## 4 Experiment and Analysis
### Optimized Parameters of Dual-Beam Laser Polishing
Figure 11a shows a flat polished sample, Figure 11b shows a freeform surface of a high-strength metal additive formed in Part B', and Figure 11c shows a freeform surface of a high-strength metal additive formed in Part C'. We used a continuous laser and a pulsed laser for multi-step polishing, which remarkably reduced the surface roughness. Additionally, the surface morphology was tested with confocal microscopy  
Figure 12: Polished specimen B’ and surface morphology of area B’. (**a**) SEM images of Part B’. (**b**) Polished specimen B’; (**c**) 3D profiles of B’. Figure 13: Polished specimen C’ and surface morphology of area C’. (**a**) SEM images of Part C’; (**b**) Polished specimen C’; (**c**) 3D profiles of C’. The lateral upper surface hardness value was higher than the original hardness value, indicating that the influence range on the surface roughness after polishing with a double laser beam was 1-2 mm; thus, the longitudinal surface hardness value was smaller. After double-laser-beam polishing metal additive manufacturing, the grain size of the material subject decreased and the lattice arrangement became neater. The surface structure of the MS metal additive steel was strengthened and improved by double-laser-beam polishing. The surface roughness in the transverse direction was obviously higher than the original hardness, indicating that the surface hardness of the laser-polished high-strength maraging steel metal material increased. The influence of the surface roughness in the longitudinal direction was 5 mm, indicating that the mechanical properties of the MS steel polished with a double laser beam had little influence in the longitudinal direction. In the laser additive manufacturing process, the surface morphology of the parts is restricted by the process parameters used, creating unique morphology characteristics. The cross-section of the LPBF outer surface is similar to those reported in the literature [53]. The hardness test results and a comparison of laser-polished MS additive parts are shown in Figure 14. Figure 14a shows a diagram of a polishing hardness test and Figure 14b shows the hardness of the as-fabricated and MS products polished by LPBF. The hardness in the horizontal direction was slightly higher than that in the vertical direction, at about 35 HRC. The horizontal cross-section hardness after polishing was much harder than the original surface, whereas the vertical cross-section hardness, with a decreased maximal surface hardness and high residual tensile stresses from 34 HRC up to about 53 HRC, can be introduced with double-laser-beam polishing. Additionally, measurement errors are noted in Figure 14b. SEM images of the original formed surface and the polished layer are shown in Figure 15. Figure 15a shows the original formed surface and the polished layer. On higher magnification, the original formed surface of LPBF reveals distinctive segregation patterns in Figure 15b,c, which shows the melt zone and the heat-affected zone, the area where the microstructure and properties of the polishing layer change significantly during laser polishing. The austenite grains grow, from martensite to austenite during this peak temperature. In the top region of the polished layer, the grain size further decreases for the high cooling rate, as shown in Figure 15d on higher magnification, which shows the polished layer grain transformation. After the double-laser-beam polishing process,  
Figure 14: Hardness test and comparison of laser-polished MS additive parts. (**a**) diagram of polishing hardness test; (**b**) hardness of the as-fabricated and polished MS parts. different grain refinement structures and a re-oriented texture are presented. In addition, it also led to a considerable change in microhardness, for the transformation of martensite to austenite, grain size, morphology, and composition.