Evaluation of Heat-Treatment-Induced Microstructural Changes in Additively Manufactured Objects Using Ultrasound Attenuation Modeling.

Additively manufactured (AM) 316L stainless steel exhibits complex microstructures, residual stresses, and scattering sources, such as grains and pores, that evolve during production and after heat treatments. In this study, we investigate the effect of heat treatments at $725~^{\circ } $ C, $900~^{\circ } $ C, and $1100~^{\circ } $ C on AM 316L using ultrasound measurements. 316L samples were printed using the same laser powder bed fusion (LPBF) parameters and then subjected to different heat treatments. Frequency-dependent attenuation and sound velocity are calculated using ultrasound backscattered signals measured before and after treatment using a 5-MHz transducer. The attenuation spectra and principal component analysis (PCA) are used as an explorative step to differentiate material states before and after heat treatments. The spectra are then modeled using a sum of power laws with cross-validation across 25 measurement points. The model shows a fit with R2 greater than 0.90. The estimated model parameters reveal contributions from multiple scattering regimes, including Rayleigh, transition-region, and absorption-related contributions, which are linked to the microstructure variations present in AM steels. This is validated by light optical microscopy (LOM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD) maps, and grain size distribution measurements, which confirm microstructural evolution and stress relief. This integrated framework demonstrates the capability of using ultrasound for physically interpretable, nondestructive evaluation of heat treatment-induced microstructural evolution in AM steel components.