experimental evaluation of co2 laser cutting quality of ultra high

EXPERIMENTAL EVALUATION OF CO2 LASER CUTTING QUALITY OF

ULTRA HIGH STRENGTH STEEL (UHSS) USING NITROGEN AS AN

ASSISTED GAS

ABDUL FATTAH BIN MOHAMAD TAHIR

A project report submitted in partial

fulfillment of the requirement for the award of the

Master of Mechanical Engineering

Faculty of Mechanical and Manufacturing Engineering

Universiti Tun Hussein Onn Malaysia

JANUARY 2015

v

ABSTRACT

Ultra High Strength Steel (UHSS) is widely used in vehicle as it able to improved

durability of the vehicle while reducing the mass. Laser cutting process has been an

alternative choice in trimming the UHSS to regain the final shape. Cutting quality is

a crucial issues in trimming the UHSS especially interm of dimensional accuracy.

This project is intended to study the effect of input parameters of CO2 laser cutting

on Ultra High Strength Steel (UHSS) focusing on the cutting quality and mechanical

properties. CO2 laser cutting machine with 22MnB5 Boron steel was used to identify

the cutting quality. The quality of the cut was monitored by measuring the kerf width

and taper ratio while on mechanical properties, heat affected zone and microhardness

were evaluated using metallographic approach. Laser power, cutting speed, assist gas

pressure and assist gas type were varied to evaluated the effect on each responses.

Result shows that power intensity at focusing point on the surface of material play

important roles in determining the quality of cut and mechanical properties. Low

laser power with high cutting speed will produce better cutting quality and

mechanical properties. Gas pressure doesn’t highly influence the cutting quality but

effecting the mechanical properties such as HAZ formation and hardness of the HAZ

region.

vi

ABSTRAK

Keluli kekuatan ultra tinggi (UHSS) digunakan secara meluas kerana ia mempunyai

ketahanan yang tinggi di samping ringan. Proses pemotongan laser telah menjadi

pilihan alternatif di dalam proses mendapatkan reka bentuk akhir. Kualiti

pemotongan merupakan isu kritikal di dalam proses ini terutama melibatkan

ketepatan dimensi. Projek ini bertujuan untuk mengkaji kesan parameter bagi mesin

laser CO2 terhadap keluli kekuatan ultra tinggi (UHSS) fokus kepada kualiti

pemotongan. Kualiti pemotongan yang dinilai adalah dengan mengukur lebar alur

pemotongan dan sudut condong manakala bagi sifat mekanikal bahan, zon terdedah

haba (HAZ) dan kekerasan mikro dinilai menggunakan kaedah metalografik. Kuasa

laser, halaju pemotongan, tekanan gas pemangkin dan jenis gas pemangkin

dirawakkan untuk mengkaji kesan ke atas semua output. Hasil menunjukkan

kekuatan tenaga di titik fokus di atas permukaan bahan memainkan peranan penting

dalam menentukan hasil pemotongan dan sifat mekanikal bahan. Halaju pemotongan

yang tinggi dan kuasa laser yang rendah akan menghasilkan kualiti pemotongan dan

sifat mekanikal yang lebih sempurna. Tekanan gas kurang memberi kesan kepada

kualiti pemotongan tetapi menyebabkan perubahan pada sifat mekanikal seperti

pembentukan zon terdedah haba (HAZ) dan kekerasan pada zon HAZ.

