The limited choice of metallic alloys suitable for additive manufacturing restricts the possibilities for new design strategies and improved materials performance in demanding applications. We applied plasma electrolytic oxidation to improve the performance of 3D printed and cast AlSi12 alloy substrates. A silicate alkaline electrolyte and a combined current mode were used. Coatings were characterised by XRD and SEM analyses and microhardness tests. Produced coatings feature microhardness level of 300–600 HV0.2, which is 3–5 times higher than the original alloy, extending the mechanical properties of 3D printed articles. The features in electrochemical behaviour of AlSi alloys with the same elemental composition, but different microstructural state, are explained by means of individual electrochemical responses of aluminium and silicon phases.