chapter 7 comparison of wedm of al/sicp and al/al2o3p...
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151
CHAPTER 7
COMPARISON OF WEDM OF Al/SICP AND Al/AL2O3P COMPOSITES
____________________________________________________________________
7.1 Scope
In this chapter, a comparative study on WEDM of A359/SiCP and A6061/Al2O3P
composites is presented. Comparisons between the characteristics of the machined
surfaces, material removal rates and dimensional accuracy in WEDM of SiC and
alumina reinforced MMCs are presented. Moreover, critical issues such as wire
breakages and wire shifting are discussed in detail. In addition, recommendations are
made to overcome these problems in machining of these difficult-to-machine
materials. Finally, a new strategy for successful machining of these difficult-to-cut
materials is proposed.
7.2 Surface Characteristics
Over the years, it has been observed that the EDM/WEDM machined surfaces are
significantly different than those machined by using any other machining process.
These surfaces have no texture or pattern which extends beyond the individual craters.
The non-directional surface patterns have proved to be one of the major advantages of
EDM over conventional machining in several applications (Huntress et al., 1986). The
examples, in particular, are making of the punches and dies, fluid control parts like
fuel jets and injectors. A clear characterization of surface topography is essential to
predict the quality and functional behaviour of the surfaces such as static and dynamic
stiffness, wear resistance, fretting corrosion etc. (Ramarao et al., 1982).
In the present section, the surface characteristics of alumina and SiC reinforced
composites are compared. The characteristics of surfaces include surface roughness
study, surface appearance, surface topography, and qualitative study of recast layer. In
addition, an attempt is made to correlate the important surface roughness parameters
with the machining process parameters.
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7.2.1 Surface Roughness Studies
A practical EDM surface is a random superposition of craters formed by the thermal
effect of successive discharges. Thus, the most common method of representing the
surfaces by Ra or maximum peak-to-valley distance (Rmax) fails to give a true picture
of the surface. Therefore, in the present study, a number of amplitude and one spacing
parameters have been investigated.
The machined specimens of the experiments reported in Chapter 4 and Chapter 5
were used for this purpose. The measurement and analysis of these specimens are
presented in this section.
The amplitude parameters are the most important parameters to characterize surface
topography. They are used to measure the vertical characteristics of the surface
deviations (Gadelmawla et al., 2002). The following surface roughness parameters,
other than the Ra, were investigated:
1. Root mean square roughness (Rq): This parameter is also known as RMS. It
represents the standard deviation of the distribution of the surface heights. This
parameter is more sensitive than the arithmetic average height (Ra) to large
deviation from the mean line. The RMS mean line is the line that divides the
profile so that the sum of squares of the deviations of the profile height form this
line is equal to zero.
2. Ten-point height (Rz): This parameter is more sensitive to occasional high peaks
or low valleys than Ra. The international ISO system defines this parameter as the
difference in height between averages of the five highest peaks and five lowest
valleys along the assessment length of the profile.
3. Maximum height of the profile (Rt or Rmax): This parameter is very sensitive to the
high peaks or deep valleys or scratches. Rmax or Rt is defined as the vertical
distance between the highest peak and the lowest valley along the assessment
length of the profile.
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4. Mean height of peaks (Rp): It is defined as the mean of the maximum height of
peaks obtained for each sampling length of the assessment length.
5. Mean depth of valleys (Rv): It is defined as the mean of the maximum depth of
valleys obtained for each sampling length of the assessment length.
6. Skewness (Rsk): The skewness of a profile is the third central moment of the
profile amplitude probability density function, measured over the assessment
length. It is used to measure the symmetry of the profile about the mean line. This
parameter is sensitive to occasional deep valleys or high peaks. A symmetrical
height distribution, i.e. as many peaks as valleys, has zero skewness. Profiles with
peaks removed or deep scratches have negative skewness. Profiles with valleys
filled in or high peaks have positive skewness. Skewness parameter can be used to
distinguish between two profiles almost same Ra or Rq values but with different
shapes. The value of the skewness depends on whether the bulk of the material of
the sample is above (negative) or below (positive) the mean line.
7. Kurtosis (Rku): Kurtosis coefficient is the fourth central moment of profile
amplitude probability density function, measured over the assessment length. It
describes the sharpness of the probability density of the profile. If Rku < 3 the
distribution curve is said to be platykurtoic and has relatively few high peaks and
low valleys. Rku > 3 the distribution curve is said to be leptokurtoic and has
relatively many high peaks and low valleys.
8. Mean spacing at mean line (Rsm): The spacing parameters measure the horizontal
characteristics of the surface deviations. The parameter Rsm is defined as the mean
spacing between profile peaks and the mean line.
The results of measured values are given in Appendix ‘E’. The results, shown in
Table E-1 to Table E-5, revealed significant differences between the machined
surfaces of A359/SiCp and A6061/Al2O3p MMCs. These distinguishing features
between two different types of MMCs might be attributed to the differences between
the type of ceramic reinforcements and the type of matrix.
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The major difference found was the characteristic of the profile of the machined
surface, i.e. the value of Rsk. This parameter is very sensitive to deep scratches or
high peaks. The values of skewness appear to be uncorrelated to the machining
parameters for both types of composites. However, skewness values of A6061/Al2O3p
and A359/SiCp were found to be significantly different. In general, the skewness of
EDM machined surfaces is positive. A positive value of Rsk indicates that the surface
profile is ‘empty’ of material. Over the years, it has been observed that EDM
machined surfaces are of positive skewness (Mamilas et al., 1987; Petropoulos et al.,
2004).
The skewness values of A6061/Al2O3p composites were found to be positive.
However, it was negative for the machined surfaces of A359/SiCp composites. The
positive skewness of alumina reinforced composites might be attributed to the
significant presence of alumina particles protruding proudly on the machined
surfaces. The negative skewness of the SiC reinforced composites, however, might be
attributed to the complete removal of the SiC particles from the machined surfaces.
These results were further assessed and checked by scanning electron microscopy of
these machined surfaces. These findings agree with the results of Seo et al. (2006) at
short on-times and low current although their work is on conventional EDM. Based
on the gathered experimental data, statistical regression analysis was employed to
study the correlation of the surface roughness parameters with the machining
conditions. The coefficients of correlations are shown on Table 7.1 for these MMCs.
Table 7.1 Correlation coefficients of quadratic regression models of selected surface
roughness parameters
Roughness
parameters
Correlation coefficient, r
A6061/Al2O3/
10p
A6061/Al2O3/
22p A359/SiC/10p A359/SiC/20p
A359/SiC/
30p
Rz 0.945 0.91 0.85 0.948 0.916
Rq 0.95 0.88 0.92 0.91 0.97
Rp --- 0.90 --- --- 0.87
Rv 0.83 --- 0.92 0.88 0.917
Rsm 0.83 --- 0.90 --- ---
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7.2.2 Surface Topography
The topography of machined surfaces of alumina and SiC reinforced composites has
been studied. Scanning electron microscopy has been employed to assess the results
of roughness measurements. Moreover, attempt was made to study the effect of spark
energy (on-time) on the crater size and surface morphology of these composites. The
effects of volume fraction and type of ceramic have also been studied so as to
investigate into the causes of significant differences between surface characteristics of
these MMCs.
7.2.2.1 Influence of Spark Energy
The influence of spark energy (on-time) on the crater size can be appreciated from
Figure 7.1 and Figure 7.2. The craters appeared to be deeper and larger in case of
longer on-times as compared to short on-times. A comparison between the craters of
alumina reinforced MMCs and the SiC reinforced composites can also be made from
these figures.
The pulse on-time was found to influence the number of ceramic particles present on
the machined surfaces of alumina particle reinforced MMCs. It has been found that at
high level of spark energy more number of particles gets dislodged from the machined
surface. This was found to be attributed to the greater volume of molten metal at
longer on-time or spark energy causing easy deepening of the craters and
comparatively easy removal of the alumina particles from the machined surface.
Scanning electron micrographs of A6061/Al2O3p composites machined at 0.2 and 1
revealed this significant difference between the densities of alumina particles on the
machined surfaces (Figure 7.3).
