Powder-mixed Electric Discharge Machining (PMEDM) of Inconel 625

Abstract

In recent times, nickel-based super alloys are widely used in aerospace, chemical and marine industries owing to their supreme ability to retain the mechanical properties at elevated temperature in combination with remarkable resistance to corrosion. Some of the properties of these alloys such as low thermal conductivity, strain hardening tendency, chemical affinity and presence of hard and abrasives phases in the microstructure render these materials very difficult-to-cut using conventional machining processes. Therefore, the aim of the current research is set to improve the productivity and surface integrity of machined surface of Inconel 625 (a nickel-based super alloy) by impregnating powder particles such as graphite, aluminum and silicon to kerosene dielectric during electric discharge machining (EDM). Initially, temperature distribution, material removal rate (MRR) and residual stress were predicted through numerical modelling of powder-mixed EDM (PMEDM) process. In the experimental investigation, particle size analysis of the as-received powder particles was carried out to identify the distribution of particles. X-ray diffraction (XRD) analysis of particles indicated the presence of various phases including small amount of impurities. An experimental setup was developed and integrated with the existing EDM system for carrying out PMEDM process. The experiments were planned and conducted by varying five different parameters such as powder concentration, peak current, pulse-on time, duty cycle and gap voltage according to the central composite deign (CCD) of response surface methodology (RSM). Effects of these parameters along with powder concentration were investigated on various EDM characteristics such as material removal rate (MRR), radial overcut (ROC) and surface integrity aspects including surface crack density (SCD), surface roughness (SR), altered layer thickness (ALT), microhardness of surface and sub-surface regions, chemical and metallurgical alterations of the machined surface and residual stress. Results clearly indicated that addition of powder to dielectric has significantly improved MRR and surface integrity compared to pure dielectric. Among the powders used, graphite has resulted in highest MRR, lowest SCD, least ALT, least microhardness of surface and sub-surface regions. Least ROC, lowest surface roughness and least residual stress were obtained using silicon powder. Aluminum performed well in terms of MRR at low concentration range (upto 6 g/l).Therefore, optimal process performance under a given operating condition depends on judicious selection of powder materials, their size, concentration and process parameters.