This image shows cells adhering to a titanium alloy created by cold-spray 3D printing, demonstrating the material’s biocompatibility. Image: Cornell University.
This image shows cells adhering to a titanium alloy created by cold-spray 3D printing, demonstrating the material’s biocompatibility. Image: Cornell University.

Forget glue, screws, heat or other traditional bonding methods. A team led by researchers at Cornell University has now developed a 3D printing technique that creates cellular metallic materials by smashing together powder particles at supersonic speed.

This form of technology, known as 'cold spray', results in mechanically robust, porous structures that are 40% stronger than similar materials made with conventional manufacturing processes. The structures' small size and porosity make them particularly well-suited for building biomedical components like replacement joints.

The team reports the technique in a paper in Applied Materials Today. Atieh Moridi, assistant professor in the Sibley School of Mechanical and Aerospace Engineering at Cornell University, is the paper's lead author.

"We focused on making cellular structures, which have lots of applications in thermal management, energy absorption and biomedicine," Moridi said. "Instead of using only heat as the input or the driving force for bonding, we are now using plastic deformation to bond these powder particles together."

Moridi's research group specializes in creating high-performance metallic materials through additive manufacturing processes. Rather than carving a geometric shape out of a big block of material, additive manufacturing builds a structure up layer-by-layer, a bottom-up approach that gives manufacturers greater flexibility in what they create.

However, additive manufacturing is not without its own challenges. Foremost among them is that metallic materials need to be heated to high temperatures that exceed their melting point, which can cause residual stress build-up, distortion and unwanted phase transformations.

To eliminate these issues, Moridi and collaborators developed a novel 3D printing method that uses a nozzle of compressed gas to fire titanium alloy particles at a substrate. "It's like painting, but things build up a lot more in 3D," Moridi said.

The particles are between 45µm and 106µm in diameter and travel at a speed of roughly 600 meters per second, faster than the speed of sound. To put that into perspective, another mainstream additive process, direct energy deposition, delivers powders through a nozzle at a velocity on the order of just 10 meters per second, making Moridi's method sixty times faster.

The particles aren't just hurled as quickly as possible. The researchers had to carefully calibrate titanium alloy's ideal speed. In cold spray printing, a particle would typically be accelerated in the sweet spot between its critical velocity – the speed at which it can form a dense solid – and its erosion velocity – the speed above which it crumbles too much to bond to anything.

Instead, Moridi's team used computational fluid dynamics to determine a speed just below the titanium alloy particle's critical velocity. When launched at this slightly slower rate, the particles created a more porous structure, which is ideal for biomedical applications such as artificial joints for the knee or hip and cranial/facial implants.

"If we make implants with these kinds of porous structures, and we insert them in the body, the bone can grow inside these pores and make a biological fixation," Moridi said. "This helps reduce the likelihood of the implant loosening. And this is a big deal. There are lots of revision surgeries that patients have to go through to remove the implant just because it's loose and it causes a lot of pain."

While the process is technically termed cold spray, it did involve some heat treatment. Once the particles collided and bonded together, the researchers heated the metal so the components would diffuse into each other and settle like a homogeneous material.

"We only focused on titanium alloys and biomedical applications, but the applicability of this process could be beyond that," Moridi said. "Essentially, any metallic material that can endure plastic deformation could benefit from this process. And it opens up a lot of opportunities for larger-scale industrial applications, like construction, transportation and energy."

This story is adapted from material from Cornell University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.