Fibre-Reinforced Materials Fabricated by Powder Extrusion
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FIBRE-REINFORCED MATERIALS FABRICATED BY POWDER EXTRUSION by HENRY MCSHANE, B.SC,(ENG.) A thesis submitted for the Degree of Doctor of Philosophy of the University of London John Percy Research Group, Department of Metallurgy and Materials Science, March 1978 Imperial College of Science and Technology, London SW7 2BP ABSTRACT The powder extrusion process as a consolidation technique for pre-alloyed aluminium and aluminium alloy powders has been thoroughly investigated. Analyses have been proffered to pre- dict. extrusion load, strain rate and "steady state" temperature during the extrusion process. Classical hot working theory has been applied to powder compacts during extrusion and to the re- sulting extrudes relating flow stress, strain rate and tempera- ture. The controlling effect of the extrusion parameters on the product structure and mechanical properties has been clearly demonstrated. Al-Mg-Si powder extrudes have been subjected to various heat treatments and the resulting mechanical properties investigated. Glass fibre reinforced rods have been produced, by this powder extrusion route, from mechanically mixed quantities of aluminium alloy powder and glass powder. The extrusion process has served three purposes:- (1) Fibre formation (in situ in the deformation zone and die neck) (2) Fibre alignment (3) Matrix consolidation The structure and mechanical properties of the resulting rein- forced extrudes have been observed in some detail. A knowledge of the glass content, extrusion temperature and ram speed required to give optimum properties has been obtained. In all)the results of several hundred extrusions, the cor- responding mechanical tests and microscopical examinations are reported in this thesis. 3 ACKNOWLEDGEMENTS The author wishes to express his sincere appreciation to:- for: - Dr. T. Sheppard His initiation of the project and his encourage- ment, supervision and guidance throughout. Borax Consolidated Limited Their generous financial support. Mr. A. Neve, The essential press main- his colleagues and all the tenance, technical help, technical staff of the equipment design and Department of Metallurgy manufacture, specimen preparation and thesis production. Dr. A.J. Wickens His advice and encouragement. Mr. D.S. Wright and Their considerable help Mr. M.G. Tutcher in computer programming. Past and Present members Their unending experimental of the John Percy Research assistance, discussion and Group evaluation of the data obtained. Mrs. G. Hopkins Her prompt, efficient typing of this thesis. His parents, Their unending moral Mr. and Mrs. J. McShane support throughout the entire duration of the project. 4 CONTENTS page Abstract .. .. • • • • 2 Acknowledgements .. • .. .. • 3 Contents .. .. .. • .. • .. • 4 List of Figures .. .. .. • • • • • 8 List of Plates • .. .. • .. .. .. 13 List of Tables . .. .. .. • .. .. 14 Chapter 1 Introduction 16 1.1.1 Powder Metallurgy 16 1.1.2 Precipitation Hardening 16 1.2.1 Fibre Reinforcement 17 1.3.1 Composite Production by powder route - 18 Present work Chapter 2 Literature Survey. 19 2.1.1. Introduction 19 2.2.1 Cold Compaction of Metal Powders 19 2.3.1 Extrusion Process 23 2.3.2 Extrusion of Powders 26 2.4.1 Prediction of load by slip line field 28 techniques 2.4.2 Plane Strain Upper Bound Solution 30 2.4.3 Axisymmetric Upper Bound Solution 32 2.5.1 Strain Rate Determination 34 2.6.1 Temperature Variation During Extrusion 36 2.7.1 Extrude Structure 41 2.8.1 Deformation Mode 44 2.9.1 The Z Parameter 46 2.10.1 Precipitation Hardening 47 2.11.1 Dispersion Strengthening 51 2.12.1 Extrusion Defects (Surface Finish) 55 2.13.1 Fibre Reinforcement of Metals 56 2.13.2 Whisker Crystals 57 2.13.3 Continuous Ceramic or Glass Fibres 59 5 Page 2.13.4 Metal Wires 61 2.13.5 Lamellar Eutectics 63 Chapter 3 Theory 3.1.1 Introduction 66 3.2.1 Plane Strain Upper Bound Solution 66 Prediction of P/K 3.2.2 Use of Plane Strain Solutions for 69 Axisymmetric Extrusion 3.2.3 Prediction of Load Using Plane 71 Strain Upper Bound Solution 3.3.1 Axisymmetric Upper Bound Solution 74 3.4.1 Prediction of Load Using Axisymmetric 79 Upper Bound Solution 3.4.2 Calculation of Flow Stress during Extrusion 84 3.5.1 Strain Rate 85 3.6.1 General Hot Working Equation 86 3.6.2 Determination of Constants In The Hot 87 Working Equation 3.6.3 The Activation Energy 88 3.7.1 Introduction to Fibre Reinforcement Theory 88 3.7.2 Stress/Strain Behaviour of Continous Fibre 90 Composites 3.7.3 Strength of Continuous Fibre Composites 93 3.7.4 Stress/Strain Behaviour of Discontinuous 94 Fibre Composites 3.7.5 Stresses Present in Discontinuous Fibre 96 Composites 3.7.6 Strength of Discontinuous Fibre Composites 100 3.7.7 Effect of Fibre Orientation 100 3.7.8 Fracture