PERFORMANCE EVALUATION OF NITROGEN GAS-ASSISTED LASER

CUTTING ON 316L AUSTENITIC STAINLESS STEEL PLATE

SHAHRIZAL BIN ABD HALIM

A project report submitted in partial fulfilment of the

requirements for the award of the degree of

Master of Engineering (Mechanical – Advanced Manufacturing Technology)

Faculty of Mechanical Engineering

Universiti Teknologi Malaysia

MAY 2010

iii

First of all, all the praises and thanks to Allah S.W.T for His Love,

This thesis is dedicated to my family

To my beloved wife and son Khariah Bt Othman and Muhammad Hasif Bin

Shahrizal

To my beloved parent, Aishah Bt Abd Malek, Abd Halim Bin Abd Rahim and

Shahliza Bt Abd Halim

Thank you very much for your support and encouragement May Allah bless

all people that I love and it’s my honour to share this happiness with my love

ones.

iv

ACKNOWLEDGEMENTS

I would like to thank Allah Almighty for His blessing and giving me strength

to accomplish this master project. A special thank to my supervisor, Prof. Dr. Safian

Sharif who greatly helped me in every way I need to go through this study, Mr.

Mohzani Mokhtar for being my co-supervisor and also for his encouragement.

Many thank to the director of KKTM Balik Pulau for giving me permission

to use the laser cutting machine in my investigation. I am also grateful to all of the

technician in Metrology Laboratory at KKTM Balik Pulau and at School of

Mechanical USM for their advice and support. I would like to express my sincere

appreciation to all of my friends and colleagues in KKTM Balik Pulau for colouring

my daily live and helped me in one-way or another.

Deepest gratitude to my beloved wife and son, Khariah Bt Othman and

Muhammad Hasif Bin Shahrizal, who give me their real love, pray, and support.

Also to my mother and father, Aishah Bt Abd Malek and Abd Halim Bin Abd Rahim

for their continuous encouragement and patience. Finally, I am grateful to University

Sains Malaysia for their support during the period of this work.

v

ABSTRACT

Austenitic stainless steel in particular 316L has played an important role in

the history of medicine for centuries. It is the most widely used material for medical

devices, implants and controlled drug delivery systems. Stainless steels are popular

class of engineered material because of its strength, corrosion resistance and

oxidation resistance at temperatures up to 550oC. Austenitic stainless steels are

typical difficult to machine by the traditional methods and also non-traditional

method. Laser cutting which is very prominent amongst the non-conventional

machining methods is expected to be used quite extensively in cutting austenitic

stainless steel due to the favourable features and advantages that it can offer. This

project was undertaken to evaluate the cutting performance of laser cutting on

austenitic stainless steel. The effects of cutting parameters such as laser power,

cutting speed and gas pressure on the machining responses such as kerf width

(KeW), surface roughness (SR) and striation width (SW) were investigated. The

experimental plans were conducted according to the design of experimental (DOE)

and analysis of variance (ANOVA) was employed in evaluating the experimental

data. Subsequently mathematical models for KeW, SR and SW were established.

Results showed that laser power and cutting speed were significant in influencing the

cutting responses during laser cutting of austenitic stainless steel 316L.

vi

ABSTRAK

Keluli tahan karat austenite, lebih terperinci 316L telah memainkan peranan

yang penting dalam sejarah perubatan berabad yang lalu. Ia adalah bahan yang

digunakan secara meluas untuk alatan perubatan, implan dan sistem pemberian ubat

terkawal. Keluli tahan karat adalah bahan kejuruteraan yang popular dalam klasnya

kerana kekuatan, tahan karat dan tahan pengoksidaan pada suhu 550 oC. Keluli tahan

karat austenit adalah sukar untuk dimesin dengan kaedah tradisional dan juga kaedah

bukan tradisional. Pemotongan laser adalah di antara kaedah pemesinan bukan

tradisional yang terkemuka dijangka akan digunakan dengan meluas untuk

pemotongan keluli tahan karat austenit disebabkan oleh ciri-ciri terpilih dan

kelebihan yang boleh ditawarkan. Projek ini telah dijalankan untuk menilai prestasi

pemotongan laser ke atas keluli tahan karat austenit. Tindakbalas apabila parameter

pemotongan seperti kuasa laser, halaju pemotongan dan tekanan gas ke atas hasil

pemotongan seperti jarak kerf (KeW), kekasaran permukaan (SR) dan jarak jaluran

(SW) telah disiasat. Plan experimen telah dijalankan secara statistic dan analisa

variasi (ANOVA) telah digunakan untuk menilai data experiment. Seterusnya model

matematik untuk KeW, SR dan SW telah diperolehi. Keputusan menunjukkan kuasa

laser dan halaju pemotongan adalah parameter mempengaruhi tindakbalas

pemotongan untuk pemotongan laser ke atas keluli tahan karat austenit 316L.

