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5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,
IIT Guwahati, Assam, India
823-1
CHEMO-ULTRASONIC ASSISTED MAGNETIC ABRASIVE
FINISHING: EXPERIMENTAL INVESTIGATIONS
NITESH SIHAG1, PRATEEK KALA2, PULAK M PANDEY3*
1M.Tech Student, IIT Delhi, 110016, E-Mail:[email protected] 2Research scholar, IIT Delhi, 110016,E-mail:[email protected]
3*Associate Professor, IIT Delhi, 110016, E-mail:[email protected]
Abstract
Chemo-Ultrasonic Assisted Magnetic Abrasive Finishing (CUMAF) is a compound finishing process, which
integrates the use of CMP(Chemo-Mechanical Polishing), ultrasonic vibrations and MAF(Magnetic Abrasive
Finishing), to finish surfaces up to nanometer order within a short span of time.The present work is focused on
designing and fabrication of experimental set up to perform CUMAF. Using this set up experiments have been
conducted on copper alloy work piece and the effects of various process parameters like percentage weight of
abrasive, oxidizing agent concentration, rotational speed of magnet, working gap and pulse on time of ultrasonic
vibration on process response namely percentage change in average surface roughness value (%∆Ra) was
recorded. The experiments were planned using response surface methodology. Experimental data was analyzed
using analysis of variance to understand contribution of various process factors and interactions on process
response. Regression model was developed to predict the percentage change in surface roughness in terms of
significant process factors and interactions. Further the developed model was validated and was optimized
using genetic algorithm to maximize the performance of the developed process.
1 Introduction
Surface finish of a product is an important
parameter as it directly influences the product life and
enhances its functionality. In the present scenario with
advancement of technology there is need of various
advanced engineering materials like tungsten,
titanium alloys, ceramics, composites etc. These
materials possess some special characteristics such as
high hardness, high wear resistance, high toughness,
high strength etc. which make these preferred over
conventional materials in modern industries. Due to
the stringent properties these materials are difficult to
process. Different conventional finishing processes
like grinding, lapping, honing, buffing etc. are
generally inefficient in finishing these materials.
Although processes like abrasive flow machining,
magnetic field assisted finishing processes and
chemo-mechanical finishing may be used but these
may be less productive. Therefore a new process
which uses combination of chemical oxidation and
magnetic field assisted abrasion (magnetic abrasive
finishing) has been conceived in the present work for
faster processing.
MAF is a process in which material from work
piece surface is removed in form of microchips by
mixture of ferromagnetic and abrasive particles in the
presence of magnetic field (Jain, 2009). The gap
between upper magnet and work piece is filled with
ferromagnetic and abrasive particles. These particles
are attracted towards each other along magnetic lines
of force and form a flexible magnetic abrasive brush
(FMAB) which acts as a multi-point cutting tool. As
the brush rotates the abrasive particles remove
microchips from the workpiece surface resulting in
finishing of work piece. MAF has a high potential to
be used widely by the industries as it can produce a
mirror like finished surface of high quality for various
simple and complex surface forms. Also dressing of
tool is not required in MAF. However, it is less
efficient on hard materials. Many researchers worked
to improve the conventional MAF process and
attempted to combine other conventional processes to
overcome the limitation. Some of the important
literature in the area is discussed below.
Yin and Shinmura (2004) applied three modes of
vibration during MAF on a stainless steel (SS 304)
sample. By experimental results they reported that
providing vibration along the direction of feed and
perpendicular to the feed together resulted in
improved surface finish with reduced finishing time.
Mulik and Pandey (2010) applied ultrasonic
vibration during MAF on a high carbon anti friction
bearing steel (AISI 52100) sample. With the help of
experimental results they reported that with
application of ultrasonic vibration during MAF
tangential cutting force increased, which helpedin
obtaining better surface finish.
Kala et al. (2013) performed experiments with
ultrasonic assisted magnetic abrasive polishing to
polish Copper alloy and analysed the results. They
reported that addition of ultrasonic vibrations with
MAF resulted in significant increase in surface finish
of paramagnetic work piece.
Li et al. (2012) incorporated ultrasonic vibrations
into fixed abrasive pellets to enhance MRR and to
improve surface finish for fused silica polishing. He
CHEMO-ULTRASONIC ASSISTED MAGNETIC ABRASIVE FINISHING: EXPERIMENTAL INVESTIGATIONS
823-2
designed a prototype ultrasonic vibrator for his
experiments.
Yan et al. (2003)incorporated electrolytic process
with MAF to produce a passive oxide film on the
work piece surface which is easier to remove as
compared to parent metal. They observed that at
higher electrolytic current the finishing characteristics
were improved as compared to MAF.Increasing both
the electrolytic current and work piece RPM increased
finishing efficiency, and the surface roughness
improved rapidly.
