Applied Physics Research; Vol. 5, No. 4; 2013
ISSN 1916-9639
E-ISSN 1916-9647
Published by Canadian Center of Science and Education
Process Anatomy for High Aspect Ratio Micro-Hole Drilling
with Short Micro-Second Pulses Using a CW Single-Mode Fiber
Jay Tu1, Ted Lehman1, Alexander G. Paleocrassas1, Wm. Ray Harp1, Satoru Noguchi2 & Etsuji Ohmura2
1
Department of Mechanical and Aerospace Engineering, North Carolina State University, EBIII, Raleigh, NC,
USA
2
Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1
Yamada-oka, Suita, Osaka, Japan
Correspondence: Jay Tu, Department of Mechanical and Aerospace Engineering, North Carolina State University,
EBIII, Raleigh, NC 27695, USA. E-mail: [email protected]
Received: May 14, 2013
doi:10.5539/apr.v5n4p1
Accepted: June 13, 2013
Online Published: July 1, 2013
URL: http://dx.doi.org/10.5539/apr.v5n4p1
Abstract
The objective of this paper is to establish a detailed process anatomy of a micro-hole drilling process using a CW
single-mode 300 W fiber laser with short micro-second pulses. For the drilling process with a one micro-second
laser pulse, the process started with drastic evaporation and concluded with melt ejection approximately 150
micro-seconds later. It was concluded that two distinct hole drilling mechanisms occurred, one as adiabatic
evaporation by the high power initial spike of the laser beam and one as melt ejection by the low power laser
energy deposition. The drilling process time line was established based on the measurements by several
in-process sensors, such as photodiodes, a high-speed camera, and a spectrometer. The theoretical modeling
zoomed in the early stage of the process to investigate the hole formation which could not be observed
experimentally. Both experimental and theoretical results were then compared to determine the laser-material
interaction mechanisms among three media involved in the process: laser, material, and vapour/plasma. Finally, a
series of temporal anatomy diagrams, denoted as process anatomy, were presented to describe the entire drilling
process, including process temperature, laser energy deposition, hole formation, and material removal
mechanisms.
Keywords: micro-hole drilling, laser ablation, microsecond pulse, fiber laser, melt ejection
1. Introduction
Laser drilling is an important industrial process to produce various sizes of holes for critical applications, such as
cooling holes in turbine components. Typical laser pulse durations for laser drilling are in the nano-, pico- and
femto-second ranges. Ultra-short-pulse lasers operate in the femto-second (10–15 sec) (fs) or pico-second (10–12
sec) (ps) ranges to produce peak power density in the range of 10–1,000 GW/cm2. The holes produced with
ultra-short pulses usually exhibit a clean finish because melting is not significant. However, the Material
Removal Rate (MRR) is usually very low, for example, in the range of 10–200 nm/pulse for steels with a Ti:
Sapphire femto-second laser (Bauerle, 2000). Breitling et al. (2004) presented fundamental aspects in machining
of metals with short and ultra-short laser pulses. Mao et al. (2007) presented detailed high speed photography
images of laser ablation mechanisms using short and ultra-short laser pulses. The micro-hole drilling process
using nanosecond (10–9 sec) (ns) pulses usually produces holes in metal with acceptable quality, but, in general,
worse than those by ultra-short lasers because melting is more significant. 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.
Laser drilling using long pulses in the range of several hundred microseconds (10–6 sec) to several milliseconds
(10–3 sec) rely on mostly on melting to remove material because their power densities typically are below 107
W/cm2. The hole quality is usually low and the hole diameter is large due to significant melting. For example, in
the experiments presented in Low and Li (2002) and Low et al. (2004), 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. Using 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, Pandey et al. (2006), 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. Kayukov et al. (1998)
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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.
In summary, the laser drilling process can be evaporation dominated or melting dominated depending on the
pulse duration. In comparison, the body of work regarding short microsecond laser drilling/ablation is small. It is
expected that the evaporation and melting both play an important role in the short microsecond drilling process.
It is the objective of this paper to establish the drilling mechanisms and the temporal characteristics of the short
microsecond drilling process, to be depicted in a series of diagrams, denoted as process anatomy.
This paper is organized as follows. First, the time line of the short microsecond drilling process is established
based on the measurements of several in-process sensors, including photo-diodes, a high speed camera, and a
spectrometer. Theoretical modeling is then used to zoom in the early hole formation stage which cannot be
observed experimentally. Both experimental and theoretical results are then compared to determine the
laser-material interaction mechanisms. Finally, a series of temporal anatomy diagrams, denoted as process
anatomy, are presented to describe the entire drilling process by a one micro-second laser pulse. The body of a
manuscript opens with an introduction that presents the specific problem under study and describes the research
strategy.
2. Review of Short Microsecond Pulse Drilling
Garnov et al. (2004) performed laser ablation using pulse durations between 150 ns and 4.5 μs on samples of
stainless steel, aluminum, alumina ceramics, and graphite with a Q-switched Nd:YAG laser that used an optical
fiber as part of the resonator to produce the desired pulse length. They reported an 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 the last decade has produced many high power lasers with very high beam
quality. For example, Trippe et al. (2004) 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 drilled holes in steel with
diameters between 80 μm and 120 μm and depths from about 120 μm to 700 μm with argon gas. Their
experimentally determined drilling speed was reported as 6 m/s. They also presented a dimensionless 2D model
of free boundary problem and they computed the time for vaporization to be approximately 100 nanoseconds.
Kreutz et al. (2007) applied this drilling technique to drill cooling holes in turbine components with multiple
pulses (percussion drilling) combined with oxygen, helium, and argon assist gasses. Walther et al. (2008)
combined the above Nd:YAG slab laser and the DPSS Nd:YAG laser to achieve very high aspect ratio percussion
drilling (5 mm through holes with 170-180 μm waist diameter). Walther et al. (2008) utilized a flash lamp
pumped Nd:YAG slab laser (FM015, LASAG) with a beam quality as M2 = 2 and it can produce pulses with
durations from 100 to 500 μs and a laser energy of 0.64 J. This excellent beam quality allows a focus spot size 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 Low and Li (2002) and Voisey et al. (2002). Walther et al.
