Focused on the crack defects commonly encountered during the manufacturing process of laser additive manufacturing (LAM), this paper introduces a laser ultrasonic imaging technique specifically tailored for these defects. This method relies on cross-correlation analysis for precise detection. Utilizing the selective laser melting (SLM) technology as a case study for printing AlSi10Mg alloy, the surface of the specimen was meticulously scanned using laser ultrasound, allowing us to capture the ultrasonic surface wave signal. Subsequently, the crack defect imaging is achieved through a cross-correlation analysis of adjacent surface waveforms. To address the challenges posed by background noise and sample surface roughness, an adaptive threshold method is introduced. This innovative approach effectively reduces noise in the defect imaging results, thereby significantly enhancing the imaging accuracy of crack defects. The findings reveal that the cross-correlation and adaptive threshold-based defect imaging method not only sharpens the boundary features of defects but also significantly diminishes the shadow area during crack defect detection. This offers a more efficient and convenient approach for defect detection and recognition, paving the way for improved quality control in LAM processes.
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.
Laser-EMAT (electromagnetic acoustic transducer) technology has the advantages of laser induced ultrasound and
EMAT simultaneously. This paper introduced a novel simulation about laser-EMAT testing the surface crack with
various depth in the aluminum block. A finite element model of laser-EMAT detection was set up for investigating the
influence of groove-type crack on ultrasonic propagation in the way of numerical simulation. Then, the response curve of
voltage in time domain was obtained by the testing coil in EMAT, which is proposed to determine the depth of crack
above. Good coupling could be found between voltage of signal received by EMAT coil and amplitude of ultrasound
generated by the laser. In addition, the snapshot of ultrasound field at different time demonstrates mode conversion
occurs when the surface wave propagated through the crack. The simulation results show the relative error of
determining crack depth by the proposed method is less than 6.5%.
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