11/27/08

Laser Machining of CarbonLaser Machining of CarbonFibre ReinforcedFibre Reinforced

Composites and FEComposites and FEModellingModelling

Professor Lin Li

Laser Processing Research Centre

The University of Manchester

Email: lin.li@manchester.ac.uk

In collaboration with SIMTech, Singapore

11/27/08

IntroductionIntroduction

• Material composition:carbon fibre, resin matrix

• Material Structure:Laminated with designed fibreorientation

• Properties of CFRP composite:High specific strength(strength/density)High specific modulus(modulus/density)

• Application of CFRP composites:Aerospace, Automotive, Marine,Sports goods, Infrastructure inweight-critical components. Airbusand Boeing have announced two newaircraft, the A350 and the B787, withcomposite content anticipated to beover 30% and 50%, respectively*.

Static

Static

Fatigue

Fatigue

Static

3

2

1

0

Rel

ativ

e st

ruct

ural

eff

icie

ncy

Aluminum(7075-T6)

Titanium(Ti-6Al-4V)

Carbon / Epoxy (AS / 3501-6)

Relative efficiency of aircraft materials*

Fatigue

* F.C Campbell, Manufacturing processes for advancedcomposites, Elsevier, UK, pp28

Example of CFRP woven cloth

* Hexcel Corporation annual report 2004

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Machining of CompositesMachining of Composites

• Trimming and routing after curing forsmooth edge

• Drilling holes for fastener and acousticdamping(a single aircraft often requires manythousands of drilled holes in variouscomposite parts*)

• Half of manufacturing process is dedicatedto machining composite parts**

*http://www.compositesworld.com/hpc/issues/2004/November/636** http://www.mmsonline.com/articles/120501.html

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• Non-contact process and no tool wearing• Easy for automation• HAZ and charring

• Best machining quality• Slow process

• Low temperature process, no heatdamage to matrix

• One jet head for all material andthickness, no tool exchange

• High noise and abrasive slurry arepotential hazardous to operator andenvironment

• Well developed process• Mechanical force induced delamination• Tool wear and dust

Advantages/disadvantages

Under development• Reduce charring/HAZ• Combining with other

technologies

• Combining withmechanical machining

• Increase pressure (700MPa)

• Combining withmechanical machining

• Tool design• Machining strategy

Development

Abrasivewaterjet

Mechanical

Ultrasonic

Laser

Techniques

Summary of Techniques for Machining CFRPSummary of Techniques for Machining CFRP

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Challenges of Laser Machining of CompositesChallenges of Laser Machining of Composites

1.8543000330071050.00Graphite

1.251000350-50012000.2Polymer

Density(gcm-3)

Heat ofVaporization (J g-1)

Vaporization/Decomposition

temperature (oC)

Heat capacity(JKg-1K-1)

Conductivity(W/m/K)

Material

Thermal properties of fibres and matrix materials*:

* V. Tagliaferri, A. Di Ilio, I. Crivelli Visconti, Laser cutting of fibre reinforced polysters, Composite 16(14) 317-325, (1985)

Factors affect laser machining quality• High differences of thermal conductivities• High differences of vaporisation temperature• Laser beam cutting direction and fibre orientation

ChallengesMatrix recession, charring, heataffected zone

v v

parallel perpendicular

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Work at Manchester/SingaporeWork at Manchester/Singapore

1. Laser machining CFRP composite using a 1 kWsingle mode fibre laser, DPSS lasers (IR, greenand UV), CO2 laser and excimer laser.

2. Examination of machining results

3. FE Modelling.

This presentation: Fibre laser cutting and 355 nmDPSS laser cutting

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FibreFibre Laser Cutting Laser Cutting

500 µm 200 µm

20 µm200 µm

1 2

3 4

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FibreFibre Laser Cutting Laser Cutting

0

200

400

600

800

1000

1200

20 30 45 50 55 60

Cutting Speed (mm/sec)

Kerf Width at Beam Entry

Kerf Width at Beam Exit

Fibre Pull Out

900 W laser power

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Effect of GasEffect of Gas

N2

O2

He

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Water Assisted Laser CuttingWater Assisted Laser Cutting

Power: 155Watt

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Kerf Width Depth of Cut HAZ

Water AssistedNo Water

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Water Assisted Laser CuttingWater Assisted Laser Cutting

