Engineers in Australia and France have created a lightweight, strong titanium metamaterial that can float in water, overcoming a long-standing limitation of open-cell metallic structures. The hybrid material combines an architected titanium alloy lattice with a lightweight polyurethane foam, allowing it to remain buoyant even when its open pores fill with water.
Open-cell metallic structures are widely used in aerospace and biomedical applications because they are extremely light and strong. However, in marine environments, water quickly fills their interconnected open pores, preventing them from floating even though their bulk density is much lower than that of water. The new material solves this problem by incorporating a foam that blocks water from entering the internal channels of the lattice.
The research, detailed in Advanced Materials, was co-led by Ma Qian, a materials engineer at RMIT University in Melbourne, along with colleagues at RMIT's Centre for Additive Manufacturing and the Conservatoire National des Arts et Métiers in France. The team developed a new measure called «skeletal density» that considers only the water-excluding parts of a structure. If this value is lower than that of the surrounding liquid, the structure will float even when water flows through its open pores.
The researchers used a hollow-strut lattice (HSL) metamaterial made from a titanium alloy, Ti-6Al-4V. Unlike conventional solid-strut designs, HSLs contain millimetre- or sub-millimetre-sized channels within their struts, making them stronger than solid-strut lattices of the same density. These channels can be filled with other materials without compromising the open-cell architecture.
By injecting expandable polyurethane foam with a density of 0.08–0.11 g/cm³ into the hollow titanium struts, the team created a hybrid lattice that floats while remaining permeable. «The closed-cell PU foam limits water ingress into the internal strut channels while the external open-cell architecture remains permeable, so safeguarding marine buoyancy without notable density gain,» Qian said.
In tests, unfilled HSLs sank immediately when immersed in freshwater, regardless of strut diameter. The foam-filled hybrids, however, stayed afloat for more than two months in freshwater and two weeks in natural seawater. They lost only about 0.15% of their mass, indicating resistance to corrosion. The foam-filled HSLs also showed higher specific strength than common marine materials such as high-density polyethylene and 316L stainless steel.
«The innovation opens a new design space for marine structures, vehicles and rescue equipment where we need strength, low weight and buoyancy all at the same time,» Qian said. He also sees potential in biomedical applications, such as combining a biodegradable magnesium scaffold with a hydrogel containing growth factors for controlled therapeutic delivery during bone regeneration. In aerospace, a hollow-strut titanium lattice could be integrated with carbon-fibre-reinforced polymer to create lightweight, stiff hybrid structures.
The researchers now plan to further develop the technology and are seeking commercial partners. «We want to assess its viability for larger marine applications, including autonomous underwater vehicles and floating platforms and investigate long-term performance under realistic marine conditions,» Qian said. They also intend to introduce the concepts into advanced manufacturing and design courses at RMIT to help engineering students and researchers develop new ideas in this emerging field.
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