While current methods rely on large metal powder beds and expensive lasers or electron beams, the Northwestern team's technique is claimed to use liquid inks and common furnaces, said to result in a cheaper, faster, and more uniform process. The team also demonstrated that its method works for a variety of metals, metal mixtures, alloys, and metal oxides and compounds.
"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 at Northwestern's McCormick School of Engineering. "Our method expands the architectures and metals we're able to print, which really opens the door for a lot of different applications."
By creating a liquid ink made of metal or mixed metal powders, solvents, and an elastomer binder, Prof Shah was able to print densely packed powder structures using a syringe-extrusion process at room temperature.
Despite starting with a liquid ink, the extruded material instantaneously solidifies and fuses with previously extruded material, enabling large objects to be created and immediately handled. Then, with collaborator David Dunand, Professor of Materials Science and Engineering, the team fused the powders by heating the structures in a simple furnace.
"By uncoupling the printing and the sintering, it appears that we have complicated the process," Prof Dunand said. "But, in fact, it has liberated us as each step is much easier separately than the combined approach."
The team predicts that many disciplines could benefit from customised, 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.
Instead of one laser working its way across a large powder bed, Shah and Dunand's method can use many extrusion nozzles at one time. They say their method has the potential to quickly 3D print full sheets that are metres wide and can be folded into large structures. The only limitation is the size of the furnace.
Another innovative component of their process is that it can be used to print metal oxides, such as iron oxide, or 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 those structures 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," Prof Dunand explained. "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."