The number of ceramic particles on the surface machined at low level of pulse on-
time was found to be greater as compared to the number of particles on the surfaces
machined at longer on-times. The height of the protrusions and depths of the craters
determine the roughness of the machined surface of alumina reinforced composites.
The roughness at the short on-time, thus, might be dominated by the height of the
particle protrusions where as the roughness values will be determined by the
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combined effect of the protrusion height as well as the depth of the discharge craters
at longer on-times. This is explained as deeper discharge craters at longer on-times
and greater protrusion heights at short on-times. This characteristic of A6061/Al2O3p
is, thus, found significantly different than the A359/SiCp MMCs. on the machined
surfaces of A359/SiCp composites, the SiC particulates have found to be absent. This
is in agreement with earlier studies (Gatto, 1997; Ramulu, 2001 and Seo et al., 2006).
Figure 7.1 Effect of pulse on-time on the size of crater in A359/SiCp composites
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Figure 7.2 Effect of pulse on-time on the size of crater in A6061/Al2O3p composites
Figure 7.3 Effect of spark energy on machined surfaces of A6061/Al2O3p
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7.2.2.2 Influence of Ceramic Particulates and Volume Fraction
The effect of volume fraction of ceramic particles on the size of craters can be
appreciated from Figure 7.1 and Figure 7.2. The crater size of 10% reinforcement
composites was found to be greatest among the A359/SiCp MMCs. However, the
craters of 20% and 30% SiC reinforced composites do not show any significant
difference. This trend is similar to the cutting speeds in machining of SiC reinforced
MMCs. The cutting speeds in machining of A359/SiC/10p were found to be greatest
as compared to the 20% and 30% composites of this type. The difference in the
cutting speed of A359/SiC/20p and A359/SiC/30p was also not very significant. This
might be attributed to the complete removal of the SiC particles from the machined
surfaces. In addition, the increase of 10% in ceramic particles beyond 20% may not be
influencing the process behaviour.
The machined surfaces of these different MMCs reveal that the mechanism of
material removal was predominantly melting and/or vaporization of the matrix alloy.
However, a number of issues are, believed to be, associated and these include: high
melt viscosity, reinforcement/matrix reactions, and segregation of the reinforcing
particles (Lloyd et al., 1989; Yue et al., 1996). In general, is has been agreed
unanimously that the ceramic particles remain unaffected during EDM/WEDM
machining and they might dislodge after removal of the surrounding matrix (Muller et
al., 2001; Yan et al., 2005). However, in the present study, the machined surfaces of
A359/SiC and A6061/Al2O3 were found to be fundamentally different from each
other.
The effect of ceramic particles on the size of the crater was found to be more
significant on the machined surfaces of alumina reinforced composites. The size of
crater was found to be limited by the surrounding ceramic particles. This might be
attributed to the plasma flushing capacity at the end of the discharge. The expelling
force at the end of the on-time might be influential to overcome the restrictions
implied by the surrounding alumina particles. This might be attributed to the high
density of ceramic particles protruding on these machined surfaces. The crater shown
in Figure 7.2 d) is an evidence of restriction to the expulsion of the molten aluminium
alloy due to the surrounding alumina particles. As shown in Figure 7.2 d), the depth of
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crater appeared to be greatest. This could be attributed to the concentration of the
spark energy on the conductive alloy trapped between the ceramic particles. These
particles might have caused poor transitivity of the discharge channel by virtue of
their electrical insulating properties. Moreover, the electrical resistivity of alumina is
significantly greater than that of the SiC.
The roughness of alumina reinforced composites was found to increase with the
increased percentage of the alumina reinforcements. The increase in the percentage of
SiC reinforcement lead, however, to reduced surface roughness. Moreover, the
ceramic particles were found protruding proudly on the machined surface of alumina
reinforced composites whereas the SiC particles were found to be absent on the
machined surfaces of A359/SiC composites.
Figure 7.4 Comparison between surfaces machined at 0.2 µs on-time A359/SiCp and
A6061/Al2O3p
The machined surfaces of SiC and alumina reinforced MMCs are shown in Figure 7.4
to appreciate the significant difference in the surface topography of these WEDM
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machined composites. The presence of alumina particles on the machined surface of
the alumina reinforced composite is evident in the Figure 7.4. However, the SiC
particles were not found to appear on the machined surfaces of A359/SiCp
composites. These findings agree with the previous studies of other researchers in
which both Al/SiCp (Gatto et al., 1997) and Al/Al2O3p MMCs have been machined
(Yue et al., 1996; Yan et al., 2005) in different separate experimental investigations.
In these studies, the roughness of SiC reinforced aluminium was found to be superior
to the unreinforced alloy, however, in Al/Al2O3p the roughness was found to be
inferior to the unreinforced alloy. Therefore, the protruding particles and effect of
concentration of discharge energy determine the roughness values in Al/Al2O3p.
It is, thus, believed that different ceramic reinforcements (SiC and Al2O3) may behave
differently in WEDM of these materials. Moreover, different matrix materials (A359
and A6061) might also be influential on the machining characteristics of these
composites. Thus, the difference between reinforcement and matrix alloy thought to
modify the machined surfaces of these MMCs. In the available literature, no
researches have been found to address these issues of major differences between
alumina and SiC reinforced aluminium matrix composites. Therefore, in view of
understanding the cause of this diverse kind of surfaces, SEM along with energy
dispersive X-ray analysis (EDX) of few machined surfaces was undertaken. The
results of these analyses are shown in Figure 7.5 and Figure 7.6. These results
revealed the significant presence of silicon and aluminium on the machined surfaces
of A359/SiCp composites, however, the presence of silicon carbide cannot be
confirmed from this analysis. This finding is in agreement with the earlier studies
(Ramulu, 2001; Seo et al., 2006). Moreover, no particles like features were observed
on the machined surfaces of A359/SiCp composites.
The absence of SiC particles on the machined surfaces of A359/SiCp MMCs could be
attributed either to the complete dislodging of the SiC particles from the metallic
molten matrix or to the chemical reaction between silicon and aluminium leading
towards absorption of silicon into aluminium matrix at high temperature environment
of WEDM process. It has been reported in few papers on solidification of Al/SiC that
silicon may dissociate from silicon carbide and get absorbed in the aluminium matrix
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and the percentage of silicon absorption was found to be dependent on the working
temperature (Lloyd et al., 1989, 1994).
Figure 7.5 EDS analysis of A359/SiC/20p MMC
001001
0.10 1.10 2.10 3.10 4.10 5.10 6.10 7.10 8.10 9.10 10.10
keV
001
0
1500
3000
4500
6000
7500
9000
10500
12000
Cou
nts
OK
a
AlK
aSi
Ka
162
Figure 7.6 EDS analysis of A359/SiC/10p MMC
Thus, it was thought that silicon has been dissociated from the silicon carbide at high
temperature conditions in the WEDM process. Further, the dissociated carbon, as it is
not found in the EDS analysis, might have been vaporized at very high temperatures
of WEDM process. However, the presence of carbon and silicon supports presence of
SiC particles on the machined surfaces of A359/SiC/30p MMC. It might be attributed
to the higher volume fraction of SiC particles on these surfaces. Thus, it can be
0.10 1.10 2.10 3.10 4.10 5.10 6.10 7.10 8.10 9.10 10.10
keV
003
0
1500
3000
4500
6000
7500
9000
10500
12000
Cou
nts
OK
a
AlK
aSi
Ka
003003
163
concluded that absence of silicon carbide on the machined surfaces of A359/SiCp
composites was attributed to the type of reinforcements. The role of matrix material,
thus, was found to be insignificant since in previous studies on WEDM of
A6061/SiCp (Gatto et al., 1997) and A356/SiCp composites (Muller et al., 2000) also
SiC particles were absent on the machined surfaces. Moreover, in the recent study by
Seo and co-workers (2006), the percentage of Si was found to be significantly greater
than that in the base alloy up to certain depth from the surface. This also agrees with
the findings of EDX of the above machined surfaces.
