Advancement in laser drilling for aerospace gas turbines white paper from JK Lasers.

1. rd
Proceedings of the 3 Pacific International Conference on Application of Lasers and Optics 2008
ADVANCEMENT IN LASER DRILLING FOR AEROSPACE GAS TURBINES
Mohammed Naeem
GSI Group, Laser Division
Cosford Lane, Swift Valley
Rugby, CV21 1QN, UK
mnaeem@gsig.com
Abstract
Aerospace gas turbines require a large number of small
diameter holes (<1mm) to provide cooling in the
turbine blades, nozzle guide vanes, combustion
chambers and afterburner. Many thousands of holes
are introduced in the surface of these components to
allow a film of cooling air to flow over the component.
Film cooling both extends the life of the component
and enables extra performance to be achieved from the
engine.
A typical modern engine will have ~ 100,000 such
holes. Drilling these cooling holes by high peak power
pulsed Nd-YAG laser is now well established. Such
holes can be successfully produced by laser trepanning
or percussion drilling.
This paper investigates laser percussion drilling with a
high peak power pulsed Nd: YAG laser (up to 20kW)
using both direct beam delivery and fiber delivered
systems. A number of holes were drilled with different
laser and processing parameters on nickel based
superalloy to quantify laser drilling times, recast layer,
taper, oxidized layer and cracking.
Introduction
Holes are drilled into gas turbines; nozzle guide vanes
and combustion rings primarily for cooling, Figure 1.
In the modern jet engine the temperature of the gases
can be as high as 20000C. This temperature is higher
than the melting point of the nickel alloy used in the
combustion chamber and turbine blades. The way that
the jet engines components are protected against these
extreme temperatures is to use boundary layer cooling.
The number of holes per component may vary from 25
to 40,000, Table 1. As the cooling air passes over the
surface it forms a cooling film, which protects the
surface of the component from the high temperature
combustion gases.
Cooling holes can be produced either by EDM
(electrical discharge machining) or by laser. EDM or
spark machining consists of an electrode, which is held
above the workpiece to produce a small gap between
the two surfaces. An increasing voltage is applied
between the electrode and the workpiece until the
electric field becomes so intense that there is an
electrical breakdown at the tip of the electrode. A
spark will discharge across the gap. Due to the very
small cross sectional area very high current densities
can result, around 1000 A/mm2. Typical temperatures
in the region of the breakdown between electrode and
workpiece are in the region of 5000 – 10 000 oC are
being achieved between electrode and workpiece. The
EDM process uses discrete discharges to drill the hole.
Although EDM is capable of producing good quality
holes it is substantially slower than the laser and other
disadvantages of this technique are:
EDM is not suited to the production of holes
at high or variable incidence angles where
multi- wire heads cannot be used.
EDM also requires reality complex
consumables tooling and electrolyte fluids,
both of which contribute adversely to cost of
hole production.
To increase temperature capability of the
engine blades and vanes, a thin coat of a heatinsulating zirconia ceramics is applied on the
surface of the blades as a thermal barrier
coating, Figure 2. EDM is not suitable for
drilling through ceramic or ceramic coated
materials
Pulsed Nd: YAG laser is now the preferred laser
choice for drilling applications in the aerospace
industry. This choice is driven by the following
considerations:

2. Good coupling of 1.06µm radiation
into part (both in terms of material
absorption and plasma avoidance)
High pulse energies and peak powers
are well suited for this application
High aspect ratio holes in a variety
of materials at very high speeds
including thermal barrier coatings
materials.
Figure 1: Laser drilled component
The frequency of the laser pulses are synchronized
with the rotational frequency of the part and the laser
drills all of the holes in a particular row virtually
simultaneously. Refereed to as “drilling on the fly”
this technique reduces the time to drill a component
but the quality of the holes produced are usually poor.
The issue of hole quality is very important but is a
subjective one. The qualities of a hole produced by
laser drilling are judged on a number of different
characteristics. The geometric factors are hole
roundness, hole taper and variation in hole entrance
diameter. The metallurgical factors are oxidation and
recast layer. The recast layer, melted material that was
not ejected form the hole by vapour pressure generated
by the laser pulse, coats the wall of the hole leaving a
thin layer of solidified metal. This layer can generate
micro-cracks, which can propagate into the parent
material. For aerospace companies like Rolls-Royce
they have a maximum allowed thickness for recast and
oxidation layer. While the hole geometric factors have
a maximum deviation value before the component can
be used in an engine. Other aerospace companies
concentrate more on the flow characteristics of an
aerospace component [2] for judging hole quality.
