New 3D printing process creates parts from “liquid rust”

A team of Northwestern Engineers has created a new way to print three-dimensional metallic objects using a wide array of metal powders and even oxides like rust. Unlike typical metal printing systems that sinter metal powder with lasers or electron beams, the new technique extrudes liquid “ink” to create 3D printed parts. The unlayered structures are then sintered into solid metal in a furnace.

“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, who led the study. “Our method greatly expands the architectures and metals we’re able to print, which really opens the door for a lot of different applications.”

The secret to Northwestern Engineering’s method lies in its liquid build material, which combines metal or mixed metal powders and solvents with elastomer binder. Dispensed through an extrusion nozzle, the metal powder liquid instantly solidifies at room temperature. While the resulting pre-sintered part – called a “green body” — can be handled after printing, it still remains flexible, researchers say.

“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.”

To solidify the part, green bodies are heated in a furnace where the entire part becomes dense uniformally as opposed to a typical laser sinter metal printer that can introduce weaker spots in the layered parts. According to the researchers, their process can also accommodate multiple extrusion nozzles working in concert to create parts limited only by the size of the furnace.

Another upside of the process is that it can print metal oxides, such as iron oxide (rust), which can then be reduced into metal using hydrogen to turn the green bodies into the respective metal before sintering in the furnace.

“It might seem like we are needlessly complicating things by adding a third reduction step where we turn rust into iron,” said David Dunand, the James N. and Margie M. Krebs Professor of Materials Science and Engineering, who collaborated with Shah. “But this opens up possibilities for using very cheap oxide powders rather than corresponding expensive metal powders. It’s hard to find something cheaper than rust.”

The team says the new method could be used for printing batteries, solid-oxide fuel cells, medical implants and mechanical parts for larger structures, such as rockets and airplanes. Their research is described in a paper published recently in the journal Advanced Functional Materials.
www.mccormick.northwestern.edu