ii

AN EXPERIMENTAL STUDY AND MODELING ON GAS METAL ARC

WELDED LAP JOINT OF A7075-T651 ALUMINIUM ALLOY TO AZ31B

MAGNESIUM ALLOY

MD. RAFIQUL ISLAM

Thesis submitted in fulfillment of the requirements

for the award of the degree of

Master of Engineering (Mechanical)

Faculty of Mechanical Engineering

UNIVERSITY MALAYSIA PAHANG

FEBRUARY 2015

vii

ABSTRACT

Innovative welding technique for joining aluminum and magnesium alloys in

automobile, aviation, aerospace and marine industries would achieve weight reduction,

high specific strength as well as increase fuel efficiency and reduce environmental

pollution. However, poor mechanical properties of welding joint between aluminum and

magnesium alloys due to the formation of AlmMgn type brittle intermetallic compounds

is the main barrier for extensive uses of these two alloys especially in transportation

sectors. No exact solution has been established yet for reliable joining while; modeling

study in this area is still much lagging. This research work was carried out to conduct

experimental study and mathematical modeling of mechanical properties for A7075-

T651 aluminum and AZ31B magnesium alloys joints welded by gas metal arc lap

welding method. Unconventional ER308L-Si stainless steel and conventional ER5356

aluminum wires were used as filler. Shielding gas used was 98% argon and 2% oxygen.

The shielding gas flow rate, tip to work distance, welding current and voltage were the

variable parameters. Box-Behnken technique in response surface methodology was used

for design of experiments. The welding torch angle, filler wire diameter and parent

metal thickness were kept constant. The ultimate tensile strength and fracture toughness

of the joints were evaluated. The fracture toughness of the welding joints was calculated

from yield strength and absorbed charpy impact energy of the joints using Rolfe-Novak-

Barsom correlation. Mathematical models were developed based on regression analysis

to relate the responses (ultimate tensile strength and fracture toughness) with welding

variable parameters and validation experiments were conducted to verify the models.

The significance and effects of variable parameters on the responses were also analyzed.

The macro and microstructures at the welding cross section were investigated by optical

microscope. The fracture surface morphologies and elements analysis at the welding

cross section were carried out by scanning electron microscopy and electron dispersive

X-ray spectroscopy. The results revealed that more significant mechanical properties

were achieved with stainless steel filler compared to aluminum filler. The maximum

yield strength, ultimate tensile strength and fracture toughness were 203.09 MPa,

249.33 MPa and 27.49 MPa√m respectively with steel filler; and 176.30 MPa, 226.28

MPa and 20.22 MPa√m respectively with aluminum filler. The analysis of variance

showed that a very good fitting and variation with data, high accuracy in predicting the

responses have been illustrated by the models for both fillers. The validation tests

revealed that the ultimate tensile strength models can predict the responses within

0.21% and 1.81% maximum error, and fracture toughness models can predict the

responses within 0.51% and 3.36% maximum error respectively. Most of the joints

failed at AZ31B alloy (with steel filler) and ER5356 aluminum nugget (with aluminum

filler). The investigation revealed that fracture occurred due to brittle fracture

mechanism by formation of micro pores, voids, cracks, MgnOm oxides, very little

amount of MgnFem and AlmMgn intermetallic compounds at AZ31B alloy, and due to

micro pores, voids, cracks, AlmOn and MgmOn oxides and a good amount of AlmMgn

intermetallic compounds at aluminum nugget. There was no evidence of fracture from

metallurgical bonding between steel or aluminum nuggets and parent alloy. The output

of this research exhibited very significant mechanical properties of the joint that can

facilitate the extensive uses of A7075-T651 and AZ31B alloys in mass production of

light weight vehicle structures in transportation industries.

viii

ABSTRAK

Teknik kimpalan yang inovatif perlu untuk menggabungkan aluminium dan magnesium aloi

untuk diaplikasikan dalam pembuatan otomobil, kapal terbang, aero-angkasa dan industri marin.

Selain itu, ia akan dapat menyumbang kearah pengurangan berat, meningkatkan kekuatan khusus (spesific strength) bahan serta peningkatan kecekapan bahan api dan mengurangkan

pencemaran alam sekitar.Walau bagaimanapun, kelemahan sifat mekanikal dalam gabungan

kimpalan di antara aluminium dan magnesium aloi disebabkan oleh pembentukan AlmMgn iaitu sejenis sebatian antara logam yang rapuh menjadi halangan utama untuk menggunakan kedua-

dua aloi ini dalam sektor pengangkutan. Belum ada penyelesaian yang khusus dihasilkan untuk

gabungan yang utuh, dan kajian pemodelan dalam bidang ini masih banyak kekurangan. Penyelidikan ini dijalankan untuk kajian eksperimen dan pemodelan matematik ke atas sifat

mekanik gabungan diantara aluminium A7075-T651 dan AZ31B aloi magnesium yang dikimpal

menggunakan teknik kimpalan gas lengai arka logam.Keluli tahan karat ER308L-Si tidak

konvensional dan aluminium ER5356 konvensional wayar pengisi digunakan sebagai pengisi. 98% argon and 2% oksigen digunakan sebagai gas pelindung. Kadar bekalan gas perisai, jarak

hujung tip dan bahan kerja, arus dan voltan kimpalan adalah parameter pembolehubah. Teknik

Box-Behnkendalam kaedah gerak balas permukaan telah digunakan untuk reka bentuk eksperimen. Kimpalan sudut obor, diameter dawai pengisi dan tebal logam adalah pemalar.

