A 3D printed shape memory alloy with superior superelasticity

May 13, 2022

(News from Nanowerk) Laser powder bed fusion, a 3D printing technique, offers potential in the manufacturing industry, especially when fabricating nickel-titanium shape memory alloys with complex geometries. Although this fabrication technique is attractive for biomedical and aerospace applications, it has rarely exhibited the superelasticity required for specific applications using nickel-titanium shape memory alloys.

The defects generated and the changes imposed on the material during the 3D printing process prevented superelasticity from appearing in the 3D printed nickel-titanium.

Researchers at Texas A&M University recently demonstrated superior tensile superelasticity by fabricating a shape memory alloy by laser powder bed fusion, nearly doubling the maximum superelasticity reported in the literature for 3D printing.

This study has just been published in the Acta Materialia journal (“Laser Powder Bed Fusion of Flawless NiTi Shape Memory Alloy Parts with Superior Tensile Superelasticity”).

An electron micrograph of nickel-titanium powder is shown on the left. Researchers can use this powder to make 3D-printed parts, like nickel-titanium lattices (right). (Image: Texas A&M Engineering)

Nickel-titanium shape memory alloys have various applications due to their ability to return to their original shape upon heating or upon removal of applied stress. Therefore, they can be used in biomedical and aerospace fields for stents, implants, surgical devices and aircraft wings. However, the development and proper fabrication of these materials requires extensive research to characterize functional properties and examine microstructure.

“Shape-memory alloys are smart materials that can remember their shapes at high temperatures,” said Dr. Lei Xue, a former doctoral student in the Department of Materials Science and Engineering and first author of the publication. “While they can be used in many ways, fabricating shape memory alloys into complex shapes requires fine tuning to ensure the material exhibits the desired properties.”

Laser powder bed fusion is an additive manufacturing technique that presents a way to produce nickel-titanium shape memory alloys efficiently and effectively, providing a route to rapid manufacturing or prototyping. This technique, similar to polymer 3D printing, uses a laser to fuse metal or alloy powders layer by layer. The layer-by-layer process is beneficial because it creates parts with complex geometries that would be impossible in traditional manufacturing.

“Using a 3D printer, we layer the alloy powder onto a substrate, then use the laser to melt the powder, forming a complete layer,” Xue said. “We repeat this layering, scanning the same or different patterns until the desired structure is formed.”

Unfortunately, most nickel-titanium materials cannot withstand today’s laser powder bed fusion process, often resulting in print defects such as porosity, warping, or delamination caused by a large thermal gradient and brittleness due to oxidation. In addition, the laser can change the composition of the material due to evaporation during printing.

To combat this problem, the researchers used an optimization framework they created in a previous study, which can determine the optimal process parameters to achieve a defect-free structure and specific material properties.

With this framework, along with the composition change and refined process parameters, the researchers fabricated nickel-titanium parts that consistently exhibited a room temperature tensile superelasticity of 6% in the as-printed state (without post-fabrication heat treatment). This level of superelasticity is almost double the amount previously seen in the literature for 3D printing.

The ability to produce shape memory alloys through 3D printing with increased superelasticity means materials are better able to handle applied deformation. Using 3D printing to develop these premium materials will reduce the cost and time of the manufacturing process.

In the future, the researchers hope their findings will lead to increased use of printed nickel-titanium shape memory alloys in biomedical and aerospace applications.

“This study can serve as a guide on how to print nickel-titanium shape memory alloys with desired mechanical and functional characteristics,” Xue said. “If we can customize the crystallographic texture and microstructure, there are many more applications where these shape memory alloys can be used.”

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