Experimental investigations into electrochemical machining of high carbon high chromium die steel.
Sekar, T. ; Marappan, R.
Introduction
Electrochemical machining (ECM) has tremendous potential because of
its versatile applications and it is expected that it would be a
promising, successful, and commercially viable machining process in the
modern manufacturing industries. ECM is a non-traditional process used
mainly to cut hard or difficult to cut metals, where the application of
a more traditional process is not convenient. Different from the other
machining processes, in ECM there is no contact between tool and
workpiece. ECM was developed initially to machine the hard alloys,
although any metal can so be machined [1]. Electro chemical reactions
are responsible for the chip removal mechanism [2]. The difficulties to
cut poor machinable materials like high carbon high chromium die steel
by conventional process have been largely responsible for the
development of the ECM process. The main components of ECM system are a
low voltage and high current power supply and an electrolyte circulation
system. The electrolyte is normally solutions of inorganic salts, like
sodium chloride (NaCl) or sodium nitrate (NaN[O.sub.3]) [3]. Machining
performance of ECM mainly depends on feed rate of the tool, applied
voltage, electrolyte discharge rate, current density and inter electrode
gap [4]. The objective of this work is to attempt an experimental study
of the influencing variables that affect the performance of
electrochemical machining of high carbon high chromium die steel with
the hardness of 63 HRC. The material removal rate and surface roughness
were studied. Three parameters were changed during the experiments: feed
rate, applied voltage and electrolyte discharge rate. The inter
electrode gap of 0.1 mm was maintained as constant throughout the
experimentation. Twenty seven experiments were conducted out in the ECM
equipment. The sodium chloride (NaCl) electrolytic aqua solution was
used. The electrochemical machining of high carbon high chromium die
steel with the sodium chloride electrolytic aqua solution presented
better results of material removal rate and surface roughness.
Experimentation
The workpiece material was high carbon high chromium die steel with
hardness of 63 HRC and the chemical composition is shown in table 1.
The high carbon high chromium die steel was chosen because of its
high hardness and low machinablility in conventional processes with high
tool wear. A Design of Experiment (DOE) is used for determining the
relationship between tool feed rate, applied voltage and electrolyte
discharge rate affecting a process and the material removal rate and
surface roughness of that process. Three factors and three level full
factorial (27) experiments were conducted using sodium chloride aqua
solution [5] as electrolyte. The material removal rate and surface
roughness obtained were studied. The actual ECM setup is shown in figure
1. The tool was made of electrolytic copper [6] with 2.45 cm as outer
diameter. The experiments were conducted for three minutes and the
results were recorded. Sartorius electronic weighing machine with 1mg
accuracy and Mitutoyo surface tester with a range of 0-150[micro]m were
used for measuring material removal rate and surface roughness (Ra)
respectively.
[FIGURE 1 OMITTED]
The complete working environment of the experiment is shown in
table 2.
Results and Discussion
The material removal rate and surface roughness of the
electrochemical machining process is primarily influenced by the
controlled variables like feed rate, applied voltage and electrolyte
discharge rate in high carbon high chromium die steel. The effects of
the above influencing parameters have been studied through the
experiments.
The intervening Parametrics on the Material removal rate
The results obtained in the experiments reveal that the effects of
the influencing parameters on material removal rate. This facilitates
analyze of the suitable parametric combinations that can be made for
achieving better material removal rate. Figures. 2-4 show the effect of
feed rate, applied voltage and electrolyte discharge rate on the
material removal rate. From figure. 2, it can be observed that the feed
rate has a significant effect on material removal rate at different
voltages. It is observed that material removal rate increases with the
increase of feed rate for all voltage conditions. The material removal
rate is found to reach a maximum value of 405 [mm.sup.3]/min at a feed
rate of 0.54 mm/min and the applied voltage of 18V conditions.
[FIGURE 2 OMITTED]
Variation of material removal rate with varying voltages is plotted
in figure. 3 for different feed rate conditions. The experimental
results show that the material removal rate increases with the increase
in the applied voltage.
[FIGURE 3 OMITTED]
This obviously indicates that a fixed inter electrode gap, the
increase in the applied voltage causes a greater machining current to be
available in the machining gap causing the enhancement of the material
removal rate. The result of the experiments reveal that the effect of
the applied voltage with higher feed rate condition gives better
material removal rate as compared to that achieved with lower feed rate
condition. Figure. 4 shows the influence of electrolyte discharge rate
for a preset inter electrode gap under various voltage conditions on
material removal rate. The figure demonstrates that the electrolyte
discharge rate increases result decrease in the material removal rate,
the pattern remaining the same for all voltage conditions. The material
removal rate decreases with electrolyte discharge rate because there is
less mobility of the ions from the metal to the solution decreasing the
speed of the chemical reactions.
