High Speed Mach. 2016; 2:26–36
Research Article
Open Access
Jay Tu*, Ted Lehman, and Nicolas Reeves
Rapid and High Aspect Ratio Micro-hole Drilling
With Multiple Micro-Second Pulses Using a CW
Single-Mode Fiber Laser
DOI 10.1515/hsm-2016-0003
Received Jun 7, 2016; accepted Aug 12, 2016
technique to produce high aspect ratio through holes with
a simple and robust setup for the production environment.
Abstract: Laser drilling is an important industrial process
for the production of various sizes of holes. In this paper,
we investigate rapid, high aspect ratio microhole drilling
using multiple microsecond pulses based on the single
pulse drilling technique reported in [17, 18]. It was established that there would be a synergistic effect if a subsequent pulse is irradiated at the target within 100 µs of
the previous pulse before the melt solidifies. However, the
peak power values of subsequent pulses decrease with
higher repetition rates. The results show that the synergistic effect could outweigh the reduction in laser power.
Another contributing factor of the synergistic effect is related to the melt ejection efficiency. As the hole deepens,
the melt ejection becomes less effective to eject the melt
completely out of the hole, resulting in a partially blocked
hole. A subsequent laser pulse needs to reopen the hole
before the hole can be deepened further. To overcome this
hole blocking problem, shooting a subsequent pulse at a
higher repetition rate also ensures that the energy absorption is more efficient when a subsequent laser pulse is irradiating at the hole blocking melt which is not yet solidified. This multiple-pulse drilling technique was applied for
through-hole drilling. It was found that the total drilling
times through an 800 µm plate were found to be 634 ms
and 21.9 ms at 13 kHz and 20 kHz, respectively. The drilling
efficiency at the 20 kHz repetition rate is drastically higher,
needing only 428 shots, compared with 8240 shots at the
13 kHz, an improvement of nearly 200 times. It is confirmed
that this multiple-pulse drilling technique with microsecond pulses using a 300 W single mode fiber laser is a viable
Keywords: Micro-hole Drilling; laser drilling Laser Ablation; Microsecond Pulse; Fiber Laser; Plasma; Melt Ejection; High Aspect Ratio Hole Drilling
*Corresponding Author: Jay Tu: Department of Mechanical and
Aerospace Engineering, EBIII, North Carolina State University,
Raleigh, NC 27695, USA; Department of Science, University West,
SE-46168, Trollhättan, Sweden; Email: [email protected]
Ted Lehman, Nicolas Reeves: Department of Mechanical and
Aerospace Engineering, EBIII, North Carolina State University,
Raleigh, NC 27695, USA
1 Introduction
Laser drilling is an important industrial process for the
production of various sizes of holes for critical applications, such as cooling holes in turbine components. For
micro-hole drilling via laser ablation, it is important to
control the material removal rate (MRR), ablation depth,
and aspect ratio. Important process parameters include
laser pulse duration, energy, peak power, the material
properties, etc. Typical pulse durations for laser ablation
are in the nano-, pico- and femto-second ranges. These
lasers operate by specifying a fixed amount of energy and
compressing it into a short pulse duration to achieve high
peak laser power. By changing the pulse duration, different peak powers can be achieved with the same pulse energy.
Micro-hole drilling using nanosecond (10−9 sec) (ns)
pulses usually produces holes in metal with acceptable
quality, but, in general, worse than those by an EDM process because melting is involved in the process. The material removal rate is usually in the order of 1–10 µm/pulse.
The power density of these lasers is in the range of
GW/cm2 .
Ultra-short-pulse lasers operate in the femto-second
(10−15 sec) (fs) or pico-second (10−12 sec) (ps) ranges
to produce a peak power density in the range of 101,000 GW/cm2 . The holes produced with these ultra-short
pulses exhibit a clean finish because melting is not significant; however, the MRR is usually very low. [2] reported
an MRR in the range of 10-200 nm/pulse for a typical
Ti:Sapphire laser for steels. [7] presented detailed high
© 2016 Jay Tu et al., published by De Gruyter Open.
