Generation and Detection of Gigahertz Ultrasound in Additively Manufactured Metal Components using Femtosecond Laser

Directed energy deposition (DED) additive manufacturing employs powders or wires with a focused heat source for the fabrication of net-shaped components or rapid prototyping. Despite the development of DED, few studies have investigated for in-situ quality assurance during the process. In particular, monitoring techniques with ultrahigh spatial sensitivity (submicrometer scale) have not been reported. In this study, a novel technique for in-situ quality monitoring during DED was developed using a gigahertz ultrasound generated and detected by a femtosecond laser. The unique contributions of this study include the (1) development of fully noncontact and nondestructive testing for DED monitoring, (2) achievement of sub-micrometer scale sensitivity using a femtosecond laser, (3) application to milling machined DED surfaces (sub-micrometer level roughness) owing to the beam-distortion detection ability, (4) characterization of microstructural features in the layered components using gigahertz ultrasounds, and (5) potential for online operation during DED. The performance of the proposed technique was validated using specimens with various surface roughness conditions created using DED.