In the electro-discharge machining of metallic materials, the energy in the spark gap,
thermal conductivity, melting temperature, heat of fusion, coefficient of thermal
expansion, thermal diffusivity of the electrode determine the depth and diameter of
the craters. This, in turn, decides the surface finish of the machined part. It is known
that higher energy in the gap is responsible for poor surface finish due to larger and
deeper craters. However, in the machining of particle reinforced materials, the surface
morphology is different. In the machining of these materials, the presence of ceramic
particles was found to influence the surface finish (Yue et al., 1996; Yan et al., 2005).
The removal of the ceramic particles from the machined surface into the spark gap
was, believed, found to affect the gap conditions. Thus, the ceramic particles and their
properties might influence the surface quality in two different ways. First, if they
remain on the machined surface the surface quality depends on their size, protrusion
and type. Second, if the particles are released along with the molten metal, the gap
conditions and fluidity of the melt will be influenced. These particles, thus, affect the
roughness values in many ways:
• If the particles remain protruding, unaffected, on the machined surface, the
height of the protrusions and corresponding crater depth determine the
roughness value. Moreover, the depth of crater might be greater due to
concentration of the discharge channel caused by poor transitivity owing to
protruding insulating alumina particulates (arcing).
• If these particles are completely dislodged from the surface, the crater depth
and the depth created due to the particle pullout will determine the roughness.
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• The particles are completely dislodged; however, the droplets of molten metal
smear these created cavities. In this case, the roughness values will be further
different.
• If the dislodged particles remain in the spark gap, the discharge power density
will reduce due to dispersion of the discharge channel and the surface finish
will be improved.
• Other alterations such, as melting and/or evaporation and chemical reaction
between Al and Si of SiC or Al and carbon might result into different
character of machined surfaces.
1. 4Al + 3SiC ↔Al4C3 + 3Si
• The influence of the particles on the fluidity of the molten metal also
contributes toward the characteristics of the machined surfaces.
Thus, based on the above discussions and EDX analysis, it has been confirmed that
SiC particulates have been dislodged from the molten alloy. In case of alumina
reinforced composites, however, the number of particles on the machined surfaces
was significantly high.
These particulates (SiC/Al2O3), thus, found to influence the surface finish in different
ways. The A359/SiCp surfaces have found to be similar to the powder-mixed
dielectric surfaces (smooth and even). This might be attributed to the release of most
of the ceramic (SiC) particulates in the spark gap. Thus, spark channel might spread
and widen. This increased diameter of the discharge channel might have reduced the
discharge power density. Thus, due to reduction in the power density, the craters
formed on A359/SiCp composites have been found to be shallow and smooth, Figure
7.1 and Figure 7.2, as compared to alumina reinforced MMCs (Wong et al. 1998;
Tzeng et al 2001). Moreover, increased volume fraction will introduce more number
of SiC particles in the spark gap resulting in smoother surfaces. This phenomenon,
however, was not possible during machining of alumina reinforced MMCs as these
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particles did not found to be dislodged into the spark gap. The significant number of
protruding particles on the machined surfaces of alumina reinforced MMCs is
evidence of no or very less release of alumina particles.
In case of alumina reinforced composites, however, due to the ‘shielding effect’ of the
particles, the spark might be striking on the same spot due to no other path of least
resistance. This could be the reason of grater fluctuations in the cutting speed during
machining of alumina reinforced composites as compared to silicon carbide
reinforced MMCs. Similarly, this continuous arcing might cause deeper craters due to
repeated sparking concentrated on same area. Therefore, the surface roughness of
alumina reinforced MMCs were dependent on the depth of crater as well as the height
of the protruding alumina particles. This is distinguishable from the surfaces of silicon
carbide reinforced composites, where the SiC particles were not found on the
surfaces.
In addition to the effects of dispersion of discharge channel (A359/SiCp) and poor
transitivity and concentration of spark (A6061/Al2O3p), the effects of change in
fluidity of the molten metal might also be significant on the surface quality of these
MMCs.
The fluidity of the molten material might also be very significant on the re-
solidification of the molten material and thus on the surface finish of the machined
composites. The fluidity of molten A359 is comparatively lower than the fluidity of
molten A6061. In addition, significant reduction in the fluidity of A356 and A6061
due to SiC particles was observed (Lloyd et al. 1989). Therefore, grater amount of the
molten A359 might have remained on the machined surface. The decrease in the
fluidity of A359/SiCp MMCs might have been greater, as compared to alumina
reinforced composites, due to the presence of released SiC particulates in the molten
metal. Moreover, the fluidity of the melt would reduce with greater ceramic particles
in the melt. In addition, the large heat of fusion of alumina reinforced MMCs might
also result in the high fluidity of this composites as compared to SiC reinforced
composites (Ravi et al. 2008). Further, as already known (Lloyd et al 1989), the
reduced fluidity with increased SiC volume fractions could also be responsible for
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improved surface roughness of the A359/SiC MMCs with increasing volume fraction
of SiC particulates.
Although, due to the ‘shielding effect’ of the particles, the flow of molten aluminium
was found to be restricted, the fluidity might be still greater as compared to that in
A359/SiC composites as the melt contained very few or no particles. Moreover, the
larger size of alumina particulates might have also contributed to higher fluidity of
this MMC even at greater volume fraction (Rohatgi et al. 1981).
Thus, it can be concluded that the type of ceramic reinforcements has been found
more significant on the surface topography of the WEDM machined surfaces of
aluminium matrix composites.
7.2.2.3 Influence of Properties of Work Materials
The craters, as shown in Figure 7.1 and Figure 7.2, can be grouped in two
morphological groups. The SiC reinforced composite can be considered as group 1
(Figure 7.1) corresponds to high thermal diffusivity as compared to alumina
reinforced composites. Higher thermal diffusivity might have caused rapid
solidification of the molten metal. Therefore, comparatively smooth, shallow craters,
surfaces might have been produced in machining of these materials as compared to
alumina reinforced composites. The machined surfaces of alumina reinforced
composites, however, appear smaller in size and deeper in depth. The low thermal
diffusivity of these materials might have caused comparatively slow re-solidification
process. Further, low heat of fusion of the A359/SiCp MMCs might have caused
greater reduction in its fluidity as compared to the alumina reinforced composites.
Thus, based on the above discussions, following factors have been considered to
influence the surface characteristics of the machined composites.
• Removal of the ceramic particles (dislodged/remaining on the surface)
• Thermo-physical properties
• Type and properties of ceramic particulate reinforcements
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• Size of the ceramic particulate reinforcements
• Volume fraction of the ceramic reinforcements
• Probable chemical reactions between the matrix and reinforcements
• Fluidity of the molten material after the discharge on-time
Based on the EDX analyses the following mechanism/reactions of material removal
during WEDM process could have been happened:
1. Due to high temperature of the WEDM process, the matrix alloy (in both cases
of alumina and SiC reinforced matrix composites) might have melted and/or
evaporated.
2. The alumina reinforcements remain unaffected and may be removed by ‘drop
out’ mechanism of material removal after weakening of bond between matrix
and the reinforcements (the number is very low).
3. The un-removed alumina particles remain protruding on the machined
surfaces (large number).
4. The significant presence of these alumina particles on the work piece surface
might adversely affect the stability of the process due to unavailability of new
discharge location easily. This, thus, might cause repeated striking of series of
discharges on the same conductive spot (Tzeng et al. 2001; Pecas et al. 2003).
This arcing would produce deeper craters and, thus, worst surface roughness
due to cumulative effect of deeper craters and higher protrusions of particles.
5. The fluidity in alumina reinforced composites, thus, remains almost unaffected
due to negligible particulates in the melt.
6. In case of SiC reinforced MMCs, however, there are few possibilities:
a. The SiC particles might remain unaffected and can be removed by the
‘dropout’ mechanism.
b. These released particles affect the spark gap conditions and improve
the surface finish similar to powder-mixed-EDM due to discharge
channel dispersion, better spark transitivity and stable machining.
c. The fluidity of the melt reduces significantly due to presence of SiC
particles in the melt.
d. The SiC may react with the matrix and form Al4C3 compound at the
very high temperatures of WEDM process. This compound (Al4C3)
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reduces the fluidity of the molten material significantly (Lloyd, 1988).