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License.
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Ratio Micro-hole Drilling With Multiple Micro-Second Pulses. . .
speed photography images of laser ablation mechanisms
using short and ultra-short laser pulses.
There are also applications using long microsecond
(ms) pulses in the range of several hundred microseconds
(10−6 sec) to several milliseconds (10−3 sec) with the power
density typically below 107 W/cm2 . For example, in the experiments presented in [14] and [13], a Q-switiched 400W
Nd:YAG laser was used to produce a pulse at 1.0 ms, with
pulse energy up to 7 J and peak power over 7 kW and with
multiple pulses and with O2 as the assist gas, it could drill
through a 2.5 mm stainless steel plate with a hole diameter
approximately 600 µm. Similarly, [15], with a similar laser,
conducted laser drilling with pulse durations ranging from
490 to 890 µs, pulse energy 0.5 J, and argon gas as the assist gas. [9] applied a single pulse from an Nd:YAG laser
for up to 40 J in energy and 20 ms in pulse duration, with
3–5 kW peak power, to drill blind holes in different metals. For chromium steel, they achieved 6.5 mm deep blind
hole with a single pulse at 17.5 J with a power density below
106 W/cm2 . The blind hole demonstrated a large opening
similar to a nail head shape over 1 mm in diameter and an
average waist diameter about 300 µm.
1.1 Laser Drilling with Short Microsecond
Pulse
The body of work regarding microsecond laser drilling/
ablation is small. [7] performed laser ablation using laser
pulses with durations between 150 ns and 4.5 µs to drill
samples of stainless steel, aluminum, alumina ceramics,
and graphite with a Q-switched Nd:YAG. They reported a
maximum MRR at 11 µm/pulse with a power density of
20 MW/cm2 and pulse duration of 4.5 µs for stainless steel.
The advance of laser technology in 2000s has produced many high power lasers with excellent beam quality, suitable for microhole drilling. For example, [16] used
a Q-switched Nd:YAG slab laser to produce pulses between
30 µs and 150 µs with power densities up to 220 MW/cm2 .
With a single pulse, they reported holes drilled in steel
plates with diameters between 80 µm and 120 µm and
depths between 120 µm to 700 µm using argon gas.
The experimentally determined drilling speed was 6 m/s.
[10] applied this drilling technique to drill cooling holes
in turbine components with multiple pulses (percussion
drilling) combined with with oxygen, helium, and argon
assist gasses. [20] combined the above Nd:YAG slab laser
and the DPSS Nd:YAG laser for very high aspect ratio percussion drilling (5 mm through holes with 170–180 µm
waist diameter). [20] utilized a flash lamp pumped Nd:YAG
slab laser (FM015, LASAG) with a beam quality as M2 = 2
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and it could produce pulses with durations from 100 to
500 µs with a laser energy of 0.64 J. This excellent beam
quality allows for a focus spot size at 45 µm in diameter, producing a power density of 220 MW/cm2 , as compared with M2 = 22–38, 600 µm spot size, and approximately 3 MW/cm2 for the laser used in [14] and [19]. [20]
also utilized a diode-pumped solid state (DPSS) Nd:YAG
laser (Powergator 1064, Lambda Physik) which had a beam
quality of M2 = 1.7, with a spot size of 42 µm, 17 ns pulse,
and 1.8 mJ pulse energy, producing a power density over
20 GW/cm2 .
1.2 Microsecond Laser Drilling/Ablation by
Modulating a CW, Single Mode Fiber
Laser
The development of high power, single-mode fiber laser established a new level of beam quality. In comparison with
ns-, ps-, and fs-lasers, the peak power of this modulated
fiber laser pulse is low but its excellent beam quality allows for tight focusing to reach very high power density
for microhole drilling.
