Ultrasonic nondestructive evaluation of laser powder bed fusion 316L stainless steel

Additive manufacturing (AM) has the potential to revolutionize manufacturing because it can fabricate complex geometries that are not possible with conventional manufacturing processes. Qualification of AM parts remains challenging due to solidification defects (e.g. porosity) and heterogeneous microstructure, both of which are strongly dependent on AM process parameters. Nondestructive evaluation (NDE) techniques are desired to provide timely evaluation of AM parts in terms of both microscopic defects and microstructure variation. This work demonstrates the capability of ultrasonic phase velocity measurements to evaluate defects and microstructural differences of AM stainless steel 316L, caused by changes in hatch spacing. We related the measured phase velocity along different directions to pore geometry and texture of AM parts, using X-ray computed tomography, electron backscatter diffraction (EBSD), and uniaxial tensile tests. Our results confirm that the phase velocity depends on both pore geometry and crystallographic texture. By estimating the phase velocity from EBSD data, our measurements suggest that the measured ultrasonic phase velocity is sensitive to changes in anisotropic elastic constants, while the measured uniaxial tensile moduli are not as sensitive to the same changes. This work can be extended to other processing parameters and other anisotropic AM materials.