In the service process, the key components of various mechanical components such as bearings, gears, and cylindrical inner walls are affected by various factors such as static load, impact, fatigue, corrosion, and radiation. Their surface areas are prone to damage defects such as cracks, delamination, and fractures, which adversely affect the performance and structural integrity of the components. Surface defects can be divided into surface defects, which are directly exposed to air, and subsurface defects, which are buried beneath the surface. In contrast to surface defects, subsurface defects cannot be detected using traditional optical and other methods; therefore, an effective non-destructive testing method is urgently required. In addition, additive manufacturing (AM), an emerging manufacturing technology, has the advantages of integrated molding, a short processing cycle, and environmental protection compared to traditional subtractive manufacturing technology, and has broad development prospects in aerospace, nuclear power, medicine, and other fields. However, because of rapid heating and cooling during AM processing, defects such as bubbles, nonfusion, spheroidization, and cracks inevitably occur inside the module, which is a key factor limiting the further application of metal AM technology. Because of the layer-by-layer stacking characteristics of the AM process, a nondestructive testing method that can be applied to a high-temperature environment with high-electromagnetic interference and effectively detect surface and sub-surface defects can improve the yield rate of additive manufactured components and the economic benefits of additive manufactured production. To address the limitations of traditional optical and other methods in detecting subsurface defects and that of ultrasound in detecting additive manufactured components , this paper proposes a laser ultrasound method to detect additive manufactured samples containing subsurface defects.

A finite element model is first established to simulate the interaction between laser-induced ultrasound and subsurface defects and to illustrate the mechanism of scattering Rayleigh (SR) waves in characterizing defect widths. The propagation properties of surface acoustic waves and the ultrasonic mode transformation characteristics caused by the defects are analyzed. Experimental verification is performed on an AlSi10Mg alloy fabricated through selective laser melting (SLM) with subsurface through defects. Subsequently, a purely optical, completely noncontact laser ultrasonic scanning system is established and used to excite the ultrasonic waves in the AM sample. The wavelengths of the ultrasonic laser surface waves are modulated by adjusting the laser spot. Subsequently, a synthetic aperture focusing technique (SAFT) is constructed using the scattering characteristics between the surface acoustic waves and subsurface defects. Finally, the improved SAFT is used to image the two subsurface rectangular defects (the burial depth of the two defects is 0.4 mm, and the sizes are 0.3 mm and 0.5 mm ) in the additive manufactured sample.

The simulation results [Fig. 2(a)] show that when the surface wave encounters a subsurface defect within the penetration depth, it interacts with the defect to produce an SR wave that propagates in the opposite direction. Figure 2(b) shows the ultrasonic field when the surface wave separates from the defect and continues to propagate backward. Spectral analysis of the laser ultrasonic signals obtained in the experiment (Fig. 6) shows that the wavelength of the excited ultrasonic surface wave is approximately 1.5 mm, and the subsurface defects in this range interact with the laser ultrasonic surface wave. After processing the signals using sliding time window filtering, the final results (Figs. 7?8) show that the size of the defect obtained by SAFT is consistent with its calibrated size, and the detection error is approximately 5%, with a rapid imaging speed realized based on the b scan data. The feasibility of the laser-ultrasound-based SAFT for AM detection and the potential application prospects of laser ultrasonic technology in AM online quality analysis are demonstrated.

Laser ultrasonic testing is a non-contact and highly precise method that can be used to detect complex environments such as high temperature, high pressure, and strong radiation, and has broad application prospects in industrial non-destructive testing. In this study, a nanosecond pulse laser is used to generate ultrasonic surface waves with a certain penetration depth, and the scattered ultrasonic surface waves interacting with subsurface defects are used to image the subsurface defects in additive manufactured samples in combination with the synthetic aperture focusing technique. Theoretical and experimental results show that the delay effect of burial depth on surface wave scattering can be ignored for subsurface defects, SAFT imaging results can be used to accurately locate subsurface defects and judge their size, and the detection system has certain industrial application prospects for the detection of subsurface defects in additive manufactured samples.