vii

CONTENTS

TITLE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

CONTENTS vii

LIST OF TABLES x

LIST OF FIGURES xi

LIST OF ABBREVIATIONS xiv

LIST OF APPENDICES xv

CHAPTER 1 INTRODUCTION 1

1.1 Project overview 1

1.2 Problem statement 3

1.3 Objectives 3

1.4 Scope of study 3

CHAPTER 2 LITERATURE REVIEW 5

2.1 Fundamental of laser 5

2.1.1 Laser beam creation 7

2.1.2 Laser mode 10

2.1.3 Material removing mechanism 13

2.2 Type of laser 15

2.2.1 CO2 laser 17

2.3 Laser cutting parameters 20

2.3.1 Laser power 22

2.3.2 Cutting speed 24

2.3.3 Assist gas 25

viii 2.4 Ultra High Strength Steel (UHSS) 26

2.4.1 Application 28

2.4.2 Mechanical properties 30

CHAPTER 3 METHODOLOGY 31

3.1 Introduction 31

3.2 Process flow chart 32

3.3 Preliminary stage 33

3.3.1 Machine 33

3.3.2 Material 35

3.3.3 Machining parameters 37

3.4 Jigs design 39

3.5 Execution stage 40

3.6 Evaluation of cutting quality 42

3.7 Heat affected zone (HAZ) 44

3.8 Microhardness 48

CHAPTER 4 RESULT AND DISCUSSION 50

4.1 Introduction 50

4.2 Cutting temperature 50

4.2.1 The effect of laser power 52

4.2.2 The effect of cutting speed 52

4.2.3 The effect of gas pressure 52

4.3 Kerf width 53




4.4 Taper angle 59




4.5 Heat effected zone 62




ix 4.6 Microhardness 68




CHAPTER 5 CONCLUSION AND RECOMMENDATION 76

5.1 Conclusion 76

5.2 Recommendation 77

REFERENCES 79

x

LIST OF TABLES

2.1 Spectrum characterizations. 7

2.2 Basic types of CO2 laser. 11

2.3 Common composition of mixture laser gas. 17

2.4 Basic configuration of CO2 laser design. 18

2.5 Input parameters. 20

2.6 Laser power and its application. 22

2.7 Chemical composition (%) of boron steel

22MnB5.

28

2.8 Mechanical properties of 22MnB5. 30

3.1 Input parameters level. 37

3.2 Constant parameters. 37

xi

LIST OF FIGURES

2.1 Laser evolution. 6

2.2 The electromagnetic spectrum. 8

2.3 Schematic illustration of amplification by

stimulated emission.

8

2.4 Schematic of CO2 laser. 9

2.5 An example of CO2 laser resonator for

Mitsubishi HVII.

10

2.6 TEM01 beam generation that also known as

“blunt tool”.

12

2.7 TEM00 beam generation that also known as

“sharp tool”.

12

2.8 Nontraditional machining processes. 13

2.9 Material removing mechanism. 14

2.10 Stages in material removing mechanism. 14

2.11 Illustration of laser cutting process. 15

2.12 Classification on type of laser. 16

2.13 CO2 laser beam creation. 19

2.14 Cycle of photon creation in CO2 laser. 20

2.15 Most investigated materials for assessing laser

cut quality.

21

2.16 Laser main input parameters. 21

2.17 Kerf effect of laser power for AHSS. 23

2.18 Relationship between cutting speed with material

thickness.

24

2.19 Effect of cutting quality due to cutting speed. 25

xii

2.20 Prediction on usage of UHSS. 27

2.21 Basic hot stamping process chain. 28

2.22 Hot stamped parts in a typical car. 29

2.23 Reduction of weight of UHSS. 29

2.24 Mechanical properties of UHSS. 30

3.1 Project flow chart. 32

3.2 Mitsubishi HVII 3015 CO2 laser cutting

machine.

33

3.3 Schematic diagram of Mitsubishi HVII laser

cutting machine head.

34

3.4 Location of B pillar for Proton Preve. 35

3.5 B pillar for Proton Preve. 36

3.6 Material extracted from parent materials. 36

3.7 Parameters and responses evaluated. 37

3.8 Focal distance and nozzle gap. 38

3.9 Jig for holding specimens. 39

3.10

3.11

Jig attached with UHSS.

Fixture plate to hold the jig.

40

41

3.12 Thermocouple location. 41

3.13 Detail of experimental operation. 42

3.14 Kerf width and taper angle. 43

3.15 Top and bottom kerf. 43

3.16 Kerf width and taper angle measurement 44

3.17 Heat affected zone. 44

3.18 Sample prepared for mounting and grinding

process.

45

3.19 Sample prepared and polishing apparatus. 45

3.20 Sample preparation process. 46

3.21 OLYMPUS STM6 High Magnifying Measuring

Microscope.

47

3.22 Location of HAZ measurement. 47

3.23 Illustration on HAZ measurement. 48

xiii

3.24 SHIMADZU HV-200 Vickers microhardness

tester.

