Titanium is well known for its excellent properties, such as high strength-to-weight ratio and outstanding corrosion resistance. However the high cost of this metal has confined its applications to those mostly within the aerospace and military industries. The high purchase price of titanium is primarily driven by the need for intricate metal extraction processes, as well as the sensitivity towards conventional metal working operations. Among the potential solutions, powder metallurgy (P/M) technology provides an economical approach to bring down the price of finished titanium products. However, there are still many problems, such as the residual porosity in the sintered body, that need to be overcome. In this thesis, a fundamental study was carried out focusing on the P/M press-and-sinter technique, using commercially pure titanium (CP Ti) as well as two binary titanium alloys, namely Ti-Ni and Ti-Sn. The influence of several processing parameters including compaction pressure, lubricant type/concentration, sintering time/temperature were performed on both the CP and binary systems. The principal tools utilized for mechanical characterization were hardness and tensile testing, whereas optical microscopy, x-ray diffraction (XRD), and scanning electron microscopy were employed to identify the microstructural features present. Press-and-sinter P/M strategies were successfully developed for all of the blends studied. For CP-Ti, a maximum tensile strength >750MPa and near full theoretical density (~99%) were achieved. Transitions in the size and the size distribution of pores and ?-Ti grains were also observed and quantified. It was found these transitions, as well as the powder impurities present (i.e. oxygen and carbon), greatly influenced the final mechanical properties. In the case of the binary alloys, it was shown that liquid phase sintering (LPS) significantly improved the sintered density for the Ti-10%Ni composition, when sintered at l100°C. A eutectic microstructure (CP-Ti + Ti2Ni), coupled with grains of CP-Ti, were identified as the principal phases present. On the other hand, the Ti-Sn alloys only showed a modest increase in sintered density compared to the CP-Ti, owing to the high solubility of Sn in Ti. In terms of crystal structure, XRD highlighted that the Sn containing samples were fully CP-Ti.

This research studies microdrilling of biocompatible materials including commercially pure titanium, 316L stainless steel, polyether ether ketone (PEEK) and aluminum 6061-T6. A microdrilling technique that uses progressive pecking and micromist coolant is developed that allows drilling of 127 micrometers diameter microholes with an aspect ratio of 10:1. The drilling parameters, dominant wear pattern, hole positioning accuracy and effect of AlTiN tool coating are experimentally determined. The experimental data trend agrees with classical Taylor's machining equation. Despite of fragile and long microdrills, the progressive pecking cycle and micromist allowed deep hole drilling on all the tested materials. Drill wear is more pronounced at outer cutting edge due to higher cutting speeds. However, when drilling 316L stainless steel attrition wear at chisel edge is dominant. Hole quality degradation due to formation of built up edge at the drill tip is observed. Coated drill improves tool life by 122% and enhances hole quality when drilling 316L stainless steel. The hole positioning accuracy is improved by 115% and total hole diameter variation decreased from 0.11% to 0.003% per mm of drilling distance.

The optimisation of compressor stage aerofoil and fan blade design remains an important area of titanium alloy research and development for aerospace gas turbines. Such research has important implications for critical and sensitive component integrity and efficiency. In particular, a better understanding of how deformation twinning interacts with microstructural features in titanium alloys is required, because such twinning facilitates plastic deformation at a higher strain rate than dislocations. To investigate this behaviour, commercial purity titanium and the titanium alloy Ti–6Al–4V were subjected to ballistic impact testing at room temperature with a high strain rate of 10³s⁻¹. In addition, a detailed analysis was conducted of three manufacturing processes of Ti–6Al–4V (wt. %) that are likely to cause deformation twinning: metallic shot peening, laser shock peening and deep cold rolling. The results presented in this thesis have furthered the understanding of the role of deformation twinning in the plastic deformation of α-phase titanium. Key findings of the research include the characterisation of deformation twinning types and the conditions that favour certain deformation twinning types. From the analysis of the ballistic testing of commercial purity titanium, the first definitive evidence for the existence of {112‾4} twinning as a rare deformation twinning mode at room temperature in coarse-grained commercial purity titanium is presented. In addition, the ballistic testing results of the Ti–6Al–4V alloy highlighted very different deformation twinning characteristics. Commercial purity titanium deformed plastically by a combination of {101‾2} and {112‾1} tensilve twinning and {112‾4} and {112‾2} compression twinning modes. By contrast, the deformation twinning of Ti–6Al–4V was limited to only the {101‾2} and {112‾1} tensile twinning modes. The two tensile deformation twinning types have very different morphologies in equiaxed fine grained Ti–6Al–4V. {112‾1} deformation twins span multiple grain boundaries and {101‾2} deformation twins reorient entire grains to a twinned orientation. This observation provides evidence for whole grain twinning of equiaxed fine grained Ti–6Al–4V by {101‾2} twinning. Grain boundary interactions between various deformation twinning types and alpha phase grain boundaries in commercial purity titanium and Ti–6Al–4V are reported and analysed. In commercial purity titanium {101‾2} as well as other deformation twinning types were observed interacting across alpha phase boundaries and higher angle alpha phase grain boundaries. The analyses of the manufacturing processes of Ti–6Al–4V highlight the very different dislocation and deformation twinning structures in surfaces processed by these techniques. A notable feature of material processed by laser shock peening is the almost complete absence of deformation twinning, contrasting with the frequent observation of extensive deformation twinning observed in the material processed by metallic shot peening and deep cold rolling. Therefore, the findings suggest that there is a strain rate limit above which deformation twinning is suppressed. The implications of this research are that a better understanding of the conditions that that favour certain deformation twinning types or propagation behaviours will enable more accurate plasticity modelling and better alloy design. This is important for the design and the manufacturing of titanium components and the high strain rate deformation to which titanium components in aerospace gas turbines can be subjected because of bird strike, foreign object debris ingestion or fan blade failures.