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The properties of the nickel based superalloy Inconel 718 (IN 718) are insufficient for its use in the higher temperature stages at the rear of the gas turbine jet engine compressor. Considerable effort is being made to adapt the microstructure and improve the properties by thermomechanical processing, to enable the cheaper IN 718 alloy to replace the current alloy, Waspaloy, in the fabrications of these rear-end compressor disks. The initial casting of the alloy leads to an unsuitably coarse microstructure. The alloy is therefore forged in order to optimise the microstructure and hence mechanical properties. Isothermal compression tests were carried out on the alloy in the as-cast condition to investigate both dynamic and static microstructural events between 900°C and 1121°C. Strain rates ranged from 0.001s<SUP>-1</SUP> to 10.0s<SUP>-1</SUP> with strains from 0.1 to 0.6. Microstructural investigations revealed that complete dynamic recrystallisation was not evident in any of the samples, however partial recrystallisation occurred in samples forged at the higher temperatures and strain rates. The amount of dynamic recrystallation was found to increase primarily with temperature but also with strain rate. Complete static recrystallation was found to have occurred in tests conducted with a post-forge hold time at temperature. The degree of this recrystallisation and the resulting grain size were found to depend primarily on temperature and strain and less so on strain rate. Stress levels varied systematically with temperature and strain rates. Higher stress levels resulted from lower testing temperatures and higher strain rates. Currently property data of material in the as-cast condition is not widely available. Constitutive analysis was performed on the Stress-strain data for subsequent implementation in the forging simulation program "BILLIE". These results would then be used to provide simulations of the industrial Inco Alloys forging process.

The present research work was to investigate the possibility of hot 'extrolling' of aluminium alloys. 'Extrolling' is a new metal forming process intended for extrusion of continuous length of prismatic sections having complex shapes. An apparatus using a die of special shape and rolls with roughened surfaces was designed and used for the experiments. Billets of commercially pure aluminium with maximum size of 18.0 nun thick, 25.0 mm wide and 125.0 mm long, heated to temperatures between 450°C and 575°C were tested. The reduction in area up to 80 per cent and having cross-section areas of 80.0 mmi to 85.0 mm2 were 'extrolled' in various shapes. The surface roughness of products was within 0.10 to 0.15 µm CIA along their lengths. Dimensions along the length were within an accuracy of 0.050 mm in width and 0.025 mm in thickness. The results of hardness and tensile tests revealed almost no difference when compared with similar results from rolling, extrusion or drawing processes. A theoretical work was developed using an existing theory for rolling which included the application of front and back tensions. In the present theory, front and back tensions were replaced by compressive forces due to extrusion and back-push forces. A computer programme to solve the extrolling problem was developed which included both extrusion and rolling to give estimates of roll forces, local shear stress, pressure distribution in arc of contact and maximum pressure at neutral plane. A mathematical model using the finite element method was developed for the numerical solution to simulate the process with high friction and high temperature values. A reasonable agreement was obtained when theoretical and experimental results were compared.

A comprehensive study of DSS SAF 2205 weldments and nature of weld metal and heat-affected zones has been undertaken. Several DSSs SAF 2205 welds were fabricated using GMAW welding process at different welding conditions. The investigation focuses on the effect of the welding parameters are current, arc voltage, welding speed and heat input on the weld geometry, microstructure and mechanical properties. Also, the technique of the determination of “Response Surface” for the welding parameters was focused to optimise the effects on them each dependent. Variable. All weldments had the UTS, the yield strength and elongation in the range of weld mechanical according to ASTM A240. The welding parameters appeared to have much effect on the tensile properties of DSS weldments tested along transverse direction than the weldment tested along rolling direction. The hardness values vary with welding parameters employed. The trends show that HAZ hardness is slightly lower than base metal when 0.53 to 1.19 kJ/mm is used. However, the hardness in WM is higher than base metal for welding conditions with HI lower than 1.30 kJ/mm. DSS weldments showed no ductile to brittle transition. The fracture location for weld TST with head-off varied between HAZ and base metal. On the contrary, the fracture location along LT was generally at the base metal. SEM photographs shows that the mode of fracture of DSS welds is because microvoids coalescence (simples). Hence the fracture is ductile in DSS specimens. AS the heat input increases the simple size also increases.

