Determination of. Metal Quality of. Aluminium and Its Alloys - PDF

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Determination of Metal Quality of Aluminium and Its Alloys by DERYA DISPINAR A thesis submitted to The University of Birmingham For the degree of DOCTOR OF PHILOSOPHY School of Metallurgy and Materials
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Determination of Metal Quality of Aluminium and Its Alloys by DERYA DISPINAR A thesis submitted to The University of Birmingham For the degree of DOCTOR OF PHILOSOPHY School of Metallurgy and Materials The University of Birmingham January 2005 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. Abstract Aluminium alloy castings are being used increasingly in safety-critical applications in the automotive and aerospace industries. To produce castings of sufficient quality, it is, therefore, important to understand the mechanisms of the formation of defects in aluminium melts, and important to have a reliable and simple means of detection. During the production of aluminium ingots and castings, the surface oxide on the liquid is folded in to produce crack-like defects (bifilms) that are extremely thin, but can be extremely extensive, and so constitute seriously detrimental defects. However, the presence of bifilms has not been widely accepted, because there has been no single metal quality test that has been able to resolve features that are only nanometres, or sometimes micrometres, in thickness. In the past, porosity has usually been held solely responsible for most failures in aluminium alloys, and hydrogen has been blamed as the actual cause. In this work it has been found that bifilms are the initiator and hydrogen is only a contributor in the porosity formation process. For the first time, evidence is presented for the contribution of air (or perhaps more strictly, residual nitrogen from air) as an additional gas, adding to hydrogen in pores in cast Al alloys. The Reduced Pressure Test (RPT) is used which is a simple and widely known test, is cost effective and involves no complicated equipment or consumables, thus recommending it for implementation on the foundry floor. Nevertheless, the discriminating use of the RPT clearly reveals the existence of bifilms, and the effect of hydrogen on porosity formation. On this basis, several Al-Si based alloys were studied: LM0 (99.5% Al), LM2 (Al-11Si), LM4 (Al-5Si-3Cu), LM25 (Al-7Si-0.4Mg), LM24 (Al-8Si-3CuFe), LM27 (Al-6Si-2Cu). A quality index -Bifilm Index- is introduced to quantify the results of the reduced pressure test which helps to asses the aluminium melt quality in best means. In addition, mechanical tests were carried out to correlate bifilm index with mechanical properties. The oxide content of recycled aluminium alloys has been a long-standing and serious problem. This thesis reports the use of the RPT test developed in this study to an industrial remelting facility that has resulted in significant benefit. Acknowledgement It is the greatest honour to be able to work under supervision of Professor John Campbell. I am grateful to be one of the luckiest persons who had a chance to work with him. I am thankful and gratified for all of his help, assistance, inspiration and guidance on the all aspects. I would like to thank Professor Nick R. Green for his support and supervision. I thank the University of Istanbul, Metallurgy and Materials Engineering, Head of Department, Prof. Dr. Ibrahim Yusufoglu, for his encouragement to help me begin my postgraduate study in University of Birmingham. Thanks to all of my colleagues, the research assistants in the department, particularly Dr. Cem Kahruman in Istanbul. I would have to acknowledge Higher Education Council of Turkey, for supporting my research. I would like to acknowledge the financial support of Norton Aluminium and particularly their assistance in the use of facilities in the foundry. The research was also partially supported