Anodic dissolution behavior of the complex microstructure of laser directed energy deposited Alloy 718 during electrolyte jet machining in NaCl-ethylene glycol electrolyte

Recently, post-processing of the laser directed energy deposited difficult-to-cut metallic materials using electrochemical machining has drawn more and more attention from both scientific and industrial aspects. How to effectively avoid selective dissolution of the complex microstructure to improve the surface quality is one of the central issues. In the present work, the microstructure and anodic dissolution behavior of the laser directed energy deposited nickel-based superalloy 718 in NaCl-ethylene glycol electrolyte were investigated. The results show that the complex microstructure comprises γ phase, Nb-segregated γ phase and secondary phases, induced by significant micro-segregation. At low current density, weak secondary passivity occurs due to the minimal amount of water in the NaCl-ethylene glycol solution, leading to the selective dissolution of γ phase and thus generating a rough surface on the micrometer scale. However, the supersaturated salt film formed at high current density can effectively homogenize the dissolution rate of the different phases to obtain a high-quality, smooth surface (Ra 0.53 µm) that is distinctly better than that in NaCl-aqueous electrolyte (Ra 1.88 µm). Furthermore, a suitable nozzle translational speed notably decreases surface roughness by promoting the formation of supersaturated salt film and avoiding stray corrosion. This investigation provides insight for improving the surface quality on the nanoscale through controlling the anodic dissolution behavior of the complex microstructure.