INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 16, No. 8, pp.

1689-1697 JULY 2015 / 1689

DOI: 10.1007/s12541-015-0222-y ISSN 2234-7593 (Print) / ISSN 2005-4602 (Online)

Drilling Temperature and Hole Quality in Drilling of

CFRP/Aluminum Stacks Using Diamond Coated Drill

Chang-Ying Wang1, Yu-Han Chen2, Qing-Long An1,#, Xiao-Jiang Cai1, Wei-Wei Ming1, and Ming Chen1
1 State Key Laboratory of Mechanical System and Vibration, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China
2 College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China
# Corresponding Author / E-mail: qlan@sjtu.edu.cn, TEL: +86-021-34206317, FAX: +86-021-3420-6317

KEYWORDS: CFRP, Stack, Drilling temperature, Hole diameter, Hole surface quality

Composite/metal stack materials, most often consisting of carbon fiber reinforced polymer (CFRP) composites and aluminum or
titanium alloy (Al or Ti), have been widely used in the aviation industry. It is still a challenge to make holes on the composite/metal
stack materials with high quality. This paper aims to investigate the influence of drilling parameters on the drilling of stack materials
consisting of T800/X850 CFRP and 7075-T651 Al with regard to drilling force, drilling temperature, hole diameter and hole surface
quality. Diamond coated drill bits with double point angles were employed in the drilling tests. The drilling temperature was measured
utilizing a rotational motion temperature measuring system. It is indicated that maximum drilling temperature increases with the
increase of spindle speed, while, it decreases with increasing feed rate. The thrust force and drilling temperature increase abnormally
during drilling of the Al layer, which can be attributed to the evacuation difficulty of the long and flexible Al chips. It is found that
sudden drops in the surface profile of CFRP are caused by the cavities owning to the resin degradation effects. The influences of
different stack sequences on the drilling temperature and hole surface quality have also been discussed.

Manuscript received: June 17, 2014 / Revised: March 12, 2015 / Accepted: April 10, 2015

materials.1 During the assembly of composite/metal components, tens

NOMENCLATURE of thousands of holes need to be drilled to meet the mechanical bolting
or riveting demand. The assembly accuracy of them is of vital
n = Spindle speed importance to the flight performance of the aircrafts. The assembly
f = Feed rate accuracy depends critically on the quality of machined holes. In order
Fz = Thrust force to obtain high quality holes, “single shot” drilling has been proposed as
T = Drilling temperature a potential solution instead of drilling each material separately.2
d = Drilling depth However, It is an extremely challenging task to drill of composite/metal
D = Hole diameter stacks due to the different machinability characteristics between them.
t = Time Various hole defects often occur which have seriously affected the
assembly accuracy.
During drilling of composite/metal stack materials, potential hole
quality issues may include: poor diameter tolerance, poor hole surface
1. Introduction quality, matrix resin degradation, fiber burrs and delamination. Different
researchers3-6 have indicated that most of the hole quality problems
From the F35 Lightning II to the Airbus A380 and the Boeing 787 mentioned above are mainly associated with an incorrect application of
Dreamliner, composite/metal stack materials, most often consisting of the tool geometry and inappropriate machining conditions.
multiple layers of carbon fiber reinforced polymer (CFRP) composites Diameter tolerance is one of the important indicators evaluating the
and aluminum or titanium alloy (Al or Ti), have been widely used in hole quality. The difference of elastic modulus between composite and
aviation industry. For example, the Boeing 787 Dreamliner is made up metal will cause different machining deformations. Therefore, the
of 50% composite, 20% Al, 15% Ti, 10% steel and 5% other diameters of different layers in the same hole are inconsistent and the

