Nonlinear ultrasonic techniques have the benefit of high sensitivity to micro or early-stage defects. Among the various nonlinear techniques, the newly proposed sideband peak count (SPC) technique investigates defect-induced nonlinearity by counting the spectral sidebands from a broadband ultrasonic response in the frequency domain. In this study, SPC analysis is transformed into the time–frequency plane through synchrosqueezed wavelet transform (SWT) for transient nonstationary ultrasonic signals. The proposed new SPC technique was then adopted for in situ porosity monitoring in directed energy deposition (DED)—a typical metal additive manufacturing process. Porosity is one of the most critical defects in DED and has detrimental effects on the mechanical properties and fatigue performance of products. For in situ porosity monitoring, a fully noncontact ultrasonic measurement was achieved with a laser ultrasonic system, and its detectability was improved by laser polishing. Time and frequency windows were properly selected to suppress the effects of wave characteristic variations on the SPC analysis in the SWT domain. The performance of the proposed technique was verified by monitoring porosity in stainless steel 316L samples manufactured in the DED process. The test results demonstrated that the proposed nonlinear technique is much more sensitive to porosity than conventional linear techniques, and hence, is more suitable for in situ porosity monitoring.

Porosity is one of the most critical defects in additive manufacturing (AM). Although porosity formation is significantly influenced by the melt pool depth (MPD), MPD estimation and control during AM is difficult to realize. In this study, a real-time MPD estimation and control system was developed to reduce porosity formation during metal-directed energy deposition (DED). First, the width, length, height, and profile slope of the melt pool were measured using an infrared camera and a line scanner during the DED process. Thereafter, an artificial neural network (ANN) was trained and adopted to estimate the MPD in real time. A feedback control system, which adjusts the power of the printing laser, was developed to instantaneously minimize the discrepancy between the estimated MPD value and the set MPD value. The effectiveness of the proposed system for porosity reduction was validated by inspecting the printed metal components using X-ray microscopy. The porosity of the components printed with the proposed system was 81% reduced in comparison with the uncontrolled parts.

Directed energy deposition (DED) is an additive manufacturing technique wherein a focused thermal energy source and a coaxial powder delivery system are combined for the fabrication of metallic parts. Although rapid progress has been made in DED, the amount of research performed for in situ quality monitoring during fabrication is limited. Grain size monitoring during DED is particularly important because the grain size is directly related to the mechanical strength and stiffness of the final products. In this study, a layer-by-layer grain size estimation technique using femtosecond laser ultrasonics is developed for in situ monitoring during DED. The proposed technique employs fully noncontact and nondestructive testing for grain size estimation and uses the relationship between the laser-induced ultrasonic waves and the grain size. In addition to the in situ operation of the technique, spatial resolution in the micrometer range was achieved. The developed technique was validated using Ti-6Al-4V specimens fabricated by DED. The results of the quantitative grain sizes measured by the developed method were consistent with those measured through independent metallography conducted after the completion of DED.