Researchers at the U.S. National Institute of Standards and Technology (NIST) have discovered a previously unknown crystal structure while studying a 3D printed aluminum alloy, a finding that could lead to stronger and more reliable additively manufactured components.
The new phase, a quasicrystal with fivefold symmetry, was identified in an aluminum-silicon-zirconium alloy produced using laser powder bed fusion (LPBF). Quasicrystals are known to exhibit unusual properties such as high strength and low thermal conductivity, making them valuable for engineering applications.
What are quasicrystals?
Quasicrystals are solids with an ordered atomic structure that does not repeat periodically. Unlike conventional crystals, which form repeating patterns in space, quasicrystals follow mathematical rules without repeating, much like a Penrose tiling. Out of the 230 possible crystal space groups, quasicrystals fit into none.
These structures are extremely rare in nature, found in places like meteorites. In 1982, Israeli scientist Dan Shechtman discovered artificially formed quasicrystals in a lab-made alloy, an achievement that won him the Nobel Prize in Chemistry in 2011.
Decades later, NIST researcher Andrew Iams identified a new quasicrystalline phase during a 3D printing process involving an aluminum alloy. The discovery was not intentional, it emerged from routine microscopic analysis of samples created via LPBF.
The discovery
Iams observed that the crystal exhibited fivefold rotational symmetry, meaning the atomic arrangement looks the same when rotated 72 degrees around an axis. This type of symmetry is considered “forbidden” in traditional crystallography, where only symmetries like twofold, threefold, fourfold, or sixfold are permitted.
By carefully rotating the sample under a transmission electron microscope, Iams found that the structure also displayed threefold and twofold symmetries when viewed from different directions. These multiple axes of symmetry, combined with the non-repeating atomic order, confirmed that the structure was a quasicrystal.
Challenges in metal 3D Printing
Although metal 3D printing allows for designs and material behaviors not possible with traditional manufacturing, it faces significant limitations, particularly with aluminum alloys.
According to NIST physicist Fan Zhang, “High-strength aluminum alloys are almost impossible to print, as they tend to develop cracks, which make them unusable.”
One of the key issues is temperature control. Aluminum melts at around 660°C, while the lasers used in LPBF can exceed 2,400°C. These extreme conditions can cause undesirable microstructures or thermal stresses, leading to part failure.
In 2017, researchers at HRL Laboratories and UC Santa Barbara found that adding zirconium to aluminum powders helped stabilize the solidification process and reduce cracking, resulting in a printable aluminum alloy. The NIST alloy used in this recent study builds on that innovation, demonstrating how alloy design continues to evolve with additive manufacturing.
Advancing aluminum alloys for additive manufacturing
The discovery of a quasicrystal in a 3D printed aluminum alloy aligns with ongoing efforts to develop aluminum alloys optimized for additive manufacturing. Traditional high-strength aluminum alloys, such as Al7075 and Al6061, are prone to cracking during LPBF processes. To address this, researchers at HRL Laboratories introduced zirconium-based nanoparticles into aluminum alloys, enabling the production of crack-free, high-strength aluminum components via LPBF.
Building on this, Constellium and Morf3D developed AHEADD CP1, an aluminum-iron-zirconium alloy tailored for LPBF. This alloy offers high strength and ductility, efficient processing, and simplified post-processing, making it suitable for applications like heat exchangers.
These advancements underscore the importance of alloy development in overcoming the challenges of metal additive manufacturing, paving the way for more reliable and high-performance 3D printed components.
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Featured image shows an electron microscope image of the aluminum alloy from the study. Image via NIST.