© KSPE and Springer 2015

 1690 / JULY 2015 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 16, No. 8

diameter deviation is usually large.7 Garrick, et al.8 studied the machined the measurement system was complex and costly.
hole diameter in drilling of CFRP/Ti stacks. The results showed that the The wear mechanism of the tool in drilling of CFRP was primarily
diameter tolerance could be up to 25 mm using a standard twist drill abrasive wear due to the abrasive nature of the carbon fibers.16,17 While,
over the course of 34 holes, while a PCD drill with the same diameter the wear mechanism of the tool in drilling of CFRP/metal stacks was
achieved a diameter tolerance of 10mm over the course of 200 holes. usually abrasive wear and adhesion wear due to metal build up edge.18-20
Park et al.9 investigated the influence of Ti layer on the size of the Diamond has ideal properties for machining of CFRP or CFRP/
machined CFRP holes in their comparative study on drilling of CFRP Alumnium stacks. The nature of hardness makes it more resistant to
and CFRP/Ti stacks. It is indicated that the drilling of Ti layer has abrasive wear than any other cutting materials.21,22 According to
significantly influences on the diameter of the CFRP holes due to the Montoya et al.,11 the use of a diamond coating can decrease the trust
increased tool instability. force (by limiting wear) by 65% for CFRP and by 35% for Al. Zitoune
In addition to hole diameter, the machined hole surface between et al.,23 Karpat et al.24-26 have studied the effect of geometry structure
CFRP and metal layer is also quite different as drilling of CFRP /metal of the drill bits during drilling of CFRP/metal stacks. The results
stacks. During drilling of the CFRP layer, the chips often generated in indicated that the double point angle drill bit outperformed the standard
the form of powders instead of ribbon chips as metal cutting proceeded, twist drill bit in terms of thrust force as well as tool wear and hole
so it is difficult to form an excellent surface finish on the hole surface surface quality. Besides, when drilling of CFRP/metal stacks, push-out
of CFRP. The surface roughness of CFRP (Ra 2~9 mm) is always much burrs and delamination of the CFRP laminates can be significantly
higher than that of metal (Ra 0.3~2 mm) due to various machining reduced due to the supporting from the metal layer(s).2,27
defects of CFRP.2,10 Cavities, micro cracks and matrix resin degradation In the present study, diamond coated drills with double point angles
on the hole surface of CFRP were observed by Xu et el.5 during drilling were employed in drilling of CFRP/Al stack materials. The aim of the
of T800S/250F CFRP laminates and Montoya et al.11 during drilling of experiments is to investigate the influence of drilling parameters on the
CFRP/Al stacks which was closely related to the too high drilling drilling force, drilling temperature, hole diameter and hole surface
temperature. quality. The influences of different stack sequences on the drilling
Drilling temperature is one of the most important factors affecting temperature and hole surface quality have also been discussed.
the machined hole surface quality, especially for the CFRP. When drilling
of CFRP, large amounts of cutting heat and friction heat will be generated
due to the abrasive nature of carbon fibers and tool wear, which usually 2. Experimental Procedure
makes the drilling temperature elevated rapidly. However, the
degradation temperature of matrix resin is usually low (150~250oC), 2.1 Workpiece materials
which is far below the drilling temperature of metals. An excessive The stack materials utilized in the present study are composed of
high drilling temperature may lead to a risk of thermal degradation of T800/X850 CFRP and 7075-T651 Al. The size of the stack plate is 300
matrix resin. Overall, this risk causes carbonization of thermosetting mm × 200 mm × 14.74 mm. The thickness of CFRP and Al is 8.74 mm
matrices and fusion of thermoplastics matrices. Sometimes, it can also and 6 mm, respectively. They are closely assembled through ten bolts.
damage the carbon fibers (burning of the carbon fibers). In addition, an The side view of the stack plate is shown in Fig. 1.
excessive high cutting speed can also significantly increase the drilling The T800/X850 CFRP composite laminate is made of 46
temperature. unidirectional-plies which are laid-up at different fiber directions (i.e.,
However, there are only few experimental studies have been done cross-ply). The laying-up sequence of the composite laminate is [±45/
on drilling temperature in drilling of CFRP or CFRP/metal stacks. M. 0/-45/0/+45/0/+45/90/-45/0/-45/0/+45/0/+45/90/-45/90/-45/0/+45/0]s.
Ramulu et al.12 studied the drilling heat induced damage on CFRP while The aluminum alloy layer, Al 7075-T651, is a typical high strength
drilling of graphite/bismaleimide (Gr/Bi) and Ti stacks. Discoloration aluminium alloy with good-corrosion cracking resistance, which is
rings around the hole exit of the composite layer due to heat effect extensively used in aircraft and aerospace industry. Table 1 and 2 present
during drilling process were observed. Drilling temperature was the detailed composition of T800/X850 CFRP and the comparative
measured by Davies, M.13 and Brinksmeier, E.14 to study the influence mechanical properties between T800/X850 CFRP and 7075-T651 Al.
of the drilling process on the surface layer of the Al/CFRP/Ti stacks.
The investigations reveal that local temperature peaks, due to high 2.2 Cutting tool
cutting speeds or evacuated Ti chips, will damage the hole surface of Diamond coated cemented carbide drill bits were utilized in this
the CFRP materials. The drilling temperature measured on the tool rake work. The geometry of the drill bit is similar to the twist drill, but it has
face was about 90oC for Al layer, 191.6oC for CFRP layer and about double point angles at the drill tip and two inner-cooling holes were
350oC for Ti layer under the cutting parameters of 40m/min for cutting designed at the flank surfaces. The drill bits were obtained from the
speed and 5 mm/min for feed rate. Le Coz et al.15 demonstrated a manufacturer KOMET GROUP. Table 3 gives the detail geometric
measurement system for rotating tools with a measurement accuracy of parameters of the used drill bits and the tool morphologies are shown
1oC. The thermocouple embedded in the tool was connected to a data in Fig. 2.
acquisition unit in tool holder and temperature signals were sent to an
amplifier by using a transmitter integrated in the tool holder and a 2.3 Experimental setup
Radio Frequency Antenna placed nearby the tool holder. Repeated Fig. 3 shows the experimental setup used in the drilling tests. The
drilling tests revealed the repeatability of the measurement. However, drilling trails were conducted on a DMU70V CNC machining center.
 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 16, No. 8 JULY 2015 / 1691