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENTS iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLE xi

LIST OF FIGURE xiii

LIST OF SYMBOL xvi

LIST OF APPENDICES xvii

1 INTRODUCTION

1.1 Overview 1

1.2 Background of Research 2

1.3 Research Statement 3

1.4 Problem Statement 4

1.5 Objective 4

1.6 Scope of study 5

viii

2 LITERATURE REVIEW

2.1 Introduction 6

2.2 Basic principle of laser 7

2.2.1 Producing gas laser (carbon dioxide laser) 9

2.2.2 The difference between light from a bulb and 11

light from a laser

2.3 Laser cutting approaches 13

2.3.1 Evaporative laser cutting 15

2.3.2 Laser fusion cutting 15

2.3.3 Reactive or oxygen-assisted laser cutting 16

2.3.4 Controlled fracture technique 18

2.4 Laser material processing 20

2.4.1 Laser cutting machine 22

2.4.2 Laser cutting parameter 23

2.4.2.1 Effect of laser power and cutting speed 24

2.4.2.2 Effect of gas assisted pressure 27

2.4.2.3 Effect of focus position 29

2.4.3 Laser cutting quality (response) 30

2.4.3.1 Striation and surface roughness 31

2.4.3.2 Kerf width 35

2.4.3.3 Heat affected zone (HAZ) 35

2.4.3.4 Dross 36

2.5 Austenitic stainless steel and their machinability 37

2.5.1 Introduction 37

2.5.2 Classification of stainless steel 37

2.5.3 Machinability of stainless steel 38

2.5.3 Laser cutting of stainless steel 39

2.6 Design of experiment (DOE) 40

2.6.1 Two-level full factorial design 41

2.6.2 Test of statistical significance 42

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3 RESEARCH METHODOLOGY

3.1 Introduction 44

3.2 Research Design Variables 44

3.2.1 Workpiece material 45

3.2.2 Machine and Equipment 49

3.3 Analysis 53

3.3.1 Statistical analysis 53

3.3.2 Surface Roughness Measurement 54

3.3.3 Measurement kerf width (KeW) and striation 55

width (SW)

3.4 Experimental Flowchart 56

4 RESULT AND DISCUSSION

4.1 Introduction 57

4.2 Experiment result 58

4.2.1 Laser Cutting Of Austenitic Stainless Steel 316L 58

4.2.2 Conformation Run 79

5 DISCUSSION

5.1 Laser Cutting Of Austenitic Stainless Steel 316L 82

5.2 Variation of Kerf Width 86

5.3 Variation of surface roughness 88

5.4 Heat affected zone 88

5.5 Summary 90

x

6 CONCLUSION AND RECOMMENDATIONS

5.1 Introduction 91

5.2 Conclusion 91

5.3 Recommendations 92

REFERENCES 93

Appendices A1 – A5 96 - 101

xi

LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Gas lasers 10

2.2 Properties of laser light and the light from an ordinary bulb 12

2.3 Various approaches of laser cutting 14

2.4 22 full factorial 42

3.1 Cutting parameters 45

3.2 Chemical properties of austenitic stainless steel 316L 46

3.3 Mechanical properties of austenitic stainless steel 316L 46

4.1 Factor and levels for laser cutting austenitic stainless 58

steel 316L

4.2 Experiment plan for laser cutting austenitic stainless steel 316L 59

4.3 Experimental result for laser cutting austenitic stainless 59

steel 316L

4.4 ANOVA table for kerf width in laser cutting process 60

4.5 ANOVA table for surface roughness in laser cutting process 67

4.6 ANOVA table for striation width in laser cutting process 73

4.7 Summary of the significant factors in laser cutting 79

experiments of 316L

4.8 Example output from the point prediction tool in laser 79

cutting of austenitic stainless steel 316L

4.9 Analysis of confirmation experiments for kerf width (KeW) 80

in laser cutting of austenitic stainless steel 316L

4.10 Analysis of confirmation experiments for Surface roughness 80

(SR) laser cutting of austenitic stainless steel 316L

xii

TABLE NO. TITLE PAGE

4.11 Analysis of confirmation experiments for striation width 80

(SW) in laser cutting of austenitic stainless steel 316L process

5.1 Selected cutting condition 89

xiii

LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 Basic principle of producing laser beam 8

(www.yourdictionary.com/computer/laser)

2.2 Process of producing CO2 laser. 11

2.3 The difference between light from a bulb and light 12

from a laser

2.4 General schematic of a laser-cutting process (Powell 1998) 14

2.5 Model geometry for laser cutting showing a control volume 16

bounded by two inclined semicircular cylinders (Kaplan 1996).