The laser beam produced by a 300 W, CW, Yb-doped
single-mode fiber laser (YLR-300, IPG) has a near perfect
beam quality M2 = 1.04 at 1065 nm and the beam can be
focused down to 10 µm with a 100 mm lens. It can also
be modulated to produce pulses from 1 µs to any length of
pulse duration. Using the above laser, [17, 18] presented a
single pulse drilling process of blind holes on a stainless
steel plate with a pulse duration from 1 to 8 µs without assist gas. With a single 1-µs pulse, it was possible to produce a blind hole 167 µm in depth and 19 µm in the opening diameter on a 0.8 mm stainless steel plate. The drilling
mechanisms were established by determining the contributions of hole drilling by evaporation and melt ejection
theoretically and experimentally. It was found that evaporation contributed approximately 1/3 of the hole drilling,
while melt ejection accounts for the rest 2/3. [17] presented
a series of diagrams, denoted as process anatomy, to illustrate the transition of this drilling process.
In this paper, we explore how this short micro-second
pulse drilling process could be extended to drill through
a stainless steel plate of 0.8 mm with multiple pulses. In
particular, the timing of a subsequent pulse in relation to
the previous pulse based on the condition of the melt will
be investigated.
This paper is organized as follows. First, a brief review of multiple-pulse through-hole drilling is provided.
The time line and the process anatomy of the single short
micro-second drilling process from [17, 18] are then pre-
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sented to establish favorable multiple-pulse drilling strategies. Experimental procedures and results are then presented, followed by discussions and conclusions.
2 Review of Multiple-Pulse Drilling
In the experiments presented in [14] and [13], a Q-switiched
400W Nd:YAG laser was used to produce a pulse of 1.0 ms,
with pulse energy up to 7 J and peak power over 7 kW.
Using multiple pulses at repetition rates 10 to 40 Hz and
O2 as the assist gas, it could drill through a 2.5 mm stainless steel plate with a hole diameter approximately 600 µm
within 1 to 2 seconds. Similarly, [15], with a similar laser,
conducted laser drilling with pulse durations ranging from
490 to 890 µs, pulse energy 0.5 J, and argon gas as the assist gas.
[12] conducted experiments to compare multiple-pulse
drilling on 0.5 mm stainless plates using µs- , ns-, ps-, and
fs- lasers at repetition rates 500, 120k, 50k, and 1 kHz, respectively. The results showed that it took 1 s, 10 s, and 5 s
to drill through a 0.5 mm stainless plate for a µs- , ps-, and
fs- laser, respectively. The result of using ns laser was not
conclusive because the hole was not shown to be drilled
through completely.
A double-pulse drilling technique was reported by [5].
This technique split a 4-ns pulse at 533 nm wave length into
two pulses. By routing the second pulse through a longer
optical path, it lagged 30 ~ 150 ns behind the first pulse.
The material removal per shot increases from 0.7 µm with
the original pulse to 1.6 µm with the double-pulse. Continuously shooting the double-pulse to a target, a high aspect ratio hole could be created. For example, a 914 µm
stainless steel sample was drilled through with a waist diameter of 40 µm after 10,000 double-shots at a repetition
rate of 5 kHz after 2 seconds. [21] implemented this doublepulse strategy by splitting a 21 ns pulse with a wavelength
of 1047 nm with a 52 ns delay from a Q-swuitched Nd:YLF
laser. Improvements in hole quality were witnessed with
an increase in the material removal rate by a factor of 2.
A 1 mm plate could be drilled through with 500 pulses at
a repetition rate of 4 kHz, or 125 ms. [1] studied the combinations of different laser wavelengths, energies and durations for better coupling between the laser beam, the
plasma plume, and the target material. Application of the
double-pulse technique using a pico-second laser was reported by [4] for drilling less than 3 µm deep holes with
eight pulses at a repetition rate of 200 Hz.
