Surface finish of additively manufactured parts using plasma electrolytic polishing

Additive manufacturing (AM) is considered a disruptive or key enabling technology. Polymer based AM using filament extrusion has attracted much attention from customer/maker side, but many industrial applications require parts made in metallic materials and consequently powder based processes. While these AM (SLM, EBM, DMD) processes become more and more reliable and the achievable accuracy shifts towards industrial applicability, achievable surface quality is still insufficient. This process-inherent challenge is based on partial melting and/or agglomeration of powder to the outside of the melt pool and the part, leading to high roughness values in the range of 10 µm ≤ Ra ≤ 30 µm. In combination with AM-specific complex designs (i.e. “complexity for free” approach) and the resulting inaccessibility of many surfaces for e.g. grinding tools, surface finishing to acceptable values of Ra ≤ 2 µm is difficult. The recently developed Plasma electrolytic Polishing (PeP) process is based on a high DC voltage applied between part and an aqueous electrolyte and the following creation of a plasma hull. Here, electrochemical and plasma reactions take place. It does not require any shaped tool and has the capability of achieving surface quality of Ra ≤ 0.02 µm when starting from milled parts. However, due to its current-density based localisation towards micro peaks, it is currently not efficient in removing large waviness. The study analyses the capabilities of PeP on SLM metal AM parts and shows current results and ongoing research. As it is shown, PeP is a suitable process to finish-machine AM parts and contributes to a tight tolerance chain, allowing to push AM of complex metal parts further towards general industrial use