MICROSTRUCTURE EVOLUTION DURING EXTRUSION OF AA3xxx ALUMINUM ALLOYS by ...

136 Pages·2011·5.35 MB·English

MICROSTRUCTURE EVOLUTION DURING EXTRUSION OF AA3xxx ALUMINUM
ALLOYS
by
YUANYUAN GENG
B.Eng., ZHENGZHOU UNIVERSITY, 2008
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR
THE DEGREE OF
MASTER OF APPLIED SCIENCE
in
THE FACULTY OF GRADUATE STUDIES
(Materials Engineering)
THE UNIVERSITY OF BRITISH COLUMBIA
(Vancouver)

March 2011
© Yuanyuan Geng, 2011
Abstract
The through process microstructural evolution of aluminum alloys AA3003
(1.27wt% Mn) and AA3102 (0.26wt% Mn) during high temperature extrusion was
investigated. The Direct Chill(DC) cast microstructure of both alloys showed
interdendritic eutectic regions consisting of rod/plate like constituent particles in
the α-Al. Prior to extrusion, the as-cast materials were homogenized for three
different conditions, i.e. 500°C for 8h, 550°C for 8h and 600°C for 24h. Optical
Microscopy was used to examine the behavior of constituent particles and
dispersoids during homogenization. Back Scattered Scanning Electron Microscopy
and Image Analysis with Clemex were conducted to study the evolution of the size,
aspect ratio, area fraction and number density of constituent particles. Moreover, to
o
investigate the evolution of constituent particles during homogenization at 600 C,
o
samples heated to and soaked at 600 C for different times were studied. It was
found that different mechanisms of microstructure evolution occurred during
homogenization including the breaking up, growth and coarsening of particles.
Extrusion trials were conducted on a laboratory scale extrusion press located
at Rio Tinto Alcan’s Avidia R&D Center in Jonquiere, Quebec. Various AA3003 and
AA3102 DC billets with different homogenization treatments were extruded at
o o
400 C and 550 C with an extrusion ratio of 130. The extrudates were examined in
different orientations to study the through-thickness microstructure profile. Grain
structures were revealed by Polarized Light Optical Microscope and Electron Back
ii
Scattered Diffraction. Back Scattered Scanning Electron Microscopy and Image
Analysis were conducted on the extrudates to quantify the characteristics of the
extruded material. It was found that the homogenization treatments have a
significant effect on the as-extruded microstructure. In particular, the presence of
dispersoids suggested significant pinning effects on the recrystallization behavior.
In addition, Gleeble tests were conducted on a low iron AA3102 alloy using a
-1 -1 -1
Gleeble 3500 Thermo-mechanical Simulator at strain rates of 0.1s , 1s and 10s
o o o
and deformation temperature of 400 C, 500 C, and 600 C. The yield stress, work
hardening and flow stress results were fit into a physically based flow stress model
of Kocks and Chen. It was found that the constituent particles had a minimal effect
on the flow stress exponent in the model.
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Table of Contents
Abstract .......................................................................................................................................................... ii
Table of Contents....................................................................................................................................... iv
List of Tables ............................................................................................................................................. vii
List of Figures .......................................................................................................................................... viii
List of Symbols ......................................................................................................................................... xii
Acknowledgements ............................................................................................................................... xiii
Chapter 1 Introduction ............................................................................................................................ 1
1.1 Homogenization .............................................................................................................................. 2
1.2 Extrusion ............................................................................................................................................ 3
1.2.1 Extrusion process .................................................................................................................. 4
1.2.2 Process parameters for extrusion .................................................................................. 4
1.3 Microstructure evolution ............................................................................................................ 5
1.4 Project objective ............................................................................................................................. 6
Chapter 2 Literature Review ................................................................................................................. 7
2.1 Characteristics of the AA3xxx aluminum system ............................................................. 7
2.1.1 Phase diagrams ....................................................................................................................... 8
2.1.2 Microstructure ..................................................................................................................... 10
2.2 The evolution of second phase particles during homogenization ......................... 11
2.2.1 Constituent particles ......................................................................................................... 12
2.2.2 Dispersoids ............................................................................................................................ 15
2.3 Recrystallization .......................................................................................................................... 17
2.3.1 Basic concepts in recrystallization .............................................................................. 17
2.3.2 Effect of second-phase particles ................................................................................... 19
2.3.3 Effect of thermo-mechanical factors ........................................................................... 21
2.3.4 Dynamic recrystallization in hot-worked aluminum .......................................... 22
2.3.5 Microstructure development during extrusion ..................................................... 25
2.4 Constitutive modeling ............................................................................................................... 27
2.4.1 Deformation mechanisms of aluminum alloy ......................................................... 28
2.4.2 Kocks and Chen Model ...................................................................................................... 29

