Enhanced surface finishing of selective laser melted 17-4PH steel by a novel two-step electrolyte plasma polishing process

Electrolyte plasma polishing (EPP) is an efficient and eco-friendly metal polishing technique that forms a discharge-generated gas layer between the component and the polishing liquid, achieving superior surface finishes. EPP has shown particular promise for applications with intricate structures and complex geometries where traditional mechanical finishing is limited. This technique not only reduces surface roughness on visible areas but also levels internal, non-visible surfaces, making it well-suited for powder bed-based printed components that typically exhibit high roughness due to powder residue. For 3D-printed flow channels, reducing non-visible roughness can also lower flow resistance. This study presents a two-stage EPP process for selective laser melted (SLM) 17-4PH steel, where powdery surface adhesions are initially removed, followed by roughness reduction. Surface roughness indicators (Sa and Sz) were improved for both visible and non-visible surfaces. Additionally, gloss coverage, as a quality measure, was assessed using a custom MATLAB-based script due to the lack of suitable measuring instruments for small components. Results demonstrated significant reductions in surface roughness and enhancements in gloss coverage. For the visible surface, the first EPP stage notably lowered Sa, though uneven Sz was observed in some areas. The second stage, with increased electrolyte temperature and processing time, further refined Sz distribution, achieving final values of Sa = 1.28 µm, Sz = 48.21 µm and gloss coverage = 0.929. For the non-visible cavity surface, initial EPP improved boundary roughness, while the second stage extended polishing coverage, reaching Sa = 4.35 µm, Sz = 151.47 µm and gloss coverage = 0.171. • Developed a two-step electrolyte plasma polishing for 3D-printed 17-4PH components. • Surface adhering particles are removed and the finishing effect is improved. • Evolution of surface roughness and gloss coverage of visible and non-visible surfaces. • Designed a gloss coverage calculation method for small-sized components.