What ever meter is used aerospace companies are
continuously striving to improve hole quality.
Table 1: Typical hoe dimensions [1]
Component
There are two basic techniques for producing holes
within a aerospace component with a laser, trepanning
and percussion drilling. Trepanning is were the laser
beam pierces the centre of the hole and then moving to
the holes circumference the laser beam or the
component rotates producing a hole. The second basic
method called laser percussion drilling, here neither
laser beam nor component is moved but by firing a
continual series of laser pulses a hole is produced. The
hole diameter is controlled by the amount of energy
used in the drilling pulse. Percussion drilling is a very
important enabling technology within the aerospace
industry as it allows for the cycle times on a
component to be reduced. This reduction in cycle time
can be further improved when drilling symmetrical
components such as a combustion ring or chamber.
Wall
Thickness
(mm)
Angle
(deg)
No of
holes
Blade
Figure 2: A stator blade of a stationary gas
turbine (Siemens Power Generation), furnished
with plasma sprayed thermal barrier coating of
YSZ (partially stabilised zirconia)
Dia
(mm)
0.3-0.5
1.0-3.0
15
25200
Vane
0.3-1.0
1.0-3.0
15
25200
Afterburner
0.4
2.0-2.5
90
40k
Baseplate
0.5-0.7
1.0
30-90
10k
Seal ring
0.951.05
1.5
50
180
Cooling
ring
0.780.84
4.0
79
4200
Cooling
ring
5.0
4.0
90
280
At present all the drilling of the aerospace components
is being carried out with direct beam deliverly systems

3. because the application of optical fibre technology in
laser drilling has progressed at a much slower pace due
to a number of technical problems. The two main
problems are the relative low damage threshold of
optical fibres and the preservation of beam quality. The
drilling parameters for aerospace components usually
use pulse widths in the millisecond range. Though
laser damage thresholds in optical materials have been
extensively reviewed, unfortunately the available data
relates generally to nanosecond laser pulses and very
little systematic data has been published in the
microsecond and macrosecond regimes. Optical fibre
can be treated to increase the damage threshold, and
this approach was taken by Kuhn et al [3] and applied
to laser percussion drilling. A 400µm fibre was treated
with a CO2 laser and holes were drilled using pulses in
the 10 – 30 J range without fibre failure. The other
problem is that as the fibre diameter is increased the
beam quality deteriorates. An M2 of 25 or better, given
the right pulse parameters should produce an
acceptable hole. Laser drilling via an optical fiber
offers many advantages over direct beam delivery
system i.e.
complex pulse shaping facilities offer greater
flexibility for drilling a range of aerospace materials
including thermal barrier coated materials.
CCTV
Optical Focus Sensor
Other Sensors
Figure 3: Schematic diagram of JK704 laser
(not to sale)
An optical fiber laser beam delivery
system offers the option of standardizing
the beam path for all CNC machines.
Optical fibers homogenize the power
distribution across the laser beam giving
a top hat profile, which can improve
drilled hole roundness and consistency.
Fiber delivered percussion drilling offers
i.e. high quality drilled holes with a
significant reduction in the production
times. This will increase throughput and
reduced the manufacturing costs.
This paper investigates laser percussion drilling with a
high peak power pulsed Nd: YAG laser (up to 20kW)
using both direct beam delivery and fiber delivered
systems. . Holes are drilled with various laser and
processing parameters on a range of nickel based
superalloy to quantify recast layer, taper, oxidized
layer cracking and drilling times.
Drilling Tests
Lasers
The direct beam drilling tests were performed with a
JK704 pulsed Nd: YAG laser (Figure 3). This laser
provides high peak power (Table 2) and very good
pulse to pulse stability ideal for drilling small diameter
percussion holes (0.25mm- 0.90mm).This laser with its
gaussian beam profile (Figure 4), enhanced control and
Figure 4: Beam profile of a JK704 laser
Table 2: Performance data of JK704 laser
Laser
Pulse
width
(ms)
Peak
power
(kW)
Energy
(J)
Power
(W)
704
LD1*
0.3-5
20
50
120
704
LD2+
0.3-5
20
50
230
* can be used to drill small holes (200-250un dia.)
