New method developed to print 3D metals and alloys
12 January 2016
A team of Northwestern University engineers 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, resulting in a cheaper, faster, and more uniform process. The Northwestern team also demonstrated that the new method works for an extensive variety of metals, metal mixtures, alloys, as well as 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 at Northwestern's McCormick School of Engineering, who co-led the study with Northwestern's Professor David Dunand. "Our method greatly expands the architectures and metals we're able to print, which really opens the door for a lot of different applications."
Northwestern Engineering's new method completely bypasses the powder bed and energy beam approach as well as uncoupling the two-stage 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 rapidly print densely packed powder structures using a simple syringe-extrusion process, in which ink dispenses through a nozzle, at room temperature.
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. The team then fused the powders by heating the structures in a simple furnace via a sintering process.
The researchers believe their novel process and 3D inks 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 ('green body') is flexible due to the elastic polymer binder containing non-bonded 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. 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.
Another innovative component of their 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 research is described in a paper published in the journal, Advanced Functional Materials.