49

3.25 Microhardness test. 49

4.1 Cutting temperature. 51

4.2 Cross sectional view of kerf formation without

assisted gas

54

4.3 Cross sectional view of kerf formation at

0.25MPa nitrogen assisted gas.

55



55



55

4.6 Kerf width at variable gas pressure. 57

4.7 Taper angle formation at different gas pressure. 61

4.8 Cross sectional view of HAZ at 0.25MPa

nitrogen assisted gas.

63



64



64

4.11 HAZ region along the material thickness. 66

4.12 Microhardness indentation location. 69

4.13 Microhardness result at different gas pressure 70

4.14 HAZ region at “C”. 72

4.15 Classification of region based on hardness value 73

xiv

LIST OF SYMBOL AND ABBREVIATIONS

AHSS - Advanced High Strength Steel

Pavg - Average laser power

BM - Base material

CO2 - Carbon Dioxide

ºC - Celcius

CW - Continuous wave

vc - Cutting speed

CNC - Computer Numerical Control

EDM - Electric Discharge Machine

HAZ - Heat affected zone

He - Helium

HV - Hardness Vickers

KW - kilo Watt

MPa - mega Pascal

mm - milimeter

µm - micrometer

nm - nanometer

mW - mili Watt

MW - mega Watt

N2 - Nitrogen

Nd - Neodymiun

TEM - Transverse electromagnetic mode

TZ - Transition zone

UHSS - Ultra High Strength Steel

YAG - Yttrium aluminium garnet

xv

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Experimental conditions. 82

B Result for cutting quality 83

C Result for mechanical properties 85

CHAPTER 1

INTRODUCTION

Laser cutting application has been evolving nowadays due to its wide range of

applications in different manufacturing processes in industries. Ability to produce

high productivity and good quality of cut has been the main element of evolution and

selection of laser cutting application. Laser cutting application has been the alternate

choice to conventional machining due to its flexibility to form a shape while

maintaining the manufacturing cost especially to advanced engineering materials.

Laser cutting machining has wide application in fine cutting of sheet metals

due to its precision and high accuracy. No tool wear and vibration as it’s a non-

contact process has been one of the major advantages in laser cutting machining.

Laser cutting machining are also known as flexible manufacturing as the beam light

able to shape out materials according to the user requirement with the certain quality

cut desired.

Cutting capabilities of thick materials and rapid processing added the value of

laser cutting approach in manufacturing industry. Fast processing, accuracy and good

cutting quality has been the aspect needed in industry related to manufacturing aspect

especially on metal shaping and cutting approach.

1.1 Project overview

Development of new material in automotive sector in order to improve the vehicle

performance has been rapidly evolving. The usage of Ultra High Strength Steel

(UHSS) in automotive body in white components are also increasing as the

2

mechanical properties of the material itself that able to withstand better absorption of

impact and higher tensile strength.

In automotive industry, UHSS were formed using hot press forming to

increase its strength. This process involve the usage of stamping die that been cooled

after the material been heated at certain temperature before quenching process takes

place. Resulting from this operation, UHSS were formed and normal stamping

operation unable to trim the materials into shape.

Development of trimming dies for UHSS is considerable high cost especially

on the maintenance side. Special materials of punch and die needed in order to

perfectly trim the formed UHSS while maintaining the quality of the trimming edge.

Laser cutting process has been that alternate choice in trimming the UHSS as

the flexibility of the laser cutting process itself. Economical and precision are also

taken note as the advantages of trimming operation using laser cutting approach.

Good cutting quality on the edge will increase the life of the part itself while

reducing the potential of fracture.

Common defect occurred during trimming process is the cutting quality

issues. Dimensional accuracy and incomplete cutting have been the major factor in

industry that creates a big issues especially during joining process that came after the

trimming process were done.

Some research has been done on similar materials that have same properties

as Ultra High Strength Steel (UHSS) and found that all input parameters influenced

the cutting quality. Lamikiz et al. (2005) has made the research on Advanced High

Strength Steel (AHSS) and conclude that the kerf width size is resulted from

selection of laser power and cutting speed.