(2008) also utilized a diode-pumped solid state (DPSS) Nd:YAG laser (Powergator 1064, Lambda Physik) which
has 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.
Most interestingly, the development of high power, single-mode fiber laser has established a new level of beam
quality. For example, Harp et al. (2008) conducted micro-hole drilling with a 300 W, CW, Yb-doped fiber laser
(YLR-300, IPG), which has a near perfect beam quality M2=1.04 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. The
modulated pulses all contain an initial spike at 1,500 W, followed by a constant power at 300 W, to reach a peak
power density of 1.9 GW/cm2 and 380 MW/cm2 at the steady state. 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.
The feasibility of laser drilling with such a CW laser via modulation control was demonstrated by Harp et al.
(2008), in which pulses from 15–40 μs were used to produce holes with diameters from 40 to 100 μm and depths
from 25 to 50 μm. This work indicated that, by using high beam quality lasers, micro-hole drilling with pulse
durations in the short microsecond range (1–10 μs) has become feasible for applications which require moderate
material removal depth per pulse, narrow opening (due to much smaller beam spot sizes), and moderate hole
quality (due to moderate melt ejection).
In this paper, we focus on a single pulse drilling process of blind holes on a stainless steel substrate with pulse
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durations oof 1–8 μs usinng a 300 W, C
CW single-modde fiber laser. IIn this study, tthe micro-holee drilling techn
nique
investigateed in Harp et al.
a (2008) has been greatly eextended and it is now posssible, with a siingle 1 μs pulsse, to
produce a blind hole 1677 μm in depth aand 19 μm in tthe opening diameter on a 0..8 mm stainlesss steel plate.
3. Summaary of Experim
mental Resultts
3.1 Profilees of Modulateed Fiber Laser Pulses
A continuuous-wave, sinngle-mode, Ybb-doped fiber llaser (YLR-3000, IPG) was used to drill bblind holes on
n 316
stainless ssteel plates (S
SUS 316). Thhis laser emittted a beam w
with a waveleength of 1,0775 nm and a CW,
steady-statte power outpuut of 300 W. IIn this study, aan external conntrol circuit waas developed w
with a capabiliity of
modulatingg laser pulsess with duratioons from 1 μ
μs to 1 s. Thiis laser moduulation controll is different from
Q-switchinng as the laser power remainns constant whiile the depositeed energy is deetermined by tthe pulse durattion.
The moduulated pulse off the laser wass measured byy an InGaAs pphotodiode (Thhorlabs 410/M
M) through a 10
0 μm
pinhole to avoid saturatiing the photoddiode. An osciilloscope (Tekktronix 3012 B
B) with a 100 M
MHz sampling
g rate
was used tto measure the signal from thhe photodiode and to determ
mine the actual laser pulse durration.
Figure 1 illlustrates the measured
m
profi
files of the 1-μ
μs and 15-μs laaser pulses. Foor the 1-μs pullse, the laser power
initially risses to about 1,,500 W at 500 ns and then faalls to the steaady state value of 300 W afteer 1 μs. The ov
verall
pulse graddually decays to zero after 110 μs. For thee 15-μs pulse, the same initial spike and the same deca
aying
pattern at the end of thee pulse are obsserved. Howevver, the laser ppower reboundds to about 5000 W after 1 μss and
then settles to the steadyy state power oof 300 W for ann additional 144 μs. The initiaal spike was m
marked by a red
d line
in a zoom--in plot in Figuure 1.
Laser pulse proofiles of the 1--s and 15-s pulses. Both ppulses have an identical initiaal spike at 1,50
00 W
Figure 1. L
whhich is approxximately five tiimes of the steaady state valuee of 300 W
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Figure 2. Hole ggeometry by thhe 1, 2, 4, 5, annd 8 microsecoond pulses
3.2 Micro--Hole Geometrry
Typical prrofiles of microo-holes producced by a singlee pulse with puulse durations at 1, 2, 4, 5, aand 8 s are sh
hown
in Figure 22. For the hole produced byy the 1 s pulsse, the diameteer of the hole is 18.9 m annd the depth of
o the
hole is 1666.7 m, equivvalent to an asppect ratio of 88.8. The result of Figure 2 iss a significant improvement over
the previoous results repoorted in Harp et al. (2008). Overall, the eexcellent holee profile and thhe narrow ope
ening
diameter is mainly due to the excellennt beam qualitty of the singlle-mode fiber laser. It is cleaar that longer laser
pulses prooduce deeper holes
h
while thee hole diameteer does not inccrease substanttially. The mellt ejection beco
omes
less effecttive when the hole depth reaaches 200 m as hole openiing blockage sstarts to appeaar. With 5 and 8 s
pulses, thee hole is complletely blocked..
4. Time Line of the Driilling Process with a Short M
Micro-Second
d Pulse
4.1 The Fiirst 5 Microsecconds
Two silicoon photodiodees (Hammatsuu S1336-18BQ
Q) were usedd to measure the intensityy evolution off the
vapour/plaasma plume whhen the laser w
was irradiated oon the substratte. The responnse time of thiss photodiode iss 5 ns.
