RMIT Engineers Make 3D-Printed Titanium Actually Float

Titanium does not float. It is denser than water, it is denser than most metals, and for as long as anyone has been 3D printing with Ti-6Al-4V, the assumption has been that titanium structures will always sink. Researchers at RMIT University just proved that assumption wrong.

The team, based at RMIT's Centre for Additive Manufacturing, designed an open-cell titanium lattice with hollow struts. They then injected those hollow channels with polyurethane foam. The result is a hybrid material with a skeletal density below that of water, meaning it floats. Not just barely: specimens remained buoyant in freshwater for more than two months without any visible leakage or water ingress.

Jordan Noronha, Joey Tallon, Raad Omar, Jason Dash, Andrey Molotnikov, Martin Leary, Milan Brandt, and Ma Qian authored the study. It is published in Advanced Materials under the title "Breaking the Surface: Buoyant Metal-Polymer Open-Cell Hybrid Lattice Metamaterials."

Why This Works

The key number is skeletal density. The team found that lattices with a skeletal density below 0.997 g/cm3 would float. That threshold applies regardless of the surrounding fluid, which makes the approach generalisable: other alloys, other polymers, other liquids all follow the same geometry-based rule.

The titanium lattice was originally developed at RMIT for load-bearing performance, not buoyancy. The hollow-strut architecture solved a stress concentration problem in earlier multi-topology designs. When the buoyancy team realised those hollow channels could also serve as injection pathways for polyurethane foam, they found a structural system that already had the physical preconditions for flotation built in.

Strength and Seawater Resistance

At matched bulk densities of around 0.27 g/cm3, the titanium-PU hybrid delivered a yield strength of 10.3 MPa. That is roughly 1.9 times the density-scaled strength of high-density polyethylene and 1.5 times that of 316L stainless steel. The hybrid outperformed conventional marine materials without being heavy.

After two weeks immersed in natural seawater from Port Phillip Bay, the specimens lost only 0.15 percent of their mass. Yield and compressive strength each dropped by less than 1 percent.

Damage tolerance was also striking. The team subjected floating lattices to controlled compression and checked buoyancy at each stage. The structures continued to float after reaching ultimate compressive strength, after initial node fracture, and even after complete layer fracture. Buoyancy was only lost when densification pushed the skeletal density back above that of water, which happened at roughly 37 percent engineering strain.

A Working Prototype

To test the concept at functional scale, the team fabricated a buoy prototype: 100 mm tall, 85 mm wide, tested in seawater under turbulent flow. It floated stably despite continuous rotation of up to 45 degrees about its central axis, with no external sealing or auxiliary buoyancy aids. That is a working object, not a lab curiosity.

The practical implications are real. Buoyant titanium structures could reduce the weight of marine equipment, underwater sensor housings, and flotation devices where corrosion resistance matters. Current titanium marine components rely on separate buoyancy materials attached to the structure. A single-piece titanium-foam hybrid eliminates that assembly step.

The skeletal density framework the team developed also opens the door to other metal-polymer combinations. The same geometry rule applies to any alloy paired with any polymer in any liquid. Researchers now have a design rule for making materials float that were never meant to.