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S. Kumari et al.
2 Materials and Methods
Experiments are carried out on electrical discharge machine (ED30 ZNC, Excetek
Technologies Co. Ltd., Taiwan) on Inconel 825 flat plates (50 × 50 × 5) mm
3 .
Chemical constituents of Inconel 825 are as follow (wt%): Ni: 38–46; Fe: 22 (min);
Cr: 19.5–23.5; Cu: 1.2–3; Mo: 2.5–3.5; Ti: 0.6–1.2; Mn: 1.0 (max); C: 0.05 (max); S:
0.03 (max); Al: 0.2 (max); Si: 0.5 (max). Two different electrodes are used: Cu and
Cu–Ni alloy (also called cupronickel alloy with 90% Cu and 10% Ni) having diameter
of 15 mm. Conventional EDM oil is chosen as dielectric media. Cu corresponds to
401 W/m K, and Cu–Ni electrode possesses 40 W/m K thermal conductivity value.
Before experiments, initial masses of workpiece, as well as tool electrodes, are
recorded using a digital balance. The workpiece is fitted on the work table, and
an electrode is fitted with a tool holder. Experiments are performed at varied peak
current (8, 16, 24, 32 and 40 A). The remaining parameters are maintained at
constant values. Constant parameters settings are gap voltage (~230 V), pulse-on time
(~1500 µs), pulse-off time (~500 µs), electrode gap distance (~50 µm) and dielectric circulation flushing pressure (~0.5 bar). Each trial experiment is continued up
to 0.5 mm machined depth, and subsequent machining time is recorded. After every
run, the final masses of the workpiece are measured. By using mass loss criteria, the
material removal rate is computed. After machining, the worn-out electrode (edge) is
viewed through optical microscopy. The surface roughness of the EDMed specimen
is measured using a surface roughness tester (Talysurf). Morphology and topographic
features of EDMed surface are analysed through scanning electron microscopy.
Elemental analysis is carried through energy-dispersive X-ray spectroscopy (EDS).
Microhardness tests are performed in Vickers hardness tester using 25 gf load and
15 s dwell time.
3 Results and Discussion
Figure 1 exhibits morphology of EDMed surface produced by using a Cu electrode
with respect to varied discharge current. Poor morphology, attributed to the machined
surface, is described by cracks, pockmarks, melted material deposition and accumulated debris (in the form of globules). These are formed due to uneven heating and
cooling of the workpiece by dielectric media [17].
During machining, huge heat energy is accumulated within the discharge gap,
which induces thermal stresses (residual stresses having tensile in nature). When
residual stresses exceed the ultimate strength of the work material, micro-cracks
are formed; it may propagate towards the depth of the recast layer. EDMed surface
containing cracks possesses lower fatigue resistance. According to Kang and Kim
[18], residual stresses are responsible for the generation of micro-cracks over
the EDMed surface. During re-solidification of the molten pool of work material
(when discharge is seized, i.e., pulse-off duration), pockmarks are developed due to
S. Kumari et al.
2 Materials and Methods
Experiments are carried out on electrical discharge machine (ED30 ZNC, Excetek
Technologies Co. Ltd., Taiwan) on Inconel 825 flat plates (50 × 50 × 5) mm
3 .
Chemical constituents of Inconel 825 are as follow (wt%): Ni: 38–46; Fe: 22 (min);
Cr: 19.5–23.5; Cu: 1.2–3; Mo: 2.5–3.5; Ti: 0.6–1.2; Mn: 1.0 (max); C: 0.05 (max); S:
0.03 (max); Al: 0.2 (max); Si: 0.5 (max). Two different electrodes are used: Cu and
Cu–Ni alloy (also called cupronickel alloy with 90% Cu and 10% Ni) having diameter
of 15 mm. Conventional EDM oil is chosen as dielectric media. Cu corresponds to
401 W/m K, and Cu–Ni electrode possesses 40 W/m K thermal conductivity value.
Before experiments, initial masses of workpiece, as well as tool electrodes, are
recorded using a digital balance. The workpiece is fitted on the work table, and
an electrode is fitted with a tool holder. Experiments are performed at varied peak
current (8, 16, 24, 32 and 40 A). The remaining parameters are maintained at
constant values. Constant parameters settings are gap voltage (~230 V), pulse-on time
(~1500 µs), pulse-off time (~500 µs), electrode gap distance (~50 µm) and dielectric circulation flushing pressure (~0.5 bar). Each trial experiment is continued up
to 0.5 mm machined depth, and subsequent machining time is recorded. After every
run, the final masses of the workpiece are measured. By using mass loss criteria, the
material removal rate is computed. After machining, the worn-out electrode (edge) is
viewed through optical microscopy. The surface roughness of the EDMed specimen
is measured using a surface roughness tester (Talysurf). Morphology and topographic
features of EDMed surface are analysed through scanning electron microscopy.
Elemental analysis is carried through energy-dispersive X-ray spectroscopy (EDS).
Microhardness tests are performed in Vickers hardness tester using 25 gf load and
15 s dwell time.
3 Results and Discussion
Figure 1 exhibits morphology of EDMed surface produced by using a Cu electrode
with respect to varied discharge current. Poor morphology, attributed to the machined
surface, is described by cracks, pockmarks, melted material deposition and accumulated debris (in the form of globules). These are formed due to uneven heating and
cooling of the workpiece by dielectric media [17].
During machining, huge heat energy is accumulated within the discharge gap,
which induces thermal stresses (residual stresses having tensile in nature). When
residual stresses exceed the ultimate strength of the work material, micro-cracks
are formed; it may propagate towards the depth of the recast layer. EDMed surface
containing cracks possesses lower fatigue resistance. According to Kang and Kim
[18], residual stresses are responsible for the generation of micro-cracks over
the EDMed surface. During re-solidification of the molten pool of work material
(when discharge is seized, i.e., pulse-off duration), pockmarks are developed due to