Ultrasonic evaluation of internal features in additively manufactured SS316L: Impact of process-induced microstructure heterogeneity

Process parameter selection in additive manufacturing (AM) is not standardized. Linear energy density (EDL = Power/Scanning speed), while commonly used, is non-unique which leads to different thermal histories and microstructures. This variability can significantly impact the performance of AM components, particularly those with internal structures. Ultrasonic nondestructive evaluation (NDE) methods are sensitive to microstructure and material properties. In this study, we assess the ability of ultrasonic NDE to resolve internal structures in AM components and evaluate the influence of microstructural heterogeneity. A systematic protocol was developed to map the viable parameter space for laser powder bed fusion (LPBF) of SS316L using a Lumex Avance 25 system. Process parameters were varied by adjusting scanning speed and laser power while maintaining a constant linear energy density to induce distinct microstructures. Samples were fabricated in two groups: solid specimens and specimens with internal structures, with each internal structure sample having a corresponding solid counterpart. Ultrasonic measurements were compared to intended geometries and X-ray CT scans to evaluate the effects of process-induced microstructural heterogeneity on feature resolution. This work highlights the potential and limitations of using ultrasound to characterize internal features in AM components, providing insights into the impact of microstructure heterogeneity on ultrasonic wave propagation.