Kekuatan tegangan muktamad (UTS) dan keliatan patah (fracture thougness) gabungan telah

dinilai. Keliatan patah gabungan kimpalan telah dikira dari kekuatan alah (yield strength) dan

serapan tenaga hentaman charpy (charpy impact energy) menggunakan korelasi Rolfe-Novak-Barsom. Model matematik telah dibangunkan berdasarkan analisa regresi untuk mengaitkan

tindak balas (kekuatan tegangan muktamad dan keliatan patah) dengan parameter

pembolehubah dan eksperimen pengesahan telah dijalankan untuk menentukan keberkesanan model.Pengertian dan kesan parameter pembolehubah keatas tindak balas telah juga dianalisia

Makro dan mikrostruktur keratan rentas gabungan kimpalan telah dicerap menggunakan

mikroskop. Morfologi permukaan patahdan analisa unsur-unsur dikeratan rentas kimpalan telah dijalankan menggunakan mikroskopi elektron imbasan (SEM) dan serakan elektron X-ray

spektroskopi. Keputusan mendedahkan bahawa sifat mekanik lebih ketara dicapai menggunakan

pengisi keluli berbanding dengan pengisi aluminium. Kekuatan alah maksimum, kekuatan

tegangan muktamad dan keliatan patah adalah 203.09 MPa, 249.33 Mpa dan 27.49 MPa√m masing-masing dengan pengisi keluli; dan 176.30 MPa, 226.28 MPa dan 20.22 MPa√m masing-

masing dengan pengisi aluminium. Analisia varians menunjukkan bahawa kepadanan yang

sangat baik serta kepelbagaian data, ketepatan yang tinggi dalam meramalkan tindak balas digambarkan oleh model untuk kedua-dua pengisi. Ujian pengesahan mendedahkan bahawa

model kekuatan tegangan muktamad boleh meramalkan tindak balas dalam 0.21% dan 1.81%

ralat maksimum, dan keliatan patah model boleh meramalkan tindak balas dalam 0.51% dan 3.36% ralat maksimum. Kebanyakan gabungan gagal pada kimpalan nugget AZ31B aloi

(dengan pengisi keluli) dan ER5356 aluminium padat (dengan pengisi aluminium).

Penyelidikan mendedahkan bahawa patah berlaku kerana mekanisme patah rapuh oleh kerana

pembentukan liang-liang mikro, lompang-lompang, retakan, MgnOm oksida, jumlah kecil sebatian antara logam MgnFem dan AlmMgn dibahagian aloi AZ31B, dan kerana liang-liang

mikro, lompang-lompang, retakan, AlmOn dan MgmOn oksida dan amaun sebatian antara logam

AlmMgn di kimpalan nugget aluminium. Tiada bukti keretakan pada ikatan logam antara kimpalan nugget keluli atau aluminium dengan aloi induk. Hasil kajian ini boleh menawarkan

ciri-ciri sifat mekanikal gabungan kimpalan yang bererti. Kajian ini boleh mempermudahkan

penggunaan A7075-T651 dan AZ31B aloi yang lebih meluas dalam pengeluaran massa struktur

kenderaan ringan dalam industri pengangkutan.

