Abstract
Ultrasonic Additive Manufacturing (UAM) is a layer-by-layer solid-state fusion process that bonds thin metal foils on top of a base substrate by using ultrasonic vibrations. In this study, longitudinal mode ultrasonic nondestructive evaluation (NDE) was used to assess the bond quality of UAM components. An interfacial spring model was developed to predict wave propagation through layered media, and the results verified by finite element simulation. Two independent interfacial stiffness coefficients are required to model the experimentally observed principal signals, one for the base/build interface (η1) and one for the rest of the layers in the UAM stack (η). An inversion model was proposed to estimate η1 and η from experimental signals. Sensitivity analysis shows that the inversion is robust even with random variations introduced into the simulated signals. With sufficiently large number of layers, it was demonstrated that a Floquet wave homogenization using the inverted interfacial spring stiffness accurately predicts the experimentally observed phase velocities. In-situ layer by layer bond quality inversion results were presented for three components of varying quality. The results of this study pave the way for utilizing ultrasonic NDE to measure the quality of UAM interface layers both in-situ and offline.
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