A tube waas placed arouund the photoddiode sensors iin order to aim
m at the vapouur plume preccisely. A short-pass
filter (Thoorlabs FES09000) was placed in front of thee photodiodes to cut off raddiations above 900 nm so tha
at the
laser radiaation (1075 nm
m) would not aaffect the meassurement of thhe plasma radiaation. Two photodiodes arra
anged
at differennt angles were used, one at a steep angle aand one paralllel to the platee. The photodiiode measurem
ments
and the lasser beam tempporal profile w
were shown in F
Figure 3. The laser pulse staarted at t = 0, aand rapidly rea
ached
its peak poower at about 500 ns. The vapour/plasma signal was firsst detected at 2200 ns. This ddelay was related to
the laser/m
material interacction time neeeded to evaporrate and dissoociate the mateerial from the solid phase to
o hot
vapour/plaasma. This obsservation is coonsistant with tthe result repoorted by Trippee et al. (2004), in which the time
needed to cause evaporration is foundd to be 100 nns. The vapour/plasma conntinued to groow in intensity
y and
reached too its peak at abbout 600-800 nns. After the ppeak, the vapouur/plasma inteensity decreaseed to nearly ze
ero at
2.8 μs as indicated by the silicon phhotodiode paraallel to the pllate, while som
me spurts of iintensity were
e still
registered by the steep angle
a
photodiode. This was bbecause the steeep angle phottodiode could oobserve the plasma
trapped insside the hole. At
A this time, thhe laser powerr dropped below
w 200 W. Anoother observatiion based on Figure
3, was thaat, under the cuurrent setup, laaser power lesss than 200 W was not capabble of producinng a vapour/plasma
with intennsity high enouugh to be regiistered by the parallel photoodiode. After 5 μs, the laserr power dropped to
below 1000 W and downn to zero afterr about 10 μs. One reason fo
for this is probbably not due to the steep cavity
which wass already form
med so that the incident anglee of the laser bbeam to the suubstrate is far from 90 degre
ees to
reduce thee energy absorpption efficiencyy.
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Figure 3. V
Vapour/plasmaa and laser intennsity measurem
ments
4.2 Entire Process Time Line
Although tthe photodiodees could not detect any signaals after aboutt 10 μs, it did nnot mean the ddrilling processs had
completedd. To observe the
t entire drilling process, a high speed ccamera was uused. The fram
me rate of this high
speed cam
mera was 40,5000 frames/secoond; thus, the time interval between each frame was 244.6 μs. The camera
was startedd before the laaser radiation aand took contiinuous frames of images throoughout the drrilling process. The
high speedd photographyy images were shown in Figuure 4. If we seet the start of the laser pulse as t = 0, the
e first
image of F
Figure 4 occurrred some timee between 0 too 24.6 μs becaause the camerra was not synnchronized with the
start of thee laser radiatioon. We denoted the first fram
me time as t1, as shown in F
Figure 4 and 0<t1<24.6 μs. In the
first framee, a coherent white
w
flash starrted. Similar w
white flash wass also observedd in frames #22 to #4, which is 72
μs later. Inn the first four frames, the CC
CD sensor of tthe high speedd camera was oobviously satuurated. At fram
me #5,
the white flash reduced to be gray andd became fainnter in frames #6 and #7, 1444 μs later. It w
was not clear if
i the
white flashh in frame #1 is the high inntensity vapouur/plasma obseerved in Figurre 3. Note thatt the high inte
ensity
vapour/plaasma in Figure 3 was no loonger detectabble by the phootodiodes afteer 3 μs when the vapour/plasma
intensity bbecame too low
w to be registerred by the phootodiodes. How
wever, the intennsity of the vaapour/plasma might
m
be too low
w for photodioodes after 3 μss, it was still sstrong enough to saturate thhe CCD sensorr of the high speed
s
camera. Because similarr saturated white flashes werre still observeed in frames ##2–#4, it was m
most likely tha
at the
first framee did not captuure the high inntensity vapouur/plasma indiccated in Figuree 3. As shownn in the red va
apour
intensity liine in Figure 4,
4 the vapour/p
/plasma was veery intensive w
within the firsst 3 μs, it then decayed gradually
into weakeer vapour and disappeared after almost 100 μs. In the seecond frame (t = t1 + 24 μs, w
where t1 is the time
for the firrst frame), ejeection of particcles could be seen, which continued in tthe subsequennt frames. Partticles
became visible in framess #6 and 7.
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Figuure 4. High speeed images of tthe vapour prooduced during laser drilling
Based on F
Figure 4, the laser
l
pulse waas completed inn 10 μs but the entire drillinng process did not complete until
more thann 144 μs laterr. Due to the existence of vvapour long aafter the laser pulse, we had to conclude
e that
significantt melting occuurred and the white flashes captured by the high speeed camera werre low temperrature
vapour hovvering over moolten metal. Thhe rise and deccay of the vapoour/plasma waas plotted in Fiigure 4 in a red
d line
under the hhigh speed cam
mera images.
4.3 Spectrooscopic Analyssis of the Initiaal Vapour/Plassma
A spectrom
meter was used to characteriize the hot vappour/plasma aat the beginninng of the proceess. Three diffferent
tests were conducted forr producing diifferent vapourr/plasma conditions, which iincluded the single pulse driilling
at 1,500 W and 1050 W, as well aas CW weldinng at 1 mm/s at 300 W. F
For the first ttwo cases, the
e hot
vapour/plaasma was not at steady statee but expandinng. Measuring expanding plaasma was diffiicult and very poor
signal to nnoise ratio wass expected. Wee did not preseent detailed infformation of thhe spectrometeer measuremen
nts in
this paper because of paage limit. Tabble 1 listed thee calculated peak vapour/plaasma temperaature based on Cr I
spectral linnes to reduce errors
e
due to seelf-absorption..
It is impoortant to note here that thee error listed was estimateed based on tthe uncertaintyy in the transsition
probabilitiies of some off the chromium
m lines. The eerror representted the worst case scenario,, assuming tha
at the
transition pprobabilities of
o the lines werre off by the m
maximum amouunt.