ix

TABLE OF CONTENTS

Page

THESIS CONFIDENTIAL STATUS i

TITLE PAGE ii

SUPERVISOR’S DECLARATION iii

STUDENT’S DECLARATION iv

DEDICATION v

ACKNOWLEDGEMENTS vi

ABSTRACT vii

ABSTRAK viii

TABLE OF CONTENTS ix

LIST OF TABLES xii

LIST OF FIGURES xiii

LIST OF SYMBOLS xvi

LIST OF ABBREVIATIONS xviii

CHAPTER 1 INTRODUCTION

1.1 Introduction 1

1.2 Problem statement 3

1.3 Research motivation 5

1.4 Objectives of research 7

1.5 Scopes of research 7

1.6 Organization of thesis 8

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction 10

2.2 Gas metal arc welding process 10

2.3 Aluminium alloys 12

2.4 Magnesium alloys 14

2.5 A7075-T651 and AZ31B alloys 17

x

2.6 Filler materials 20

2.7 Lap welding 21

2.8 Welding parameters 22

2.8.1 Gas flow rate 22

2.8.2 Tip to work distance 23

2.8.3 Welding voltage 24

2.8.4 Welding current 24

2.9 Welding of aluminum and magnesium alloys 25

2.9.1 Mechanical properties 25

2.9.2 Modeling of welding joints 28

2.9.3 Metallurgical properties 29

2.10 Summary 32

CHAPTER 3 METHODOLOGY

3.1 Introduction 34

3.2 Research framework 34

3.3 Materials 35

3.4 GMA lap plug welding process 36

3.4.1 Experimental setup 38

3.5 Preliminary experiments 40

3.6 Design of experiments 40

3.7 Sample preparation and experimental work 42

3.8 Mechanical testing 43

3.9 Estimation of fracture toughness 44

3.10 Mathematical modeling 44

3.10.1 First order model 45

3.10.2 Second order model 47

3.10.3 Analysis of variance, ANOVA 49

3.11 Main effects plot 51

3.12 Metallurgical investigation 52

3.13 Summary 52

CHAPTER 4 RESULTS AND DISCUSSION

xi

4.1 Introduction 54

4.2 Preliminary experimental results 54

4.3 Mechanical properties of the joints 56

4.4 Mathematical modeling 60

4.5 Validation of models 64

4.6 Effects of welding parameters on UTS and FT 66

4.7 Macro and microstructural analysis of joints 70

4.8 Locations of fracture in welding joints 74

4.9 Fracture surface analysis with SS filler 75

4.10 Fracture surface analysis with Al filler 86

4.11 Metallurgical bonding with SS filler 91

4.12 Metallurgical bonding with Al filler 94

4.13 Summary 97

CHAPTER 5 CONCLUSION AND FUTURE WORK

5.1 Introduction 98

5.2 Conclusion 98

5.3 Future work 100

REFERENCES 102

LIST OF PUBLICATIONS 113

xii

LIST OF TABLES

Table No. Title Page

2.1 Strength of A7075 alloy with various treatments 18

2.2 Physical and mechanical properties of A7075-T651 alloy 19

2.3 Physical and mechanical properties of AZ31B alloy 19

2.4 Mechanical properties of ER308L-Si SS filler 20

2.5 Mechanical properties of ER5356 Al filler 21

2.6 A summary of previous highlighted researches 33

3.1 Chemical compositions of A7075-T651, AZ31B alloys and ER308L-Si SS,

ER5356 Al fillers (wt %)

36

3.2 List of parameters with low and high levels 41

3.3 Design of experiments and welding power 41

4.1 Welding appearances with first round of preliminary experiment 55

4.2 Welding appearances and toughness with revised preliminary experiment 55

4.3 Mechanical properties of parent metals and welding joints with SS and Al fillers

57

4.4 Estimated regression coefficients for models with SS and Al fillers 61

4.5 ANOVA for ultimate tensile strength with SS and Al fillers 62

4.6 ANOVA for fracture toughness with SS and Al fillers 62

4.7 Actual UTS, predicted UTS and error percentage with SS and Al fillers 64

4.8 Actual FT, predicted FT and error percentage with SS and Al fillers 65

4.9 Validation and accuracy of UTS models 65

4.10 Validation and accuracy of FT models 66

4.11 Location of fracture in tensile and toughness testing with SS and Al fillers 75

4.12 EDX analysis results for AZ31B/ER308L-Si SS and A7075-T651/ER308L-Si SS metallurgical bonding

93

xiii

LIST OF FIGURES

Figure No. Title Page

2.1 GMA welding torch 12

2.2 Relation between strength and toughness for various materials 27

3.1 Structure of research plan 35

3.2 Illustration of overall GMA lap plug welding process set up 37

3.3 Experimental setup with necessary components 39

3.4 Illustration of welding layout 39

3.5 Mechanical and metallurgical specimens’ preparation plan in EDM 42

3.6 (a) ASTM E8M-04 standard tensile specimen and (b) ASTM D5045 standard

impact toughness specimen (with SS filler), (c) ASTM E8M-04 standard

tensile specimen and (d) ASTM D5045 standard batch of impact toughness specimens (with Al filler).

43

4.1 Welding appearances; (a) for minimum and (b) for maximum heat input with

first round preliminary experiment

55

4.2 Welding appearances; (a) for minimum and (b) for maximum heat input with

revised preliminary experiment

55

4.3 (a) Optical microscope image of welding macro cross section and (b)

macrostructure between AZ31B alloy and ER5356 weld pool with revised preliminary experiment

56

4.4 Comparison of stress-strain curves for minimum and maximum joint strength,

and parent metals; (a) with ER308L-Si SS filler and (b) with ER5356 Al filler

59

4.5 (a-d) Effects of gas flow rate on UTS and FT for both fillers 67

4.6 (a-d) Effects of tip to work distance on UTS and FT for both fillers 68

4.7 (a-d) Effect of welding voltage on UTS and FT for both fillers 69

4.8 (a-d) Effect of welding current on UTS and FT for both fillers 69

4.9 Welding macro cross sections; (a) with complete nugget, (b) incomplete

nugget (SS filler), and (c) with complete nugget, (d) with incomplete nugget

(Al filler)

