Analysis of chip breaking during orthogonal machining of Al/SiCp
composites
S.S. Joshi a, N. Ramakrishnan a,*, P. Ramakrishnan b
b
a
Department of Mechanical Engineering, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India
Abstract
With the advent of large number of composite materials, a systematic study of machining characteristics of these new materials
is necessary for their rapid adoption in the actual engineering applications. This paper is an attempt to understand the machining
characteristics of Al/SiCp composite material, which is the most widely used and projected to have large application in the near
future. A detailed study of fundamental aspects of chip formation during orthogonal machining is carried out. From the visual
observation of the physical form of chips, a systematic chip breaking pattern was observed. Also, the chip breaking phenomenon
was related to the mechanical properties of composites by a chip breaking criterion. An understanding of chip curling and
breaking can be very helpful in designing cutting tools with constrained chip flow space (like drills, taps, broaches, etc.).
Keywords: Chip breaking; Orthogonal machining; Al/SiCp composites
1. Introduction
Among the various types of Metal Matrix Composites (MMCs), the aluminium alloy based composites are finding increasing engineering applications.
They are being used as a replacement materials in
various engineering applications such as for cylinder
block linears, vehicle drive shafts, automotive pistons,
bicycle frames, etc. [1 – 3]. All these and many other
engineering applications of MMCs necessitate an adequate knowledge of technology of machining these
materials. Here, the composite material differs from
the conventional material by the way of presence of
harder and isolated reinforcements in the path of the
tool during machining. This characteristic leads to
difficulties in machining of these composites, primarily of rapid wear of the cutting tools. Therefore, the
available literature concentrates on the study of wear
characteristics of various tool materials during ma-
chining of aluminium alloy based MMCs [4–7]. However, in view of the growing engineering applications
of these composites, a need for detailed and systematic study of their machining characteristics was envisaged. Such kind of study would be helpful for the
efficient and economic machining of these materials
to the required tolerances or characteristics. It is
known from the theory of metal cutting that the
study of chip formation is the most effective and
cheapest way of understanding machining characteristics of a material. This work is an attempt to understand the machining characteristics of Al/SiCp
composite material, which is most widely studied and
projected to have large applications in a near future.
A detailed study of fundamental aspects of chip formation and chip breaking was carried out during orthogonal machining of these composites. Focus of the
study is on the influence of volume of reinforcement
on machining characteristics of these composites. Initial understanding of chip formation based on the
visual observations of physical form of chips is presented in this paper. It was observed that the chips
91
showed a systematic breaking pattern depending upon
the volume of reinforcement in the composite material and the chip breaking can be related to their
mechanical properties by a chip breaking criterion.
Whereas, further understanding of the chip formation
involving the microstructural analysis is presented in
the another paper [8].
2. Experimental procedure
It involved orthogonal machining of Al/SiCp composites containing 10, 20 and 30% volume of SiCp
and the unreinforced alloy on shaping as per the experimental details given in the Table 1. A large number of chips per parametric condition were collected
during orthogonal machining operations. Radius of
chip curvature and chip thickness were measured using Nikon Tool Makers’ Microscope. At least 20
measurements of radius of chip curvature and chip
thickness were taken per parametric condition.
Initially, the composite material required for these
experiments was fabricated using the liquid metallurgy
route consisting of rheocasting, squeeze casting and
hot extrusion. The extruded composites were given
solution treatment so that the matrix in the composites is devoid of the second-phase alloy precipitates such as copper in the present matrix alloy. This
facilitated a clear understanding of the effect of reinforcement on the machining characteristics of composites. It is important to note here that the all the
processing steps including the heat-treatment have
been performed on the matrix material so as to make
the comparison easier. A block diagram of steps in
the process of fabrication of these composites is
shown in Fig. 1. Carbide inserts of specification
TPUN 160308 and grade THM (equivalent to ISO
Table 1
Experimental specifications
Parameters
Specifications
Machining process
Work material
Shaping
Al/SiCp composites with 10, 20
and 30% volume of SiCp
Matrix aluminium alloy
50 (long)×5 (width)
Tungsten carbide
+ 6, 0, −6
5.8, 16.5
0.04, 0.08, 0.12
Workpiece size (mm)
Tool materials
Tool rake angles (°)
Cutting speed (m min−1)
Feed (mm rev−1)/
Depth of cut (mm)
K10 and K20) were supplied by (WIDIA, India) used
during the machining tests.