This project is intended to study the effect of CO2 laser cutting main input

parameters on Ultra High Strength Steel (UHSS) used by Miyazu (M) interm of

cutting quality and mechanical properties. Specimens will be checked on the kerf

width, taper angle, Heat Affected Zone (HAZ) and microhardness of UHSS.

Discussion was made on the effect of main input parameter that is laser power,

cutting speed, assist gas type and assist gas pressure.

3

1.2 Problem statement

The changes of mechanical properties of UHSS were significant drawbacks in

trimming process. Hot pressed material contains more martensitic content and higher

tensile strength after undergo quenching process. The component tends to become a

ductile mode due to the increases of tensile strength subsequently impair the

component’s performance. Secondary process such as trimming the side edge is

unable to be done by conventional stamping die due the changes of the materials

properties.

Laser cutting process has becomes one of the alternative approaches in

trimming process of UHSS due to its flexibility. Multi axis cutting and precision is

one of the added values in using laser cutting process for trimming UHSS. High

accuracy and good productivity were the main target in the production process, thus

reduces the production cost. However, the major issue in trimming process using

laser is the cutting quality especially on the dimensional accuracy and surface

integrity. Defect such as incomplete penetration and out of tolerance was usually

occurred during the trimming process of the UHSS. The out-of-tolerance component

subsequently affects the incoming process such as joining process and assembly.

1.3 Objectives

The objectives of this project are:

(i) to investigate on the effect of laser processing parameters on the cutting

quality in terms of kerf width and taper angle of UHSS.

(ii) to evaluate on the changes of mechanical properties such as heat affected

zone (HAZ) and microhardness of UHSS after laser cutting.

1.4 Scope of study

The scope of study will cover on:

(i) B pillar Boron steel 22MnB5 with the thickness of 1.7mm used in Proton

Preve.

(ii) Mitsubishi HVII 3015 CO2 laser cutting machine equipped with 4kW

resonator with standard focusing lens of 190.5mm.

4

(iii) Main input parameter in CO2 laser cutting application is the laser power,

cutting speed and assisted gas pressure.

(iv) Metallographic study will be done on HAZ, Vickers harness on

microhardness and metrological process on cutting quality.

CHAPTER 2

LITERATURE REVIEW

2.1 Fundamental of laser

Light amplification by stimulated emission of radiation or famously known as

“LASER” was invented by Schawlow and Towness in 1958 where potassium vapour

were used as the active medium to produce the violet radiation light. Experimental

were done by using two half silvered plane mirror which monochromatic light

undergo multiple reflection before moving away from the partial reflecting mirror.

The research done by Towness and Company were mostly focusing on the

infrared laser than popularize by Gordon Gould during the conference in 1959 where

he published the term “LASER” in the paper “The LASER, Light Amplification by

Stimulated Emission of Radiation”.

The development of laser has been evolving since it was first introduce as the

strength of the laser processing itself that able to simplify human activities. High

industry demands and invention of new technology has made the laser development

rapidly evolving as the potential of the laser itself to wide range of industry sectors.

Figure 2.1 shows the timeline of the laser development during the

intervention stages until the millennium era. The evolution of laser cutting

technology has been one of the factors in development of manufacturing industries.

6

7

2.1.1 Laser beam creation

Laser beam is the most important elements in laser cutting process where it

represents the strength of the laser itself. Generated by light, spectrum has been the

major key element in generation of laser beam. Spectrum has been divided into

portions as shown in Table 2.1.

Table 2.1 : Spectrum characterization. Type of waves Sources Frequency Energy (eV) Wavelength (nm)

radio waves antena low

frequency 10-5 – 10-9 108 – 1014

microwaves electrical

oscillation

low

frequency 10-3 – 10-5 105 – 108

Infrared electronic

vibration

middle

frequency 10 – 10-3 102 – 105

Ultraviolet high electronic

vibration

middle

frequency 102 – 10 10 – 102

X rays deep electronic

vibration

high

frequency 104 – 102 10-1 – 10

Gamma rays radioactive decays high

frequency 1011 – 104 10-7 – 10-1

Figure 2.2 shows the electromagnetic spectrum of the waves. Normal eyes

able to see the visible light that range approximate between 10 till 1000nm that

consist of variable colour such as violet (390 – 430nm), indigo (430 – 455nm), blue

(455 – 492nm), green (492 – 577nm), yellow (577 – 597nm), orange (597 – 622nm)

and red (622 – 780nm) and it was produced from corresponding between energy

states in the valence electrons of atoms (Ion, 2005).