71

4.10 Overall microstructure at welding cross section with SS filler; (a-c) at A7075-

T651 alloy, (d-f) at AZ31B alloy, (g) at SS nugget, and with Al filler; (h)at

A7075-T651 alloy, (i) at AZ31B alloy, (j) at Al nugget

73

4.11 Location of fracture in tensile and impact toughness testing with SS filler: (a) 74

xiv

and (d) at AZ31B alloy; (b) and (e) at SS nugget; (c) and (f) at A7075-T651

alloy respectively

4.12 Location of fracture in tensile and impact toughness testing with Al filler: (a)

and (c) at ER5356 Al nugget, (b) and (d) at AZ31B alloy respectively

75

4.13 (a) Fracture at AZ31B alloy, (b) Close-up of AZ31B alloy fracture (symbolic), and SEM images of fracture surface at (c) unaffected AZ31B alloy and (d)

PMZ of AZ31B alloy

76

4.14 (a) Close-up of AZ31B alloy fracture surface top view, and SEM images at

AZ31B alloy fracture surface for EDX analysis, (b) at point 1 of unaffected alloy, (b) at point 2 of PMZ (c) at point 3 of PMZ with SS filler

78

4.15 EDX results at point 1 of unaffected AZ31B parent metal fracture surface 78

4.16 EDX results at area “x” of point 2 at AZ31B PMZ fracture surface 79

4.17 EDX results at area “y” of point 2 at AZ31BPMZ fracture surface 79

4.18 EDX results at area “z” of point 3 at AZ31B PMZ fracture surface 79

4.19 (a) Fracture at A7075-T651 alloy, (b) Close-up of A7075-T651 alloy fracture

(symbolic), and SEM images of fracture surface at (c) unaffected A7075-T651 alloy and (d) PMZ of A7075-T651 alloy

80

4.20 (a) Close-up of A7075-T651 alloy fracture surface, and SEM images at

A7075-T651 alloy fracture surface for EDX analysis, (b) at point 1 of unaffected alloy, (c) at point 2 of PMZ (d) at point 3 of PMZ with SS filler

81

4.21 EDX results at point 1 of unaffected A7075-T651 alloy fracture surface 82

4.22 EDX results at area “x” of point 2 at A7075-T651 PMZ fracture surface 82

4.23 EDX results at area “y” of point 2 at A7075-T651 PMZ fracture surface 82

4.24 EDX results at area “z” of point 3 at A7075-T651PMZfracture surface 83

4.25 (a) and (b) Fracture at ER308L-Si nugget, (c) Close-up of ER308L-Si nugget

fracture (symbolic), and (d) SEM image ER308L-Si nugget fracture surface

84

4.26 Optical microscope images of ER308L-Si SS nugget’s cross section, (a) the

poor bonding and (b) proper bonding in the nugget

84

4.27 (a) Close-up of ER308L-Si nugget fracture surface, and SEM images at ER308L-Si nugget fracture surface for EDX analysis, (b) at point 1 and (b) at

point 2

85

4.28 EDX results at point 1 of ER308L-Si SS nugget fracture surface 85

4.29 EDX results at point 2 of ER308L-Si SS nugget fracture surface 85

4.30 (a) and (b) Fracture at AZ31B alloy, and SEM images of fracture surface at (c)

unaffected AZ31B alloy, (d) PMZ of AZ31B alloy (e) PMZ of AZ31B alloy

86

xv

where AlmMgn were formed more with ER5356 Al filler

4.31 (a) Close-up of AZ31B alloy fracture surface, and SEM images at AZ31B alloy fracture surface for EDX analysis (b) at point 1 of unaffected alloy, (c) at

point 2 of PMZ (d) at point 3 of PMZ with Al filler

87

4.32 EDX results at point 1 of unaffected AZ31B alloy fracture surface 88

4.33 EDX results at area “x” of point 2 of AZ31B alloy PMZ fracture surface 88

4.34 EDX results at point 3 of AZ31B alloy PMZ fracture surface 88

4.35 (a) and (c) Fracture at ER5356 Al nugget, (b) fracture surface at around

ER5356 nugget, and SEM images of fracture surface at (d) around ER5356 nugget, (e) at middle of ER5356 Al nugget with ER5356 Al filler

90

4.36 (a) Close-up of ER5356 nugget fracture surface, and SEM images at ER5356

nugget fracture surface for EDX analysis (b) at point 1 and (c) at point 2

90

4.37 EDX results at point 1 of ER5356 Al nugget fracture surface 91

4.38 EDX results at point 2 of ER5356 Al nugget fracture surface 91

4.39 (a) Welding cross section with SS filler; (b) and (c) microscopic images

showing A7075-T651/SS nugget and AZ31B/SS nugget metallurgical bonding respectively; (d) and (e) SEM images showing A7075-T651/SS nugget and

AZ31B/SS nugget metallurgical bonding respectively where EDX analysis

was carried out

92

4.40 EDX results at bonding between AZ31B alloy and ER308L-Si SS nugget 93

4.41 EDX results at bonding between ER308L-Si SS nugget and A7075-T651 alloy 94

4.42 (a) Welding cross section with Al filler, (b) and (c) SEM images showing AZ31B/ER5356 nugget bonding, (d) and (e) SEM images showing A7075-