To meet stringent European and U.S. environmental legislation, the automotive industry has been actively researching methods of improving the fuel economy of their vehicles, in order to reduce harmful pollutant emissions. One of the most logical ways to achieve this is to reduce the weight of the kerbside weight. This is becoming increasingly difficult; as customers demand more comfort and safety, such as air-conditioning, side impact bars etc. Despite the demands of the customer there have been significant advances in technology that have allowed vehicle weight to be considerably reduced. One significant area of light-weighting technology, which as been researched in this Engineering Doctorate thesis, is that of fatigue in relation to possible down gauging. The materials studied in this thesis were high strength sheet steels for application in structural body-in-white and suspension components. Fatigue has been a much research phenomenon since the early 1800's. This thesis begins with an in-depth practical and finite element analysis study of simple small-scale tests carried out on unwelded and spot welded sheet steels. Following on from this, tests were carried out on more complex components, through to the testing of a full-scale automotive production component. With increasing demand for safety and weight-reduction, the fatigue of automotive components has never been considered to be as important. With many automotive manufacturers inclining towards lighter vehicles through the use of thinner higher strength sheet steels, it is vital that the durability of components made from high strength sheet steel be analysed. It is hoped that this thesis will go some way in assisting and reassuring manufacturers in the development of durable down gauged automotive components.

The majority of spot welds in a wide range of sheet metal products e.g. automotive body in white structures and general engineering applications involve the joining of two or more dissimilar sheet thickness. Research to date has concentrated on the welding of two similar thicknesses. Work was therefore carried out to determine the differences in the heat patterns developed in two and three thickness welding and the effect on initial melting and weld nugget growth. Metallographic examination of partially completed welds, temperature profiles determined using thermal imaging techniques and measurement of the dynamic resistance at the electrode-sheet and sheet-sheet interfaces were used for this purpose. In addition, an FE model of the process has been developed to study the effect of contact characteristics and temperature dependent material properties on weld formation. The heat produced from the electrode-sheet and sheet-sheet interfaces as a result of current flow is conducted into the bulk of the material being welded leading to the thermal runaway cycle of increasing temperature/resistivity which aids the development of a weld nugget. This effect is considered essential to the process. Whilst the above accounts for heat generation, the end result in terms of weld nugget geometry can be markedly influenced by the extent of heat abstraction through the water-cooled electrodes and along the sheet thickness. The cooling effect can be the determining factor whether a weld nugget is formed. The results presented in this thesis indicate that cooling effects need to be given greater emphasis when developing a model to account for all the various factors of weld growth than has been given in the past.

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Finite element formulations dealing with geometric and material non-linearities have been well developed and a significant amount of work has been accomplished for the numerical simulation of metal forming processes. Nevertheless, standard finite element approaches can be sometimes ineffective in handling bulk material deformation owing to severe mesh distortion or mesh entanglement. In the past, some finite element methods such as the Arbitrary Lagrangian Eulerian (ALE) method have been introduced to allow continuous remeshing during computation. Though rather effective in handling large deformation and keeping track of moving boundaries, these methods required extensive computational effort. In this thesis an attempt is made to address the aforementioned problems by using particle based Lagrangian techniques in the numerical simulation of large deformation metal forming processes. For this purpose a particle method called Corrected Smooth Particle Hydrodynamics (CSPH) is considered in the present work. CSPH method is developed from Smooth Particle Hydrodynamics (SPH) techniques which originated twenty years ago. Like most of the particle methods the CSPH also requires no explicit mesh for the computation and therefore avoids mesh direction difficulties in large deformation analysis. In addition, CSPH can achieve similar order of accuracy as any other modern mesh-less methods while retaining the simplicity of the original SPH technique. The simplicity and robustness of SPH method are demonstrated in the first few chapters of this thesis. As a first step of the present research, the SPH method is studied for evaluating its consistency, accuracy and other characteristics. As a consequence of these analyses various correction procedures are introduced in the original SPH method to enhance its performance. The resulting method is referred to here as the Corrected SPH technique. The CSPH is then used to formulate the viscoplastic forming problems with the aid of flow formulation technique.