Material removal and surface finishing of additively manufactured Ti-6Al-4V coupons by cyclic process of plasma electrolytic polishing

Abstract

A cyclic plasma electrolytic polishing (PEP) process was applied to Ti-6Al-4V coupons fabricated by laser powder bed fusion (LPBF). In the initial stage of each cycle, the anode (i.e., the workpiece) was slowly immersed (~2.5 mm/s) into the electrolyte with the applied voltage (300 V) turned on. By introducing process interruptions between adjacent cycles, the electrolyte temperature was able to be maintained between 75 and 94 °C, which, in turn, reduced electrolyte evaporations during the PEP process. The slow immersion lowered the current density threshold required to form vapor-gaseous envelope (VGE) and to initiate microarc discharge and plasma in the VGE layer surrounding the coupon. The shortened processing time (2 min) in each cycle shifted the PEP operation from a stable to an initiation-transition stage. Material removal (in terms of volume and weight) and surface roughness reduction (from R_a > 10 μm to R_a ≈ 1.0 μm) were strongly correlated with each other, both increasing with the number of PEP cycles. Microstructural and surface chemical analyses provided valuable insights into the plasma electrochemical reaction on the coupon surface, revealing its relationship with material removal and localized strain relaxation as the processing cycles increased.