T651/ER5356 nugget bonding

95

4.43 EDX results at area “x” of point 1 at AZ31B/ER5356 Al nugget bonding 95

4.44 EDX results at area “y” of point 2 at ER5356 Al nugget/AZ31B alloy bonding 96

4.45 EDX results at point 3 of A7075-T651/Al nugget bonding 96

4.46 EDX results at point 4 of Al nugget/A7075-T651 alloy bonding 96

xvi

LIST OF SYMBOLS

A Ampere

A0 Constant

Ai, Aii, Aij Regression coefficient

Al Aluminium

Bi, Bii, Bij

Regression coefficient

B0 Constant

β0, βi, βij, βii

Regression coefficient

Co Cobalt

CO2 Carbon Di-oxide

Cr Chromium

Cu Copper

o C Degree centigrade

E Young’s modulus

ε Experimental error

g/cm3 Gram per cubic centimeter

GPa Giga Pascal

J Joule

KIc Fracture toughness

L/min Litre per minute

Mg Magnesium

mm Millimeter

Mn Manganese

Mo Molibdenum

MPa Mega Pascal

xvii

MPa√m Mega Pascal square root meter

Ni Nickel

O2 Oxygen

R2 Coefficient of determination

R2

Adjusted Coefficient of determination (Adjusted)

S Standard error of regression

Si Silicon

Tm Melting temperature

σs

Tensile strength

σy

Yield strength

V Volt

W/m-K Watt per meter-Kelvin

Y Response

Zn Zinc

% Percentage

εb Percentage elongation

xviii

LIST OF ABBREVIATIONS

AISI American Institute Standard Instruments

AIE Absorbed impact energy

CMT Cold metal transfer

DOE Design of experiment

EDM Electrical discharge machining

EDX Energy dispersive X-ray

FSW Friction stir welding

FT Fracture toughness

GMA Gas metal arc

GFR Gas flow rate

GTA Gas tungsten active

HAZ Heat affected zone

HT Heat treated

IMCs Intermetallic compounds

MIG Metal inert gas

PMZ Partially melted zone

PRESS Predicted sum of squares

RSM Response surface methodology

SEM Scanning electron microscope

SS Stainless steel

TIG Tungsten inert gas

TWD Tip to work distance

TS Tensile strength

YS Yield strength

xix

UTS Ultimate tensile strength

WC Welding current

Wr. Wrought

WV Welding voltage

CHAPTER 1

INTRODUCTION

1.1 INTRODUCTION

Welding is one of the most important secondary manufacturing processes that

has been and will continue to be more essential for the survival, comfort and

advancement of human race. It is the foremost used and realistic joining method within

the manufacturing industries. Its traces are found nearly in all the sectors of production

at present. The buildings, bridges, streets, sewer and transportation systems, petroleum

pipelines, aerospace, defensive industries and most of the everyday use stuffs are

produced by a variety of welding processes. From the day the welding process has been

discovered, this has widen the standard of life and has allowed us to make our world

(Messler, 1999). Welding means the construction of continuous solid body from two or

more separate pieces of materials. The description of welding can be stated as: forming

an atomic bonding between materials by putting their surfaces close enough so that a

new atomic arrangement can be created through inter atomic or inter molecular forces

by having a common electron configuration (Erturk, 2011; Messler, 2004). Among

numerous welding methods, mankind has found gas metal arc (GMA) welding to be the

most effective one. The materials to be joined are melted by the heat of an electric arc.

The filler material is deposited between the work pieces. Upon solidification of the

molten bath, atomic bonds and continual surface are produced (Blondeau, 2013; Erturk,

2011).

Aluminium (Al) and magnesium (Mg) alloys are the most promising materials

for manufacturing industries at present. This two alloys have been continously and

extensively researched during the past decades for their practical industrial applications.

2

In fact, various Al and Mg alloys will be counted as the fundamental materials in almost

all structures in near future. In most of the land, water and air transportation systems;

especially in the automotive industries, both Al and Mg alloys will be used in the same

structure. In such cases, a successful welding technique is of utmost necessity to

combine these two alloys together. The main problems in welding of these two alloys

are the difference in physical and chemical properties between them. Moreover, Al-Mg

reaction causes formation of very brittle and fragile AlmMgn type intermetallic

compounds (IMCs). Therefore; the joint possesses very low strength and can be very

easily fractured even by bare hand. In order to achieve a joint with good mechanical and

metallurgical properties by reducing or avoiding the formation of AlmMgn IMCs at the

welding joint, an appropriate welding technique should be introduced.

The welding parameters play an important role in the performance of welding

joints. GMA welding parameters and their appropriate settings can produce better

quality joints. The selection of welding parameters and their ranges are provided by the

machine manufacturers, which are only applicable for the common materials in similar

welding. The parameter settings for welding of dissimilar Al and Mg alloys need to be

optimized experimentally. Modeling is an effective way to solve the welding problems

by relating the process parameters to the responses. It was revealed from the study of

previous researchers that it is difficult to develop a universal mathematical model for

almost any kind of material and welding method to predict the responses. Therefore, it

is not acceptable to use existing models when advanced dissimilar materials are welded.