+ Used to drill large holes up to 900um dia.
The fiber delivered drilling tests were performed with
GSI latest high peak power pulsed laser, JK300D

4. (Figure 5). This laser with its high peak power coupled
with top hat beam profile (Figure 6) is ideal for
percussion drilling aerospace alloys.
The beam from the laser was transmitted in a 10m x
300µm diameter fiber, which terminated in 160mm
right-angled output housing fitted with focusing optics.
The laser specification matrix is highlighted in Table 3.
Drilling trials
The drilling tests were performed with various laser
and processing parameters for both laser systems
(Table 4).
Table 4: Drilling tests parameters
Laser parameters
Processing parameters
Peak power
Pulse energy
Pulse width
Pulse frequency
Power density
Pulse shape
Assist gas
Assist gas pressure
Focus position
Nozzle tip standoff
Angle of incidence
Spot size
These tests are intended to compare the drilling
performance of the both laser systems when percussion
drilling aerospace nickel based super alloys.
Results and Discussion
Figure 5: Schematic diagram of JK300D laser
(not to sale)
Of primary concern to the component designer is
achieving adequate airflow through the holes so that
the appropriate cooling is provided. Airflow is
governed principally by the size and shape of the hole
and hence the need for tight control of size, roundness
and taper. There are other factors also to consider;
holes are often very closely positioned to one another
on a component and any deviation in size may
adversely encroach on other holes or even weaken the
component locally. Excessive bell- mouthing or
barrelling is therefore undesirable in addition to recast
layer and heat-affected zone. The geometrical features
and the metallurgical characteristics of each laser
drilled hole generated during the present study were
carefully investigated. The prominent results are
briefly disused below.
Drilling times
Figure 6: Top hat beam profile of a JK300D laser
Table 3: JK 300D Specification Matrix
Average laser power
Maximum peak power
Maximum pulse energy
Maximum frequency
Pulse width range
Fiber size
Beam quality (M2)
300W
20kW
35J
1000Hz
0.2-20ms
300µm
42
Holes produced at 90 degrees to surface for 2mm thick
material were less than 0.5 second for both laser
systems. Figures 7-8 show the drilling times for 20 and
10 degrees to surface for fiber delivered system. The
results show that with a long focal length (160mm)
with its bigger spot size (300µm) and better depth of
focus produced holes in the shortest time compared to
120mm focal length lens. Also there appears to be a
correlation between pulse width and the drilling times.
Longer pulse widths and hence higher pulse energies
produced holes at faster times compared to short pulse
widths and low pulse energies. The drilling tests
carried out with JK704 LD1 laser show that because of
its better beam quality i.e. M2 of 16 compare to M2 of

5. Drilling time (sec)
42 for JK300D, the drilling times were much shorter
(Figures 9-10). High beam quality allows use of long
focal length lens (200-250mm), whilst maintaining the
power density required for fast drilling times. The
main advantages of using longer focal length lenses are
reduced damage to the optics from the spatter
generated during drilling hence extending the life of
the cover glass slide, which protects the focussing
optics. Additionally, the high beam quality gives a
greater depth of focus, allowing greater tolerances to
variations in workpiece or motion system positioning.
5
4
3
2
1
0
5
10
15
20
25
Peak power (kW)
0.5ms
0.7ms
1.0ms
Figure 10: Drilling times for different pulse widths
(10 degrees to surface, JK704LD1, & O2 assist gas)
Drilling time (sec)
2.5
Taper and Hole Roundness
2
1.5
1
0.5
0
5
10
15
20
25
Peak power (kW)
0.5ms
0.7ms
1.0ms
Figure 7: Drilling times for different pulse widths
(20 degrees to surface, JK300D, & O2 assist gas)
Drilling time (sec)
8
6
4
2
0
5
10
15
20
25
Peak power (kW)
0.5ms
0.7ms
1.0ms
Figure 8: Drilling times for different pulse widths
(10 degrees to surface, 300µm spot, & O2 assist gas)
Drilling time (sec)
2.5
Figures 11-12 show typical taper for 2mm thick
materials at different incident angles for both laser
systems. Although both systems produce very similar
taper, however the holes drilled with the fiber
delivered laser were much rounder than those produce
with the direct beam delivery system, because the fiber
circularizes and homogenizes the laser beam. Typical
cross sections of holes drilled with both lasers are
highlighted in Figure 13. Holes drilled with both
lasers at 90 degrees to the surface show that the taper is
not uniform along the depth of the hole and varies
particularly substantially in the centre of the hole.
