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.

Selective Laser Melting (SLM) was used to develop a manufacturing route for high temperature aero-engine components from the Ni-superalloys CM247LC, focussing on improving the microstructure, mechanical properties, and processing route. A statistical design of experiments approach was applied to determine the optimum processing parameters leading to the least structural defects. High-speed imaging was used to observe the melt pool during SLM. Microstructural investigations showed that certain elements were selectively evaporated, then condensed in the form of particles. These were then re-incorporated within the build. Cracks and pores were found in SLM-processed samples and these were sometimes associated with these condensed particles. Residual stresses, developed within SLMprocessed samples, were measured using neutron diffraction, highlighting the role of the scanning strategy on the residual stress development. The solidification microstructures formed in SLM-processed samples were characterised using analytical scanning and transmission electron microscopy. Cells, with identical orientation and 700 nm in width containing a high density of very small γʹ (up to 20 nm), were observed. Cell boundaries and grain boundaries were found to contain high densities of dislocations, Hf/Ti/Ta/W-rich precipitates and γ/γʹ eutectic containing larger particles of γʹ up to about 50 nm. The cooling rate derived from the cell size was estimated at 106 K/s, but the cooling rate, derived from the size of γʹ within grains was estimated as 104 K/s based on Jominy end-quench test. SLM-processed samples also showed high yield strength due to their fine microstructures, alongside poor ductility resulting from the presence of cracks. Post-SLM heat treatments were used to reduce the extent of cracking and porosity by Hot Isostatic Pressing (HIPping) and also to promote the precipitation of γʹ. These treatments improve the ductility in vertically built samples, but the ductility in horizontally built samples remains low. Though SLM-processed samples subjected to post-processing heat treatments showed poor creep strength, this was improved by HIPping. A novel approach for netshape SLM/HIP processing was assessed for manufacturing a blisk using powder CM247LC or dual materials.

Resistance against fatigue failure is a major requirement for critical rotating parts, which could be sensitive to surface condition after machining such as roughness, modification of microstructure and residual stress. The speed, tool material and other machining parameters can also have a significant effect on the surface quality of the parts. The effects of baseline and damage machining on the surface integrity of Alloy 720Li have been characterised comprehensively using metallography, EDX, surface roughness, micro hardness, nano-indentation, electron backscattered diffraction, differential scanning calorimetry and transmission electron microscopy. It was found that machining damage imparted a work-hardened layer, poorer surface roughness and surface features containing recrystallised material. A test matrix was constructed to establish whether machining induced changes of surface integrity would impact the fatigue performance of the material. Baseline samples and samples damaged by machining both with and without shot peening, and at different Kt factors were tested. Low cycle fatigue tests were conducted. Some damage machined samples exhibited a lower fatigue life. This was attributed to brittle surface features, caused by high temperatures and forces of machining, which resulted in early crack initiation.

This thesis focused on the development of Ti-Ni-based shape memory alloys using a novel alloy development process based on laser melting technique, suspended droplet alloying (SDA), and on development of a manufacturing route Ti-Ni-based structures using selective laser melting (SLM), specifically for Negative Poisson’s ratio (NPR) auxetic structures. To assess the SDA process, a series of Ti-Ni-based binary, ternary and quaternary SMA have been built to analyse the chemical and microstructural homogeneity of the builds produced by the SDA process. The shape memory performance in terms of the transformation temperature, thermal stability and thermo-mechanical stability was also investigated and compared with commercial SMA and literatures. It proved that the SDA built sample was comparable to the bulk SMA built by other alloy development processes. To assess the processability of Ti-Ni-based alloys, SLM process parameters were optimised for Ti-Ni SMA and the SMA NPR structure was built for mechanical testing. It was found that the SLM process parameters can severely alter the microstructure and shape memory properties due to different cooling rate and Ni vaporisation. Finally, TiNiCuNb and TiNiHfCuNb alloys were developed using SDA to improve the shape memory properties by altering the precipitates. The analysis of these alloys indicated that the addition of Cu, Hf and Nb altered the precipitate types and morphology altogether, resulting in a unique shape memory behaviour during thermal cycling.