## 3 Numerical Simulations and Experiment Validation
### Experiment Validation  
To experimentally verify the feasibility and practical applicability of the proposed grating laser ultrasonic method, aluminum alloy specimens fabricated via wire arc additive manufacturing (WAAM) were employed. By adjusting the WAAM process parameters, two types of specimens with distinct internal conditions were obtained, as visually illustrated in Figure 7. One exhibited minimal internal porosity (estimated to be near 0%) and is hereafter referred to as the low-porosity specimen, serving as a dense reference. The other, referred to as the high-porosity specimen, exhibited a visibly greater internal void content, corresponding to an estimated porosity level of approximately 1-2% based on process experience and prior observations. A pulsed laser beam was used to generate SAWs through a microlens array with a spatial period of 0.5 mm, producing a periodic grating pattern on the specimen surface. A laser interferometer was used to capture the SAW signals for subsequent analysis. The excitation and detection points were separated by approximately 2 mm, consistent with the simulation configuration. Importantly, the use of laser-based excitation and interferometric detection enabled a fully non-contact measurement scheme that demonstrates strong potential for integration into real-time or online defect monitoring systems in metal AM. To validate the simulation results and further assess the applicability of SAW signals for evaluating porosity in metal AM components, experiments were carried out on aluminum alloy specimens with relatively low and high porosity levels. Figure 8 presents a side-by-side comparison between the simulated SAW signal for a porosity level of 1.29% (Figure 8a) and the experimentally measured signal obtained from the high-porosity WAAM specimen  
Figure 5: Frequency domain analysis and velocity–porosity correlation: (**a**) SAW frequency spectra for varying porosity levels; (**b**) center frequency and corresponding velocity as function of porosity. Figure 6: (**a**) Comparison between the theoretical SAW velocity–porosity curve and simulated data points; (**b**) correlation between evaluated porosity values derived from SAW velocities and actual porosity values in the numerical models. (Figure 8b). In both cases, distinct reflected waves appear following the main SAW packet in the time-domain response, suggesting a notable interaction between the wave and internal voids. The corresponding frequency-domain signals also display comparable spectral shapes and bandwidths, indicating that the simulation can reasonably reproduce the frequency characteristics of SAW propagation under similar porosity conditions. Figure 9 further presents a comparison between the experimental signals obtained from the low- and high-porosity specimens. In the time domain, the signal from the high-porosity sample shows a more evident reflected wave and a slight decrease in the amplitude of the main wave packet. In the frequency domain, a shift of the spectral peak toward lower frequencies is observed along with a mild increase in the amplitude of the high-frequency components. These trends are consistent with the simulation results and  
Figure 8: Comparison of simulated and experimental SAW signals: (**a**) SAW signals from simulation (porosity = 1.29%); (**b**) SAW signals from experiment (high porosity). Figure 7: AM specimens with different porosity levels: (**a**) low porosity; (**b**) high porosity. align with the theoretical expectation that increased porosity leads to reduced effective wave velocity and greater signal scattering. It should be noted that although the experimental results show agreement with the simulations in terms of waveform evolution and spectral shift, a precise quantitative analysis (i.e., porosity estimation from SAW velocity) is not conducted here. This is primarily due to the lack of reliable material property data (e.g., elastic modulus and density) for the as-built specimens, which can significantly affect their absolute wave velocities.