However, this possibility might be ruled out in case of A359/SiC/10p
and A359/SiC/20p composites since no carbon was found on the
machined surfaces of these composites.
e. The carbon part of SiC reinforcement might be dissociated and then
vaporized at the high temperature environment of WEDM process.
Then only Al alloy and Si particles could be present on the machined
surfaces.
f. The probability of formation of oxide also cannot be ruled out. Oxide
formation would reduce the fluidity of the melt drastically.
g. Larger heat of fusion of A6061 as compared to A359 might have also
contributed to the greater fluidity of the alumina reinforced
composites.
Based on the above discussion and the EDX results, it can be said that the carbon has
been dissociated and the Si may remain in particle form in the aluminium alloy. The
presence of aluminium carbide has been assessed by X- ray diffraction analysis
(XRD). The results revealed no evidence of formation of aluminium carbide (Al4C3).
In addition, all the machined surfaces were found to show the presence of spinel
(MgAl2O4). This could be the cause of avoid of the reaction between aluminium and
carbon to form aluminium carbide (Yaghmaee and Kaptay, 2002; Lee et al., 2000).
The magnesium aluminium oxide layer may cover the silicon carbide and do not
allow any reaction between aluminium and SiC even at such temperature environment
of the WEDM spark gap. The absence of aluminium carbide causes undesirable
influence on the fatigue life, corrosion strength and fluidity of these composites.
However, in the present study, this compound was not found to form even at the high
temperatures of the WEDM process. These findings disagree with the recent findings
of Yue and colleagues on EDM and Electro-chemical discharge machining (ECDM)
of Al/SiC composites (Liu et al. 2010). Besides, the greater content of Si relative to
A359 alloy was found to agree with the findings of Seo et al. (2006). This finding also
suggests partial melting of the SiC reinforcements. The results of XRD analysis are
shown in Figure 7.7 to Figure 7.10. The detailed list of peaks is given in Appendix F.
Finally, it is believed that, the release of SiC and/or Si in the inter-electrode gap as
well as lower fluidity of the A359/SiC composites has contributed toward smooth
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surfaces of A359/SiC composites as compared to alumina reinforced composites. The
alterations in the gap conditions (discharge channel spreading and spark gap
widening) due to ceramic particles have reduced the depth of craters in the SiC
reinforced composites. The worst surface roughness of alumina reinforced composites
have been found to be attributed to the concentrations of sparks, poor transitivity of
the discharge channel and protrusions of ceramic particulates on the machined
surfaces.
Position [°2Theta]
20 30 40 50 60 70 80 90
Counts
0
1000
2000
3000
4000
SiC 10
Figure 7.7 XRD analysis of A359/SiC/10p composite
170
Position [°2Theta]
20 30 40 50 60 70 80 90
Counts
0
200
400
600
SiC 20
Figure 7.8 XRD analysis of A359/SiC/20p composite
Position [°2Theta]
20 30 40 50 60 70 80 90
Counts
0
500
1000
SiC 30 WS
Figure 7.9 XRD analysis of A359/SiC/30p composite
171
Position [°2Theta]
20 30 40 50 60 70 80 90
Counts
0
500
1000
1500
Alumina 20
Figure 7.10 XRD analysis of A6061/Al2O3/22p composite
7.2.3 Study of Recast Layer
A layer is formed by molten metal which is not flushed away by the dielectric, but
resolidifies on the sample’s machined surface during cooling. The properties of this
recast layer are critical in most of the engineering applications. The, integrity of
machined surfaces has become more and more important for the newly engineered
materials since they are being put to severe service conditions. In this section, the
study on the cross-section of the machined composites is presented.
The machined surfaces of A359/SiCp, and A6061/Al2O3p were investigated for the
presence recast layer, cracks, and any other surface defect. The investigations on the
recast layer were conducted by using scanning electron microscope. It has been found
that on-time and ceramic volume fraction have significant influence on the thickness
of the recast layer. Figure 7.11 show the recast layer on the machined surfaces of
A359/SiCp at different levels of pulse on-time.
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The presence of recast layer was significant on the A359/SiCp composites. Moreover,
the effect of pulse on-time was also found to show significant influence on the
thickness of the recast layer. It has been found that the thickness of recast layer
increases with increased on-time. It is known that the amount of molten metal which
can be flushed away by the dielectric is almost constant (Bozkurt et al., 1996).
Therefore, with increased on-time, since supplied energy is increased, the amount of
molten material also increases. The recast layer thickness, thus, was found to increase
with increased on-time. The thickness of recast layer was also found to be influenced
by the volume fraction of ceramic reinforcements. The thickness was found to
decrease with increased volume fraction of SiC/Si particulates. It might be attributed
to the SiC particulates released in the spark gap producing the powder-mixed WEDM
effect. During machining of high volume SiC reinforced MMCs, greater number of
particulates might be present in gap causing better dispersion of the spark resulting
into thinner recast layers as compared to lower volume of SiC reinforcements (Tzeng
and Chen, 2005; Syed and Kuppan, 2013). The decreased recast layer thickness was
found to increase with the reduced thermal conductivity of the work material of the
molten material. The change in the form of the recast layer, rather, was very
distinguishable in alumina and SiC reinforced composites. Thus, the type of
reinforcements was also found to show prominent influence.
The presence of significant recast layer on the machined surfaces of A6061/Al2O3p is
shown in Figure 7.12. It is evident that the pulse on-time influences the thickness of
the recast layer. The recast layer was found to vary with the volume fraction of the
alumina reinforcements. The prominent presence of alumina particles on the
machined surfaces of the A6061/Al2O3p composites could be the cause of increase in
the recast layer. This could be attributed to greater power density caused by spark
concentration relative to SiC reinforced MMCs.
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Figure 7.11 Recast layer of A359/SiCp MMCs
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Figure 7.12 Recast layer of A6061/Al2O3p composites
It can be observed in Figure 7.11 and Figure 7.12 that recast layer of silicon carbide
reinforced composites are significantly even as compared to the uneven layer of
alumina reinforced MMCs. This has found to be attributed to the presence of
significant number of Al2O3 particles on the machined surfaces of these MMCs. On
the other hand, the recast layer thickness of A359/SiC composites was found to less as
compared to A6061/Al2O3 composites. This was found to be attributed to the released
SiC particulates in the spark gap during machining of Al/SiCp MMCs.
Thus, finally, it can be stated that the thickness of the recast layer depends on the
pulse energy (pulse current & on-time), fluidity of the molten metal (volume fraction
and type of ceramic reinforcements), and properties and volume fraction of ceramic
reinforcements along with matrix properties. In addition, the effect of ceramic
particles (if present in the gap) on the gap conditions is also very influential.
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7.2.4 Surface Appearance and Defects
In this section, discussion on the observed defects on the machined surfaces is
presented. The high energy of the WEDM process induces various defects on the
machined surfaces of the metallic components. These defects include cracks, voids,
black patches, and bump marks in addition to surface softening. These defects have
been considered as the outcome of various process parameters including spark energy
and the electrode specific thermo-physical properties such as thermal and electrical
conductivities, melting temperature, boiling temperature, coefficient of thermal
expansion, heat of fusion and density of the cathode and anode materials. The surface
defects, in machining of MMCs, however, were found to be significantly influenced
by the type, size and volume fraction of the ceramic particulate reinforcements.
In the present study, the cracks, voids, pock marks and black patches were observed
on the machined surfaces. Moreover, particle pull-outs were observed in all types of
MMCs. Cracks were found to be induced due to high thermal stresses induced on the
machined surfaces due to rapid cooling of the molten metal. The cracks could also be
generated due to high electrical forces applied during the machining process. The
appearance of cracks, however, was significant on the machined surfaces of
A359/SiCp composites only. The machined surfaces of alumina reinforced composites
were found to be free from cracks. This finding agrees with some of the earlier studies
by various researchers (Brahmankar et al., 2000). The differentiating feature of the
machined surfaces of different MMCs might be attributed to the different types of
matrix materials. Since the matrix of SiC reinforced MMCs is a cast aluminium alloy,
the inherent defects in the cast material might be responsible for the occurrences of
cracks on the surface of A359/SiC MMCs. The observed cracks on the surfaces of
A359/SiCp are shown in Figure 7.13.