Based on the above result, the double-pulse drilling
strategy via optical splitting is effective but it requires ad-
ditional optical setup for splitting the pulse. Similarly, the
double-laser strategy is effective, especially for very thick
plates, but the setup is complicated and expensive for
combining two different lasers. In this paper, we investigate a rapid, high aspect ratio micro-hole drilling using
only one CW single mode fiber laser without assist gas and
without beam splitting.
3 Experimental Setup
A 300 W Ytterbium, Single-Mode, Fiber Laser (YLR-300,
IPG) was used for this research. The laser beam is fiber
delivered and the raw beam size is 5 mm. The laser beam
quality is near Gaussian (M2 ~1.04). An optical isolator is
attached to the collimator to divert reflected laser beam
away in order to avoid damages to the laser. The beam diameter and beam quality become 7 mm and M2 = 1.14, respectively. In this study, an external control circuit was designed to modulate the laser beam into pulses with durations from 1 µs to 1 s. This laser modulation control is different from Q-switching because the laser power remains
constant while the deposited energy is determined by the
pulse duration. This control circuit also allows for producing consecutive pulses at different repetition rates, for
specific number of pulses, and at different grouping of
pulses. A 1-µs and a 15-µs pulse are shown in Figure 1. Both
pulses have an initial spike at 1,450 W followed by a constant power at 300 W to achieve a peak power density of
1.9 GW/cm2 and 380 MW/cm2 , respectively. Note that a 1µs pulse lasted about 8 µs while its initial spike was only
1 µs long. On the other hand, the 15-µs pulse contains the
same initial spike but the 300 W power lasts for 13 µs, with
a total duration of 23 µs.
The vapor/plasma intensity measurements were obtained using a Hamamatsu silicon S1336-18BQ photodiode.
This photodiode is most sensitive (80–90% transmission)
in the visible spectrum. However, it also has a moderate
sensitivity (50–60% transmission) at 1,065 nm. Therefore,
a short-pass filter (Thorlabs FES0900) was placed in front
of the photodiodes to cut off radiations above 900 nm so
that the laser radiation (1065 nm) would not affect the measurement of plasma radiation. The modulated pulse of the
laser was measured by an InGaAs photodiode (Thorlabs
410/M) through a 10 µm pinhole to prevent saturating the
photodiode. The diode’s sensitivity at 1,075 nm is 75%,
while insensitive to visible light. In addition to measuring
the beam pulse profile, this photodiode was also placed
under the sample plate to detect the instant when a hole
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Figure 1: Profiles of 1-µs and 15-µs laser pulses. Both pulses have an identical initial spike at 1,450 W which is approximately five times of
the steady state power of 300 W.
is drilled through. An oscilloscope (Tektronix 3012B) with
100 MHz sampling rate was used to record the signals.
4 Microsecond Drilling Process
Anatomy
4.1 Time Line of the Drilling Process with a
Short Micro-Second Pulse
As reported in [17, 18], the entire drilling process using a
1-µs pulse was recorded by a high speed camera as shown
in Figure 2. Based on Figures 1 and 2, the laser pulse was
completed in 10 µs but the entire drilling process did not
complete until more than 144 µs later. The white flashes
captured by the high speed camera were low temperature
vapor hovering over the molten metal, indicating that the
melt still existed 100 µs after the laser radiation started.
4.2 Process Anatomy of Single Pulse Drilling
the microsecond laser drilling, using the single-mode fiber
laser, was compiled. Four stages of the drilling process
are depicted in Figure 3 to depict the laser/material interaction mechanisms, hole formation, material removal
mechanisms, and vapor/plasma properties. This process
anatomy diagram is also color-coded to provide the temperature values during the process. According to Figure 3,
up until stage #2, the hole drilling is mainly due to the
energies from the initial spike of the laser beam and the
induced plasma, which account for 1/3 of the total hole
depth. In stages #3, the hole is further deepened by the
steady state laser power at 300 W. Finally, at stage #4, the
vapor pressure has decreased, which allows the melt to explode away as droplets seen in Figure 2. Stages #3 and #4
account for 2/3 of hole drilling.
