Plasma electrolytic polishing-induced surface chemical and structural evolutions of additively manufactured Ti-6Al-4V coupons

Abstract

Plasma electrolytic polishing (PEP) was performed on Ti-6Al-4V alloy coupons, which were fabricated by laser powder bed fusion (LPBF) followed by heat treatment. X-ray fluorescence (XRF), X-ray photoelectron spectroscopy, and X-ray diffraction were employed to study the surface chemical and crystallographic evolutions associated with material removal and surface smoothening. Unintentional surface contaminations due to post-LPBF heat treatment, resulted in iron and titanium oxynitrides. XRF revealed that chemical etching using Kroll's reagent led to a decrease in the relative content of Al compared to Ti and V. In contrast, the relative content of Al and V, especially the former, increased while that of Ti decreased after the PEP process, with the voltage ranged from 275 to 350 V. Remarkable surface smoothening (up to 59.8 % and 46.6 % reductions in R_a and R_z, respectively) was achieved by the PEP process for 5 min, regardless of the contamination and high initial surface roughness of over 12 (120) μm in R_a (R_z). The surface smoothening mechanism was discussed based on the removal of nonmelted and/or semi-melted particles as well as the oxidation/oxynitridation and dissolution of large surface features. These observations shed new light on PEP for surface finishing of additively manufactured near-net-shape metallic components.