A team of engineers from Northwestern University in the US has created a new way to print three-dimensional metallic objects using rust and metal powders.

While current methods rely on metal powder beds and lasers or electron beams, Northwestern’s new technique uses liquid inks and common furnaces. The Northwestern team said that the new method works for a wider variety of metals, metal mixtures, alloys, and 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. ‘Our method greatly expands the architectures and metals we’re able to print, which really opens the door for a lot of different applications.’

Conventional methods for 3D printing metallic structures can be time and cost intensive. The process takes an energy source, such as a focused laser or electron beam, that moves across a bed of metal powder, defining an object’s architecture in a single layer by fusing powder particles together. New powder is placed on top on the previous layer, and these steps are repeated to create a 3D object.

Liquid powder ink

Northwestern Engineering’s method bypassed the powder bed and energy beam approach by creating a liquid ink made of metal or mixed metal powders, solvents, and an elastomer binder. The method makes it possible to print densely packed powder structures using a syringe-extrusion process, in which ink dispenses through a nozzle, at room temperature.

Despite starting with a liquid ink, the extruded material solidifies and fuses with previously extruded material, enabling very large objects to be created and handled. The powders are then sintered in a furnace.

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. It could also be used for on site manufacturing that bypasses the sometimes slow-moving supply chain, the team suggests.

The process could also make it easier to develop 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 more flexible, due to the elastic polymer binder containing unbonded metallic powders.

‘We used a biomedical polymer that is commonly used in clinical products, such as sutures,’ Shah said. ‘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.’

A copper lattice structure 3D printed using the new method.
A copper lattice structure 3D printed using the new method.

Uniform structures

Heating the completed green bodies in a furnace where all parts of the structure densify simultaneously can also lead to more uniform structures. In traditional methods that scan powder beds with a laser, the heat is localized. As powder is added on layer by layer, more heat is applied, which can create localized heating and cooling stresses. Using a furnace, however, helps ensure a more uniform temperature.

Instead of one laser, the method can use many extrusion nozzles at one time, making it possible to potentially 3D print full sheets that are meters wide and can be folded into large structures.

The process can also be used to print metal oxides, such as iron oxide (rust), which can then be reduced into metal. 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 research is described in a paper published last month in the journal Advanced Functional Materials.

This story uses material from Northwestern University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier.