Grain size estimation in additively manufactured metal thin-wall using noncontact laser ultrasonic and zero-group-velocity Lamb waves

Grain size is a critical factor influencing the performance of fabricated structures, particularly their elastic modulus and yield strength. In metal additive manufacturing (AM), unique thermal properties such as thermal accumulation, thermal gradients, and cooling rates can lead to grain size variations throughout the structure. This study proposes a grain size estimation technique for additively manufactured metal thin-walled structures. The developed technique employs pulsed and continuous wave (CW) lasers to generate and measure zero-group-velocity (ZGV) Lamb waves. By extracting ZGV resonance frequencies from the measured waves, Poisson’s ratio is determined and correlated with grain size using a pre-established model. The developed technique offers several key advantages: (1) Localized ZGV Lamb wave modes enable precise, localized grain size estimation. (2) The advanced and localized nature of the technique allows it to accommodate the complex geometries of compact and lightweight AM designs. (3) As a non-contact and non-destructive measurement method using lasers, it is suitable for both in-situ monitoring and post-processing investigation. The technique was experimentally validated on additively manufactured stainless steel 316L thin-walled samples, demonstrating its feasibility for real-time in-situ grain size monitoring in AM.