This thesis is concerned with the porosity formation mechanism during electron beam welding of titanium-based alloys. During the welding of titanium alloys for aerospace engine applications, porosity is occasionally found in the solidified welds. Hence the key factors responsible for porosity formation need to be identified, and guidance to minimise porosity occurrence needs to be provided. The research conducted in this work is twofold. First, porosity formed in electron beam welded titanium samples is characterised to rationalise the porosity formation mechanism. Second, models based on sound physical principles are built to aid understanding of porosity formation, and to provide predictive capability. Porosity formed in electron beam welds is characterised using metallographic sectioning, high resolution X-ray tomography, residual gas analysis (RGA), scanning electron microscopy (SEM) and energy and wavelength dispersive spectroscopy (EDS/WDS) analysis. The results confirm porosity formed in electron beam welded titanium-based alloys is associated with gas dynamics; hydrogen is very likely to be responsible for porosity formation. A coupled thermodynamic/kinetic model is proposed to study the hydrogen migration behaviour during electron beam welding process, and then the effect of hydrogen on bubble formation is investigated via quantitative modelling, backed up by targetted experimentation

Low temperature plasma surface engineering has been a useful method for increasing the hardness and wear resistance of austenitic stainless steel without reducing the corrosion resistance of this alloy. Plasma carburising is of particular interest as it produces thicker hardened layers than plasma nitriding, and an equivalent improvement in the tribological and corrosion performance of the base material. In this project, the active screen (AS) plasma technique was used to carburise austenitic stainless steel AISI 316 and the obtained layer of carbon expanded austenite was compared with the one produced by conventional DC plasma treatments. The hardening and wear resistance produced by AS and DC plasma carburising were equivalent. With regard to corrosion, the AS treated material performed better than its DC counterpart as a consequence of the improved surface quality of the former. The mechanism of AS carburising was comparatively studied with its AS nitriding counterpart. Different experimental arrangements and two plasma diagnostic techniques were used for this purpose: optical emission spectroscopy and electrostatic probes. The evidence shows that AS nitriding relies on the deposition of iron nitrides and the active species in the plasma to produce hardening, whilst AS carburising requires the plasma activation and moderate ion bombardment.

The compaction, sintering and heat treatment processing conditions for the commercial powder, Ecka Alumix 231 with the nominal composition of Al-15Si-2.5Cu-0.5Mg(wt%) have been optimized in this study. It has been found that densification of Ecka Alumix 231 depended largely on the sintering atmosphere and the amount of liquid phase presented. Sintering in nitrogen atmosphere was found to be beneficial due to the formation of AIN which later induced pore filling effect. Presence of hydrogen in the sintering atmosphere, however, was not desirable. Decomposition of MgH2 at higher temperature was suggested to increase the pore pressure and caused detrimental effect on compact densification. The effects of alloy additions (i.e. Cu and/or Ni) on sintering and heat treatment response were also investigated in this study. Addition of copper was found to enhance the sintering response by a significant increase in the sintered density,reduced the peak ageing temperatures and time. Nickel addition, however, was detrimental to both sintering and heat treatment response since it reduced the amount of copper content dissolved in the α-aluminum due to the formation of Al\(_3\)(Ni,Cu)\(_2\) phase. The precipitation sequence for Al-Si-Cu-Mg-(Ni) was suggested to be: Supersaturated solid solution (SSS)→GP zones → θ” → θ’ → θ.

An investigation into the fundamentals of the deposition of silicon carbide within porous silicon carbide fibre preforms using microwave-enhanced chemical vapour infiltration has been carried out. The study of the kinetics of deposition revealed an Arrhenius behaviour of the matrix growth rate against the temperature in the range 800-1000°C and a linear dependence on the pressure in the range 20 - 70 kPa. This is typical of a surface-reaction limited regime. The morphology of the SiC deposited changed with both temperature and pressure. Increases in both lead to a transition from a smooth, globular deposit morphology to something that was rougher and more angular; this corresponded to the transition from a nucleation to a growth regime. Stoichiometric SiC was predominantly found in the central region of the samples infiltrated at 1000°C, but the deposit became more silicon-rich (up to 2.6 at %) the farther from the initial deposit. Dielectric properties showed that ZMI Tyranno silicon carbide fibres readily absorbed microwave energy. In specific conditions of temperatures and pressures, 900-950°C and 50 kPa, an inside-out deposition pattern was observed indicating a temperature gradient across the preform. Deposition of silicon carbide and silicon caused the gradual flattening of the temperature gradient.

