Scientists see inside metal printing process

Combining special, high-energy X-rays with thermal imaging and visible light, engineers at the University of Wisconsin-Madison are looking inside 3D metal prints made with electron beam powder bed fusion to better understand, and eventually refine, this promising new metal 3D printing method.

With a technology as limitless as 3D printing, it is only a matter of time before the impossible becomes possible. Building geometrically complex or one-of-a-kind designs, artificial joints replicated from a scan of the original human bone, or strong lightweight components for rovers sent to other planets, 3D printing has quickly moved from novelty to necessity for manufacturing and design.

Among the popular techniques used in metal 3D printing, one that has gained traction for its ease of use and efficiency is called electron beam powder bed fusion.

"For electron beam powder bed fusion, right now, there's pretty fast growth," said 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 then melts and fuses additional powder layers to construct any shape or part from the bottom up. While the process sounds easy and straightforward, it is recent enough to be less than well understood.

There are many aspects at play, and defects hidden within the many thousands of 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," said Chen. "With a deeper understanding of the process, we can design better technology to move the process to a much higher level."

Completing its system in early January, the team tested successfully at Argonne National Laboratory's Advanced Photon Sources, 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 then allows the team to study how the temperature affects during the process, while a visible light camera enables them to observe the part's evolving surface structure.

"It is quite fascinating," said Luis Izet Escano, a mechanical engineering 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 colleagues designed and fabricated their system from scratch, drawing on extensive experience building tools that employ a synchrotron to study and improve another popular additive manufacturing technology called laser powder bed fusion.

The team overcame several technical challenges associated with studying electron beam powder bed fusion-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.

Not only do these results provide the world's first window into the electron beam powder bed fusion printing process, but also the most versatile.

"Development and integration of the system has been a great challenge, as it requires expertise in multiple engineering areas," Escano said. "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."