The adoption of lithium-ion and/or super-capacitor battery technologies is a current hot topic in the automotive industry. For both battery types, the terminals and busbars are manufactured from copper (Cu) and/or aluminium-based (Al-based) alloys, as a result of their high electrical and thermal conductivities. Laser welding is considered an attractive process to industry due to its easy auto-motability, high processing speed and highly repeatable cost-effective processing. However, laser welding of monometallic and dissimilar Cu and A1 presents several difficulties due to the high surface reflectivity at infrared (IR.) wavelengths. Three main areas were investigated in this research work: • Laser beam lap-welding of monometallic Cu sheets, with the aim of validating the developed welding procedures against target specifications addressed by the automotive industry, in line with production environment setups. • The suitability of using tailored energy distributions, produced using a 1070nm laser source delivered through a laser beam scanner, for welding multiple overlapping Al or Cu foils. • The effects of different laser processing parameters on the formation of deleterious brittle intermetallic phases when welding Alto Cu with a continuous-wave Yb-fibre laser.

Linear friction welding (LFW) is a solid-state welding process (i.e. the melting temperature of the material is not reached) that is used for the fabrication of titanium alloy bladed discs (Blisks) in the fan and compressor stage of modern aero engines. Blisk technology enables a 20-30 % weight saving through removing the need for the dovetail attachment thus enabling slimmer, lighter and more streamlined disc architecture. This significant weight saving helps the aero engine meet environmental targets (ACARE, 2000) by reducing fuel burn, noise and emissions as well as eliminating fretting fatigue around the dovetail attachment and extending component life. LFW is considered to be a self-cleaning process where contaminants trapped within the plasticised layer are expelled into the flash thus producing a high integrity weld that is stronger than the parent material. This high integrity is critical for Blisk application as contaminants in the final weld joint may result in reduced component life or sudden unexpected failure. Despite the importance of weld integrity for Blisk applications, little is known about the underlying process physics of the cleaning regime therefore the aim of this thesis is to provide a thorough mechanistic understanding of the weld evolution of Ti- 6Al-4V LFW by empirically evaluating weld efficiency, material flow behaviour and weld cleaning behaviour over a range of appropriate key process variables.

An evaluation of inertia friction welds in RR1000 has been conducted through microtensile and sustained load crack growth tests on pre-cracked testpieces. As-welded and two post weld heat treatment conditions have been considered. The fatigue crack growth behaviour of γ TiAl has also been considered, with a focus on establishing fatigue crack growth thresholds.

In this work, a portfolio of novel active screen plasma surface engineering technologies have been developed including active screen plasma surface modification of carbon paper for gas diffusion layer (GDL) and active screen plasma surface co-alloying of 316 austenitic stainless steel with both interstitial alloying element of nitrogen and such substitutional alloying elements as silver (Ag), niobium (Nb) and platinum (Pt). The active screen plasma surface modification of GDL carbon paper at a low temperature for a short period of time can effective activate the carbon paper surface mainly due to the removal of the hydrophobic PTFE coating and introduction of many functional groups, thus contributing to the improved growth of Pt nano-wires. Accordingly, the electrochemical and catalysis performance can be effective improved. The novel ASP surface alloying technique developed from this research has been applied to modify the 316 stainless steel surface using nitrogen for active screen plasma nitriding (ASPN); nitrogen and silver (N&Ag); nitrogen and niobium (N&Nb); and nitrogen and platinum (N&Pt). The experimental results have demonstrated that the layer structure of the ASP treated 316 SS surfaces can be tailored by using different alloying elements and/or adjusting treatment parameters. The surface electrical conductivity of 316 can be reduced significantly. The ASPN, ASPA(N&Ag) and ASPA(N&Nb) increase the corrosion potential, lower the corrosion current density, but raise the passive current density of 316 SS. Among all the surface alloying treatments, the ASPA(N&Pt) treatment has delivered the best performance and fulfilled the technique target set by the Department of Energy (DoE), USA.

A comprehensive study of microstructure, texture and mechanical properties of Ti-6Al-4V joints produced by inertia friction welding is addressed in this thesis. Axis-symmetric tubular welds of different wall thicknesses were produced using different welding parameters. In the present study, there are two thin wall welds and three thick wall welds. A martensitic transformation occurred in the thin wall welds, resulting from the fast cooling rate, whereas it is prevented in the thick wall welds. The acicular microstructure in the Centre Weld Zone is finer and plastic deformation in the Thermal Mechanical Affected Zone is more severe with a higher energy input. In thick wall welds, prior β grain boundaries and grain boundary α are more clearly observed. Based on the reconstruction of prior β grains, the average β grain size can be related to the exponential of the forged pressure over the average input power density. The mechanical behaviour of both thin wall and thick wall welds for the varied welding parameters is also investigated, including tensile tests, fracture toughness tests, low cycle fatigue and fatigue crack growth resistance tests.