Power: 270Watt

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Kerf Width Depth of Cut HAZ

Water Assisted

No Water

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Water Assisted Laser CuttingWater Assisted Laser Cutting

No water With water

900 W 50 mm/s

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355 nm DPSS Laser Cutting355 nm DPSS Laser Cutting

Laser:Wavelength: 355 nmMaximum power: 10 WPulse frequency: 40 kHzBeam size: 20~25 µm

Galvanometer

F-theta lens

Workpiece

X

Y

Z

Galvanometer

Mirror 1

Mirror 2

Laser beam

Material:carbon fibre reinforced plastic(CFRP) compositesThickness: 0.3~7 mm

Atmosphere: Air

11/27/08

V= 50 mm/s V=200 mm/s V=800 mm/s

Drilling of Composite:Drilling of Composite: effects of scanning speedeffects of scanning speed

Hole size: 2 mm. Sample thickness 0.3 mm

• HAZ reducing with increase scanningspeed.

• Number of passer required to drill throughincrease with scanning speed.

• Time of drilling through decreasing withscanning speed.

0

5

10

15

20

25

30

35

40

45

50

-100 400 900

Scanning speed (mm/s)

Num

ber

of

pas

ses

requir

ed t

o d

rill

thro

ug

h

0

0.2

0.4

0.6

0.8

1

1.2

Tim

e of d

rilling th

rough (s)

drilling

through

without serious

damage

burning

not

drilling

throug

h

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Drilling of Composite:Drilling of Composite: effects of laser beam scanning spacingseffects of laser beam scanning spacings

1-ring 2-ring 3-ring

spacingLaser beam path:

• Material removal rate is higher using 3-ring than 2-ring beam paths• Optimum beam spacing is 100 µm which is great than effective beam size 35 µm

0

10

20

30

40

50

60

70

80

90

100

0 0.1 0.2 0.3

Laser bream trace spacing (mm)N

um

ber

of

pas

ses

req

uir

ed t

o d

rill

th

rou

gh

0

4

8

12

16

20

24

28

32

36

40

Tim

e of d

rilling

thro

ug

h (s)

2-ring

3-ring

2-ring

3-ring

Hole size 2mm. Sample thickness 1 mm

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Material Removal Mechanism:Material Removal Mechanism:Fibre ejection by heat conductionFibre ejection by heat conduction

1 2 3

M1 M2

T

Laser beam trace

M1 M2

Laser machining ofconventional material

Laser machining of CFRP

• Fibres are chopped intosmall pieces.

• Heat is conducted intoM1and M2.

• Heat is constrained in M1and M2.

• Surrounding polymer matrixare heat up to hightemperature.

• Polymer matrix loss itsholding power.

• Fibres are ejected.Fibre redeposited on sample surface

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Laser Machined CFRP CompositesLaser Machined CFRP Composites

No serious HAZ and delamination free are observed

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Bearing Strength of Drilled CompositesBearing Strength of Drilled Composites

0

2

4

6

8

10

12

0 2 4Displacement (mm)

Load

(kN

)

Laser drilledMechanical drilled

0

1

2

3

4

5

6

7

0 0.5 1 1.5 2 2.5

Displacement (mm)

Load

(kN

)

5.3 kN

Mechanical drilled Laser drilled

11/27/08

Finite Element Analysis of Heat TransferFinite Element Analysis of Heat Transferduring laser machining CFRPduring laser machining CFRP

Governing equations for heat conduction:

( ) ( )( )

( ) ( ) ( )( )!!

"

!!

#

$

%+&'

()*

+

,

,

,

,=

,

,

%+&'

()*

+

,

,

,

,=

,

,

RtQn

tnTK

nt

tnTC

RtQn

tnTK

nt

tnTC

fff

mmm

1,,

1),(,

-

-

1. Transient irradiated surface

; If the interaction surface is on the matrix.

; If the interaction surface is on the fibre.

2. The remaining solid

( ) ( )!"

#$%

&

'

'

'

'=

'

'

n

tnTK

nt

tnTC

ccc

,,(

( )( )

( )( )!!"

!!#

$

%

%=&

%

%=&

n

TKRtQ

n

TKRtQ

f

m

1

1 ; If the interaction surface is on the matrix.

