UNIVERSITE DU QUEBEC MEMOIRE PRESENTE A L'UNIVERSITÉ DU QUÉBEC À CHICOUTIMI COMME EXIGENCE PARTIELLE DE LA MAÎTRISE EN INGÉNIERIE PAR LIN ZHANG

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1 UNIVERSITE DU QUEBEC MEMOIRE PRESENTE A L'UNIVERSITÉ DU QUÉBEC À CHICOUTIMI COMME EXIGENCE PARTIELLE DE LA MAÎTRISE EN INGÉNIERIE PAR LIN ZHANG A KINETIC STUDY OF HYDROGEN ABSORPTION AND DEGASSING BEHAVIOUR OF DURALCAN COMPOSITES NOVEMBRE 1996

2 bibliothèque Paul-Emile-Bouletj UIUQAC Mise en garde/advie Afin de rendre aessible au plus grand nombre le résultat des travaux de reherhe menés par ses étudiants gradués et dans l'esprit des règles qui régissent le dépôt et la diffusion des mémoires et thèses produits dans ette Institution, l'université du Québe à Chioutimi (UQAC) est fière de rendre aessible une version omplète et gratuite de ette œuvre. Motivated by a desire to make the results of its graduate students' researh aessible to all, and in aordane with the rules governing the aeptation and diffusion of dissertations and theses in this Institution, the Université du Québe à Chioutimi (UQAC) is proud to make a omplete version of this work available at no ost to the reader. L'auteur onserve néanmoins la propriété du droit d'auteur qui protège e mémoire ou ette thèse. Ni le mémoire ou la thèse ni des extraits substantiels de eux-i ne peuvent être imprimés ou autrement reproduits sans son autorisation. The author retains ownership of the opyright of this dissertation or thesis. Neither the dissertation or thesis, nor substantial extrats from it, may be printed or otherwise reprodued without the author's permission.

3 RESUME Ce travail s'insrit dans le adre d'un programme de reherhe visant à étudier l'impat de l'hydrogène dans les alliages d'aluminium et à mesurer par différents méthodes la solubilité de e gaz dans le métal liquide. L'objet de e mémoire de maîtrise porte partiulièrement sur la inétique de dégazage et d'absorption de l'hydrogène dans deux omposites de type DURALCAN MD, produits par une usine pilote de la ompagnie ALCAN: l'un pouvant être mis en forme par filage [W6D.22A ou 6061/Al 2 O 3(p) ] et l'autre développé spéialement pour les appliations en fonderie [F3S.20S ou Al-Si-Mg/SiC {p) ]. L'objetif prinipal est d'étudier l'effet des partiules de renfort (A1 2 O 3, SiC) sur la inétique et le méanisme d'absorption de l'hydrogène dans es matériaux hétérogènes. Il s'agit d'un travail original et d'intérêt tant sur le plan sientifique qu'industriel. Les variations des teneurs en hydrogène en fontion du temps dans les omposites susmentionnées sont évaluées dans différentes onditions expérimentales favorisant soit l'absorption ou le dégazage de e gaz. Un appareil de type "Alsan" est utilisé pour évaluer le dosage d'hydrogène. Pour fins de omparaison, les mêmes expérienes sont réalisées parallèlement ave les alliages onstituant les matries de es deux omposites. Les résultats expérimentaux sont présentés sous forme de ourbes montrant l'évolution de la teneur en hydrogène en fontion du temps. L'analyse de es données à l'aide d'un modèle mathématique simplifié permet de mettre en évidene et d'évaluer les paramètres inétiques des proessus d'absorption et de dégazage des deux omposites et de leur matries. Les paramètres ainsi déterminés sont nettement différents d'un omposite à l'autre; ils montrent notamment que la vitesse d'absorption d'hydrogène dans le omposite ontenant des partiules d'alumine est plus grande que dans le as où le arbure de siliium onstitue les éléments de renfort. Ces résultats disutés en onsidérant le rôle de la ouhe d'oxyde qui flotte sur le métal liquide et la omposition des alliages onstituant les matries des deux omposites. L'effet des onditions environnementales sur la inétique d'absorption d'hydrogène est étudié. On observe que la teneur d'hydrogène dissous dans le liquide atteint une valeur d'équilibre qui est fontion de la pression partielle de la vapeur d'eau dans l'atmosphère; la relation [H] - p HO obéit à la loi de Sieverts. La vitesse d'absorption d'hydrogène est évidemment fontion du taux d'humidité dans l'air, prinipale variable et fore motrie du proessus; ependant, les oeffiients de transfert de masse développés dans le modèle mathématique demeurent les mêmes pour un omposite ou un alliage donné, peu importe les onditions environnementales.

