Inconel 718 (a nickel based superalloy) is widely used in the aerospace industry for the manufacture of aeroengine components that are subjected to relatively high temperature and stresses during operation, such as turbine disks, shafts, compressor blades and combustion chamber casings. Aeroengine manufacturers have recently begun to evaluate alternative production methods for these parts as opposed to the conventional route, which can involve arduous and environmentally hazardous chemical milling processes. High speed machining (HSM) involving the use of end / ball end mills is one such technology that has been earmarked as a potential substitute after successful trials on titanium components yielded significantly improved productivity and component quality. Metal cutting research has traditionally been a laboratory-based activity requiring massive investment in equipment, manpower, time and workpiece materials. Over the course of the last two decades however, computer based techniques such as finite element (FE) modelling have been shown to provide an acceptable and cost effective tool for metal cutting process simulation. Unfortunately, up until recently, the majority of published FE work on metal cutting has been confined to 2D orthogonal turning. Following an extensive literature review on analytical modelling techniques and FEM in metal cutting together with HSM technology, details are given of the development of a Lagrangian based, 3D finite element model to simulate the high speed ball nose end milling of Inconel 718 using the commercial FE package ABAQUS Explicit. Preliminary 2D, plane strain analysis of orthogonal turning was carried out utilising ABAQUS Standard followed by 3D turning using ABAQUS Explicit as a precursor to full scale milling research. ABAQUS Standard was deemed to be inappropriate for segmented / discontinuous chip and 3D formulations due to the nature of its code, which was not suited to highly non-linear and large deformation type problems. Outputs from FE models were validated against corresponding experimental data. Chip morphology studies were performed using an explosive quick stop device for orthogonal turning. This was used to investigate the effect of cutting speed (20,50 and 80m/min) and tool rake angle on chip formation, but also to obtain input 'parameters such as shear angle & deformed chip thickness for analytical modelling of cutting temperatures as a comparison with predicted FE results. Force measurements were carried out using Kistler piezoelectric dynamometers while chip surface temperatures were measured using infrared techniques. Flow stress data ofInconel 718 at elevated strain rates (up to lOOS-I) and temperatures (up to 850°) were determined through uniaxial compression tests on a Gleeble 3500 thermomechanical simulator. Based on this Gleeble data, material property constants for the overstress power law constitutive model were calculated. This was then applied in the finite element models in order for flow behaviour at high strain rates and temperatures to be reasonably defined. The 3D turning model in general provided reasonable predictions of tangential cutting forces and chip surface temperatures, with a difference of approximately 14% and 5% respectively to experimental results. The model however failed to describe the segmented chip morphology that was expected when turning at 50 & 80m/min. This was due to the lack of a user subroutine containing a suitable fracture criterion to define the adiabatic shear localisation and failure within the chip.

The mechanical and toughness properties of six structural steels have been investigated in detail over a range of temperatures between 20°C and -196°C. The true stress - true strain characteristics have been analysed using several different approaches. Strain hardening exponent and transition strain valves were determined from Hollomon's equation σT = K εTn using the simple log σT - log εT plot and the more complex Crussard analysis. The crussard analysis was found to be unreliable in prediction of both transition strain and strain hardening exponent. As deformation temperature was reduced below ambient, the strain hardening exponent values tended to a maximum at some critical ternperature below which the strain hardening exponent values decreased. Transition strain tended to increase with decreasing deformation temperature, until at some critical temperature two stage hardening was replaced by single stage hardening. The variation of true work hardening rate (d σT/d εT) with strain and temperature was determined. At high strains, the true work hardening rate increased lineally with decreasing deformation telnperature whilst at low strains the same behaviour occurred until a critical deformation temperature was attained, when the true work hardening rate decreased. Critical crack opening displacement (C.O.D.) values were deterlnined for all the alloys over the temperature range. The critical event was defined as the onset of slow crack growth, or fast fracture, whichever occurred first. Onset of slow crack growth was detected by a potential drop technique, which was found to give good reproducibility in critical C. O. D. values. The normal transition behaviour was observed in these tests and the effect of increasing grain size on reducing toughness was investigated. No direct correlation between work hardening behaviour and toughness could be deduced and the relationship previously derived by Hahn and Rosenfield, and by Thomason, relating material mechanical properties to plane strain fracture toughness were found to be unacceptable in the light of the data obtained. It has been assumed in the past that the plastic instability strain, as measured in a tensile test, is the relevant strain when considering slow crack growth as the failure mechanism. It is proposed in the present study that a more realistic approach is to consider the critical strain as being that strain at which void linkage occurs.