In centrifugal casting, molten metal is introduced into a mould which is rotated at high speed. The centrifugal force helps to fill thin sections but this benefit may be offset by the effect of the turbulent flow on the casting quality. In this research, the effect of direct and indirect gated mould designs on the quality and reliability of aluminium alloy investment castings made by centrifugal casting was investigated. The scatter in the ultimate bend strength and the modulus of elasticity was analyzed using the Weibull statistical technique, which showed that the Weibull modulus of both properties was significantly improved for the indirect gated cast test bars compared to the direct gated bars. A detailed microstructural characterization was carried out on the cast test bars, which included grain size, dendrite cell size and porosity. Scanning electron microscopy was used to examine and analyze the presence of defects on the fracture surfaces such as shrinkage pores, entrapped bubbles and oxide films resulting from surface turbulence during mould filling. The results indicated a clear correlation between the mechanical properties and the presence of casting defects. Water modelling experiments were carried out using purpose-built experimental centrifugal casting equipment and filling sequences recorded using a high speed video camera. The water modelling results showed that the general tendency for the direct and indirect gated mould designs was that the higher the rotational velocity, the lower the filling length and consequently the lower the filling rate. Subsequently, this information was used to validate the computer software ANSYS CFX. An excellent correlation was obtained between the experimental water modelling and simulation results for both direct and indirect gated moulds.

The microstructure evolution of a sand cast AZ91 magnesium alloy during heat treatment (solution treatment and subsequent ageing) were characterized quantitatively using a combination of optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The area fraction of discontinuous precipitates and number density of continuous precipitates (N\(_V\)) in the AZ91 alloys with and without pre-deformation were measured using OM and TEM, respectively. Based on these metallurgical evidences, the existing precipitation strengthening mode for AZ91 was modified and the effect of pre-deformation on the precipitation strengthening of AZ91 was investigated. Al-Mn-(Mg) particles in the size range of 20-200 nm have been found in the as-cast AZ91. Their morphologies, chemical composition and structures were investigated using TEM. It was found that these particles have a chemical composition of (Al \(_6\)\(_.\)\(_5\)\(_2\)Mn) \(_1\)\(_-\)\(_x\)Mg\(_x\) (x < 0.13) and a decagonal quasi-crystalline structure. These particles were stable during a solution treatment and acted as preferential nucleation sites for continuous Mg\(_1\)\(_7\)Al\(_1\)\(_2\) precipitates during the subsequent ageing. The results obtained using two electron tomography (ET) approaches were also summarized in this thesis. One uses HAADF-STEM for Mg\(_1\)\(_7\)Al\(_1\)\(_2\) precipitates on Al-Mn-(Mg) particles. The other technique is BF-STEM applied to study Mg\(_1\)\(_7\)Al\(_1\)\(_2\) precipitates on the dislocations.

