F. KARACA, B. AKSAKAL: EFFECT OF THE TiBN COATING ON A HSS DRILL WHEN DRILLING THE MA8M Mg ALLOY
75–79
EFFECT OF THE TiBN COATING ON A HSS DRILL WHEN
DRILLING THE MA8M Mg ALLOY
VPLIV TiBN PREVLEKE NA HSS SVEDRU PRI VRTANJU MA8M
Mg ZLITINE
Faruk Karaca1, Bünyamin Aksakal2
1Firat University, Technology Faculty, Dept. Mech. Eng., Elazig, Turkey
2Yildiz Technical University, Faculty of Chemical and Metallurgy, Dept. Metallurgy and Mater. Eng., Istanbul, Turkey
fkaraca@firat.edu.tr
Prejem rokopisa – received: 2014-11-30; sprejem za objavo – accepted for publication: 2015-01-28
doi:10.17222/mit.2014.290
Mg alloy MA8M is used in the aerospace and food industries and in biomedical applications due to its lightness and
biocompatibility. This paper presents the drilling performance of the standard HSS and TiBN-coated drill bits during the
machining of the MA8M Mg alloy at various drill rotational speeds and feed rates. After the experiments, the surface roughness,
topography and chip formation were analyzed. Atomic-force microscopy (AFM) and surface profilometer were used for this
purpose. It was observed that higher drilling feed rates and drill rotational speeds lead to lower surface-roughness values.
TiBN-coated drill bits exhibited undesired surface qualities.
Keywords: MA8M Mg alloy, TiBN coating, twist drills, surface quality, multiple regression analysis, chip formation
Mg zlitina MA8M se uporablja v letalstvu, prehrambeni industriji in v biomedicini, ker je lahka in biokompatibilna. ^lanek
predstavlja obna{anje pri vrtanju z normalnimi HSS in s svedri s TiBN prevleko na konici, pri obdelavi Mg zlitine MA8M pri
razli~nih vrtljajih svedra in hitrostih podajanja. Po eksperimentih je bila analizirana hrapavost povr{ine, topografija in nastanek
ostru`ka. V ta namen sta bila uporabljena mikroskop na atomsko silo (AFM) in profilometer povr{ine. Ugotovljeno je, da ve~ja
hitrost podajanja in vrtenja svedra povzro~i manj{o hrapavost povr{ine. S TiBN prevle~eni svedri so povzro~ali neustrezno
kvaliteto povr{ine.
Klju~ne besede: MA8M Mg zlitina, TiBN prevleka, vija~ni svedri, kvaliteta povr{ine, multipla regresijska analiza, nastanek
ostru`ka
1 INTRODUCTION
Nowadays, reducing the energy consumption and
providing a better surface quality in several manufac-
turing industries are vital for economic production
cycles. Many manufacturing industries substituted the
materials, e.g., steel was replaced with light metals or
plastics to decrease the energy consumption and/or
increase the strength/weight ratio. Although light metals
such as aluminum or magnesium are easier to machine,
the magnesium alloys have a higher specific strength and
stiffness than aluminum alloys.1–3 Metallic implants
made of stainless steel, titanium or cobalt-chromium
alloys are used for stress shielding and revision sur-
geries, improving the quality of life and the healthcare
system. On the other hand, due to their low density and
compatibility, magnesium alloys are also very promising
as orthopedic biomaterials compared to the other me-
tallic alloys such as stainless steel and titanium alloys.4
However, the unsatisfactory corrosion resistance of mag-
nesium alloys limit their application to a great extent.5
To overcome these undesired problems, in some
studies, the microstructures and mechanical properties of
magnesium alloys were processed with cyclic closed-die
forging. Using this method under various processing
conditions resulted in the desired grain size, microstruc-
tural parameters and growth mechanical properties.3 The
surface integrity of a machined magnesium alloy used
for biomedical implants could have a critical impact on
its corrosion resistance. The influence of the cutting edge
radius and the cooling method on the surface integrity
was investigated. Cryogenic machining using a large
edge-radius tool led to a thicker grain-refinement layer
that remarkably enhanced the corrosion performance of
the magnesium alloy.5 During another surface-integrity
treatment, synergistic dry cutting/finish burnishing of
magnesium-calcium implants resulted in a good surface
finish, high compressive hook-shaped, low-residual
stress profile and extended strain hardening of the sub-
surface with little change in the grain size.4
The high-speed dry-machining process investigated
with a finite-element model predicts that the most hazar-
dous outcome, the chip ignition during machining mag-