Table 1. Ellectron temperratures (in Kelvin) calculatedd from Boltzm
mann plots (95 % confidence level)
Usiing Cr I lines
1 µs pulsing, 1,500 W
43,611 ± 27,623
1 µs pulsing, 1,050 W
29,482 ± 12,624
1 mm/s weldding, 300 W
8,290 ± 998
Accordingg to Table 1, with
w 95% confiddence level, thhe plasma tempperature at the quasi-static w
welding conditiion at
300 W waas estimated to
t be 7,000 too 9,000 K. The peak tempperature of thee expanding vvapour/plasma was
estimated to be betweenn 17,000–42,0000 K at a peakk power of 1,0550W and 16,000 K–71,000 K at 1,500 W peak
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power. Based on the results of Table 1, we can conclude that a hot vapour/plasma over 16,000 K and as high as
71,000 K did exist in the first 1 μs of the process during the laser initial spike. Note that even at the highest
temperature estimated in Table 1, the vapour/plasma was still optically thin with respect to the fiber laser
wavelength of 1075 nm. Therefore, the study of Table 1 also allows us to conclude that the laser beam did not
heat the vapour/plasma and nearly all laser energy was used to either evaporate or to melt the material. The
vapour temperature measured for the steady state welding condition at 300 W indicated that a weaker vapour at
temperature between 7,000 to 9,000 K could exist at the later stage of the drilling process.
4.4 Melt Ejection and the Expanding Vapour/Plasma
The existence of a high temperature vapour/plasma indicates that its pressure would be high as well. Because the
micro-hole was covered by this high temperature and high pressure plasma, the melt is likely to be superheated
in the early stage of the drilling process. As the vapour/plasma cooled down to weaker vapour (after at least 24
microseconds), the pressure also dropped. The drop of pressure could cause the superheated melt to boil and to
eject as fine droplets as seen in the high speed images of Figure 4.
5. The Hole Formation Simulation during the First One Microsecond
Figures 3 and 4 provide the timeline of this drilling process; however, the actual hole formation could not be
observed directly. Therefore, computer simulation based on a 3D finite-element thermo-hydrodynamic model
was used to study the hole formation due to evaporation by the initial laser spike.
5.1 3D Finite-Element Thermo-Hydrodynamic Model
This 3D finite-element based thermo-hydrodynamic model accounts for the effect of multiple reflections inside
the cavity, the movement of the liquid-vapour interface in time, the effect of recoil pressure on the liquid-vapour
interface and the mass loss by evaporation during the drilling process (Ohmura & Noguchi, 2010).
The model contained the continuity, Navier-Stokes and energy equations:

v  0


 
 v 
   v  v   p   2 v  F
 t



k

 H 

 v   H      H   w
c

 t

 p




(1)
(2)
(3)
where v was the velocity vector of the material in the molten state, ρ was the density of the material, p was the
pressure, μ is the material viscosity, F comprised the body force vector for all the recoil pressure force and the
surface tension force at work on an element of material, H was the enthalpy, k was the thermal conductivity, cp
was the specific heat and w is the source term which was the heat input into the system by the laser.
The computational domain (Figure 5) was a 3D rectangular domain with the z axis parallel to the axis of
propagation for the laser beam and the x-y plane parallel to the material surface. The bounds of the domain are:
x(-42 m, +42 m), y(-42 m, +42 m), z(-180 m, +30 m). The plane z = 0 was considered the surface of the
material and the point (0, 0, 0) was the center of the laser beam where it irradiated at the surface (see Figure 5,
indicated by a blue dot). The whole domain is at the room temperature as the initial condition and the boundaries
of the domain remain at the room temperature during the simulation. The laser beam was considered to be
Gaussian in space. A triangular function was used to approximate the actual laser pulse shape (the red profile in
Figure 1) so that only the effect of the initial spike was simulated. The conditions at the material boundaries were
considered to be insulated with zero flow velocity.
The model used a volume of fluid (VOF) method for handling of the liquid-vapour interface by computing the
volume fraction of liquid, F, in each element. The range of F was from 0 to 1, with a value of 0.5 indicating that
the element was an interface element. As the interface position was calculated, the normal vectors to the surface
were determined and used to compute the angle of reflection for the surface. Using the Fresnel equation, the
reflection coefficient was determined for each surface and the amount of reflected energy was calculated.
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Figure 5. Computational domain
It was also assumed that, when the surface material was vaporized, the vaporized mass carried energy with it and
traveled away from the surface. The amount of mass vaporized was calculated using an energy balance along the
surface of the material.
t  t
t  t
t
t
k H
S c p n
 Qdt  
mv 
dSdt

Lv
Sin
(4)
where mv was the mass of the vaporized material, Q was the laser power absorbed at the surface area S and Lv
was the latent heat of vaporization.
As the vaporized mass traveled away from the surface, it generated a recoil pressure on the surface. To calculate
the recoil pressure generated by the vaporization of the material, the recoil pressure, pr, was determined as the
mass evaporated per unit area per unit time multiplied by the velocity of the vapour:
pr 
mv
vT
St
(5)
where vT was the vapour velocity at the edge of the Knudsen layer and was defined by Anisimov et al. (1971) as
one-quarter of the square mean velocity:
vT 
1 8kbTd
4 ma
(6)
where Td was the surface temperature and ma is the atomic mass of the vapour. This generated recoil pressure
created a force on the surface of the material and acted against the surface tension force. These two forces acted
as surface forces, whereas the momentum equation (Equation 2) required the input to be body forces. To convert
the surface forces to body forces a continuum surface force (CSF) model was used (Brackbill, 1992).
The pressure-correction method for the flow velocity-pressure coupling used was the Simplified
Marker-And-Cell (SMAC) method by Harlow and Amsden (1971). The algorithm for the model involved a
series of steps. The first step was to calculate the normal vectors for the surface. Next, the surface forces were
converted to body forces through the CSF method and used in the momentum equation, which was solved with
the continuity equation through the SMAC method. The volume liquid fraction was then determined for the
elements and the multiple reflections were calculated for the walls of the hole. The energy absorbed at the
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surface waas used as the input source inn the energy eqquation, in whhich the enthalppy was compuuted. The vaporized
mass and the recoil preessure were thhen calculatedd. The time sttep was then advanced andd the computa
ations
re-perform
med until the siimulation was completed.
