## 4 Conclusions  
In this experiment, LINE scanning, CHESS scanning, and STRIPE scanning strategies combined with a directional angle offset scanning strategy were studied to explore the surface quality, mechanical properties, and microstructure of L-PBF samples with different scanning strategies. Our intention was for the optimal combination method to directionally enhance the mechanical properties of a TC4 titanium alloy with better surface quality. The conclusions are as follows:  
The surface of the L-PBF specimen under the CHESS scanning strategy combination was relatively flat, and the surface roughness was the lowest. The surface roughness of the CHESS&45\({}^{\circ}\) scanning strategy combination was the lowest, reaching 14 \(\upmu\)m. The sample's surface collapsed under the scanning strategy with a single angular offset, and the protruding melt channel was parallel to the angular offset direction. Due to the repeated action of the laser on the surface of the sample, the voids between the melt channels were continuously accumulated and enlarged, resulting in the collapse of the sample's surface. The surface hardness of the samples was generally higher with the LINE scanning strategy and the angle offset of 90\({}^{\circ}\), and the highest hardness combination was LINE&90\({}^{\circ}\), reaching 409 HV, which was much higher than that of the forgings. The overall surface hardness of the samples combined with the CHESS scanning strategy was the lowest because the CHESS scanning strategy could reduce heat accumulation, reduce the temperature gradient, and generate less residual tensile stress and lower surface hardness. The tensile properties of the L-PBF samples were the best under the combination of CHESS scanning strategies. Among them, the tensile properties of the sample were the best under the combination of CHESS&45\({}^{\circ}\): the tensile strength reached 1125 MPa, the yield strength was 912 MPa, and the elongation was 8.2%. The fracture mode of the sample was a ductile fracture, and there were many dimple structures in the fracture. The performance of the CHESS&45\({}^{\circ}\)scanning strategy combined specimen was greater than that of the single scanning strategy formed specimen. Compared with the CHESS scanning strategy, the tensile performance was improved by 12.8%. Compared with the 45\({}^{\circ}\) scanning strategy sample, the tensile performance was improved by 57.3%, which enhanced the tensile properties of the sample to a certain extent. The density of the L-PBF samples was higher under the combination of the CHESS scanning strategy, among which the sample density of the CHESS&0\({}^{\circ}\) combination was the highest, reaching 99.88%. The microstructure of the L-PBF sample mainly included the primary \(\beta\) phase and \(\alpha^{\prime}\) martensite phase. The upper surface of the CHESS scanning strategy combination sample had a clear melt channel, and the distribution of each phase was uniform. A certain number of columnar \(\beta\)-crystals were distributed in the longitudinal section paralleled to the build direction, and the columnar \(\beta\)-crystals of CHESS&45\({}^{\circ}\) were relatively coarse. W.S. : conceptualization, resources, investigation, methodology, formal analysis, writing--original draft, writing--review and editing; J.L. : validation, writing, writing--original draft, reviewing and editing, writing--review and editing; Y.J. : conceptualization, investigation, validation, visualizing, writing--original draft; Y.L. ([PERSON]): conceptualization, investigation, methodology, validation, visualization, investigation, resources; Y.L. ([PERSON]): conceptualization, investigation, methodology, validation, visualization; Y.H. : conceptualization, investigation, methodology, validation, visualization. All authors have read and agreed to the published version of the manuscript. This research was funded by National Natural Science Foundation of China grant number 51975006, 51505006.