Modeling of the damage mechanisms in AlMgSi alloys

Understanding the role of homogenization on the extrudability
Première édition

With the growth in importance of the aluminium industry, has come increased demand to invest into the quality improvement of the different aluminium based hot extruded products. One of the main mechanisms, which can influence deformation at high temperature within the 6xxx aluminium, is linked to the presence of the AlFeSi intermetallic phases. These phases severely restrict hot workability when present as hard and brittle plate-like precipitates b-AlFeSi. Damage initiation occurs in these alloys by decohesion or fracture of these intermetallic inclusions. The understanding and modeling of the deformation and fracture behavior of aluminium alloys at room and at hot working temperature is very important for optimizing manufacturing processes such as extrusion. The ductility of 6xxx aluminium alloys can be directly related to chemical composition and to the microstructural evolution occurring during the heat treatment procedures preceding extrusion if proper physics based deformation and fracture models are used. In this thesis, room temperature and hot tensile tests are adopted to address the problem xperimentally. The damage evolution mechanisms is defined at various temperatures and a micromechanics based model of the Gurson type considering several populations of cavities nucleated by different second phase particles groups is developed on the basis of the experimental observations. This model allows relating quantitatively microstructure and ductility at various temperatures strain rates and stress triaxialities. Finite element simulations based on an enhanced micromechanics-based model are used to validate the model. Finally, the effect of some key factors that determine the extrudability of aluminium is also discussed and a correlation between the ductility calculations in uniaxial tension and the maximum extrusion speed is developed for one defined profile.


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Spécifications


Éditeur
Presses universitaires de Louvain
Partie du titre
Numéro 87
Auteur
Denis Lassance,
Collection
Thèses de l'École polytechnique de Louvain
Langue
anglais
Catégorie (éditeur)
Sciences appliquées > Science des matériaux et procédés
BISAC Subject Heading
TEC000000 TECHNOLOGY & ENGINEERING
Code publique Onix
06 Professional and scholarly
CLIL (Version 2013-2019 )
3069 TECHNIQUES ET SCIENCES APPLIQUEES
Date de première publication du titre
01 janvier 2006
Subject Scheme Identifier Code
Classification thématique Thema: Technologie, ingénierie et agriculture, procédés industriels
Type d'ouvrage
Thèse

Livre broché


Details de produit
1 Couverture pelliculée
Date de publication
01 janvier 2006
ISBN-13
9782874630170
Ampleur
Nombre de pages de contenu principal : 266
Code interne
73161
Format
16 x 24 x 1,5 cm
Poids
432 grammes
Prix
34,00 €
ONIX XML
Version 2.1, Version 3

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Sommaire


General introduction 1

1 Theoretical background 7

1.1 Background on extrusion . . . . . . . . . . . . . . . . . . . . 9

1.2 Background on the metallurgy of AlMgSi alloys . . . . . . . . 11

1.3 Background on homogenization treatment . . . . . . . . . . . 14

1.3.1 Intermetallic phases in the ENAA 6xxx series . . . . . 15

1.3.2 Morphological changes . . . . . . . . . . . . . . . . . . 16

1.3.3 Inuence of the alloy content . . . . . . . . . . . . . . . 18

1.3.4 Improvement of extrudability . . . . . . . . . . . . . . 18

1.4 Surface quality considerations . . . . . . . . . . . . . . . . . . 19

1.4.1 Surface cracking . . . . . . . . . . . . . . . . . . . . . . 20

1.4.2 Pick-up and die-lines . . . . . . . . . . . . . . . . . . . 20

1.5 Micro-mechanisms of ductile fracture . . . . . . . . . . . . . . 21

1.6 Micromechanics-based constitutive model for ductile fracture 23

1.6.1 Local approach to fracture . . . . . . . . . . . . . . . . 23

1.6.1.a Void nucleation . . . . . . . . . . . . . . . . . 24

1.6.1.b Growth of isolated void . . . . . . . . . . . . . 25

1.6.1.c Micromechanical model by Gurson for porous solids . . . . . . . . . . . . . . . . . . . . . . . 26

1.6.1.d Inuence of the void shape . . . . . . . . . . . 28

1.6.1.e Void coalescence conditions . . . . . . . . . . . 28

1.6.2 Uni ed extended model for void growth and coalescence 30

2 The starting point 33

2.1 Introduction on industrial extrusion . . . . . . . . . . . . . . 36

2.2 Material and Industrial testing . . . . . . . . . . . . . . . . . 37

2.2.1 Material . . . . . . . . . . . . . . . . . . . . . . . . . . 37

2.2.2 Homogenization tests . . . . . . . . . . . . . . . . . . . 38

2.2.3 Extrusion tests . . . . . . . . . . . . . . . . . . . . . . 38

2.3 Results of the extrusion tests . . . . . . . . . . . . . . . . . . 39

2.3.1 Surface defect A . . . . . . . . . . . . . . . . . . . . . . 41

2.3.2 Surface defect B . . . . . . . . . . . . . . . . . . . . . . 43

2.3.3 Evolution of the extrusion temperature . . . . . . . . . 43

2.4 Microstructure investigation around the surface defects . . . . 45

2.4.1 TEM investigation at the defects locations . . . . . . . 45

2.4.2 DSC investigation at the defects locations . . . . . . . 46

2.4.2.a The DSC methodology . . . . . . . . . . . . . 48

2.4.2.b DSC measurements . . . . . . . . . . . . . . . 49

2.5 First conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 49

3 Microstructure evolution during homogenization 53

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

3.2 Materials and Experimental techniques . . . . . . . . . . . . . 57

3.2.1 Material . . . . . . . . . . . . . . . . . . . . . . . . . . 57

3.2.2 Experimental techniques for microstructure analysis . . 58

3.2.2.a Scanning Electron Microscopy (SEM) . . . . . 58

3.2.2.b Quanti cation of the relative
-AlFeMnSi and

-AlFeSi fraction by SEM and EDS . . . . . . 58

3.2.2.c Measurements of the intermetallics shape by Image analysis (IA) . . . . . . . . . . . . . . . . . 60