The time line of the process (Figure 2) and the anatomy
(Figure 3) indicate that when a multiple-pulse strategy is
used for drilling, it would be synergistic to shoot a subsequent pulse within 100 µs before the melt is solidified
so that the laser does not have to re-melt the material. In
addition, the melt at high temperature is more efficient in
absorbing the laser energy. Based on this observation, the
minimum repetition rate is then set as 10 kHz. Note that
Based on the experimental and simulation results presented in [17], a temporal process anatomy diagram of
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Figure 2: High speed images of the vapor produced during 1 µs single pulse laser drilling.
Figure 3: Process anatomy of the rapid drilling process using a one micro-second laser pluse generated by a single-mode CW fiber laser.
the fiber laser’s specification allows a repetition rate up to
50 kHz.
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Ratio Micro-hole Drilling With Multiple Micro-Second Pulses. . .
Figure 4: The peak power of subsequent pulses at different repetition rate.
5 Multple-Pulse Drilling with
Microsecond Pulses
5.1 Repetition Rate vs peak power
The fiber laser used for this investigation has a repetition
rate up to 50 kHz. However, the peak power of the pulse
is affected by the repetition rate. The peak power of the
first pulse will remain as 1450 W but the subsequent pulses
would have a reduced peak power when the repetition rate
is higher than 1 kHz. The peak power of the pulses were
measured at different repetition rates, as shown in Figure 4. When the repetition rate is higher than 30 kHz, the
peak power dropped substantially to less than 400 W after the first pulse. At repetition rates lower than 20 kHz,
the peak power of subsequent pulses would be still over
1000 W. As a result, the repetition rate should be kept below 20 kHz and higher than 10 kHz based on the process
anatomy of Figures 2 and 3 for the synergistic effect.
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the hole drilling by the group of the three pulses of Figure 5. The results are shown in Figure 6. A clean blind hole
with a depth of 233 µm was drilled by the three 1450 W
pulses with 5 seconds delay (left, Figure 6), compared with
a partially blocked hole of 248 µm created by three pulses
at 13 kHz. Note that the second and third pulses of Figure 5 are at 1000 W peak power, or 31% lower but the hole
depth is actually 6.4% higher, confirming the synergistic
effect described above. However, with a deeper hole and
a lower peak power, making the melt ejection less effective. The melt which was not ejected completely out of the
hole could block the hole at the entrance of the hole, preventing the laser energy from deepening it further. This observation is confirmed by the plasma radiation measurement shown in the lower chart of Figure 5. The radiation
from plasma was very high from the first laser pulse when
the solid metal was evaporated and ionized, as shown in
the first three stages of the process anatomy in Figure 3.
However, the second pulse created almost no detectable
plasma radiation. This is because that the second laser
pulse was guided into the microhole through multiple reflections. Because the laser was not irradiating at a 90 degree angle along the wall of the microhole, the energy
absorbed at each reflection point was lower, enough to
widen the hole but not enough to create hot plasma, confirmed by the fact that no plasma radiation was measured
by the photodiode. When the laser energy was guided toward the bottom of the hole, the laser energy was lower
but still capable of deepening the hole. However, when the
hole was deeper with the laser power lower, the molten
metal could not be completely ejected, resulting a partially
blocked hole. The third laser pulse would irradiate at the
hole blocking melt at a straight angle, creating detectable
plasma, as shown in Figure 5.
5.3 Ten-shot drilling
5.2 Three-Pulse Drilling
Microhole drilling with three pulses was investigated. The
pulse profiles are shown in the upper chart of Figure 5.
The actual repetition rate is defined by the time interval
between the second and the third pulse because the time
interval between the first pulse and the second pulse could
vary depending on the exact starting time when a “START”
push button was pressed. The interval between the second
and the third pulses is 76 µs in Figure 5 which is equivalent
to 13 kHz.
