**4. Conclusions**

A new α phase was generated by the as-PBF Ti-6Al-4V specimen and the heat-treated specimen after the erosion. The heat affected the composite effect of Al2O3 particle erosion, and the erosion surface adopted a serial formation: softened zone, hardened zone, and hardness stabilization zone. The ratio of the new α phase induced by particle erosion in ASE became higher than that in HTE, resulting in a difference of hardness between the softened and the hardened zones.

After the PBF Ti-6Al-4V was subjected to a heat treatment at 800 ◦C for 4 h, the phase transformation α' α + β resulted in a decrease of the strength. The mechanical properties of both the as-PBF Ti-6Al-4V and the heat-treated specimens were affected by particle erosion-induced phase transformation. The strength and the ductility of the ASE test specimens became lower than those of AS, and titanium–aluminum phase particles were observed in the softened zone. There were no titanium–aluminum phase particles found in HTE. Compared with the HT test specimen, the strength decreased, but the ductility increased.

**Author Contributions:** Methodology, J.-R.Z.; investigation, J.-R.Z.; data curation, J.-R.Z.; writing—original draft preparation, J.-R.Z.; writing—review and editing, F.-Y.H. and T.-S.L.; supervision, F.-Y.H. and T.-S.L.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors are grateful to The Instrument Center of National Cheng Kung University and the Ministry of Science and Technology of Taiwan (Grant No. MOST 107-2221-E-006-012-MY2) for their financial support for this research.

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