## 1 Introduction
However, the technology has not attracted attention before, due to significantly high costs of installation and complexity of the data interpretation. With the advent of advanced manufacturing processes, such as metal-additive manufacturing, it is possible to integrate the laser with the printing system, to provide real-time monitoring solution. The LUT method uses a pulsed laser source to generate ultrasound waves on a sample surface. Once the pulsed laser irradiates on the sample surface, a tiny area around the incident point is instantaneously heated by the high-energy pulsed laser [14]. The strain and stress fields induced by thermoelastic mechanism (in a few cases, thermal ablation mechanism) on the sample surface cause fluctuations in the heated zone, generating ultrasonic waves inside the sample [15]. A laser Doppler vibrometer (LDV) can be used to detect these ultrasonic waves as micro-displacements on the sample surface [16]. Compared with conventional UT methods, ultrasonic waves generated by pulsed laser have broader bandwidth and higher energy. Moreover, the non-contact characteristic makes LUT suitable for online quality monitoring in complicated working conditions, such as the SLM and LENS processes [17]. [PERSON] et al. presented a laser ultrasonic method for the analysis of transient Lamb waves in a thin-plate structure, which can directly estimate the thickness of the plate [18]. [PERSON] et al. utilized an electromagnetic acoustic transducer (EMAT) to detect ultrasonic waves in LUT of internal hole defects of aluminum parts, finding that only defects with a diameter larger than 1 mm could be identified [19]. In addition, a numerical model based on the finite element method was developed for crack inspection, using laser-induced ultrasonic waves; cracks with various depths were evaluated by the shadow method [20]. [PERSON] et al. improved the damage evaluation algorithm of UPI technology through the introduction of a wavelet-transformed ultrasonic propagation imaging method [21] and investigated the internal delaminations in wind-turbine blades. Delaminations with length of 20 mm were identified by using the proposed LUT system [22]. [PERSON] et al. utilized a constraint layer on the aluminum sample surface to change the propagation direction and the generation efficiency of the longitudinal wave. Defects with a diameter larger than 0.8 mm could be identified by using the transmission mode, indicating that the generation laser and detection laser were located on opposite sides of the sample [23]. [PERSON] et al. utilized a flexible ultrasonic transducer (FUT) for non-destructive characterization of metallic pipes at high temperature of up to 176 \({}^{\circ}\)C. Compared with normal ultrasound transducers, the FUT had great potential for non-destructive evaluation of structures in high temperature environments [24]. [PERSON] et al. developed a nonlinear ultrasonic testing method for delamination detection of lined anticorrosion pipes [25]. [PERSON] et al. demonstrated a rapid non-contact ultrasonic inspection technique by visualization of Lamb wave propagation for detecting barely visible impact damage (BVID) in carbon fiber reinforced polymer (CFRP) laminates [26]. [PERSON] et al. utilized laser-induced ultrasonic guided wave technology to detect corrosion of the reinforced concrete structures [27]. [PERSON] et al. presented a non-destructive damage detection, visualization, and quantification technique based on laser-generated ultrasonics and state-space predictive models [28]. To date, few works have reported the application of the LUT method in the field of AM. [PERSON] et al. evaluated the use of laser-generated surface waves to detect artificially set-in holes in titanium alloy (Ti-6 Al-4V) samples fabricated by AM; only holes with a diameter larger than 0.725 mm were identified from B-Scan images [29]. [PERSON] et al. proposed a self-established LUT system to evaluate AM parts, with hole defects of 2 mm diameter identified in B-Scan and C-Scan images [30]. [PERSON] et al.