Pulse-echo-based third-order nonlinear ultrasound enhancement and its applications to microstructural characterizations in metal additive manufacturing

Second harmonic-based nonlinear ultrasonics is an emerging nondestructive testing technology that has recently gained attention owing to its high sensitivity for characterizing microstructures. However, this technique operates exclusively in the through-transmission mode and cannot be applied using the pulse-echo (PE) method, which is highly suitable for field applications owing to the phase-inversion effect. To address this limitation, we develop a PE-based third-order nonlinear ultrasonic technique for characterizing the microstructures of additive manufacturing (AM) components. Experiments demonstrate that the PE-based third-harmonic technique is highly sensitive to microstructural changes, including micro-oxide inclusions and grain structures formed during AM, whereas the conventional second-harmonic technique is not. An extensive discussion, supported by numerical simulations, is provided to explain these findings. The developed technique shows potential for in-situ monitoring in various industrial fields, including AM, because it enables single-sided measurements.