Breaking the Surface, RMIT’s Floating Titanium Lattice Lets Water Through and Still Refuses to Sink

Photo credit: Sara Tan | RMIT
Researchers at RMIT University’s Centre for Additive Manufacturing have printed a titanium lattice that floats while remaining open so water can pass straight through. Lead researcher Dr Jordan Noronha and project leader Distinguished Professor Ma Qian worked with Professors Andrey Molotnikov, Milan Brandt and Martin Leary, along with partners at France’s Conservatoire National des Arts et Métiers. Their hybrid starts with Ti-6Al-4V, an alloy that normally sits around 4.43 grams per cubic centimeter, more than four times the density of freshwater.

Laser powder bed fusion resulted in simple cubic lattices of 4 by 4 by 4 stacks of 10 mm cells, all neatly stacked. Instead of thick solid beams, we used hollow struts with walls that were only 0.2 mm thick and interiors that measured 2.5 to 4 mm across. We next filled those hollows with expandable polyurethane foam, allowing it to swell, lock onto the rough inner walls, and form a combination of tiny closed cells ranging in size from 10 to 200 microns. This resulted in a small 5.9 to 7.3 percent increase in bulk density while still trapping a lot of gas inside the metal.

Metallic lattices have a bit of a problem in that they can achieve bulk densities lower than water while still sinking because liquid fills all of the gaps. Researchers at RMIT developed a new concept called skeleton density, which counts only the titanium walls and sealed-foam filled insides while ignoring all of the empty space on the outside. If the skeleton density in freshwater remains below around 0.997 grams per cubic centimeter, the entire thing will float, even if water only seeps in through the exterior gaps.

They kept them in freshwater aquariums for over two months to test how long they could stay afloat. After only two weeks, seawater from Melbourne’s Port Phillip Bay lost 0.15 percent of its mass and less than 1 percent of its strength. Compression tests revealed that lattices with foam within had almost the same yield strength, ultimate compressive strength, and stiffness as hollow strut equivalents without foam. Interestingly, in terms of specific strength, they outperformed both high-density polyethylene and 316L stainless steel. At the same overall density, our hybrid design was around 70% stronger than steel or HDPE, placing it close second.

Even after tiny cracks, fissures, and the removal of an entire layer of lattice, these hybrids remained buoyant. They only sank toward the end, at the last densification step, when the pressure was high enough to compress the volume and raise the skeleton density above water. They tested an 85 x 100 mm printed buoy in a turbulent seawater tank, allowing it to swing up to 45 degrees without the need for a sealed outer shell.

Jobs such as maritime buoys, floating sensors, piers, and offshore platforms are likely to profit from hardware that can withstand wave impacts and resist corrosion better than typical plastics or steel of comparable weight. Distinguished Professor Ma Qian mentioned that the same titanium structure may be used for other fillers to absorb energy, manage heat, or minimize vibration. The Australian Research Council and the RMIT School of Engineering provided funding for this work. They’re now looking to create larger sections and test them in real-world sea conditions for a few years.
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