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Forming of Ultra-Thin Ferritic Stainless Steel Sheets

봉혁종 (Bong, Hyuk Jong, 포항공과대학교)

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초록 moremore
The present thesis investigates the forming of ultra-thin ferritic stainless steel (FSS) sheets developed for the bipolar plates (BPs) of proton exchange membrane fuel cells (PEMFCs). Due to a lack of standard practices and research on the forming of ultra-thin materials, forming such thin sheets is...
The present thesis investigates the forming of ultra-thin ferritic stainless steel (FSS) sheets developed for the bipolar plates (BPs) of proton exchange membrane fuel cells (PEMFCs). Due to a lack of standard practices and research on the forming of ultra-thin materials, forming such thin sheets is challenging. Current works cover topics from the forming limit diagram (FLD), the most basic concept to evaluate the formability of sheet materials, to the application of forming technologies for the forming of ultra-thin FSS sheets. Chapter 1 presents an extensive literature review describing the historical development of FLD determinations and predictions. It also describes the forming technologies that can be adopted for use in ultra-thin materials. In Chapter 2, the materials investigated in the present thesis are summarized to show them all in a single view. The materials are labeled according to their sheet thickness and the year they were produced. In Chapter 3, a robust test method to determine FLD of ultra-thin FSS sheets is proposed based on rigorous experiments, and a theoretical FLD prediction is provided. The FLDs of two FSS sheets of thicknesses 1 and 0.1 mm were determined experimentally. For the 0.1 mm sheet, the modified Marciniak test and the conventional ASTM standard test were used for the FLD determination. The advantages of the Marciniak test compared with the ASTM standard test for assessing ultra-thin FSS sheets are described. In this section, the FLD was also predicted theoretically using a modification of the Parmar-Mellor-Chakrabarty (PMC) model, which incorporates the effects of surface roughness. A non-quadratic anisotropic yield function, YLD2000-2d, was implemented in this model to represent the anisotropy of the sheet metals. The FLDs predicted with the conventional Marciniak-Kuczyński (M-K) and the modified PMC models were compared with the FLDs determined experimentally. The FLD calculated with this modified model was in better agreement with the measured data than that computed with the M-K model for both the thin and the thick sheets. In Chapter 4, the experimental and finite element (FE) simulation results for forming an ultra-thin FSS sheet using servo-press technology are given. Forming experiments on a 0.15 mm thick FSS sheet sample were conducted using a direct-drive digital servo-press. Four different slide motions available in the servo-press were proposed: V-shaped (V), holding (H), W-shaped (W) and oscillating (O) motions. The effects of the type of slide motion on the micro-channel depth and shape accuracy were investigated. In addition to these experiments, FE simulations of forming for the ultra-thin FSS sheets were performed. The FE simulation results validated the influence of the slide motion. In particular, the proposed FE model successfully predicted the maximum thinning as well as the thickness profile across the micro-channel of the BP. In Chapter 5, the two-stage micro-channel forming results are presented. The two-stage micro-channel forming experiments were performed using 0.1 and 0.075 mm ultra-thin FSS sheets. A forming depth at the first forming stage was chosen as the process variable, and its effect on the formability of the micro-channel at the second forming stage was experimentally investigated. In addition to these experiments, FE simulations for the two-stage forming process were conducted to optimize the punch radius and forming depth at the first forming stage for improving the formability. The comparative study between the FE simulations and the experimental results could validate improvements in the formability by using the two-stage forming approach. This work could also support the existence of an optimum forming depth at the first forming stage. Based on the FE simulation results, mathematical modeling was used to identify the dominant factor needed for formability improvements and to propose a methodology for the process optimization of the particular multi-stage forming. Chapter 6 describes the optimization of the two-stage forming using a mathematical and statistical algorithm, central composite design (CCD). Three factors, the punch radius, die radius, and forming depth, were optimized by CCD in the first stage. The optimized results from Chapter 6 are consistent with the results presented in Chapter 5.
목차 moremore
ABSTRACT I
LIST OF TABLES V
LIST OF FIGURES VII
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ABSTRACT I
LIST OF TABLES V
LIST OF FIGURES VII
ABBREVIATIONS XIV
CONTENTS XVI