The focus of this research was to design, optimise and deploy a practical multi-measurand sensor (MMS) for process monitoring of glass and carbon fibre preforms in an autoclave. The MMS design was based on the optical fibre-based extrinsic Fabry-Perot interferometric (EFPI) strain sensor. The EFPI sensor consisted of a pair of cleaved optical fibres, with a defined gap that were housed in a precision bore capillary. Interferometric interrogation was used to measure the cavity length hence, permitting the strain to be determined. The basic EFPI sensor design was modified in the following manner to enable multi-measurand monitoring. Firstly, a fibre Bragg grating (FBG) was inscribed on one of the optical fibres that were destined to be housed in the capillary; the end-face of this cleaved optical fibre was sputter coated with Au/Pd to produce a reflective surface. This FBG is in a relatively strain-free condition and only responds to temperature. Secondly, a technique was developed to manufacture optical-quality end-faces of the capillary and this too was sputter coated with Au/Pd. Thirdly, secondary cleaved optical fibres were packed around the lead-in optical fibre of the EFPI sensor with a defined (secondary) cavity thus, permitting transmission/reflection FTNIR spectroscopy. Finally, cleaved secondary optical fibres were also secured to the lead-in fibre and these served as Fresnel reflection sensors. The interrogation of the MMS was carried out using a conventional fibre-coupled multi-channel FTNIR spectrometer. The feasibility of monitoring strain, temperature, cross-linking kinetics and refractive index simultaneously during the processing of glass and carbon fibre preforms was demonstrated.

Complex heat treatments and manufacturing processes such as welding involve a wide range of temperatures and temperature rates, affecting the microstructure of the material and its properties. In this work, a diffusion based approach to model growth and shrinkage of precipitates in the alpha + beta field of Ti-6Al-4V alloys is established. Experimental heat treatments were used to validate the numerical predictions of the model for lamellar shrinkage, whilst data from literature have been used to evaluate the numerical model for the growth of equiaxed microstructures. The agreement between measurements and numerical predictions was found to be very good. Experimentally-based approaches were used both to describe the growth of alpha lamellae and martensitic needles while cooling down from temperatures above the beta transus, and beta grain growth for temperatures remaining above the beta transus. Such models were coded in the commercial FE software Visual-Weld for the prediction of microstructure evolution during welding simulations. Experimental welding tests were carried out to validate the predictions. The metallurgical models developed were linked with a mechanical physically based model to predict the flow properties and the initial implementation of the coupled models in Visual-Weld is discussed.

Transparent conducting coatings are used in a wide range of applications, particularly displays and photovoltaic devices, and demand is anticipated to increase with the rise of plastic electronics. Indium tin oxide, deposited by sputter coating, is widely used. Concerns over the cost and long term availability of indium, together with environmental concerns over production, are driving a search for more efficient deposition methods and alternative materials. This work represented the early stage development of industrially applicable coatings, hence the work was performed with a supply chain partnership to facilitate commercial uptake. Sol-gel technology was used to facilitate direct printing of patterned layers. Precursors were selected and a processing route developed. Values for glass transition and crystallization temperature were found to be considerably lower than the accepted bulk value. Dip-coated parts yielded indium tin oxide coatings with transparency of 98% and resistivity of 1.5x10\( -\)\( 2\)\(\Omega\)cm fired in nitrogen and 98% transparency, 3.0x10\( -\)\( 2\)\(\Omega\)cm resistivity fired in air. A printed coating was produced with 93% transparency and 7.0\(\Omega\)cm resistivity. It was established that the limitations to conductivity arose from low film thickness and lack of macro-scale connectivity within the layer. Initial work was also conducted in preparing tin oxide and silicon-doped zinc oxide coatings.

The Bauschinger effects in X70, X80 and X100 high strength low alloy pipeline steels were presented. The microstructure of the as-received alloys was characterized. A variety of microstructures was present across the different alloys, ranging from a refined granular ferrite with small amounts of perlite to a bainitic structure containing martensite/austenite islands, retained austenite and small cementite constituents along with a small amount carbides. Similarly, the dislocation structures varied from homogeneously distributed across the ferrite grains to clusters/walls of dislocations. Mechanical tests on macro and micro sized samples were carried out up to 1% and 2% plastic strains. A micro-device for Bauschinger test was designed and manufactured using micro-electro-mechanical-system (MEMS) technology and was incorporated onto a FIB/SEM in order to prepare the micro sized samples and perform the micro Bauschinger tests. The Bauschinger stress parameter showed that the Bauschinger effect becomes more obvious in samples with higher yield strength and also with increasing pre-strain. In addition, the Bauschinger effect remained similar on the samples of different sizes in the current study. The results indicate that the cause of the early yielding during reverse loading of these alloys is dominated by the dislocation-dislocation interaction.