3D-Printing of Large Metallic Glasses

The cylinder shown here is an amorphous iron alloy, or metallic glass, made using an additive manufacturing technique. (Source: Z. Mahbooba)

Researchers have now demon­strated the ability to create amorphous metal, or metallic glass, alloys on large scales using three-dimen­sional printing tech­nology, opening the door to a variety of applications – such as more efficient electric motors, better wear-resistant materials, higher strength materials, and lighter weight structures. “Metallic glasses lack the crystal­line structures of most metals – the amorphous structure results in exceptio­nally desirable pro­perties,” says Zaynab Mahbooba, a Ph.D. student in North Carolina State Uni­versity’s Depart­ment of Materials Science and Engi­neering.

Unfor­tunately, making metallic glass requires rapid cooling to prevent the crystal­line structure from forming. Historically, that meant researchers could only cast metallic glasses into small thick­nesses. For example, amorphous iron alloys could be cast no more than a few milli­meters thick. That size limi­tation is called an alloy’s critical casting thickness. “The idea of using additive manu­facturing, or 3D-printing, to produce metallic glass on scales larger than the critical casting thickness has been around for more than a decade,” Mah­booba says. “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.”

The technique works by applying a laser to a layer of metal powder, melting the powder into a solid layer that is only 20 microns thick. The “build platform” then descends 20 microns, more powder is spread onto the surface, and the process repeats itself. Because the alloy is formed a little at a time, it cools quickly – re­taining its amorphous qualities. However, the end result is a solid, metallic glass object – not an object made of laminated, discrete layers of the alloy. “This is a proof-of-concept demon­strating that we can do this,” says Ola Harrysson, Professor of Industrial Systems and Engi­neering at NC State.

“And there is no reason this technique could not be used to produce any amorphous alloy,” Harrysson says. “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.”

“It will take some trial and error to find the alloy compo­sitions that have the best combi­nation of proper­ties for any given application,” Mah­booba says. “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.” “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,” Harrysson says. (Source: NCSU)

Reference: Z. Mahbooba et al.: Additive manufacturing of an iron-based bulk metallic glass larger than the critical casting thickness, App. Mat. Today 11, 264 (2018); DOI: 10.1016/j.apmt.2018.02.011

Link: Center for Additive Manufacturing and Logistics, North Carolina State University, Raleigh, USA

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