In summarry, this model assumes that eenergy couplinng between thee laser and the material was mainly through the
Fresnel abbsorption. The absorption beccame more effficient as the caavity forms, w
which allowed m
multiple reflec
ctions
of the laseer beam. Whenn the material was removedd by the evapooration processs, only a portion of the abso
orbed
energy waas conducted away
a
in the rradial directionn. Some laserr energy used to induce meelting in the radial
r
direction ccould be reclaiimed by the addvance of the ccavity as an addiabatic evapooration processs. In other words, if
a material mass was evaaporated due too the irradiatioon of a laser beam with a veery high powerr density, this mass
could be suuper-heated annd dissociated into plasma, cconsistent withh the measurem
ments discussedd in Section 4.
5.2 Hole F
Formation Simulation
Figure 6 illlustrates the formation
f
of thhe hole in diffferent time stepps by the simuulation. The siimulation cond
dition
matched thhe experimenttal condition uusing a 1-s ppulse as show
wn in Figure 1,, except that tthe pulse proffile is
simplified as the red linne profile in F
Figure (1). In tthis simplifiedd profile, the ppulse drops to zero at 1 s. This
profile waas used to studyy the effect off the initial spike alone. The hole was seenn to form very quickly during the
laser pulsee, but stopped abruptly whenn the pulse ennded at 1 s. T
This result wass due to a simpplified laser prrofile
was used and only evvaporation wass considered as the material removal m
mechanism byy this model. The
contributioon of the moddel is its abilitty to show thee evolution off the hole form
mation via adiaabatic evapora
ation,
which is nnot observable via experimenntation.
Figure 6. Holee formation byy adiabatic evaaporation by sim
mulation
t opening diiameter of the hole did not cchange after 5500 ns, which indicated that little
Accordingg to Figure 6, the
heat transffer occurred inn the radial ddirection once the hole was established. T
This could alsso be explaine
ed by
multiple reeflections as shown
s
in bottoom figures in Figure 6. Oncce the hole became sufficienntly deep, the laser
energy waas reflected to the
t bottom of tthe cavity to fuurther deepen tthe hole.
Another innteresting obseervation of Figgure 6 was thaat the majorityy of material rremoval by adiiabatic evaporration
was complleted by 750 ns.
n Because thee evaporated m
material receivved a large amoount of laser eenergy deposite
ed by
the initial laser spike, it
i was dissociaated into hot plasma, as coonfirmed in Section 4.3. Thhis result was also
consistent with the meassurement of vaapour/plasma inntensity whichh peaks at abouut 600 to 800 nns (Figure 3).
5.3 Data aand Model Com
mparison
The experiimentally obtaained hole had a depth of 166.7 m, an opening diameteer of 18.9 m, and an aspect ratio
of 8.8 (Figgure 2). In com
mparison, the simulation prooduced a hole with a depth of 50 m, a ddiameter of 27
7 m,
and an asppect ratio of 1..8. Therefore, the simulationn produced a hhole wider andd shallower thaan the experim
mental
result. As the model coonsidered onlyy the adiabaticc evaporation, the discrepanncy between thhe experimentt and
simulationn indicated thee important rolle of other meechanisms negglected by the model, such aas the laser en
nergy
deposited after 1 s, melt ejection, expplosive boilingg, and the enerrgy re-radiatedd by the vapouur/plasma. With
h this
comparisoon, the adiabatiic evaporation mechanism iss estimated to ccontribute to abbout 1/3 of thee material remo
oval.
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Figurre 7. Volumetrric removal ratte comparison
Figure 7 pplots the Volum
metric Materiaal Removal R
Rate (VMRR) during the earrly hole formaation. To provide a
comparisoon, a one-dimennsional drillingg model by voon Allmen and Blatter (1995)) was used to ccalculate its VM
MRR.
This modeel assumes thaat adiabatic evvaporation occcurs at the botttom of a cylinndrical volumee; thus, there is no
heat transffer in the radiaal direction. Baased on this moodel, the drillinng speed equattion is:
u  Ia
Vd
H (Td )  H LV
(7)
where Ia w
was the absorbed power denssity, Vd is the molar volumee, ΔH(Td) was the enthalpy nneeded to reach the
surface tem
mperature Td, and ΔHLV waas the enthalpyy of vaporizattion. Using ann absorbed laseer power of 80
00W,
which wass the average value
v
of the laaser power ovver the pulse, aand the area oof the cylinder equal to the beam
b
spot size, the corresponnding VMRR w
was calculatedd to be 8.81x110-9 m3/s, which was also pplotted in Figu
ure 7.
t VMRR preedicted by the 1D model waas only slightlyy lower than thhe average valu
ue of
Accordingg to Figure 7, the
the 3D sim
mulation. Becaause these twoo predictions matched eachh other very w
well, the mechhanism of adia
abatic
evaporatioon could be assumed in the early stage (beefore 750ns) oof the drilling process. The fact that the actual
a
hole depthh was much deeper
d
than thhat of the sim
mulated hole, it again suggeested significaant contributio
on of
material reemoval by mecchanisms otherr than evaporaation.
6. Other M
Material Rem
moval Mechaniisms
Based on tthe analysis inn Section 5, it iis estimated thhat adiabatic evvaporation conntributes to onnly 1/3 of the actual
a
hole formaation. The rolees of other maaterial removall mechanisms,, such as the laaser energy deeposited after 1 s,
melt ejectiion, explosive boiling, and thhe energy re-raadiation by thee vapour/plasm
ma, need to be iidentified.