El-Taweel(2008) integrated electro-chemical
turning (ECT) process with MAF to finish 6061
Al/Al2O3 (10% wt) composite. It was reported that
addition of ECT with MAF resulted in 33%
improvement in surface quality and machining
efficiency is increased by 147.6% as compared to that
was achieved with traditional MAF.Kim and Choi
(1997) analysed the finishing characteristic of a Cr-
coated roller with newly developed magnetic
electrolytic abrasive polishing (MEAP) system. They
showed that the finishing efficiency was increased by
addition of magnetic field.
Nanz and Camilletti(1995) compared different
CMP models to understand and investigate
assumptions of the models. They reported that both
primary removal mechanisms(mechanical and
chemical) will depend on the passive removable layer
and hence accurate modeling of the process is very
crucial to understand the process. Wrschka et al.
(2000) evaluated performance of two alumina based
slurries and two silica based slurries during CMP of
copper sample. Finished copper samples were
analysed using scanning electron microscopy. They
concluded that low etch rates of the slurry chemistry
is favourable for better removal rates.
Literature review discussed above showedthat
previous attempts were made to improve the
performance of MAF by incorporating ultrasonic
vibrations into it. Also some researchers have applied
electrolyte to the machining zone. But while using
electrolyte work piece must be electrically conducting
for electrolysis. UAMAF process may also be less
productive for non ferromagnetic hard materials due
to its low magnetic flux density. Also in the literature
discussed above, researchers have used ferromagnetic
work piece which produced high finishing forces for
effective finishing due to its high permeability. But
finishing non ferromagnetic material using MAF or
UAMAF may take long time to obtain the desired
results. Therefore it was planned to combine Chemo
mechanical polishing (CMP), ultrasonic vibration and
MAF. The new hybrid process is named as Chemo-
Ultrasonic Assisted Magnetic Abrasive Finishing
(CUAMAF).
The objective of the present study is to design and
fabricate a set up to conduct experiments on Cu-alloy
work piece using CUMAF process. Five parameters
namely percentage weight of abrasive, oxidizing
agent concentration, rotational speed of magnet,
working gap and pulse on time for ultrasonic
vibration, were selected as process parameters to
evaluate the performance of CUMAF process and
their effect on response variable namely percentage
change in surface roughness was recorded.
Furthermore, the overall change in work piece
morphology after finishing was observed with SEM
images.
2 Experimental set up
The developed experimental set up consisted of
two permanent magnetic tools, specially designed and
fabricated fixtures to hold the workpiece and an
ultrasonic generator unit. A permanent magnetic tool
was made of aluminium disk having four magnetic
poles. Each magnetic pole consists of set of ten
NdBFe magnetic disks(Φ25mm X 3 mm thick). The
magnetic poles have arrangement of alternative South
and North Pole. Magnetic flux density varies with
working gap and radial distance from cent of magnet.
The upper magnet was attached to spindle of CNC
Bevermill machine to provide required rotation to
magnet. Lower magnet was held above the machine
table with the help of specially designed fixture so
that the magnet was unaffected by machine table
movement.
Before starting the experiments, workpiece was
exposed to oxidizing agent for 30 minutes. Then
workpiece was held with help of specially designed
fixture. The gap between workpiece and upper magnet
was filled with ferromagnetic abrasive particles.
These particles formed a flexible magnetic abrasive
brush at four magnetic poles. The FMAB formed on
four magnetic poles of upper magnet has been shown
in top left of figure 1.
While performing experiments the workpiece held
in designed fixture was excited by ultrasonic
vibrations produced by ultrasonic generator through
horn. Figure 1 shows actual photograph of fabricated
experimental set up with various components.
Figure 1 Experimental set up
Fixture to hold lower magnet
Ultrasonic horn
Upper magnet
Lower magnet Work piece holding fixture
5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,
IIT Guwahati, Assam, India
823-3
3 Selection of process parameters
In the first step of work some preliminary
experiments were conducted to get the idea about the
range of different process variables. Based on these
experiments the finishing time was fixed at 20
minutes for all the experiments. In the present work
five process variables were selected. Details and
levels of process factors are given in table 1.
Percentage change in surface roughness was
selected as process response, which was calculated by
the formula shown below:
%∆Ra� ������� ���� ��� �� ����� ��� ���� ����� ������� ���� ��� x100
Details of the parameters which were kept
constant aregiven in table 2.
4 Experimental procedures
After selection of levels of process factors design
of experiments is second most crucial step. Here
central composite design methodology was used as it
needs smaller number of experiments with reliable
predictions. A second order model was involved with
central composite design technique, also known as
response surface methodology.
