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Attenuation of Ultrasonic Waves Generated from Laser Ultrasound during Annealing of Steel; a Comparison between Theory and Experiment and Potential Application to Additive Manufacturing
Christopher M. Kube1, Ricardo Rodriguez2, Ed Habtour1, and Larry Holmes2, 1Vehicle
Technology Directorate, Army Research Laboratory, Aberdeen Proving Ground MD, 21005 and
2Weapons and Material Research Directorate, Army Research Laboratory, Aberdeen Proving
Ground MD, 21005
The advancement of additive manufacturing methods for the production of metallic parts has initiated the potential development of materials with tailored microstructures to enhance their material properties. To help facilitate the development, methods based on ultrasonic grain
scattering are proposed to provide in-situ monitoring of the microstructure’s evolution during the build process. In this work, the longitudinal attenuation coefficient is considered, theoretically and experimentally, as a function of temperature during an annealing process of steel.
Theoretically, an iterative solution to the attenuation model of Stanke and Kino [1] is given. The theory is compared against experimental measurements of the longitudinal attenuation
coefficient for a steel sample taken at various stages of annealing. Laser ultrasound was
employed because it is a remote technique that minimizes unwanted temperature related effects. The annealing process brought the sample from room temperature to 950 oC. A phase
transformation from ferrite to austenite occurred at 800 oC, which caused a significant drop in the measured attenuation coefficient. The theoretical attenuation model borrowed previously
measured temperature-dependent single-crystal elastic constants of pure iron as model inputs. A mixing formula that considers the volume fraction of ferrite to austenite was applied near the 800
oC mark where the drop in attenuation appeared. Remarkably, the theoretical attenuation model
almost exactly reproduced the experimental data points. This concurrence supports: (1) the employment of laser ultrasound for measurement of the attenuation during heating of materials, (2) the suitability of theoretical ultrasonic grain scattering models during highly transient temperature behavior, and (3) the ability of the theoretical attenuation model to represent the effect of a phase transformation.
References:
1. F. E. Stanke and G. S. Kino, “A unified theory for elastic wave propagation in polycrystalline materials,” J. Acoust. Soc. Am., 75 (3), 665-681 (1984).