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Figure 7.13 a) Particle pullout, b) Cracks on machined surfaces of A359/SiCp
composites
Figure 7.14 Cracks on machined surfaces of A359/SiC/30p composites resembling
thermal spalling mechanism
The cracks observed on the machined surfaces of A359/SiC/30p could be attributed to
the thermal spalling mechanism of material removal, which is generally dominant
material removal mechanism in EDM machining of ceramic materials. The major
reason of this mechanical mode of material removal is very high melting temperature
of ceramic materials as well as poor performance of these materials in tension.
Although, the presence of electrically conductive aluminium alloy in the A359/SiC
composites determines the material removal mechanism in the present study, the
possibility of spalling cannot be completely ruled out in MMCs with higher volume
fraction of ceramic reinforcements such as in case of A359/SiC/30p. Recent
investigations in machining of Al/SiC composites using EDM and ECDM has also
claimed thermal spalling as the major material removal mechanism (Liu et al., 2010).
The type of cracks shown in Figure 7.14 has been observed on the machined surfaces
of A359/SiC/30p MMCs.
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In addition, voids, chimney, pock marks, re-solidified globules, and black patches
were observed on the machined surfaces of Al/Al2O3p. The black patches, attributed to
abnormal arcing, were, however, observed only on the machined surfaces of alumina
reinforced composites. This might be due to high arcing tendency during machining
of this material. Similar black patches were reported in the earlier studies on
WEDM/EDM machining of alumina reinforced aluminium matrix composites and
cemented carbides (Gadalla et al., 1992; Brahmankar et al., 2000; Mahdavinejad et
al., 2005).
The observed black patches are as shown in Figure 7.15. These patches have found to
be attributed to abnormal arcing. It was found, during machining, that arcing was very
frequent in machining of the alumina reinforced composites only. This difference in
the frequency of arcing might also be the cause for the lower cutting speed in
machining of the alumina reinforced composites.
Arcing is not desirable in WEDM since it could lead the process towards wire
breakage and also damage the machined surface. Arcing is determined by the
magnitude of the ignition delay time and it is either due to poor transitivity of
discharge channel or excessive debris in the gap and continuous repeated discharge at
the same spot or location. Low or almost zero ignition delay might result due to
highly contaminated spark gap and ease in initiation of discharge. The spark gap
contamination is determined by the volume of the molten metal in the previous pulse
and also the flushing efficiency of the process. However, the reason of arcing might
be unavailability of sufficient conductive path on the surface (due to protruding
alumina particles) as well as excessive debris, due to concentration of spark, in the
gap to assist easy ignition. Thus, the black patches appeared on the alumina reinforced
composites are attributed concentration of spark energy repeatedly on the same spot
or discharge location. In addition, it is also found to be attributed the low flushing
efficiency at relatively small gap size of the process in machining of this material.
This phenomenon of burning due to arcing was, however, not found on the machined
surfaces of SiC reinforced composites. This could be attributed to the relatively stable
machining conditions due to widening of the spark gap and dispersion of the
discharge channel caused by the presence of SiC/Si particulates in the inter-electrode
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gap. The burning marks are the evidence of abnormal arcing during machining these
materials. The EDS analysis further confirm this by the presence of carbon in the
vicinity of the black spots on the machined surfaces. It is interesting to note that these
black spots are located almost surrounding the alumina particles. This fact also
support the phenomenon of arcing caused by the discharge concentration due to poor
transitivity of the discharge channel caused by the shielding effect produced by
surrounding alumina particles.
Figure 7.15 Black patches on the machined surface of Al/Al2O3/10p
7.3 Cutting Rate The cutting rates were found to be significantly high in machining of A359/SiCp
composites as compared to the A6061/Al2O3p composites. This difference in the
cutting rates of these MMCs have found to be attributed to the thermo-physical
properties of the ceramic reinforcements (especially electrical resistivity), composites
materials, the matrix materials along with the gap alterations due to the presence of
different ceramic particulates in the MMCs.
The machining efficiency of the WEDM process depends mainly on the stability of
the process. The stability, generally, is attributed to the spark gap distance, effective
removal of debris from the gap and presence of debris in the gap to facilitate the
ignition at a larger spark gap (Wong et al., 1998). However, the role of debris is very
complicated. The absence of debris can result in arcing owing to the lack of a precise
feeding mechanism (or cause servo hunting). This happens in the initial stages of
ignition. On the other hand too much debris is, generally, believed to be the dominant
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cause of spark concentration (arcing) that leads to unstable machining (Luo et al.,
1997 and Mohri et al., 1991).
A stable machining process, however, demands evenly dispersed discharge locations
that depend mainly upon debris concentration and distribution. Therefore, gap debris
have been reported to have most significant effect on the discharge transitivity, gap
size, breakdown strength and deionization (Luo et al., 1997; Tzeng et al., 2001).
Based on the above discussion and knowledge, the following factors are, believed to
be, influential for significantly different cutting rates during machining of these
composites:
• The alumina particles were found to be unaffected during the process. These
insulating particles were found protruding on the machined surfaces. Thus, the
initiation of ignition might be difficult due to poor electrical conductivity of
alumina compared to SiC. Therefore, the transitivity and even dispersion of
the discharge channel could also be adversely affected. Thus, spark
concentrations, arcing and unstable process behaviour was obvious as
discussed above. The black patches shown in Figure 7.15 supports the
concentration of sparks adjacent to the alumina particles.
• Due to high protrusions of the alumina particles on the machined surfaces, the
flow of the molten matrix might have been impeded. This can also be seen
from Figure 7.2. This inefficient flushing might result into excessive
conductive molten material in the spark gap. During this type of gap
contamination, abnormal arcing may take place due to reduced or even no
ignition delay time. The control system of the WEDM machine may retract the
wire electrode back, in such situations, to avoid wire breakages by reducing
the supply current.
• The two conditions discussed above were observed frequently during
machining of the alumina reinforced composites. The process was not stable
during machining this type of material and the electrode retraction was
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frequently observed on the machine tool monitor. If the indicator shown
positive value, the gap is too large and the speed increases. Moreover, if it
shows a negative value, the gap is too short and the control action reduces the
wire forwards speed or retract the wire away from the work piece.
• Severe fluctuations in such gap status were observed during machining of
alumina reinforced composites especially at low pulse on-time (0.2 µs). This
could be attributed to the lack of debris in the gap to initiate the ignition and
insufficient energy to break down the dielectric strength. The servo might be,
therefore, hunting for the discharge location and also accompanied by spark
concentration when the spark is initiated.
• However, these fluctuations were not observed during machining of the SiC
reinforced composites. The process was found to be stable during the total
cutting length of these materials. The stability of SiC reinforced composites
have found to be attributed to the complete release of the SiC and/or Si
particles from the machined surface. These particles once released, could
remain in the gap to assist initiation of the next spark at a relatively large spark
gap. This is well supported by experimental studies on powder mixed EDM
process (Wong et al., 1998; Schumacher et al., 2004; Tzeng et al., 2001).
Moreover, earlier studies by Gatto (1997); Ramulu (2001) and Seo et al.
(2006) also reported similar results although the powder mixed effect was not
been discussed.
• The stability during machining of SiC reinforced MMCs was found to be
attributed to the increased gap size, effective removal of the debris, and thus
better transitivity of the discharge. Moreover, with the increased number of
SiC and/or Si particles released in the gap might cause greater stability of the
process.
Based on the above discussions, it appears that the SiC particles alter the gap status
favourably as compare to the alterations made by the alumina particles. It is possible
because the SiC particles have been easily removed in WEDM gap. The machined
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surfaces also support this easy removal of SiC particles as compared to the removal of
Al2O3 particles.
