## 4 Experimental Validation  
The proposed data-driven adaptive controller was implemented in the laser-based DED system and validated experimentally. As listed in Table 1, different materials, geometries, and deposition tool paths were tested to validate that the proposed controller could automatically adjust its parameters and was adaptable to different deposition situations without the necessity to conduct system identification. Specifically, three experiments were conducted: (1) solid semicylinder with profile tool paths in 316 L stainless steel, (2) solid semicylinder without profile tool paths in 316 L stainless steel, and (3) thin-wallpipe with a continuous spiral tool path in LPW-35N nickel alloy (a customized material designed by the authors' organization). Experiments 1 and 2 each comprised three samples. The first sample was fabricated using the constant nominal process parameters listed without the employed control system in Table 2. The second sample was fabricated using a conventional PID controller, where constant PID gains were used. The PID gains were determined by experiment-based system identification and trial-and-error. The third sample was fabricated using the proposed adaptive control method, where PID gains were automatically optimized and updated during the process. These three samples were compared with each other, and the effect of the adaptive controller was analyzed. Experiment 3 was conducted to validate that the proposed adaptive control method was still effective even when the powder material and part geometry were changed (compared to Experiments 1 and 2), while no additional experiment was needed to recalibrate the controller. The MPW data and the corresponding laser voltage output in these three experiments were recorded, and the results are discussed below. Figure 5 shows the results of Experiment 1, comparing the samples of depositing the 30 mm diameter semicylinder structure using 316 L stainless steel. The part's nominal height (\(H_{N}\)) was 9 mm, and the nominal semicylinder diameter (\(D_{N}\)) was 30 mm, as indicated in Figure 5. In Figure 5a, the semicylinder structure was fabricated using the constant laser voltage signal (6.2 V) without control. A laser profiler was used to scan the surface of the part and produce its 3D point cloud using the method introduced in [26; 27]. The scanned surface reconstructed from the point cloud is shown in Figure 5b, where the color bar indicates the distance in the z-direction from the points to the reference plane [26]. The larger distance shown in the graph means the lower dimensional accuracy of the DED-fabricated part. A bulging area at the center of the surface that is significantly higher than the edges can be seen in Figure 5b, which was caused by the unstable laser energy density and hence the uneven heat accumulation in the part. Figure 5c shows the sample deposited using a conventional PID controller with constant PID gains ((\(K_{P}\), \(K_{I}\), \(K_{D}\)) = (0.04, 0.02, 0.00)). The PID gains were tuned based on Ziegler-Nichols criteria [28] via trial-and-error experiments, which was considerably time-consuming and ineffective in terms of manpower and cost. The top surface of the conventional PID controlled sample is shown in Figure 5d. It can be seen that its surface was flatter and the central bulge height was smaller than that in the uncontrolled sample. This is due to the effect of the closed-loop control on stabilizing the heat input and reducing the localized heat accumulation. Figure 5e shows the sample deposited with the proposed adaptive controller, and its surface was \begin{table}
\begin{tabular}{c c c c} \hline
**Experiment Number** & **Powder Material** & **Geometry** & **Deposition Tool Path** \\ \hline
1 & 316 L stainless steel & Solid semicylinder & Zigzag infill with the profile tool path \\
2 & 316 L stainless steel & Solid semicylinder & Zigzag infill without the profile tool path \\
3 & LPW-35N nickel alloy & Thin-walled hollow pipe & Continuous spiral single-bead tool path \\ \hline \end{tabular}
\end{table}