A team of engineers at Northwestern University (Evanston, IL) has created a new way to 3D print metal components using rust and metal powders with a polymer binder, according to McCormick News, a publication of Northwestern's McCormick School of Engineering. The new method bypasses the powder bed and energy beam approach and decouples the two-step process of printing the structure and fusing its layers.
Northwestern's new technique uses liquid inks and common furnaces, resulting in a cheaper, faster and more uniform process, said the information. The Northwestern team also demonstrated that the new method works for an extensive variety of metals, metal mixtures, alloys, metal oxides and compounds.
"This is exciting because most advanced manufacturing methods being used for metallic printing are limited as far as which metals and alloys can be printed and what types of architecture can be created," said Ramille Shah, Assistant Professor of materials science and engineering in the McCormick School of Engineering and of surgery in the Feinberg School of Medicine. "Our method greatly expands the architectures and metals we're able to print, which really opens the door for a lot of different applications," said Shah, who led the study.
Despite starting with a liquid ink, the extruded material instantaneously solidifies and fuses with previously extruded material, enabling very large objects to be quickly created and immediately handled. Then, with collaborator David Dunand, James N. and Margie M. Krebs Professor of materials science and engineering, the team fused the powders by heating the structures in a simple furnace, rather than using a very intense energy source such as a focused laser or electron beam, in a process called sintering whereby powders merge together without melting.
The researchers' unique 3D inks and process open doors for more sophisticated and uniform architectures that are faster to create and easier to scale up. After the object is printed but before it is densified by heating, the structure, called a "green body," is flexible because of the elastic polymer binder containing unbound metallic powders.
"We used a biomedical polymer that is commonly used in clinical products, such as sutures," Shah explained. "When we use it as a binder, it makes green bodies that are very robust, despite the fact that they still comprise a majority of powder with very little binder. They're foldable, bendable and can be hundreds of layers thick without crumbling. Other binders don't give those properties to resulting 3D-printed objects. Ours can be manipulated before being fired. It allows us to create a lot of different architectures that haven't really been seen in metal 3D printing."
Heating the completed green bodies in a furnace, where all parts of the structure densify, simultaneously also leads to more-uniform structures. In traditional methods that scan powder beds with a laser, the heat is localized. As powder is added layer by layer, more heat is applied, which can create localized heating and cooling stresses, leading to undesirable microstructures and, ultimately, suboptimal properties. Using a furnace, however, ensures uniform temperature, resulting in structures that sinter uniformly without warping or cracking, said the report.
"To me, as a metallurgist, I'm amazed that the structure does not deform or break apart, despite shrinking extensively during densification," Dunand said. "That is not something that I see often."
Another innovative component of the process is that it can be used to print metal oxides, such as iron oxide (rust), which can then be reduced into metal. Rust powder is lighter, more stable, cheaper and safer to handle than pure iron powders. Shah and Dunand's team discovered that they could first 3D print structures with rust and other metallic oxides and then use hydrogen to turn the green bodies into the respective metal before sintering in the furnace.
The team believes that many disciplines could benefit from customized, quickly printed metals, with the possibility of using the new method for printing batteries, solid-oxide fuel cells, medical implants and mechanical parts for larger structures such as rockets and airplanes. It could also be used for on-site manufacturing that bypasses the sometimes slow-moving supply chain.
The full paper was published in the December issue of Advanced Functional Materials.