Residual Stress Measurement of Laser-Engineered Net Shaping AISI 410 Thin Plates Using Neutron Diffraction
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INTRODUCTION
PAST studies have employed different techniques for the measurement of internal residual stresses in parts manufactured by the laser-engineered net shaping (LENS) rapid fabrication process.[1,2] The results of these studies revealed high levels of type I macroscopic residual stress induced within the parts by the net shaping process. However, little is yet known of how the LENS input parameters affect the resulting levels and distributions of internal stresses. A more thorough understanding of the relationship between these parameters and residual stress could lead to better optimization of the LENS process and the production of higher quality components. Using the method of neutron diffraction, strain measurements were taken at the High Flux Isotope Reactor Neutron Residual Stress Mapping Facility (NRSF2) at Oak Ridge National Laboratory for seven LENS-produced thin wall plates of stainless steel AISI 410. The dimensions of the plates were between 22- and 38-mm wide and 15 mm in height, with a width of 1 to 3 mm depending on the laser power. One of these plates P. PRATT, Graduate Student, Department of Mechanical Engineering, S.D. FELICELLI, Associate Professor, Department of Mechanical Engineering and Center for Advanced Vehicular Systems, and L. WANG, Postdoctoral Researcher, Center for Advanced Vehicular Systems, are with Mississippi State University, Mississippi State, MS 39762. Contact e-mail: [email protected] C.R. HUBBARD, Leader of Diffraction and Thermal Physical Properties Group, is with the Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. This article is based on a presentation given in the symposium entitled ‘‘Neutron and X-Ray Studies for Probing Materials Behavior,’’ which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee. Article published online October 15, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
is shown in Figure 1. The samples were produced at the facilities of Optomec (Albuquerque, NM). The plates were made by varying three process parameters: laser power, laser travel speed, and metal powder flow rate, according to Table I. The goal of the neutron measurements was to determine how the magnitude and distribution of residual stresses in the plates were affected by varying these input parameters.
II.
OVERVIEW: NEUTRON DIFFRACTION
The neutron diffraction method for strain measurement at a reactor source uses a monochromatic beam of neutrons of known wavelength from a monochromator crystal. The atomic planes of certain crystallographic orientation {hkl} diffract the neutrons at a scattering angle of 2h. Then, using Bragg’s law of diffraction, k ¼ 2dhkl sin h
½1
where k is wavelength, dhkl is lattice spacing, and h is diffraction angle. The spacing between the lattice planes, dhkl, was found by measurement of 2h. The diffracting lattic
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