Micro-arc oxidation for improving high-temperature oxidation resistance of additively manufacturing Ti₂AlNb

The poor formability and high-temperature oxidation resistance of Ti₂AlNb-based intermetallics (for example, Ti–22Al–25Nb at. %, nominal composition) are considered to be the main obstacles that need to be overcome in critical high-temperature applications. In this study, micro-arc oxidation (MAO) was applied to Ti₂AlNb additively manufactured by selective laser melting (SLM). SLM offered advanced forming capabilities, while MAO enhanced the mechanical and chemical properties of the materials. The MAO coating comprised an amorphous layer, a nanocrystalline oxide layer, and a continuous TiO₂ layer. The multi-layers structure of MAO coating significantly improved the high-temperature oxidation resistance by blocking inward oxygen diffusion. The pulse frequency had significant effects on the microstructure, hardness, and wear resistance of the MAO coatings. Low pulse frequency led to thick MAO coatings with coarse surface roughness, large crater-like pores, and serious surface cracks. High pulse frequency improved the hardness and wear resistance of the MAO coatings. A pulse frequency of 1000 Hz deteriorated the wear resistance of MAO coating caused by the poor metallurgical bonding between the substrate and MAO coating. This study demonstrated the possibility of employing MAO to enhance the critical properties of the Ti₂AlNb-based intermetallics fabricated by SLM.