Evidence for contributions of lack-of-fusion defects and dislocations to acoustic nonlinearity and loss in additively manufactured aluminum

Abstract

Resonant acoustic nonlinearity and loss have previously been found to be correlated with porosity in additively manufactured (AM) commercially pure aluminum and stainless steel, and this effect offers a potential basis for rapid nondestructive qualification of AM metal parts with complex geometries in industrial settings. This study explores possible physical mechanisms for this correlation and its observed anisotropy, using a combination of measurements and modeling. Acoustic anisotropy and porosity dependence are characterized with noncontacting nonlinear reverberation spectroscopy (NRS) and Ritz modeling of resonant modes that would be degenerate in isotropic material. Information on pore geometries, crystallographic texture, and densities of geometrically necessary dislocations (GNDs) is obtained from X-ray computed tomography and SEM-based measurements. The results are consistent with two physical mechanisms dominating the acoustic nonlinearity and loss in this material: (1) an anisotropic and porosity-dependent mechanism involving hysteretic motion within lack-of-fusion defects under acoustic stress and (2) bulk dislocation nonlinearity/anelasticity that is approximately independent of porosity.