Strip casting is a rapid processing technique used in the production of alloys for both sintered and HDDR bonded NdFeB magnets. During strip casting, molten alloys are gravity fed onto a rotating copper wheel. The rapid cooling rate produces NdFeB microstructures which have many advantages over conventionally cast NdFeB book-mould alloys such as less alpha iron, near stoichiometric alloy compositions and a finer grain structure. The wheel texture of a pilot scale strip caster was altered in order to optimize the microstructure and improve the consistency of strip cast flakes for the production of NdFeB sintered magnets. This made it possible to reduce the flake thickness distribution and alpha iron content, improve the grain size consistency and increase lamellar alignment. In NdFeB alloys for HDDR (Hydrogenation Disproportionation Desorption and Recombination) powders, which are used in bonded magnets, a large grained material is preferable with little or no alpha iron. By implementing a combination of surface texture and reduced wheel speed, the grain width was increased from ~5 to ~40 μm, whilst maintaining a low alpha iron content. The magnetic performance has been shown to be comparable to conventional book-mould alloys that have been heat treated for 10 hours at 1140°C.

The construction of a multi-layered, multi-turn coated conductor cylinder encompasses several aspects that may limit its performance unless they are designed and fabricated in a suitable way. This project investigates the optimum thicknesses of YBa\(_2\)Cu\(_3\)O\(_7\)\(_-\)\(_8\) (YBCO) superconductor and SrTiO\(_3\) (STO) insulator layers, interconnect design between YBCO layers and the fabrication process for defining tracks in the YBCO. Test samples were produced by pulsed laser deposition (PLD), photolithographic and ion-beam and chemical etching techniques and were characterised by AC susceptibility, transport measurements, atomic force microscopy (AFM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and x-ray diffraction (XRD). The growth conditions produce a YBCO film that develops a strong texture even over an ion-beam milled edge. Additional steps were required to remove contaminants from the surface after photolithographic processes, with both ion-beam milling and alkaline etch proving effective. Interconnects were successfully fabricated and were most effective when a large step was ion-beam milled into the first YBCO layer, rendering a critical current density (Jc) of 8.58x10\(^5\)A/cm\(^2\). Electrical transport through a crossover was made possible by the application of an additional etching process to create a more gentle slope although further optimisation is required to improve epitaxial growth on the track edge.

The objective of the work reported in this thesis was to tailor a colloidal processing technique called electrophoretic deposition (EPD) for use within the investment casting shell formation process, where the EPD coating procedure would be used to form the primary ceramic coating on the melt-out substrate. EPD takes place due to the presence of an electric field within the suspension medium, which attracts charged particles in the suspension towards an electrode of opposite charge, onto which they are deposited. For the complex structures created using investment casting, the die cast patterns used as the substrate for the ceramic have to be easily removed, and so substrates materials that can either be melted or dissolved out the material to leave the hollow ceramic shell used. To implement EPD into the investment casting process, this substrate needed to be conducting, and so conducting particle-filled investment casting waxes were created and analysed. Carbon black and graphite filler were incorporated into waxes, and the conductivity and rheology of the resultant composites were studied, to gauge their suitability as an investment casting pattern material. On the basis of both cost and for environmental reasons, the use of aqueous suspension media for EPD was preferred over the more commonly used organic systems. EPD was carried out using zircon in aqueous suspension, and the low particle concentration suspensions were stabilised through pH modification and anionic dispersant addition. The effect of suspension parameters and EPD set-up parameters on the coatings formed on compressed graphite electrodes and conductive wax electrodes were studied, through yield measurements and cross sectional analysis using scanning electron microscopy.

The autogenous friction welded- titanium alloys, Ti-6Al-4V and Ti-6Al-2Sn-4Zr-6Mo, were studied during this Ph.D. degree. An investigation was conducted on the microstructure evolution, fracture toughness and fatigue crack propagation behaviour of autogenous inertia welded Ti-6Al-2Sn-4Zr-6Mo. The microstructure and fatigue threshold behaviour of linear friction welded Ti-6Al-4V were also assessed in both the as-welded and the post weld heat treated conditions, and the fracture toughness was assessed at room temperature in post weld heat treated condition only. The fatigue threshold behaviour of the autogenous linear friction welded Ti-64 in the as-welded condition was assessed to help understand the edge cracking phenomenon which was observed in the as-welded linear friction welded Ti-6Al-4V BLISK joints.