Therefore, finally, it can be concluded that the abnormal gap status and arcing is the
main cause of low cutting rates in machining of alumina reinforced composites. The
larger size of alumina particles might also be responsible for this significant
difference in the surface morphology as well as cutting rate.
7.4 Kerf Width
Although the difference is not very significant, as compared to the kerf width of SiC
reinforced composites kerf were found to be on lower side in alumina reinforced
composites. This could be attributed to the material properties, stability of wire
electrode, and wear of wire electrode. In addition, the discharge gap widening caused
by the presence of SiC/Si particles released in the spark gap has also been found to
influence the gap width. Thus, effects of material properties on kerf width might be
reduced by the widening of the spark gap. Although, the kerf width was expected to
be greater in case of A359/SiCp composites due to gap widening, the difference was
found to be very small. This could be attributed to reduced lateral gap at greater
cutting rates as observed in these composites compared to alumina reinforced MMCs.
7.5 Wire Electrode Breakages
The technologies of WEDM have grown rapidly in last two decades owing to the
requirement in various manufacturing fields. However, a few problems involved in
WEDM are still far from solved. Among them, wire breaking during machining
process is one of the serious problems in the application of WEDM. The occurrence
of wire rupture would result in a great increase of machining time, a decrease of
machining accuracy, and the deterioration of the machined surface.
If gap voltage is the feedback for the servo system to decide on the gap status and
consequently on the forward feed speed of the wire electrode, then it may fail in
conditions when the wire comes across non-conducting ceramic particles. This is
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explained like this: even at small actual gap width, the gap voltage sensor may not fed
back the real status of the gap when the gap is contaminated by a number of non-
conducting particles and therefore the system may fed the wire very rapidly after a
feedback signal of larger gap. This rapid forward speed of wire may cause the wire
electrode to collide with the work material (ceramic particles). This blind forward
feed of the wire electrode is termed as ‘blind feeding’. The collision of the blindly fed
wire electrode into the cluster of ceramic particles might lead to wire breakage.
If the spark gets a conductive aluminium alloy, instead of spark, an arc can be formed.
This arc, however, would lead to damaging effects on the electrode. In both these
cases, the wire breaks. This kind of behaviour was observed during machining of
alumina reinforced composites at low level of energy, i.e. at 0.2 μs on-time. The gap
was very much fluctuating and the cutting speed was not stable during machining of
alumina reinforced MMCs. This unstable wire forward feed is attributed to the
presence of non-conductive particles in the wire path, since the electrical resistivity of
alumina particulates is significantly higher as compared to that of SiC particulates.
Thus, although the servo voltage setting is selected as of that for steel or aluminium,
the chances of wire breakages are very high as the wire electrode is subjected to
greater heat due to spark concentration and arc leaving more molten aluminium in the
gap. The increase in the upper and lower flushing pressure during machining of
alumina reinforced MMCs could be the reason of this excessive gap contamination
causing abnormal arcing. Further, the colour of spark was also significantly abnormal
and deteriorated during machining of these MMCs. Therefore, one of the options is to
increase the value of servo voltage for machining of ceramic reinforced composites.
However, this will result in lower cutting speed and low productivity. Figure 7.16
shows the surface and end of the broken wire electrode. The broken part reveals a
fluid like nature of the wire surface due to excessive energy supplied to the wire
electrode due to abnormal arcing. Moreover, the EDS analysis reveals presence of
aluminium on the surface of the wire electrode. It supports the possibility of arcing
and even bridging of the electrode gap by the molten aluminium alloy caused by
concentration of spark energy at the same spot due to poor discharge transitivity
associated with the presence of insulating alumina particulates on the work surface.
The wire breakages, therefore, have been found to occur more frequently during
machining of alumina reinforced MMCs. Thus, it can be concluded that wire
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breakages, mainly, occur due to abnormal arcing and the role of mechanical failure
due to blind feeding was found to be insignificant. Therefore, to avoid wire breakages
in machining of ceramic reinforced metal matrix composites, abnormal arcing should
be avoided by maintaining proper feedback about the gap status as well as by proper
flushing of the gap.
Figure 7.16 Micrograph of broken wire electrode
Figure 7.17 EDS analysis of the broken wire electrode
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7.6 Wire Electrode Shifting
Wire shifting is one of the major issues associated in machining of ceramic reinforced
composites. It affects the integrity of the machined surfaces adversely. The presence
of non-conducting ceramic particles such as silicon carbide or aluminium oxide was
considered as the major cause of wire shifting. Wire shifting leaves a bump mark on
the machined surfaces. This mark is undesirable and need to be addressed by
employing post processing. Wire shifting, therefore, has been identified as big
problem as breaking of wire electrodes during machining.
If the servo voltage is kept at high values and off-times are increased to avoid wire
breakages, the productivity is adversely affected due to long ignition delay time and
reduced pulse frequency. Longer ignition delay period leads not only to low
machining rate but also adversely affects wire electrode equilibrium. Over the years, it
has been accepted that wire electrode slackness takes place at low pulse frequency
and wire vibration is a problem at higher pulse frequency. The off-time contributes
toward the wire equilibrium and consequently wire shifting since it increases the
ignition delay time, reduces the pulse frequency and consequently increases the wire
slackness (Altpeter et al., 2004; Giandomenico et al., 2008). This unbalance of wire
electrode further leads to streaks and band marks on the work piece surface.
Wire shift, generally take place when the wire do not get any conductive path. In this
case, the wire search for conductive path and may get it before the front gap is too
short to form arc or collide with the insulating ceramic particles. Thus, the wire
follows a new path of least resistance. Moreover, this new path can only be found and
achieved if ignition is possible by break down of the insulated gap. In general, it has
been observed that the gap width must be smaller to allow the wire to find a new
conductive path (Yue et al., 1996; Altpeter et al., 2004; Giandomenico et al., 2008).
Moreover, the wire may also be able to find a new path if the open circuit or no-load
voltage is high enough to breakdown the dielectric strength. However, in machining
of particulate reinforced MMCs, the presence of ceramic particulates (SiC/Al2O3)
might also assist the ignition of new spark. Therefore, the shifting may require greater
ignition or open circuit voltage to overcome the insulating properties. In the present
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study also all wire shifting have taken place when the open voltage was set at high
value (100V and 120V) during the screening experiments.
Previous studies in machining of steel or other monolithic materials have reported the
phenomenon of wire shifting. The authors of these studies claimed that wire shift is
attributed it to the presence of non-conducting ingredients in the machined material
(Klocke et al., 2005). In the present study, the work piece materials are aluminium
matrix composites reinforced with SiCp or Al2O3p. The reinforcements of these
MMCs are electrically insulators. It is known that WEDM/EDM process can machine
only electrically conductive materials and insulators cannot be machined by WEDM.
Thus, it is believed that the concentration and focus of the supplied spark energy
remains on the conductive matrix material only.
The presence of the ceramic particles in the small machining gap may alter the status
of the inter-electrode gap. The alteration of the inter-electrode gap can be of different
types. Clustering of ceramic particles, in the spark gap, might increase the dielectric
strength of the spark gap. However, if they don’t form a significantly large cluster,
these particles might also assist ignition even at larger gap width.
This study has revealed the important fact that the number of occurrences of wire
shifting is significantly more during machining of SiCp MMCs as compared to
alumina reinforced composites. This is attributed to the release of SiC/Si particulates
in the inter-electrode gap. However, during machining of alumina reinforced MMCs,
the lateral ignition (wire shifting) was not found to be assisted due to the absence of
released alumina particles in the spark gap. Therefore, arcing and wire breakages were
found to be more frequent during machining of A6061/Al2O3p composites.
