By  Materials Science and Engineering

Lin Gao, a Ph.D. student of materials science and engineering, earned first place in The Minerals, Metals & Materials Society best paper contest for graduate students.Gao conducts research to build high-performance metallic materials using additive manufacturing as a member of the research group led by Tao Sun, associate professor of materials science and engineering.
Gao earned the best graduate paper award for "Exceptional Ductility Induced by Intrinsic Grain Boundary Engineering." Gao will present the paper at the TMS 2023 annual meeting and exhibition, which will convene in San Diego next March, and will receive his honor during the TMS-AIME awards ceremony.
“Additive manufacturing, or 3D printing, is a very popular domain,” Gao said. “We can custom-print a complex object with the desired material composition and geometry in a short period of time.” 3D printing is widely used to manufacture devices and parts needed in the energy, aerospace and medical fields.
Gao specializes in an additive manufacturing technique for metals called directed energy deposition (DED). His research focuses on understanding and controlling the melt pool dynamics, solidification behavior and microstructure evolution during printing.
In structural applications, printed parts need to withstand load-bearing and repetitive stress, a measure of material strength. These objects also need to continuously adapt their dimensions to the work environment in which they are placed. The specific ability of a material to be drawn or deformed without fracture is called ductility. Ductility extends the lifetime of the printed object by delaying if not preventing fractures and cracks.
“Typically, if you want to increase the ductility of a material, you have to sacrifice some strength,” Gao said. “Our laser 3D-printing strategy increases both ductility and strength of the 316L stainless steel synergistically.”
Gao's innovation is how the steel wire feedstock is deposited. Gao researched the fundamental principles of the laser wire DED process, such as laser absorption, melt flow and solidification. His new deposition strategy enables the engineering of grain-scale microstructure by creating special melt pool behavior and solidification conditions. It endows the as-deposited samples with exceptional ductility.
Gao's strategy is also very economic. In the standard practice, it is often necessary to add a nucleating agent during printing or perform post-heat treatment over the printed piece to improve its mechanical performance, which considerably adds to the production cost. Gao's process yields strength and ductility without additional overhead.
Gao's research leverages Sun's discoveries of the physics that govern complex energy-matter interactions in laser additive manufacturing. Sun's collaborative research, published in Science magazine, opened multidisciplinary research areas that attract students like Gao, the first graduate student to join Sun's research group at UVA.
Gao, who earned his master's degree in materials science from Central South University in China, was excited to learn from Sun's experience in additive manufacturing.
“There are so many unknowns and so much to explore,” Gao said. “I am optimistic about the opportunities for various manufacturing industries to adopt our deposition approach.”
Gao benefited from Sun's relationship with Argonne National Laboratory, where he performed operando synchrotron X-ray imaging and diffraction experiments to illustrate and explain how his wire-based printing method altered melt pool behavior and solidification conditions, along with the multi-physics simulation and in situ thermal imaging experiments conducted at UVA.