from the infrared to hard x-ray portions of the electromagnetic spectrum, which has enabled it to become one of the most important research tools for the study of matter in all its various forms. The intense x-rays penetrate up to tens of um in most metals and can be used as Spatially Resolved (SRXRD) or Time Resolved (TRXRD) x-ray diffraction noncontact probes to monitor real-time microstructural evolution under the extreme temperatures and harsh conditions that surround welds (Ref. 2). Figure A shows a schematic of one type of experiment for simulating welding that is used at the Advanced Photon Source (APS), which is situated at Argonne National Laboratory. This technique probes the surface of a metal alloy in real time at 50-ms resolution as the sample is heated and cooled under controlled conditions, to study welding-related phase transformations in steels, stainless steels, and titanium alloys (Ref. 2). Neutron radiation interacts with matter differently than x-rays, and is particularly relevant to welding research where it can spatially probe residual stresses within bulk welded components. Unlike x-rays that interact with electrons, neutrons interact with atomic nuclei and can be captured or scattered depending upon the kinetic energy of the neutron and intrinsic characteristics of the sample’s nuclei. Because of this and in general, neutrons travel further into the crystal lattice than x-rays, penetrating up to tens of mm in typical metals. While neutron diffraction measurements cannot currently compete with high energy x-rays in time or spatial resolution, the ability to probe the interior of application-sized components on multiple length scales is unique to neutron radiation. Hence, throughthickness measurements of texture, dislocation density, and — more relevant — residual stress, become accessible in geometrically complex parts. “Aside from their ability to penetrate thick, high atomic number welded metals, neutrons can be used to detect nonmetallic impurities (e.g., oxides and carbides), as these can have coherent neutron-scattering cross sections comparable to the metallic matrix,” remarked Donald W. Brown, Team Leader and Instrument Scientist at Los Alamos National Laboratory. Further, “engineering beamlines at neutron sources can map residual stresses near and through weldments, which is essential for model validation and refinement,” noted Thomas R. Watkins, Senior Research Staff and the Scattering and Thermophysics Group Leader, Oak Ridge National Laboratory (ORNL). A schematic of a neutron beam line is shown in Fig. B, where data have been collected to help understand residual stress formation in welds as well as to provide verification of weld modeling efforts (Ref. 3). Auspices This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52- 07NA27344 and has been assigned the document release LLNL-JRNL- 691945. References 1. Spooner, S. 1992. Using federal x-ray, electron and neutron facilities. Journal of The Minerals, Metals & Materials Society 44(10): 72–76. 2. Elmer, J. W. 2008. A new path forward for understanding microstructural evolution during welding. Welding Journal 87(6): 149-s to 166-s. 3. Spooner, S. 2003. Neutron residual stress measurement in welds, in Analysis of Residual Stress by Diffraction Using Neutron and Synchrotron Radiation. New York, N.Y.: Taylor & Francis, Inc., pp. 296–318. 54 WELDING JOURNAL / JULY 2016 JOHN W. ELMER (elmer1@llnl.gov) is the Group Leader for Materials Processes, and AMANDA S. WU is a Materials Scientist, Materials Engineering Division, Lawrence Livermore National Laboratory, Livermore, Calif. Fig. A — APS weld simulation setup showing synchrotron based TRXRD experiments being performed in an environmental chamber to prevent oxidation of the heated sample. Fig. B — A schematic of the experimental setup at the VULCAN neutron beam line at ORNL’s Spallation Neutron Source. The specimen (blue rectangle) is translated relative to the gauge volume, allowing for mapping studies. The two detector banks can collect signals in two orthogonal directions (LD & ND) simultaneously from the gauge volume (red square), which is defined by the incident beam slits and radial collimators. (Courtesy of Dr. Ke An, ORNL.) WJ
Welding Journal | July 2016
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