## 4 Experiment and Analysis
### Optimized Parameters of Dual-Beam Laser Polishing  
In the parts created by laser metal additive manufacturing, the surface roughness is not always ideal. A surface roughness (Sa) of less than 5 \(\upmu\)m for laser-selective melting martensitic aging die steel is currently considered to be relatively low level [17; 46; 47; 48]. Both of the laser beams that are used in these processes are flat-topped beams. The power of the infrared continuous laser is strong and it is mainly used for rough polishing. Additionally,  
Figure 8: SEM images and 3D profiles of Part B’: (**a,b**) horizontal plane; (**c,d**) vertical surface. Figure 9: SEM images and 3D profiles of Part C’: (**a,b**) horizontal plane; (**c,d**) vertical surface. the power of the infrared pulse laser is weak and it is mainly used for sub-mirror or mirror polishing after rough polishing. In this process, firstly, the free-form surfaces of the high-strength parts formed by metal additive manufacturing are polished to a roughness of Ra < 4 \(\upmu\)m by an infrared continuous laser. The free-form surface is then polished to Ra < 2 \(\upmu\)m by an infrared pulse laser, to sub-specular or specular degree. Optimizing the laser polishing process parameters to ensure the peaks fill valley is difficult. The tension of the molten pool will be too large or too small, meaning that the peak cannot \"just\" fill the trough, and further reducing the surface roughness is difficult. When using Gaussian laser-beam polishing, the energy density in the center of the spot is too large, directly leading to metal gasification and leaving many pits on the polished surface. Using a flat-topped beam instead of a Gaussian beam ensures that the microscopic crest is only melted and avoids vaporization. Optimized parameters of dual-beam laser polishing are shown in Table 5.  
The polished roughness value relative to the initial roughness value is shown in Figure 10. The graph shows the roughness of the double-laser-beam polished surfaces corresponding to each initial rough ground surface with a double-laser-beam polishing power varying from 200 W to 600 W by a single factor test method experiment. The surface roughness after polishing is not only related to the polishing process parameters, but also to the original roughness. Figure 10 shows that after a double-laser-beam polishing with the optimal parameters (CW 400 W + 60 W, v = 800 mm/s), the surface roughness was significantly reduced to improve the service life of the high-strength MS metal materials from an initial roughness of Sa = 6.198 \(\upmu\)m to Sa = 0.98 \(\upmu\)m and from an initial roughness of Sa = 7.92 \(\upmu\)m to Sa = 1.37 \(\upmu\)m, with a CW laser power of 400 W and a pulsed laser power of 60 W. Scale Point C's error bar with scale is shown in Figure 10, which reveals the difference in the roughness measured by polishing in different regions of three samples of high-intensity MS laser additive manufacturing. Figure 10: Polished roughness related to initial roughness. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Factor Name** & \multicolumn{6}{c}{**Optimized Value or Feature**} \\ \hline Roughness of initial state (nm) & 6198 & 6365 & 6888 & 6922 & 7128 & 7920 \\ Scanning speed (mm/s) & 1000 & 900 & 850 & 800 & 750 & 650 \\ CW power & & & 200 W, 400 W, 600 W & & & \\ Pulsed laser (PW) & & & 60 W & & & \\ Top-hat beam profile & & & Between medium and ideal & & & \\ Polishing parameters & & & Wavelength: 1080 nm; step-over: 0.1 mm & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Optimized parameters of dual-beam laser polishing. Current studies focus on the horizontal and vertical building orientations considering different LPBF modes [49; 50; 51; 52]. In LPBF, we distinguish the surfaces \"horizontal\" and \"vertical\" with respect to the building direction. Figures 11-13 shows that three samples formed LPBF after horizontal polishing.