The function of the compressor wheel in a turbocharger is to compress the air drawn by the turbine. During this process the wheel reaches temperatures up to 230°C. Aluminium alloys have been developed to withstand both the elevated temperatures and the cyclic loading that the wheel experiences during its working life. The 2618-T6 aluminium alloy is used due to its good combination of price, mechanical properties and suitability at temperatures up to 300°C. However, the manufacturer found during inspections of catastrophic failed wheels a considerable contribution of "intergranular" fracture (fracture along grain boundaries). This mode of failure could be the result of a complex combination of factors, such as a creep mechanism operating at high temperature, and/or the effect of microstructural features such as second phase particles, quenched induced intergranular and transgranular preCipitates, and precipitate free-zones located along grain boundaries. A quantitative metallography analysis was made within the compressor wheel net-shape to understand the influence of the process on the grain size and the distribution of second phase particles. It was found that the grain size is smaller in the sections where the wheel tends to fail, and that the percentage of second phase particles bigger than 5~lm is less than 10% in the same zones. Fatigue tests at both room and elevated temperature (230°C) were carried out to understand the fatigue behaviour of the material under such conditions. It was found that "intergranular" failure does not depend on temperature. A new methodology was developed during this research to quantify the percentage of "intergranular" failure (flat areas) on the fracture surfaces. The methodology is a combination of stereo-photogrammetry and image analysis. Stereo-pairs were randomly taken along the crack paths and were analysed using the MeX software. Results showed a higher contribution of flat areas at room temperature than at 230°C. The mechanisms which could promote this "intergranular" fracture were discussed by means of extensive literature review and observations during this research. The effect of these areas on the fatigue life was explored using a model based on a modification of the Paris law. The main idea of this model was to consider the material as the mixture of two phases associated with the two modes of failure observed on the fracture surfaces. The properties of a softer material were selected to simulate the "intergranular" (flat areas) behaviour. The model gave a good prediction of both the fatigue lives measured during this research and the fatigue lives provided by Cummins Turbo Technologies. Results from the model showed that the fatigue life is overestimated by a factor 10 if the presence of the flat areas is not accounted for.

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This thesis is concerned with developing numerical models to predict the mechanical behaviour of closed-cell aluminium (AI) foams under uniaxial loading. Many of the existing models produce responses which are very stiff initially and fall sharply after an initial peak (Type-II response). In contrast, the real Al foams exhibit a reduced stiffness and give a flat-topped curve in the neighbourhood of peak-load (Type-l response). Thus, the weakness of the existing unit-cell models in relation to the two qualitatively different load displacement responses was identified. A link has been established between the stretching dominated mode of deformation to the Type-II response and the bending dominated to the Type-l response respectively. The absence of Type-l response in the current unit-cell models in the literature moved the present research in a direction to identify an improved new 3D unit-cell, which can be used by future researchers to represent closed-cell Al foam in loading scenarios with large deformations. The unit-cell identified has a right polyhedra structure. Furthermore, the deformation for this model is bending dominated thus giving Type-l response in three principal loading directions. Two sets of polyhedra unit-cell based finite element (FE) models for the crushing of closed-cell Al foam were presented. The first set was constructed by stacking up to 6-cells along principal material loading directions. Perpendicular to the loading direction, the mechanical behaviour for an infinite system was simulated by using periodic boundary conditions (PBC). In the second set, a finite domain sample containing up to 64 unit-cells was used to capture the compressive response behaviour. The crush features of this system were obtained using 3D array of many cells (MC). Additionally, the application of the second type of FE models was extended further to study the performance of Al foam containing imperfections under low-velocity impact by a rigid indenter. A simple imperfection in the form of reduced cell-wall thickness was introduced into the unit-cell Al foam and a set of two different populations of imperfections were considered. All these aforementioned FE foam model predictions were compared with experimental results. Finally, the new 3D unit-cell model results were compared with the contemporary stretching dominated (Type-II response giving) models in literature namely, the truncated-cube and cubic-spherical. A parametric study was conducted to quantify the mechanical properties of aforementioned unit-cell models alongside the model developed in this thesis. It was demonstrated that the plateau phase stress-strain response of the developed 3D unit-cell model was more representative of real closed-cell Al foams. Further, it was shown that the crushing resistance and energy absorption features of this new 3D unit-cell model were higher, compared to the truncated-cube and cubic-spherical models.

In an attempt to achieve the high strength requirements for modem applications, today's advanced metallic materials are enhanced through strengthening mechanisms such as solid solution strengthening or cold workmg. The disadvantage associated with these mechanisms is the subsequent loss in ductility. This reduction also has a significant effect on the formability of materials. For example this is a particular disadvantage in the aerospace industry were the curvature of sheet components are limited by their formability. This project investigates an alternative strengthening mechanism in an attempt to maintain ductility.

The influence of secondary processing conditions on an aluminium metal matrix composite, comprising of an AA2124 matrix and 3 Jlm particulate SiC reinforcement at 25 volume percent was investigated. The metal matrix composite (MMC) was extruded at three different temperatures, 350°C, 450°C and 550°C, at a ratio of 20:1 and at three different ratios, 5:1, 10:1 and 20:1, at a temperature of 450°C. It was subsequently solution heat treated and naturally aged. A mechanical property assessment was carried out using standard tensile and rotating bend fatigue test methods to determine the properties of the material extruded under each condition. A novel technique using a Focussed Ion Beam (FIB) Microscope was developed to prepare polished specimens and microtextural analysis was performed by FIB imaging. Additionally, techniques were successfully established, through the use of FIB milling and polishing, to provide site-specific electron transparent films, permitting detailed examination ofthe microstructure with a transmission electron microscope. Material extruded at 550°C exhibited a lower yield strength than material extruded at 350°C and 450°C, which was attributed to grain coarsening and recrystallisation. Evidence of recrystallisation was found during texture analysis by X-Ray diffraction, where there was a reduction in the intensity of the fibre texture in the extrusion direction. The phenomenon was also observed during irticrostructural analysis work, where recrystallised grains at grain boundaries were observed. Higher extrusion ratios offered a small improvement in tensile properties, due to an enhanced fibre texture within the microstructure. Microtextural examination gave evidence of the existence of both high angle grain and low angle grain boundaries for the material extruded at 350°C. It is believed that a subgrain structure was partially transformed during extrusion, through subgrain rotation, leading to the formation of high angle grain boundaries. This microstructure was found to offer the optimum mechanical properties.

