Resonant acoustic nonlinearity and loss in additively manufactured stainless steel

Abstract

Resonant acoustic nonlinearity and loss in additively manufactured stainless steel were measured with the aim of assessing the potential of such measurements for nondestructively sensing defects that degrade mechanical performance. The material was fabricated by laser powder bed fusion (L-PBF) with intentional differences in spacing between laser tracks, which produced differences in porosity and amount of incompletely melted powder. The composition of the material was that of nominal 17-4 stainless steel. X-ray computed tomography was employed to image the structure of pores, including incompletely melted parti-cles. Porosities were determined to be in the range of approximately 0.05 % to 11.9 %, and unmelted powder fractions were in the range of approximately 0 % to 2.3 %. Noncontacting electromagnetic-acoustic transduction was employed to excite the lowest-order cutoff torsional mode in cylindrical specimens. Resonant frequencies as a function of RF amplitude were determined through time-domain analysis of decaying waveforms following tone-burst excitation, and acoustic loss was determined from an exponential fit of amplitude vs. time. Resonant frequencies were found to decrease with time during ringdown, corresponding to an unusual positive dependence on vibrational amplitude. Amplitude-dependent frequency shifts and acoustic loss increased between specimens monotonically with porosity and unmelted powder fraction.