[FIGURE 4 OMITTED]
The Intervening Parametrics on the Surface Roughness
Fig.5 shows the pattern of results for all the three voltage
conditions. When sodium chloride aqua solution was used, the machined
surface showed a general trend that the surface roughness starts
decreasing with increasing feed rate for all voltage conditions. A feed
rate ranges from 0.32 mm/min to 0.54 mm/min seems to give better surface
roughness as compared to that achieved with lower feed rate condition.
This is because the increase in feed rate at a particular applied
voltage condition causes greater electrolyzing current to be available
in the machining gap, as well as causing a greater current intensity. At
low feed rates, the process of material removal may be instable
resulting in poor surface roughness. The roughness can be affected, if
machining gap is not maintained at the specified value which implies
that the shape of the tool will not be accurately duplicated in the
workpiece.
[FIGURE 5 OMITTED]
Figure. 6 indicates that the significant effect of the applied
voltage on surface roughness is pronounced at the highest applied
voltage.
[FIGURE 6 OMITTED]
This result is due to the higher applied voltage values causing a
greater current intensity during higher feed motion of tool, which leads
to decrease the spike formation on the machined surface. At 18 V
condition, the value of surface roughness decreases with the ratio of 27
% that compared to 15V and 0.54 mm/min conditions. The best surface
roughness has been achieved as Ra. 2.19[micro] under the conditions of
18 V and 0.54 mm/min. Figure 7 shows the influence of electrolyte
discharge rate on surface roughness under varying voltage conditions.
The experimental results reveal that the electrolyte discharge rate
increases here also cause decreases in surface roughness and the pattern
remaining the same for all voltage conditions. Higher order electrolyte
discharge rate minimizes dissolution rate and flushes out the residues
in the machining gap and hence lower surface roughness is obtained.
[FIGURE 7 OMITTED]
Conclusions
Based on the results obtained in this experiment, the following
conclusions are drawn.
The material removal rate increases with the increase of the feed
rate and the applied voltage and decreases with the increase in the
electrolyte discharge rate.
The full factorial experiments used in the present work has proved
its adequacy to be an effective tool for the analysis of the ECM
process.
Machining of High carbon high chromium die steel by using sodium
chloride aqua solution at the tool feed rate of 0.54 mm/min, the applied
voltage of 18 V and the electrolyte discharge rate of 8 lpm gives the
maximum MRR of about 410 [mm.sup.3]/min.
Surface roughness decreases with the increase of the applied
voltage, feed rate and electrolyte discharge rate.
The lowest value of surface roughness Ra 2.19 [micro]m has been
achieved at the feed rate of 0.54 mm/min, the applied voltage of 18V and
the electrolyte discharge rate of 12 lpm.
The NaCl aqua electrolyte solution presented better results of
material removal rate and surface roughness in the machining of high
carbon high chromium die steel in ECM.
References
[1] McGeough, J.A., 1974, Principles of electrochemical machining,
Chapman and Hall, London.
[2] Hewidy, M.S., 2005, "Controlling of metal removal
thickness in ECM process," J. Mat. Process. Tech, pp. 348-353.
[3] El-Dardiry, M.A., Asfoor, M.A., and Osman, H.M., 1984,
"Experimental investigation into the performance of electrochemical
machining processes," Part-II Proc.5th International Conference on
Production Engineering, Tokyo, pp. 382-388.
[4] Joao Cirilo da Silva Neto., Evaldo Malaquias da Silva and
Marcio Bacci da Silva., 2006, "Intervening variables in
electrochemical machining," J. Mat. Process. Tech., pp. 92-96.
[5] Mohan Sen and H.S Shan., 2005, "A review of
electrochemical macro to micro hole drilling processes," Int. Jr.
Mach Tools & Manufact, pp.137-152.
[6] Pandey, P.C., and Shan, H.S., 1980, Modern Machining Processes,
Tata McGraw Hill, New Delhi.
T. Sekar
Faculty of Mechanical Engineering, Advanced Manufacturing
Laboratory Government College of Engineering, Salem - 636011. India.
[email protected]
R. Marappan
Director (Academic), Paavai Institutions, Pachal Namakkal, India.
Table 1: Chemical composition of High carbon high chromium die steel
Element C Cr Ni Si Mn
Wt % 1.8 12.23 0.416 0.492 0.205
Element Co V Mg Mo S P Fe
Wt % 0.192 0.042 0.01 0.01 0.047 0.038 84.42
Table 2: Complete working conditions
Voltage (V) 12,15 &18
Current (A) 0 - 280
Current density (A/[cm.sup.2]) 0-25
Inter electrode gap (mm) 0.1
Feed rate (mm/min) 0.1,0.32 & 0.54
Power supply--DC Continuous
Electrolyte discharge rate (lpm) 8, 10 & 12
Electrolyte type NaCl--aqua solution
Electrolyte concentration 150 g/l
Tool material Copper
Tool outer diameter 2.45 cm
Electrolyte temperature 20-30[degrees]C
Work piece High carbon high chromium die steel
Workpiece hardness 63 HRC
Machining time (min) 3