by University of Birmingham. I would like to thank Adrian Caden for his assistance during the experiments and rest of the IRC group for their warm and kind friendship. Although we could not finalise this project together, I would like to thank to Dr. Simon Fox for his effort and support on the start of my thesis. I would like to express my gratitude to Nazım Karadağ for his commitment and continued support as my best friend. My mom and dad, and my whole family; there are no words to express my feelings for your never-ending support and belief in me. Above all, I would like to express my deepest gratitude to my wife and best friend Muzi, for her love and support and everything during this work and ever. TABLE of CONTENTS CHAPTER 1 INTRODUCTION...1 CHAPTER 2 LITERATURE REVIEW History of Casting and Aluminium Casting Processes Metal Quality The Dissolution of Hydrogen Formation of Oxide Film The Concept of Surface Entrainment: Bifilms Incorporation of Surface Films into Melts Compacting and Unfurling (Ravelling and Unravelling) of Bifilms Porosity Phenomena Nucleation Growth The New Approach to Pore Formation Porosity Types Shrinkage Porosity Gas Porosity Factors Affecting Porosity An Inrease in the Hydrogen Content of the Melt An Increase in the Entrained (exogenous) Inclusion Content Oxide Structure Alloying Cooling Rate Intermetallics Measurement of Metal Quality Inclusion Detection Tecniques LIMCA (Liquid Metal Cleanliness Analyser) PoDFA PREFIL (Porous Disk Filtration Analysis) Ultrasonic Summary of Quality Assessment Tecniques Reduced Pressure Test (RPT) Secondary Remelting - Recycling Melting Rotary Furnace Tilt Rotary Furnace Electric Furnace Fluxing Degassing...32 Summary...33 CHAPTER 3 EXPERIMENTAL PROCEDURE Alloys Reduced Pressure Test (RPT) Laboratory Tests Industrial Tests Hydrogen Measurement Moulds Density Measurement Image Analysis Tensile Tests Weibull Analysis Microscopy and Microanalysis Quality Index Studies...43 CHAPTER 4 RESULTS Laboratory Tests Effect of Binder Type and Content on RPT RPT Optimisation of Temperature and Pressure The Assessment of Effect of Bifilms on RPT Industrial Trials Fluxing and Degassing Studies Studies at Casting Area: Two Different Casting Heights Studies in Holding Furnace: Trials on use of Diffusers Quality Index Results and Evaluation of Bifilm Index Methods Investigated to Quantify RPT Laboratory Test Results Industrial Test Results Relationship between Bifilm Index and Number of Pores Calculation of Air Gap between Bifilm Tensile Tests Optimisation of Tensile Test Pattern Comparison of Different Conditions...57 CHAPTER 5 DISCUSSION The RPT Mould Effect Study Pore Formation and Growth...59 5.3 Effect of Time and Temperature on Pore Morphology Effect of Different Pouring Height Effect of Different Gas Flow Rates during Fluxing and Degassing Comparison of Effect of Ceramic Diffusers During Fluxing Evaluation of Bifilm Index The Relationship between Number of Pores, Hydrogen and Bifilm Index Image Analysis Studies Optimum Pressure Considerations Effect of Bifilms on Mechanical Properties Parameters Affecting the RPT Sensitivity Sample Pouring Temperature Microstructure Chamber Pressure Shrinkage Melt Gas Content...85 CHAPTER 6 CONCLUSIONS...86 FUTURE WORK...88 PUBLICATIONS...89 REFERENCES...91 APPENDICES I Calculation of Theoretical Hydrogen-Density Relationship II Comparison of Image Analysis Software III Calculation of Air Gap between Bifilm FIGURES TABLES...192 List of Illustrations Figure 2.1: Surface turbulence; probably the most common mechanism of introducing bifilm into the melt: The folding of the film, dry side to dry side, will trap gas between the surfaces[5] Figure 2.2: Solubility of hydrogen in pure aluminium [34] 115 Figure 2.3: (a) Supersaturation of hydrogen in the absence of bifilm (b) pore formation and diffusion of hydrogen into the gap between the bifilm Figure 2.4: Oxygen partial pressure and oxide structure[42] Figure 2.5: the effect