Proceedings of the 3rd

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 heat-

insulating 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:

� 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.

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.

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 Dia

(mm)

Wall

Thickness

(mm)

Angle

(deg)

No of

holes

Blade 0.3-0.5 1.0-3.0 15 25-

200

Vane 0.3-1.0 1.0-3.0 15 25-

200

Afterburner 0.4 2.0-2.5 90 40k

Baseplate 0.5-0.7 1.0 30-90 10k

Seal ring 0.95-

1.05

1.5 50 180

Cooling

ring

0.78-

0.84

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

Figure 1: Laser drilled component

Figure 2: A stator blade of a stationary gas

turbine (Siemens Power Generation), furnished

with plasma sprayed thermal barrier coating of

Y S Z ( p a r t i a l l y s t a b i l i s e d z i r c o n i a )

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.

� 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

complex pulse shaping facilities offer greater

flexibility for drilling a range of aerospace materials

including thermal barrier coated materials.

* 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

CCTV

Optical Focus Sensor

Other Sensors

Figure 3: Schematic diagram of JK704 laser

(not to sale)

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

(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.

Table 3: JK 300D Specification Matrix

Average laser power 300W

Maximum peak power 20kW

Maximum pulse energy 35J

Maximum frequency 1000Hz

Pulse width range 0.2-20ms

Fiber size 300µm

Beam quality (M2) 42

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 Assist gas

Pulse energy Assist gas pressure

Pulse width Focus position

Pulse frequency Nozzle tip standoff

Power density Angle of incidence

Pulse shape 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

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

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 M

2 of

Figure 5: Schematic diagram of JK300D laser

(not to sale)

Figure 6: Top hat beam profile of a JK300D laser

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.

0

0.5

1

1.5

2

2.5

5 10 15 20 25

Peak power (kW)

Dri

llin

g t

ime

(sec

)

0.5ms 0.7ms 1.0ms

0

2

4

6

8

5 10 15 20 25

Peak power (kW)

Dri

llin

g t

ime

(sec

)

0.5ms 0.7ms 1.0ms

0

0.5

1

1.5

2

2.5

5 10 15 20 25

Peak power (kW)

Dri

llin

g t

ime

(sec

)

0.5ms 0.7ms 1.0ms

0

1

2

3

4

5

5 10 15 20 25

Peak power (kW)

Dri

llin

g t

ime (

sec)

0.5ms 0.7ms 1.0ms

Taper and Hole Roundness

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

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

Figure 7: Drilling times for different pulse widths

(20 degrees to surface, JK300D, & O2 assist gas)

Figure 8: Drilling times for different pulse widths

(10 degrees to surface, 300µm spot, & O2 assist gas)

Figure 9: Drilling times for different pulse widths

(20 degrees to surface, JK704LD1, & O2 assist gas)

Figure 10: Drilling times for different pulse widths

(10 degrees to surface, JK704LD1, & O2 assist gas)

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.

0

1

2

3

4

5 10 15 20 25

Peak power (kW)

Tap

er (

%)

90 deg 30deg 20 deg 15 deg 10 deg

Figure 11: % Taper as a function of PP (JK300D)

0

1

2

3

4

5 10 15 20 25

Peak power (kW)

Tap

er (

%)

90 deg 30deg 20 deg 15 deg 10 deg

Figure 12: % Taper as a function of PP (JK704LD1)

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

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.

• 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

References

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 1-85423-

047-6. Page No 237-247.

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 30-

November 2 2006

JK704 JK300D