4 Le temps néessaire pour absorber l'hydrogène jusqu'à la moitié de la valeur d'équilibre est évalué dans différentes onditions expérimentales. Ces aluls montrent que e temps est de l'ordre d'une demi-heure et qu'il est plus ourt pour les omposites que leur matries orrespondantes en alliage d'aluminium. L'effet du rapport surfae libre (A) de métal fondu sur son volume (V) fait également l'objet d'expérienes. Les résultats montrent que le taux d'absorption de l'hydrogène est diretement proportionnel au rapport (A/V); on doit don en onlure que le proessus est ontrôlé par les réations et la diffusion aux interfaes ave la ouhe d'oxyde qui se forme au-dessus du matériau liquide. 11

5 ABSTRACT The kineti behaviour of hydrogen absorption and degassing has been experimentally investigated by aquiring hydrogen onentration-time profiles with Alsan apparatus in molten DURALCAN omposites, W6D.22A (6061/Al 2 O 3(p) ) from wrought alloy family and F3S.20S (Al-Si-Mg/SiC (p) ) from foundry alloy family, and their matrix alloys under ertain experimental onditions. This work is a part of the projet on hydrogen absorption and solubility in molten aluminum alloys and aluminum matrix omposites. The main objetive of the present work is to study the effet of the reinforing partiles on hydrogen absorption behaviour in these materials. The kineti parameters of the proesses of hydrogen absorption and degassing for the omposites and their matries have been evaluated with the experimental data aording to the mathematial models developed on the basis of the mathematial analysis of these proesses. The effet of the reinforing erami partiles in the omposites on the kinetis of hydrogen absorption and degassing has been studied by omparing the kineti parameters of the omposites to those of their matries. It has been found that the erami partiles an inrease the rate of hydrogen absorption and the effet of A1 2 O 3 partiles is stronger than that of SiC partiles. The mehanism of the effet of these erami partiles has been disussed. The effet of the environmental onditions on the rate of hydrogen absorption and degassing has been studied. It was found that the equilibrium value of hydrogen onentration in the proess of hydrogen absorption inreases with inreasing water partial pressure in the atmosphere and their relationship obeys Sieverts' law. The environmental ondition an influene the rate of hydrogen absorption by hanging the value of the driving fore of the proess, while the rate onstant, or the mass transfer oeffiient, for hydrogen absorption was found to be independent of the atmospheri onditions. The rate onstant for degassing is also independent of the atmospheri onditions above the melt during degassing. A half-reover time of hydrogen absorption has been evaluated for the omposites and their matries. The results show that the rate of hydrogen absorption is higher for the omposites than that of their matries and the time needed to absorb hydrogen to a half of the equilibrium level is shorter for the omposites than the matries. Finally, the effet of the ratio of melt surfae area, A, to its volume, V, has been evaluated by the experiments with various values of A/V ratio. The results indiate that the rate of hydrogen absorption will be higher if the area of the melt surfae is inreased with respet to a same volume of metal.

6 ACKNOWLEDGEMENT First of all I would like to thank my diretor Dr. Mihel Bouhard and o-diretor Dr. Hua Liu, professors with Université du Québe à Chioutimi, for their supervision for the entire work of this projet. I would like to aknowledge Dr. Mihriban O. Pekguleryuz, diretor of Alan-UQAC Chair in Solidifiation and Metallurgy of Aluminum, for her ontinuous support on this projet. I also thank Mr. Celil Aliravi and Dr. Dariyoush Emadi for their helpful advie and disussions. I would like to thank Mr. Gilles Lemire and also Mr. Régis Bouher for their help in the laboratory to prepare the neessary setups for the experiments. The Arvida Researh and Development Center of Alan International Ltd. should be gratefully aknowledged for their ritial evaluation of this work and useful advie. The Dubu Plant of Alan Ltd. and Bomem In. should also be aknowledged for supplying the materials. Finally, I would like to thank all the persons who have made their ontributions, in one way or another, to this work as well as my family and my friends.