Fig. 1 Stack materials used in the present study consisting of T800/

X850 CFRP and 7075-T651 Al

Table 1 Composition of T800/X850 CFRP composite laminates

Epoxy Fiber volume Fiber
Reinforcing Thickness
Matrix content bundle
T800 X850 65% 5 µm, 12K 8.74 mm
Fig. 2 Tool morphologies of the used drill bits (a) side view, (b) drill
Table 2 Mechanical properties of the stack materials tip view
Properties T800/X850 CFRP 7075-T651 Al
Tensile strength (MPa) 5490 570
Strain (%) 1.9 11 Table 3 Geometric parameters of the used drill bit
Thermal conductivity Diameter 9.53 mm
- 130
(W/(m•k)) Cutting edge length 58 mm
Single-beam intensity
445 (12K) - First drill point angle 130o
(g/1000m) Second drill point angle 60o
Helix angle 30o
The travel range of the machined tool is defined as: X-710 mm, Y-520 Coating Diamond (4 µm)
mm, Z-520 mm, five axes (X, Y, Z, B and C); spindle speed range: 20~
12000 rpm. A full factorial experiment with 12 cutting conditions was
designed which is detailed illustrated in Table 4. In the tests, the holes
were drilled into from the CFRP layer and drilled out from the Al layer
except for Section 3.5. Constant feed rate is employed to drilling of the
CFRP and Al. Besides, all the tests were performed under dry cutting
conditions.
In terms of measurement devices, a four-component force
dynamometer Kistler 9272 was used to gather cutting force signals.
The measured signals were transmitted to a multichannel charge
amplifier Kistler 5070A, then to an A/D board and recorded on a
personal computer using data acquisition software Labview 7.1.
A rotational motion temperature measuring system was utilized in
the experiments to obtain the drilling temperature. In the measuring
system, two K-type standard thermocouples were mounted in the inner-
cooling holes at the flank surfaces of the drill bit. The location and size
of the inner-cooling holes are shown in Fig. 2. Each of the thermocouples
was connected with a data storage device though the inner-cooling hole
and the tool holder, as shown in Fig. 3. The sample frequency of the Fig. 3 Experimental setup for the drilling tests
temperature measuring system was 1 Hz. The temperature measurement
range of the K-type standard thermocouple is -260~1370oC and its
accuracy is ±0.5oC. After the tests, the temperature data storage device A three point internal micrometer was used to measure the diameter
can be connected to the computer and the temperature data can be of the holes after the drilling tests. The measurement range of the
exported. micrometer is 8~10 mm with a measurement accuracy of 0.004 mm
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Table 4 Drilling parameters and their levels