In order to verify the synergistic effect, three pulses of
1450 W with a five-second delay in between was used for
drilling on the same stainless plate for comparison with
A ten-shot drilling was conducted. The pulse profiles and
the plasma radiation are shown in Figure 7. Notice that the
plasma radiation measurements in Figure 7 indicate a similar hole block behavior. As the hole deepens, the melt ejection becomes inefficient. The subsequent laser pulse was
either used to deepen the hole (when low plasma radiation
was measured) or to open the hole (when high plasma was
measured.) Based on the observations of Figures 5 and 7,
the process anatomy diagram of Figure 3 can now be extended to include the drilling anatomy involving multiple
pulses.
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Figure 5: The three-shot pulse profiles and the resulting plasma radiation during drilling.
5.4 Multiple-Pulse Process Anatomy
Continuing with the Figure 3, Figure 8 illustrates the
drilling anatomy for the 2nd , 3rd , 4th , and subsequent
pulses.
and to deepen the hole (third image). This phenomena of
deepening and opening repeated themselves depending
on the condition of the blockage (fourth image).
5.5 Repetition Rate Effect
5.4.1 Second Pulse Stage:
The second pulse started 57 µs after the first pulse when a
hole was formed with a molten hole wall. The second pulse
deepened the hole substantially as shown in first image of
Figure 8. However, as the melt began to move up the cavity,
it also began to solidify near the top of the hole without
being ejected completely.
5.4.2 Subsequent Pulses Stage:
The third pulse vaporized the newly formed blockage near
the hole’s entrance, producing high plasma radiation (second image). With the hole cleared by the third pulse, the
fourth pulse could again reach down the cavity to widen
The ten pulse scheme was used at 5 repetition rates
(13 kHz, 16 kHz, 32 kHz, 43 kHz, and 48 kHz) to investigate the effect of the repetition rate on the drilling efficiency. The pulse duration was kept the same to investigate if the synergistic effect of using a higher repetition rate
can compensate for the loss of laser power. Figure 9 shows
that at higher repetition rates, both the hole diameter and
depth increase, while blockage becomes more prominent.
Note that at repetition rates of 32, 43, and 48 kHz, the peak
power values of the laser pulses from 2nd to 10th are below 400 W, causing less melt ejection and more substantial
blockage. Based on the results of Figure 9, it was decided
to keep the repetition rate at or below 20 kHz for the use of
multiple pulse scheme to achieve through hole drilling.
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higher at 20 kHz, the hole quality is worse with the aspect
ratio (depth over average diameter) decreases from 34 at
13 kHz to 12 at 20 kHz. The choice of the repetition rate
would depend on the production requirement.
Finally, aside from the hole quality, the merit of
drilling speed can be compared. The drilling times through
a 800 µm stainless plate (Table 1, 634 ms and 21.9 ms, respectively) with this technique are much shorter than the
times required for the ps- and fs- lasers to drill through a
500 µm stainless plate (10 s and 5 s, respectively) as reported by Leitz et al. and the double-shot scheme by [5]
through a 914 µm plate (2 s), while comparable to the ns
double-shot scheme by [21] through a 1 mm plate (500
pulses or 125 ms). However, the setup of this technique is
simpler, at lower cost, and more robust for the production
environment. Note that the drilling time and plate different
thicknesses are not linear. Direct drilling time comparison
for the drilling of higher plate thickness over 800 µm is not
meaningful.
6 Discussion and Conclusion
Figure 6: Hole Profile Produced by two different three-Pulses strategies.
Table 1: Total drilling time and number of shots at two repetition.
