Experimental Studies on Machinability of Inconel Super Alloy during Electro-Discharge Machining: Emphasis on Surface Integrity and Metallurgical Characteristics of the EDMed Work Surface

Abstract

Inconel alloys are Nickel-Chromium based high temperature super alloys widely applied in
aerospace, marine, nuclear power generation; chemical, petrochemical and process industries.
Execution of traditional machining operations on Inconel super alloy is quite difficult due to
its very low thermal conductivity which increases thermal effects during machining
operations. Inconel often exhibits strong work hardening behavior, high adhesion
characteristics onto the tool face, and thereby alters cutting process parameters to a
remarkable extent. Additionally, Inconel may contain hard abrasive particles and carbides
that create excessive tool wear; and, hence, surface integrity of the end product appears
disappointing. The extent of tool life is substantially reduced. Thus, Inconel super alloys are
included in the category of ‘difficult-to-cut’ materials.
In view of the difficulties faced during conventional machining, non-traditional machining
routes like Electro-Discharge Machining (EDM), Wire Electro-Discharge Machining
(WEDM), micro-machining (micro-electro-discharge drilling) etc. are being attempted for
processing of Inconel in order to achieve desired contour and intricate geometry of the end
product with reasonably good dimensional accuracy. However, low material removal rate and
inferior surface integrity seem to be a challenge.
In this context, the present dissertation has aimed at investigating machining and
machinability aspects of Inconel super alloys (different grades) during electro-discharge
machining. Effects of process control parameters (viz. peak discharge current, pulse-on time,
gap voltage, duty factor, and flushing pressure) on influencing EDM performance in terms of
Material Removal Rate (MRR), Electrode Wear Rate (EWR) and Surface Roughness (SR) of
the EDMed Inconel specimens have been examined. Morphology along with topographical
features of the EDMed Inconel work surface have been studied in view of severity of surface
cracking and extent of white layer depth.
Additionally, X-Ray Diffraction (XRD) analysis has been carried out to study metallurgical
characteristics of the EDMed work surface of Inconel specimens (viz. phases present and
precipitates, extent of grain refinement, crystallite size, and dislocation density etc.) in
comparison with that of ‘as received’ parent material. Results, obtained thereof, have been interpreted with relevance to Energy Dispersive X-ray Spectroscopy (EDS) analysis, residual
stress and micro-indentation hardness test data.
Effort has been made to determine the most appropriate EDM parameters setting to optimize
MRR, EWR, along with Ra (roughness average), relative Surface Crack Density (SCD), as
well as relative White Layer Thickness (WLT) observed onto the EDMed work surface of
Inconel specimens.
Moreover, an attempt has been made to examine the ease of electro-discharge machining on
Inconel work materials using Deep Cryogenically Treated (DCT) tool/workpiece. A unified
attempt has also made to compare surface integrity and metallurgical characteristics of the
EDMed Inconel work surface as compared to the EDMed A2 tool steel (SAE 304SS) as well
as EDMed Titanium alloy (Ti-6Al-4V).