Levels
Drilling parameters
Level 1 Level 2 Level 3 Level 4
Spindle speed, n (rpm) 1000 2000 3000 -
Feed rate, f (mm/rev) 0.02 0.04 0.06 0.08

and a display accuracy of 0.001 mm. The surface roughness (Ra) of the
holes was measured by surface roughness measurement instrument
Mitutoyo SJ-210 with a sampling length (cut-off length) of 0.8 mm. The
number of the samples was set to five and the measurement distance
was 4.0 mm. The travelling speed of the probe was 0.5 mm/s when
measuring Ra of the workpiece. A digital microscope system Keyence
VHX-500FE combined with a scanning electron microscope (SEM)
JEOL JSM-6390LV was used to study the hole surface quality.

3. Experimental Results and Discussion

3.1 Thrust force

Fig. 4(a) shows the evolution of the thrust force (Fz) recorded during
the drilling of CFRP/Al stacks as a function of the feed rate. It can be
noted that each point of the Fig. 4 represents an average value of the
thrust force curves for CFRP or Al. It can be indicated that the thrust
force is almost proportional to the feed rate for both CFRP and Al. Fig. 4 Evolution of the thrust force as a function of the (a) feed rate
Furthermore, the thrust force recorded during drilling of Al was found and (b) spindle speed
to be about two times higher than these recorded during drilling of
CFRP. For example, while drilling with a spindle speed of 2000 rpm
and a feed rate of 0.06 mm/rev, the thrust force increased from 143 N machining parameters. Fig. 5(a) illustrates the thrust force and drilling
in CFRP to 286 N in Al. temperature curves as a function of drilling depth measured during
Fig. 4(b) shows the evolution of the thrust force (Fz) recorded drilling of the stacks under the drilling parameters of n = 2000 rpm and
during the drilling of CFRP/Al stacks as a function of the spindle f = 0.02 mm/rev. The starting point (A) on the thrust force and drilling
speed. It can be seen that the thrust force is slightly elevated with the temperature curves corresponds to the location where the chisel edge of
increase of the spindle speed. While drilling with a constant feed rate the drill bit enters the material. It is indicated that when drilling of the
(0.06 mm/rev) and the spindle speed changed from 1000 rpm to 3000 CFRP layer, the temperature rises rapidly and reaches the peek value
rpm, the thrust force increased by 22% for CFRP and 16% for Al. (232oC) at the end of the region AB. However, as drilling of the Al
Therefore, feed rate is the most important factor influencing the thrust layer, the drilling temperature drops quickly from the peek value. The
force no matter for CFRP or Al. drilling temperature drops from 232oC to 115oC and the decline
amplitude reaches 117oC. It is revealed that the difficult-to-cut
3.2 Drilling temperature machinability of the CFRP material. It can be attributed to the abrasive
Matrix resin in the CFRP is an amorphous polymer material. nature of the carbon fibers and the poor thermal conductivity of CFRP.
According to the temperature of the matrix resin, it can generally be During drilling of CFRP, large amounts of heat was generated and
divided into three different mechanical states: glassy, high elastic state accumulated. However, the material of Al possesses the excellent
and viscous flow state. The transition between the glassy state and high thermal conductivity property and the accumulated heat in drilling of
elastic state is called glass-transition. The corresponding transition the CFRP layer can be diffused rapidly. Therefore, the drilling
temperature is called glass-transition temperature. At room temperature, temperature did not rise but decreased.
the resin is in the glassy state. When its temperature reaches the glass- The thrust force for CFRP slightly decreases after all the drill tip
transition temperature (typically 200oC for epoxy resin), the mechanical immerged into the CFRP. As soon as the chisel edge enters the Al layer,
properties will changed significantly which will cause the resin the thrust force increases rapidly. It must be noted that the thrust force
degradation and debonding with carbon fibers. Therefore, in drilling of is elevated suddenly as the drill tip approaching the hole exit in the
the CFRP, appropriate drilling parameters should be employed to avoid region BC. It can be attributed to the problem of chip removal during
the drilling temperature reaching or exceeding its glass-transition drilling of the Al. In the lower right of Fig. 4(a), the picture of Al chips
temperature. is present under the machining parameters of n = 2000 rpm and f = 0.02
The thrust force and drilling temperature obtained during drilling of mm/rev. The long and flexible Al chips can accumulate in the spiral
the CFRP/Al stacks reveal important information about the drilling groove of the drill bit, which is difficult to discharge. The Al chips
process, which can be used to monitor the drilling state and optimize rotate with the drill bit in a high speed and squeeze with the hole
 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 16, No. 8 JULY 2015 / 1693

