Experimental Study to Assess the Effect of Electrode Bottom Profiles while Machining Inconel 718 through EDM Process

Abstract

Electrical Discharge machining (EDM) is a non-conventional machining process, in general adopted to produce components with complex profiles that are difficult to be achieved through conventional manufacturing processes. Since long, EDM is widely used in industries to machine ‘difficult to machine materials’ like HCHC steel (tool and die material). INCONEL 718 is one of the alloys that have relatively poor machinability in the conventional machining processes, due to its work-hardening nature, retention of high strength at high temperature (700 k) and low thermal conductivity. For Inconel alloy, EDM is a preferred material removal process due to its advantages like reduced machining stresses, lesser work-hardening effects and lesser metallurgical damage. While analysing the possible improvements in the EDM process, it was observed that the bottom surface profile of the electrode was contributing towards many aspects like Material Removal Rate (MRR), Electrode Wear Rate (EWR), surface roughness and surface integrity. Certainly such process improvements would contribute a lot in the shop-floor in terms of productivity and product-quality, while machining Inconel 718 alloy. To understand the effect of the electrode bottom profile and also its extent of influence on machining Inconel alloy, experimental study was carried out through EDM. Electrodes of different bottom profiles were used and the machined surfaces were analysed in terms of recast layer, surface topology, form tolerance and MRR. Electrodes having Convex, Concave and Flat profile at their bottom surface were chosen for the experimental study; electrodes were made of copper rod of 12mm diameter with convex or concave profile at their bottom with three different radii of curvature namely, 6,8 or10mm. The surface roughness of the machined surface was measured and the nature of recast layer formed was evaluated and characterized using scanning electron microscope (SEM). The observations and results of the above experimental study are discussed and analysed in this paper. Nature of the machined surface obtained using the above mentioned electrodes are compared and discussed. It is concluded that the adverse effects caused due to the erosion of flat profile electrodes on the machined surfaces could be overcome by employing convex profile electrodes; concave profile electrodes almost simulate the condition of eroded flat-profile electrode; convex profile electrodes produce machined surfaces of better quality in terms of higher surface finish, thinner recast-layer and closer geometry, in addition to higher MRR compared to flat profile or concave profile electrodes.