Powerful X-Rays Put to Use in Study of Physics Behind Metal 3D Printing Process as Researchers Seek to Eliminate Structural Defects


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Lianyi Chen, assistant professor of mechanical and aerospace engineering in his office and Toomey lab. [Image: Sam O’Keefe/Missouri S&T]

From medical tools and aerospace turbines to components for race cars and satellites, it may seem like metal 3D printing can be used to magically make just about anything, for less money and weight. However, we know that the technology is not foolproof, and definitely not magic – issues like micro cracking and spattering abound, and many times metal 3D printed materials differ from their original designs, and are susceptible to structural defects. Metal parts with defects can mean safety issues and compromised functionality, so engineers have to either repair the piece or start all over again, wasting precious time and money. Now, an assistant professor from the Missouri University of Science and Technology (Missouri S&T) is working with researchers from Carnegie Mellon University and the Department of Energy’s Argonne National Laboratory to learn more about the physics behind the technology.

Dr. Lianyi Chen with Missouri S&T, together with the rest of the research team, is taking a very close look at the entire laser powder bed fusion 3D printing process, in the hopes of potentially eliminating structural defects in 3D printed materials. Thanks to the “intense synchrotron X-rays” available at the Advanced Photon Source (APS), a DOE Office of Science User Facility, the team was able to get a real-time look inside the materials formed by 3D printing as the laser printed EOS Titanium Ti64 powder into components.

In these high-speed x-ray images, the 3D printer is using a laser to melt metal powder, which causes a ‘keyhole’ defect within the cooled material. Researchers at Argonne are studying this process and developing guidelines to avoid such errors. [Image: Argonne National Laboratory]

Tao Sun, a physicist at Argonne, explained, “The laser-metal interaction happens very quickly. Fortunately, we captured the process at 50,000 frames a second using the high-speed X-ray instrument at the Advanced Photon Source. We can study the resulting movie frame by frame to examine how the material’s microstructure, especially defects and pores, form.”

Argonne National Laboratory [Image: APS]

APS, located in Argonne, Illinois, is the brightest X-ray source in the entire Western Hemisphere, and scientists from all over the world use the X-rays to make breakthroughs in many diverse fields. The researchers studying metal 3D printing processes used the 32-ID-B beamline at APS for their work.

To find out how defects form, and how to get around them, the researchers are looking at all aspects of the 3D printing process, including the material properties of the metal powders and how the laser prints them into components. Using the Advanced Photon Source X-rays, the team can observe, and quantify, many of the characteristics of metal 3D printing, such as solidification, porosity formation, powder ejection, the shape and size of the melt pool, and phase transformations.

The team published their work in a paper, titled “Real-time monitoring of laser powder bed fusion process using high-speed x-ray imaging and diffraction,” in Scientific Reports; in addition to Sun and Chen, co-authors include Cang Zhao, Kamel Fezzaa, Ross W. Cunningham, Haidan Wen, Francesco De Carlo, and Anthony D. Rollett.

According to the abstract, “We employ the high-speed synchrotron hard X-ray imaging and diffraction techniques to monitor the laser powder bed fusion (LPBF) process of Ti-6Al-4V in situ and in real time. We demonstrate that many scientifically and technologically significant phenomena in LPBF, including melt pool dynamics, powder ejection, rapid solidification, and phase transformation, can be probed with unprecedented spatial and temporal resolutions. In particular, the keyhole pore formation is experimentally revealed with high spatial and temporal resolutions. The solidification rate is quantitatively measured, and the slowly decrease in solidification rate during the relatively steady state could be a manifestation of the recalescence phenomenon. The high-speed diffraction enables a reasonable estimation of the cooling rate and phase transformation rate, and the diffusionless transformation from βto α phase is evident. The data present here will facilitate the understanding of dynamics and kinetics in metal LPBF process, and the experiment platform established will undoubtedly become a new paradigm for future research and development of metal additive manufacturing.”

Schematic of the high-speed X-ray imaging and diffraction experiments on laser powder bed fusion process at the 32-ID-B beamline of the Advanced Photon Source. A short-period undulator generates a pseudo pink beam with first harmonic energy of 24.4 keV (λ = 0.508 Å). The laser impinges on the miniature powder bed sample from the top, and the X-rays penetrate the sample from the side. The imaging and diffraction detectors are placed downstream, about 300 mm away from the sample. The inset surrounded by the dashed circle enlarges the view of the laser-sample and X-ray-sample interaction. The distance of each component from the source is labeled on top.

Using Argonne’s supercomputer, the team then processed the information gleaned from the X-rays to make virtual models of how objects actually print. Aaron Greco, a principal materials scientist at Argonne and its additive manufacturing project co-leader, is working to enhance these models to gain an accurate understanding of the “underlying materials physics required to make 3D-printing truly reliable.”

“After printing, we examine the product’s resulting microstructure and defects. We use a variety of techniques including optical and electron microscopy and even tomography at the Advanced Photon Source, to validate the models,” Greco said.

This results in a virtuous loop: the experimental data feeds into additive manufacturing models, and the improved models are then tested in more elaborate experiments.

“Our goal is to explore new possibilities. Industries are currently limited to a certain set of metal alloys. But what about new ones? If you understand the physical properties related to how to print new alloys, you can adopt these into the process and speed up the reliability of printing,” Greco said.

The detailed models that are necessary to define the 3D printing process for more complex components limit industries, so by including only a fraction of the elements that affect reliability and quality in the models, the research team hopes that the new and improved models will be more useful.

Greco said, “Our work will not only help industries improve efficiency and performance, but increase the likelihood that metal additive manufacturing will be more widely adopted in other applications.”

Sun will share the paper’s conclusions with other national laboratories and academic partners who are working to build models that can predict printed material characteristics and process dynamics. This innovative 3D printing process work at Argonne will not only help scientists understand the many mysteries of metal additive manufacturing, it will also give industries blueprints that can be used to quickly 3D print reliable, cost-effective products. Discuss in the Metal 3D Printing forum at 3DPB.com.

[Sources: Missouri S&T, Argonne National Laboratory]


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