Parameter optimization and electrochemical passivation study for recast-layer-free microhole machining on DD9 superalloy in hybrid EDM-ECM process

This study investigates the hybrid electro-discharge and electrochemical machining (EDM-ECM) process for fabricating microholes on DD9 superalloy, focusing on the effects of parameters and electrochemical passivation on recast layer removal. In the EDM stage, discharge capacitance predominantly influences both the material removal rate and recast layer thickness, selecting a high-voltage, low-capacitance configuration (110 V, 80 nF, 6 μs/12 μs) as the optimal parameter combination. Furthermore, the electrochemical passivation kinetics are studied via electrochemical (PdP, EIS) and spectroscopic (XPS) analyses. The DD9 exhibits a wide passivation range in NaNO₃, consistent with its high charge-transfer resistance in EIS tests, confirming that NO₃⁻ promotes an oxide-rich passive film. Cl⁻ in NaCl penetrates the passive barrier, leading to active dissolution that removes the recast layer. XPS analysis reveals the root cause in the chemical composition. The NaNO₃-formed film consists of stable oxides (Cr₂O₃, Al₂O₃, etc.), whereas the NaCl-formed film is porous, even discontinuous. The structural-electrochemical correlation influences the machining outcomes. A NaNO₃-NaCl composite electrolyte can achieve recast-layer-free and high-precision machining, though intergranular corrosion increases surface roughness within an acceptable range. Ultimately, micro-holes with a high aspect ratio (>10), recast-layer-removed, and small taper are fabricated, providing a feasible framework for hole machining on the DD9 superalloy.