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S. Kumari et al.
(e.g. excessive tool wear and elevated cost of tooling) due to higher strength, hardness,
toughness and strong work-hardening tendency [1]. In addition, the poor thermal
conductivity of the workpiece generates huge temperature during machining. Such
an extreme cutting temperature is responsible for the formation of built-up-edge
(BUE), which in turn adversely affects the surface integrity of the end product [2]. In
contrast, non-conventional machining methods are recommended to machine these
superalloys.
Amongst different non-conventional methods, electrical discharge machining
(EDM) is one of the promising routes for machining of difficult-to-cut aerospace
alloys like Inconel 825 because EDM can produce complex parts of intricate geometry with good dimensional accuracy and reasonable surface finish. During EDM,
material from the workpiece is removed through melting as well as vaporization
mechanism due to the generation of periodic sparks in between electrode and
workpiece in the presence of a dielectric medium. EDM is advantageous as it can
successfully machine any conductive material irrespective of its hardness with tight
dimensional tolerance. Mostly, EDM is applied for producing micro-drills, dies and
moulds. As compared to conventional machining methods, EDM offers lower material removal efficiency and, thus, consumes high specific cutting energy. For many
years, research is being carried out to understand the mechanisms of the EDM process
in the purview of process responses such as MRR and surface roughness (SR).
These responses are controlled by choosing suitable process parameters such as
peak discharge current, gap voltage, pulse-on time, pulse-off time, polarity and duty
cycle [3]. Apart from aforesaid electrical parameters, non-electrical parameters such
as tool material, type of dielectric media, flushing condition, dielectric circulation,
flushing pressure, etc., influence EDM performance. D’Urso et al. [4] studied the
influence of electrode material (brass and tungsten carbide) during EDM of tungsten
carbide and stainless-steel workpieces on EDM responses, including MRR and TWR.
Li et al. [5] explored the performance of Cu-SiC composite electrodes during EDM
of Inconel 718. Authors experienced that Cu-SiC electrode offered higher material
removal efficiency, better surface finish, and lower tool wear than traditionally used
copper electrode.
Literature is quite a rich focusing dependence of EDM process responses: MRR,
TWR, SR, cylindricity and wear ratio on EDM parameters when machining nickelbased superalloys [6–8]. In addition, aspects of machined surface integrity, including
crater, recast layer (or white layer), and surface cracks developed on EDMed/WireEDMed surface, were extensively studied in [9–14]. Surface cracks are developed
due to the generation of excessive stresses within the workpiece during the EDM
process. On the other hand, the recast layer is obtained because of the re-solidification
of molten work material during pulse-on duration when discharge is seized. The existence of surface cracks as well as the recast layer degrades EDMed surface integrity.
However, the severity of surface cracking and depth of the recast layer can be
controlled by appropriate settings of EDM parameters; but, complete elimination
of these is impossible. EDMed surface containing cracks and recast layer exhibits
degraded fatigue strength. According to Newton et al. [10], the recast layer extends
is with increased energy per spark.
S. Kumari et al.
(e.g. excessive tool wear and elevated cost of tooling) due to higher strength, hardness,
toughness and strong work-hardening tendency [1]. In addition, the poor thermal
conductivity of the workpiece generates huge temperature during machining. Such
an extreme cutting temperature is responsible for the formation of built-up-edge
(BUE), which in turn adversely affects the surface integrity of the end product [2]. In
contrast, non-conventional machining methods are recommended to machine these
superalloys.
Amongst different non-conventional methods, electrical discharge machining
(EDM) is one of the promising routes for machining of difficult-to-cut aerospace
alloys like Inconel 825 because EDM can produce complex parts of intricate geometry with good dimensional accuracy and reasonable surface finish. During EDM,
material from the workpiece is removed through melting as well as vaporization
mechanism due to the generation of periodic sparks in between electrode and
workpiece in the presence of a dielectric medium. EDM is advantageous as it can
successfully machine any conductive material irrespective of its hardness with tight
dimensional tolerance. Mostly, EDM is applied for producing micro-drills, dies and
moulds. As compared to conventional machining methods, EDM offers lower material removal efficiency and, thus, consumes high specific cutting energy. For many
years, research is being carried out to understand the mechanisms of the EDM process
in the purview of process responses such as MRR and surface roughness (SR).
These responses are controlled by choosing suitable process parameters such as
peak discharge current, gap voltage, pulse-on time, pulse-off time, polarity and duty
cycle [3]. Apart from aforesaid electrical parameters, non-electrical parameters such
as tool material, type of dielectric media, flushing condition, dielectric circulation,
flushing pressure, etc., influence EDM performance. D’Urso et al. [4] studied the
influence of electrode material (brass and tungsten carbide) during EDM of tungsten
carbide and stainless-steel workpieces on EDM responses, including MRR and TWR.
Li et al. [5] explored the performance of Cu-SiC composite electrodes during EDM
of Inconel 718. Authors experienced that Cu-SiC electrode offered higher material
removal efficiency, better surface finish, and lower tool wear than traditionally used
copper electrode.
Literature is quite a rich focusing dependence of EDM process responses: MRR,
TWR, SR, cylindricity and wear ratio on EDM parameters when machining nickelbased superalloys [6–8]. In addition, aspects of machined surface integrity, including
crater, recast layer (or white layer), and surface cracks developed on EDMed/WireEDMed surface, were extensively studied in [9–14]. Surface cracks are developed
due to the generation of excessive stresses within the workpiece during the EDM
process. On the other hand, the recast layer is obtained because of the re-solidification
of molten work material during pulse-on duration when discharge is seized. The existence of surface cracks as well as the recast layer degrades EDMed surface integrity.
However, the severity of surface cracking and depth of the recast layer can be
controlled by appropriate settings of EDM parameters; but, complete elimination
of these is impossible. EDMed surface containing cracks and recast layer exhibits
degraded fatigue strength. According to Newton et al. [10], the recast layer extends
is with increased energy per spark.