Fig. 6 Drilling temerature with the evolution of spindle speed and feed
rate during drilling of CFRP/Al stacks

and “second rise” of the drilling temperature are disappeared.

However, the drilling temperature decreases with the increase of the
feed rate, which is quite different from that in metal cutting.28 This
phenomenon can be attributed to the abrasive effect and the friction
between the tool and CFRP. During the machining of CFRP, the carbon
fibers act as the abrasive grains, which accelerate the friction effect and
the tool wear. When a smaller feed rate is employed, the abrasive wear
will be aggravated and the drilling period is also be prolonged,
therefore, more cutting heat will be generated in the drilling process
and the drilling temperature will be higher. As a whole, lower spindle
speed and larger feed rate can achieve a lower drilling temperature.

3.3 Hole diameter

Tight dimensional and geometric tolerances are vital important in
the assembly of the stack materials, especially in aviation industry.
Usually, 30 mm and lower diameter tolerances are required for
mechanical bolting of stack materials.7 When drilling of the stack
Fig. 5 Drilling temperature and thrust force curves with the evolution materials, the difference of hole diameters among different layers is
of drilling depth during drilling of the CFRP/Al stacks. (a) 2000 rpm, usually large due to the great differences in physical and mechanical
0.02 mm/rev, (b) 2000 rpm, 0.08 mm/rev properties among them. In order to investigate the effect of the
machining parameters on the diameter difference, the drilled hole
diameter was measured by employing a three point internal micrometer.
surface of Al and CFRP. On the one hand, the friction between the Al The diameter was measured at two locations in CFRP (hole entry and
chips and the hole surface will elevate the thrust force. On the other hole exit) and mid-thickness in Al. The measurement was performed
hand, the friction heat will also increase the temperature of the stack four times at four different angle locations of the holes and the average
materials and the drill bit. Therefore, the phenomenon of “second rise” of the four diameters is taken as the final result. The schematic
of the drilling temperature as the drill tip approaching the hole exit in illustration of the diameter measurement is shown in Fig. 7. The
the region BC. After the drill tip drilling out of the Al (location C), the measurement results of Al entry, CFRP entry and CFRP exit under
drilling process is still continued due to the length of the drill tip. After different drilling parameters are given in Fig. 8.
location D, the drilling temperature decreases gradually under the It can be seen that diameters of Al, CFRP exit and CFRP entry
cooling air. show an increasing trend with the increase of feed rate under three
Fig. 5(b) gives the thrust force and drilling temperature curves as a spindle speed levels. It can be attributed to the cutting vibration and the
function of drilling depth measured during drilling of the stacks under instability of the cutting condition as the feed rate increased. It can be
the drilling parameters of n = 2000 rpm and f = 0.08 mm/rev. As we can indicated that the diameter of CFRP is always bigger than that of Al.
see, the evolution of the thrust force and drilling temperature curves is The phenomenon was also observed by Montoya et al..11 It can be
similar to that of Fig. 5(a). It must be noted that short C-type Al chips attributed to the increased tool instability when drilling the bottom Al
are generated under the drilling parameters of n = 2000 rpm and f = layer. With a spindle speed of 1000 rpm, (see Fig. 8 (a)), the maximum
0.08 mm/rev. This greatly reduced the difficulty of the chip removal diameter difference between CFRP and Al is 50 mm as a feed rate of
issue. Therefore, both the phenomena of sudden rise of the thrust force 0.08 mm/rev is employed. While, the diameter difference with the
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Fig. 7 Schematic illustration of the diameter measurement (a) three