8

Figure 2.2 : The electromagnetic spectrum (Ion, 2005).

Optical cavity is a requirement in generating laser beam using amplification

process after being heated to stimulate the emission. Amplification only happen

when stimulated emission bouncing in the optical cavity thus increasing the number

of photons. This action also reported by McGeough (1987) where light is propagated

in a direction parallel to the axis of interferometer, being reflected by the mirror

back. This occasionally amplifies and provides it to maintain the same phase at

successive reflection.

Figure 2.3 : Schematic illustration of amplification by stimulated emission.

9

For CO2 laser, creation of laser beam is mostly the same as common laser

where full reflective and partial mirror were used to generate the laser beam as

shown in Figure 2.3. All CO2 laser applied the principle of lasing action and the

differences is the ways of exciting and cooling (Powell, 1998).

Figure 2.4 : Schematic of CO2 laser.

All CO2 laser using a high energy stream of electron that passed across a

specific low pressure gas mixture known as laser gas. Attached with high voltage

supply, creation of electron is needed to stimulate the gas mixture thus producing the

laser beam. Figure 2.4 shows the schematic of CO2 laser attached with the high

voltage supply. Consisting of CO2, He and N2, laser gas is a mixture gases to render

the lasing action to be more efficient as collision between molecular generate a

parallel beam of infrared light known as laser beam. Figure 2.5 shows the cross

schematic view for the Mitsubishi HVII laser resonator.

10

Figure 2.5 : An example of CO2 laser resonator for Mitsubishi HV II.

2.1.2 Laser mode

Output laser is classified according to its pattern of energy density that also known as

the laser mode. Variable laser mode has been developed to widen the application of

laser cutting especially to the manufacturing sectors.

This mode can be determined by exposing a block of acrylic to the focus

beam for few second. Contour and profile produced is determined by the transverse

electromagnetic mode (TEM) that is the variation of beam intensity with position in

a plane perpendicular to beam propagation. (Ion, 2005).

Powell(1998) has stated that CO2 Laser Cutting that TEM commercially were

labeled as TEMxy and the x value represent the beam print from left to right and y

value represent the beam print from top to bottom. Meanwhile, Ion (2005) mentioned

that TEM is determined by the geometry, alignment, spacing and gain distribution of

the active mediums. He also added that, the TEM should be symmetrical and evenly

distributed. Table 2.2 shows the description of each mode in most CO2 laser.

Total reflective mirror Partial reflective

mirror

Heat exchanger

Electrodes

Gas circulation

Laser beam

11

Table 2.2 : Basic types of modes for CO2 laser. Mode Description TEM Intensity distribution

Gaussian

- Single concentration

point of power at the

middle

- Known as the lowest

order mode

- The beam size could be

define by the spot point

TEM00

Doughnut

- Circular in cross section

but hollow in the center

- Shape as the doughnut

where hole in the middle

- Happen due to unstable

optical due to the mirror

used

TEM01

Multimode

- Combination of Gaussian

and Doughnut mode

- Known as the 1st order

mode

- Certain laser able to

change the mode from

Gaussian to multimode if

poor quality of cut

TEM10

In laser mode development, type and orientation of mirror used plays

important roles in deciding the mode obtained. Most commercial laser especially for

CO2 used TEM00 and TEM01* as the processing laser. The optical arrangement

influence the mode produce thus determining the size of the resonators.

TEM01* generated from curvy mirror that describe as “unstable” and designed

to release light past the smaller area after lasing process. Large output power are

possible for doughnut mode (Powell, 1998). Experimental has shown that doughnut

mode power intensity is lower that Gaussian mode as the focusing point of this mode

is null.

12

TEM01 has been chosen for its power than focused spot size. Powell (1998)

has mentioned that a doughnut energy density of 5kW could be the same as TEM01

with the power as low as 1kW as the mode itself that not focusing at single point.