nesium alloys, does not occur during high-speed dry
cutting with sharp PCD (polycrystalline diamond) tools.6
The effect of coated drills on the minimum-quantity-
lubrication drilling of magnesium alloys was experi-
mentally investigated using a carbon-coated HSS drill
and the AZ91 magnesium alloy. Such a coating and the
minimum-quantity-lubrication condition limited the
temperature to below the hazardous level and, hence,
Materiali in tehnologije / Materials and technology 50 (2016) 1, 75–79 75
UDK 669.721.5:621.95:620.179.11 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(1)75(2016)
both the drill wear and the magnesium adhesion were
successively reduced.7 On the other hand, the machining
of the AZ91 magnesium alloy with both the TiN-coated
and PCD tools was conducted to find the influence of
tool coatings in the machining of magnesium. It showed
an excessive tool wear of the TiN carbides even at low
cutting speeds, while the PCD coatings showed better
results at low film thicknesses.1
A limited number of the investigations were per-
formed in the field of machinability of magnesium
alloys. In this study, experimental drilling of the MA8M
magnesium-alloy sheet was conducted. The effects of the
drilling parameters such as the drill speed, the diameter,
the feed rate and the TiBN coating on the hole-surface
quality were analyzed with respect to the surface rough-
ness, the burr and chip formation and the hole accuracy.
After these evaluations, the optimum surface quality was
determined. I was also found that an increased drill feed
rate increased the roughness, an increased drill speed de-
creased the roughness and the TiBN coating increased
the surface roughness. The present study could be refe-
renced to similar studies.
2 EXPERIMENTAL WORK
In this study, MA8M Mg-alloy sheets having dimen-
sions of 100 × 100 × 8 mm3 were used. Two different
kinds of the cutting-tool material, namely HSS and
TiBN-coated HSS twist drills were used to compare their
effects on the surface roughness of the machined holes
and the chip formation. Some pilot experiments were
performed. Since the best results were obtained with a
6-mm drill diameter of the HSS tools, the TiBN coating
was considered only with this diameter and the others
were ignored. The tool diameter, feed rate and rotational
speed of the cutting tool were changed to explore their
effects. A full factorial experimentation was applied
using the parameters. In order to statistically identify the
correlation between the applied parameters and the sur-
face roughness, the multiple-regression-analysis method
with a confidence interval of p = 0.05 was used. The
drill-bit coating parameter variation of the regression
was analyzed separately. The surface-roughness
measurements were performed with a Mitutoyo SJ 210
profilometer with a 0.5 mm/s measuring speed and 0.25
× 4 mm length in line with the ISO 1997 standard. The
average surface roughness Ra (μm) was used to evaluate
the hole-surface roughness after the measurements were
repeated three times. Furthermore, an atomic force
microscope (AFM) was used to determine the 3D surface
topography with a scanning area of 40 × 40 μm2 and a
rate of 0.15 Hz (Park System XE-100).
3 RESULTS AND DISCUSSION
The surface roughness, surface topography, chip
formations and drill-bit coating were under consideration
when evaluating the machinability characteristics of the
MA8M Mg alloy. The measured values are discussed in
the following sections.
3.1 Average surface roughness (Ra)
The surface integrity is an important parameter in
machinability and is directly related to the surface
roughness.4,5 The effects of all the experimental para-
meters including the drill diameter, the feed rate, the drill
speed and the drill coating on the hole-surface rough-
ness, analyzed in a body with contour graphics, are given
in Figure 1. In order to plot these contour graphics, the
weighted least square method was used. As seen in
Figure 1, the surface roughness of the machined holes
has a tendency to decrease with an increase in the feed
rate for all drill types. When three drill diameters were
evaluated, the maximum surface roughness was obtained
at a 4-mm drill diameter, while 6-mm and 8-mm drills
presented similar values. The roughness reached the
highest values at 1710 min–1 when the 4-mm-diameter
drill bit was used and then became reduced at 2730
min–1, as shown in Figure 1.
