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
4.3.1 The effect of laser power 56
4.3.2 The effect of cutting speed 58
4.3.3 The effect of gas pressure 58
4.4 Taper angle 59
4.4.1 The effect of laser power 60
4.4.2 The effect of cutting speed 60
4.4.3 The effect of gas pressure 62
4.5 Heat effected zone 62
4.5.1 The effect of laser power 65
4.5.2 The effect of cutting speed 67
4.5.3 The effect of gas pressure 67
ix 4.6 Microhardness 68
4.6.1 The effect of laser power 71
4.6.2 The effect of cutting speed 74
4.6.3 The effect of gas pressure 74
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
4.4 Cross sectional view of kerf formation at
0.50MPa nitrogen assisted gas.
55
4.5 Cross sectional view of kerf formation at
0.75MPa nitrogen assisted gas.
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
4.9 Cross sectional view of HAZ at 0.50MPa
nitrogen assisted gas.
64
4.10 Cross sectional view of HAZ at 0.75MPa
nitrogen assisted gas.
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
REFERENCES
ASTM International (1999). Standard Test Method for Microindentation Hardness of Materials. United States: E 384-99.
ASTM International (2001). Standard Test Method for Preparation of
Metallographic Specimens. United States: E 3-01. ASTM International (2000). Standard Test Method for Macroetching Metals and
Alloys. United States: E 340-00.
Ahn DG, Byun KW, Kang MC. (2010). Thermal characterisitn in the cutting of Incornel 718 superalloy using CW Nd:YAG laser. Journal of Material Science and Technology, 26(4), 362-366.
Aspacher, J., (2008), Forming hardening concepts. In : 1st International Conference on Hot Sheet Metal Forming of High-Performance Steel, Kassel, Germany, pp.77-81.
Aziz, N., & Aqida, S. N. (2013). Optimization of quenching process in hot press forming of 22MnB5 steel for high strength properties for publication in. IOP Conference Series: Materials Science and Engineering, 50, 012064. doi:10.1088/1757-899X/50/1/012064
Berglund, G,.(2008), The history of hardening of boron steel in Northern Sweden. In : 1st International Conference on Hot Sheet Metal Forming of High-Performance Steel, Kassel, Germany, pp.175-177
Babu, P. D., Buvanashekaran, G., & Balasubramanian, K. R. (2012). Experimental studies on the microstructure and hardness of laser, 36(3), 241–258.
Bharatish, A., Murthy, H. N. N., Anand, B., Satyanarayana, B. S., Nagaraja, S., & Sunil, R. Y. (2014). Laser Microdrilling of Thermal Barrier Coatings. Procedia Materials Science, 5, 1005–1014. doi:10.1016/j.mspro.2014.07.389
Bharatish, A., Narasimha Murthy, H. N., Anand, B., Madhusoodana, C. D., Praveena, G. S., & Krishna, M. (2013). Characterization of hole circularity and heat affected zone in pulsed CO2 laser drilling of alumina ceramics. Optics & Laser Technology, 53, 22–32. doi:10.1016/j.optlastec.2013.04.010
80
Cekic, A., Begic-Hajdarevic, D., Kulenovic, M., & Omerspahic, A. (2014). CO2 Laser Cutting of Alloy Steels Using N2 Assist Gas. Procedia Engineering, 69, 310–315. doi:10.1016/j.proeng.2014.02.237
Chen, S. (1999). The effects of high-pressure assistant-gas flow on high-power CO2 laser cutting, 88(1), 57–66.
