## 2 Materials and Methods
### Process Parameters  
To achieve the deposition of uniform beads, basic experiments were conducted to determine the optimal welding parameters, aiming for layer thicknesses of 3 mm and thin wall thicknesses of 6 mm, as presented in Table 2. Figure 2 shows a schematic diagram of the WAAM-Rolling process. SS308L was deposited from the 1 st to the 20 th layer and Inconel625 from the 21 st to the 40 th layer. Rolling pressure was applied to all the layers with simultaneous deposition.  
To compare the effects of pressure, various pressure parameters were selected, as shown in Table 3. Figure 3 shows the pressure strategies for each experimental case of SS308L (1-20 th layer) and Inconel625 (21-40 th layer). Four experimental cases were conducted under identical deposition parameters. In the case of Inconel625, only the non-pressure and maximal pressure were applied, as the effect of pressure was insignificant at a low pressure.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Unit** & \
\begin{tabular}{c} **SS 308L** \\ **(1–20 th Layer)** \\ \end{tabular} &
\begin{tabular}{c} **Inconel 625** \\ **(21–40 th Layer)** \\ \end{tabular} \\ \hline Voltage & V & 17.2 & 14.5 \\ Current & A & 115 & 160 \\ Feed rate & m/min & 4.3 & 6.6 \\ Travel speed & mm/s & & 7 \\ Contact Tip to Work & mm & & 15 \\ Distance (CTWD) & Degree & & 90 \\ Torch angle & Second & & 30 s \\ Interlayer offset & mm & & 3 \\ Wire diameter & mm & & 1.2 \\ Shielding gas & - & & Ar 100\% \\ Temperature & \({}^{\circ}\)C & Room temperature \\ Shielding gas flow rate & L/min & & 20 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Deposition parameter.  
Figure 2: Schematic diagram of process.  
The cooling time between layers was kept below 30 s to ensure a high productivity. The distance between the center of the roller and the welding torch was set to 70 mm. Before the process began, all the substrates were cleaned with acetone. During the process, the scale was not removed and the deposition was continuous. Additionally, WAAM was deposited in one direction to reduce the thermal effects.  
To observe the microstructure, the specimens were prepared by polishing with abrasive papers ranging from P400 to P2000 grits, followed by a final polish using 3 \(\upmu\)m and 1 \(\upmu\)m diamond suspensions. For the metallographic analysis, the Inconel625 region was etched by Kalling 2 (5 g of CuCl\({}_{2}\), 100 mL of HCl, and 100 mL of C\({}_{2}\)H\({}_{5}\)OH) for 2 min at room temperature, and the SS308L region was etched by Nital (95 mL of Ethanol and 5 mL of Nitric acid) for 3.5 min at room temperature.  
The cross-sectional microstructures at the interface (IF) region of the WAAM-processed bimetallic structure were analyzed using an optical microscope (Olympus, GX53, Tokyo, Japan) and FE-SEM (Tescan, MIRA I LMH, Brno, Czech) equipped with an energy-dispersive spectroscopy (EDS) instrument. The dissolute ion of the main elements (Fe, Ni, Nb, Mn, Cr, and Mo) within the interface (IF) region was investigated via EDS line scanning and EDS mapping.  
The hardness was measured using a hardness tester (Mitutoyo, HM-200, Kawasaki, Japan) with a load of 1 kgf, applied along the build direction at 5 mm intervals.  
Tensile tests were performed using a tensile testing machine (Zwick, Z250 machine, Ulm, Germany) at room temperature. Sub-sized specimens conforming to the ASTM A370-14 standard were adopted and three samples from each experimental case were tested to determine the average tensile properties.