Advances in Synchrotron-Based High-Speed x-ray Imaging and Diffraction of Additive Manufacturing Processes.
High-speed x-ray imaging and diffraction experiments were carried out at the 32-ID-B beamline of the Advanced Photon Source (APS) using a short-period undulator that generates polychromatic x-rays with a first harmonic energy at 24.4 keV. The samples studied in the experiments were miniature Ti-6Al-4V powder bed systems, which consisted of two pieces of glassy carbon plates as the container walls, one piece of Ti-6Al-4V plate (450 µm thick) sandwiched between the glassy carbon as the base, and one layer of Ti-6Al-4V powder (~100 µm thickness). High-speed x-ray images of the melting process with two different laser power conditions were taken with a frame rate of 50 kHz and an exposure time of 350 ns for each individual image. The cavity and melt pool were well defined in the radiographs, enabling measurement of melting and solidification rates and the latter approached a value of 1 m/s. The presence of a cavity indicates that key-holing can be studied directly with the technique. Powder particles were observed to move into the melt via surface tension forces. Particle ejection was also readily apparent and a spectrum of individual velocities was measured with individual speeds up to 15 m/s. Diffraction data revealed the formation and disappearance of the bcc beta phase; the intensity of diffraction from the hexagonal alpha was approximately complementary. The Bragg peaks for the beta were sparse and large compared to the diffraction from the alpha, which is consistent with the columnar solidification mode commonly observed in Ti. Tests on a non-standard flaky titanium powder revealed similar melting behavior as for standard powders, which indicates that irregularly shaped powders can be used in additive manufacturing. In general, this new and exciting technique is capable of revealing many details in powder bed additive manufacturing and is likely to also be useful for studies of high speed welding.
Dr. Rollett has been a Professor of Materials Science & Engineering at Carnegie Mellon University (CMU) since 1995 and was the Department Head 1995-2000. Prior to CMU he worked for the University of California at the Los Alamos National Laboratory (1979-2005). He spent ten years in management with five years as a Group Leader (and then Deputy Division Director) at Los Alamos, followed by five years as Department Head at CMU (1995-2000). He has published about two hundred peer reviewed papers on a wide variety of topics in materials science, many of which are cited frequently in the literature. The main focus of his research is on additive manufacturing and the measurement and computational prediction of microstructural evolution especially in three dimensions. His interests include additive manufacturing, strength of materials, constitutive relations, microstructure in 3D, texture, anisotropy, grain growth, recrystallization, formability and stereology.
Prof. Rollett’s honors include: Bain Award of the Pittsburgh Chapter of ASM International, 2016; Member of Honor of the French Metallurgical Society, 2015; Cyril Stanley Smith award from TMS, 2014; Chercheur d'Excellence (Outstanding Researcher) at the University of Lorraine in Metz, France, 2012; Brahm Prakash Professor at the Indian Institute of Science (Bangalore), 2011; Fellow of TMS, 2011; Fellow of the Institute of Physics (UK), 2004; Howe Medal (Best Paper in Metallurgical & Materials Transactions), 2004; Fellow of ASM-International, 1996; Award for Technology Transfer from the Federal Laboratories Consortium, 1989. He was the Chair of the International Conference on Texture (ICOTOM-15), which was held on campus at CMU in 2008 and, as of 2014, is the Chair of its International Scientific Committee. From 2001-2013 he was the Chair of the International Committee of the conference on Grain Growth and Recrystallization. He was a co-Chair of the 13th International Conference on Aluminum and its Applications in 2012. He is a co-author of the texture analysis package popLA, and the polycrystal plasticity code, Lapp. He is also a contributor to the well-known textbook Texture & Anisotropy edited by Kocks, Tomé and Wenk and is working on a new edition of Recrystallization and Related Annealing Phenomena.