Australian researchers create floating titanium metamaterial

Australian researchers have built a 3D-printed titanium structure that floats in water and keeps floating even after significant damage, according to a study led by RMIT University.

The material is a lattice made of hollow, interconnected titanium struts filled with polyurethane foam. Researchers said it withstands seawater exposure and is stronger than the stainless steel or high-density plastic currently used in jetties, buoys and floating sensors.

“Although metallic lattices can be incredibly light, with densities less than one-tenth the density of water, their open, interconnected spaces allow water to enter, causing them to sink,” said Dr. Jordan Noronha, lead researcher at RMIT’s Centre for Additive Manufacturing. “This has made these strong, lightweight structures unsuitable for marine infrastructure, until now.”

“By filling only the hollow titanium struts with polyurethane foam, we created a structure that allows water to flow through it while remaining buoyant even after significant cracking and damage,” Noronha said.

The researchers said the work is the first reported demonstration of a floating metal-hybrid lattice metamaterial. Samples floated in freshwater for more than two months during testing, according to the study.

The project team: Associate Professor Andrey Molotnikov, Distinguished Professor Ma Qian, Distinguished Professor Milan Brandt, Dr Jordan Noronha and Professor Martin Leary at RMIT’s Centre for Additive Manufacturing. Credit: Sara Tan, RMIT.

To design the structure, the team introduced a measurement they call skeletal density. Conventional density calculations count all the open space inside a lattice, including space that water can fill and that therefore does not help the structure float. Skeletal density instead counts only the titanium walls and the sealed, foam-filled channels, the parts of the structure water cannot enter.

“This gives engineers a simple design rule: if the skeletal density is lower than that of the surrounding liquid, the structure will float, even when water flows through all its external openings,” Noronha said.

In compression testing, the titanium structure was 70% stronger than stainless steel or high-density polyethylene at the same overall density, the researchers said. In a two-week corrosion test using seawater from Melbourne’s Port Phillip Bay, the lattice lost 0.15% of its mass and less than 1% of its strength.

The structure also kept floating after damage that included cracking, failure at connection points and the fracture of an entire lattice layer, according to the study. It sank only after being severely crushed and compacted.

“Tiny, sealed cells in the foam trap gas and prevent water from flooding the hollow struts,” Noronha said. “In this way the foam acts as a distributed barrier that helps the structure remain afloat after damage, unlike conventional hollow marine structures, which can rapidly fill with water after cracking.”

The 3D-printed buoy used in testing. Credit: Sara Tan, RMIT.

The team built a 3D-printed marine buoy to test the concept and placed it in a turbulent seawater tank, rotating it up to 45 degrees. The researchers said the buoy remained stable without a sealed casing, protective coating or added flotation.

Project leader Distinguished Professor Ma Qian said the next steps include scaling up the demonstration parts and testing long-term performance under realistic marine and deep-sea conditions. Qian said the structure’s properties can be tailored by changing the material inside the titanium framework, opening potential uses beyond marine infrastructure in areas such as energy absorption, thermal management and vibration control.

RMIT’s Centre for Additive Manufacturing led the project with the Conservatoire National des Arts et Métiers in France. The Australian Research Council and RMIT’s School of Engineering supported the research.

The study, “Breaking the surface: buoyant metal-polymer open-cell hybrid lattice metamaterials,” was published in the journal Advanced Materials.