3. Results and discussion
The results are discussed based on the physical
characteristics of chips such as the number of circles
through which chip curls, initial and final radius of
chip curvature and a criterion for chip breaking. The
microstructural analysis of reinforcement in the composite material carried out earlier showed fairly uniform distribution of reinforcement.
3.1. Number of chip curls
Chips produced as per above mentioned experimental conditions are depicted in Fig. 2 (a–c). In these
figures, the chips are arranged from top to bottom
with increasing volume of reinforcement in composites. These figures show that the number of circles
through which chips curl before breaking decreases as
the volume of reinforcement increases. Table 2 gives
Fig. 1. Block diagram of process of fabrication of Al/SiCp composites.
92
Fig. 2. Physical appearance of chips (on shaper, speed — 16.5 m min − 1, depth of cut — 0.12 mm).
approximate number of circles through which a chip
curls before breaking. Chips of unreinforced alloy
break after curling through 2 to 3 circles. While the
number of chip circles decreases to 1 to 1 – 1/2 in case
of Al/SiC/10p composites, chips of Al/SiC/20p and
30p composites show similarity by curling through
about 3/4th of a circle. This trend can be explained
from the mechanical properties of these materials-by
observing the variation in the strain at failure during
tension testing. Decrease in the number of chip curls
is in direct proportion with the decrease in the strain
at failure during tension testing, refer Table 2. For
Al/SiCp/10p composites The number of chip curls decrease drastically to, 1 to 1 – 1/2 from 2 to 3 as the
volume of reinforcement increases from 0 to 10 so is
the strain at failure during testing. Similar trend can
be observed for other two composites, refer Table 2.
Another observation from these photographs in
Fig. 2 (a–c) is that the chips of aluminium alloy
show a dependence on the rake angle, whereas, the
chips of composites do not. As the rake angle decreases, the chips of aluminium alloy curl through
widely spaced and larger diameter circles. It has been
reported that the process of chip curling depends
upon the ratio of length of plastic contact of chip to
the total length of contact of the chip on the tool
face [9].
The higher the ratio, the flatter the chips. At negative rake angles, the length of contact of chip on the
tool face is higher, hence the chips could be flatter
and larger in diameter. Besides, if the cutting speed is
low, the shear strength of a work hardening material
is high resulting in an early breakage of chips. Thus,
the phenomenon also decreases the number circles (or
the length of chip) through which a chip curls. This
can be more prominently seen from the chips produced at − 6° rake angle. This trend is not seen in
case of the chips of composites because they are too
small in length and break very early due to the poor
ductility of composites.
93
Table 2
Number of chip curl and strain at failure during tension test
Volume of reinforcement (% of SiCp)
00
10
20
30
Strain at failure during tension test
Number of chip curls
0.30
2 to 3
0.049
1 to 1–1/2
0.027
3/4 th to 1
0.019
3/4 th to 1
3.2. Radius of chip cur6ature
Initial and final radii of curvature of chip were
measured for quantifying the above observations and
relating them to the mechanical properties of the composite material. Variation of these radii with the volume
of reinforcement and the rake angle of the tool is
shown in Figs. 3 and 4, respectively. The data are an
average of a large number measurements, at the same
time, the complexity of chip radius measurement need
not be emphasized here.
The initial radius of chips appear to be independent
of the rake angle, refer Fig. 3. As the initial radius of
chip curvature is formed out of a smaller chip length, it
could be independent of the rake angle. On the other
hand, the initial radius of chips do show material
dependence. As the volume of reinforcement in the
composite material increases, the chips curl through the
smaller radii. It may be due to the similar effect of
length of contact already explained in case of the
aluminium alloy. Further, if the volume of reinforcement in composite material is small, material will have
more tendency to stick on to the tool surface applying
restrain on the further chip movement. This may force
the chip to take a longer path by curling through larger
diameter circles. This effect is very prominent at − 6°
rake angle tools and gradually decreases, as the rake
angle increases towards +6°, refer Fig. 3.