iv
Chapter 3 Scope and Objectives ........................................................................................................ 31
3.1 Scope ................................................................................................................................................. 31
3.2 Objectives ........................................................................................................................................ 32
Chapter 4 Experimental Methodology ........................................................................................... 33
4.1 Initial materials ............................................................................................................................ 33
4.2 Homogenization ........................................................................................................................... 34
4.3 Extrusion trials ............................................................................................................................. 36
4.4 Microstructure characterization .......................................................................................... 39
4.4.1 Sample preparation............................................................................................................ 39
4.4.1.1 Billet sample .................................................................................................................. 39
4.4.1.2 Extruded sample ......................................................................................................... 40
4.4.2 Metallographic preparation procedure ..................................................................... 42
4.4.3 Microscopy ............................................................................................................................. 44
4.5 Image analysis .............................................................................................................................. 45
4.6 Compression tests ....................................................................................................................... 46
Chapter 5 Experimental Results ....................................................................................................... 49
5.1 Microstructure characteristics in as-cast billets ........................................................... 49
5.1.1 As-cast AA3003 .................................................................................................................... 49
5.1.2 As-cast AA3102 .................................................................................................................... 51
5.2 Microstructure evolution during homogenization ....................................................... 52
5.2.1 Evolution of constituent particles and segregation in the matrix ................. 52
5.2.2 Observations on dispersoids for different homogenization temperature . 57
5.2.3 Optical micrographs of as-homogenized AA3102 ................................................ 62
5.3 Microstructure characteristics of extruded Samples .................................................. 63
5.3.1 As-extruded AA3003 ......................................................................................................... 63
5.3.1.1 Constituent particles ................................................................................................. 63
5.3.1.2 Grain structure ............................................................................................................. 69
5.3.2 As–extruded AA3102 ........................................................................................................ 73
5.4 Effect of constituent particles on flow stress .................................................................. 76
5.4.1 SEM micrograph of low iron AA3102......................................................................... 76
5.4.2 Stress-Strain curves ........................................................................................................... 77
Chapter 6 Discussion ............................................................................................................................. 80
6.1 Evolution of second phase particles during homogenization and extrusion .... 80v
6.1.1 Constituent particles ......................................................................................................... 80
6.1.2 Dispersoids ............................................................................................................................ 85
6.2 Effect of extrusion conditions on as-extruded microstructure ............................... 85
6.2.1 Time scheme for extrusion and the through thickness variation in
processing ......................................................................................................................................... 86
6.2.2 Coarse surface grain region ............................................................................................ 87
6.2.3 Effect of homogenization treatment ........................................................................... 89
6.3 Physically-based flow stress model .................................................................................... 93
6.3.1 The model ............................................................................................................................... 93
6.3.2 Constitutive modeling of low iron AA3102 ............................................................. 94
6.3.3 Discussion on the fit parameters for the flow stress model ............................. 96
Chapter 7 Summary and Conclusions ............................................................................................ 99
7.1 Summary ......................................................................................................................................... 99
7.2 Future work ................................................................................................................................ 102
References ............................................................................................................................................... 103
Appendices .............................................................................................................................................. 113
Appendix A Repeatability of compression results and determination of yield stress
and flow stress .................................................................................................................................. 113
A.1 Repeatability of stress-strain curves .......................................................................... 113
A.2 Determination of yield stress and steady state flow stress .............................. 114
Appendix B Summary of compression tests results ......................................................... 116
Appendix C Image analysis method and sensitivity test for constituent particles
quantification .................................................................................................................................... 117
C.1 Clemex Routine ..................................................................................................................... 117
C.2 Sensitivity test on threshold value ............................................................................... 118
Appendix D Calculation of Burger’s vector and Young’s Modulus ............................. 120
D.1 Temperature dependent Young’s Modulus ............................................................. 120
D.2 Temperature dependent Burger’s vector ................................................................. 121
Appendix E Strain and strain rate calculation .................................................................... 123