In the present study, based on the observations during machining process, the first
type of alteration of the spark gap is, believed to be, influencing the occurrences of
wire shifting. Significant change in the wire forward feed rate was observed and noted
during screening experiments of both types of composites. The change in the wire
forward feed rate was a negative change, i.e. the wire forward feed was found to be
reduced significantly for a fraction of moment. The location of wire, in the work
material, at time of reduction in the cutting rate was noted carefully. These machined
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samples were then carefully observed and also assessed by using Scanning electron
microscopy (SEM) for further investigations. Figure 7.18 show bump or band mark
on the machined surfaces. Figure 7.19 show the results of EDS analysis of these band
marks.
Figure 7.18 Band marks on a) A359/SiC/30p and b) A6061/Al2O3/22p composite
a)
b)
Figure 7.19 EDS of Band marks on a) A359/SiCp and b) A6061/Al2O3p composite
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Based on the observations made during machining experiments, and the study of
machined surfaces, these band marks have found to be attributed to the presence of
insulating ceramic particles in the machined composites. Finally, the following
sequences are thought to have taken place during machining of these samples.
• A cluster of ceramic particles might have come across the forward travelling
wire electrode during machining. This clustering could be due to high energy
in the discharge channel.
• The spark might not ignite when the wire electrode come across these
electrically insulating particles.
• The feedback system might send a misleading signal about the status of the
gap due to the insulating particles in the gap. The control action could then be
to reduce the forward feed speed of the wire electrode. However, although, the
lateral gap width is not small enough, at higher open circuit voltage the wire
may shift laterally by breaking the strength of the spark gap.
• The wire electrode now searches for conductive path of least resistance. At
this point of time, if the lateral gap width is low enough to overcome the
dielectric strength of the lateral gap, the wire may shift laterally due to
sparking laterally instead of front sparking.
• In other case, if the wire forward speed is increased due to the difference in the
actual gap voltage and the set value of the gap voltage, i.e. Aj, the forward
moving wire might collide with the work piece. This could be termed as ‘blind
feeding’ of the wire electrode. This collision, however, could be avoided if the
lateral gap is small enough to overcome its dielectric strength. In addition, the
presence of debris (SiC/Si) might also assist the lateral ignition.
• Increase in wire forward feed speed was found to take place in alumina
reinforced MMCs prior to wire breakages due to significantly higher electrical
resistivity of alumina particulates as compared to SiC particles. Thus, in SiC
reinforced MMCs, these released SiC and/or Si particles could assist the spark
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to initiate by formation of a bridge of relatively conductive path. Therefore,
the wire forwards feed was found to be reduced drastically prior to the wire
shifting.
The above sequences of actions taking places in the gap were also observed during
machining of these materials at certain level of process parameters. Based on these
observations the following parameters are found to be significant on the occurrences
of wire shifting.
• Pulse off-time
• Pulse on-time
• Open circuit voltage
• Wire tension
• Volume fraction and type of ceramic reinforcements
Thus, it can be, thus, concluded that pulse off time, servo voltage, on time and open
circuit voltage might be very influential on the phenomenon of wire shifting (Altpeter
and Perez, 2004; Schumacher et al., 2004). In addition, wire mechanical tension
contributes towards the wire vibration and wire slackness. Therefore, by counteracting
the wire slackness and vibrations, higher wire mechanical tension may help reducing
wire shifting.
7.7 Recommendations for strategy to machine MMCs
Based on the experiments, observations and literature survey, it has been found that
two major problems, wire breaking and shifting, arise during machining of ceramic
reinforced aluminium matrix composites. Therefore, it is necessary to develop entirely
separate machining strategy to machine these materials. The results show that the
pulse frequency may vary significantly during machining of MMCs. These significant
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variations might be attributed to the presence of entirely dissimilar phases of these
materials (metal and ceramic), and the significant difference in the thermo-physical
properties of the reinforcements and the metallic matrices. It is believed that the
presence of non-conducting ceramic particles, excessive molten material or almost no
debris in the spark gap lead to either wire shifting or to breakages of wire electrode.
The conservative machining setting to avoid wire breakages include longer off-time,
high servo voltage, greater wire speed, low wire tension and low pulse on-time.
However, at these machining settings the productivity and dimensional accuracy of
the machined components is lost. In addition, the integrity of the machined surface is
adversely affected due to shifting of the electrode wire at low wire tension, high open
voltage, short pulse on-time and longer off-time.
Based on the observations of the machining process, it has been found that ignition
delay time might be affected due to the presence of ceramic particles in the machined
materials. It was observed, during experiments, that the sparking frequency was too
low or rather negligible during machining at conservative machining settings such as
longer off-time, low on-time, and low wire tension. It is believed to be attributed to
the excessive time required to initiate the spark at some point of time and place during
the forward cutting motion of the wire electrode. As discussed earlier in the section of
wire shifting, these locations were noted and SEM investigations of such locations on
the machined surfaces were conducted. The SEM and EDX analysis of these specific
locations of the machined surfaces revealed that the presence of cluster of silicon
carbide was found to be excessive in A359/SiC composites. Similarly, the alumina
particles were found to be significant near the wire shift mark in the A6061/Al2O3
composites.
Therefore, the observed reductions (almost zero) in the cutting speed, the wire shift
mark at the location of the reduced wire forward speed and the presence of clustered
ceramic particles are, believed to be, interrelated and interdependent. The reduction in
the wire cutting speed is believed to be attributed to the changed gap status. The
ignition delay might be attributed to the presence of insulating ceramic particles in the
spark gap. Thus, based on the experimental setting during wire shift in the screening
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experiments, of the following conditions are believed to promote the shifting of wire
electrode:
1. Low Wire tension: at lower wire mechanical tension, the wire can shift toward
the lateral surfaces to find new path of least resistance for ignition. Thus, low
wire tensions assist the shifting of wire electrode.
2. Longer Pulse off-time: At longer off-times, the spark gap might be completely
flushed during the off-time. This might increase the spark gap dielectric
strength causing increase in the ignition delay time of the next spark. In
addition, wire electrode slackness is probable at low pulse frequency.
3. Higher Open circuit voltage: at higher open voltage, the spark can easily jump
on the side wall surfaces on the work material. Thus, even at wider spark gap,
the wire shifting is possible at greater open voltages. This is possible due to
small lateral gap width as compared to front gap width at significantly
moderate cutting speeds.
4. Longer Pulse on-time: at longer on-times, the removed molten material is
larger, thus, the possibility of longer ignition delay is reduced. Therefore, at
longer on-times, the wire shifting was, generally, not noted.
5. Greater volume fraction of ceramic particles: the frequency of occurrences of
wire shifting was found to be more in machining of MMCs with higher
volume fractions of ceramic reinforcements. It could be due to the ineffective
removal of these particles as well as their clustering in the gap or on the
machined surfaces during machining. Thus, the volume fraction of ceramic
reinforcements assists the increased ignition delay time and therefore the
shifting of wire. However, the electrical resistivity of the ceramic particulates
might have also caused the different gap status in these MMCs.
Similarly, the above mentioned factors also contribute toward the breakages of wire
electrode.
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1. High wire tension: it might cause breakages of the wire electrode due to high
tensile load at even higher temperature during machining. Further, at increased
ignition delay time, the wire may not shift and collide with the work material
(ceramic cluster) because of high mechanical wire tension. Thus, the greater
wire tension assists the wire breakages.
2. Longer pulse off-time: it may avoid the wire breakages by increasing the
ignition delay and consequently the reducing the pulse frequency. Thus, the
longer off-times assist to reduce wire breakages.
3. High open voltage: at high open voltage, the wire may find path along the side
wall as well as may also break the dielectric strength during machining. Thus,
the ignition delay could be reduced. In addition, the wire breakages could also
be reduced due to easier ignition at comparatively wider gap width. However,
at excessively high open voltage abnormal arcing or even bridging of the spark
gap by the molten metal and ceramic particles might cause severe wire
breakages. Thus, the open voltage little more than that employed for the
aluminium alloy should be used for aluminium MMCs.
4. Low pulse on-time: at longer on-times, the energy in the gap is greater.
Therefore, the wire breakages could be severe at these high values. Thus, low
on-times generally avoid wire breakages. However, in MMCs, the wire may
feed blindly in to the ceramic particles and collide at too low on-time. During
machining of the alumina reinforced MMCs, the concentration of the spark
energy due to poor discharge transitivity was found to assist the wire
breakages even at low on-times.
