Amorphous metal, or metallic glass, is a solid metal material with a disordered atomic-scale structure, not dissimilar to conventional glass. The material is not like the glass you see in windows, but because of its atomic structure, metallic glass can be up to three times as strong as conventional metal alloys, which are made up of very ordered, crystalline atom structures. Two years ago, two companies developed a way to 3D print amorphous metal components, and researchers from North Carolina State University (NC State) recently published research in this area, but with one major difference – they can 3D print metallic glass alloys in bulk.
Zaynab Mahbooba, a PhD student in NC State’s Department of Materials Science and Engineering, explained, “Metallic glasses lack the crystalline structures of most metals – the amorphous structure results in exceptionally desirable properties.
“The idea of using additive manufacturing, or 3-D printing, to produce metallic glass on scales larger than the critical casting thickness has been around for more than a decade. But this is the first published work demonstrating that we can actually do it. We were able to produce an amorphous iron alloy on a scale 15 times larger than its critical casting thickness.”
Until now, researchers were only able to cast metallic glasses into small thicknesses – amorphous iron alloys could not cast more than a few millimeters thick, which is called the critical casting thickness of an alloy. This is because rapid cooling is needed to keep the crystalline structure from forming when making metallic glass.
But the NC State team, working with collaborators from Liquidmetals and North Carolina startup Sindre Metals, have used a laser powder bed technique, which melts powder into a layer only 20 microns thick, to 3D print metallic glass in bulk. The material is able to retain its amorphous qualities, since the alloy is formed a little bit at a time, and results in a solid, metallic glass object.
Ola Harrysson, the Edward P. Fitts Distinguished Professor of Industrial Systems and Engineering at NC State, said, “This is a proof-of-concept demonstrating that we can do this.
“And there is no reason this technique could not be used to produce any amorphous alloy. One of the limiting factors at this point is going to be producing or obtaining metal powders of whatever alloy composition you are looking for.
“For example, we know that some metallic glasses have demonstrated enormous potential for use in electric motors, reducing waste heat and converting more power from electromagnetic fields into electricity.”
This work could introduce numerous applications, like more lightweight structures and stronger materials, materials with better wear resistance, and more efficient electric motors.
The team recently published a paper on their research, titled “Additive manufacturing of an iron-based bulk metallic glass larger than the critical casting thickness,” in the Applied Materials journal; co-authors include Mahbooba, Lena Thorsson, Mattias Unosson, and Peter Skoglund of Sindre Metals; Harvey West, Timothy Horn, and Christopher Rock from NC State; Evelina Vogli with Liquidmetal Coatings; and Harrysson.
“It will take some trial and error to find the alloy compositions that have the best combination of properties for any given application. For instance, you want to make sure you not only have the desirable electromagnetic properties, but that the alloy isn’t too brittle for practical use,” explained Mahbooba.
“And because we’re talking about additive manufacturing, we can produce these metallic glasses in a variety of complex geometries – which may also contribute to their usefulness in various applications.”
The abstract reads, “Fe-based bulk metallic glasses (BMG) are of increasing research interest, driven in part by a unique combination of mechanical, magnetic and chemical properties. However, the maximum thickness and geometry of BMGs achievable in traditional manufacturing processes is limited. This work examines the capabilities of laser based powder bed additive manufacturing (AM) to produce relatively large Fe-based bulk metallic glass specimens. AM fabricated specimens exceed the critical casting thickness of the material by a factor of 15 or more in all dimensions. Resulting microstructural and mechanical properties are reported. Despite decreasing quench effect with increasing build thickness, X-ray diffraction analysis suggests that a fully amorphous structure was maintained throughout the build. However, a low concentration of sparsely distributed nano-grain clusters was discovered using a high-resolution electron backscatter diffraction scan. The results pave the way for novel applications of metallic glasses achievable through appropriate material design and optimization of existing additive manufacturing processes.”
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