2.6 Physical mechanism of oxygen assisted laser cutting. 18

(Schuöcker 1986)

2.7 Mechanism of laser cutting by controlled fracture for 19

asymmetrical cutting. (Tsai and Chen 2004)

2.8 Classification of laser material processing 21

(Jdutta majumdar and Imanna)

2.9 Schematic diagram of laser cutting machine 23

2.10 Effect of applied power and cutting speed 25

(K. Abdel Ghany, M. Newishy 2005)

2.11 Optimum laser cutting area which gives clean thorough cuts. 26

(Lamikiz et al. 2005)

2.12 Effects of the assistant gas pressure 28

(Shang-Liang Chen 1997)

2.13 Effect of the focus position (K. Abdel Ghany, 29

M. Newishy 2005)

xiv

FIGURE NO. TITLE PAGE

2.14 Typical striation patterns (Powell 1998) 32

2.15 Effect of cutting speed on the surface profile of striations 32

2.16 Schematic mechanism of formation of striations during 33

laser cutting. (Powell 1998.)

2.17 Kerf width 35

3.1 (a) Jig on Working Table 47

3.1 (b) Uncut workpiece on Jig 47

3.1 (c) Workpiece material after Cut 48

3.2 Specimen for experiment 48

3.3 Cutting position 49

3.4 TRUMPF CNC (CO2) laser cutting centers 50

3.5 Surface roughness testers 50

3.6 Optical 3D microscope 51

3.7 Profile projector 52

3.8 Metallurgical specimen cutters 53

3.9 Flow chart for the analysis steps 54

3.10 Typical geometry of striations from top view 55

(Ivarson et al. 1994.)

3.11 Experimental flow chart 56

4.1 Normal probability plots of residuals for Kerf width in 62

Laser Cutting process

4.2 Residual vs predicted response for Kerf Width in Laser 62

cutting process

4.3 Outlier T plot for Kerf width in Laser cutting process 63

4.4 Interaction of main effects plot for kerf Width and laser power 64

4.5 Interaction of main effects plot for Kerf Width and gas 65

pressure

4.6 3D surface graph for kerf width in laser cutting process 66

4.7 Normal probability plots of residuals for surface roughness 68

in Laser Cutting process

xv

FIGURE NO. TITLE PAGE

4.8 Residual vs predicted response for surface roughness in 68

Laser cutting process

4.9 Outlier T plot for Surface roughness in Laser cutting process 69

4.10 Interaction of main effects plot for Surface roughness and 70

laser power

4.11 Interaction of main effects plot for Surface roughness and 71

gas pressure

4.12 3D surface graph for surface roughness in laser cutting process 72

4.13 Normal probability plots of residuals for striation width in 74

Laser Cutting process

4.14 Residual vs predicted response for striation width in Laser 74

cutting process

4.15 Outlier T plot for striation width in Laser cutting process 75

4.16 Interaction of main effects plot for striation width and 76

laser power

4.17 Interaction of main effects plot for striation width and gas 77

pressure

4.18 3D surface graph for striation width in laser cutting process 78

5.1 Perturbation plot for kerf width 83

5.2 Perturbation plot for surface roughness 84

5.3 Perturbation plot for striation width 86

5.4 Kerf width measurement point 87

5.5 Trend Of Kerf Width 87

5.6 Graft of HAZ width 90

xvi

LIST OF SYMBOLS

ANOVA - Analysis of variance

CI - Confidence Interval

CO2 - Carbon dioxide

DOE - Design Of Experiment

HAZ - Heat affective zone

He - Helium

KeW - Kerf Width

MRR - Material Removal Rate

N2 - Nitrogen

SR - Surface Roughness

SW - Striation Width

xvii

LIST OF APPENDICES

APPENDIX TITLE PAGE

A.1 Box-Cox Plot for Kerf width in laser process 96

A.2 Box-Cox Plot for Surface roughness in laser process 97

A.3 Box-Cox Plot for Striation width in laser process 98

A.4 Measured kerf width value 99

A.5 Picture of HAZ at the cutting edge 100

CHAPTER 1

I�TRODUCTIO�

1.1 Overview

For centuries metals have played an important role in the history of

engineering and medicine. The first stainless implant in modern medicine was dated

back in 1926 by E.W. Haey Groves that used 18-8 (austenitic 304) stainless screws

in fixation of femoral neck fracture. Following this achievement, several advances

have been made in the use of stainless steel devices and implant in the human body.