From this viewpoint, mechanical properties of the welding joints have been investigated

and development of mathematical models have been attempted in order to correlate the

welding process parameters to certain responses. The validation of the models by

conformation experiments was attempted. The metallurgical properties of the joints

have also been studied.

This research work was carried out in three major steps. Primarily, the most

important parameters such as welding voltage, current, gas flow rate and tip to work

distance were selected as process parameters. For that, the behavior of the materials to

different welding process parameters was investigated. Then, joints with good welding

appearances and with no visible defects were produced. The mechanical properties of

3

the welding joints were investigated. Secondly, mathematical models were developed

through statistical analysis to relate the process variables to the responses. Validation

experiments were conducted to verify the models. The effects of the variable process

parameters on the responses were also studied. Finally, metallurgical properties of the

welding joints and fracture surface morphology after mechanical testing were

investigated. To accomplish these processes; Box-Behnken design of experiment (DOE),

GMA lap plug welding of Al and Mg alloys, tensile and charpy impact toughness tests

for the joints, statistical analysis, mathematical modeling and validation, macro and

micro structural examinations of welding cross sections, fracture surface morphology

study, and elements analysis at the welding cross were carried out step by step. The

technical aim of this research was to create a lap welding joint between A7075-T651

aluminium and AZ31B magnesium alloys by new technique of GMA plug welding

method with better joint performance. Production of lighter and more fuel efficient

vehicle structures for automobiles, aviation, aerospace and marine industries was aimed

by such joint.

1.2 PROBLEM STATEMENT

A variety of attempts have been taken so far to find out the best method to join

Al to Mg alloys such as arc, metal inert gas (MIG), tungsten inert gas (TIG), friction stir

welding (FSW), diffusion, laser, laser-TIG hybrid, laser and electron beam welding

(Hayat, 2011, Zhang and Song, 2011). But, most of these methods particularly gas metal

arc welding method failed to come up with complete satisfactory outcomes (Yan et al.,

2005). When two dissimilar alloys like Al and Mg are used in the same product, it is

very important to have a proper bonding between them so that significant mechanical

proppeties of the joint can be obtained (Liu et al., 2008). But due to the difference in

physicochemical properties between Al and Mg alloys, it is very difficult to obtain a

sound joint through conventional welding methods as large amount of extremely brittle

AlmMgn type IMCs like Mg17Al12 and Mg2Al3 etc., cracks, defects, pores, oxide,

cavities, coarse grains are always formed at the interface of these alloys. Most

importantly, AlmMgn IMCs have a strong negative, unacceptable deteriorative effects on

the mechanical and metallurgical properties of the welding joints (Zhang and Song,

2011). Even in high energy density laser welding, harmful AlmMgn IMCs can form

4

which greatly decreases the ductility of the weld zone and makes it very fragile (Chang

et al., 2011; Chang and Kim, 2011; Sato et al., 2004; Yan et al., 2010; Yong et al.,

2010). The maximum strength of the welding joint between Al and Mg alloys so far is

only around 60% of Mg base alloy which is very low. Some methods like,

electromagnetic impact and cold metal transfer-MIG (CMT-MIG) welding were used to

eliminate or reduce the formation of AlmMgn IMCs but the strength is still insignificant.

At present FSW, diffusion bonding, laser and hybrid laser-TIG welding are used to

obtain higher strength of Al/Mg alloys joint but still could not fulfil the requirements for

practical applications (Borrisutthekul et al., 2005). The strength of Al/Mg welded joints

need to be further improved (Liu and Ren, 2011). Therefore, the formation of AlmMgn

IMCs must be controlled or stopped to develop a reliable joint (Chang and Kim, 2011;

Qi and Liu, 2012; Zhang and Song, 2011). The details about these drawbacks have been

discussed in section 2.10.1 and 2.10.3.

Moreover, the flexibility of FSW is poor, diffusion bonding is less efficient and

needs to be put in a vacuum chamber. Laser and hybrid laser-TIG welding need

expensive equipment and complex welding procedures. Therfore, these methods are

some what limited in applications. Meanwhile; TIG, MIG, GMA, laser and other

conventional fusion welding inevitably make fusion zone to generate a thick AlmMgn

IMCs layer, which leads to the formation of cracks seriously affecting the mechanical

and metallurgical properties of the joint (Shang et al., 2012; Zhang and Song, 2011).

Additionally, a few research was reported so far to investigate the fracture tougness

(FT) of the joints betwen any Al to Mg alloy. But, FT is a very important mechanical

property that describes the impact resistance of materials and welding joints. Therefore;

from the above discussions, it can be stated that most of the fusion and non fusion

welding methods for joining Al to Mg alloys have drawbacks mainly because of the

formation of AlmMgn IMCs. Considering these drawbacks and taking the simple

conventional arc welding method into account, the principal technical challenge was to

combine Al and Mg alloys by a reliable joining technique of GMA plug welding for

their extensive structural applications (Liu et al., 2009; Sato et al., 2004; Shang et al.,

2012). GMA welding method is simple, can be automated and involves inexpensive

machineries. In this research, the generation of AlmMgn IMCs was reduced in lap joint

between A7075-T651 and AZ31B alloys by a new technique of GMA plug welding

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method using low welding power with ER308L-Si stainless steel (SS) and ER5356 Al

fillers.