Repetition Rate
13 kHz
20 kHz
Thickness 0.8 mm
8240 shots
629 ms
438 shots
21.9 ms
5.6 Through Hole Drilling
Stainless plates of 800 µm thickness were used to conduct
the through hole drilling experiments. The laser pulses
were similar to the ones in Figure 7 except that the subsequent pulses were unlimited until the plate was drilled
through, indicated by a photodiode to detect laser radiation’s presence at the bottom of the plate. Two repetition
rates were used at 13 and 20 kHz. The total drilling times
through the 800 µm plate were found to be 634 ms and
21.9 ms at 13 kHz and 20 kHz, respectively (Table 1). The
drilling efficiency at the 20 kHz repetition rate is drastically
higher, needing only 428 shots, compared with 8240 shots
at the 13 kHz, an improvement of nearly 200 times.
The hole profiles at these two repetition rates are
shown in Figure 10. Although the drilling efficiency is
In this paper, we investigate rapid, high aspect ratio microhole drilling using multiple microsecond pulses based
on the single pulse drilling technique reported in [17, 18].
The single pulse drilling mechanisms were summarized in
Figures 1 – 3, illustrating the laser pulse profiles and the
anatomy of the drilling process. It was established that
the melt solidified at about 100 µs after the laser radiation. Therefore, it was synergistic for the second pulse to
be fired within 100 µs or at a repetition rate higher than
10 kHz. The peak power values of subsequent pulses at
different repetition rates were illustrated in Figure 4 and
their profiles in Figures 5 and 7. It was found that the
peak power decreased from 1450 W above 1 kHz repetition
rate, to about 1000 W at 20 kHz, and to less than 400 W
above 30 kHz. Figure 6 confirmed that the reduction in
peak power at 13 kHz repetition rate achieved a hole which
is 6.4% deeper despite that the 2nd and 3rd pulses were
31% lower in peak power. This confirms that the synergistic effect could outweigh the reduction in power. However,
deeper holes also made melt ejection less efficient, causing the hole to be blocked as shown in Figures 6. Subsequent pulses often expended their energy to reopen the
hole than to deepen the hole. This new drilling mechanism was illustrated in an extended process anatomy for
the multiple-pulse drilling (Figure 8). The effect of different
repetition rates was investigated for 13 kHz, 16 kHz, 32 kHz,
43 kHz, and 48 kHz, the synergistic effect of higher rep-
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Figure 7: Ten shots pulse profiles and plasma radiation measurements.
Figure 8: Subsequent process anatomy with multiple pulse drilling.
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Figure 9: Hole depth versus repetition rate for ten-shot drilling at 13 kHz, 16 kHz, 32 kHz, 43 kHz, and 48 kHz, respectively (a to e).
Figure 10: Through hole profiles at 13 kHa (left) and 20 kHz (right)
repetition rates.
etition rates began to affect the hole quality adversely at
rates higher than 20 kHz as shown in Figure 9. Finally, this
multiple-pulse drilling technique was applied to throughhole drilling. It was found that the total drilling times
through an 800 µm plate were 634 ms and 21.9 ms at 13 kHz
and 20 kHz, respectively (Table 1). The drilling efficiency
at the 20 kHz repetition rate is drastically higher, needing
only 428 shots, compared with 8240 shots at the 13 kHz, an
improvement of nearly 200 times. It was found that the total drilling times through an 800 µm plate were found to be
634 ms and 21.9 ms at 13 kHz and 20 kHz, respectively. The
drilling efficiency at the 20 kHz repetition rate is drastically
higher, needing only 428 shots, compared with 8240 shots
at the 13 kHz, an improvement of nearly 200 times. It is
confirmed that this multiple-pulse drilling technique with
microsecond pulses using a 300 W single mode fiber laser
is a viable technique to produce high aspect ratio through
holes with a simple and robust setup for the production
environment.
Finally, the heat affected zone (HAZ), hole shape, or
burr at the entrance and exit would be investigated further for specific applications depending on the precision
requirement.
Acknowledgement: This investigation is supported by the
Department of Mechanical and Aerospace Engineering at
North Carolina State University, NSF Grants CMS-0402857,
DMI-0355481, DMI-0355214, and DMI- 0944509 as well as
Knowledge Foundation of Sweden (KKS).
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