Wire electrochemical trimming the recast layer on René 88DT for aero-engine applications

Abstract

Surface integrity has an important impact on the fatigue life of aero-engine parts, especially the recast layer formed by electrical discharge machining. Wire electrochemical trimming is a method to remove the recast layer (WECT) by utilizing a wire electrode that sweeps along the surface to remove material through an electrochemical reaction, which can ensure the requirements of accuracy and surface integrity simultaneously. An electric-field model was established to investigate current distributions in the WECT machining zone. The results indicated that when the wire electrode sweeps the surface, to accomplish high-precision WECT, it is essential to control three stages including rapid breakdown of the passive film, complete dissolution of the recast layer, and diminishment of overcut of stray current. To this end, the microstructure and polarization behavior of the René 88DT recast layer was first studied and the breakdown behavior of its passive film was analyzed. Results indicate that the crystallite size of recast layer is far less than that of the substrate, and the passive film of recast layer is more difficult to dissolve. Passive film models were established, revealing that the surface of the recast layer adheres to a loose outer passive film (Ni(OH)2) and a dense inner passive film (TiO, TiO2, Cr2O3). The experimental results were then used to derive the critical conditions for the dissolution of the recast layer: when the current density i > 2 A.cm−2, the recast layer starts to dissolve, and when the electric quantity Q = 500 C.cm−2, the recast layer is completely removed. Finally, the preferred parameters were selected by the critical conditions, and the wire electrical discharge machining surface was trimmed to a smooth surface without a recast layer. This paper is expected to provide research ideas for future high-precision electrochemical fabrication of aero-engine applications.