**Particle Erosion Induced Phase Transformation of Di**ff**erent Matrix Microstructures of Powder Bed Fusion Ti-6Al-4V Alloy Flakes**

#### **Jun-Ren Zhao, Fei-Yi Hung \* and Truan-Sheng Lui**

Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan **\*** Correspondence: fyhung@mail.ncku.edu.tw; Tel.: +886-6-275-7575 (ext. 62950)

Received: 9 June 2019; Accepted: 26 June 2019; Published: 28 June 2019

**Abstract:** In this study, powder bed fusion Ti-6Al-4V alloy flake was subjected to heat treatment at 800 ◦C for 4 h for inducing the complete transformation of the α' phase into the α<sup>+</sup>β phases. An erosion experiment with 450 μm mean particle diameter of Al2O3 particles at a 90◦ impact on both the as- powder bed fusion (PBF) Ti-6Al-4V and the 4-h 800 ◦C heat-treated specimens to clarify the particle erosion-induced phase transformation behavior and its effect on mechanical properties. Particle erosion-induced phase transformation to the α phase was observed on both the as-PBF Ti-6Al-4V and the heat-treated specimens. It brought about a sequential formation from the surface to the bottom: (1) a surface softened zone, (2) a hardened zone, and (3) a hardness stabilization zone. The as-PBF Ti-6Al-4V was positively eroded by erosion particles, decreasing strength and ductility. In the case of the heat-treated specimens, we found decreased strength ye<sup>t</sup> an increased ductility.

**Keywords:** powder bed fusion (PBF); Ti-6Al-4V; erosion; phase transformation; tensile
