A penetrating gaze: System allows world’s first X-ray look at electron-beam 3D-printing process

Combining special, high-energy X-rays with thermal imaging and visible light, engineers at the University of Wisconsin–Madison are looking inside metal items made with a new 3D-printing technique to better understand (and eventually refine) the promising manufacturing method.

Building geometrically complex or one-of-a-kind designs — artificial joints designed from a scan of the original human bone, for example, or strong, lightweight components for rovers sent to other planets — are among the applications for a 3D-printing process with metals called electron beam powder bed fusion.

“For electron beam powder bed fusion, right now, there’s pretty fast growth,” says Lianyi Chen, an assistant professor of mechanical engineering at UW–Madison. “It’s an important technology to make parts for aerospace—for example, for jet engines, with titanium aluminide. We can’t make these with any other 3D-printing technology.”

Electron beam powder bed fusion begins with a base of metal powder on a substrate. An electron beam melts and fuses additional powder layers to construct a part from the bottom up. While the process sounds straightforward, it’s new enough to be less than well understood. There are lots of physical phenomena at play, and defects hidden within metal layers could cause failures without warning.

Chen and a team of UW–Madison mechanical engineers have pioneered the integration of several imaging technologies into a system that can study the fundamental mechanics of electron beam powder bed fusion in real time.

“It is the first time we have the ability to see what happens beneath the surface—what are the defect formation mechanisms,” says Chen, the Charles Ringrose Assistant Professor in mechanical engineering and a fellow of UW–Madison’s Grainger Institute for Engineering. “With a deeper understanding of the process, we can design better technology to move the process to a much higher level.”

The team completed its system in early January and has tested it successfully on Argonne National Laboratory’s Advanced Photon Source, which uses a particle accelerator to produce ultra-bright, high-energy X-rays for exacting scientific studies.

The UW–Madison system combines the synchrotron X-ray imaging and diffraction — a process that uses the way materials scatter X-rays to reconstruct their shape — with more conventional techniques. The high-energy synchrotron X-rays give the researchers a look at how material is behaving in its otherwise hidden interior in unprecedented detail as the printing system works. A thermal camera allows them to study how the temperature evolves during the process, while a visible light camera enables them to study the part’s evolving surface structure.

“It is quite fascinating,” says Luis Izet Escano, the mechanical engineering graduate student in Chen’s group who led development of the system. “With only one run on our machine, we are able to see several aspects of the printing process simultaneously.”

Escano and his colleagues designed and fabricated their system from scratch, drawing on extensive experience building tools that employ a synchrotron to study and improve another additive manufacturing technology called laser powder bed fusion.

The team overcame several technical challenges associated with studying the electron beam powder bed fusion process—among them, maintaining the high vacuum needed for the process, mitigating vibrations from the vacuum pump in their measurements, and manufacturing special viewports so that the synchrotron’s X-rays could pass through them effectively.

The result is not only the world’s first window into the electron beam powder bed fusion printing process, but also its most versatile.

“Development and integration of the system has been a great challenge, as it requires expertise in multiple engineering areas,” says Escano. “Now, the flexibility of our machine allows us to run experiments and collect data quite fast—and this will accelerate our research toward the fundamental understanding and perfection of this printing technology.”

Graduate students in Chen’s group who also contributed to the system’s development include Xinhang Zhang, Qilin Guo, William Dong and Minglei Qu. Samuel J. Clark and Kamel Fezzaa, who are beamline scientists at the Advanced Photon Source, assisted with the group’s synchrotron X-ray experiments. Funding for the work came from the National Institute of Standards and Technology and Chen’s faculty startup package at UW–Madison.