Spatial evolution of broadband Rayleigh waves indicative of material state

Abstract

Laser ultrasound is well suited to monitor metal additive manufacturing processes. Pulse laser-generated Rayleigh waveforms evolve with propagation distance due to material nonlinearity, making them a powerful tool for nondestructive evaluation, particularly for assessing microstructure. Unlike narrow-band Rayleigh waves, where the relative acoustic nonlinearity parameter is commonly used to evaluate material degradation, a pulse laser generates broadband unsymmetrical V-shaped waveforms whose spatial evolution we have characterized by a steepness parameter. Thermal aging precipitates multiple phases (including ${\gamma }^{\prime }$ and ${\gamma }^{\prime \prime }$) in Inconel718 samples that we documented by X-ray diffraction. These precipitates are associated with increased material nonlinearity. Comparing waveform spatial evolution, through changes in steepness, in samples before and after thermal aging revealed significant sensitivity to the material state. Thus, the technique has strong potential to provide unique insight into a material’s microstructure and the mechanical properties dictated by that microstructure.