1.3 RESEARCH MOTIVATION

Light weight structure is a great concern in automobile, aviation, aerospace and

marine industries because of issues like fuel economy, reduction of green house gas

emissions (Karunakaran and Balasubramanian, 2011). The transportation industries

have been affected continually by the energy and financial crises in recent time.

Economic precautions have been taken due to the limited reserves of fossil fuels and

fluctuations in fuel prices experienced in last 30 to 40 years. Though, steel is so far the

principal material used in transportation industries; recently, materials like Al, Mg,

plastic and composites have gained attraction due to the issues like reduction of body

weight and fuel saving (Hayat, 2011). At present, the increasing demand for more light

weight vehicles to reduce fuel consumption and air pollution is a big challenge for the

transportion industries (Miller et al., 2000). Therefore, the interests on the use of light

weight materials such as Al and Mg alloys are rising (Karunakaran and

Balasubramanian, 2011). Both Al and Mg alloys have many attractive physical,

mechanical and chemical properties like light weight, very good thermal and electrical

conductivity, high specific strength, high stiffness, good formability, high durability,

recyclability, excellent corrosion resistance, low cost maintenance, high recovery

potential. Infact, Mg is the lightest (around two-thirds of Al and one fifth of steel) of all

structural metals on earth with some other properties like good castability, workability

and electromagnetic shielding capability (Casalino, 2007; Chang and Kim, 2011; Chen

and Nakata, 2008; Gourier-Fréry and Fréry, 2004; Hayat, 2011; Liu et al., 2008; Miller,

2000; Mofid et al., 2012; Qi and Liu, 2012; Toros et al., 2008; Yan et al., 2010; Yong et

al., 2010; Zhang and Song, 2011). With the requirements of fuel economy and

environmental conservation, the specific strength is the key feature for materials

selection and will continue to be of major importance (Yan et al., 2010). Recently, in

many industries much attention has been paid to Al and Mg alloys as they have high

potential usability almost for all types of applications in high technology manufacturing

industries like automobiles, aviation, aerospace, marine and electronics (Hayat, 2011;

Kwon et al., 2008; Liu et al., 2009; Mofid et al., 2012; Shang et al., 2012; Qi and Liu,

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2012). For extensive uses as structural material, innovative way to join these two alloys

are very essential (Chen and Nakata, 2008; Yan et al., 2010; Mofid et al., 2012;

Casalino, 2007; Chang and Kim, 2011; Yan et al., 2005). Because, the ability to join

these two alloys would allow further design flexibility, reduce the weight and cost,

increase the fuel efficiency, reduce the environmental pollution and enhance the quality

of components through compound structures (Liu et al., 2009; Shang et al., 2012). Liu

et al. (2009) stated that the welding of Mg alloy to Al alloy would improve flexibility

and availability of components substantially (Qi and Liu, 2012). In this token,

effectiveness of joint between Al and Mg alloys is essential. Therefore, problems in Al

to Mg welding must be faced and need to be solved (Chang and Kim, 2011; Chen and

Nakata, 2008; Mofid et al., 2012; Yan et al., 2005; Yan et al., 2010).

Currently, the welding technology between Al and Mg alloys has been an

important research field in transportation industries. The intersection of modern

industrial materials has also made the welding technology of Al and Mg alloys urgently

(Shang et al., 2012). The welding joint is also one of the most important factors

affecting the life span, safety, endurance and quality of the structures (Hayat, 2011).

Thus, for extensive practical applications of Al and Mg alloys; efficient and reliabile

welding technologies should be established in addition to consider issues such as alloy

design, microstructure control, plastic forming and surface treatment (Casalino, 2007;

Kwon et al., 2008; Zhang and Song, 2011). Welding joints are also very important for

wide applications of advanced materials like Al and Mg alloys. At present, many

researchers have been going on to develop reliable joints between different Al and Mg

alloys using diferrent welding methods such as TIG, FSW, laser, electron beam

welding, CMT and laser-TIG welding to establish them as potential structural materials

(Liu et al., 2011). But most of the attempts achieved either partial satisfactory or

unsatisfactory results because of very brittle AlmMgn type IMCs formation at the Al-Mg

interphase which deteriorate the mechanical properties of the joints. Certain welding

configurations and selection of apropriate parameters with their appropriate ranges

could be one of the solutions to reduce or stop the formation of AlmMgn IMCs.