7 TABLE OF CONTENTS CHAPTER 1 INTRODUCTION A DESCRIPTION OF THE PROBLEM OBJECTIVE METHODOLOGY 5 CHAPTER 2 LITERATURE REVIEW HYDROGEN IN ALUMINUM Solubility The Soures of Hydrogen during Melting and Casting Hydrogen-Generated Defets Porosity in Aluminum Castings Effets on Mehanial Properties DEGASSING ALUMINUM MELTS Introdution Mehanisms of Hydrogen Elimination An Overview of the Methods of Degassing Degassing Effiieny and Determining Fators HYDROGEN ABSORPTION IN ALUMINUM MELTS Introdution Adsorption of Hydrogen on Melt Surfaes Diffusion of Hydrogen in Liquid Metals Mehanism of Hydrogen Absorption Method of Avoiding Hydrogen Absorption 43 CHAPTER 3 EXPERIMENTAL PROCEDURE MATERIALS The Seletion of Materials Desription of Materials DESCRIPTION OF EXPERIMENTAL SETUP The Furnae and the Cruible The Stirrer and the Protetion Gas DEGASSING AND RE-ABSORPTION Melting of the Sample Degassing Aluminum Melt 53

8 3.3.3 Hydrogen Re-absorption of the Melt Measurement of Humidity Level EFFECT OF SURFACE-TO-VOLUME RATIO ON HYDROGEN ABSORPTION MEASUREMENT OF HYDROGEN CONTENT Introdution Theory of Operation A Desription of Alsan Analyzer The Probe The Reirulation Loop Measurement Proedure 65 CHAPTER 4 APPROACH TO MATHEMATICAL ANALYSIS OF DEGASSING AND ABSORPTION INTRODUCTION INTERPHASE MASS TRANSFER Mixing The Mass Transfer Coeffiient Mass Transfer in a Two-phase System Effet of the Ratio of Surfae Area to Melt Volume MATHEMATICAL MODELS FOR HYDROGEN ABSORPTION AND DEGASSING Melt-Side and Gas-Side Mass Transfer Resistanes Pik-up of Hydrogen from Water Vapour Mathematial Models for Hydrogen Absorption and Degassing DATA PROCESSING Curve Fitting with the Least-Square Method A Desription of the Program 91 CHAPTER 5 RESULTS AND DISCUSSIONS EXPERIMENTAL RESULTS Charateristis of the Variation of Hydrogen Conentration The Kineti Parameters ANALYSES AND DISCUSSIONS Rates of Degassing and Hydrogen Absorption Effets of the Reinforing Partiles Effets of the Atmospheri Conditions The Half-Reover Period of Hydrogen Absorption Effet of the Ratio of Melt Surfae Area to its Volume SOURCES OF ERROR 139 CONCLUSIONS 144 VI

9 REFERENCES 149 APPENDIX A LIST OF THE PROGRAM USED FOR DATA ANALYSIS 155 vu

10 LIST OF FIGURES Figure 1-1. Solubility of hydrogen at atmospheri pressure in aluminum 3 Figure 1-2. Figure 2-1. Porosity as a funtion of hydrogen ontent in sand ast aluminum and aluminum alloy bars 3 Hydrogen solubility in liquid pure aluminum at 700 C at various pressures 9 Figure 2-2. Interdendriti porosity in an ingot of 99.2 % Al 19 Figure 2-3. Figure 2-4. Ultimate tensile strength versus hydrogen porosity for sand ast bars of three aluminum alloys 21 Effet of ingot porosity on short-transverse tensile properties of 3-inh thik D.T.D. 5020A plate hot rolled from 11 in. x 44 in. D.C. ingot 22 Figure 2-5. Gas purging ontrol volume 34 Figure 2-6. Degassing effiieny (Z 0 ) 2 vs the dimensionless bubble ontat area Figure 2-7. Degassing effiieny as a funtion of purge gas bubble size for two melt 36 hydrogen onentrations 37 Figure 2-8. Degassing performane of a lane, porous plug, and an impeller 37 Figure 3-1. Figure 3-2. Photomirographs of two DURALCAN omposites, F3S.20S (above), and W6D.22A (below) 48 The ingots of DURALCAN omposites and their matries, a) omposites, and b) matries 49 Figure 3-3. A shemati showing the furnae and ruible used in the experiments Figure 3-4. The use of stirrer and nitrogen in the experiment 52

