Metal additive manufacturing technology has several advantages such as efficient formation, short processing cycle, and cost effectiveness. Components with complex spatial structures can be produced via additive manufacturing, thereby overcoming the limitations of traditional manufacturing. Therefore, this technology is favored by the automotive, aerospace, and medical equipment industries. However, possible internal defects, such as lack of fusion, cracks, and holes during the forming process of additive manufacturing, limit its promotion and wide application in the industry. Furthermore, the microstructure of the components changes significantly with the variation in the laser power, process approaches, and scanning parameters during the additive manufacturing process. Furthermore, the stability of the phase and characteristic microstructures are affected by the protective gas, while controlling the surface topography. Therefore, the quality control of metal additive manufacturing products, particularly online monitoring, is of great strategic significance. Several approaches of nondestructive evaluation of flaw inspection and material characterization, such as X-ray computed tomography, fluorescent penetrant inspection, and ultrasonic testing, have attracted much interest. Particularly, ultrasonic testing is one of the most commonly used nondestructive methods for detecting internal defects. Compared to traditional ultrasonic nondestructive testing technology, laser ultrasonic nondestructive testing has the advantages of no-contact, high sensitivity, and suitability for harsh environment, which can realize rapid online monitoring.
In this paper, the characteristics of metal additive manufacturing and nondestructive testing on additive manufacturing are briefly introduced, highlighting the fact that applying laser ultrasonic testing on metal additive manufacturing has great strategic significance. Then, two kinds of laser ultrasonic mechanisms are analyzed: thermoelastic mechanism and ablation mechanism. Under the laser ultrasonic simulations and experimentations, the thermoelastic mechanism is chosen without destroying the integrity or performance of the additive manufacturing components. Next, the finite element simulation studies on laser ultrasonic detection are introduced. Based on the finite element method (FEM), the complex models can be processed and the global numerical solution can be obtained by solving heat conduction and thermoelastic equations. Afterward, the principle of laser ultrasonic nondestructive testing and the testing system are introduced, in which common detection methods on laser ultrasonic are listed and briefly explained. Some improving methods on laser ultrasonic testing systems are also discussed. Yan et al. proposed an experimental method of a no-contact all-optical laser ultrasonic detection and built the optical differential detection system using the beam deflection technique, which improved the antinoise ability of the optical path. Finally, the application progress of laser ultrasonic nondestructive testing of metal traditional and additive manufacturing material at domestic and foreign industries is systematically summarized. Moreover, we have analyzed the research progress—both home and abroad—for reference. Studies on laser ultrasonic began earlier abroad. One example is the study by Pierce et al. (1993), who successfully used a pulsed Nd∶YAG laser to excite ultrasonic waves in metal aluminum blocks and increased the laser ultrasonic signals by modulating the frequency of laser source, thus excavating the great prospect of laser ultrasonic in nondestructive testing. In comparison, relevant research in China only started in 2006. For instance, Shen et al. detected rectangular metal aluminum blocks with an artificial surface defect with depth of 0.71 mm and width of 2.00 mm by constructing an optical differential detection system based on the beam deflection method, accurately locating the surface defect position. In addition, laser ultrasonic can be used for detecting the defects and measuring other significant parameters or monitoring other important processes, such as characterizing the elastic modulus, measuring residual stress of additive manufacturing alloy parts, and monitoring the changes in additive manufacturing process like recrystallization.
Several studies have shown the feasibility of laser ultrasonic nondestructive testing on metal additive manufacturing. Given that the effect of additive manufacturing process parameters on quality has been widely studied and reported, developing a link between controllable process parameters and the required process characteristics to support feedforward and feedback control is the best way to achieve the goal of its application in future control systems. However, various challenges still exist. For example, the relationships among the parameters must be identified, including the parameters measured from experiments and those important parameters for monitoring and characterizing but cannot be obtained directly. Furthermore, an algorithm for quickly identifying different defects should be explored, and a corresponding feedback control scheme must be established to improve the quality of the additive manufacturing components. Moreover, the feedback data obtained from the entire online monitoring of the additive manufacturing process are expected to be massive and difficult to deal with. Thus, we should find an algorithm that can efficiently process these data, detect the anomalies in real time, and provide corresponding feedback to the closed-loop detection system, thus allowing us to control the manufacturing engineering of the workpiece in real time and ultimately improve the quality of finished products.
