Constitutive behaviour of magnesium-aluminium extrusion alloys

154 Pages·2011·14.17 MB·English

by  Floris Slooff

Constitutive behaviour of
magnesium-aluminium extrusion alloys

Proefschrift

ter verkrijging van de graad van doctor
aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof.ir. K.C.A.M. Luyben,
voorzitter van het College voor Promoties,
in het openbaar te verdedigen op dinsdag 19 april 2011 om 10:00 uur

door

Floris Alexander SLOOFF

materiaalkundig ingenieur
geboren te Lage Zwaluwe.
Dit proefschrift is goedgekeurd door de promotor:

Prof. ir. L. Katgerman

Copromotor: Dr. J. Zhou

Samenstelling promotiecommissie:

Rector Magnificus, voorzitter
Prof. ir. L. Katgerman, Technische Universiteit Delft, promotor
Dr. J. Zhou, Technische Universiteit Delft, copromotor
Prof. dr. K.U. Kainer, Technische Universiteit Hamburg
Prof. dr. D.J. Schipper, Universiteit Twente
Prof. dr. ir. L.A.I. Kestens, Technische Universiteit Delft , Universiteit
Gent
Prof. dr. I.M. Richardson, Technische Universiteit Delft
Prof. ir. F. Soetens, Technische Universiteit Eindhoven

Keywords: wrought magnesium alloy, deformation, constitutive analysis,
hot working, extrusion.

ISBN: 978-94-6108-157-5

Copyright © 2011 by F.A. Slooff

All rights reserved. No part of the material protected by this copyright
notice may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying, recording or by any
information storage and retrieval system, without the prior written
permission from the author.

Printed in the Netherlands
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Table of Contents

Chapter 1. Introduction
1.1 Introduction 1
1.2 Background, application and future 2
1.3 Alloyed magnesium 6
1.4 Deformation mechanisms of magnesium 10
1.5 Hot working thermomechanical simulation 14
1.6 Thesis outline 20
1.7 References 20

Chapter 2. Experimental set-up and data processing of
uniaxial hot compression tests for
constitutive analysis
2.1 Introduction 25
2.2 Experimental set-up for uniaxial hot compression tests 26
2.3 Data correction and processing 30
2.4 Conclusions 37
2.5 References 38

Chapter 3. Constitutive analysis of wrought
magnesium-aluminium alloys
3.1 Introduction 41
3.2 Constitutive behaviour of AZ61 42
3.3 Development of strain dependent constitutive analysis 54
3.4 Strain dependent constitutive analysis of AZ41, AZ61 and AZ80 60
3.5 Conclusions 70
3.6 References 71

(cid:1) i
Chapter 4. Hot workability analysis of extruded AZ
magnesium alloys with processing maps
4.1 Introduction 75
4.2 Processing maps 76
4.3 Compression tests and microstructure analysis 79
4.4 Results and discussion 80
4.5 Conclusions 102
4.6 References 103

Chapter 5. Prediction of pressure required to extrude a
wrought magnesium alloy using optimized
strain dependent constitutive parameters
5.1 Introduction 107
5.2 Extrusion modelling 108
5.3 Compression and extrusion experiments of AZ41 111
5.4 Predicting the extrusion pressure for various profile shapes 114
5.5 Conclusions 121
5.6 References 122

Chapter 6. Conclusions and recommendations
6.1 Introduction 125
6.2 Conclusions 126
6.3 Recommendations 127

Summary 131
Samenvatting 135
List of publications 141
Dankwoord 143
Curriculum vitae 145
(cid:1) ii
(cid:1) iii
(cid:1) iv
Chapter 1
Introduction
1.1 Introduction
Because of their high strength-to-weight ratio, wrought magnesium alloys
have a great potential for various applications where mass reduction is
desired. By increasing the use of magnesium alloys in the automotive
industry, for example, CO emissions will be reduced, thereby making a
2
positive impact on the environment. However, the peculiar deformation
behaviour of most magnesium alloys has so far limited their applications.
Due to the difficulties to deform many wrought magnesium alloys, high
extrusion pressures are required and an unacceptable surface quality is
likely to develop. Finding a proper window of processing, realizing an
optimum microstructure and being able to preset processing parameters
on a rational basis will all help in expanding the utilization of these
promising materials.
Chapter 1

1.2 Background, application and future
Magnesium alloys have a great potential for various applications. In the
past, cast alloys have been put into numerous automotive applications
due to their low density combined with the high specific stiffness and
strength. Developing wrought magnesium alloys will increase the
applicability of this group of materials in modern automobiles. Table 1.1
gives a brief overview of the differences between magnesium and the two
other common structural metallic materials: aluminium and steel. The
main differences can be found in the crystal structure (and therefore the
slip planes available for deformation) and the specific strength, which is
the ratio of yield strength to density. The table clearly shows that the
specific strength of magnesium is superior to that of aluminium or steel.

Table 1.1.
Comparison of material properties between magnesium, aluminium and steel.
Property Magnesium Aluminium Steel
Hexagonal Face Body
closed centred centred
Crystal Structure
packed cubic cubic
(hcp) (fcc) (bcc)
Density at RT [*103 kg/m3] 1.74 2.70 7.86
Thermal Expansion Coefficient
25.2 23.6 11.7
[*106/°C]
Melting Point [°C] 650 660 1536
Elastic Modulus [GPa] 44 69 207
Tensile Strength [MPa] 260 310 800
Yield Strength [MPa] 200 275 600
Specific Strength [GPa*m3kg-1] 115 102 76

To illustrate the properties given in Table 1.1, a comparison between the
weight and stiffness of different materials is given in Figure 1.1. The figure
shows that when beams of equal weight are compared, magnesium-
2