Sensitivity of acoustic nonlinearity and loss to residual porosity in additively manufactured aluminum

Acoustic nonlinearity and loss are found to be positively correlated with porosity at industrially relevant levels of less than half a percent in commercially pure aluminum produced by laser powder bed fusion (L-PBF) with several different power levels. The technique employed for acoustic measurements involves nonlinear reverberation spectroscopy (NRS) with noncontacting electromagnetic-acoustic transduction, which offers advantages of adaptability to complex part geometries and short inspection times for industrial qualification of additively manufactured (AM) parts of arbitrary size. Porosity and microstructure are characterized with the Archimedes technique, X-ray computed tomography, and scanning electron microscopy. Fit parameters of nonlinearity and loss vs. porosity are found to vary significantly with the height of material in the build, consistent with an hypothesis that the correlations are indirect and involve dislocations as the principal nonlinear/anelastic elements. Nonlinearity and loss decrease with time under acoustic excitation, while being relatively insensitive to pauses in excitation of similar duration, indicating that acoustic excitation at inspection levels induces changes in nonlinear/anelastic defects without predominant involvement of thermal excitation. This remarkable behavior is not seen as a fundamental impediment for the application of the technique to nondestructive AM part qualification because of the brief time required for a measurement.