11 Figure 3-5. A shemati illustration of degassing and hydrogen re-absorption, (a) degassing, (b) re-absorption 56 Figure 3-6. A sketh of an Alsan probe 64 Figure 3-7. The reirulation loop of an Alsan analyzer 65 Figure 3-8. A shemati illustration of the funtion of Alsan analyzer 66 Figure 3-9. Alsan dynami urves for W6D.22A omposite and its matrix alloy 68 Figure 4-1. Figure 4-2. Figure 4-3. Veloities are low along the interfae so that the transfer of H atoms depends on the slow mehanism of diffusion. 73 A melt-gas system shows the onentrations Cj and j; in the melt and pressures p^ and p bj in the gas phase 76 Some of the steps involved in the transfer of H aross the aluminum meltgas interfae 84 Figure 4-4. Flow hart of the program 92 Figure 5-1. Figure 5-2. Figure 5-3. Figure 5-4. Figure 5-5. Figure 5-6. Figure 5-7. Alsan readings of hydrogen onentration of W6D.22A omposite during degassing and hydrogen re-absorption (Experiment No. WC1) 96 Alsan readings of hydrogen onentration of the matrix alloy of W6D.22A (Experiment No. WM3) 97 Alsan readings of hydrogen onentration of F3S.20S omposite during degassing and hydrogen re-absorption (Experiment No. FC3) 98 Alsan readings of hydrogen onentration of the matrix alloy of F3S.20S omposite (Experiment No. FM2) 99 The end yle Alsan readings of hydrogen onentration of W6D.22A omposite (Experiment No. WC1) 100 The end yle Alsan readings of hydrogen onentration of the matrix alloy of W6D.22A omposite (Experiment No. WM3) 100 The end yle Alsan readings of hydrogen onentration of F3S.20S omposite (Experiment No. FC3) 101 IX

12 Figure 5-8. Figure 5-9. Figure The end yle Alsan readings of hydrogen onentration of the matrix alloy of F3S.20S omposite (Experiment No. FM2) 101 The fitted urves for hydrogen absorption in W6D.22A omposite under different environmental onditions 103 The fitted urves for hydrogen absorption of the matrix alloy of W6D.22A under different environmental onditions 104 Figure The fitted urves for hydrogen absorption of F3S.20S omposite under different environmental onditions 105 Figure Figure Figure Figure The fitted urves for hydrogen absorption of the matrix alloy of F3S.20S under different environmental onditions 106 The fitted urves for degassing of W6D.22A omposite under different environmental onditions 108 The fitted urves for degassing of the matrix alloy of W6D.22A under different environmental onditions 109 The fitted urves for degassing of F3S.20S omposite under different environmental onditions 110 Figure The fitted urves for degassing of the matrix alloy of F3S.20S under different environmental onditions Ill Figure Figure Figure Figure The hydrogen absorption rate of W6D.22A and its matrix alloy at different times during the proess of absorption 117 The hydrogen absorption rate of F3S.20S omposite and its matrix alloy at different times during the proess of absorption 118 The rate of degassing for some of the experiments at different times during the degassing proess 120 A omparison of the rate onstant of hydrogen absorption, k a, for different materials (markers represent the mean values) 123 Figure A omparison of the degassing rate onstant, k d, for different materials (markers represent the mean values) 124 Figure A diagram showing that k a is onstant with the water partial pressure in the atmosphere 127 x

13 Figure Figure Figure Figure Figure A diagram showing that the k d values were onstant with the flow rate of N Relationship of the equilibrium hydrogen ontent of absorption and the water partial pressure in the atmosphere 130 Relationship between the equilibrium values of hydrogen ontent and water partial pressure for degassing 131 A graphial illustration of the half-reover period for hydrogen absorption of molten W6D.22A omposite and its matrix alloy 134 A graphial omparison of the half-reover period, t 1/2, for hydrogen absorption of the omposites and their matries 134 Figure A diagram showing the effet of A/V on the rate of hydrogen absorption. 138 Figure Figure Failure Alsan readings during hydrogen absorption of molten W6D.22A omposite 140 A omparison of the alulated values of hydrogen ontent to the expeted values at the beginning of degassing 141 XI