On the other hand, the TiBN-coated HSS drill bit
enhanced the surface roughness to the highest rates
(Figure 1) compared to the other three drill bits. There
was a significant difference in the roughness between the
holes drilled with the TiBN-coated and uncoated 6-mm
drill bits. On the other hand, the roughness was reduced
with the increased drill diameter when the contour
graphics were analyzed. However, a drill speed of 1080
min–1 caused the lowest roughness as the TiBN coating
had an inverse effect on the surface roughness during the
drilling of the MA8M alloy. Although the increased drill
speed caused a decrease in the surface roughness at all
three feed rates and with the 6-mm and 8-mm drills, the
surface roughness increased with the TiBN coating as
shown in Figure 1.
F. KARACA, B. AKSAKAL: EFFECT OF THE TiBN COATING ON A HSS DRILL WHEN DRILLING THE MA8M Mg ALLOY
76 Materiali in tehnologije / Materials and technology 50 (2016) 1, 75–79
Figure 1: Variation in surface roughness with feed rate, drill speed,
drill diameter, in comparison with TiBN-coated drill bits
Slika 1: Spreminjanje hrapavosti povr{ine s podajanjem, hitrostjo
vrtenja svedra, premerom svedra, v primerjavi s svedrom s TiBN
prevleko
The drill speed of 1710 min–1 led to the peak rough-
ness from among the three drill speeds when using the
TiBN-coated drill. This speed was suggested as a switch
point aspect for the surface roughness (Figure 1). Espe-
cially the rates of 80 mm/min and 40 mm/min exhibited
similar roughnesses with all three drill speeds. The peak
value of the surface roughness (2.84 μm) occurred at the
speed of 1080 min–1, the feed rate of 20 mm/min and the
diameter of 4 mm, as shown in Figure 1. The drill speed,
feed rate and diameter were also presented as rather
parallel results when statistically analyzed. The drill
speed had the maximum effect on the surface roughness
when partial correlation values of the three parameters
were analyzed. The partial correlation values identifying
the effectiveness of the drill speed, the feed rate and the
diameter were Rpart(speed) = –0.62, Rpart(force) = –0.45
and Rpart(dia.) = –0.29, respectively. Statistical results
confirmed the above contour plots of the experiment.
The most effective parameter was the drill speed, and the
least effective parameter was the diameter of the drill bit,
also resulting from multiple regressions.
It was established with the analysis that the surface
roughness increased with a decreased drill diameter.
When Mg alloys are machined in a dry drilling con-
dition, a flank build-up occurs on the cutting tool and the
workpiece due to the adhesion.1 The flank build-up
formation on the Mg alloys causes sufficient temperature
at the tool-workpiece contact and the adhesion of the
workpiece material on the cutting tool leads to an in-
creased cutting-edge radius and to an increase in the
machining forces.1,8 The flank build-up formation, in the
current study, increased the cutting forces and, hence, the
performance of the cutting tool was reduced. This
suggests that the smallest drill diameter was not enough
to sufficiently overcome the cutting forces and a drilling
failure occurred. On the other hand, the largest drill
diameter was found to be more successful at removing
the chips, resulting in a lower surface roughness. In order
to achieve smooth surfaces when drilling MA8M, both
higher drill speed and feed rate were required.
3.2 TiBN Coating
In the present study, the surface roughness was in-
creased to a specific value at a drill speed of 1710 min–1,
and then it was diminished with the increasing drill
speed up to 2730 min–1. The cutting energy exceeded the
plastic-deformation force of the chip and a more effec-
tive drilling can be the reason for this fact. Moreover, it
was reported that higher cutting temperatures obtained
with higher cutting speeds cause material softening on
the shear plane, easier cuts and a smoother machined
surface.4,9,10 Hence, the maximum drill speed of 2730
min–1 caused a decrease in the surface roughness after
the peak value reached at 1710 min–1. On the contrary,
for the TiBN coating, the critical level of the roughness
did not distinctly differ from the uncoated drill bits.
According to the general trend, a roughness decrease was
observed via the increased drill speed. On the other hand,
the surface roughness was increased with a decreased
feed rate under all experimental conditions.