; If the interaction surface is on the fibre.

Under boundary conditions of:I. T(n,0)=293KII. T(∞,t)=293KIII. At laser irradiated surface:

IV. At all other surfaces: ( )!""=

#

#TTh

n

TK

sc

11/27/08

ModellingModelling Considerations Considerations

Fibre diameter: 7 µm

Spacing: 1 µm

0

20

40

60

80

100

120

0 200 400 600 800 1000 1200Temperature (°C)

Wei

gh

t%

320 º C

424 ºC

500 º C

715 º C

N 2

Air

Thermal Gravimetric Analysis

polymer decomposes at 424°C.Carbon burns at 882 °C

11/27/08

Laser Beam Reflectivity SpectrumLaser Beam Reflectivity Spectrum

0

5

10

15

20

25

30

20

0

24

4

28

8

33

2

37

6

42

0

46

4

50

8

55

2

59

6

64

0

68

4

72

8

77

2

81

6

86

0

90

4

94

8

99

2

10

36

10

80

Wavelength(nm)

Ref

lect

ivity

(%)

11/27/08

Material PropertiesMaterial Properties

6981153Decomposition Temperature (K)1884710Specific Heat(J.kg-1.K-1)

0.150Thermal conductivity in ambient

temperature(W.m-1.K-1)

12001800Density(kg/m3)40%60%Volume Fraction

EpoxyFibreProperty

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Effect of Cutting Speed on Heat AffectedEffect of Cutting Speed on Heat AffectedZoneZone

400 mm/s (24 m/min) 800 mm/s (48 m/min)

Cut geometry pre-determined

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Effect of Cutting Speed on Heat AffectedEffect of Cutting Speed on Heat AffectedZoneZone

50 mm/s 200 mm/s 800 mm/s

Cut geometry calculated

11/27/08

Comparison of FEA and Experiment ResultsComparison of FEA and Experiment Results

The HAZ overestimation observed from FEA canbe explained by thermal conductivity of fibresdecrease considerably at high temperatures.

Heat Affected Zone Ablation depth

Effect of speed on heat transfer mechanisms:Low speed: long heating time leads to low heatloss rate and more material removal.High speed: heat input decreasing results in heatloss rate increase and less material removal.

0

10

20

30

40

50

0 500 1000Scanning Speed (mm/s)

HA

Z (!

m)

FEA

EXP

µ

0

10

20

30

40

50

60

70

80

0 500 1000Scanning Speed (mm/s)

Abl

atio

n D

epth

(!m

)

FEA

EXP

µ

11/27/08

Effects of Laser BeamEffects of Laser BeamScanning SpacingsScanning Spacings

Simulation pathof the laser beam

Computationdomain (Red

section)

Previously CutGroove

100 µm spacing 150 µm spacing

75 µm Spacing

BlockEnd

Far Side

BeamPath

100 µm

BlockEnd

Far Side

Beam Path

150 µm

Block End

Far Side

Beam Path

75 µm

11/27/08

Effects of Laser BeamEffects of Laser BeamScanning SpacingsScanning Spacings

200 µm spacing

11/27/08

Removal DepthRemoval Depth

0

5

10

15

20

25

30

75 100 150

Exp.FEA

RemovalDepth (µm)

Beam Scanning Spacing (µm)

11/27/08

SummarySummary

• Minimum HAZ and delamination free are achieved by usingUV laser machining of CFRP composites.

• Bearing strengths of machined CFRP composites are similarbetween laser and mechanical drilled samples.

• Introduced New Material removal mechanism: combinationof laser ablation and heat conduction.

• Process speed could be increased by optimising laserscanning spaces.

• FEA prediction showed that the HAZ and ablation depthwere more sensitive to lower speed as compared to higherranges.

• FEA predicted that at 150µm distance i.e. 5 times the activebeam spot, the chip formation of fibres takes place.

11/27/08

The University of Manchester:Dr. S. Marimuthu,Mr. R. Negarestani,Dr. M. Sheikh,Dr. P. Mativenga,

Singapore (SIMTech)Dr. Zhongli Li,Mr. Pau Loong Chu,Dr. Hongyu Zheng,Dr. Gnian Cher Lim

Acknowledgements