Watching a 3D printer at work can be mesmerizing. Guided by a digital data file, the machinery moves rhythmically back and forth, steadily adding layer upon layer of material until a custom three-dimensional shape begins to take form. 

Additive manufacturing technology has been on the rise, and it’s easy to understand why. The process produces geometrically and compositionally complex parts using less energy and material waste than traditional manufacturing processes. Prototyping costs much less, and small production runs are faster and less expensive. The technology is able to provide on-demand fabrication and offer repair capabilities whether the user is on land, at sea, or even in space. 

As with any new technology, there is still much to learn about its potential uses and opportunities for innovation. This material, whether polymer or metal-based, undergoes quite a lot of stress during the manufacturing process. As each new layer is applied, the material rapidly solidifies and endures complex thermal cycles throughout the entire process. These factors can lead to unique microstructural features and defects in the final part.

Virginia Tech’s Yao "Yolanda" Fu wants to know more about these unique microstructural features and their effects on fracture and cracking in a corrosive environment over the lifespan of the material. 

Fu, an assistant professor in the Kevin T. Crofton Department of Aerospace and Ocean Engineering, is the recipient of two prestigious research grants to study the environmental-related behaviors of additively manufactured metals. This year, she received a National Science Foundation Faculty Early Career Development Program (CAREER) award, as well as a Young Investigator Program (YIP) award from the Office of Naval Research.

“Corrosion Fatigue Behavior of Additively Processed Metallic Alloys”

With her $594,948 CAREER grant from the National Science Foundation, Fu looks to answer two relevant unknowns about additive manufactured metals: how the fatigue behavior of these alloys differs from that of their conventional counterparts under both normal and corrosive environments and how the additive manufactured alloy’s unique microstructures contribute to those differences in behaviors.

Using both experimentation and computational methods, Fu and her research team will scrutinize the behavior of the additive manufactured metals under various conditions in the new Materials and Manufacturing Design Lab and the Aerospace Structures and Materials Laboratory in Randolph Hall. 

To better understand the material’s strengths and weaknesses as compared to traditional metallic alloys, the team will investigate how the material reacts under tensile/compressive testing and high-cycle fatigue testing. Following the initial cycle of experimentation, the team will subsequently perform similar tests in corrosive environments, and take a closer look at environmental factors, such as varying temperatures, humidity, or salinity levels. 

Insights gained from Fu’s research will guide the design and manufacturing of additively manufactured parts, helping to prolong their service life by limiting the causes of fatigue failure and reducing the financial losses related to corrosion damage. 

Microstructural features of laser powder bed fused 316L stainless steel samples: (a) Sample design for the crack propagation test and its relation with the build, (b) crystallographic planes and (c) orientation of the subgrain dislocation network on three coordinate planes. Courtesy of Yao Fu.
Microstructural features of laser powder bed fused 316L stainless steel samples: (a) Sample design for the crack propagation test and its relation with the build, (b) crystallographic planes and (c) orientation of the subgrain dislocation network on three coordinate planes. Model courtesy of Yao Fu.

'Microstructural Effects on the Environmental-Assisted Cracking of Advanced Manufactured Stainless Steels in Marine Environment'

Fu has also been awarded a grant of $509,878 from the Office of Naval Research’s YIP. While this project also focuses on additively manufactured alloys, Fu will specifically examine stainless steels often used in marine environments. She will also dive deeper into the performance of hybrid structures that consist of partially printed and partially conventionally processed parts. 

Additive manufactured 316L stainless steel is of particular interest for naval engineering and marine applications. Significant saltwater exposure can be unforgiving to metals because corrosion is inevitable. Fu and her team will take a closer look at hybrid conventional and printed stainless steel structures that contain a number of varied printed layers and evaluate how their environmentally assisted cracking behavior differs from that of their conventional or bulk counterparts. In addition, Fu will investigate how the material’s solidification texture and grain directionality have a direct effect on corrosion-related properties over a wide range of temperatures. 

Fu will investigate corrosive behaviors in sodium chloride solutions with concentrations close to that of seawater. She plans to complete an electrochemical analysis of the corrosion characteristics, stress corrosion cracking, high cycle fatigue, and crack propagation testing in the corrosive environment. Fu will also characterize the microstructural features before and after cracking, as well as conduct multiphysics modeling of the electrochemical mechanical processes for understanding the link between the microstructure and electrochemical/mechanical properties.

By understanding the underlying mechanisms that lead to the corrosion, cracking, and failure of additive manufactured alloys, in both normal and in humid or saltwater environments, researchers will be better equipped in controlling the most critical features and defects during the manufacturing process. As an added benefit, the multiphysics computational framework established by Fu and her team will aid in reducing the cost of corrosion-informed materials design and manufacturing, and make a significant contribution to the Office of Naval Research’s corrosion control and related technology.

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