3.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

3.3.1 SEM investigations of the microstructure evolution . . 60

3.3.2 Quanti cation of the microstructure evolution . . . . . 62

3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

4 Growth and coalescence of penny-shaped voids 71

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

4.2 Void cell calculations with initially penny-shaped voids . . . . 78

4.2.1 Numerical procedures . . . . . . . . . . . . . . . . . . . 78

4.2.2 Results of the unit cell calculations without particle . . 79

4.2.3 Results of the unit cell calculations with particle . . . . 85

4.3 The constitutive model . . . . . . . . . . . . . . . . . . . . . . 89

4.3.1 The model . . . . . . . . . . . . . . . . . . . . . . . . . 90

4.3.1.a Before void nucleation . . . . . . . . . . . . . . 90

4.3.1.b Void nucleation condition . . . . . . . . . . . . 91

4.3.1.c Void growth model . . . . . . . . . . . . . . . . 92

4.3.1.d Void coalescence condition . . . . . . . . . . . 93

4.3.2 Assessment of the model . . . . . . . . . . . . . . . . . 94

4.3.2.a Assessment of the void coalescence condition . 94

4.3.2.b Assessment of the void growth model . . . . . 94

4.4 Parametric study and discussion . . . . . . . . . . . . . . . . 95

4.4.1 No nucleation stage - low to large stress triaxiality . . . 96

4.4.2 No nucleation stage - very low stress triaxiality . . . . 99

4.4.3 Delayed void nucleation . . . . . . . . . . . . . . . . . . 103

4.4.4 Simple model for the ductility of metals with penny shape voids . . . . . . . . . . . . . . . . . . . . . . . . . 106

4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

5 Micromechanics of low and high temperature fracture in 6xxx Al alloys 113

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

5.2 Materials, Experimental methods and Mechanical data reduction scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

5.2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . 118

5.2.2 Characterization of the damage nucleation and evolution 119

5.2.3 Tensile tests . . . . . . . . . . . . . . . . . . . . . . . . 121

5.2.4 Inverse procedure to determine uniaxial ow properties 122

5.3 Experimental results . . . . . . . . . . . . . . . . . . . . . . . 125

5.3.1 Mechanical tensile tests . . . . . . . . . . . . . . . . . . 125

5.3.1.a Nominal stress-strain relationships . . . . . . . 125

5.3.1.b Yield stress evolution . . . . . . . . . . . . . . 129

5.3.1.c Uniform elongation - Hardening behavior . . . 129

5.3.1.d Inverse identi cation of the materials ow curves 133

5.3.1.e True average fracture strain f . . . . . . . . . 133

5.3.2 Fracture mechanisms . . . . . . . . . . . . . . . . . . . 140

5.3.2.a In situ tensile tests at room temperature . . . 140

5.3.2.b Interrupted tensile tests at high temperature . 143

5.3.2.c Fracture surface . . . . . . . . . . . . . . . . . 144

5.4 Micromechanical modeling . . . . . . . . . . . . . . . . . . . . 146

5.4.1 Physical model . . . . . . . . . . . . . . . . . . . . . . 146

5.4.2 Micromechanics-based full constitutive model . . . . . 149

5.4.2.a Introduction . . . . . . . . . . . . . . . . . . . 149

5.4.2.b Before void nucleation . . . . . . . . . . . . . . 151

5.4.2.c Void nucleation condition . . . . . . . . . . . . 151

5.4.2.d Void growth model . . . . . . . . . . . . . . . . 152

5.4.2.e Onset of void coalescence . . . . . . . . . . . . 156

5.4.2.f Void coalescence . . . . . . . . . . . . . . . . . 158

5.4.3 Numerical method and problem formulation . . . . . . 158

5.4.4 Results of the modeling . . . . . . . . . . . . . . . . . . 160

5.4.4.a Parameters indenti cation . . . . . . . . . . . 160

5.4.4.b Deformation at room temperature . . . . . . . 162

5.4.4.c Deformation at high temperature . . . . . . . 163

5.5 Discussion and conclusions . . . . . . . . . . . . . . . . . . . 166

6 Parametric study and optimization of extrusion conditions171

6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

6.2 Inuence of the homogenization temperature and holding time 174

6.3 Inuence of the Mg and Si content . . . . . . . . . . . . . . . 176

6.4 Inuence of the Mn content . . . . . . . . . . . . . . . . . . . 177

6.5 Inuence of Fe content . . . . . . . . . . . . . . . . . . . . . . 178

6.6 Inuence of the thermal treatment on Mg2Si dissolution . . . 179

6.6.1 Mg2Si phase dissolution/growth . . . . . . . . . . . . . 180

6.6.1.a The dissolution/growth model . . . . . . . . . 180

6.6.1.b Results . . . . . . . . . . . . . . . . . . . . . . 183

7 Conclusions 189

Bibliography 199

A E
ect of the anisotropy distribution parameter 0 207

B Important geometrical relationships 213

C Expressions for the parameters of the void growth model 217

D Klocker formulation for void aspect parameter k 223

E Flow curves identi cation 227