point internal micrometer used in the tests, (b) four angle locations of
the three point internal micrometer measuring a hole
Fig. 9 Surface roughness Ra as a function of feed rate for the hole
surface of CFRP and Al

spindle speed of 2000 rpm (Fig. 5(b)) and 3000 rpm (Fig. 5(c)) are 35
mm and 11 mm respectively. It is indicated that a higher spindle speed
can achieve a smaller diameter tolerance between CFRP and Al, which
is beneficial to the final assemblies. In addition, the diameter of the
CFRP exit is always bigger than that of CFRP entry. On the one hand,
it can be attributed to the increased tool run out when drilling the
bottom Al layer. On the other hand, the produced Al chips will erode
the resin of CFRP exit, which will be discussed in detail in Section 3.4.

3.4 Hole surface quality

The quality of the hole surface is also an important aspect influencing
the assembly accuracy. The surface roughness of the hole was measured
on the longitudinal direction of the hole by the surface roughness
measurement instrument Mitutoyo SJ-210. The value of surface
roughness is the average of six measurements at different locations of
the corresponding machined surface. The surface roughness (Ra), which
is mostly employed in industries, was taken for the present study.
Currently, the maximum acceptable surface roughness (Ra) in aviation
industry is 1.6 mm for Al and 3.2 mm for CFRP.11
Fig. 9 shows the evolution of Ra as a function of feed rate with a
constant spindle speed of 2000 rpm for Al and CFRP. It can be seen
that the increase of feed rate leads to a slightly increase in the value of
the roughness for both of the Al and CFRP layers. The drilling
experiments performed by Zitoune10 shown that, the feed rate had a
significant influence on the surface roughness Ra. However, the values
of the surface roughness Ra measured in the CFRP layers are
significantly higher than those measured in the Al layers. This can be
attributed the generation of the machined defects in CFRP. Due to the
anisotropic and non-homogeneous material structure, machinability
characteristics of CFRP are significantly different for different fiber
directions. Various machined defects lead to the uneven surface of
CFRP. Fig. 10 presents the surface profiles and surface morphologies
of the hole surfaces of the CFRP and Al under the drilling parameters
of 2000 rpm and 0.06 mm/rev. It can be seen that several sudden drops
appear in the surface profile of CFRP corresponding to an uneven
machined surface. Conversely, the hole surface of Al is very smooth
Fig. 8 Hole diameter with the evolution of the feed rate under different and the evaluation profile is flat and no sudden drops. In terms of the
spindle speed (a) 1000 rpm, (b) 2000 rpm, (c) 3000 rpm value of Ra, the value for CFRP (Ra = 2.664) is far greater than that
 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 16, No. 8 JULY 2015 / 1695

Fig. 12 Hole surface of CFRP under the drilling parameters of n =

1000 rpm, f = 0.08 mm/rev. (a) Hole surface of CFRP drilling from
CFRP to Al, (b) Hole surface of CFRP drilling from Al to CFRP