of increasing height on a falling stream of liquid [5]; 116 a) the oxide film remains intact, b) the oxide film being detached and accumulating to form a dross ring, c) the oxide film and air being entrained in the bulk melt Figure 2.6: confluence geometry: separation and rejoining involves the formation of films [5] a) at the side, b) randomly, c) on the top 116 Figure 2.7: Energies of a growing pore [5] Figure 2.8: Geometry of a bubble [5]: (a) wetting and contact angle between liquid and solid (b) energy association between homogeneous and heterogeneous nucleation 117 Figure 2.9: Feeding mechanisms in a solidifying casting [5] Figure 2.10: Volumetric shrinkage in long freezing range alloy depending on the thickness of casting [5] Figure 2.11: Typical pore microstructure in an Al-Si alloy (a) interdendritic [172], (b) intereutectic [174] Figure 2.12: Porosity morphology associated with the hydrogen level of the melt [88] (a) High hydrogen content: diffusion into bifilms starts in liquid (b) High-medium hydrogen content: high liquid fraction (c) Medium-low hydrogen content: growing in interdendritic structure (d) Low hydrogen content: not enough driving force to expand the bifilms Figure 2.13: Schematic demonstration of entrainment of an inclusion [5] 119 Figure 2.14: Effects of alloying elements on hydrogen solubility in liquid Al at 973K, 1 atm partial pressure of hydrogen [36] 120 Figure 2.15: The relation between porosity, hydrogen and cooling rate[80] 120 Figure 2.16: Porosity formation and intermetallics 121 a) nucleating on bifilm [174], b) nucleation on oxides [211], c) shrinkage and β-phase[215], d) β-phase on oxide [105] Figure 2.17: Schematic representation of LiMCA [220] Figure 2.18: Schematic representation of PoDFA test [220] Figure 2.19: Schematic diagram of ultrasonic technique Figure 2.20: Schematic RPT pparatus 123 Figure 2.21: bifilms and porosity formation [5] Figure 2.22: Schematic section of a 5t capacity Rotary Furnace (Size: 3 to 14 m 3 ) 123 Figure 2.23: Schematic drawing of Tilt Rotary Furnace (Size: 3 to 14 m 3 ) Figure 2.24: 2.5t capacity Induction Furnace. 124 Figure 2.25: different type of degassers (illustrated by air in water) bubble size and distribution in a) lance, b) rotary degassing head [236] Figure 2.26: different type of diffusers (illustrated by the use of air bubbles in water) stationary ceramic diffusers a) disc-type, b) T-type [237] 125 Figure 3.1: (a) RPT machine and (b) schematic circuit drawing 126 Figure 3.2: a schematic representation of aluminium ingot production 126 Figure 3.3: schematically shown surface turbulence during fluxing and degassing 127 Figure 3.4: Dimensions of the holding furnace and location of diffusers Figure 3.5: Schematic illustration of changes made at the casting trials (a) - 1 The launder was lowered to be as close to the casting mould possible - 2 The casting and filling speed were decreased (b) - 3 More care was taken to avoid the severe turbulence on tapping Figure 3.6: Sand moulds that were used in RPT 129 a) resin binder, b) sodium silicate + coal dust mixture; and metal samples cast in the moulds c) an outer view (50x35x15 mm); d) a section through the centre. Figure 3.7: Pattern design: (a) core box, (b) dimensions 130 Figure 3.8: a) sand particles adhered on surface of RPT sample, b) better surface finish with addition of coal dust 130 Figure 3.9: Schematic diagram of tensile test piece mould showing the dimensions of the optimised mould Figure 3.10: Tensile test bar dimensions used for mechanical tests Figure 3.11: Real time X-Ray studies with different gating designs; entrapment of bubbles and severe turbulence was observed (dark regions are gaps and white regions are liquid metal in the marked points). 