A 3D printing technique was developed 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.
Instead of using only heat as the input or the driving force for bonding, the researchers used plastic deformation to bond the powder particles together.
Additive manufacturing is not without its challenges — foremost among them is that metallic materials need to be heated at high temperatures that exceed their melting point, which can cause residual stress buildup, distortion, and unwanted phase transformations.
To eliminate these issues, the team developed a method using a nozzle of compressed gas to fire titanium alloy particles at a substrate. The particles were between 45 and 106 microns in diameter and traveled at roughly 600 meters per second — faster than the speed of sound. Typically, in cold spray printing, a particle would accelerate in the sweet spot between its critical velocity — the speed at which it can form a dense solid — and its erosion velocity: when it crumbles too much to bond to anything.
Instead, the team used computational fluid dynamics to determine a speed just under 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 bio-medical applications such as artificial joints for the knee or hip, and cranial/ facial implants.
For implants made with these porous structures, when inserted in the body, the bone can grow inside the pores and make a biological fixation. This helps reduce the likelihood of the implant loosening, eliminating the need for revision surgeries that patients have to go through to remove the implant because it is loose and causes 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. The team focused on titanium alloys and biomedical applications but the applicability of the process could extend to any metallic material that can endure plastic deformation.