Larger cutting forces act more effectively in closing
surface cracks and pores. However, there is a limit to the
positive effect of the rolling force and beyond certain
levels, such a force acts as the initiating source of cracks
and cold welds, deteriorating the surface.4 However, the
TiBN coating had a minor effect on the surface-rough-
ness increment that was also found with the statistical
analysis (the partial correlation of 0.2745). On the other
hand, the feed rate had an extremely strong effect (the
partial correlation of -0.66) on the surface-roughness
reduction when compared with the drill speed and the
coating. In the present study, the TiBN coating of HSS
did not have the desired effect on the surface quality
because of a lower wear resistance. As the Mg alloys
caused the tool wear of the TiN-coated cutting tools, as
reported before,1,11 this approach had similar effects on
the MA8M drilling operation.
3.3 Chip formation
The achievement of a successful and better drilling
operation is also indicated by removed chip formations.
Moreover, the chip formations can show some variations
due to the workpiece material and operation parameters
such as cutting speed, feed rate or depth of cut.12 For in-
stance, when a regular broken chip or an irregular broken
chip is formed on a workpiece with elastic properties, no
chip breakers should be provided. Similarly, a workpiece
material with elastoplastic properties produces a conti-
nuous fragmentary chip or a continuous chip with a
wedge-shaped texture, and if the workpiece has plastic
properties then the result is a continuous type of chip.12
Long chips are usually not desirable because they can
tangle along the drill body and have to be removed ma-
nually.13 Instead, well broken chips are associated with a
smooth drilling process.
The diameter slightly influenced the chip formation,
and it seemed that a larger diameter provided for a better
chip formation. The feed rate seemed a more effective
parameter than the drill diameter. Most of the continuous
and a few irregular small-particle chips were produced
when the TiBN-coated drill bits were used. The analysis
suggests that the TiBN coating spoiled the drilling-pro-
cess performance and the hole quality, which was also
confirmed by the chip formation. The chip-formation
impairment was derived from an unstable material
adhesion on the drill, i.e., the surface roughness was
increased by the TiBN coating. Higher drill speeds
caused longer chip lengths and higher radii of helical
spirals caused flutes that made it hard to drill and hard to
push the chips away. Small-string chips are presented in
Figure 2a obtained with drilling at the speed of 1080
min–1, the 4-mm diameter and the 40-mm/min feed rate.
By comparing Figure 2b with Figure 2a and by
keeping the other parameters constant, it is seen that the
F. KARACA, B. AKSAKAL: EFFECT OF THE TiBN COATING ON A HSS DRILL WHEN DRILLING THE MA8M Mg ALLOY
Materiali in tehnologije / Materials and technology 50 (2016) 1, 75–79 77
smaller and irregular chips occurred as the drill diameter
changed from 4 mm to 6 mm. Since the drill diameter
was changed exclusively from 4 mm to 8 mm, a slight
difference was observed in the chip formation (Figures
2a and 2b). An insignificant difference in the chip for-
mation was observed when Figures 2b and 2c were com-
pared, being almost identical with Figure 2a. Obviously,
no significant difference in the chip formation was
observed between Figures 2a, 2b and 2c. Therefore, the
chip formation was not influenced by the drill diameter
and the feed rate. Figure 2d shows rather different chip
formations formed due to the TiBN coating including a
few long helical spiral chips that mostly had short
strings. An increased drill speed increased the amount of
the long helical spiral chips as seen from Figures 2d, 2e
and 2f. Long-string chip formations were obtained espe-
cially at 2730 min–1 (Figure 2f) though small-diameter
helical chips were observed for the holes drilled at 1710
min–1 (Figure 2e). Irregular, small chip formations
presented in Figure 2f are also seen in Figure 2e. In this
case, the drill speed and the TiBN coating were observed
to be more effective than the feed rate in the chip for-
mation.
