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.

Applications for titanium metal matrix composites (TiMMCs) are currently being developed by Rolls-Royce plc in gas turbine engine components. With any component manufacturing process there is a probability of defects, as much of the work on TiMMCs to date has been laboratory scale there is as yet no study that looks specifically at the defects that can arise in full scale components. This work set out to investigate the influence of a selection of defects on the fatigue properties of titanium diffusion bonds – an integral joint type in TiMMC components – using conditions derived from TiMMC component stress analysis. The study found that cladding material microstructure and texture greatly affected the fatigue life of the bond. This was characterised by a new technique called Spatially Resolved Acoustic Spectroscopy (SRAS). Airborne debris and residual degreasing agent staining were found to be contaminants the most detrimental to fatigue life and methods of modifying the manufacturing process have been suggested to eliminate them. A number of other methods have been discussed for reducing the sensitivity of the TiMMC components to defects of this type through the control of residual stresses microstructure and texture.

Aerospace applications typically require electronic products with not only higher levels of reliability than the consumer electronics industry but also longer service lives within demanding working environments. The transition will inevitably mean changes to design and manufacturing procedures, which is likely to incur a significant cost to the business. For example, the best candidate Pb-free solder alloys have been shown to require higher soldering temperatures and have higher surface tensions. Moreover, a reduction in product safety and reliability is not acceptable to the industry. This present work is divided into three sections. Firstly, the effect of increased component soldering temperatures on the integrity of the epoxy laminate material used for manufacture of printed circuit boards (PCB) has been assessed. Secondly, the required changes in soldering process parameters have been investigated for a range of solders and PCB finishes, largely due to the different wetting characteristics brought about by the increased surface tension of the Pb-free solders. Thirdly, the reliability of SnAgCu solder is assessed in comparison to the currently utilised SnPbAg solder alloy. This has been achieved firstly by accelerated thermal cycling, as the dominant mode of failure in a solder joint is typically thermo-mechanical fatigue and as such is already well researched. In addition, the mechanical fatigue properties have been assessed using a novel accelerated vibration test method and then finally, the two individual accelerated environmental tests of thermal cycling and vibration have been combined in a novel way to assess whether the combination is especially dangerous for SnAgCu solder reliability. A secondary objective of the combined environment test was to see if the well established thermal cycling test method for demonstration of product reliability can be further accelerated while still producing solder joint failure representative of those in-service. The present work shows that SnAgCu solder has inferior thermo-mechanical and mechanical fatigue life to SnPbAg solder. A combined environment test has been developed which effectively combines the single environments of thermal and vibration. The combination of thermal cycling with superimposed vibration is especially dangerous for SnAgCu solder, where an 89% reduction in the characteristic life is observed when compared to the equivalent thermal cycling characteristic life. It is suspected that a large reduction in life will be observed in SnPbAg solder, but not as pronounced as SnAgCu due to SnPbAg solders ability to better withstand plastic deformation that is induced by thermal cycling.

To improve the efficiency of turbine aero engines, higher operating temperatures and weight savings are being investigated. Alloys such as RR1000 are being trialled as they perform better at higher temperatures than current nickel-based superalloys. To achieve weight savings, inertia welding is being trialled for turbine discs but current post weld heat treatments reduce fatigue life. In this investigation, a number of novel post weld heat treatments were trialled aimed at improving post weld microstructure and fatigue properties. Extensive microstructural characterisation and mechanical testing were used to assess the effect of these treatments on both parent and weld materials. Post weld heat treatment (PWHT) was found to have a great effect on the size and distribution of γ' and carbides, particularly when a PWHT temperature of 980ºC or above was used. The effect of this microstructural change on the hardness of the weld and parent material has also been characterised. Extensive total life fatigue testing was carried out at 650ºC. It was found that failure can occur in both the parent and weld material, although it is deduced that the yield stress of the weld needs to be surpassed to see weld failure (plasticity in the weld). Increasing dwell time at peak load reduced the life of these components. Two mechanisms for crack growth were observed with initiation either at the surface or at a large Hf rich particle subsurface. Oxidation was found to have a large effect on both initiation and growth of fatigue cracks. By introducing a sharp pre-fatigue crack into samples, static load testing was used to determine a threshold value of K (stress intensity factor) for growth and growth rates were plotted at different K values. It was seen in these tests that PWHT had a large effect on growth rates and threshold values of K.

This work covers two projects, with the common theme being fine metallic powders and the use of polymer binder systems to aid processing.