CHAPTER 1. INTRODUCTION 1
1. Introduction 2
1.1. Fuel Cell 2
1.2. FLD 5
1.2.1. Experimental FLD determination 7
1.2.2. Theoretical FLD prediction 12
1.3. Servo-Press Technology 15
1.4. Multi-Stage Forming Technology 23
1.5. Response Surface Methodology 25
1.6. Objectives of Current Study 27

CHAPTER 2. MATERIALS 28
2. Materials 29
2.1. Basic Material Properties 30

CHAPTER 3. THE FORMING LIMIT DIAGRAM OF FERRITIC STIANLESS STEEL SHEETS: EXPERIMENTS AND MODELING 32
3. The Forming Limit Diagram of Ferritic Stainless Steel Sheets: Experiments and Modeling 33
3.1. Experimental Procedures 34
3.1.1. Grain size measurement and surface roughness analysis 34
3.1.2. FLD determination 35
3.1.3. Results of FLD determination and discussions 40
3.2. Theoretical Prediction 46
3.2.1. M-K model 46
3.2.2. PMC model 49
3.2.3. Comparison of the predicted results with the experimental results 52
3.3. Conclusions 60

CHAPTER 4. FORMING AN ULTRA-THIN FERRITIC STAINLESS STEEL BIPOLAR PLATE OF PEMFC: DIGITAL SERVO-PRESS TECHNOLOGY 62
4. Forming an Ultra-thin Ferritic Stainless Steel Bipolar Plate of PEMFC: Digital Servo-Press Technology 63
4.1. Experimental Procedures 64
4.1.1. Uniaxial tension test 64
4.1.2. Experimental setup 64
4.1.3. Slide motions 71
4.1.4. Observation of cross-section 74
4.2. FE Simulation Setup 75
4.2.1. General FE model 76
4.2.2. FE Model for slide motion 79
4.2.3. FE Model for twist springback 79
4.3. Results and Discussion 85
4.3.1. SR uniaxial tension test 85
4.3.2. Micro-channel cross-section 87
4.3.3. FE simulation results 96
4.4. Role of Slide Motions 104
4.4.1. SR modeling 105
4.4.2. H motion 112
4.4.3. O motion 114
4.4.5. Comparison of slide motions 116
4.5. Conclusions 118

CHAPTER 5. DESING OF OPTIMUM CONDITIONS IN TWO-STAGE FORMING OF ULTRA-THIN FERRITIC STAINLESS STEEL SHEETS FOR BIPOLAR PLATE OF PEMFC 120
5. Design of Optimum Conditions in Two-Stage Forming of Ultra-Thin Ferritic Stainless Steel Sheets for Bipolar Plate of PEMFC 121
5.1. Experimental Procedures 122
5.1.1. Experimental setup 122
5.1.2. A main design parameter 125
5.2. FE Analysis 127
5.2.1. FE model 127
5.2.2. Optimization of process variables 128
5.3. Results 130
5.3.1. Formability enhancement using the two-stage forming approach 130
5.3.2. Coulomb’s friction coefficient for the FE analysis 133
5.3.3. Optimization of process variables by FE analysis 133
5.3.4. Validation of FE simulation results 136
5.4. Discussion 138
5.4.1. Formability improvement in the two-stage forming approach 140
5.4.2. Thinning balance between the punch and die corners 143
5.4.3. Factors affecting the thinning balance 145
5.4.4. Determination of optimum pre-forming depth 151
5.4.5. Possible optimization strategy 158
5.5. Conclusions 161

CHAPTER 6. APPLICATION OF CENTRAL COMPOSITE DESIGN FOR TWO-STAGE FORMING PROCESS OPTIMIZATION 163
6. Application of Central Composite Design for Two-Stage Forming Process Optimization 164
6.1. Two-Stage Forming Processes 165
6.1.1. Design of the FE simulation 165
6.1.2. Design of the FE simulation 169
6.2. Results and Discussion 171
6.2.1. Statistical analysis and model fitting 171
6.2.2. Model validation 174
6.2.3. Visualization of model equations 177
6.2.4. Optimization 180
6.3. Conclusions 190

CHAPTER 7. CONCLUDING REMARKS AND FUTURE WORKS 192
7. Concluding Remarks and Future Works 193
7.1. Concluding Remarks 193
7.2. Future Works 196

APPENDIX A 205
APPENDIX B 206
REFERENCES 207
요약문 (SUMMARY IN KOREAN) 220
ACKNOWLEGEMENT 223
PUBLICATIONS 225