In situ and layer-by-layer grain size estimation in additively manufactured metal components using femtosecond laser ultrasonics

Directed energy deposition (DED) is an additive manufacturing technique wherein a focused thermal energy source and a coaxial powder delivery system are combined for the fabrication of metallic parts. Although rapid progress has been made in DED, the amount of research performed for in situ quality monitoring during fabrication is limited. Grain size monitoring during DED is particularly important because the grain size is directly related to the mechanical strength and stiffness of the final products. In this study, a layer-by-layer grain size estimation technique using femtosecond laser ultrasonics is developed for in situ monitoring during DED. The proposed technique employs fully noncontact and nondestructive testing for grain size estimation and uses the relationship between the laser-induced ultrasonic waves and the grain size. In addition to the in situ operation of the technique, spatial resolution in the micrometer range was achieved. The developed technique was validated using Ti-6Al-4V specimens fabricated by DED. The results of the quantitative grain sizes measured by the developed method were consistent with those measured through independent metallography conducted after the completion of DED.