As additive manufacturing (AM) technologies become ever more prevalent and important in various industries, efforts to better characterize and understand the complex phenomena of various AM techniques are accelerating to keep pace. Such efforts are essential to improve and expand the utility of AM methods. The major AM technique for fabricating metal parts is laser powder bed fusion (LPBF), which shares similarities with laser welding. At the heart of both are laser-metal interactions, which because of their optical opacity have only been indirectly studied, leaving important details of their underlying dynamics incompletely defined. Recently, however, synchrotron x-ray imaging techniques have been developed that allow in situ investigations under real-time processing conditions, offering vital new insights. Investigators used ultra-high-speed transmission x-ray imaging at the U.S. Department of Energy’s Advanced Photon Source (APS) to study LPBF phenomena in two commonly-used AM alloys. Achieving the most complete understanding possible of the unseen intricacies of laser-based additive manufacturing processes will allow their continued improvement, resulting in increasingly reliable and versatile AM-fabricated metal components. Their results were published in Materials Today Advances.
Working at the Dynamic Compression Sector 35-ID x-ray beamline at the APS (the APS is an Office of Science user facility at Argonne National Laboratory), researchers from Lawrence Livermore National Laboratory (LLNL) studied a range of laser heating mechanisms in alloys of aluminum (AL6061) and titanium (Ti-6Al-4V) at very fine temporal resolution from 0.3 to 30 µs. Such resolution at the nano- and microsecond level afforded detailed characterization of laser-metal interactions below the materials’ surface, which is beyond the reach of other in situ methods. The experiments were complemented by multi-physics simulations.
Images of high-energy-density, 400 W laser irradiation scanning at a 500-kHz frame rate in Al6061 show the rapid formation of a keyhole vapor depression, which displays dynamic instabilities including the formation of ledges and humps of material. Some of these quickly change shape and vaporize within the melt void while others reattach or are absorbed into the vapor depression wall. Some form close to the metal surface, where waves create chevron patterns on the surface similar to those seen in laser welding. Simulations show that laser energy is reflected from the front of the vapor depression toward the rear wall, vaporizing and expanding material and strongly contributing to the observed instabilities. Similar phenomena are seen at all laser powers utilized and in the Ti-6Al-4V alloy, both under vacuum and in an argon environment.
Pore formation under steady-state laser scanning was also studied in both alloys. Experimental observations show similar findings in an argon environment, indicating that pores generally form near the base of the vapor depression, and are then trapped as the melt pool front solidifies. In the vacuum environment, however, pore entrainment also occurs closer to the surface and pores are driven deeper into the metal by Maragoni convection. The experimental observations and multi-physics simulations confirm that pores can be difficult to prevent or remove and that further post-processing can drive them deeper into the metal.
When the laser power is shut off, the vapor depression collapses in less than 5 µs and a solidification front occurs that can trap pores near the base of the depression. Cracks can also form when solidification fronts from the back and front of the depression converge. Experiments in Ti-6Al-4V under an argon environment in the end-of-track region also show that pores and bubbles can become trapped as the laser shuts off, the metal suddenly cools, and the vapor depression collapses. These oscillate in shape and size due to acoustic cavitation until the metal solidifies. The investigators note that such end of track effects could be minimized or avoided by optimizing processing conditions, perhaps by varying laser output or scanning speed in ways that would better control laser power density in the melt pool.
Even with the detailed picture of the range of laser-metal heating phenomena and defect formation offered by the ultrafast x-ray imaging techniques used in these experiments, the research team states that further such experiments will be needed to fully characterize the complex dynamics in play, in conjunction with multi-physics simulations.
See: Aiden A. Martin1*, Nicholas P. Calta1, Joshua A. Hammons1, Saad A. Khairallah1, Michael H. Nielsen1, Richard M. Shuttlesworth1, Nicholas Sinclair2, Manyalibo J. Matthews1, Jason R. Jeffries1, Trevor M. Willey1, and Jonathan R.I. Lee1**, “Ultrafast dynamics of laser-metal interactions in additive manufacturing alloys captured by in situ X-ray imaging,” Mater. Today Adv. 1, 100002 (2019). DOI: 10.1016/j.mtadv.2019.01.001
Author affiliations: 1Lawrence Livermore National Laboratory, 2Washington State University
This work was performed under the auspices of the U.S. Department of Energy (DOE) by Lawrence Livermore National Laboratory under Contract No. DE-AC52e07NA27344. Projects 17-ERD-042 (experimental) and 18-SI-003 (simulation) were funded by the LDRD Program at LLNL. The Dynamic Compression Sector is operated by Washington State University under the U.S. DOE/National Nuclear Security Administration award no. DE-NA0003957. The authors gratefully acknowledge the support (x-ray imaging at DCS) of P. Rigg, D. Rickerson, J. Klug, and N. Weir (DCS). This research used resources of the Advanced Photon Source, a U.S. DOE Ofﬁce of Science User Facility operated for the DOE Ofﬁce of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357.
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