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Researchers from Argonne National Laboratory and Carnegie Mellon University have identified the cause of defects in 3D printing, opening the way to better quality and better control of finished products.
Argonne’s Advanced Photon Source (APS) was used to take superfast video and images of laser powder bed fusion (LPBF), a process in which lasers are used to create the product by scanning each layer of powder and fusing metal where it is needed. Researchers watched what happened as the laser moved across the metal powder bed to create each layer of the product. The APS experimental platform included a laser apparatus, specialized detectors, and dedicated beamline instruments.
Argonne scientists inside a hutch at Argonne’s Advanced Photon Source, in front of a specialty system that can simulate the laser powder bed fusion process in a commercial 3D printer. Pictured, clockwise from top left, are Kamel Fezzaa, an APS beamline scientist; Tao Sun, an APS beamline scientist; Cang Zhao, an APS postdoc; and Niranjan Parab, an APS postdoc. Courtesy of Argonne National Laboratory.
“Most people think you shine a laser light on the surface of a metal powder, the light is absorbed by the material, and it melts the metal into a melt pool. In actuality, you’re really drilling a hole into the metal,” said Carnegie Mellon professor Anthony Rollett.
Under perfect conditions, the melt pool shape is shallow and semicircular. But during the actual printing process, the high-power laser, often moving at a low speed, can change the melt pool shape to something like a keyhole: round and large on top, with a narrow spike at bottom. This “keyhole mode” melting can potentially lead to defects in the final product, the researchers found. They showed that keyholes form when a certain laser power density is reached that is sufficient to boil the metal, underlining the importance of the level of laser power used in the additive manufacturing process.
“Based on this research, we now know that the keyhole phenomenon is more important, in many ways, than the powder being used in additive manufacturing,” said researcher Ross Cunningham. “Our research shows that you can predict the factors that lead to a keyhole — which means you can also isolate those factors for better results.”
The formation of keyholes, or vapor-filled depressions, during laser welding presents a large problem for additive manufacturing. The team’s research shows that these “vapor depressions” exist under nearly all conditions in the 3D printing process, and how to predict when a small depression will grow into a large, unstable depression that could potentially lead to a defect.
This image, taken under the synchrotron at Argonne National Laboratory, shows a keyhole void about to be formed during the metal 3D-printing process. During laser powder bed fusion, a 3D printer “drills” a hole into the metal. Courtesy of Carnegie Mellon University.
The team believes this research could motivate makers of additive manufacturing machines to offer more flexibility when controlling the machines and that the improved use of the machines could lead to a significant improvement in the final product. In addition, if these insights are acted upon, the process for 3D printing could get faster.
“It’s important because 3D printing in general is rather slow,” Rollett said. “It takes hours to print a part that is a few inches high. That’s OK if you can afford to pay for the technique, but we need to do better.”
The research was published in Science (http://dx.doi.org/10.1126/science.aav4687).READ MORE