Electrochemical polishing with glycol-based electrolyte of the curved internal channels fabricated via laser powder bed fusion

Abstract

Electrochemical polishing (ECP) is used to process metal additive manufacturing parts with poor surface qualities arising from powder adhesion, spheroidal effect, and step effect during forming. Conventional ECP uses stationary acid-based solutions and very low current densities, which makes it challenging to efficiently polish complex-shaped internal channels. Typically, an oxide layer forms in the conventional aqueous-based electrolyte, which prevents uniform dissolution and consequently deteriorates surface quality. In this study, we proposed a modified ECP method for polishing curved internal channels in laser powder bed fusion (LPBF) 316 L stainless steel (SS) specimens using a high-speed flow of a NaCl–ethylene glycol (EG) solution at low current densities. During electrochemical tests, LPBF 316LSS specimens did not exhibit passivation in the NaCl–EG solution, and the current efficiency was high at low current densities (i < 4 A·cm−2). The reaction mechanism of ECP LPBF 316LSS in the NaCl–EG solution was clarified by observing the experimental phenomena and testing the composition of the reaction products. The ECP experiments showed that the decrease of roughness value is directly related to the charge. A small roughness value can also be obtained at low current densities (i < 2 A·cm−2). Finally, S-shaped curved holes with a diameter of Φ3 mm were efficiently polished using a flexible cathode in the NaCl–EG solution. In the case of an aperture increase rate of only about 7 %, it would only take 400 s to drastically reduce the surface roughness value Sa from 12.73 to 12.94 μm to 2.16– 2.43 μm.