LLNL Provides Update on Collaborative Research into Why Flaws Occur in Metal 3D Printing Processes

May 28, 2018 by Sarah Saunders 3D Printing Metal 3D Printing Science & Technology

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While the potential that metal 3D printing has to truly shake things up in the manufacturing world is great, it’s not without its faults. Many facilities are putting forth significant resources to determine why flaws in metal 3D printed parts occur, and how to stop them…perhaps none more so than the innovative researchers at Lawrence Livermore National Laboratory (LLNL). This winter, LLNL teamed up with scientists from Ames Laboratory and the Department of Energy’s SLAC National Accelerator Laboratory to use X-rays to study the metal 3D printing process and figure out where these flaws come from, and how they can be prevented.

The DOE’s Energy Efficiency and Renewable Energy (EERE) Advanced Manufacturing Office is funding the multi-year partnership between the three laboratories, an offshoot of its annual National Laboratories Big Ideas Summit (BIS) that facilitates strategic technical planning and collaboration through the national laboratory complex.

(L-R) LLNL researchers Phil Depond, Nick Calta, Aiden Martin, and Jenny Wang. [Image: Julie Russell, LLNL]

This partnership hopes to generate more interest from US industry for collaborative projects like this one, for which LLNL has just provided an update.

Adoption of metal 3D printing has been slow in several industries, such as aerospace and automotive, because part quality and certification cannot be anything less than perfect for these critical parts. LLNL researchers have conducted multiple experiments in order to pull the covers back on many complex mechanisms that can cause defects to form in metal parts, and this latest with SLAC and Ames is a more direct examination of what factors in particular lead to these defects, and how to avoid these flaws.

[Image: SLAC]

This month, one of the first scientific papers resulting from this partnership was published, as an Editor’s Pick, in the Review of Scientific Instruments journal. The paper details how the researchers used X-ray imaging and diffraction to get a look inside metal parts while they’re being 3D printed in a common laser powder bed fusion (LPBF) process.

“It’s a really nice team because each partner brings a strength. The team is building a capability that is somewhat unique and providing information you can’t get any other way,” Tony Van Buuren, deputy division leader for S&T in LLNL’s Materials Science Division, said about the project. “Together we’re bringing in the diagnostics, spinning in science and spinning out the ability to look at new materials.”

Lead author Nick Calta designed and built a portable diagnostic machine that’s able to probe the melt pool. The device and method were both tested and evaluated at SLAC’s Stanford Synchrotron Radiation Lightsource, and Calta’s team was able to successfully observe dynamics of the melt pool under the surface.

“A vast majority of diagnostics use visible light, which are extremely useful but also limited to analyzing the surface of the part. If we’re going to really understand the process and see what causes flaws, we need a way to penetrate through the sample. This instrument allows us to do that,” said Calta.

LPBF chamber design details.

Calta explained that it was difficult for the collaborative team to build a portable in situ diagnostic machine on an aggressive timeline. First, LLNL researchers had to transport the device to SLAC, and use the synchrotron to create the X-ray flux and high energy X-rays that were needed to investigate the samples, which provide useful data on X-ray diffraction and imaging so the researchers could see how the metal solidifies, which helps determine the strength of a part. But they are already reaping the benefits of all that hard work, and gathered useful data that they’re still working to analyze.

LLNL physicist and Laser Materials Science group leader Ibo Matthews, who has years of experience in developing experiments to understand the physics behind LPBF, said, “We’re getting information about the melt pool structure and what can go wrong during a build. The vapor plume created by the laser heating the melt pool can create pockets and pores. These pore defects can serve as stress concentrators and compromise the mechanical properties of the part.”

Because the team was able to actually see the layers formed at the melt pool, and compare the X-ray images to simulations, Matthews said that they are able to confirm predictions of how lasers can create defects through its gas plume, path, and heat buildup. If they combine this new information with modeling in continued experiments, improvements and confidence in metal 3D printed parts could increase.

“Success would be learning more about the physics in ways that let us modify the process to avoid defects. So far we’re getting promising results,” explained Calta. “We want to continue to optimize the instrument and apply it to different material systems. We already have a big body of knowledge based on optical data, this lets us branch out and complement that knowledge.”

[Image: SLAC]

Calta said the team has already started to map out pore formation and elicit cooling rates information. Hopefully, this device will result in a better understanding of the LPBF 3D printing process, and lead to more interest in metal 3D printing from industry.

“You can’t tell what’s inside the box by looking outside the box. The purpose of this project is to accelerate the adoption of additive manufacturing (AM) for metallic components across the manufacturing sector by developing sophisticated in-situ tools to enable rapid process development of the AM components,” said Van Buuren. “With new materials, we don’t yet understand the properties and we need to be able to look at the process in real-time. It’s a bit different focus than what we usually do at the Lab. We want to build up a capacity that industry would come in and use.”

The team hopes to add optical diagnostics, which are often used on commercial machines, to help correlate with the X-ray imaging.

Co-authors of the paper, “An instrument for in situ time-resolved X-ray imaging and diffraction of laser powder bed fusion additive manufacturing processes,” include: