Microstructure and Mechanical Properties of Welded AA6082 Aluminium Alloys

106 Pages·2016·7.3 MB·English

by  Jonas Vestfjell Jakobsen

Microstructure and Mechanical
Properties of Welded AA6082 Aluminium
Alloys
Jonas Vestfjell Jakobsen
Materials Science and Engineering
Submission date: June 2016
Supervisor: Trond Furu, IMTE
Co-supervisor: Ole Runar Myhr, IMT
Norwegian University of Science and Technology
Department of Materials Science and Engineering

Abstract
The present work have investigated variants of an AA6082 aluminium alloy. Five different
alloys with various amounts of Mn, Cr and Zr have been subjected to three homogenization
procedures. After homogenization, the aluminium billets were extruded into plates that were
artificially aged to T6-temper. Further, the profiles were MIG welded to obtain a heat affected
zone. The effect of these parameters on microstructure and mechanical properties of the
various alloys in welded state have been studied. The microstructure after extrusion and
welding were investigated by optical microscope and hardness measurements. In addition,
subsequent heat treatments have been performed in an attempt to further increase the density
of dispersoids for one specific alloy.
Welding of age-hardened aluminium alloys change the microstructural hardening phase that
can be a challenge for the mechanical properties in the heat affected zone (HAZ) and lead to a
reduction of the yield strength. The strength of the ageing effect is crucial for the load-bearing
capacity of welded structure. The degree of work hardening in the HAZ is important to
achieve the same yield strength level as the base material. Introduction of dispersoids in the
alloys can affect the microstructure and strength of the aluminium alloys, and in the present
work, an attempt to increase the strength by the introduction of dispersoids is carried out.
Results from this study show that chemical composition, homogenization procedure, artificial
ageing and welding influence the grain structure and mechanical properties of these AA6082-
alloys. Density of dispersoids increases with increasing amount of Mn, Cr and Zr, low
homogenization temperature and short holding time. Dispersoids retard recrystallization
effectively after extrusion and welding. A typical welding profile were obtained for all of the
tested alloys and the different levels of dispersoids formed during this work have not
improved the welding properties significantly.

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Sammendrag
Denne avhandlingen undersøker varianter av AA6082 aluminiumslegering. Fem ulike
legeringer med ulike nivå av Mn, Cr og Zr har blitt utsatt for tre homogeniserings prosedyrer.
Etter homogeniseringen, ble boltene ekstrudert til plater som ble utherdet til maksimal styrke
(T6). Videre ble profilene sveist for å oppnå en varmepåvirket sone. Effekten av disse
parameterne på mikrostruktur og mekaniske egenskaper av de forskjellige legeringene i sveist
tilstand har blitt studert. Mikrostrukturen etter ekstrudering og sveising ble undersøkt ved
lysmikroskop og hardhetsmålinger. I tillegg er det blitt utført etterfølgende
varmebehandlinger for å øke tettheten av dispersoider av en utvalgt legering.
Sveising av utherdet aluminiumslegeringer endrer mikrostrukturen til herdefasen som kan
være en utfordring for de mekaniske egenskaper i den varmepåvirkede sonen (HAZ) og fører
til en reduksjon av flytespenningen. Styrken som oppnås ved utherdingen er avgjørende for
bæreevnen av sveiste strukturer. Graden av arbeidsherding i HAZ er viktig for å oppnå samme
strekkfasthet som grunnmaterialet. Introduksjon av dispersoider i legeringene kan ha en effekt
på mikrostrukturen og mekaniske egenskaper av aluminiumslegeringene, og i dette arbeidet,
er det blitt gjort forsøk på å øke styrken ved å introdusere dispersoider.
Resultatene viser at kjemisk sammensetning, homogeniseringsprosess, utherding og sveising
påvirker kornstrukturen og mekaniske egenskaper til AA6082-legeringene. Tettheten av
dispersoider øker med økt mengde Mn, Cr and Zr, lav homogeniseringstemperatur og kort
holdetid. Dispersoidene hemmer rekrystallisering effektivt etter ekstrudering og sveising.
Typiske sveiseprofiler er oppnådd for alle testede legeringer og de ulike nivåene av
dispersoider dannet i dette arbeidet har ikke forbedret sveiseegenskapene signifikant.

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Preface
This project has been a part of collaboration between the Norwegian University of Science
and Technology (NTNU) and Hydro Aluminium. It was carried out at the Department of
Materials Science and Technology at NTNU, during the final semester of the 5 year Master’s
Degree Program in Material Science and Engineering.
This study is a part of a larger collaboration project involving Hydro, Reinertsen and Snøhetta
where the focus is to project a floating bridge with underwater tunnel and where various
aluminium alloys are included in the solution. The components must be joined together and
the problems of welding aluminium alloys must be enhanced. In this study, various variations
of alloy AA6082 are welded and investigated considering mechanical properties in the heat
affected zone and the base material.
The main aim of this study has been to contribute to a better understanding of which effects
the alloying elements Mn, Cr and Zr and dispersoids have on the microstructure and
mechanical properties of the welded aluminium alloys.

Jonas Vestfjell Jakobsen

Trondheim, June 2016
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Aknowledgement

I would like to offer special thanks to my supervisors, Adjunct Professor Dr. Trond Furu1 and
Dr. Ole Runar Myhr2 for their guidance and encouragement about aluminium and throughout
this study. Their commitment and knowledge provided me with necessary input to carry out
this thesis.

My appreciation goes to Chief Engineer Trygve Schanche3 for his help with various needs
and difficulties encountered in the metallurgical laboratory and optical microscope. I would
like to thank Senior Research Scientist Martin Lefstad4 and Senior Technician Arne Gellein4
for help with the extrusion and artificial ageing of the specimens. Thanks to Research
Scientist Hans Fostervoll4 for help with the welding of the samples. Further, I would like to
thank Research Fellow Magnus Remøe3 for conversations regarding measurements of
dispersoids. Finally thanks to Senior Engineer Yingda Yu3 for instructions and education
concerning use of the Scanning Electron Microscope.

1 Hydro Aluminium, Sunndalsøra.
2 Hydro Aluminium, Raufoss.
3 Norwegian University of Science and Technology (NTNU), Trondheim.
4 SINTEF Materials and Chemistry, Trondheim.

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