14 LIST OF TABLES Table 2-1 Temperature Dependene of Hydrogen Solubility in Liquid Pure Aluminum Table 2-2 Temperature Dependene of Hydrogen Solubility in Some Liquid Aluminum Alloys 13 Table 2-3 Interation Coeffiients in Liquid Al-H-i Alloys 16 Table 3-1 Chemial Compositions of the Matries of W6D.22A and F3S.20S 47 Table 3-2 A Summary of the Experimental Conditions 59 Table 3-3 Values of the Ratio of Melt Surfae Area to its Volume 60 Table 5-1 Evaluated Hydrogen Absorption Parameters 113 Table 5-2 Evaluated Degassing Parameters 114 Table 5-3 The Half-reover Period for the Composites and their Matries 135 Table 5-4 Results of k a from Experiments with various A/V values 137

15 LIST OF SYMBOLS A a ; B C Ç, Cj CF(A) CF(T) D d f G AG [H] A[H] [H] o [H] e AH 0 j K k, k M k a k d k t L M- N A N AX P n "atm area onstant defined in Eqn. (2.7) ativity of i in solution onstant defined in Eqn. (2.7) total molar onentration molar onentration of i alloy orretion fator temperature orretion fator diffusion oeffiient diameter ativity oeffiient of hydrogen flow rate of inert gas hange of free energy hydrogen ontent driving fore for hydrogen absorption or for degassing initial hydrogen ontent equilibrium hydrogen ontent hange in enthalpy molar flux equilibrium onstant mass transfer oeffiient hydrogen absorption rate onstant degassing rate onstant total mass transfer oeffiient harateristi vessel dimension moleular weight of i total molar flux of A molar flux of A in x-diretion molar flux pressure atmospheri pressure

16 PE gas " met p x shrink p surfae Pi Pinert R r r 2 S s, T t *l/2 V V Ax V v max V x* x, (Z 0 ) 2 total external pressure equilibrium hydrogen pressure inside a pore pressure of metal head pressure due to the ontration of the metal during solidifiation pressure assoiated to surfae tension partial pressure of i internal pressure inert gas partial pressure gas onstant rate oeffiient of regression hydrogen solubility in liquid aluminum standard deviation of the value of i temperature in Kelvin time half-reover period volume veloity of A in x-diretion maximum veloity loal molar average veloity in x-diretion mole fration of i in solution degassing effiieny Y ativity oeffiient of i «2 thikness of the melt boundary layer K B Boltzmann onstant A, ratio between the resistanes to diffusion in the gas phase and in the melt phase u visosity p seond order interation oeffiient density x m mixing time \]//[%H] dimensionless ontat area e first order interation oeffiient é energy xiv

17 CHAPTER 1 INTRODUCTION

18 CHAPTER 1 INTRODUCTION 1.1 A DESCRIPTION OF THE PROBLEM Hydrogen is appreiably soluble in both solid and molten aluminum [1]. Molten aluminum reats with water vapour in the atmosphere, adsorbed water, water present as hydrated oxide films on srap, water adsorbed on or ombined in furnae refratory and even the surfae ontamination of the initial harge. Solid aluminum also reats with moisture in a furnae atmosphere to form hydrogen. These reations are the soures of hydrogen both in solid and liquid aluminum aused by diffusion from the surfae. The amount of hydrogen present in solid aluminum an be signifiantly greater than the equilibrium solubility value either beause of the presene during solidifiation of suh exess amount, or beause of reations of furnae moisture on the surfae of the metal and subsequent diffusion of hydrogen into the body of the solid. The former probably is the dominant fator whih leads to an exessive amount of hydrogen remained in solid aluminum alloys during asting due to the big differene of hydrogen solubility in liquid and solid aluminum, although alloy omposition, inlusion ontent and asting tehnique are also important fators. The solubility of hydrogen at atmospheri pressure in pure aluminum is illustrated in Fig Hydrogen in exess of the solid solubility in aluminum and aluminum

19 INTRODUCTION alloys is generally onsidered deleterious, and its effets have long been reognized and well doumented [1-6]. These may be diretly attributed to the formation of porosity in astings, ingots, and wrought produts, although other hydrogen indued defets, suh as bright flakes in forging, blisters on aluminum sheet, and high-temperature deterioration during heat treating as well as hydrogen embrittlement are also found and reported. A large amount of researh has been made with respet to the influene of gas ontent in molten aluminum on the formation of porosity. For molten aluminum and its alloys, threshold o 1 O 0.5 u Temperature," F I Temperature, C Figure 1-1. Solubility of hydrogen at atmospheri pressure in aluminum [1] n A - B C -D AI, AI -1.7 Si, Al- 6 Si and Al-12 Si 99.2 Al AI-4.6 Cu AI 1 1 y/ Ly B II /, T / / i / f A l / i D IS 1 / Hydrogen Content, m 3 /100g of Metal Figure 1-2. Porosity as a funtion of hydrogen ontent in sand ast aluminum and aluminum alloy bars [3]. I

