Morphological evolution and anodic electrochemical behaviors of Ni-based single crystal superalloy and its recast layer in NaNO3 electrolyte

Abstract

Ni-based single crystal superalloy is favored for turbine blades. In order to adapt to the high temperature of the working environment, film cooling technology is widely used. The film cooling holes of turbine blades require no recast layer residuals and high machining efficiency. Combined or hybrid methods of electrical discharge machining (EDM) and electrochemical machining (ECM) show prospective applications. Here, the recast layer generated by discharge during high-speed drilling is removed by electrochemical dissolution. But the dissolution behavior of it is not investigated. In this paper, The anodic electrochemical behavior of a Ni-based single crystal superalloy and its recast layer in NaNO3 electrolyte is reported, and the surface morphology change of electrochemical dissolution is elucidated. The results showed that the γ' phase disappeared in the recast layer after EDM. Its oxide content within the passivation film is significantly elevated, exhibiting a stronger corrosion resistance than the matrix. For the material removal characteristics, the crater on the recast layer surface preferentially undergoes corrosion, adopting an alveolate structure, with the bulges later dissolved or removed by shedding due to oxide enrichment. For the matrix, the γ phase undergoes electrochemical dissolution, leaving a block γ' phase. A qualitative electrochemical dissolution model is established.