Nucleation and growth process of a softened-spark layer during microarc oxidation on a selective laser melted Ti alloy.

A two-step method for preparing composite coatings with different composition on Ti alloys using softened spark microarc oxidation technology was proposed. The nucleation and growth processes of a softened spark layer, as well as the influence of softened sparks on the deposition of anions in electrolytes, were studied. The results show that the process voltage of the softened spark discharge on the selective laser-melted Ti6Al4V alloy was lower than the breakdown voltage of its anodic oxide film. The softened sparks prioritized nucleation at the coating/substrate interface in the initial spark discharge area rather than in the microarc discharge area. On one hand, the softened spark layer grew towards the Ti6Al4V substrate, and on the other hand, the molten oxide generated by the softened sparking was transferred into the external porous layer. The softened sparks generated inside the coating promoted the rutile phase formation and linear growth in the thickness of the softened spark layer. Ca and P are mainly distributed in the external porous layer or at the interface between the softened-spark and external porous layers. Nevertheless, softened sparking had little effect on the initial micro/nanoporous structures of the coatings.