High temperature oxidation of NiCrAlY coated Alloy 625 manufactured by selective laser melting

The high temperature oxidation of fully SLM-processed NiCrAlY coated Alloy 625 parts was investigated between 800 degrees C and 1000 degrees C. For comparison, bulk NiCrAlY and bulk Alloy 625 were also fabricated using SLM. Two thin layers of NiCrAlY powder were lasered at the surface of Alloy 625 to form the bi-materials. Bulk NiCrAlY and bi-materials showed an improved oxidation behavior compared to Alloy 625. The oxidation of the NiCrAlY coating results in regions with a dense and continuous external Al2O3 layer and regions composed of a mixture of external Cr2O3 and internal Al2O3. Large EDS maps at the surface and cross-sections of the bi-material highlighted a heterogeneous distribution of constitutive elements of the NiCrAlY coating, resulting in some regions with an Al activity lower than the one required for the formation of a continuous and dense Al2O3 layer (<= 10 atomic percent). Low Al activity and high Al activity regions were related to the topography of the SLMed surface and correspond to hill and valley regions, respectively. In addition, cracks, mainly occurring in high Al activity regions, were observed across NiCrAlY specimens. The fabrication of coated but small components with a brittle coating by SLM is not trivial and needs further investigations.