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.

Linear friction welding (LFW) has been selected as a screening method to investigate the solid state joining of the Ni-based superalloys: IN718, RR1000, Alloy10, IN100, IN713LC and CMSX4, with the volume fraction (Vf) of γʹ range from 20-70%. Similar welds of each alloy and one dissimilar weld, IN718-IN713, are studied in this project. The extreme thermomechanical history during LFW results in dynamic recrystallisation in the weld zone, dissolution and reprecipitation of the strengthening phase in the HAZ. Sustained load crack growth (SLCG) threshold testing was carried out at 650°C in air for all the similar welds. Weld IN718 which had a low Vf of γʹ+γʹʹ, had the highest threshold (25MPa√m) and highest SLCG resistance. However, there was no such clear effect of γʹ size and volume fraction on threshold value (9-19MPa√m) in welds with a high γʹ content. The crack growth rates of all these welds accelerated sharply from 10-5 to 10-2mm/s over a limited increase in K. A comprehensive mechanical assessment of dissimilar weld IN718-IN713 was carried out, including tensile tests at room temperature and 630ºC, low-cycle and high-cycle fatigue tests at 630ºC on notched specimens, fatigue crack propagation testing at 630ºC, and stress rupture testing at 650ºC.

The steel industry is facing significant competition on a global scale due to the drive for light-weighting and cheaper more sustainable construction. Not aided by oversupply in geographic sectors of the industry, there is significant competition within the slowly shrinking sector. The recent growth in developing countries through installation of modern plant technology has led to the reduction in unique selling points for mature steelmaking locations. As such, to compete with the equalling product capability and innate cheaper production costs within developing areas the industries in Europe and North America require significant improvements in productivity and agile resource management. To date the basic oxygen furnace has been somewhat treated as a black box within industry, where only control parameters are monitored, not the fundamental mechanisms within the converter. Studies over the past 30 years have shown the basic oxygen furnace is unable to attain the thermodynamic minimum phosphorus content within the output liquid steel. Coupled with the need to drive down resource cost, with a potential for high content phosphorus ores the internal dynamic system of the basic oxygen furnace requires more rigorous understanding. With the aid of in-situ sampling of a pilot scale basic oxygen furnace, and laboratory studies of individual metal droplets suspended in a slag medium (known to be a key driving environment for impurity removal) the present project aims to provide insight into the transient interfacial area between slag and liquid metal through basic oxygen steelmaking processing. Initially the macroscopic dynamics including the amount of metal suspended in the gas/slag/metal emulsion, the period of time it is suspended for, and the speed at which it moves, is investigated. It was found that these parameters vary greatly through the blow, with a normal peak in residence times near the beginning of the blow and a dramatic increase in metal circulation rates at the end of the blow, when foaming is reduced or collapsed. Further to this, a method of interrogating the size of metal droplets within the slag layer using X-ray computed tomography is introduced. The study then progresses into the microscopic environments that individual droplets are subjected to during steel processing. Initially the cause of spontaneous emulsification in basic oxygen furnace type slags is investigated through high temperature-confocal scanning laser microscopy/X-ray computed tomography led experimentation, with the addition of null experiments conducted to rationalize the experimental technique. It was found that the flux of oxygen across the interface was the cause and thus the confirmation of material transfer across the interface being the driving force. Furthermore the physical pathway of emulsification is interrogated and quantified, with in-situ observation of spontaneous emulsification in the high temperature-confocal scanning laser microscope enabled through use of optically transparent slags. The life cycle of perturbation growth, necking and budding is observed and quantified through high-resolution X-ray computed tomography. In addition a phase-field model is developed to interrogate slag/metal systems in 2D and 3D variations, giving rise to the ability to track the cause of emulsification and to predict its occurrence. Finally the project progresses with the in-situ investigation of spontaneous emulsification as a function of initial metal composition. The behaviour of droplet spontaneous emulsification is seen to reduce in severity and subsequently to decline into a non-emulsifying regime below a critical level. Free energy calculations coupled with a measure of the global interfacial tension increase give quantifiable reasoning as to the behaviour seen.

Vermiculite is a clay mineral that is generally used for a wide range of applications such as in agricultural, horticultural and construction industries. This is due to its various properties which include high porosity, lightweight, thermo-insulating, non-toxic and good absorption capacity when exfoliated. The objective of this research was to critically evaluate the fundamental interaction of electromagnetic waves with vermiculite from different source locations and to understand the mechanism of exfoliation in an applied microwave field. When vermiculite minerals are placed under the influence of high electric fields, they expand due to the rapid heating of their interlayer water, which subsequently builds up pressure that pushes apart the silicate structure. The degree of exfoliation is directly related to the intensity of the applied electric field. The principal areas covered in this thesis include: a detailed review of the fundamentals of microwave processing and issues surrounding scale up; a critical literature review of vermiculite mineralogy, and previous methods of vermiculite processing and their limitations; understanding the interaction of microwave energy with vermiculite by carrying out mineralogical and dielectric characterisation; microwave exfoliation tests of vermiculite minerals from different source locations and a comparative energy and life cycle analysis of microwave and conventional exfoliation of vermiculite. A detailed review of the literature revealed that conventional exfoliation of vermiculite by gas or oil fuelled furnaces has significant limitations such as emissions of greenhouse gases, high-energy requirements (greater than 1 GJ/t), health and safety issues and poor process control. All work reported so far on microwave exfoliation of vermiculite has been limited to laboratory scale using domestic microwave ovens (2.45 GHz, power below 1200 W) and the route to scale up the process to industrial capacity has not given due consideration. Mineralogical characterisation of vermiculite from different geographical locations (Australia, Brazil, China and South Africa) revealed that only the sample from Brazil is a pure form of vermiculite while the other samples are predominantly hydrobiotite. All the samples have varying degrees of hydration with the Brazilian sample having the highest total water content. The presence of water in any form in a material influences its dielectric response and ultimately the microwave absorbing properties. The dielectric characterisation carried out on the different vermiculite samples shows that the vermiculite mineral structure is effectively transparent to microwave energy, but it is possible to selectively heat microwave absorber, which is the interlayer water in the vermiculite structure. The continuous microwave exfoliation tests carried out at both pilot scale at 53-126 kg/h and the scaled up system at 300-860 kg/h demonstrated that microwave energy can be used for the industrial exfoliation of vermiculite at high throughputs and is able to produce products below the specified product bulk densities standard required by The Vermiculite Association (TVA). The degree of vermiculite exfoliation depends on factors such as power density, feedstock throughput, energy input, interlayer water content, particle size of the feedstock, and vermiculite mineralogy. The highest degree of exfoliation was recorded for the Brazilian sample, which also had the highest water content. Life cycle analysis (LCA) frameworks by the International Organisation for Standardisation (The ISO 14040: principles and framework and ISO 14044: Requirements and guidelines) and British standards institution (PAS2050) were used to carry out comparative life cycle analysis of vermiculite exfoliation using microwave heating and conventional (industrial and Torbed) heating systems. The results showed that the microwave system potentially can give an energy saving of about 80 % and 75 % over industrial and Torbed Exfoliators respectively, and a carbon footprint saving potential of about 66 % and 65 %. It can be concluded that the reduced dust emission and noise from the microwave system would improve the working conditions, health and safety. Furthermore, the methodology discussed in this project can be used to understand the fundamental of microwave interaction with perlite and expanded clay, which are minerals with similar physical and chemical compositions as vermiculite.