132 Figure 3.12: Schematic illustration of mould fillings taken from real time X-Ray studies(figure 3.11) Left row: Different patterns, Right Row: same pattern as left row but with filters placed under sprue (dark regions are liquid metal and white regions are air entrapment and gap) Figure 4.1: RPT sample density change with hydrogen (solidified at 100 mbar alloy LM4) Figure 4.2: density index change at 100 mbar alloy LM Figure 4.3: RPT sample density change with hydrogen (solidified at 10 mbar alloy LM4). 135 Figure 4.4: density index change at 10 mbar alloy LM Figure 4.5: density index variation at different pressures alloy LM Figure 4.6: Sectioned surface of the reduced pressure test samples cast at moulds with different binder contents (alloy LM4) Figure 4.7: theoretical density-hydrogen relationship at different vacuum levels 138 Figure 4.8: comparison of theoretical relationship of density change with hydrogen from RPT samples obtained at different vacuum levels alloy LM4 (a)1000 mbar, (b)200 mbar, (c)100 mbar, (d)50 mbar, (e)10 mbar Figure 4.9: different morphology and size of pores in RPT sample at different vacuum levels and casting temperatures Figure 4.10: schematic bursting of bubbles from the surface Figure 4.11: Examples of bursting shown from sectioned surfaces of the reduced pressure test samples at 10 mbar 142 Figure 4.12: SEM images of inside the pores of RPT samples (LM4) 142 Figure 4.13: EDS analysis of roughened areas shown in Figure (a) Some thick oxides gave only Al and O peaks (b) In some cases Mg, Si and Cu phases under the thin oxide were detected Figure 4.14: density-hydrogen relationship at different vacuum levels; LM0 melt at 750 o C 145 Figure 4.15: Sectioned surfaces of 99% Al (LM0) samples cast at 750 o C: clean and unclean melts (200 mbar, 100 mbar, 50 mbar) 146 Figure 4.16: SEM pictures of LM0: oxide covered surfaces inside the pores (a) young oxides, (b) old oxides Figure 4.17: Alloy LM27: degassing studies with high gas flow rate and low gas flow rate (50 and 100 mbar) Figure 4.18: fluxing and degassing with different diffusors (alloy LM4-50 and 100 mbar) 147 Figure 4.19: The predicted bubble distribution with different diffusors 147 a) lance: coarse and few bubbles, b) disc type ceramic: finer and more bubbles, c) T-type ceramic: finer and evenly distributed Figure 4.20: density-hydrogen relationship at different casting levels (LM24); a) 50 mbar, b) 100 mbar Figure 4.21: histogram of density of the RPT samples (LM24) (a) 50 mbar, (b) 100 mbar Figure 4.22: the effect of casting height and bifilms on the pore morphology (LM24) Figure 4.23: SEM images inside the pore (LM24). 150 a) bottom filled: fragments of a bifilm in between dendrites, b) top poured: close up of an internal crumpled oxide Figure 4.24: The density change of the reduced pressure test samples cast at different heights (alloy LM mbar) 151 Figure 4.25: The density histogram of the reduced pressure test samples cast at different heights (alloy LM25). 151 Figure 4.26: Sectioned surface of the reduced pressure test samples cast at different heights (alloy LM25; RPT 100 mbar) 151 Figure 4.27: Density change of RPT samples (100 mbar) from start, mid to end when running:(a) 0 diffuser, (b) 1 diffuser, (c) 2 diffusers 152 Figure 4.28: Density of RPT samples comparing different number of diffusers (100 mbar) Figure 4.29: The density of RPT samples comparing the different techniques (100 mbar) 153 (a) 0 diffuser, (b) 1 diffuser, (c) 2 diffusers (SET 1: non-quiescent conditions, SET 2: quiescent conditions) Figure 4.30: X-RAY images of some of RPT samples 154 Figure 4.31: Schematic illustration of sectioning of RPT samples. 155 Figure 4.32: shape factor change with the hydrogen level (LM4: RPT at 200 mbar). 155 Figure 4.33: shape factor change with the hydrogen level (LM4: RPT at 100 mbar). 155 Figure 4.34: shape factor change with the hydrogen level (LM4: RPT at 50 mbar) 156 Figure 4.35: shape factor change with the hydrogen level (LM4: RPT at 10 mbar) 156 Figure 4.36: shape factor change with temperature at different vacuum levels (LM4) Figure 4.37: average pore area with temperature and pressure (LM4). 