While the figures reflects the differences in percent
taper with laser parameters, the influence of peak
power density on the taper and the shape of the hole is
seen to very substantial. During the present study, the
extent of barrelling formation, mainly observed at the
centre of the hole, was found to be consistently more in
the case high power densities. This is presumably
because of plasma formation which significantly
decreases the contribution of vaporisation to the
material removal process during the hole formation.
Holes drilled at acute angles to surface show no
barrelling effect. This may be due to spot size which
tends to elongate at an angle and therefore the power
density is greatly reduced.
Recast layer
2
1.5
1
0.5
0
5
10
15
20
25
Peak power (kW)
0.5ms
0.7ms
1.0ms
Figure 9: Drilling times for different pulse widths
(20 degrees to surface, JK704LD1, & O2 assist gas)
Apart from oxide layer, the recast layer is one of the
main metallurgical characteristic of interest in laser
drilling and this has been comprehensively
investigated with the fiber delivered system [4]. The
result show that a typical recast layer in laser drilled
sample at 90 degrees to surface was between 25-35µm.
The recast layer was very similar with the direct beam
delivery laser. 2mm thick material. The oxide layer
was between 10-15 µm for all the laser parameters
tested for both lasers. Holes drilled at acute angles to
surface, the recast layer thickness is seen to vary
substantially with location [4]. Greater recast layer

6. formation near the entry- side of the hole is possibly
the result of the fact that molten material ejection
during percussion drilling takes place from this side.
As may be expected, the thickness of the recast layer
was found increase with low pulse energies and peak
powers.
established benchmark in industrial laser drilling. The
new high peak power fiber delivered driller offers a
number of advantages over current direct beam
delivery systems i.e.
•
•
Taper (%)
4
3
2
•
•
1
0
5
10
15
20
25
Peak power (kW)
90 deg
30deg
20 deg
15 deg
10 deg
Figure 11: % Taper as a function of PP (JK300D)
•
Taper (%)
4
•
3
2
•
1
0
5
10
15
20
25
15 deg
10 deg
References
Peak power (kW)
90 deg
30deg
20 deg
Very Compact, lower-cost, high peak-power,
fiber-delivered drilling laser
Capable of percussion drilling a range of hole
sizes for aerospace applications. Typical hole
sizes from 0.4mm to 0.8mm and thicknesses
of over 6mm.
Very round holes can be achieved
High beam quality, allowing transmission of
the energy through a 300µm diameter fiber.
Therefore, typical drilling lens focal lengths
(i.e. 120-160mm) can be used which offers:
– Fast material removal rates
– Possible to drill at shallow angles
– Good depth of focus
– Less damage to focusing optics from
spatter generated during drilling
Possible to drill down to 10 degrees from the
surface
Easier laser integration, simpler motion
systems, possibility of robotic delivery,
simple Time-Share Multiplexing
Ability to drill on-the-fly with varying pulse
frequency and skipped sections
1. .H.H van Dijk, D de Vilrger, J.E.Brouwer.
Laser Precision Hole Drilling in Aero-engine
Components. Proc 6th Conf lasers in
Manufacturing. May 1989 ISBN 1-85423047-6. Page No 237-247.
Figure 12: % Taper as a function of PP (JK704LD1)
2.
P.J.Disimile, C.W.Fox, An experimental
investigation of the airflow characteristics of
laser drilled holes. Journal of Laser
Applications. Vol 10 No 2, April 1998. Page
78 – 84.
3.
Kuhn.A, French.P, Hand.D.P, Blewett.I.J,
Richmond.M, Jones.J.D.C; Preparation of
fibre optics for the delivery of high-energy
high-beam-quality Nd: YAG laser pulses.
Appl Optics Vol 39, No 33. 20th Nov 2000.
4.
M.Naeem, Laser Percussion Drilling of
Aerospace Material with High Peak Fiber
Delivered Lamp- Pumped Pulsed Nd: YAG
Laser,
Conference
Proceeding
2006,
Scottsdale Arizona, USA; October 30November 2 2006
JK300D
JK704
Figure 13: Laser drilled holes with both laser systems
Summary
GSIL have been producing drilling lasers for the
aerospace industry since the early 1980’s and JK704