3.4 Atomic-force-microscopy (AFM) observation
The maximum surface height of 500 nm was reached
(Figure 3). The longitudinal grooves were unclear and
shallow when Figures 3 to 5 were compared. The
drill-speed effect obtained at 2730 min–1, the 6-mm drill
diameter and the 40-mm/min feed rate was clearly ob-
served in both Figures 3 and 4. The maximum height of
F. KARACA, B. AKSAKAL: EFFECT OF THE TiBN COATING ON A HSS DRILL WHEN DRILLING THE MA8M Mg ALLOY
78 Materiali in tehnologije / Materials and technology 50 (2016) 1, 75–79
Figure 4: AFM topography of a specimen drilled at 2730 min–1, 6 mm
and 40 mm/min
Slika 4: AFM-topografija vzorca, vrtanega pri 2730 min–1, 6 mm in
40 mm/min
Figure 2: Chip formations for drilling conditions of: a) 1080 min–1,
4 mm dia and 40 mm/min feed rate, b) 1080 min–1, 6 mm dia and
40 mm/min feed rate, c) 1080 min–1, 6 mm dia and 80 mm/min feed
rate, d) 1080 min–1, 6 mm dia, 80 mm/min feed rate by TiBN-coated
drill bits, e) 1710 min–1, 6 mm dia, 80 mm/min feed rate by
TiBN-coated drill bits, f) 2730 min–1, 6 mm dia, 80 mm/min feed rate
by TiBN-coated drill bits
Slika 2: Nastanek ostru`kov pri pogojih vrtanja: a) 1080 min–1, 4 mm
premera in hitrostjo podajanja 40 mm/min, b) 1080 min–1, premer
6 mm in hitrost podajanja 40 mm/min, c) 1080 min–1, premer 6 mm in
hitrost podajanja 80 mm/min, d) 1080 min–1, premer 6 mm in hitrost
podajanja 80 mm/min pri svedru s TiBN prevleko, e) 1710 min–1,
premer 6 mm in hitrost podajanja 80 mm/min pri svedru s TiBn
prevleko, f) 2730 min–1, premer 6 mm in hitrost podajanja 80 mm/min
pri svedru s TiBN prevleko
Figure 5: AFM topography of a specimen drilled at 1710 min–1 and
40 mm/min with a TiBN-coated drill
Slika 5: AFM-topografija vzorca, vrtanega pri 1710 min–1, s TiBN
prevleko in 40 mm/min
Figure 3: AFM topography of a specimen drilled at 1710 min–1, 6 mm
and 40 mm/min
Slika 3: AFM-topografija vzorca, vrtanega pri 1710 min–1, 6 mm in
40 mm/min
the surface was elevated at about 1000 nm, and the
longitudinal grooves are clearly seen in Figure 4. The
TiBN-coating effect on the surface topography was
observed at about 1750 nm, being measured as the maxi-
mum height and rare longitudinal grooves of the surface
(Figure 5). The drill speed and feed rate were 1080
min–1 and 20 mm/min, respectively, and the lowest level
of the present experimental study led to the surface
topography of Figure 5. When Figures 3 to 5 are
compared it is found that the drill speed and TiBN
coating had an excessive negative effect on the surface
topography during the drilling of the MA8M Mg alloy.
On the AFM graphs, the maximum undulation was
observed for the TiBN-coated drill.
4 CONCLUSION
The experimental work and the analysis showed that
the MA8M alloy has a lower machinability capacity.
Although the drill-speed increasing was performed rather
well, the results for the roughness and chip formation
were not obtained with the AFM graphs. The drill speed
has a direct relationship with the cutting speed and an
increase in the cutting speed resulted in smoother sur-
faces compared with lower cutting speeds.13 But the in-
crement of the cutting speeds led to higher cutting tem-
peratures, and the temperature increase also decreased
both the cutting performance of a drill bit and the surface
integrity. For this reason, the drill speed should not be
performed at extremely high levels. On the other hand,
the feed rate can be increased by increasing the drill
speed if smooth surfaces are required. However, the
feed-rate increment has a positive effect on the surface
roughness, and it should be used carefully with smaller
diameters of drill bits. According to the results of the
experimental work, the TiBN coating was not appro-
priate for the MA8M drilling operation when compared
with the HSS drill bit. However, the wear resistance of
the TiBN-coated drilling tools should be investigated.
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F. KARACA, B. AKSAKAL: EFFECT OF THE TiBN COATING ON A HSS DRILL WHEN DRILLING THE MA8M Mg ALLOY
Materiali in tehnologije / Materials and technology 50 (2016) 1, 75–79 79