6.1 Materiial Removal byy Laser Energyy Deposited att a Lower Laseer Power
The effectt of the laser ennergy depositeed after 1 s att a low laser ppower was deteermined in thiss section. In Figure
8, a 1-μs ppulse was plottted against a 44-μs pulse. Forr the 1-μs pulsse, the laser ennergy deposited between 1–1
10 s
(the area bbelow the laserr power curve)) was calculateed to be approxximately 2.4 m
mJ, while the eenergy depositted in
the first 1 s was 0.8 mJJ. The laser ennergy depositedd at this low laaser power perriod was not abble to produce high
temperaturre plasma (Figgure 3) but it coould induce melting and mildd evaporation to advance thee hole depth.
To determ
mine the materiial removal byy the lower pow
wer energy, w
we compared thhe holes produuced by a 1, 2,, 4, 5
and 8-μs ppulses (Figure 2). The additional energy deeposited by lonnger laser pulsses was 0.3, 0..9, 1.2, and 2.1
1 mJ,
respectivelly. The extra laser energies led to 19.5, 338.4, 52.5, andd 80.7 μm addditional hole ddepth, respectiively.
Dividing tthe additional depth by the additional eneergy, we obtainned an average depth versuss additional en
nergy
ratio as 655.00, 42.6, 43.8, and 38.4 μ
μm/mJ, respecttively. It appeeared that the energy deposiited near the initial
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spike was more effective. It should bee noted that the energy depoosited after thee initial spike iin a 1-μs pulse
e was
not at a coonstant powerr. Due to this ffact, we use thhe average off the above rattios, 47.2 μm/m
mJ, for estima
ation.
Based on tthis averaged ratio, the addiitional energy deposited betw
ween 1 to 10 μ
μs (2.4 mJ) forr a 1-μs laser pulse
p
likely conttributed to 1133 m, which aaccounted for 668% of the tottal hole depth of 166.7 μm. On the other hand,
h
based on tthe simulation,, the initial spiike of the 1-μss likely was reesponsible for 30 % of the hole drilling (50
0 μm
of 166.7 μ
μm). Both resullts were consisstent.
Figure 8. E
Energy differeence between tw
wo pulse durattions
However, material remooval by meltinng needs effecttive melt ejecttion mechanissms. As shownn in Figure 2, with
laser pulsees over 5-s and
a hole depthh over 220 m
m, significant blockage at thhe opening byy re-solidified melt
started to aappear.
6.2 Melt Ej
Ejection
As the holle depth becam
me deeper, it w
would take morre pressure to ddrive the melt at the bottom of the hole upward
and out off the hole. Thiss melt ejectionn process mustt be effective iin order to keeep the blink hoole open at the
e top.
In this studdy, we did nott attempt to im
mprove melt ejeection by an asssist gas jet, blowing side w
way to enhance melt
ejection. H
However, suchh strategy shouuld be investiggated for multipple-pulse drillling to achievee hole depth ov
ver 1
mm or asppect ratio over 20.
7. Three M
Media Interacctions
From the aabove analysiss, it became evvident that the ddrilling investiigated in this ppaper involvedd three media, laser,
bulk materrial, and vapoour/plasma. Reegarding energgy sources for this drilling pprocess, the hiighly focused laser
beam was the primary energy
e
source, depositing ennergy at two diifferent powerr levels, while the vapour/plasma
was a secoondary energy source, whichh re-radiate som
me energy bacck to the bulk material to prromote melting
g and
is the onlyy energy sourcce after 10 ss. Note that alll energy camee from the lasser beam, but the vapour/plasma
helped to ppreserve some energy. A disccussion of the interactions beetween these thhree media is iin order.
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7.1 Laser/B
/Bulk Material Interaction
Initial Lasser Spike on thhe Bulk Material: The initiall spike (1,500 W) of the laseer beam offereed sufficient power
density too initiate a cavity into the bulk materiall through adiaabatic evaporaation (Figure 6). As the en
nergy
absorbed bby this evaporrated material far exceeded the latent heatt of the materrial due to veryy high laser power
density (~ GW/cm2), it superheated
s
annd partially ionized the vapoour into hot annd high pressuure plasma at about
a
16,000–711,000 K. The expansion
e
of tthis vapour/plaasma also com
mpressed the am
mbient air to fform a shock wave
w
which it eexpanded. A popping
p
soundd was clearly audible durinng the drillingg process. Thee initial spike was
estimated tto produce 1/33 of the depth oof the micro-hoole.
The Bulk Cavity on thee Laser: The ccavity of the bbulk material ccreated favorabble multiple rreflections to direct
d
substantiall portion of thee laser beam tooward the caviity bottom to fu
further deepen the cavity (Figgure 6).
Lower Pow
wer Laser on the
t Bulk Mateerial: As the laaser beam pow
wer fell from itts peak to the steady state power
(300 W), tthe power dennsity of the laseer beam was nno longer suffiicient to evapoorate and ionizze the bulk into hot
plasma. Thherefore, the energy
e
depositted by the low
wer power laseer beam inside the cavity m
mainly was used to
create superheated melt inside the cavvity. Without ffurther feedingg, the vapour/pplasma started to cool (Figurre 3).
The hole ddepth contributted by the lower power portiion was estimaated to be 2/3 oof the total hole depth.
Melt on thhe Laser Beam
m: Due to multiiple reflectionss, substantial aamount of laseer energy was ddirected towarrd the
bottom off the cavity (Fiigure 6) and tthe lower beam
m power contrributed more m
melt at the bottom. On the other
hand, becaause of its low
wer reflectivity,, the melt coulld absorb the bbeam energy m
more efficientlyy, cutting down the
beam refleection. As a result,
r
as the hhole became ddeeper, the lasser energy thaat could be delivered to the hole
bottom beecame less annd the hole drrilling becamee less effectivee. As shown in Figure 2, a 4-μs laser pulse
p
contained about 30% moore energy thann that of a 1-μss pulse; howevver, it only prooduced 16% moore in hole dep
pth.