vi
List of Tables

Table 2-1 Wrought aluminum alloy composition limits, reproduced with permission
from (Woldman and Frick 2000) ......................................................................................................... 8
Table 2-2 Crystal structure and density of the main phases in Al-Mn-Si alloy ................ 9
Table 4-1 Chemical composition of experimental alloys (wt %) ....................................... 33
Table 4-2 Parameters for the extrusion trials ............................................................................ 37
Table 4-3 Metallographic procedure for grinding and polishing ....................................... 43
Table 6-1 The measured fraction of constituent particles and equilibrium phase
fraction from Thermo-Calc ................................................................................................................. 83
Table 6-2 The SEM data and simulated (Du et al. 2011) volume fraction of the
constituent particles .............................................................................................................................. 84
Table 6-3 Comparison of fit parameters for low iron AA3102 alloys with those from
previous AA3003 and AA3102 alloys (Kubiak 2009); note the data from Kubiak is
represented by the shadowed boxes. ............................................................................................. 97
Table B-1 Flow stress and yield stress value (MPa) for as cast low iron AA3102 ... 116
Table B-2 Stress value (MPa) for homogenized low iron AA3102 ................................. 116
Table C-3 Selected range of threshold value for one micrograph from homogenized
o
sample (600 C, 48h) ........................................................................................................................... 119
Table C-4 Selected range of threshold value for all micrographs from homogenized
o
sample (600 C, 48h) ........................................................................................................................... 119
Table D-5 Parameters meaning and references for Modulus equations ..................... 120
Table D-6 Temperature dependent Shear Modulus and Young’s Modulus values .. 121
Table D-7 Parameters meaning and references for Burger’s vector equations ........ 121
Table D-8 Coefficient of thermal expansion data from (Fickett 1971) ......................... 122
Table D-9 Calculated temperature dependent lattice parameter and Burger’s vector
values ......................................................................................................................................................... 122
vii
List of Figures

