## 1 Introduction  
Additive manufacturing (AM) is a type of technology in which an entity part is manufactured by means of a gradual accumulation of materials. Information is obtained via computer-aided design (CAD), and the part is built in a layer-by-layer style. In recent years, AM processes such as selective laser melting (SLM) and laser-engineered net shaping (LENS) have gained increasing attention as a type of AM technology. The AM process offers many advantages over conventional machining technology, such as the diversity of materials available and no limitation on the structural complexity of modeling part [1]. Despite these advantages, material discontinuities can be caused due to the complex environment of the AM process. The most common material discontinuities are voids and pores situated in the bulk of the fused material. Such defects seriously affect the mechanical properties of the workpiece, limiting the development and application of the AM process [2]. Therefore, there is an urgent need for non-destructive quality evaluation technology for the process control of AM. At present, the most commonly used technologies for the characterization of internal defects in AM part are X-ray computed tomography (XCT) and ultrasonic testing (UT) [3]. XCT is a test technology that provides information on the spatial distribution of X-ray absorption in analyzed structures. The sample can be reconstructed in three dimensions, and its internal porosities can be assessed [4]. [PERSON] et al. utilized XCT to detect the discontinuity and porosity of 316 L stainless-steel parts produced by AM, showing that the porosity of AM samples ranged from 0.15% to 0.39% and that the highest resolution of the defects was 70 \(\upmu\)m, depending upon the voxel size of the XCT machine [5]. [PERSON] et al. used ultrahigh-speed synchrotron X-ray imaging to quantify the phenomenon of vapor depression during the laser melting of metals. The evolution of melt pool and vapor depression under stationary laser illumination were well observed [6]. UT uses the signals reflected from defects, to characterize the internal structure of the sample. [PERSON] et al. accessed several features of UT such as the sound velocity and its variation due to microstructural parameters, using a well-designed AM part. The minimum internal defect that can be detected is of 1 mm thickness [7]. [PERSON] et al. set an ultrasonic transducer at the bottom side of the building platform. Different from the traditional methods, the transducer fixed under the bottom side of the platform can avoid the interference from the complex environment of building chamber. Therefore, it can be utilized for online monitoring. By plotting A-Scan signals after each layer-wise melting, discontinuities in the interfaces between adjacent layers can be identified but not quantitatively evaluated [8]. [PERSON] et al. used high-frequency ultrasonic signals to investigate AM parts. It was found that ultrasonic parameters such as signal amplitude and velocity were very sensitive, with small physical changes leading to drastic changes in these parameters [9]. [PERSON] et al. used both XCT and UT methods to evaluate the porosity of titanium samples fabricated by gas tungsten arc welding (GTAW), finding that a change in process parameter, namely wire feed, significantly affected the porosity of the sample fabricated by GTAW [10]. However, owing to the relatively large size of XCT equipment and the coupling agent needed for UT, their application has been limited [2]. The laser ultrasonic testing (LUT) technique has been around for about 20 years, which can be applied to the non-contact and non-destructive inspection in many areas, including nondestructive inspection, quality assurance testing, and structural health monitoring [11; 12]. The performance of LUT technique has been constantly improved in recent years by advances in material sciences and fabrication technology [13].