Vibration, poor optical component quality and local variations in pressure of the gas

can reduce the performance of Gaussian mode thus corrupting the nature of Gaussian

mode. Figure 2.6 shows the beam generation for TEM01 mode.

Figure 2.6 : TEM01 beam generation that also known as “blunt tool”.

TEM00 beam generated by two parallel mirrors are known as total reflective

mirror and partial mirror. Stimulated emission that bombard and with lasing gas will

pass through the partial mirror thus producing the laser beam with highly

concentrated power at the middle of the beam. Powell (1998) has described this

optical configuration as “stable cavity” that implies the statistical chance that a

particular photon may remain in the cavity for a period of time.

Figure 2.7 : TEM00beam generation that also known as “sharp tool”.

13

2.1.3 Material removing mechanism

Laser cutting process is categorized as non-contact machining that grouped as

thermal processes machining. Mostly advance or nontraditional machining offer

better output compared to conventional machining. It reduces the tooling cost and

increase the productivity.

Figure 2.8 shows the group categorized for nontraditional machining.

Thermal machining operated by using thermal effect to heat and thus melting the

material before blowing away the molten material away from the parent materials.

El-Hofy (2005) reported that secondary phenomena relating to surface quality occur

during machining such as micro cracking, formation of heat affected zones and

striations.

Figure 2.8 : Nontraditional machining processes.

There are three main processes involved in material removal mechanism in

laser machining, namely heating, melting and vaporizing. Many theories arise for the

material removing mechanism as shown as Figure 2.9 where Ion (2005) categorized

that mechanism into five stages. Even though many theories proposed, all theories

has come to the conclusion that three main stages stated before are the causes of

material removal processes.

14

Figure 2.9 : Material removing mechanism (Ion, 2005).

Material removing processes started as the high intensity beam of infrared

light is focused onto the surface of the workpiece by lens. The surface of material

was heated by the unreflected light from the lens. This stage is called heating stage

where power density must be greater than the loss and material heat conductivity.

McGeough (1987) reported that the typically energy absorbed about 0.1µm for

metals while it takes less than 0.1µm for most organic compound.

Melting stage takes place after heating stage, where the heated area melts as

the temperature rise above the materials melting temperature. Melting process

happen as the materials absorb sufficient heat and the material temperature

increasing. A pond of melting metals start to appear before it penetrate full all over

the material thickness.

Figure 2.10 : Stages in material removal mechanism.

Figure 2.10 shows the stages involved in material removing mechanism for

laser cutting process where the final stage of laser processing is the ejection stage. In

this stage, the materials were removed by vaporization and ejection using a

pressurized gas jet. Mostly during processing metal, molten metal is flushed away by

assist gas as in Figure 2.11 while vaporization takes place mostly for polymer and

woods. El-Hofy (2005) reported that most nonmetal that have low thermal

conductivity and have the ability to absorb CO2 laser better compared to other laser

due to the bigger CO2 laser wavelength.

15

Figure 2.11 : Illustration of laser cutting process.

2.2 Type of laser

Various type of laser was developed since 1960s due to its extended function and

usage. The development of laser is still growing rapidly as the market needed on fast

application and accuracy is high especially on laser the able to run at lowest cost as

possible.

Mostly laser cutting application has been categorized as three major elements

that is solid state, gas and liquid. Solid state and gas laser has been widely used in

most manufacturing industries from cotton industry to heavy manufacturing industry.

Figure 2.12 shows the classification of laser based on its medium. Gases medium has

been the most variation of laser as the gases acted as the stimulated catalyzer to

generate the laser beam.

Among all type of laser, Nd:YAG and CO2 are most widely used in laser

beam machining application. CO2 has a longer wavelength that is 10.6µm and

produce better efficiency and good beam quality. It is suitable for fine cutting of

sheet metal at high speed (Choudhury & Shirley, 2010).

Meanwhile Nd:YAG lasers have low beam power but when operating in

pulsed mode high peak powers enable it to machine even thicker materials. Also,

shorter pulse duration suits for machining of thinner materials. Due to shorter

wavelength it can be absorbed by high reflective materials which are difficult to

machine by CO2 laser (Dubey & Yadava, 2008).

