Quantitative characterization of microstructure in additively manufactured metals with nonlinear ultrasound

Our work employs nonlinear ultrasonic techniques to track microstructural changes in additively manufactured metals. One study is based on the use of the second harmonic generation technique with Rayleigh surface waves used to measure the acoustic nonlinearity parameter, β. Stainless steel specimens made through laser-powder bed fusion and laser engineered net shaping were compared with traditional wrought manufactured specimens. The β parameter is measured through successive steps of an annealing heat treatment intended to decrease dislocation density. In agreement with fundamental material models for the dislocation-acoustic nonlinearity relationship in the second harmonic generation, β drops for each specimen throughout the heat treatment before recrystallization. Geometrically necessary dislocations (GNDs) are measured from electron back-scatter diffraction as a quantitative indicator of dislocations; average GND density and β are found to have a statistical correlation coefficient of 0.852 showing the sensitivity of β to dislocations in additively manufactured metals. Moreover, β shows an excellent correlation with hardness, which is a measure of the macroscopic effect of dislocations. Ongoing work continues trying to characterize additively manufactured stainless steels with nonlinear ultrasound through non-collinear mixing.