Figure 1-1 Industrial manufacturing process of AA3xxx for heat exchanger
application ..................................................................................................................................................... 2
Figure 1-2 Schematic of direct extrusion press, reproduced with permission from
(Davis 1993) ................................................................................................................................................. 4
Figure 2-1 Sections of Al-Mn-Si phase diagram with a) 1.5%Mn; b) 0.5% Mn,
reproduced with permission from (Belov et al. 2005) .............................................................. 9
Figure 2-2 Area fraction and number density of constituent particles during
o
homogenization at 600 C, reproduced with permission from (Li and Arnberg 2003)
......................................................................................................................................................................... 13
Figure 2-3 Effect of deformation on PSN, reproduced with permission from
(Humphreys and Hatherly 2004) ..................................................................................................... 20
Figure 2-4 Polarized Light Optical Micrograph of partly extruded 6060 billet,
reproduced with permission from (Kayser et al. 2010) ......................................................... 26
Figure 4-1 Recirculation air furnace ............................................................................................... 35
Figure 4-2 Homogenization treatment profiles ......................................................................... 35
Figure 4-3 Extrusion trials in ARDC 1) extruded strip exits here, 2) billet position for
extrusion, 3) mechanical billet loading system, 4) ram ......................................................... 36
Figure 4-4 Water quench unit located at die exit 5) Extrudate being water quenched
......................................................................................................................................................................... 37
Figure 4-5 Principle extrusion variables ...................................................................................... 38
Figure 4-6 I-beam Extrudate a) dimension b) actual extrudate ........................................ 38
Figure 4-7 Position indication of billet samples ........................................................................ 40
Figure 4-8 Dimension of a) AD sample with the observation plane indicated as the
red color b) tapered sample with the observation plane indicated as the blue color
......................................................................................................................................................................... 40
Figure 4-9 Dimension of Tapered samples .................................................................................. 41
viii
o
Figure 4-10 AA3102 as-cast, and extruded at 400 C a) schematic representation of
the combined 3-D profile, b) picture from AD sample, and c) picture from tapered
sample .......................................................................................................................................................... 42
Figure 4-11 Picture of anodizing equipment .............................................................................. 44
Figure 4-12 Example of manually plotting on as-cast AA3003 ........................................... 46
Figure 4-13 Gleeble Thermal-mechanical Machine for compression tests ................... 47
Figure 4-14 Jaw setup 1) jaws, 2) thermocouples, 3) sample, 4) load cell .................... 47
Figure 5-1 As-cast AA3003 a) centre of the billet, b) half radius section of the billet,
c) outer section of the billet ................................................................................................................ 50
Figure 5-2 Grain structure of as-cast AA3003 ............................................................................ 50
Figure 5-3 Higher magnification of Back Scattered SEM micrographs showing views
of constituent particles in as-cast AA3003 a) x2000 b) x7000 ........................................... 51
Figure 5-4 OM micrograph of as-cast AA3102 ........................................................................... 52
Figure 5-5 SEM micrograph of as-cast AA3102 ......................................................................... 52
Figure 5-6 As-cast AA3003: a)backscattered electron image, b)x-ray map for Mn,
c)x-ray map for Fe, d)x-ray map for Si ........................................................................................... 53
o
Figure 5-7 AA3003 homogenized at 600 C for 24h: a)backscattered electron image,
b)x-ray map for Mn, c)x-ray map for Fe, d)x-ray map for Si ................................................ 53
Figure 5-8 Backscattered electron micrographs for samples with a heating rate of
o o o o o o
150 C/h and quenched from : a) 200 C, b)350 C, c) 400 C, d) 500 C and e) 550 C . 54
o
Figure 5-9 Backscattered electron micrographs for samples soaked at 600 C for :
a)0min, b)10min, c)20min, d)40min,e)1h,f)12h,g)24h, and f)48h ................................... 55
o
Figure 5-10 Size and aspect ratio of constituent particles in AA3003 soaked at 600 C
......................................................................................................................................................................... 56
Figure 5-11 Area fractions and number density of constituent particles in AA3003
o
during homogenization at 600 C ..................................................................................................... 57
o
Figure 5-12 a) OM micrographs of AA3003, 500 C, 8h homogenized ............................. 58
Figure 5-13 Backscattered electron micrographs of homogenized AA3003 for a)
o o o ix
500 C, 8h b) 550 C, 8h and c) 600 C, 24h .................................................................................... 60
Figure 5-14 Area fractions of constituent particles in AA3003 .......................................... 61
Figure 5-15 Number density and circular diameter of constituent particles in
AA3003 after different homogenization treatments ............................................................... 61
o o
Figure 5-16 OM Micrographs of Homogenized AA3102 a) 500 C, 8h b) 550 C, 8h c)
o
600 C, 24h .................................................................................................................................................. 62
o
Figure 5-17 SEM micrographs of constituent particles in AA3003, extruded at 400 C
o o o
a) as-cast, and homogenized for b) 500 C, 8h c) 550 C, 8h d) 600 C, 24h ..................... 64
o
Figure 5-18 SEM view of constituent particles in AD sample from AA3003, 600 C
o
and 24h homogenized and extruded at 400 C a) surface b) center of the extrudate 65
Figure 5-19 the number density variation of the constituent particles of the entire
o o
AD plane (AA3003, 600 C and 24h homogenized and extruded at 400 C) ................... 65
Figure 5-20 Average circular diameters of constituent particles a) before extrusion
o
b)after extrusion at 400 C ................................................................................................................... 66
o
Figure 5-21 Area fraction of constituent particles before and after extrusion (400 C)
......................................................................................................................................................................... 67
Figure 5-22 Number density of constituent particles a) before extrusion b)after
o
extrusion at 400 C .................................................................................................................................. 68
o
Figure 5-23 Microstructure of AA3003 I-Beam extruded at 400 C .................................. 70
o o
Figure 5-24 EBSD mapping of 400 C as-extruded (24h@600 Chomogenized) .......... 71
o
Figure 5-25 Microstructure of AA3003 I-Beam extruded at 550 C .................................. 72
o
Figure 5-26 Microstructure of AA3102 I-Beam extruded at 400 C .................................. 74
o
Figure 5-27 Microstructure of AA3102 I-Beam extruded at 550 C .................................. 75
Figure 5-28 SEM micrograph of as-cast low iron AA3102 .................................................... 76
Figure 5-29 Stress-strain curves from as-cast low iron AA3102 for selected
compression conditions ....................................................................................................................... 77
o
Figure 5-30 Stress-strain curves from 24h@600 C homogenized low iron AA3102
for selected compression conditions .............................................................................................. 78
Figure 5-31 Selected stress-strain curves of as-cast and homogenized low iron
x
AA3102 ........................................................................................................................................................ 78