Simultaneous Compositional and Grain Size Measurements Using Laser Opto-Ultrasonic Dual Detection for Additive Manufacturing

A Review of Non-Destructive Testing (NDT) Techniques for Defect Detection: Application to Fusion Welding and Future Wire Arc Additive Manufacturing Processes

In Wire and Arc Additive Manufacturing (WAAM) and fusion welding, various defects such as porosity, cracks, deformation and lack of fusion can occur during the fabrication process. These have a strong impact on the mechanical properties and can also lead to failure of the manufactured parts during service. These defects can be recognized using non-destructive testing (NDT) methods so that the examined workpiece is not harmed. This paper provides a comprehensive overview of various NDT techniques for WAAM and fusion welding, including laser-ultrasonic, acoustic emission with an airborne optical microphone, optical emission spectroscopy, laser-induced breakdown spectroscopy, laser opto-ultrasonic dual detection, thermography and also in-process defect detection via weld current monitoring with an oscilloscope. In addition, the novel research conducted, its operating principle and the equipment required to perform these techniques are presented. The minimum defect size that can be identified via NDT methods has been obtained from previous academic research or from tests carried out by companies. The use of these techniques in WAAM and fusion welding applications makes it possible to detect defects and to take a step towards the production of high-quality final components.

Investigation of the mechanism and influence of laser wavelength and energy on laser opto-ultrasonic dual detection

Laser opto-ultrasonic dual (LOUD) detection, which uses laser irradiation of samples to generate spectral and ultrasonic signals simultaneously, can perform multimodal detection of element composition and structural property. As such, it has been applied to the detection of additive manufacturing (AM) components. Further, optimized parameters lead to better detection results. To the best of our knowledge, however, there is no study on the effect of laser properties on LOUD detection. Therefore, we studied the mechanism and influence of laser wavelength and energy on LOUD detection. In this work, the intensity, signal-to-noise ratio (SNR), and stability evolution of the laser excitation spectrum and ultrasonic signals at different wavelengths and energies were analyzed. It was found in the plasma evolution that high electron number density means a large amount of ablated mass generated, which was favorable for laser ultrasonic excitation and can produce higher SNR and a more stable signal. However, it also led to more atoms of the ground-state, which resulted in the self-absorption effect and reduced spectrum intensity in the spectrum analysis. Therefore, with self-absorption correction, better stability, and higher signal intensity, an SNR of spectral and ultrasonic signals can be obtained using 355 nm laser excitation at optimal energy. As a result, in the quantitative analysis of Cu and Si elements by LOUD detection, the determination coefficients (R²) were higher than 0.995, and the average relative errors were less than 2.5%, the limit of detection could reach the order of 100 ppm. Further, the defect size of 0.55 mm in the wire +arc additive manufacturing sample was detected by LOUD detection, and the average relative error was 5.59% compared with the digital radiography results, which indicate that laser wavelength and laser energy affect the intensity and stability of spectral and ultrasonic signals in LOUD detection, which means selecting appropriate laser parameters is important to obtain a high precision detection.