Therefore, more research should go on to establish reliable welding techniques between

Al and Mg alloys. Proper parameters selection and their appropriate ranges are also very

much important for the better performance of the welding joints and this is a difficult

7

job. Usually, for that purpose we have to rely on operator’s experience and inadequate

general information provided by welding machine manufacturers which are only

applicable for common materials viz. steel, copper and Al. The selection of parameters

for welding depends extremely on parent metal-filler wire combinations. Usually high

mechanical properties are desired. The proper combination of input parameters can

achieve significant mechanical properties. Some researchers have developed models to

predict welding joints’ performances for different Al or Mg alloys using different

methods, such as RSM, Taguchi, artificial neural network (ANN) and so forth. It is

found that the models were developed for particular materials in similar welding and it

cannot be used for other dissimilar materials. The modeling in Al to Mg alloys welding

joints is still much lagging. In this circumstance, an effort has been made to study the

ultimate tensile strength (UTS) and fracture toughness (FT) of joints between A7075-

T651 Al and AZ31B Mg alloys by a new technique of GMA lap plug welding method.

Mathematical models to correlate UTS and FT with the process parameters were also

developed. Microstructure and fracture surface morphology at welding cross sections

were also studied by optical microscope, scanning electron microscope (SEM) and

energy dispersive spectroscopy (EDX) analysis.

1.4 OBJECTIVES OF RESEARCH

The objectives of this research are as follows:

i. To develop welding joint between dissimilar A7075-T651 and AZ31B alloys

using ER308L-Si stainless steel and ER5356 aluminium filler wires by new

technique of gas metal arc lap plug welding method.

ii. To investigate weldability, effects of welding parameters on ultimate tensile

strength and fracture toughness, and develop mathematical models

1.5 SCOPES OF RESEARCH

The scopes of this research are as follows:

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i. Box-Behnken design of experiments for welding.

ii. A new technique of gas metal arc lap plug welding method to join dissimilar

A7075-T651 and AZ31B alloys using ER308L-Si stainless steel and ER5356

aluminium filler wires.

iii. Investigation of the mechanical properties (ultimate tensile strength and fracture

toughness) of the welding joints.

iv. Statistical analysis and mathematical modeling in response surface

methodology.

v. Studying the macro and microstructures at welding joints.

vi. Studying fracture surface morphology after mechanical testing and analyzing

chemical composition at welding joints by scanning electron microscope and

energy dispersive X-ray spectroscopy.

1.6 ORGANIZATION OF THESIS

The current research has been organized to provide features on the particulars,

observations, logics, interpretations and ways to meet the objectives. Chapter 1 is an

introduction to GMA welding, Al and Mg alloys, problem statement, motivation,

objectives and scopes of the research. Chapter 2 presents about GMA welding,

dissimilar Al and Mg alloys in brief along with the drawbacks of previous researches on

welding (mechanical and metallurgical properties of the joints) between different Al and

Mg alloys. This chapter also presents about the properties of selected materials, merits

and demerits of lap welding, welding parameters, significance of Al/Mg dissimilar

welding. Several attempts were taken by the previous researchers in modeling studies

for different Al and Mg alloys welding using distinct methods. Chapter 3 describes the

research framework, materials and sample preparation, GMA lap plug welding method,

experimental set up, preliminary experiments to select the welding parameters and their

ranges, and the design of experiments (DOEs). This chapter also deals with the

procedure to carry out the experiments and measurements for the process outputs.

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Eventually, Chapter 3 delivers the detail process of model development through RSM

Box-Behnken approach. Chapter 4 describes the preliminary experimental, experimental

and analytical results, discussions and analysis. This chapter presents the results

obtained from preliminary experiments, mechanical testing. The RSM modeling as well

as the significance of the welding parameters on joint’s performance was also presented.

In addition, this chapter provides the results of model validation by statistical and error

analysis. Finally, micro-structural investigation and analysis at welding cross sections

and fracture surface morphology investigation by SEM and EDX spectroscopy were

also presented. Chapter 5 reports the concluding remarks and further recommendations

that would extend the current research in future.

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

This chapter presents a review on GMA welding, Al and Mg alloys; the

characterization and properties of dissimilar A7075-T651 and AZ31B alloys; filler

materials; merits and demerits of lap welding over other welding; variable process

parameters; previous studies on mechanical and metallurgical properties of dissimilar

Al-Mg alloys joints and their drawbacks; brief summary of previous studies on

modeling of Al and Mg welding. The purpose of this chapter is to provide a review of

past research efforts linked with welding process parameters, joints’ performance

characteristics, and methods attempted to correlate process parameters and joints’

performance characteristics by mathematical modeling. Previous researches on Al and

Mg dissimilar alloys welding by conventional and unconventional methods are also

presented. Among the various conventional welding processes, GMA is one of the most

attempted, simple and easy welding methods. Therefore, research in dissimilar Al-Mg

joining with GMA welding method has become very important. A review of relevant

studies connected with modeling in various welding methods for different Al and Mg

alloys are also presented. From this elaborate review, the research gap and the scope of

current research work can easily be understood.

2.2 GAS METAL ARC WELDING PROCESS

Different kinds of welding methods were attempted to increase the productivity

in industries. The advanced, highly efficient and economical processes have always

been targeted. Amongst numerous methods, GMA welding process is so far the best