The promise of metal three-dimensional (3-D) printing is hard to overstate, with the opportunity to revolutionize the manufacturing of a vast array of components, from the automotive to the medical industries. However, the technology has yet to mature to a point where metal 3-D printers reliably churn out consistently high-quality products at high volumes. One common approach to metal 3-D printing is called “laser powder bed fusion additive manufacturing.” In this method, a laser melts a metal powder, fusing the particles together. However, controlling the process dynamics can be tricky, and instability can cause defects in the printed material. To capture metal 3-D printing dynamics, a team of researchers combined high-fidelity simulations with x-ray imaging data collected at the U.S. Department of Energy’s Advanced Photon Source (APS) at Argonne National Laboratory. Using this information, the researchers came up with a set of criteria to stabilize the melted metal and reduce defects in the finished product.
In laser powder bed fusion additive manufacturing, a laser scans over a flat pattern of metal powder, melting and fusing the microscopic particles together bit by bit. This layer then forms a melt pool that trickles down a track to join with previous layers. The process is repeated thousands of times until the 3-D metal object is fully formed. But the accumulation of pores and other randomly generated defects leads to variability in the finished product, and variability is the enemy of high manufacturing standards, leading to quality and safety issues.
Combining the high-fidelity simulation and x-ray data collected at the Dynamic Compression Sector located at Sector 35 at the APS using transmission x-ray imaging, the researchers in this study, from Lawrence Livermore National Laboratory, the Air Force Research Laboratory, UES, Inc., and The Barnes Group Advisors, came up with a strategy for reducing spatter and other sources of variability. They used a physics-based stability criterion to set critical stability limits on the laser scan strategy (laser power and scan speed). They called this approach the “power map strategy,” which is essentially a way to modulate the laser’s power level to stabilize the melt pool dynamics and thus reduce spatter. This study demonstrates how the combination of simulation and experiment may help usher in the metal 3-D printing revolution.
The team realized that understanding and controlling the interdependency between the laser, the powder, and the melt pool may help address the metal 3-D printing variability issue. They turned to a powerful simulation algorithm to capture the underlying physics of the laser powder bed fusion additive manufacturing process. The researchers conducted virtual experiments, on a microscale, to look for the source of defects under various conditions. Through these virtual experiments, the team discovered a new way that spatter is produced as part of the metal 3-D printing process.
Spatter consists of bits of metal that are ejected from the laser beam and can be a source of defects in the final product. The researchers observed spatter in their simulation that was related to the particulars of the laser scanning strategy, among other factors.
To anchor the simulation's findings to the real world, the researchers also performed ultra-fast x-ray experiments at the X-ray Science Division Imaging Group’s 32-ID beamline, also at the APS, an Office of Science user facility at Argonne using Ti-6Al-4V, a grade-5 titanium alloy. The x-ray experiments imaged the surface of the metal particles and melt pools, as well as beneath the surface, on a time scale that allowed the researchers to track structural changes on the fast time scale of a laser. Thus, they could observe spatter and other defect-related phenomena in the x-ray experiments. ― Erika Gebel Berg
See: Saad A. Khairallah1*, Aiden A. Martin1, Jonathan R. I. Lee1, Gabe Guss1, Nicholas P. Calta1, Joshua A. Hammons1, Michael H. Nielsen1, Kevin Chaput2, Edwin Schwalbach2, Megna N. Shah2, Michael G. Chapman2,3, Trevor M. Willey1, Alexander M. Rubenchik1, Andrew T. Anderson1,Y. Morris Wang1, Manyalibo J. Matthews1, and Wayne E. King4, “Controlling interdependent meso-nanosecond dynamics and defect generation in metal 3D printing,” Science 368, 660, (8 May 2020). DOI: 10.1126/science.aay7830
Author affiliations: 1Lawrence Livermore National Laboratory, 2Air Force Research Laboratory, 3UES Inc., 4The Barnes Group Advisors
Correspondence: *khairallah1@llnl.gov
We thank N. Sinclair, P. Rigg, and D. Rickerson (x-ray imaging at DCS); and K. Fezzaa and A. Deriy (beamline 32-ID). Work was performed under the auspices of the U.S. Department of Energy (DOE) by Lawrence Livermore National Laboratory (LLNL) contract DE-AC52-07NA27344. Lawrence Livermore National Laboratory directed research and development, project 17-ERD-042, 18-SI-003. The Dynamic Compression Sector is operated by Washington State University under (DOE)/National Nuclear Security Administration award no. DE-NA0002442. This research used resources of the Advanced Photon Source, a U.S. DOE User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357.
The U.S. Department of Energy's APS is one of the world’s most productive x-ray light source facilities. Each year, the APS provides high-brightness x-ray beams to a diverse community of more than 5,000 researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. Researchers using the APS produce over 2,000 publications each year detailing impactful discoveries, and solve more vital biological protein structures than users of any other x-ray light source research facility. APS x-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from batteries to fuel injector sprays, all of which are the foundations of our nation’s economic, technological, and physical well-being.
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