This research was aimed at exploring different surface engineering techniques towards cost-effective and scalable fabrication of graphene-based coatings on metallic surfaces. Graphene oxide (GO) was employed as the coating precursor, and various coating routes, including electrophoretic deposition (EPD), self-assembly, electroplating and electro-brush plating were investigated in the project. The results have showed that both EPD and self-assembly are capable for depositing thin and uniform GO coatings on steel. The tribological and corrosion tests on EPD-GO coatings have showed promising properties of the GO coatings. It has also been found that pre-modification of the steel surface with a self-assembled monolayer of silane can also enhance the bonding of the GO sheets to the steel surface, thus improving the wear performance of the ultra-thin GO coating. The electro-brush plated Ni-GO nano-composite coatings have exhibited significantly improved compactness and homogeneity. A grain refinement and improvement in mechanical properties were observed. More importantly, the GO-containing nano-composite coatings have exhibited enhanced thermal stability after annealing at elevated temperatures up to 600 ℃. Meanwhile, the Ni-GO composite coatings showed clearly improved tribological and anti-corrosion performance. The findings indicate that electro-brush plating is a very promising technique to produce novel nano-composite coatings.

A novel process in which individual turbine blades are produced by the high rate solidification method has been developed. The technology, with an optimised radiation baffle, gave a thermal gradient of 11.64 x 10\( 3\) K.m\( {-1}\), whilst maintaining a flat solidification front, calculated using a specially created and validated process model. The corresponding primary dendrite arm spacing was reduced to 165 x 10\( {-6}\) m and the calculated freckle potential was below the critical threshold limit identified by Beckermann et al. (2000), even in highly freckle prone alloys. Low angle grain boundaries formed when misorientation accumulates in growing dendrite envelopes which subsequently converge were studied experimentally. While extensive dendrite branching or steady state growth was found to lead to an average primary dendrite misorientation of 2.3 ° that was random in nature, enhanced growth kinetics accompanying non steady state growth, found in platforms, produced a monotonic increase in accumulated misorientation of up to 10 °. It was concluded that the latter is due to mechanical moments arising from extensive growth of unsupported tertiary dendrite branches growing laterally across the platform normal to the direction of gravity. The degree of misorientation is therefore dependent on local geometry and mushy zone shape.

Forging of components for gas turbine application from nickel-based superalloys remains a challenge, due to the exceptional high temperature properties of these alloys. In order to enhance formability and prevent oxidation at high temperature forging, a glass coating is applied to protect the alloy. However, the contribution and significance of glass coating to the material flow during forging is not fully understood. In addition, increased competitive pressure requires a better understanding of the root cause for geometrical part variability in the forging of aerospace components. Particularly, reduction of manual rework is highly desirable, as it increases the manufacturing costs of these components significantly. The aim of this work was to identify the Key Process Variables (KPVs) of the forging of aerofoils. Therefore, numerical modelling of a full factorial design of experiment was conducted to study the significance of various process parameters. The contribution of glass coating as a process variable has been studied experimentally. The double cup extrusion test has been employed to study the influence of glass coating thickness and chemistry in hot forging condition. Ring compression tests have been conducted to investigate the contribution of graphite lubricant. It was found, that in the presence of graphite lubricant friction reduces markedly. However, in order to establish a stable lubrication system the fusion behaviour of glass coating becomes crucial. In the absence of graphite lubricant, friction increases with increased glass coating thickness. Numerical modelling revealed that the interfacial heat transfer coefficient increases with a reduction of glass coating thickness. A good agreement between experimentally and numerically derived interfacial heat transfer coefficient has been found. In contrast, in a rapid deformation process such as blade forging, the effect of heat loss becomes less important compared to the contribution of friction. The initial variability of glass coating thickness has been characterised in the industrial forging of Inconel 718 aerofoils. Glass coating thickness varies considerably across the workpiece and the batch. This variability, alongside the variability of manual process parameters such as transfer and resting time, have a notable effect on the forming temperature and thus on the resulting aerofoil geometry.