Response equation of following form is yielded by
central composite design methodology.
� � �� � ����
����� � ������
�
���� ����!���! � "
!#�
Where, Y represents the response variable, k is
number of factors, ��,��,���,��! are constants, �� , �� , ���! are linear, square and interaction terms of
process factors respectively, and " is the random
error.
Initially, Ra was measured at three random points
and then average of these values was calculated. After
finishing the work piece again,Ra values were
measured at the same three points and average was
calculated. Based on inputs mentioned in table
1,experiments were carried out and the surface
roughness values and percentage change in Ra were
recorded.
5 Statistical model to predict change in
surface roughness
A statistical model for %∆Ra is obtained by
performing regression analysis. The results were
analysed with help of analysis of variance (ANOVA).
Initially model contained some insignificant terms
also. In second step the insignificant terms were
dropped and ANOVA was performed again with
significant terms. The final ANOVA table has been
shown in Table 3.The obtained model to predict
%∆Ra is given below.
%∆Ra = - 74.6 + 0.361 X1 + 45.1 X2 + 6.64 X3 +
3.97 X4 + 14.3 X5 - 0.000840 X12 - 11.7 X2
2 - 0.862
X32 - 1.70 X4
2 - 0.122 X5
2 (1)
Table 1: Process factors and levels
Description Level
-2 -1 0 1 2
Rotational speed
(RPM) (X1)
50 100 150 200 250
Working gap
(mm) (X2)
1 1.5 2 2.5 3
Conc. of NaOH
(X3) (gm/l)
1 3 5 7 9
Pulse on time for
ultrasonic
vibration, ton (s)
(X4)
1 2 3 4 5
%Weight of
abrasive(X5)
10 15 20 25 30
6 Effect of process parameters on %∆Ra
The main effect plot of different factors
considered in present study has been shown in figure
2. The main effect plot demonstrates the effect of
various process parameters on %∆Ra. It can be seen
from figure 3 that the rotational speed (22.89%) has
the highest impact on %∆Rafollowed by %weight of
abrasive (16.11%), ton (12.71%), concentration of
NaOH (12.06%) and working gap (0.72%).
Figure 4 shows the effect of various process
parameters on the response variable. It can be seen
from figure 4 that %∆Raincreases with increase in
rotational speed.Itmay be because athigher RPM the
striking rate of MAP’s increases which helps in fast
shearing of surface peaks resulting in increase in
%∆Ra. From figure 4 it can be seen that %∆Rais
maximum at some intermediate value of working gap.
Lower gap may cause excessive indentation resulting
in deterioration of work piece surface. At higher gaps
finishing forces may be insufficient to shear the peaks
of work piece surface which may lead to decrease in
%∆Ra. Some intermediate value of NaOH
concentration is favourable for optimum surface
finish. This may be due to the fact that very low
concentration of NaOH may not be able to oxidize the
required amount of copper atoms to form a uniform
oxide film, which may lead to surface deterioration.
At very high concentration of NaOH, copper atoms
start dissolving in NaOH solution, which deteriorates
the surface finish. Increasing ton causes long lasting
impact of abrasive particles and hence %∆Raincreases
with increase in ton. Increase in percentage weight of
abrasive results in increase in %∆Rainitially and after
sometime%∆Ra decreases.
CHEMO-ULTRASONIC ASSISTED MAGNETIC ABRASIVE FINISHING: EXPERIMENTAL INVESTIGATIONS
823-4
Table 2: process parameters kept constant in
experiments
S.
No.
Process parameter Value
1 Pulse off time of ultrasonic
vibrations, Toff (s)
2.0
2 Mesh number of Al2O3
powder
1200
3 Ultrasonic power supply (W) 720
4 Time for each experiment
(min)
20
This may be due to the reason that at very low
percentage weight of abrasive, cutting edges required
for finishing are less, which results in poor cutting
action and hence low %∆Ra. As percentage weight of
abrasive increases, after a certain value ratio of
magnetic particles become low and hence overall
magnetic force required to form the chain reduces and
the chain starts disintegrating. Hence effectiveness of
the process reduces.
7 Process Optimization
The change in surface roughness obtained as
equation (1) is maximized to predict the best
performance level of CUAMAF. The function is
optimized using genetic algorithm toolbox in
MATLAB 12 software and the results are presented in
Table 4.
Figure 2 Main effect plot of process
parametersduring CUAMAF
Figure 3 Percentage contributions of significant
factors on %∆Ra
Figure 4 Effect of (a) Rotational speed, (b)
working gap, (c) conc. of NaOH, (d) pulse on time,
and (e) % weight of abrasive; on %∆Ra
The surface roughness profiles obtained at
optimum process conditions are shown in figure 5. It
can be observed that by CUMAF the maximum peak
to valley height is reduced to approximately 1/6th
after finishing.
