## 2 Materials and Methods  
The ER4220 aluminum alloy welding wire with a diameter of 1.6 mm manufactured by North East Industrial Materials & Metallurgy Co., Ltd. (NEIMM, Fushun, China) was selected as the raw material, and the substrate was 6061-O aluminum alloy plate with the dimensions of 300 mm \(\times\) 150 mm \(\times\) 10 mm. The chemical compositions of the raw material and substrate are listed in Table 1. The WAAM system shown in Figure 1 was built by CMT advanced 4000 R power supply of Fronius Company and ABB 1410 robot. The WAAM alloys were prepared by the single layer and multichannel forming path. Figure 2 shows the forming process. The length and height of the WAAM alloy were 200 mm and 140-150 mm. During the WAAM process, the interlayer temperature was maintained in the range of 160-180 \({}^{\circ}\)C, and the shielding gas flow at 25 L/min. The process parameters of the additive process are listed in Table 2. The equation \(H_{i}=\eta\big{(}\overline{U}\,\overline{I}\big{)}/TS\) is used to calculate the heat input of each group of experiments, where HI (J/mm) is the heat input of the welding process, \(\overline{U}(V)\) is the average value of each stack voltage, \(\overline{I}(A)\) is the average value of each stack current, \(\eta\) is the thermal efficiency of CMT process set to 0.8 [24], and TS (mm/s) is the welding speed (WS).  
The as-deposition alloys were treated via T6 heat treatment (540 \({}^{\circ}\)C, 12 h, 175 \({}^{\circ}\)C, and 4 h). Three parallel tensile samples and the metallographic sample were intercepted from the WAAM alloy using water jet, band saw, wire cutting, milling machine, grinder, and other tools. The sampling position is presented in Figure 3 and the dimensions of the tensile sample are presented in Figure 4. The thickness and total height of the WAAM alloy was measured using a Vernier caliper with an accuracy of 0.02 mm; the chemical compositions of the raw materials, substrates, and WAAM alloy were analyzed via direct reading spectrometer (FOUNDRY-MASTER Xpert, GER). A universal testing machine (WDW-30, CHN) was used to test the tensile properties of the sample, and a scanning electron microscope (SEM, Quanta FEG 250, USA) was used to observe its microstructure and for the energy spectrum analysis (EDS, USA) of the phase. The metallographic samples were etched with [PERSON]'s reagent (2 mL HF, 6 mL HNO\({}_{3}\), 92 mL H\({}_{2}\)O), and the microstructure was observed under a metallographic microscope (OM, Axio Imager A2m, GER).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{\begin{tabular}{c} **Wire Feeding** \\ **Speed (m/min)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Welding** \\ **Speed (mm/s)** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Current** \\ **\(\overline{\mathbf{I}}(A)\)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Voltage** \\ **\(\overline{\mathbf{U}}(V)\)** \\ \end{tabular} } & \multirow{2}{*}{**HI (J/mm)**} \\ \hline
1\# & 5 & 48.0 & 195 & 22.5 & 73.13 \\
3\# & 5.5 & 35.9 & 215 & 23.0 & 110.19 \\
4\# & 6 & 35.9 & 236 & 23.5 & 123.59 \\
6\# & 5.5 & 24.3 & 215 & 23.0 & 162.80 \\
7\# & 5.5 & 19.8 & 215 & 23.0 & 199.80 \\
8\# & 6 & 19.8 & 236 & 23.5 & 224.08 \\
9\# & 5 & 15.6 & 195 & 22.5 & 225.00 \\
10\# & 5.5 & 15.6 & 215 & 23.0 & 253.59 \\
11\# & 6 & 15.6 & 236 & 23.5 & 284.41 \\
12\# & 5 & 10.4 & 195 & 22.5 & 337.50 \\
13\# & 5.5 & 10.4 & 215 & 23.0 & 380.38 \\
14\# & 5 & 8.2 & 195 & 22.5 & 428.05 \\
15\# & 6.5 & 24.3 & 255 & 24.0 & - \\
17\# & 5.5 & 8.4 & 215 & 23.0 & 470.95 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Process parameters.  
Figure 2: Schematic of WAAM process.