Application of laser ultrasound in the detection of metal microstructure

Laser ultrasonic backscattering is of great significance to nondestructive evaluation of the microstructure of strong-scattering metal materials. However, it is difficult to extract the microstructural backscattering signals because of high-level electrical noises. In this manuscript, the laser ultrasonic backscattering characteristics of metal microstructure are analyzed and studied based on the Empirical Mode Decomposition (EMD). TA2 titanium alloy was heat-treated at 800°C for different times to obtain single-phase titanium alloy specimens with different grain sizes. The laser-ultrasonic waveforms transmitted by the bottom surface of the sample were obtained. The EMD was performed on the backscattering noise signal between two successive pulse echo signals, and the multi-order Intrinsic Mode Function (IMF) was obtained. The correlation between the average spectrum of multiple measurements of each order IMF and the spectrum of the first longitudinal wave pulse echo was analyzed. and the IMF with the largest correlation coefficient was selected as the effective IMF, which was the most relevant to the microstructure. Subsequently, the backscattering levels of each sample were calculated based on the variance analysis of the effective IMF measured at multiple points. The results have shown that the backscattering level is positively correlated with the grain size of the metal structure in a specific frequency range.