Uncovering microstructural heterogeneities in binder jet printed SS316L through ultrasonic testing and X-ray computed tomography

Samples extracted from a binder jet printed stainless steel 316L block with variations in grain and pore characteristics were studied using X-ray computed tomography (XCT) and ultrasonic testing (UT). Ultrasonic group velocity (wave speed) and attenuation were mapped over the gauge region of tensile samples, and 3D pore reconstructions were developed with XCT. The limits of ultrasonic testing to detect porosity were probed by drawing correlations between 2D pore metrics and attenuation and wave speed. In areas where porosity was low and grain size large, the grain size was found to dominate the ultrasonic response, a dependence validated using scattering-based models. In high porosity areas, attenuation correlated to the pore area perpendicular to the wave displacement and, as expected, to the volume fraction of porosity. Wave speed decreased more drastically in the presence of networked pores than for equal volume fractions of isolated pores, a finding linked to the effect of interconnected pores on effective elastic stiffness. Tensile properties and fracture location were analyzed relative to the XCT and UT results. Converting the 3D pore reconstructions to 2D heat maps aided in prediction of failure location with high accuracy, while ultrasound showed promise in identifying regions of interest for possible fracture.