MS Defense by Aurelio Bellott
Master's Thesis Presentation
Thursday, June 25, 2020, 1:00 p.m.
https://bluejeans.com/1170627115, Room Online
Nonlinear Ultrasonic Technique for the Characterization of Microstructure in Additive Materials
Dr. Laurence Jacobs, Chair (ME)
Dr. Jin Yeon Kim (CEE)
Dr. Christopher Saldana (ME)
This thesis employs nonlinear ultrasonic techniques to track microstructural changes in additively manufactured metals. The second harmonic generation technique based on the transmission of Rayleigh surface waves is used to measure the acoustic nonlinearity parameter, β. Stainless steel specimens are made through three procedures: traditional wrought manufacturing; powder bed fusion (PBF); and laser engineered net shaping (LENS). The β parameter is measured through successive steps of an annealing heat treatment intended to decrease dislocation density. Dislocation density is known to be sensitive to manufacturing variables. In agreement with fundamental material models for the dislocation-acoustic nonlinearity relationship in the second harmonic generation, β drops in each specimen throughout the heat treatment before recrystallization. Geometrically necessary dislocations (GNDs) are measured from electron backscatter diffraction (EBSD) as a quantitative indicator of dislocations; average GND density and β are found to have a statistical correlation coefficient of 0.852 showing sensitivity of β to dislocations in AM metals. Moreover, β shows an excellent correlation with hardness, which is a measure of the macroscopic effect of dislocations.
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