5. Lower volume fraction of ceramic particles: this will not alter the gap status
significantly. The wire breakages as well as wire shifting can be avoided with
the machining settings near to the settings recommended by the machine
manufacturer.
Although wire shifting and breakage found to pose limitations in machining of these
MMCs, they cannot be easily avoided by selecting a set of machining parameters due
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to random behaviour of the WEDM process as well as the random nature of the
ceramic particles in the work piece. Therefore, it is believed that a new machining
strategy is necessary to machine these materials. This strategy should take care of the
gap status and take corrective action during the machining process.
The major parameter identified to measure for determining the gap status is ignition
delay time. Based on the on-line measurement of the ignition delay time the status of
the gap could be interpreted and then corrective actions can be taken to avoid either
wire breakages or the wire shifting. The proposed machining setting, adjustable
parameters, for controlling the gap are open voltage, wire tension, pulse off-time,
pulse on-time, servo voltage, flushing pressure and wire speed. According to the
symptoms reflected in the variations of machining conditions during the wire shifting
and rupture process, based on the results obtained in this study and the experiences in
WEDM, a scheme is proposed to prevent wire breakages as well as wire shifting in
machining of ceramic particulate reinforced aluminium matrix composites. Figure
7.20 illustrates the flow chart of the scheme.
To apply this strategy, the ignition delay period is to be measured accurately and fed
back if the measured ignition delay time (td) is too short than the reference ignition
delay time frequency (Td) or less than the lower limit of the ignition delay time (TDL),
the discharge parameters at the "microseconds" order should be adjusted immediately
to decrease of discharging energy and pulse on-time to reduce the pulse frequency. In
addition, the off-time and wire mechanical tension should be reduced to avoid wire
breakages. Further, the flushing pressure and wire speed should be increased to
prevent wire breakages. The pulse on-time and short pulse time should be reduced
when the measured ignition delay is too small than the reference delay time. The
range of ignition delay time should be determined by doing extensive experimental
measurements of the delay time in machining of these composite materials.
If the ignition delay time is very long as compared to the reference delay time or
greater than the higher limit of the reference ignition delay time (TDH) to be
determined by experimental measurement of the ignition delay, a bad gap condition,
i.e. presence of nonconductive particles in the gap is encountered. In this case, wire
breakages due to “blind feeding” are possible. Moreover, wire shifting is also
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probable due to lateral sparking. If this state of process lasted for a relatively long
period of time, the open circuit voltage, flushing pressure, wire tension, wire speed
and short pulse time should be increased to avoid lateral shifting of the wire electrode.
In addition, parameters such as servo voltage, water conductivity and maximum no
load feed rate which have long time response on the process should be adjusted at
once to cope with the such bad gap condition. In both cases, the ignition delay time
should be kept within a certain range so that high metal removal rate can be achieved.
The present strategy can be further developed by doing extensive experiments and
collections of data related to peak current, ignition delay time, gap voltage and
machining frequency.
In addition, the application of the powder mixed dielectric also show promise in
machining of these MMCs. Appropriate powder and mixture may lead to successfully
avoid wire breakages as well as wire shifting in machining of such materials. A study
will need to correlate the ignition delay and powder mixture so as to improve the
process stability and performance.
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Figure 7.20 Flow chart of proposed strategy for machining of ceramic reinforced
MMCs.
START
INITIALIZATION
DATA INPUT
INCREASED: Flushing pressure, Pulse off-time, wire speed
DECREASED: Pulse on-time, Wire tension
IF td ≤ TDL
IF td ≥ TDH
YES
NO
INCREASED: Open circuit voltage, flushing pressure, pulse on-time, short pulse time, wire
tension and wire speed
DECREASED: Pulse off-time
YES
NO
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7.8 Conclusions
In this chapter, comparison on machinability of two different aluminium matrix
composites is reported. In addition, discussions on vital observations during
machining of these composites are presented. The critical issues such as wire
breakages and wire electrode shifting are discussed at length. A comparative study on
the machined surfaces of these composites is also presented. Finally, a machining
strategy, based on ignition delay time, is also proposed as one step towards
developing machining technology for ceramic reinforced metal matrix composites.
Based on this study, the following conclusions may be drawn:
• The WEDM performance of A359/SiCp and A6061/Al2O3p MMCs were found
to be significantly different. Although, melting and/or evaporation of
aluminium alloy matrix were found as the major material removal mechanism
in these MMCs, thermal spalling could be the reason of material removal in
A359/SiC/30p composite.
• Cutting rate and surface finish were found to be better in machining of
A359/SiCp as compared to WEDM of A6061/Al2O3p composites. This was
found to be attributed to the better stability of the process during machining of
SiC reinforced composites. The observed stability in machining of this type of
MMCs could be due to gap widening, discharge channel spreading and
relatively better transitivity of discharge channel as compared to alumina
reinforced composites.
• The skewness parameters of machined surfaces of A359/SiCp and
A6061/Al2O3p have found to be significantly different exhibiting the release
and/or partial melting of SiC and protrusions of unaffected alumina
particulates.
• In A359/SiCp MMCs the surface roughness was found to reduce with
increased volume fraction of SiC particles. However, surface roughness was
found to increase with increased volume fraction of the alumina particles in
A6061/Al2O3p composites. The combined effect of deeper craters caused by
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spark concentration and the height of the particulate protrusions was found to
increase the roughness of A6061/Al2O3p composites with increased ceramic
volume fraction.
• Significant presence of recast layer was found on the machined surfaces of
both types of MMCs. Thickness of recast layer was found to be attributed to
pulse on-time and volume fraction of ceramic reinforcements. Moreover, the
recast layer was found to be very even in A359/SiCp composites as compared
to the uneven recast layers found in A6061/Al2O3p MMCs.
• Defects such as pock marks, cracks, voids and black patches were found on
the WEDM machined surfaces of these MMCs. However, A359/SiCp MMCs
were found free from the black patches and cracks were not found on the
machined surfaces of A6061/Al2O3p composites. This was found to be
attributed to the stable machining conditions during machining of SiC
reinforced MMCs. On the other hand, the concentration of spark energy due to
poor discharge transitivity caused severe arcing resulting in black patches was
found in case of alumina reinforced MMCs.
• Wire breakages were found to pose limitations on the cutting speed in
machining of these composites. The wire breakages were found to be severe
during machining of A6061/Al2O3p composites as compare to A359/SiCp
MMCs. The wire breakages were found to be attributed to the abnormal arcing
caused by the presence of insulating ceramic reinforcements on the work
surfaces (alumina). These particles have been found to resist the discharge
channel to form causing concentration of spark at the same spots. The
significant difference between the thermal and electrical properties of
aluminium matrix and the ceramic reinforcements was found to be responsible
for the frequent wire breakages during machining of these materials.
• Wire shifting was found to deteriorate the machined surfaces of both types of
composites. The number of occurrences of wire shifting was found to be more
in machining of A359/SiC/30p composites. The presence of ceramic particles
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in the inter-electrode gap and/or machined surface was found to be responsible
for the shifting of the wire electrode.
• On-time, off-time, open voltage, wire speed, and wire tension were found to
influence the phenomenon of wire breakages and wire shifting. It was also
attributed to the volume fraction, size, and type of ceramic reinforcements and
thermo-physical properties of these composites.
• To successfully machine these MMCs a new strategy is proposed. Ignition
delay time is proposed as the measure of the status of the inter-electrode gap.
• Wire breakages could be reduced by reducing the on-time, open voltage, wire
tension and by increasing flushing pressure, and wire speed.
• Wire shifting was found to be avoided by increasing the on-time, wire tension,
and flushing pressure. Moreover, it can be avoided by reducing the open
voltage and wire speed.
• The range of variations of these process parameters would, however, be
determined on the basis of ignition delay time. Therefore, to determine the
range of ignition delay time, to avoid wire breakages as well as wire shifting,
extensive experimentation and measurements of ignition delays during
machining these materials is necessary.