weldability of high strength aluminium alloys

119 Pages·2015·8.77 MB·English

Muyiwa Olabode
WELDABILITY OF HIGH STRENGTH
ALUMINIUM ALLOYS
Thesis for the degree of Doctor of Science (Technology) to be presented with
due permission for public examination and criticism in lecture hall 1382 at
Lappeenranta University of Technology, Lappeenranta, Finland on the 1st of
December, 2015, at noon.
Acta Universitatis
Lappeenrantaensis 666
Supervisors Professor Jukka Martikainen
Laboratory of Welding Technology
LUT School of Energy Systems
Lappeenranta University of Technology
Finland

Associate Professor Paul Kah
Laboratory of Welding Technology
LUT School of Energy Systems
Lappeenranta University of Technology
Finland

Reviewers Professor Leif Karlsson
Department of Engineering Science
University West
Sweden

Professor Thomas Boellinghaus
Department of Component Safety
Federal Institute of Material Research and Testing
Germany

Opponent Professor Leif Karlsson
Department of Engineering Science
University West
Sweden

ISBN 978-952-265-865-4
ISBN 978-952-265-866-1 (PDF)
ISSN-L 1456-4491
ISSN 1456-4491

Lappeenrannan teknillinen yliopisto
Yliopistopaino 2015
Abstract
Muyiwa Olabode
Weldability of high strength aluminium alloys
Lappeenranta 2015
59 pages
Acta Universitatis Lappeenrantaensis 666
Diss. Lappeenranta University of Technology
ISBN 978-952-265-865-4, ISBN 978-952-265-866-1 (PDF), ISSN-L 1456-4491, ISSN
1456-4491

The need for reduced intrinsic weight of structures and vehicles in the transportation
industry has made aluminium research of interest. Aluminium has properties that are
favourable for structural engineering, including good strength-to-weight ratio, corrosion
resistance and machinability. It can be easily recycled saving energy used in smelting as
compared to steel. Its alloys can have ultimate tensile strength of up to 750 MPa, which
is comparable to steel. Aluminium alloys are generally weldable, however welding of
high strength alloys like the 7xxx series pose considerable challenges.
This paper presents research on the weldability of high strength aluminium alloys,
principally the 7xxx series. The weldability with various weld processes including MIG,
TIG, and FSW, is discussed in addition to consideration of joint types, weld defects and
recommendations for minimizing or preventing weld defects.
Experimental research was carried out on 7025-T6 and AW-7020 alloys. Samples were
welded, and weld cross sections utilized in weld metallurgy studies. Mechanical tests
were carried out including hardness tests and tensile tests. In addition, testing was done
for the presence of Al O on exposed aluminium alloy.
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It was observed that at constant weld heat input using a pulsed MIG system, the welding
speed had little or no effect on the weld hardness. However, the grain size increased as
the filler wire feed rate, welding current and welding speed increased. High heat input
resulted in lower hardness of the weld profile. Weld preheating was detrimental to AW-
7020 welds; however, artificial aging was beneficial. Acceptable welds were attained
with pulsed MIG without the removal of the Al O layer prior to welding. The Al O
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oxide layer was found to have different compositions in different aluminium alloys.
These findings contribute useful additional information to the knowledge base of
aluminium welding. The application of the findings of this study in welding will help
reduce weld cost and improve high strength aluminium structure productivity by
removing the need for pre-weld cleaning. Better understanding of aluminium weld
metallurgy equips weld engineers with information for better aluminium weld design.
Keywords: Aluminium alloys, aluminium welding processes, high strength aluminium,
anodising, Al O , 7025-T6, AW-7020
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Acknowledgements
I would like to express my appreciation to the many people that have in one way or the
other helped me in the completion of this thesis. I gratefully acknowledge the efforts of
Dr. Paul Kah for his input in the form of research methodology, article corrections,
availability and readiness to guide. I thankfully acknowledge the efforts of Esa
Hiltunen, Antti Heikkinen and Antti Kähkönen in carrying out laboratory weld
experiments. I would also like to thank Dr. Liisa Puro and Toni Väkiparta for their
assistance in carrying out O composition experiments on the Al O layer. I would like
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to thank Peter Jones for the valuable input on the academic presentation of this thesis. I
wish also to extend my thanks to Martin Kesse for all his support.
I would like to thank the pre-examiners of this work, Professor Leif Karlsson and
Professor Thomas Boellinghaus for their valuable comments and suggestions that
helped in improving the quality of this work.

I wish to acknowledge the encouragement of friends and families. My special thanks are
extended to my families, the Olabodes, the Pöllänens and the Olamilehins for their
encouragement and support. Additionally, my appreciation goes to Bidemi Orebiyi,
Samuel Okunoye, Kevin Eyiowuawi, Edith Emenike, and others that have in one way or
the other supported my journey.
My sincere appreciation is expressed to my immediate family, especially my wife,
Olaitan Olabode, for all the encouragement, forbearance and understanding exercised
during the course of this research. I feel blessed, thank you all.

Muyiwa Olabode
October 2015
Lappeenranta, Finland

Dedication

Dedicated to almighty God, The one who was, is, and is to
come; allowing the acquisition of knowledge and giving the
wisdom to know when, where and how to apply the acquired
knowledge.

Contents
List of publications 11
Author's contribution 11
Nomenclature 13
1 Introduction 15
1.1 Research problem and research questions ............................................... 16
1.2 Scope and limitations of the study .......................................................... 17
1.3 Contribution of the work ......................................................................... 18
1.4 Social and environmental impact ............................................................ 18
1.5 Thesis outline .......................................................................................... 18
2 State of the art of Al welding 19
2.1 Alloy designation .................................................................................... 19
2.2 HSA ......................................................................................................... 19
2.3 Weldability of HSA ................................................................................. 22
2.3.1 Joint types and process limitations .............................................. 22
2.3.2 Work preparation ........................................................................ 26
2.3.3 Welding defects in HSA ............................................................. 30
2.4 Hybrid laser beam welding (HLBW) of HSA ......................................... 32
2.4.1 HLBW focusing head .................................................................. 33
2.4.2 Challenges of HLBW of Al ........................................................ 36
3 Experimental work 37
3.1 Welding metallurgy of HSA (7025-T6) .................................................. 37
3.2 Investigation of the Al O layer in Al alloys ........................................... 39
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3.3 Effect of Al O layer on HSA (AW-7020) weld metallurgy ................... 39
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4 Results 41
4.1 Findings on the welding metallurgy of HSA (7025-T6) ......................... 42
4.2 Findings on the Al O layer of Al alloys ................................................. 45
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4.3 Findings on the effect of Al O on HSA (AW-7020) weld metallurgy .. 45
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5 Discussion 49
5.1 Welding metallurgy of HSA (7025-T6) .................................................. 49
5.2 Effect of Al O on HSA (AW-7020) weld metallurgy ............................ 50
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6 Conclusions 53
7 Future work 55
References 56