20 INTRODUCTION hydrogen values were reported under whih the ingots or astings will be free of porosity [4,6-8]. A relationship of hydrogen ontent and porosity in aluminum and its alloys is shown in Fig It an be seen from the previous work that to prevent the formation of porosity in astings and the existene of other hydrogen indued defets, the hydrogen ontent in aluminum melts should be redued to a very low level (<0.05 ml/100 g) before asting. Sine industrial asting proess is onduted in an environment ontaining soures of hydrogen, as those mentioned above, hydrogen an easily be piked up and absorbed by molten aluminum. A proess is needed in industrial asting operations to remove hydrogen from aluminum melt and to ontrol the hydrogen ontent in molten aluminum alloy before asting. This proess is usually referred to as degassing in foundry pratie and beomes imperative to a suessful asting operation. Degassing and hydrogen absorption are dynami proesses whih involve several hemial and kineti relations. In order to well understand these important proesses in industry, it is neessary to study them from both a hemial and a kineti perspetive. The hemial and kineti study on the proesses of degassing and hydrogen absorption in molten pure aluminum and in some aluminum alloys have been performed by several investigators [9-16]. However, information onerning degassing and hydrogen absorption behaviour in molten DURALCAN omposites is very limited in published literature. This hydrogen behaviour in molten DURALCAN omposites has been believed to be different from that in pure aluminum and aluminum alloys due to the presene of a large onentration of reinforing SiC and A1 2 O 3 partiles (10% vol. to 20% vol. for ommerial DURALCAN

21 INTRODUCTION omposites). 1.2 OBJECTIVE The objetive of this work is to study the kinetis of degassing and hydrogen absorption of molten DURALCAN omposites and their matrix alloys. This will help to reate a better understanding of the effet of reinforing SiC and A1 2 O 3 partiles on hydrogen absorption behaviour in these materials. In partiular, the results of the present work will ontribute to the management of industrial proess and failitate the quality ontrol of astings of DURALCAN omposites. 1.3 METHODOLOGY The following steps are performed in this present investigation: 1. Selet typial wrought and foundry DURALCAN omposites and their matries for the investigation of degassing and hydrogen absorption. 2. Establish an experimental setup for the aquisition of degassing and hydrogen absorption data under various atmospheri onditions. 3. Study the environmental and proess parameters that influene degassing and hydrogen absorption in the experiments. Determine relevant experimental proedures. 4. Derive mathematial models for hydrogen absorption and degassing of aluminum melts aording to the relevant hemial reations and physial proesses. Determine kineti

22 INTRODUCTION parameters for degassing and hydrogen absorption of aluminum melts. 5. Carry out experiments of degassing and hydrogen absorption on both DURALCAN omposites and their matries with fixed proess parameters and under various environmental onditions. Monitor and reord hydrogen ontent-time profiles of the melts for the proesses of degassing and absorption. 6. Calulate the kineti parameters of hydrogen absorption and degassing for DURALCAN omposites and their matries with the observed values of hydrogen ontent at different times of degassing and absorption, by means of mathematial regression aording to the derived mathematial models for degassing and hydrogen absorption. 7. Study the degassing and hydrogen absorption harateristis, with given proess onditions, of both wrought and foundry DURALCAN omposites as well as their matries by evaluating the orresponding kineti parameters. Evaluate the effets of reinforing SiC and A1 2 O 3 partiles on degassing and hydrogen absorption behaviour in these materials. 8. Investigate the effets of partiular proess parameter on the kineti behaviour of degassing and hydrogen absorption of aluminum melts.