3.5 Influence of different drilling sequence on drilling

temperature and hole quality
In industrial production, the drilling sequence of CFRP/metal (CFRP
is placed on the top of metal, i.e., the holes are drilled from CFRP and
drill out from metal.) is usually employed in most cases in order to avoid
the hole exit burrs and delamination defects of CFRP. However, in
some cases, the drilling sequence of metal/CFRP has to be employed.
The influences of those two drilling sequence on the drilling process
are different. Fig. 12 present the hole surface photographs of CFRP at
Fig. 10 Evaluation profile and SEM photographs of the hole surface
critical locations under different drilling sequences. As shown in Fig.
under the drilling parameters of 2000 rpm and 0.06 mm/rev. (a) Ra =
12, at the hole exit of CFRP (i.e., interface between Al and CFRP),
2.664 for CFRP, (b) Ra = 0.488 for Al
severe edge failure and surface cracks and were observed. However,
when drilling from the Al layer, a neatly and sharp hole edge was
obtained and the surface cracks almost disappeared, in other to say, the
hole quality of CFRP is significantly improved without the support of
the Al layer. On the one hand, it has been confirmed that the double
point angle drill bit can achieve a smaller thrust force, reduce the
delamination defects and obtain an excellent hole exit even without
support.25,26 On the other hand, this can probably be attributed to the
more efficient evacuation of Al chips from the hole. Similar experimental
results were reported by Wertheim.29 It was shown that during drilling
of the CFRP/Al stacks, the flow of Al-chips may cause a “washing out”
effect of the shim material between the CFRP layer and the Al layer.
Fig. 11 Hole surface defects on the CFRP layer When drilling from the CFRP layer, the chip removal issue will be
occurred. The long and flexible Al chips generated during drilling of
the bottom Al layer may accumulated in the spiral grooves and rotate
for Al (Ra = 0.488). Taking no account of sudden drops, the evaluation with the drill bit. They will erode the matrix resin of the hole exit in
profile seems to be flat and steady. Therefore, it is precisely because of the CFRP layer, which leads to edge failure and surface cracks defects
these sudden drops that the surface roughness Ra of CFRP is elevated. and enlarges the hole diameter of the CFRP layer. The Al chips will
Obviously, these sudden drops are caused by the cavities on the surface also scrape the machined surface of CFRP and cause the degradation
of some plies of the CFRP. These cavities are composed of the matrix of matrix resin. However, when drilling from the Al layer, the Al chips
resin depressions and carbon fiber fill-out. There are mainly two reasons usually can be removed from the hole quickly and almost have no
that cause these machining defects. On the one hand, the mechanism of influence on the following drilling process. Therefore, the hole exit
material removal is different for different carbon fiber cutting directions. edge of the CFRP is neatly. The comparative drilling temperature curves
On the other hand, the excessive drilling temperature will lead to resin may give us some help to understand this detail.
degradation directly and cause interfacial deboning of resin and carbon Fig. 13 shows the drilling temperature curves measured under those
fiber which may result in the surface cracks, matrix resin depressions two drilling sequences. As we can see that the maximum drilling
and carbon fiber pull-out defects, and in turn matrix resin cavities temperature of Al/CFRP sequence is lower than that of CFRP/Al
occurred, as shown in Fig. 11. sequence. When the Al is on the bottom of the stacks, the produced Al
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(5) Severe edge failure and surface cracks are found at the exit of
the CFRP holes when drilling of the CFRP/Al stacks.

ACKNOWLEDGEMENT

This work was supported by the National High Technology Research

and Development Program of China (2013AA040104), National Natural
Science Foundation of China (No. 51105253) and Important National
Science & Technology Specific Projects (2013ZX04009-031).

Fig. 13 Comparative analysis of drilling temerature with different REFERENCES

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4. Conclusions
4. Davim, J. P., Reis, P., and Antonio, C. C., “Experimental Study of
The experimental investigation on drilling of CFRP/Al stacks using Drilling Glass Fiber Reinforced Plastics (GFRP) Manufactured by
a diamond coated drill bit with double tip point angles was conducted. Hand Lay-up,” Composites Science and Technology, Vol. 64, No. 2,
The influence of drilling parameters on the drilling process were pp. 289-297, 2004.
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