Choudhury, I. a., & Shirley, S. (2010). Laser cutting of polymeric materials: An experimental investigation. Optics & Laser Technology, 42(3), 503–508. doi:10.1016/j.optlastec.2009.09.006
Dubey, A. K., & Yadava, V. (2008). Laser beam machining—A review. International Journal of Machine Tools and Manufacture, 48(6), 609–628. doi:10.1016/j.ijmachtools.2007.10.017
El-Hofy, H. (2005). Advanced Machining Processes (1st ed., p. 286). United States of America: McGraw-Hill. doi:10.1036/0071466940
Eltawahni, H. a., Olabi, a. G., & Benyounis, K. Y. (2010). Effect of process parameters and optimization of CO2 laser cutting of ultra high-performance polyethylene. Materials & Design, 31(8), 4029–4038. doi:10.1016/j.matdes.2010.03.035
Eltawahni, H. a., Olabi, a. G., & Benyounis, K. Y. (2011). Investigating the CO2 laser cutting parameters of MDF wood composite material. Optics & Laser Technology, 43(3), 648–659. doi:10.1016/j.optlastec.2010.09.006
Hasc, A. (2013). Optics & Laser Technology CO2 laser cut quality of Inconel 718 nickel – based superalloy, 48, 554–564. doi:10.1016/j.optlastec.2012.11.003
John C. Ion. (2005). Laser Processing of Engineering Materials (1st ed., p. 589). Oxford, UK: Elsevier Butterworth-Heinemann.
Karbasian, H., & Tekkaya, A. E. (2010). Journal of Materials Processing Technology A review on hot stamping. Journal of Materials Processing Tech., 210(15), 2103–2118. doi:10.1016/j.jmatprotec.2010.07.019
Lamikiz, A., Lacalle, D., Sa, J. A., & Lo, L. N. (2005). CO2 laser cutting of advanced high strength steels ( AHSS ). Journal of Applied Surface Science, 242, 362–368. doi:10.1016/j.apsusc.2004.08.039
Miroslav Radovanovic, M. M. (2011). Experimental investigations of CO2 laser cut quality : a review. Nonconventional Technologies Review, (4).
Powell, J. (1998). Laser Cutting. (J. Powell, Ed.) (Second., p. 248). London: Springler-Verlag. doi:10.1007/978-1-4471-1279-2
81
Riveiro, a., Quintero, F., Lusquiños, F., Comesaña, R., & Pou, J. (2010). Parametric investigation of CO2 laser cutting of 2024-T3 alloy. Journal of Materials Processing Technology, 210(9), 1138–1152. doi:10.1016/j.jmatprotec.2010.02.024
Stournaras, a., Stavropoulos, P., Salonitis, K., & Chryssolouris, G. (2009). An investigation of quality in CO2 laser cutting of aluminum. CIRP Journal of Manufacturing Science and Technology, 2(1), 61–69. doi:10.1016/j.cirpj.2009.08.005
Tang, B. T., Bruschi, S., Ghiotti, a., & Bariani, P. F. (2014). Numerical modelling of the tailored tempering process applied to 22MnB5 sheets. Finite Elements in Analysis and Design, 81, 69–81. doi:10.1016/j.finel.2013.11.009
Thomas, D. J. (2011). The influence of the laser and plasma traverse cutting speed process parameter on the cut-edge characteristics and durability of Yellow Goods vehicle applications. Journal of Manufacturing Processes, 13(2), 120–132. doi:10.1016/j.jmapro.2011.02.002
Thomas, D. J., Whittaker, M. T., Bright, G. W., & Gao, Y. (2011). The influence of mechanical and CO2 laser cut-edge characteristics on the fatigue life performance of high strength automotive steels. Journal of Materials Processing Technology, 211(2), 263–274. doi:10.1016/j.jmatprotec.2010.09.018
Yilbas, B. S. (2007). Laser cutting of thick sheet metals : Effects of cutting parameters on kerf size variations, 1, 285–290. doi:10.1016/j.jmatprotec.2007.11.265
Yilbas, B. S. (1996). Experimental investigation into laser cutting parameters. Journal of Materials Processing Technology, 58, 323 – 330.
Yilbas, B. S. (2004). Laser cutting quality assessment and thermal efficiency analysis. Journal of Material Processing Technology, 15, 155-156.
Zantour, S., Gasbar, M., Mikhaylov, A., & Kharroubi, H. (2013). Influence of the speed in advance and the laser ’ s power on the zone affected thermically for steel C45, 6(3), 5–14.
Top Related