Passivation characteristics and recast layer removal of DD9 nickel-based superalloy in hybrid electro-discharge-chemical machining process

DD9 nickel-based superalloy is an advanced material for aircraft engine turbine blades. Its high-temperature strength and mechanical properties also bring a challenge for micro-hole fabrication, especially with good surface integrity and high efficiency. The hybrid electro-discharge-chemical (EDM-ECM) process combining EDM's high efficiency and ECM's superior surface integrity is a promising solution. This study investigates the anodic passivation behavior and dissolution kinetics of DD9 to achieve recast layer removal and high surface integrity. Through potentiodynamic polarization, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy analyses, the passivation behavior of DD9 in neutral solutions are revealed. NaNO₃ induces an oxide-rich passive film that broadens the passivation range and suppresses the anodic dissolution. The porous hydroxide-rich passive film generated in NaCl is susceptible to breakdown, efficiently removing the recast layer. A NaNO₃-dominated solution mixture with NaCl is selected to achieve a dynamic passivation mechanism, resulting in moderate anodic dissolution of the recast layer and controllable stray corrosion. With the optimal conditions, the fabrication of micro-holes with high-aspect-ratio (>10), a small taper (below 0.8°) and high surface integrity (recast layer removed) is achieved using the hybrid EDM-ECM process, providing a feasible technological path for micro-hole manufacturing on DD9 superalloy.