23 CHAPTER 2 LITERATURE REVIEW

24 CHAPTER 2 LITERATURE REVIEW 2.1 HYDROGEN IN ALUMINUM Solubility Hydrogen is the only gas with measurable solubility in pure aluminum. Values of hydrogen onentration in equilibrium with the gas at atmospheri pressure ( Pa) are shown in Fig The solubility is muh higher in liquid than in solid metal. At the melting point, 660 C, the onentrations of hydrogen in equilibrium are 0.7 and 0.04 m 3 per 100 grams of metal for the liquid and solid aluminum, respetively. It is well known that hydrogen solubility is proportional to the square root of hydrogen partial pressure, p H. This proportionality, also true for the solution of other diatomi gases like N 2 and O 2 in liquid metals provided the solution is dilute, has been found for a long time in suh suffiient number of experiments that it has been alled a law of Sieverts. Some of the results on hydrogen solubility in liquid pure aluminum from three different researh groups are shown 8

25 LITERATURE REVIEW in Fig It an been seen that Sieverts' law is ompletely followed. Aording to the work of Opie and Grant [16], Sieverts' law is also obeyed by molten aluminum-opper alloys from 0 to 50.5 perent opper, and by molten aluminum-silion alloys from 0 to 18 perent silion with melt temperature ranging from 700 C to 1000 C. The most reent experimental values of hydrogen solubility in liquid aluminum-opper binary alloys from 0 to 10 perent opper have been obtained by Liu et al [22], whih are in good agreement with that of Opie and Grant. Similar researh work was onduted by Anyalebehi et al [18] in liquid aluminum-lithium binary alloys ontaining 1, 2 and 3 perent lithium, and the results show that Sieverts' law is obeyed by these alloys with melt temperature ranging from 640 C to 800 C and pressure from 5.3 x 10 4 to 10.7 x 10 4 P n. This >M.2 Opie and Grant [16] A Talbot and Anyalebehi [17] O Liu et al [22] relationship between o.s hydrogen solubility in aluminum and its partial pressure diretly leads to a Pg (mm Hg) onlusion that hydrogen Figure 2-1. Hydrogen solubility in liquid pure aluminum at 700 C under various pressures. exits in its atomi form in aluminum solution, i.e., its absorption proess ould be expressed by the following equation:

26 LITERATURE REVIEW 10 H 2 (gas)^h(in aluminum) (2.1) In other words, the dissoiation of diatomi hydrogen in liquid aluminum is the fundamental feature of the whole dissolution proess in liquid aluminum. No analysis and explanation to this phenomena have been given exept in the paper of Talbot and Anyalebehi [17] that the dissoiation of hydrogen is attributed to interation of hydrogen atoms with the interatomi eletroni environment in metals. It should be noted that Eqn. (2.1) desribes the dissoiation of diatomi hydrogen in ontat with liquid aluminum. Suh a reation (H 2 (g) > 2H(g)) ould hardly our without aluminum, sine its equilibrium onstant at 700 C is in the order of 10" 18. Like the dependene of hydrogen solubility on pressure, its dependene on temperature is also of great importane. Consider the absorption equation {Eqn. (2.1)}, the equilibrium onstant K an be expressed as [20]: (2.2) where a H is hydrogen ativity, y H is ativity oeffiient, X H is hydrogen mole fration in liquid aluminum, and p H is hydrogen partial pressure, in atm. For a dilute solution like the one of hydrogen in liquid aluminum, the y H in Eqn. (2.2) ould be

27 LITERATURE REVIEW 11 regarded as onstant beause Henry's law is valid. Sine hydrogen solution in aluminum is suffiiently dilute and hydrogen gas in atmosphere an be regarded as ideal, we an make use of Eqn. (2.3) whih is an alternative form of Van't Hoff equation [19,20]: ]nk= AH 0 RT + onstant (2.3) or In AH 0 RT + onstant (2.4) In Eqns. (2.3) and (2.4), AH 0 is the hange in enthalpy ours when 0.5 mole of H 2 dissolves, aording to Eqn. (2.1), in aluminum to form a dilute solution with hydrogen ontent of 1 m 3 in standard state (273 K and 1 atm.) per 100 grams of metal whih in engineering pratie is usually hosen as the referene state for hydrogen solution in aluminum. Sine y H is onstant, Eqn. (2.4) ould be rewritten as: In 0 AH RT + onstant (2.5) The hydrogen mole fration, X H, in Eqn. (2.5) an be replaed by its equilibrium ontent represented by a symbol S whih is hydrogen solubility. Eqn. (2.5) then beomes: In ( s AH 0 RT onstant (2.6) or

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