**5. Conclusions**

In this study, taking the geometric characteristic size into consideration, samples consisting of solid struts with different section characteristic sizes (0.4 to 1.4 mm) were fabricated by selective laser melting with three scan speeds (700, 1300, and 1900 mm/s). The morphology features, relative density, microstructure, and mechanical properties were measured and a simple thermal model was presented to systematically explain the effect of geometric characteristic size under different scan speed conditions. The following was determined:

(1) The geometric characteristic size influenced the physical characteristics of the LPBF-produced struts by affecting the cooling condition. A small geometric characteristic size resulted in a poor cooling condition. The effect may only become obvious when the geometric characteristic size is small (<1 mm).

(2) The relative density of the solid strut with an *L*s value of 0.4 mm reached the highest value of 96.8% when the *v* was 1900 mm/s, but the relative density of the solid strut with an *L*s value of 1.4 mm reached the highest value of 98.1% when the *v* was 1300 mm/s. The solid strut with smaller geometric characteristic size had a more obvious size error. To a certain degree, the nanohardness and elastic modulus increased with increasing scan speed, and decreased with the decline of the geometric characteristic size when the geometric characteristic size was smaller than 1 mm.

(3) In contrast to the solid bulk forming process, for a superior forming quality, a higher scan speed should be set for the LPBF-manufacturing of porous structures, especially for structures with small geometric characteristic size in solid struts.

**Author Contributions:** Experiment design and writing, H.L. and D.X.; data analysis of μCT, Y.M.; data analysis of microstructure and nanoindentation, J.S. and C.W.; writing—review and editing, L.S. and Z.T.

**Funding:** This research was funded by the Advanced Research Project of Army Equipment Development (301020803), the Key Research and Development Program of Jiangsu (BE 2015161), the Young Scientists Fund of the National Natural Science Foundation of China (51605473), the Jiangsu Provincial Research Foundation for Basic Research, China (BK 20161476), the Science and Technology Planning Project of Jiangsu Province of China (BE 2015029) and the Science and Technology Support Program of Jiangsu (BE 2014009-1, BE 2014009-2, BE 2016010-3).

**Conflicts of Interest:** The authors declare no conflicts of interest.