Nowadays biomaterials are becoming important as they are used to replace a part or

a function of the body in a safe, reliable, economic and physiologically acceptable

manner. Concurrent with this progress the demand for improved materials are

increasing rapidly.

Stainless steels are popular class of engineered materials known for their

strength, corrosion resistance and oxidation resistance at temperatures up to 550oC.

Stainless steels are the most widely used material for medical devices, implants and

controlled drug delivery systems. The most widely used stainless steel in medical

applications today is austenitic stainless steels type 316 (Y.V. Murty 2003).

2

Austenitic stainless steels material are typical difficult to machine by the

traditional methods (T. Akasaw et al. 2003). Poor surface finish and short tool life

are common problem in machining of these materials that leads to high

manufacturing cost. Since the cost of using conventional machining is generally

prohibitive, non-conventional machining such as laser machining probably amongst

the ideal technique in dealing with these materials because of its flexibility, rapid

material removal and ability to produced intricate shape.

Laser cutting has proved its capability especially in cutting difficult to cut

material such as austenitic stainless steels probably because of its high strength, low

thermal conductivity, high ductility and high work hardening tendency (Ihsan Korkut

et al. 2003). Although laser cutting technology is widely used in the manufacturing

of mechanical components, its optimum cutting conditions in producing high quality

product is still under investigation by many researchers.

1.2 Back ground of research

Laser cutting is a non-traditional process that is used in a two-dimensional

machining process in which material removal is obtained by focusing a highly

intense laser beam on the workpiece. The heat of the laser beam subsequently

melts/vaporizes the workpiece throughout the thickness or depth of the material thus

creating a metal cutting process. It is a noncontact process, whereby the workpiece

doesn’t need to be clamped or cantered on precise fixtures as in conventional

machining. Accurate positioning of the workpiece on the X–Y table with defined

direction of cut can be easily obtained during laser cutting thus facilitating machining

of flimsy and flexible materials such as thin sheet metal. Demand for this process is

increasing because of its ability to produce geometrical complex shapes as well as its

ability to cut hard materials that are extremely difficult to machine when using

conventional process.

3

Previous study (Shang-Liang Chen 1997) shows that oxygen cutting is still

the best, although argon and nitrogen may be used instead for the cutting of 3 mm

thick mild steel plate with high-pressure assistant-gas flow on high-power CO2 laser .

In the previous study, mild steel is the most used material being investigated for the

optimum cutting conditions during laser cutting process. K. Abdel Ghany and M.

Newishy (2005) reported that the laser cutting quality depends mainly on the laser

power, pulse frequency, cutting speed and focus position when cutting 1.2 mm

stainless steel sheet using pulsed and CW Nd:YAG laser.

It was found that research study on the effect of cutting conditions on a 3 mm

austenitic stainless steels (316) sheet using CO2 laser cutting was not reported so far.

New machining data on laser cutting of austenitic stainless steels (316) sheet served a

great significance and could be further exploited especially for continuous cutting

condition. This project is undertaken to study the effect of cutting parameters such as

laser power, cutting speed and gas pressure on surface roughness, striation and kerf

width.

1.3 Research Statement

Laser cutting of austenitic stainless steel 316L with changes of significant

cutting parameters will lead to improvement in cutting responses such as surface

roughness, size of striation and kerf width.

4

1.4 Problem Statement

Some of the issues that are encountered by laser cutting users are as follows:

a) Challenging task in setting up the initial laser cutting parameters for material

that are not listed in the default database/library of the laser cutting machine.

b) Extensive laser trial and error in order to obtain good cutting conditions and

this can be very wasteful with respect to time and material, subsequently

increasing production cost.

c) Laser cutting process parameter are interrelated, the right combination of

optimum process parameters are desired to produce good quality

characteristics.

1.5 Objective

The main objectives of this study are:

a) To investigate the effect of laser cutting parameters on the cutting responses

such as kerf width (KeW), surface roughness (SR) and striation width (SW)

when laser cutting 316L austenitic stainless steel.

b) To establish mathematical model for the cutting responses using DOE

approach.

5

1.6 Scope of study

The scope of the study includes the following:

a) TC L2530 Plus TRUMPF CNC (CO2) laser cutting center at Kolej

Kemahiran Tinggi MARA (KKTM) Balik Pulau was used to conduct the

experimental works.

b) The work piece was 316L austenitic stainless steel with 3 mm thickness.

c) Process parameters were cutting speed, laser power output, gas assisted

pressure and focal length.

d) Cutting responses are surface roughness (SR), striation width (SW) and kerf

width (KeW).

e) Two level full factorial experimental plan was employed to reduce the

number of experiment.

f) ANOVA was used to identify the significant contribution of laser cutting

parameters against the response variables.