16

17

2.2.1 CO2 Laser

CO2 laser is classed under gases laser that oriented as molecules that produce energy

by vibration and rotational of the molecules. McGeough (1987) has stated in his book

that laser transition between two vibration energy of CO2 molecules that is between

the orientation of 0001 and 1000.

In CO2 laser beam generation, the mixtures of laser gases were used to

improve the efficiency of the laser. Nitrogen and helium were added to increase

about 10 – 25% more efficiency and increasing the lasing action as reported by

Powell (1998) . The common composition of the mixture laser gas is shown in Table

2.3.

Table 2.3: Common composition of mixture laser gas. Gases Percentage Proportion Purity

CO2 1 – 9% 1 99.9995%

N2 13 – 35% 5 99.995%

He 60 – 85% 20 99.990%

CO2 can store energy by becoming distorted such as a spring as the structure

of the molecules of CO2 itself that is a triatomic and contain two oxygen atoms

attached at the side of carbon atom. Most industrial CO2 laser using an electrical

supply as the medium to generates electron for CO2 excitement. The circulation of

the energy releasing for CO2 laser related to the atomic content in the laser gas is

simplify as shown in Figure 2.13.

The nitrogen was used as a catalyze the excitement of CO2 molecule to ensure

the excited molecules remain its level for a long time. Ion (2005) reported that CO2

molecule may excite directly in an electrical discharge but the level of lasing is low

and the efficiency of the process is low. He also reported that the since nitrogen lies

as two molecule atom, only one vibration occurred and it is easily collided high

energy discharge from CO2 molecules.

Helium is added as the cooling units and energy restoration to continue the

stimulated emission. Reaction with nitrogen molecule and other CO2 molecule has

made the CO2 molecule lost its energy and needed to recharge. Helium acted as

cooling units for CO2 molecule before its excited again and produce photon laser.

18

This happen as the properties of helium itself have higher thermal conductivity than

other molecules in the mixture gases and enable to dissipate heat from other

molecules. The cycle of photon creation is illustrated in Figure 2.14.

The operation of excitement of CO2 molecules will exhaust the capability of

the CO2 molecules itself after a certain period. Change of new laser gas mixture is

needed to ensure the power of the beam produced maintained. Laser gas mixture is a

major running cost and needed to minimize the wastage while producing good beam

quality.

Invention of multiple configuration in CO2 laser has reduce and increase the

lasting of laser gas mixture thus reducing the laser machine running cost. Table 2.4

categorized the basic configurations of CO2 laser design.

Table 2.4 :Basic configuration of CO2 laser design (Ion, 2005).

Sealed TEA Slow flow Fast flow Transverse flow

Optical design Stable Stable /

unstable Stable

Stable /

unstable Unstable

Gas composition in % volume

(He-N2-CO2-O2-CO)

He = 72 N2 = 16 CO2 = 8 O2 = 0 CO = 4

He = 72 N2 = 16 CO2 = 8 O2 = 0 CO = 4

He = 72 N2 = 19 CO2 = 9 O2 = 0 CO = 0

He = 67 N2 = 30 CO2 = 3 O2 = 0 CO = 0

He = 60 N2 = 25

CO2 = 10 O2 = 5 CO = 0

Gas flow rate (ms-1) - - 5-10 300 20

Gas pressure

(mbar) 6-14 1000 6-14 70 50

Wavelength (µm) 10.6 10.6 10.6 10.6 10.6

19

Figure 2.13 : CO2 laser beam creation.

20

Figure 2.14 : Cycle of photon creation in CO2 laser.

2.3 Laser cutting parameters

Laser cutting quality and process involve many parameters that influence the

outcome produced. Parameter for laser cutting are mostly the same for all laser

especially for common laser that is CO2 and Nd:YAG. The laser cutting process and

cut quality depend upon proper selection of laser parameters and workpiece

parameters (Yilbas, 2007). Cekic et al. (2014) reported that some input parameters

that affect the product quality are stated as shown in Table 2.5.

