A team of Northwestern University engineers have created a way to print three-dimensional metallic objects using metal powders and rust.

While current methods rely on metal powder beds and lasers or electron beams, the new technique uses liquid inks and common furnaces, resulting in what the developers say is a less expensive, faster and more uniform process. The Northwestern team also demonstrated that their method works for a 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,” says Ramille Shah, assistant professor of materials science and engineering, who led the research.

A copper lattice structure created via the new 3D printing process. Image credit: Northwestern University.A copper lattice structure created via the new 3D printing process. Image credit: Northwestern University.Conventional methods for 3D printing metallic structures can be both time and cost consuming. The process takes an intense 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 of the previous layer and these steps are repeated to create a 3D object.

Any unfused powder is removed. This prevents certain architectures, such as those that are hollow and enclosed, from being created. This method also may be limited by the types of compatible metals and alloys that can be used.

The new method bypasses the powder bed and energy beam approach and uncouples the two-step process of printing the structure and fusing its layers. By creating a liquid ink made of metal or mixed metal powders, solvents and an elastomer binder, Shah was able 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 large objects to be quickly created and immediately handled. Then, the team fused the powders by heating the structures in a simple furnace in a process called sintering, in which powders merge without melting.

The team believes that many disciplines could benefit from customized, quickly printed metals. The 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 supply chain.

The researchers’ 3D inks and process may open doors for more sophisticated and uniform architectures that are faster to create and easier to scale. After the object is printed but before it is densified by heating, the structure, called a “green body,” is 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,” says Shah. “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.”

Another 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, less expensive and safer to handle than pure iron powders. Shah's team found 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.

“It might seem like we are needlessly complicating things by adding a third reduction step where we turn rust into iron,” says research collaborator David Dunand, professor of materials science and engineering. “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.”

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