## 4 Experiment and Analysis  
### Parameters of Dual-Beam Laser Polishing  
The EOS M290 laser metal additive manufacturing system is a direct metal sintering system for the mass production of molds, metal parts, and fast-forming parts. The DMLS direct metal sintering system uses a laser beam to sinter ultrafine metal powder in layers, enabling complex designs such as complex surfaces, bending deep grooves, and three-dimensional channel designs to be realized. Except for the scanning speed, the surface  
Figure 5: Dual-beam laser polishing system: (**a**) schematic diagram of the EOS M290 LPBF system, (**b**) photograph of dual-beam laser polishing system.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Set-Up Laser Parameters** & **CW Laser** & **Pulsed Laser** \\ \hline Power & 600 W & 80 W \\ Wavelength & 1080 nm & 1064 nm \\ Pulse duration & N/A & 1.3 μs \\ Spot diameter & 0.47 mm & 0.32 mm \\ Scanning speed & 800 mm/s & 2000 mm/s \\ Step-over & 0.1 mm & 0.1 mm \\ Scanning route & zigzag & Zigzag-square wave \\ \hline \hline \end{tabular}
\end{table}
Table 3: Parameters of the dual-beam laser polishing.  
metal additive manufacturing method is similar to that used in previous works [44,45], with the MS1 metal additive manufacturing parameters optimized and shown in Table 4.  
The optimized process parameters of powder laser additive manufacturing were as follows: laser power, 300 W; laser scanning speed, 720 mm/s; pulse width, 2.0 ms to 4.0 ms; and scanning interval, 0.1 mm. Three sample parts formed by LPBF and which underwent high-strength additive manufacturing are shown in Figure 6. Figure 6a shows the high-strength steel additive manufacturing plane parts, Figure 6b shows the high-strength steel additive manufacturing small amplitude curved part, and Figure 6c shows the three-dimensional sample with large curvature. As can be seen from Figure 6, under this combination of optimized process parameters, the formed parts by laser additive manufacturing had essentially smooth surfaces and a high formability. Area A', B', C' are scale representative of Parts A, B and C respectively, representing different molding angles.  
Figures 7-9 show the SEM images and 3D profiles of Parts A', Part B', and Part C'. The microstructures of the formed parts were dense, without macroscopic cracks and cavities, and their cell structures were at the submicron level, as can be seen from images taken by a high-power microscope. The additive manufacturing process parameters, properties of the materials, and geometry and shape of the parts all affected their surface morphology after forming. The initial Part A' surfaces had roughness values of 6.198 um to 6.635 um, the initial Part B' surfaces had roughness values of 6.888 um to 6.922 um, and the initial Part C' surfaces had roughness values of 7.128 um to 7.92 um. Therefore, it can see that with different forming structures and angles, the surface roughness is different.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Laser Power** & **Scanning Speed** & **Scanning** & **Powder** & **Energy Density** \\
**(W)** & **(mm/s)** & **Interval (mm)** & **Thickness (\(\mu\)m)** & **(J/mm\({}^{3}\))** \\ \hline
300 & 750 & 0.12 & 55 & 69.4 \\ \hline \hline \end{tabular}
\end{table}
Table 4: MS1 laser additive process parameters and metal laser additive process parameters.  
Figure 6: Three samples of high-intensity MS laser additive manufacturing. **(a)** High-strength steel additive manufacturing plane in Part A, **(b)** high-strength steel additive manufacturing small amplitude curved in Part B, and **(c)** three-dimensional sample with large curvature in Part C.  
Figure 8: _Cont._  
Figure 7: SEM images and 3D profiles of Part A’: (**a,b**) horizontal plane; (**c,d**) vertical surface.