Experimental studies on graphite powder-mixed electro-discharge machining of Inconel 718 super alloys: Comparison with conventional electro-discharge machining

Inconel 718 is a nickel-based super alloy widely applied in aerospace, automotive, and defense industries. Low thermal conductivity, extreme high temperature strength, strong work-hardening tendency make the alloy difficult-to-cut. In contrast to traditional machining, nonconventional route like electro-discharge machining is relatively more advantageous to machine this alloy. However, low thermal conductivity of Inconel 718 restricts electro-discharge machining from performing well. In order to improve the electro-discharge machining performance of Inconel 718, powder-mixed electro-discharge machining was reported in this paper. It was carried out by adding graphite powder to the dielectric media in consideration with varied peak discharge current. The morphology and topographical features of the machined surface including surface roughness, crack density, white layer thickness, metallurgical aspects (phase transformation, crystallite size, microstrain, and dislocation density), material migration, residual stress, microindentation hardness, etc. were studied and compared with that of the conventional electro-discharge machining. Additionally, effects of peak discharge current were discussed on influencing different performance measures of powder-mixed electro-discharge machining. Material removal efficiency and tool wear rate were also examined. Use of graphite powder-mixed electro-discharge machining was found to be better in performance for improved material removal rate, superior surface finish, reduced tool wear rate, and reduced intensity as well as severity of surface cracking. Lesser extent of carbon migration onto the machined surface as observed in powder-mixed electro-discharge machining in turn reduced the formation of hard carbide layers. As compared to the conventional electro-discharge machining, graphite powder-mixed electro-discharge machining exhibited relatively less microhardness and residual stress at the machined surface.