Toughness of Composites 102 3.8.1 Effect of Temperature on Extrude Flow Stress 106 3.9.1 Temperature Change During Extrusion 108 3.10.1 Torsion Testing 115 Chapter 4 Experimental Procedure 4.1.1 Introduction 117 4.2.1 Metal Powder Production 117 4.2.2 Glass Powder Production 118 6 Page 4.2.3 Powder Size lln 4.2.4 Powder Shape and Structure 119 4.2.5 Powder Chemical Analysis 120 4.3.1 Mixing and Compaction of Billets 120 4.3.2 Strength of Compacted Billet 120 4.4.1 The Extrusion Press 123 4.4.2 The Heaters 123 4.4.3 Lubrication 123 4.4.4 Billet Preheat 124 4.4.5 Reduction Ratio 125 4.4.6 Ram Speed 125 4.4.7 Extrusion Load 127 4.4.8 Extrusion Procedure 127 4.4.9 Heat Treatment 128 4.5.1 Product Properties 128 4.5.2 Hardness Tests 128 4.5.3 Tensile Tests 129 4.5.4 Impact Tests 129 4.5.5 Torsion Tests 130 4.6.1 Optical Microscopy 130 4.6.2 Transmission Electron Microscopy 131 4.6.3 Scanning Electron Microscopy 132 4.6.4 Partially Extruded Billets 132 Chapter 5 Results and Discussion 5.1.1 Sieve Analyses of Metal Powders 133 5.1.2 Sieve Analyses of Glass Powders 133 5.1.3 Chemical Analyses 133 5.1.4 Shape of Powder Particles 134 5.1.5 Structure of Powder Particles 135 5.1.6. Strength of Compact 135 5.2.1 Theoretically Predicted Loads 141 5.3.1 Strain Rate Prediction :(From Upper 147 Bound Solution) 5.4.1 Extrusion Parameter Measurements 151 5.4.2 Effect on the Load of Extrusion Parameter 154 Variation 7 Page 5.5.1 Torsion Test Results 165 5.6.1 Determination of the Hot Working 171 constants and Activation Energy 5.7.1 Partial Extrusions-Flow Patterns 180 5.8.1 Effect of Extrusion Conditions on 101 Extrude Surface Finish 5.9.1 Stress-Strain Behaviour of the Extrude 184 5.9.2 Analysis of Fracture Behaviour 188 5.9.3 Effect on Extrude Properties of 189 Variations in Extrusion Conditions 5.9.4 Dependence of Extrude Properties on 198 Test Conditions 5.9.5 Dependence of Extrude Properties on 202 Powder Particle Size 5.9.6 Effect on Extrude Properties of Variations 205 in Glass Content 5.10.1 Hardness Results 209 5.11.1 Fracture Toughness 210 5.12.1 Heat Treatment Results 214 5.12.2 Effect of Soaking Temperature Variation 216 on Extrude Properties 5.12.3 Effect of Ageing Time/Temperature Variation 218 on Extrude Properties 5.13.1 Structure'of Extrudes-Optical Micrographs 221 5.13.2 Structure of Extrudes-Transmission_ 225 Electron Micrographs 5.14.1 Nature of Glass-Metal Interface 229. Tables • .. 230 Chapter 6 Conclusions .- •• • 259 Appendix 1 .. • .. • 266 Appendix 2..• • .. • • 267 Appendix 3 • 271 ' .. • .. Appendix 4 • • • • • • • • • • 281 Appendix 5 • .. • .. • .. .. 283 Appendix 6 .. .. • • 284 Appendix 7 Errors . • • 205 Appendix 8 Recommendations for further work • • 287 Nomenclature .. .. .. • . .. 291 References .. • .. .. .. 295 8 LIST OF FIGURES FIG 1 Relationship between compacting pressure and relative density FIG 2 Extrusion pressure as a function of ram displacement (Idealised) FIG 3 Velocity fields for different types of extrusion FIG 4 Temperature rise model involving stepwise movement of the billet FIG 5 Phase diagram for ideal precipitation hardening system FIG 6 Effect of temperature on age hardening curve FIG 7 (a) Al-Mg-Si phase diagram (b) Al-Mg2Si pseudo "binary" phase diagram FIG 8 Effect of second phase particles on dislocation motion FIG 9 Plane strain upper bound double triangle velocity field and hodograph FIG 10 Extrusion hodograph showing incident velocity components FIG 11 Axisymmetricupperbound velocity field and hodograph FIG 12 Nomenclature and element details of elemental upper bound solution FIG 13 Definition of mean flow stress FIG 14 Stress-strain behaviour of Al-glass composite FIG 15 > E M Stress-strain behaviour of composite in which EF FIG 16 Stress-strain behaviour of composite subjected to repeated loading FIG 17 Relationship between composite strength and fibre content FIG 18 Stress build up in fibre FIG 19 Strain distribution across matrix-fibre interface FIG 20 Stress distribution in matrix FIG 21 Composite with fibres inclined to stress axis FIG 22 Effect of fibre-stress axis inclination on composite strength FIG 23 Effect of notch depth on composite fracture toughness 9 FIG 24 Stress distribution as a function of distance from crack tip FIG 25 Three stages in fibre delamination by crack propagation FIG 26 Schematic representation of obstacle to dislocation movement as an energy barrier FIG 27 Effect of temperature on flow stress FIG 28 Control volumes and heat losses employed in integral profile temperature rise model FIG 29 Apparatus used to determine compact strength FIG 30 Extrusion die FIG 31 Locations of samples taken from an extrude FIG 32 Compact fracture force as a function of theoretical density FIG 33 Relative density of compact as a function of compaction pressure FIG 34 Relationship between compact fracture force