This thesis addresses aspects of the development of both processing methods and the assessment of the mechanical properties of titanium metal matrix composites in order for the material to be introduced with confidence into aero-engine applications. Assessment of the SM1140+ fibre has been carried out and compared with the SCS-6 and Trimarc fibres in order to gain an appreciation of the performance of these fibres in relation to each other to aid fibre selection and to aid further development of composite components. The SM1140+ fibre is found to fail almost always from the core and is consistent with a statistical distribution that can be modelled by a unimodal Weibull approach. The development of the SM2156 fibre was made in an effort to produce both a UK source and a lower cost source of fibre. Mechanical testing of fibre in both as-received and composite form revealed a decrease in strength when compared with results for the virgin, uncoated fibre and by deduction from SCS-6 composite mechanical behaviour. The deterioration of fibre properties appears to be caused by the rough surface of the SiC fibre causing a ‘keying’ effect that inhibits interfacial sliding. The high rate sputtering deposition process has been developed in order to obtain an alternative, lower cost method of producing matrix coated fibre. Testing of the MCF showed a mild deterioration of fibre strength during processing (due to fibre spooling), but still demonstrated the composite shows potential for production given further development.

The aim of this study was to develop a new castable and heat treatable alloy, based on the A205 alloy composition, with improved thermal stability at 230 °C. Several unique combinations of alloying elements, chosen from groups IIIB, IVB and VB, up to 0.5 wt. %, were added to the A205 alloy. These elements were chosen to nucleate high temperature precipitates, with a cubic L12 structure. The additions were made in group of two, with one elements being always the zirconium. After 1000 hours holding at 230 °C, several of these alloys resulted to have better mechanical properties when compared to the A205 alloy. The IDEAL alloy was chosen and further studies were carried out. This alloy showed the same tensile proprieties of the A205 alloy at peak-aged condition, while at elevated temperature, the IDEAL alloy had 15 % improved UTS and YS. An investigation was carried out to understand the reasons of this improvement. Two strengthening mechanisms were found. Firstly, the microstructure of the alloy showed a reduced size of the Al-Cu precipitates. Secondly, two different high temperature precipitates were nucleated.

To prevent the formation of ice on aircrafts, de-icing systems are incorporated into various structures. These can consist of multi functional composite structures. To maintain control over the production of these systems, the composite must be formed in stages. Although there is a significant body of literature describing the infusion of PEEK into fibre reinforcement, there are many inconsistencies regarding the selection of hold temperature, time and pressure. This work sought to identify a set of optimum process conditions for the infusion of PEEK into a glass fibre fabric. The key considerations were the effect of process conditions on degradation and subsequent crystallisation of PEEK. The repeatability of DSC analysis was determined through the use of thermal cycling through a range of temperatures from above melt (350°C) to above T m 0. The number of runs obtainable, from a sample of PEEK before the effects of degradation occurred, decreased when hold temperature increased. However, a distinct change in degradation rate was observed above 380°C. Results show that multiple runs of PEEK can be obtained at 380°, without the effects of degradation, while ensuring the removal of residual nuclei in the melt. The process of degradation affects the chemical structure of PEEK. Through Fourier-transform infrared spectroscopy (FTIR) of PEEK the effect of degradation can be measured in the variation in the height of peak 1653cm·1 (crystalline phase) and 1648cm·1 (amorphous phase). As the amount of nuclei available for crystallisation decreases the amorphous peak will increase in ratio in comparison to the crystalline peak.

Stress-assisted grain boundary oxidation (SAGBO) is one potential mechanism accounting for the detrimental effect of oxygen on dwell fatigue crack growth behaviour. This mechanism is assumed to weaken the grain boundaries by formation of oxide intrusion along the grain boundary ahead of the crack tip. In this thesis, finite element analyses of oxide formation along grain boundary ahead of a stationary crack have been carried out to study the stress-assisted oxidation behaviour and the corresponding evolution of stress state near the crack tip during oxide formation. A coupled stress-reaction model was established, of which the mechanical constitutive behaviour was represented by a microstructure-explicit constitutive model and the oxide formation by multicomponent diffusion and thermodynamics. Two microstructures with different particle distribution were involved in the simulations to study the effect of microstructure on the oxide formation. A criterion for oxide fracture was proposed. In combination with finite element simulations, it was used for prediction of crack growth rate during dwell period. The predicted results were in the same order of magnitude as experimental data and indicates a same trend of the influence of microstructure on crack growth rate as what was observed in literature.

The aim of the work reported in this thesis is to develop friction stir welding for superplastic titanium alloys. A number of studies have shown that friction stir welding is capable of retaining the fine-grained superplastic microstructure of the base metal and so there has been much commercial interest in combining it with superplastic forming processes. Within this programme friction stir welding was performed with a variety of process parameters and in both the conventional and stationary shoulder configuration. This meant that a number of welds were created with various rates of heat input. The elevated temperature and intense plastic deformation associated with the friction stir welding process substantially alters the base metal microstructure. As a result the influence of the process parameters on the microstructure, texture and residual stress development has been identified within this investigation. Tool wear and deformation also remains a hindrance in the commercialisation of friction stir welding titanium alloys. Wear and deformation of the tool not only changes its shape, but it can have further implications on the structural integrity of the weld. Thus the influence of the process parameters on wear and deformation of the tool have also been identified.