The parametric investigation and microstructural characterization of laser directed energy deposited NiCrAlY powder

Abstract

NiCrAlY as a coating is often used to improve the high temperature strength and oxidation performance of superalloys. Traditionally these coatings are deposited using various thermal spray techniques. Producing these coatings with Additive Manufacturing methods like the Directed Energy Deposition with Laser Beam (DED-LB) process improves the coating-substrate bonding. Subsequently, it enhances the high-temperature capability. To achieve this, understanding the correlation between process parameters and microstructure is crucial. The commercially available NiCrAlY powder is deposited on the alloy 718 substrate in this study. A two level three factor design of experiments was conducted for single track and three-layer track depositions. Statistical analysis was used to understand the effect of the primary process parameters (laser power, scan speed and powder feed rate) on the geometrical characteristics such as height and width of the deposition, and dilution of the substrate. Microstructural studies revealed the transformation in grain morphology from intra-dendritic mode of precipitation to inter-dendritic mode of precipitation with dilution from the substrate. From the Energy Dispersive Spectroscopy analysis, the formation of γ-Ni matrix phase and β-NiAl precipitate phase was confirmed. Samples with low dilution exhibited high hardness in accordance with high β phase content. In the three-layer tracks, the presence of hard and brittle β phase along the dendritic boundaries led to surface cracking due to stress build up.