3D-Printed Titanium Lattice Floats After RMIT Fills Hollow Struts With Foam

RMIT University engineers have produced the first floating 3D-printed titanium lattice. The trick: fill the hollow struts with polyurethane foam. The result is a metal-polymer hybrid that stays buoyant even when cracked, while outperforming the stainless steel and high-density plastic currently used in marine infrastructure.

Why titanium sinks

Metallic lattices can be incredibly light, with densities below one-tenth that of water. Their open, interconnected structure, however, lets water flow straight through. That is why strong, lightweight metal lattices have never worked for buoys, jetties, or floating sensors. The water fills the voids and the structure sinks.

Dr. Jordan Noronha, who leads the RMIT Centre for Additive Manufacturing, said the foam-filled design changes the math. "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."

A new way to measure buoyancy

The team also introduced skeletal density, a metric that ignores open spaces water can enter. Conventional density counts every void in a lattice, even though water occupying that space does not add weight. Skeletal density counts only the material that actually displaces water: the titanium walls and the sealed foam-filled channels.

The rule is simple. If skeletal density is lower than the surrounding liquid, the structure floats. That holds even when water rushes through external openings.

Real-world testing

Samples floated in freshwater for more than two months. After two weeks in seawater from Melbourne's Port Phillip Bay, the lattice lost only 0.15% of its mass and its strength dropped less than 1%. The structure was 70% stronger than stainless steel or high-density polyethylene at the same overall density.

The team also built a 3D-printed marine buoy. It remained stable in a turbulent seawater tank tilted up to 45 degrees, with no sealed casing, protective coating, or extra flotation. Even after cracking and breaking a connection point, the buoy stayed afloat. It sank only after being severely crushed.

The foam acts as a distributed barrier, trapping gas in tiny sealed cells and preventing water from flooding the hollow struts. That is a different failure mode from conventional hollow marine structures, which can rapidly fill with water after a crack opens.

What comes next

Distinguished Professor Ma Qian, who led the project, wants to scale the demonstration parts and test them under realistic deep-sea conditions. The material is also tailorable. Swap the foam for something with different properties and the same lattice could handle energy absorption, thermal management, or vibration control.

The research, published in Advanced Materials, was a collaboration between RMIT and the Conservatoire National des Arts et Metiers in France. For now, the most tangible result is a titanium lattice that floats, survives saltwater, and is strong enough to replace heavier materials in places where corrosion and weight both matter.