*Article* **The Size E**ff**ect on Forming Quality of Ti–6Al–4V Solid Struts Fabricated via Laser Powder Bed Fusion**

#### **Huixin Liang 1,2, Deqiao Xie 1, Yuyi Mao 3, Jianping Shi 4, Changjiang Wang 5, Lida Shen 1 and Zongjun Tian 1,\***


Received: 4 March 2019; Accepted: 3 April 2019; Published: 6 April 2019

**Abstract:** Laser powder bed fusion (LPBF) is useful for manufacturing complex structures; however, factors affecting the forming quality have not been clearly researched. This study aimed to clarify the influence of geometric characteristic size on the forming quality of solid struts. Ti–6Al–4V struts with a square section on the side length (0.4 to 1.4 mm) were fabricated with different scan speeds. Micro-computed tomography was used to detect the struts' profile error and defect distribution. Scanning electron microscopy and light microscopy were used to characterize the samples' microstructure. Nanoindentation tests were conducted to evaluate the mechanical properties. The experimental results illustrated that geometric characteristic size influenced the struts' physical characteristics by affecting the cooling condition. This size effect became obvious when the geometric characteristic size and the scan speed were both relatively small. The solid struts with smaller geometric characteristic size had more obvious size error. When the geometric characteristic size was smaller than 1 mm, the nanohardness and elastic modulus increased with the increase in scan speed, and decreased with the decline of the geometric characteristic size. Therefore, a relatively high scan speed should be selected for LPBF—the manufacturing of a porous structure, whose struts have small geometric characteristic size.

**Keywords:** selective laser melting; Ti–6Al–4V alloy; metallurgical quality; mechanical properties
