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Acknowledgements List of Figures Introduction Chapter I Formin g at elevated temperature 3 1.1. General 3 1.1.1. Effect of temperature and flow properties 4 1.1.2. Effect of strain rate and flow properties 5 1.1.3. Mechanics of stable deformation at elevated temperatures 8 1.1.4. Ductility at elevated temperature 10 1.2. Metallurgy of warm forming 12 1.2.1. Dynamic recovery and recrystallisation 14 1.2.2. Cavitation 16 1.2.3. Fracture at elevated temperature 19 1.3. Biaxial forming 20 1.3.1. Deep draining 20 1.3.1.1. Deep drawing at elevated temperature 21 1.3.2. Biaxial bulge and stretch forming 23 1.3.2.1. Bulging at elevated temperature 25 1.3.2.2. Stretching at elevated temperature 28 Chapter II Materials and experimental techniques 41 2.1. Materials 41 2.2. Experimental techniques 41 2.2.1. Uniaxial tests 41 2.2.2. Hot pressing tests 42 2.2.3. Hot bulging tests 42 2.2.4. Microstructure examinations 43 ChapterIII Experimental results 48 3.1. Commercial purity aluminium 48 3.1.2. Al-2.2% Mg alloys 48 3.1.3. Al-3% Mg alloys 49 3.1.4. Al-4.4% Mg alloys 49 3.1.5. Al-6.6% Mg alloys 50 3.1.6. Results from all tests 52 3.2. Experimental results in biaxial stretch forming 53 3.2.1. Results of Al/2 Mg alloy 53 3.2.2. Results of Al/6.6% Mg alloy 54 3.2.3. A comparison between 2 Mg and 6 Mg hot pressings 55 3.2.4. Resuilts of hot bulging tests 55 3.3. Discussion 57 3.3.1. Uniaxial tensile test 57 3.3.2. Biaxial press-stretching at elevated temperatuures 58 3.4. Metallography of Al/6 Mg alloys 62 3.4.1. optical microscopy 62 3.4.2. Scanning electron microscopy 64 3.4.3. Transmission electron microscopy 64 3.5. Discussion 66 3.5.1. Optical metallography 66 3.5.2. Transmission electron microscopy 69 3.6. Determination of activation energy 72 3.6.1. Activation energy for tensile deformation 72 3.6.2. Activation energy for dynamic recovery 3.7. Discussion Chapter IV Gene ral discussions, conclusions and-recommen3 ations 4.1. General discussion and conclusions 77 4.2. Recommendations for future work 81 Tables 82 References 85 4 I gratefully acknowledge the courtesy of the Department of Indust-rual Metallurgy, University of Birmingham for the use of their hot pressing facility.

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The regime of sheet metal deformation extends from pure shear, ϵ1= - ϵ2, through plane strain, ϵ2=0, to balanced biaxial tension, under which ϵ1=ϵ2. During deformation a material passes into different states, termed (1) uniform straining, (2) diffuse straining, and (3) localized straining, respectively. Depending on the strain ratio, (1) alone, (1) & (2) & (3), (1) & (3) or (1) & (2) can be experienced by a sheet between zero strain and fracture. In this work the deformation behaviour, in tensile and hydraulic bulge testing, of steel, aluminium and 70-30 brass has been studied. The onset of state (3) has been correlated with strain measurements, taken from a cine film of the deformation, indicated by a grid electrochemically marked on the sheet surface. Various theories predict the major and minor strains at the initiation of these states, and the theoretical and experimental results are compared for the various materials; also the implications with respect to forming-limit diagrams are discussed.