Figure 9. Interaction among thrree media durinng rapid microo-hole drilling
7.2 Laser//Vapour-Plasm
ma Interaction
Laser Heatting of Vapourr/Plasma: The IB absorption coefficient caalculated basedd on the electroon temperature
e also
indicates llow laser/plasm
ma interactionn (Table 1) evven at the peakk temperaturee of the vapouur/plama. The laser
beam did nnot heat the vaapour/plasma ddirectly and the energy contaained in the plaasma was origginally absorbed via
the laser/bbulk interactionn. The above sstatement is alsso supported bby the photo-diiode measurem
ment of the pla
asma.
For exampple, in the one-microsecond ablation, the m
measurement oof the plasma emission droppped to almost zero
at approximately 3 micrro-second evenn though the llaser power staayed over 2000 W during thee same time period
(Figure 3)..
Vapour/Plaasma on Laserr Beam: Basedd on the plasmaa measuremennt of Table 1, thhe existence oof plasma insid
de the
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cavity likeely did not blocck or affect thee laser/bulk intteraction.
7.3 Vapourr-Plasma/Bulkk Interaction
As the higgh temperaturee plasma was a high energy source in itseelf, the plasmaa re-irradiated some of its en
nergy
back to thhe bulk materiial to enhance the hole drillling. The highh pressure, couupled with thee high temperrature
plasma, m
might also allow the superheeating of the m
melt. Subsequuently, the mellt would boil when the pressure
dropped aas the plasma expanded andd exited out off the cavity. T
The reduction of pressure ppromoted explo
osive
ejection off melts as fine droplets long after the laser irradiation, as observed in F
Figure 4.
8. Processs Anatomy
Based on the experimenntal and simullation results ppresented in pprevious sectioons, a temporaal process anatomy
diagram of the short miccrosecond laseer drilling, usinng the single-m
mode fiber lasser, was compiiled. Four stag
ges of
the process were depicteed in Figure 100, from shortlyy after the laseer irradiation too long after thee completion of
o the
laser irraddiation. This process anatom
my diagram waas also color-ccoded to illusttrate corresponnding tempera
atures
during thee process. Thiss process anatoomy diagram summarizes thhe laser/materrial interactionn mechanisms, hole
formation,, material rem
moval mechannisms, and vappour/plasma ccharacterizatioon as discusseed in the prev
vious
sections. A summary of time
t
line, togeether with the pprocess anatom
my, is discussedd below:
ya
Figure 110. Process anaatomy of the raapid drilling prrocess using a one micro-seccond laser plusse generated by
single-moode CW fiber llaser
my Diagram #11): The laser ppower rapidly increased to be high enoug
gh to
Stage 1 (00~250 ns) (Proocess Anatom
evaporate a small amounnt of solid into hot vapour/pllasma, first dettected at 200 nns. At 250 ns, tthe laser powerr was
600 W andd the cavity waas estimated too be 5 μm deepp (Figures 3 annd 6).
Stage 2 (2250~500 ns) (P
Process Anatoomy Diagram #2): The intennsity of the vapour/plasma continued to grow
along withh the increase of
o the laser pow
wer. When thee laser power inncreased to itss peak value off 1,500 W at 50
00 ns,
the vapourr signal reacheed 1/4 of its peaak intensity. The hole was deeepened to 20 μm.
Stage 3 (500 μs ~ 1 μs) (Process Anattomy Diagram #3): The laserr power rapidlly dropped from
m its peak to about
a
400 W at 1 μs. Howeverr, the vapour/pplasma intensitty continued too grow and reaached its peak value at about 600
to 800 nss. After 800 ns,
n the vapouur/plasma inteensity started to drop. Based on the eleectron temperrature
measurem
ments and subseequent calculaations of the ellectron densityy and absorptioon coefficient, the vapour/plasma
was opticaally thin and the
t laser energgy could be deeposited over tthe cavity walll without beinng absorbed by the
vapour/plaasma, even at its
i peak. The hhole was deepeened to 50 μm by evaporationn alone.
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Stage 4 (1 μs ~ 2.5 μs) (Process Anatomy Diagram #4): The laser power fell to its steady state (300 W) at 2.5 μs.
The vapour/plasma intensity continued to fall but at a slower rate than that of the laser power. The intensity of
the vapour/plasma was still high enough to be registered by the photodiodes, which indicated that its temperature
was still high, so was its pressure. The melt produced by the laser at the cavity wall was likely super-heated
because it was covered by the high temperature and high pressure vapour/plasma. The hole continued to deepen
via mild evaporation and melting.
Stage 5 (2.5 μs ~ 10 μs) (Process Anatomy Diagram #4): Laser power dropped below 200 W and continued to
drop to zero after 10 μs. At 2.8 μs, plasma had turned into weaker vapour, not detected by the horizontal
photo-diode. The laser energy deposited at lower power could still produce some spurts of vapour/plasma inside
the micro-hole registered by the steep angle photodiode. This part of laser energy deposited at lower power most
likely produced more melt than vapour. The hole continued to deepen mostly via melting.
Stage 6 (10 μs ~ 80 μs) (Process Anatomy Diagram #4): No laser energy was deposited in this stage, but hot
vapour still covered the micro-hole. This intensity of this hot vapour was too low to be measured by the
photodiodes but was high enough to saturate the CCD sensor of the high speed camera (first four frames of
Figure 4. Therefore, the temperature of the vapour should be much lower than the vapour/plasma observed in
Stages #1-5 to render its pressure lower. The drop of vapour pressure likely would cause the superheated melt to
boil and eject outward as seen in frames #2-4 of Figure 4 and the 4th process anatomy diagram. The hole likely
stopped deepening and was partially filled with melt, ejecting upward and out.
Stage 7 (80 μs ~ 150 μs): Vapour was no longer visible after around 80 μs. The last melt ejection was observed
around 150 μs, which was the end of the drilling process. Some melt was re-solidified around the hole as recast.