Table 2.5 : Input parameters. Parameters Details

laser type

laser operating mode continuous / pulsed

power density of beam

distribution of power density TEM mode

quality of laser beam

polarized beam

cutting speed

assist gas type / pressure / purity

materials type / thickness

Various studies have been conducted on the laser input parameter to

understand the result and effect for each parameter. Radovanovic (2011) has compile

21

the distribution of research done based on CO2 laser cutting and found that metals

has been most investigated. 58.3% of the research made on laser cutting were using

metal as the tested material as in Figure 2.15. In the report, author also conclude that

the most input parameter analyze on cut quality is laser power, cutting speed and gas

pressure.

Figure 2.15 : Most investigated materials for assessing laser cut quality (

Radovanovic, 2011).

Figure 2.16 : Laser main input parameters.

Figure 2.16 shows the type of input parameters that affects the outcome

product. Research on input parameters has been done staring early 1980s especially

on the laser power and cutting speed. Cut quality is strongly influence by the setting

parameters used and this statement is supported by the research done by

Radovanovic (2011) reported that 54% of the researchers were able to determine the

optimal cutting parameter settings.

22

2.3.1 Laser power

Laser power play one of the most important parameters in cutting process and it

needed sufficient amount to heat up the material before penetrate and cutting process

were done. Chen (1999) reported that cutting quality and performance depend on the

laser power. Selection of suitable laser power is critically important as the low power

will prevent full cutting option while high power will result burn area at the cutting

edges.

Laser power is classed into several levels depending on its functional

approach. Table 2.6 shows the power application using laser. Radovanovic (2011)

reported in his paper for 50 reviewed made on CO2 laser cutting experiment, 40

paper or 80% of the researcher using laser power as the main input parameters as this

show that laser power is the most important factors in laser cutting process.

Table 2.6 : Laser power and its applications. Power Use

1 – 10mW - Laser pointer

- CD – ROM drive

- DVD player

10 – 500mW - CD-RW burner

- DVD burner

1 – 20W - Laser for micro machining

- Green laser in holographic

versatile disc

30 – 100W - Surgical CO2 laser

100 – 5000W - CO2 laser cutting machine

100KW - For military purpose

Lamikiz et al. (2005) conducted a research on advanced high strength steel

(AHSS) with the laser power within 200 – 600 watt and found that as the laser power

increases, the kerf width will also increase proportionally. Figure 2.17 shows the

effect of laser power on the kerf width formation.

23

Figure 2.17 : Kerf effect of laser power for AHSS (Lamikiz et al., 2005).

Hasc (2013) concluded that laser power contributes about 26% of the kerf

taper compared to cutting speed. He proved that research made by Rajaram et al.

(2003) is not fully correct with his experiment when cutting Inconel with the laser

power up to 4000W.

Yilbas (2004), Stournaras et al. (2009) and Eltawahni et al. (2010) have

concluded that kerf width increase at higher laser power give an effect on the size of

kerf width, heat affected zone and surface quality of the cutting materials. Lamikiz et

al. (2005) has made experimental research on AHSS and conclude that the optimum

laser power for cutting AHSS with the thickness more than 1mm should be higher

than 300W. Increment of laser power is needed for thicker material and should be

supported by suitable assist gas pressure.

Yilbas (2007) conducted a study on the effect of laser cutting parameters on

kerf width. They found that laser power and gas pressure were significantly

influenced the kerf width. With the laser power within 500 – 2000 W, he concludes

that lower laser power increases the thermal efficiency.

24

2.3.2 Cutting Speed

Besides laser power, cutting speed influence the cutting quality for all type of

machining either subtractive machining or additive machining. Ion (2005) stated that

cutting speed for materials is inversely proportional to its thickness. Powell (1998)

also has come to conclusion that decrement of cutting speed is needed as the

thickness of material increased as Figure 2.18. Suitable cutting speed is needed to

avoid excessive burning of the cut edge, to reduce the HAZ and to prevent dross

formation during cutting process.

Figure 2.18 : Relationship between cutting speed with material thickness (Powell,

1998).

Cutting speed, or sometimes known as scanning speed is defined as the rate

the laser beam move along desired profile. Mostly in laser cutting, rate of cutting

speed were defined in mm/min or m/min. Cutting speed effect the energy input time

during cutting process at the cutting line. High cutting speed subsequently affects the

79

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experimental evaluation of co2 laser cutting quality of ultra high