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Since mechanical properties of cast Al-Si-Mg alloys are directly influenced by microstructural defects, understanding the characteristics of these defects and any interactions between them is important for improving the properties of castings. This research studied the effect of the most common defects normally found in cast aluminium alloys namely, double oxide films, Fe-rich phase, and porosity. A different level of each defect was introduced into the castings to investigate their effects on tensile properties with the results analysed by Yate’s algorithm and Weibull statistical analysis to determine their relative effects. The most important defect in this research was the oxide film that had an effect on UTS and elongation of about 21 MPa and 0.9% respectively and on the UTS Weibull modulus and elongation Weibull modulus of about 37 and 3 respectively. Increasing Fe content from 0.1 to 0.5 wt% influenced the tensile properties of the castings by decreasing the UTS and elongation by about 28 MPa and 1.7 % respectively. Although Fe-rich phases produced the greatest effect on tensile properties, their relatively high Weibull modulus showed that the reliability of the castings was at least predictable compared to the effects associated with oxide films. The porosity defects caused by an increase in hydrogen content from 0.1 to 0.45 ml/100g metal were the least detrimental to tensile properties. The most important interaction found in this research was the interaction between oxide films and porosity suggesting a mechanism for porosity formation in which entrained oxide film acted as initiation sites for pore formation in the castings. The main factor in the formation of porosity was hydrogen and shrinkage, since both could encourage the expansion of the oxide film defects to become gas porosity or shrinkage porosity in the castings. The other interaction between the microstructural defects observed in this research was that oxide films were found to be substrates for the nucleation and growth of Fe-rich phases, particularly the β-Al5FeSi phase. An interaction between all three defects were also observed and it further influenced the tensile properties of the cast Al-Si-Mg alloys by decreasing UTS and elongation by about 5 MPa and 0.5% respectively and the UTS Weibull modulus and elongation Weibull modulus of about 8 and 0.6 respectively.

Ce travail constitue une contribution à l’étude de l’effet du cumul de dommage en fatigue multiaxiale polycyclique des matériaux métalliques (l’alliage d’aluminium 6082T6). Généralement, cet effet peut être interprété par différentes approches des lois de cumul d’endommagement. En 2003, Mesmacque et Santos Garcia ont développé une nouvelle loi, damage stress model (DSM), en introduisant un nouvel indicateur non linéaire de dommage dans le cas de fatigue uniaxiale. Cet indicateur ne nécessite que la connaissance de la courbe de Wöhler et la résistance maximale du matériau. Le travail présenté ici constitue une extension du modèle DSM au cas de la fatigue multiaxiale. Cette extension est rendue possible par l’utilisation de critères de fatigue multiaxiale de Sines, DangVan et Robert à durée de vie limitée et par la mise en œuvre d’une méthode d’itération programmée sous Matlab fournissant les indicateurs de dommage. Elle ne nécessite aucun paramètre supplémentaire mise à part la connaissance de la courbe de Wöhler uniaxiale. Les résultats des travaux expérimentaux en fatigue biaxiale, qui ont servi de support pour la validation de notre modèle ainsi que la comparaison avec le modèle de Miner, ont été obtenus grâce à la plateforme d’essais multiaxiaux à quatre vérins à implantations modulaires. Une base de données expérimentale est disponible pour chargement biaxiale effectués sur des éprouvettes cruciformes amincies au centre. Par ailleurs, une simulation numérique sous Abaqus a permis d’accéder au champ de déformation/contrainte au centre. Les résultats de cette simulation ont été corroborés à l’aide d’une éprouvette instrumentée par des jauges de déformation. / This work is a contribution to the study of the effect of cumulative damage in multiaxial high cycle fatigue in metallic materials (the aluminum alloy 6082T6). Generally, this effect can be interpreted by different cumulative damage models. In 2003, Mesmacque and Santos Garcia have developed a new damage stress model (DSM), introducing a novel nonlinear damage indicator in the case of uniaxial fatigue. This indicator requires previous knowledge of the Wohler curve and the ultimate tensile strength of the material. The work presented here is an extension of the DSM model to the case of multiaxial fatigue. This extension is made possible with the assumption of multiaxial fatigue criteria: global type (Sines) and critical plane type (Dang Van and Robert) under finite life regime and the implementation of an iteration method in Matlab programming, which provides damage indicators. In particular, it requires no additional parameters besides the uniaxial Wöhler curve, which makes it easy to use. The results of experimental biaxial fatigue tests were obtained with the aid of the multiaxial test platform of our laboratory (four-cylinder modular-implementations platform) and an original test mode based on “modal control”, providing ground for the validation of our model and its comparison against other most current models. An experimental database is available for constant-level and block fatigue loadings using cruciform specimens thinned in the center. In addition, a numerical simulation in Abaqus allowed the analysis of the strain/stress field in the center of the specimen. The simulation results were corroborated using a specimen instrumented with strain gages.