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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

[Display omitted] Subtle differences among additive manufacturing (AM) processing parameters lead to variations in pore networks and complicate the prediction of void-sensitive mechanical behaviors, including location of fracture. The current work expands upon a recently developed pore metric, the void descriptor function (VDF), by accounting for interactions among neighboring pores and stress concentrations induced by non-spherical pores or voids. The modified VDF is evaluated against 120 computationally generated fracture simulations and six physical tensile specimens of as-built laser powder bed fused IN718. The latter set of experiments, which include X-ray computed tomography measurements before and after deformation, enables evaluation against pore populations that are representative of defects commonly observed in AM metals. The modified VDF accurately predicts fracture location (within ± 5% tolerance) for 94 out of 120 simulated specimens, representing 3.3%, 62.1%, and 59.3% increases in the number of accurate predictions in comparison to predictions based on the original VDF, the location of maximum cross-sectional area reduction, and the largest-pore location, respectively. In the experimental data set, the modified VDF accurately predicts the location of fracture in five out of six specimens compared to only two out of six using the original VDF, maximum cross-sectional area reduction, or largest-pore location. Also, the maximum value of the modified VDF was found to be more highly correlated than fraction porosity, pore size, reduced-cross section area, and total number of pores to the ultimate tensile strength, elongation to failure, and toughness modulus, suggesting that the modified VDF presented in this work could serve as a promising metric to assist with characterizing unique pore networks and predicting fracture-related properties in AM components. [ABSTRACT FROM AUTHOR]