Hot deformation behavior of Al-Mg-Si and Al-Mg alloys modified with CaO-added Mg

Title
Hot deformation behavior of Al-Mg-Si and Al-Mg alloys modified with CaO-added Mg
Authors
이지운
Keywords
hotdeformationbehaviorofalmgsiandalmgalloysmodifiedwithcaoaddedmg
Issue Date
2016
Publisher
인하대학교
Abstract
The thermomechanical processes have been considered as one of the ways to obtain the best combination of material properties, such as high strength, good ductility and good corrosion resistance. As the properties of the materials are mainly influenced by the microstructural development, which is governed by the dynamic softening mechanisms, the precise understanding of the hot deformation behavior is required. In Al alloys, the formation of Mg oxides causes several deleterious effects during casting or thermomechanical processes. Mg oxides degrade the fluidity of the melt and the mechanical properties of the final product. Recently, CaO-added Mg with improved oxidation resistance is utilized as a new Mg additive for Al alloys to reduce the detrimental effects of Mg during casting or thermomechanical processes. In the present thesis, the hot torsion tests were performed on Al-Mg-Si and Al-Mg alloys in order to investigate the effect of CaO-added Mg on the hot deformation behavior. The dynamic recrystallization (DRX) mechanism was identified by the microstructure observation. The constitutive equations and the processing maps are established to optimize the processing conditions. In addition, the kinetics models were established for the prediction of the microstructural evolution.
Al-Mg-Si alloy modified with CaO-added Mg showed fewer second phase particles than conventional alloy. Flow curves showed that the failure strain of the modified alloy was higher than that of the conventional alloy. The deformed microstructure and the misorientation angle distribution revealed that the continuous dynamic recrystallization (CDRX) is a main restoration mechanism for the Al-Mg-Si alloy modified with CaO-added Mg. The modified alloy had a greater volume fraction of recrystallized grains compared with conventional alloy. The higher failure strain and CDRX fraction of modified Al-Mg-Si alloy may be due to fewer second phase particles. In addition, the kinetics model for CDRX was proposed, and the model correlated well with the experimental data.
The constitutive equations of modified and conventional Al-Mg-Si alloys were established and fitted well with the hyperbolic sine law. The activation energy for deformation of the modified alloy was lower than that of conventional alloy. The processing maps were established at high strains over 1 (ε=100%). The power dissipation efficiency increased as the stain increased. The gap between contour lines was decreased with increasing strain, which means the values of power dissipation efficiency became more sensitive with increasing strain. The optimum deformation condition of the modified Al-Mg-Si alloy was in the temperature range of 400 - 500 °C with strain rates of 0.05 - 0.5 s-1.
The ‘necklace’ structure and the nucleation of strain-free grains were found in the deformed microstructure of the Al-7Mg alloy. These features strongly support that the DDRX occurs for Al-7Mg alloy as the dynamic restoration mechanism during hot deformation because of the reduced SFE from the high Mg content. The DRX kinetics model was established by the modification of Avrami equation. The fraction of DRXed grains increased as the temperature increased and the strain rate decreased. The relationship between the deformation conditions and the size of DRXed grains was established for the prediction of the grain size during hot working. The size of grain DRXed grains increased as the temperature increased and the strain rate decreased.
Description
Abstract I
Table of contents III

Chapter 1 Introduction 1

1.1 General introduction 1
1.2 Aluminum alloys modified with CaO-added Mg 3
1.3 Dynamic restoration mechanism 5
1.3.1 Dynamic recovery (DRV) 6
1.3.2 Discontinuous dynamic recrystallization (DDRX) 8
1.3.3 Continuous dynamic recrystallization (CDRX) 10
1.3.4 Geometric dynamic recrystallization (GDRX) 11
1.4 DRX kinetics model 12
1.5 Purpose and outline of this study 16
References 18

Chapter 2 Continuous dynamic recrystallization behavior and kinetics of Al-Mg-Si alloy modified with CaO-added Mg 21

2.1 Introduction 21
2.2 Experimental procedure 24
2.3 Results 26
2.3.1 Distribution of second phase particles in the initial state 26
2.3.2 Flow behavior 26
2.3.3 Microstructure evolution 30
2.4 Discussion 35
2.4.1 Effect of CaO-added Mg 35
2.4.2 Continuous dynamic recrystallization (CDRX) 38
2.4.3 CDRX kinetics 42
2.5 Conclusions 48
References 50

Chapter 3 Hot Deformation behavior of Al-Mg-Si alloy modified with CaO-added Mg using constitutive analysis and processing maps 53

3.1 Introduction 53
3.2 Experimental procedures 55
3.3 Results 57
3.3.1 Flow curves 57
3.3.2 Constitutive analysis 59
3.3.3 Processing maps 63
3.3.4 Microstructures 64
3.4 Discussion 69
3.5 Conclusions 74
References 76

Chapter 4 Dynamic recrystallization behavior and kinetics of Al-7Mg alloy under hot working 79

4.1 Introduction 79
4.2 Experimental procedure 80
4.3 Results 82
4.3.1 Flow curves 82
4.3.2 Deformed microstructure 84
4.3.3 DRX kinetics 90
4.4 Discussion 93
4.4.1 Flow behavior 93
4.4.2 DRX behavior 94
4.4.3 Prediction of microstructure 97
4.5 Conclusions 97
References 99

Chapter 5 Conclusions 101
URI
http://dspace.inha.ac.kr/handle/10505/35595
Appears in Collections:
Graduate School (일반대학원) > Theses(금속공학 석박사 학위논문)

Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated.

Browse