Although the overall trend in the final radius of
curvature of chips is qualitatively similar to that in the
initial radius, there are some displacements of the
Fig. 3. Initial radius of curvature of chip vs. rake angle.
curves, refer Fig. 4. This could be due to more uncertainty in final breaking of the chip.
3.3. Analysis of chip breaking
Above discussion shows that the chip breaking in
case of the composite material can be related to their
mechanical properties. Correspondingly, a chip breaking criterion is proposed.
3.3.1. Criterion for chip breaking
It is evident from the literature review the two important criteria for chip breaking are given by [10,11].
According to Nakayama, [10], the chip breaks when the
strain on chip increases beyond some critical value
given by
ob B
n
t2 1
1
−
2 RO RL
(1)
Zang and Peklenik, [11] have analyzed the process of
chip breaking in ‘difficult-to-cut’ materials and proposed a criterion based on their mechanical properties
as
2ssc
h
1
+ ob = c 1−
where, k= rg/rc
(2)
Ec
rg
2k
n
n
In the above criterion, the left hand side estimates
strain on chip based on its mechanical properties;
whereas, the right hand side gives the estimated strain
on chip based on the geometry of chip breaker. It was
also suggested that the mechanical properties of chip
material could be related to the mechanical properties
Fig. 4. Final radius of curvature of chip vs. rake angle.
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Table 3
Tensile test data and properties of chip material
Vol. of SiCp (%)
sw (MPa)
ssc (MPa)
Ew =Ec (GPa)
ow
ob
om
10
20
30
259.13
274.03
292.63
388.695
411.05
438.945
57.34
59.20
61.61
0.049
0.027
0.019
0.00735
0.00405
0.00285
0.0209
0.0179
0.0171
of the parent workpiece material by certain proportions
[11].
A careful observation of right hand sides of Eqs. (1)
and (2) shows that both these equations differ in the
coefficients of the first term if arranged in terms of
similar symbols as explained below. The right hand side
of Eq. (1) gives
1 hc
1 hc
−
2 rg
2 rc
4.
(3)
and the right hand side of Eq. (2) gives
hc
1 hc
−
2 rg
rc
5.
(4)
Thus, in the first equation, the coefficient of the first
term is 1 and in the later it is 1/2. If it is assumed that
the critical strain on the chip suggested by Nakayama is
same as that of the one derived from the mechanical
properties in Zang’s model, combined chip breaking
criteria can be given as follows
om =
1 hc
hc
−
2 rg
rc
…Zang and Peklenik, [11]
=
1 hc
1 hc
−
2 rc
2 rg
6.
(5)
Peklenik, [11] evaluated the mechanical properties of
chips of steel and found that the strain in the chip
material is about 11–20% of that of the workpiece
material. In the present analysis, the strain on chip
material (ob) is taken in the same range as used in
the analysis of Zang, i.e. 15% of that of the tensile
strain (ow).
Strain on chips based on its mechanical properties
(om): It is evaluated using the left hand side of Eq.
(2) based on the mechanical properties of chips as
evaluated above.
Initial Radius of curvature of chip (rc): In Zang’s
analysis, the chip is forced to pass through some
restrictions such as chip breaker grooves; and the
radius of chip curvature is estimated based on the
geometry of these restrictions. However, as this
experiment is aimed at analyzing the chip breaking
process rather than designing chip breakers, the
cutting tool was not provided with any kind of chip
breakers. Hence, rc is measured as initial radius of
curvature of chip on the completely formed chip for
a fixed chord length of 1 mm. Results are plotted in
Fig. 3.
Final radius of curvature of chip (rg): Similarly, the
final chip curvature radius (rg) is measured on the
chip for a fixed cord length of 2 mm. Results are
plotted in Fig. 4.
Thickness of Chip (hc): It is directly measured on
the chip.
…Nakayama, [10]
where, om-strain on chip estimated using its mechanical
properties from left hand side of Eq. (2)
7.
