Materials Science Addresses Need to Recycle and Re-Use Metal Powders

Sustainability principles guide modern manufacturing. Newcomer industries such as additive manufacturing eagerly incorporate the “3Rs”: to reduce, re-use and recycle materials. 

Additive manufacturing, a form of 3-D printing, creates metal objects by melting or sintering metal powder together in a layer-by-layer method. A specific type of this process called laser powder bed fusion uses a laser to melt powder in a 2-D geometry, much like a stencil, and then uses a roller to spread another layer of powder. Through these repeated steps of laser melting, solidification and powder coating, the object slowly takes shape.     

Additive manufacturing reduces the overall amount of metal materials consumed. Whereas mid-century manufacturing relies on subtracting material from a stock piece to form a useable part, laser powder bed fusion builds parts by adding consecutive layers of melted powder. This additive approach conserves much of the raw material because the unused powder can be recovered, sieved, and reused for later builds.        

Additive manufacturing has yet to realize the full potential to reduce, recycle and re-use powders; it may be necessary to use a pristine powder to meet quality standards of the finished part. A team from the University of Virginia’s Department of Materials Science and Engineering has discovered why some powders left behind after a build process may fail to meet specifications. Ph.D. students Jonathan Skelton and Connor Headley conducted experiments to understand why powder particles degrade.

A National Science Foundation grant supports their research. Jerrold Floro, professor of materials science and engineering, is the lead principal investigator; James Fitz-Gerald, also a professor of materials science and engineering, is a co-principal investigator for this collaborative project. Skelton and Headley are first and second authors of the team’s research paper, “On the Morphology Changes of Al and Al-Cu Powder After Laser Melting,” published in Metallurgical and Materials Transactions B, a journal of the Minerals, Metals and Materials Society and ASM International.

“When we heated the particle, we saw that it turned into a raisin-looking thing, like a shriveled pea. We wanted to know why these particles changed from a sphere to a collapsed shape,” Skelton said.

When re-used powder is rolled out, the buckled particles will clump together in unhelpful ways. The more spherical the particle, the better it will flow and spread; a buckled particle will tend to clump more during the recoating process. An uneven powder coating will lead to defects in a layer and potentially degrade the properties of the finished part itself.

“Jonathan and Connor made a smart selection of materials for their experiments,” Floro said. The pair used an alloy of aluminum and copper, which is stronger than either aluminum or copper by itself. The aluminum-copper alloy is eutectic, a technical term for alloys that melt at a temperature that is lower than any single element’s melting point.

Eutectic composites are advantageous in additive manufacturing because their microstructure can be directly correlated to how fast the material undergoes solidification. “The faster you solidify the metal liquid, the finer the spacing between the phases, the stronger the alloy will be,” Headley said.

Skelton and Headley used a sequential approach to examine how particles in aluminum and aluminum-copper eutectic alloys changed shape after being heated with a high-power laser. They found that the left-over powder contained misshapen or buckled particles compared to the pristine feedstock.

To gain a more quantitative and controlled perspective, they dispersed a new batch of aluminum and aluminum-copper powders onto a glass substrate and irradiated them with a low-power laser diode. This second approach permitted Headley to characterize specific particles before and after the laser treatment, which clearly showed dents and rifts in the particles.

From these experiments, Skelton and Headley deduced that isolated melting and resolidification of particles—each particle contained within an oxide shell—can occur. Thermal stresses developing in the oxide shell during cooling can account for the buckling.

“These experiments help us explain how powder particles can melt while completely contained inside a native oxide shell that’s only ten nanometers thick, and why the oxide shell buckles as the particles cool and solidify,” Skelton said.

Headley conducted these experiments as a fourth-year undergraduate in the UVA Department of Chemistry. Recognizing this and other accomplishments, Headley earned the 2020 undergraduate student chemistry award from the Virginia section of the American Chemical Society for his outstanding record of academic achievement.

“Chemistry was a natural choice for me,” Headley said. A native of Richmond, Virginia, he racked up AP credits in chemistry and followed the path taken by his mother, who also majored in chemistry at UVA. Headley enrolled in Floro’s Introduction to Materials Science and Engineering - Guided Inquiry course and was hooked. He chose materials science as his concentration for his bachelor’s degree in chemistry. 

“Chemistry is a great way to gain exposure to materials science, and I am still fascinated by the interactions of atoms at the nano-scale,” Headley said. The application-driven questions asked by materials scientists and his desire to do hands-on research led Headley to continue his research at UVA as a graduate student in materials science and engineering.

Headley is advised by Ji Ma, assistant professor of materials science and engineering. Ma’s students explore ways of using additive manufacturing or 3-D printing techniques to create materials with novel properties that cannot be achieved through conventional means. They also incorporate these materials in unique geometries, such as hollow cylinders, pyramids and combs, to produce functionally unique parts.

“Additive manufacturing is such a new field,” Headley said. “There are so many big questions that are not easy to answer, with potentially big pay outs.” A UVA Engineering Distinguished Fellowship award supports his current study and research.

Headley met Skelton during his second year of undergraduate study. Headley sought Floro’s advice for a summer research internship; Floro thought Headley and Connor would make a good team.

“It helped that Jonathan also came from chemistry and was brand new at UVA. We made the transition from chemistry to materials science together,” Headley said.

Skelton, who earned his Bachelor of Science in chemistry from Longwood University, is passionate about teaching. After earning his bachelor’s degree, Skelton taught middle school math for the Northumberland County, Virginia, school system. This experience motivated him to pursue a career in teaching and to further his own education

Skelton began working toward his Master of Science degree online at UVA. He later enrolled full time in the Ph.D. program for materials science and engineering after visiting UVA and meeting Floro and Fitz-Gerald, joining Fitz-Gerald’s group as an advisee.

“I am very thankful to have the opportunity to work with two professors who, though having different teaching styles, share a dedication to their students. I’ve learned a lot from them both and remain inspired by their example.” Skelton said.

During his three years here at UVA, Skelton has had the chance to hone his own teaching skills through various TA positions, mentoring undergrads in the lab, and involvement in the Summer Enrichment Program directed by the UVA School of Education and Human Development. After graduation, Skelton hopes to continue to help students see the amazing world around them through teaching, whether that be volunteering through outreach programs, or teaching at a college or high school.