9. Conclusion
This paper presented a single-pulse micro-hole drilling technique using short micro-second pulses by modulating
a CW single-mode 300 W fiber laser. The capability of this drilling technique was demonstrated and several
in-process monitoring experiments, such as photodiode measurements of the vapour intensity, high-speed
photography, and spectroscopic plasma measurements, were used to identify the laser/material interaction
mechanisms. For the drilling process with a 1-μs laser pulse, the process started with drastic evaporation and
concluded with melt ejection approximately 150 μs later. It is concluded that two distinct drilling mechanisms
occurred, one as adiabatic evaporation by the high power initial spike and one as melt ejection by the low power
energy deposition. The time line of this drilling process was established experimentally based on measurements
by several in-process sensors, such as photodiodes, a high-speed camera, and a spectrometer. The theoretical
modeling zoomed in the early stage of the process to investigate the hole formation which could not be observed
experimentally. Both experimental and theoretical results were then compared to determine the laser-material
interaction mechanisms among three media involved in the process: laser, material, and vapour/plasma. Finally, a
series of temporal anatomy diagrams, denoted as process anatomy, were presented to describe the entire drilling
process by a one micro-second laser pulse. This process anatomy summarizes the laser/material interaction
mechanisms, hole formation, material removal mechanisms, and vapour/plasma characterization of this short
micro-second laser drilling process.
Acknowledgments
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, DMI-0944509. The authors would
like to acknowledge the assistance of Prof. W. Roberts of MAE, NCSU for the loan of the high speed camera.
References
Bauerle,
D.
(2000).
Laser
Processing
http://dx.doi.org/10.1007/978-3-662-04074-4
and
Chemistry
(3rd
ed.).
Springer.
Brackbill, J. D., Kothe, D. B., & Zemach, C. (1992). A continuum method for modeling surface tension. J. Comp.
Phys., 100(2), 335-354. http://dx.doi.org/10.1016/0021-9991(92)90240-Y
Breitling, D., Ruf, A., & Dausinger, F. (2004). Fundamental aspects in machining of metals with short and
ultrashort laser pulses. Pro. SPIE, 5339, 49-63. http://dx.doi.org/10.1117/12.541434
Garnov, S. V., Konov, V. I., Kononenko, T., Pashinin, V. P., & Sinyavski, M. N. (2004). Microsecond Laser
Material Processing at 1.06 μm. Laser Physics, 14(6), 910-915.
Griem, H. R. (1964). Plasma Spectroscopy (1st ed.). New York: McGraw-Hill.
Harp, W. R., Dilwith, J. R., & Tu, J. F. (2008). Laser ablation using long pulsed, high fluence, CW single mode
14
www.ccsenet.org/apr
Applied Physics Research
fiber
laser.
Journal
of
Materials
http://dx.doi.org/10.1016/j.jmatprotec.2007.06.062
Processing
Vol. 5, No.4; 2013
Technology,
198(1),
22-30.
Harlow, F. H., & Amsden, A. A. (1971). A numerical fluid dynamics calculation method for all flow speeds. J.
Comp. Phys., 8, 197-213. http://dx.doi.org/10.1016/0021-9991(71)90002-7
Kayukov, S. V., Gusev, A. A, Zaichikov, E. G., & Petrov, A. L. (1998). Drilling of narrow holes in metals by
millisecond pulses of Nd:YAG lasers. Laser Physics, 8(2), 527-531.
Kreutz, E. W., Trippe, L., Walther, K., & Poprawe, R. (2007). Process development and control of laser drilled
and shaped holes in turbine components. Journal of Laser Micro/Nanoengineering, 2(2), 123-127.
Low, D., Li, L., & Byrd, P. (2004). The influence of temporal pulse train modulation during laser percussion
drilling. Opt Lasers Eng, 35, 149-164. http://dx.doi.org/10.1016/S0143-8166(01)00008-2
Low, D., & Li, L. (2002). Effects of inter-pulse an d intra-pulse shaping during laser percussion drilling. Proc
SPIE, 4426, 191-194. http://dx.doi.org/10.1117/12.456852
Mao, X., Wen, S., & Russo, R. E. (2007). Time resolved laser-induced plasma dynamics. Applied Surface
Science, 253, 6316-6321. http://dx.doi.org/10.1016/j.apsusc.2007.01.053
Ohmura, E., & Noguchi, S. (2010). Laser drilling simulation considering multiple reflection of laser, evaporation
and melt flow. In Andreas Öchsner, Lucas Filipe Martins da Silva, & Holm Altenbach (Eds.), Materials
with Complex Behaviour-Modelling, Simulation, Testing, and Applications, Advanced Structured Materials,
3, 297-310. Springer-Verlag.
Pandey, N. D., Shan, H. S., & Bharti, A. (2006). Percussion drilling with laser: hole completion criterion. Int. J.
Adv. Manuf. Technol., 28, 863-868.
Trippe, L., Willach, J., Kreutz, E. W., Schulz, W., Peteriet, J., Kaierle, S., & Poprawe, R. (2004). Melt ejection
during single-pulse drilling and percussion drilling of micro holes in stainless steel and nickel-based
superalloy by pulsed Nd:YAG laser radiation. SPIE, 5662, 609-615.
Voisey, K. T., Klocker, T., & Clyne, T. W. (2002). Measurement of melt ejection velocities during laser drilling,
ACTA Materialia, 4219-4230. http://dx.doi.org/10.1016/S1359-6454(02)00249-5
Von Allmen, M., & Blatter, A. (1995). Laser-Beam Interactions with Materials. Berlin: Springer.
http://dx.doi.org/10.1007/978-3-642-57813-7
Walther, K., Brajdic, M., & Freutz, E. W. (2008). Enhanced processing speed in laser drilling of stainless steel by
spatially and temporally supported pulsed Nd:YAG laser radiation. Int. J. Adv. Manuf. Technol., 35, 895-899.
http://dx.doi.org/10.1007/s00170-006-0768-z
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