model. Then, this model is digitally sliced into many thin layers using a software tool," explained Qian. "This layer-based fabrication process involves laser melting of metal powders, rapid solidification of the liquid metal (the melted metal powders) and repeated heating and cooling processes of the solidified metal."

Much like most printing techniques and technologies, the process still takes time, which Qian says currently takes around 18 hours.

Metamaterial specs

So, what makes its new metamaterial so strong?

As mentioned before, the team sought to use hollow struts, along with thin plates, that are ultimately responsible for the high strength of the new materials.

Unlike most cellular materials, which contain weak points where stress concentrates, these two complementary lattices evenly distribute stress while also providing support.

"Ideally, the stress in all cellular materials should be evenly spread," Qian explained. "However, for most topologies, it is common for less than half of the material to mainly bear the compressive load, while the larger volume of material is structurally insignificant."

"This multi-topology design also promotes the deflection of crack paths to enhance the toughness," he added. "Instead of the cracks occurring directly through the lattice, which occur in most cellular materials, in our thin-plate hollow-strut lattice topology, the struts and plates work together to divert the cracks along a longer path."

In current commercial applications, magnesium alloys are often used as they require higher strength and lighter weights. Compared to the strongest cast magnesium alloy available, WE54 (wrought magnesium with yttrium and other rare earth elements), a sample of RMIT's titanium metamaterial with comparable density was much stronger.

Given that magnesium alloys are also not amenable to laser powder bed fusion or other metal 3D printing techniques, owing to vaporization of the powder, the prospects of this latest titanium alloy already give it a clear manufacturing advantage.

Potential applications

Before the material can be commercialized, Qian and his team first want to ensure that the material can perform at its maximum efficiency.

To do this, the group plans to improve the current design for strength and lighten the titanium metamaterial even more. For example, based on present simulations, adjusting the proportion of thin plates to hollow struts will allow more uniform stress distribution throughout the entire structure.

Further, if the metamaterial is fabricated from a high-temperature titanium alloy, it can be used at temperatures up to 600 degrees Celsius (1,112 degrees Fahrenheit). This feature, along with its corrosion resistance, makes the material suitable for use in high-speed flying aircraft or missiles, which must be able to withstand the intense heat generated by their high speeds; titanium drones used to closely monitor or fight wildfires would also benefit from the light weight, strength, and heat-resistance of the metamaterial.

Because the metamaterial is also biocompatible, it could be used in medical devices such as bone implants. However, the technology is not yet ready at its present stage, said the researchers, so its adoption by industry will still take some time.

"Our most important limitation is the exclusivity of our technology, and the cost of fabrication could be another important concern," Qian stated.

"Traditional manufacturing processes are not practical for the fabrication of these intricate metal metamaterials, and not everyone has a laser powder bed fusion machine in their warehouse," he added. "However, as the technology develops, it will become more accessible, enabling a larger audience to implement our high-strength multi-topology metamaterials in their components."