Table 4: Optimization results
Sample Cu Alloy
RPM 215
Gap (mm) 1.9
Conc. of NaOH
(gm/l)
4
Pulse on time (s) 4.2
%weight of
abrasive
16
% ∆Ra 82.86
25020015010050
70
65
60
55
50
3.02.52.01.51.0 97531
54321
70
65
60
55
50
3025201510
RPM
Mean
Gap Conc. of NaOH
Pulse on time %wt. of abrasive
Main Effects Plot for %∆Ra
Data Means
RPM Working Gap (mm) Conc. of NaOH (gm/l)
Pulse on time (s) % wt of abrasive
5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12
IIT Guwahati, Assam, India
Table 3: Analysis of variance (ANOVA) after
Source DF Seq SS
Regression 10 2952.1
Linear 5 2018.47
Square 5 933.63
Interaction 0 0
Residual error 21 158.88
Lack-of-Fit 16 108.36
Pure error 5 50.52
Total 32 3129.86
Figure5Surface roughness plot of specimen
with (a) Ra =0.3571 µm (b) Ra =0.
The surface roughness profiles alone do not
explain the exact mechanism of material removal and
the quality of surface generated. Therefore, scanning
electron microscope (SEM) images of rough and
finished surfaces were taken. Figure 6(a) shows SEM
image of unfinished Cu sample. Figure 6(b
SEM images of the same sample when it is finished
using processing conditions given in table 5.
Table 5 Detail of each parameter selected for
SEM images shown in figure 6
It is observed that CUMAF has remarkably
finished the surface and very fine machining marks
are left after finishing as compared to initial rough
surface.
Fig
No. RPM
Gap
(mm)
Conc.
of
NaOH
(gm/l)
ton
(s)
%wt of
abrasive
6.b 150 2 5 3
6.c 200 2.5 3 4
6.d 150 2 5 3
6.e 200 1.5 3 2
6.f 200 1.5 7 4
(b) R(a) Ra = 0.3571µm
All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12
Analysis of variance (ANOVA) after dropping insignificant terms
MS F P R2 Remark
295.21 39.02 0 94.92%
173.163 22.89 0
186.725 24.68 0
7.566
6.733 0.67 0.75
10.104
model is adequate and lack of fit
is insignificant
Surface roughness plot of specimen
0.0617µm
The surface roughness profiles alone do not
explain the exact mechanism of material removal and
generated. Therefore, scanning
electron microscope (SEM) images of rough and
. Figure 6(a) shows SEM
Cu sample. Figure 6(b-f) show
SEM images of the same sample when it is finished
given in table 5.
Detail of each parameter selected for
images shown in figure 6
t is observed that CUMAF has remarkably
finished the surface and very fine machining marks
ng as compared to initial rough
Figure 6SEM image (2000 X)
finishing; (b-f) after finishing with the
parameters given in table 5.
8 Conclusions
1) In the present work, paramagnetic
copper alloy is successfully finished using unbounded
magnetic abrasive particles. Ultrasonic vibration
caused enhanced interaction of abrasive grains with
the peaks on the work piece and NaOH formed a
softer oxide layer over the surface. Combination of
%wt of
abrasive
%
∆Ra
20 79.5
15 79.2
20 74.8
15 67.1
20 60.1
(b) Ra = 0.0617µm 10 µm
10 µm
10 µm
All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,
823-5
insignificant terms
Remark
model is adequate and lack of fit
is insignificant
(2000 X) (a) before
f) after finishing with the
parameters given in table 5.
1) In the present work, paramagnetic work piece i.e.,
copper alloy is successfully finished using unbounded
magnetic abrasive particles. Ultrasonic vibrations
caused enhanced interaction of abrasive grains with
and NaOH formed a
softer oxide layer over the surface. Combination of
=2.34
>
=2.156
<
10 µm
10 µm
10 µm
CHEMO-ULTRASONIC ASSISTED MAGNETIC ABRASIVE FINISHING: EXPERIMENTAL INVESTIGATIONS
823-6
these two resulted into better surface quality with
minimum surface defects.
2) It is observed that rotational speed of magnet has
maximum percentage contribution (22.89%) on
surface finish during CUMAF.
3) With optimum parameters, 82.86% improvement in
surface finish is observed during CUMAF. Surface
finish of a Cu alloy sample with initial surface
roughness 357 nm can be improved to final surface
roughness 61 nm using the developed process. It is
observed from surface roughness profiles obtained
using Talysurfthat the maximum peak to valley height
has been reduced to approximately 1/6th after
finishing.
9 References
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