3.3.2. E6aluation and measurement of model
parameters
Magnitude of various terms in the Eq. (5) is evaluated based on the proportions proposed by Zang [11]
for the evaluation of mechanical properties of chip
material from the properties of work material, refer
Table 3. The procedure of evaluation is mentioned
below;
1. Yield strength of chip material (ssc): Yield strength
of the chip material is taken as 1.5 times that of the
work material (sw).
2. Elastic modulus of chip material (Ec): Elastic modulus of the chip material is taken as the same as the
work material (Ew).
3. Strain on chip at the time of breaking (ob): Zang and
3.3.3. Comparison of results
Comparison of strains on chips evaluated based on
above two models (right hand side of Eq. (5)) and the
mechanical properties of chip material (left hand side of
Eq. (2)) for various composites is shown in Figs. 5–7.
These figures show that the strains on chips estimated
using both the above models remain fairly constant for
Al/SiC/10p and 20p composites for various rake angles,
barring some decrease only at + 6° rake angle in case
of Al/SiC/30p composites. Hence, it would be appropriate to compare these with the strains on chips based on
their mechanical properties.
Another observation from these figures is that strains
estimated using the Zang’s model deviated much more
than those estimated using Nakayama’s model for all
95
Fig. 5. Comparison of chip strains for Al/SiC/10p composites.
Fig. 7. Comparison of chip strains for Al/SiC/30p composites.
the composites, refer Figs. 5 – 7. It was felt that these
deviations may be due to the following reasons;
“ In the estimation of strains using Zang’s model,
the radii of chips were directly measured on chips.
These radii are of naturally curling chips rather
than the estimation of these radii based on the
geometry of chip breaker. Thus, there is a difference in the experimental procedures.
“ Effect of spring back in the chip material and its
effect on the chip dimension may lead to some
alterations in the chip dimensions which is not
taken care of in the Zang’s model.
“ On the other hand, there are no differences in the
definitions of terms occurring in the Nakayama’s
model and their actual measurements. Hence, these
results could be closer to the mechanical properties.
From the above discussion it can be concluded
here that the chip breaking criteria, based on the mechanical properties is found to be satisfactory for the
composite materials also.
Fig. 6. Comparison of chip strains for Al/SiC/20p composites.
4. Concluding remarks
Following conclusions can be drawn based on above
study of physical characteristics of chips
“ Number of circles through which chip curls is proportional to the strain which material can withstand during tension test.
“ Chips of aluminium alloy are found to curl
through circles of wider and larger diameters as
rake angle decreases. It may be due to sticking of
workpiece material on the tool face. Initial radius
of chip increases with decrease in the volume of
reinforcement, especially, at lower rake angles. This
could be due to the changes in the length of contact on to tool face.
“ Since strains on chips estimated using two models
remain fairly constant, they can be compared with
those of the strains estimated based on the mechanical properties. Estimation of strains using
Nakayama’s model lie closer to those estimated using mechanical properties than using Zang’s model.
A chip breaking criterion based the mechanical
properties of chip is found to be satisfactory for
composite materials also.
“ In developing appropriate cutting tool geometry,
breaking of the chips is necessary for compaction
of chips in constrained environment (flutes of
drills, tooth cavity in broaches, taps, etc. Curled
and continuous chips not only occupy more volume but also creates difficulties in chip disposal
system. The understanding of chip curling and
breaking will thus help in developing better tools
for composite materials.
96
5. Nomenclature
ob
t2
RO
RL
omax
ssc
Ec
hc
k
rg
rc
om
strain in the chip material
chip thickness (mm)
radius of initial chip curl (mm)
radius of final chip curl prior to fracture (mm)
maximum strain in the chip
yield strength of the chip material (MPa)
modulus of elasticity of chip material (GPa)
chip thickness (mm)
rg/rc
radius of chip curvature at the breaking point
(mm)
radius of chip curvature when chip flows out of
the chip breaker groove (mm)
strain on the chip based on the mechanical
properties of material
Acknowledgements
Authors wish to express their gratitude to (WIDIA,
India), Bangalore, for their financial and technical support.
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
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