157 Figure 4.38: porosity % versus temperature and pressure (LM4) Figure 4.39: Porosity % versus density and pressure (LM4: at all temperatures) Figure 4.40: relationship between porosity %, hydrogen and pressure (LM4: at all temperatures) Figure 4.41: relationship between average pore area with hydrogen and pressure (LM4: at all temperatures) Figure 4.42: average pore area change with density (LM4: at all temperatures) 159 Figure 4.43: alloy LM4: (a) shape factor change with density at different vacuum levels (b) shape factor distribution 160 Figure 4.44: quality index (QI 2 ) change with the RPT density (Alloy LM4) (a: QI 2 =5, density=2600 kg/m 3, b: QI 2 =1, density=2700 kg/m 3 ) 161 Figure 4.45: quality index (QI 2 ) change with the density (Alloy LM24); Comparison of different castings heights (a) 100 mbar, (b) 50 mbar. 161 Figure 4.46: Concept of QI 3 change with density. 162 Figure 4.47: QI 3 change with density - data from experiments (LM4) Figure 4.48: Bifilm index versus density at different vacuum levels (LM4) (all temperatures) Figure 4.49: Bifilm index versus density at different vacuum levels (LM4) (a) 700 o C, (b) 800 o C, (c) 900 o C Figure 4.50: Bifilm index versus hydrogen at different vacuum levels (LM4) (a) 700 o C, (b) 800 o C, (c) 900 o C. 164 Figure 4.51: Bifilm index versus hydrogen at different vacuum levels (LM4) (all data from Figure 4.50 superimposed here on the same graph). 164 Figure 4.52: Bifilm index versus density as a function of casting height; LM24 (a) 100 mbar, (b) 50 mbar Figure 4.53: Weibull distribution change of bifilm index of different casting heights (LM24). 165 Figure 4.54: Bifilm index versus density; LM25 (100 mbar) 166 Figure 4.55: Weibull distribution change of bifilm index of different casting heights (LM mbar) Figure 4.56: Bifilm index versus density (LM mbar). 167 Figure 4.57: Bifilm index (determined at 100 mbar) change from start, mid to end Comparing different number of diffusers (LM24) Figure 4.58: Comparison of bifilm index change (determined at pressure 100 mbar) between non-quiescent (SET 1) and quiescent conditions (SET 2) (a) 0 diffuser, (b) 1 diffuser, (c) 2 diffusers Figure 4.59: Weibull distribution change of bifilm index of different casting conditions. 169 (LM mbar) (a) Non-quiescent conditions (SET 1) (b) Quiescent conditions (SET 2) Figure 4.60: The relationship between the number of pores and bifilm index (determined at pressure 100 mbar) 170 Figure 4.61: Schematic illustration of sectioned surface of RPT samples illustrating the relationship between the number of pores and bifilm index Figure 4.62: The relationship between bifilm index (determined at 100 mbar) and hydrogen content of the melt 171 Figure 4.63: Change in the average air gap between bifilms with temperature Figure 4.64: Comparison of Weibull distribution of tensile properties developed under conditions of quiescent and non-quiescent fillings: Alloy LM27 (a) UTS, (b) elongation Figure 4.65: Comparison of Weibull distribution of mechanical test results of bars obtained by machining of ingots and by re-melting and casting into test bars, Alloy LM4. (a) The Weibull plot of UTS and (b) the Weibull plot of elongation e 173 Figure 4.66: Sectioned surface of RPT samples alloy LM2 (100 mbar) (a) High Mg, (b) Low Mg. 173 Figure 4.67: Alloy LM24 Weibull distribution change of two different casting heights showing (a) UTS, (b) elongation 174 Figure 4.68: Alloy LM2 (low Mg) Weibull distribution change of two different casting heights showing (a) UTS, (b) elongation Figure 4.69: Alloy LM2 (high Mg) Weibull distribution change of two different casting heights showing (a) UTS, (b) elongation Figure 4.70: Alloy LM2 (Bottom Filled): Weibull distribution change of different Mg content (a) UTS, (b) elongation values. 177 Figure 4.71: Alloy LM2 (Top poured): Weibull distribution change of different Mg content (a) UTS, (b) elongation values 178 Figure 4.72: All
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