The project aim was to develop a lean method for bimetallic fuel pump bearing manufacture using powder metallurgy (PM), as an alternative to flame spraying. Specifically, the sintering and diffusion bonding of aluminum powder alloy to 30% leaded bronze included: • 'Press and sinter': AI-6%Si alloy. 7075 alloy (AI-5.5%Zn-2.5° oMg-1.6%Cu), \(\leq\)500 °C AI-8%Mg-4%Zn alloy, \(\leq\) 500 °C. • HIP (Hot Isostatic Pressing) - AI-6%Si alloy. • 'Hot compaction' - AI-8%Mg-4%Zn alloy. 'Press and sinter' of AI-6%Si alloy resulted in residual porosity and expansion during cooling, due to silicon density reduction during solidification. HIP enabled simultaneous compaction, sintering and diffusion bonding of AI-6%Si alloy to leaded bronze; bearing 'blanks' were manufactured, with an intermetallic layer (approximately 100\(\mu\)m) at the interface.'Press and sinter' of 7075 alloy resulted in swelling during transient liquid phase formation and residual porosity. However, using AI-8%Mg-4%Zn alloy this process demonstrated improvement, as it was closer to its solidus temperature at the chosen processing temperature. Test pieces and bearing 'blanks' were produced, with an intermetallic layer (approximately 20 \(\mu\)m) at the interface, of tensile strength > 20 MPa, superior to flame sprayed bearings. Further work involved optimising HIP and 'hot compaction' processing for batch production.

The aim of this project is developing a near net-shape processing route for Al-alloy compressor components with Selective Laser Melting (SLM). Design of experiments (DOE) techniques such as the Response Surface Method, and statistical analysis using the analysis of variance (ANOVA) were applied for processing parameter optimisation in order to minimise the defects (pores or cracks) and studying the influence of powder, such as chemical composition, particle shape/ morphology and particle size and SLM parameters, such as laser power, laser scan speed, hatch spacing, laser scan strategy, island size. The tensile, fatigue and creep properties of the samples built using the optimum parameters were assessed. The microstructures were assessed using optical, scanning and electron transmission microscopy. The influence of building directions (0°, 45°, and 90°) and post processing (T6 heat treatment and Hot Isostatic Pressing (HIPping)) on microstructure and mechanical properties were also investigated.

The global demand for Mg alloys continually grew in the last 20 years, motivating a wide interest in the improvement of the mechanical properties of Mg-alloy castings. In addition, the existence of double oxide film defects, which were widely recognised as a major factor in the quality and reproducibility of the properties of light-alloy castings, has been demonstrated in Mg-alloy castings. Thus it became important to understand behaviour of double oxide film defects formed in Mg-alloys. In the work reported here, three different Mg alloys (commercial pure Mg, AZ91 alloy, and Mg-Y alloys) and two cover gases (SF6/air and SF6/CO2), were used, in order to involve different doubled oxide films which may have different behaviours. Direct and cross-sectional observations of the double oxide film defects formed the Mg-alloy castings protected by different cover gases were obtained via a Scanning Electron Microscopy (SEM), and the focus ion beam milling (FIB) technique. In addition, oxide films growing on the corresponding Mg-alloy melt surfaces were also investigated. Based on the observed film structures in conjunction with a thermochemical calculation, evolution processes of the different double oxide film defects were suggested. The quality of Mg-alloy castings was evaluated by the Weibull modulus, which is popularly used to discriminate “good” and “bad” castings. A shortcoming of the traditional Weibull estimation method (i.e. linear least square method) was demonstrated, and a new estimation method was therefore come up with. The Weibull modulus result revealed that air can confer an improvement in the quality of AZ91 castings, compared with CO2.