The aims of this research were to study the effects of electrolyte concentration, surface finish of samples and different types of corrosion on electrochemical noise. Basically, electrochemical noise is a term used to describe the spontaneous fluctuations of potential or current at electrode surfaces in electrochemical processes. In order to achieve the research aims, several experiments involving different corrosion systems were carried out. The corrosion systems were chosen to produce known types of corrosion. For the effects of varying electrolyte concentration on uniform corrosion, two systems were chosen; mild steel in hydrochloric acid, HCI and in sulfuric acid, H2S04 solutions. As for pitting corrosion, four systems were chosen, namely mild steel in a mixture of calcium hydroxide, Ca(OH)2 and sodium chloride, NaCI, mild steel in a mixture of sodium nitrite, NaN02 and NaCI, stainless steel 303 and stainless steel 304 in NaCI solution. Electrolyte concentration was varied in experiments involving the two former systems in order to see the effect of concentration on pitting corrosion. For the other two systems, the surface of the electrodes was varied to three finishes. 320. 600 and 1200 grit. This was to see the effect of surface finish on pitting corrosion. Analysis of the data obtained was carried out by examining the time records, the power spectra (including the impedance spectra). corrosion parameters (noise resistance, Rpo characteristic charge, q and characteristic frequency,ln, which were derived from simple statistical parameters). Maps of pairs of the corrosion parameters and the cumulative probability of each parameter occurring were plotted. Evolvement of corrosion parameters with time was also investigated. Analysis of the time record showed that they could distinguish uniform corrosion from pitting corrosion. From power spectra analysis, it was found that the technique could not be used to see the effects of changes in concentration and surfaces finish nor could it be used to distinguish uniform corrosion from pitting corrosion. It was found that parameter maps were not able to distinguish the changes in solution concentrations and surface fInish. Parameter maps were not able to distinguish between uniform and pitting corrosion of similar type of samples from different systems but were able to distinguish uniform corrosion from pitting corrosion. Cumulative probability method was not able to differentiate the variation in solution concentrations and in surface finish. It was also not able to distinguish between uniform and pitting corrosion. Impedance spectra analysis was not suitable to indicate the types of corrosion occurring. It was also unable to distinguish the variations in the solution concentrations and surface finishes used. Finally, it was found that, evolvement of corrosion parameters is independent of time.

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Electrohydrodynamic atomisation (EHDA) spraying is a promising materials deposition technique as it allows uniform and regular deposition, and offers a range of other advantages, such as low cost compared with other current techniques, easy set-up, high deposition rate, ambient temperature processing and the capability to generate specific surface topographies. This research is aimed at using EHDA spraying to produce hydroxyapatite (HA) deposition with desirable chemical, topographical and biological characteristics for bone implant. In principle, the EHDA process involves the flow of liquid/suspension from a needle under the influence of an electric field which results jetting and droplet formation. In this work, phase-pure nano-sized hydroxyapatite (nHA) was synthesised and taken up in ethanol to prepare a suitable suspension for electrohydrodynamic flow processing. A range of key EHDA process control parameters, such as needle size, needle to substrate distance, suspension flow rate, applied voltage and spraying time, were studied and optimised. A uniform nHA coating with nanometer scale topographical features was successfully prepared on a commercially pure titanium substrate. Furthermore, due to the significance of the surface structure to the cellular response, a novel technique, namely template-assisted electrohydrodynamic atomisation (TAEA) spraying, was innovated to prepare a well-defined surface topography for guiding cell attachment, spread and growth of osteoblasts. A range of precise micro-scale uniform nHA geometries with high resolution were prepared on implants materials. Finally, to systematically investigate the effect of needle geometry to the electrospraying process, which has not been documented in the research field of this technique, an in-depth study was carried out to uncover the relationship between the needle exit angle and the droplet relic size. The droplet relic size, which is crucial for the deposition properties, has been significantly reduced via engineering the needle geometry during the electrospraying process. The results of this work have demonstrated that EHDA deposition routes show great potential for the commercial preparation of nHA coatings and patterns for bone implants with enhanced bioactivity.

A review of the literature on corrosion fatigue and fatigue testing is followed in this work by an analysis of the variables involved and of the various mechanisms which have been advanced. Particular reference is made to the corrosion fatigue of surgical implant alloys. An experimental programme was devised to be particularly relevant to the environmental variations possible in implant/body systems. A multi-specimen, reverse-bend, strain facility was designed and produced for this research. Small smooth specimens were tested at a 'walking pace' of 1.7 Hz. A multi-channel, mini-potentiostat was also designed and produced for electrochemical studies during corrosion fatigue. Scanning and transmission electron microscopy was used to examine fracture surfaces. The corrosion fatigue performance of some alloys (stainless steel type 316, titanium 130 and 318) currently used for surgical implants has been evaluated. A comparative study of the performance of mild steel, a material which has in the past been used for implants, was also made. A series of experiments was conducted to simulate normal conditions with a 0.17M saline solution buffered at pH 6.5. Some extreme conditions were also investigated at pH 1.5 and pH 11.5. In order to evaluate the comparative influence of saline, additional experiments were conducted in air and in distilled water for each material.A considerable number of metal/environment combinations was thereby investigated, involving corrosion fatigue in both the passive-state and the active state. Since it is common place for some implants to be drilled for fitting, a further series of experiments was conducted using specimens containing a drilled hole. The influence of such stress concentrating features was thus established for each material. The behaviour of mild steel in all the environments tested, and that of S316 in pH 1.5 saline was described using a single model for interaction between stress concentration and corrosion during fatigue. A different model was found to represent the behaviour of all the other metal/environment combinations tested. The two models were seen to represent, respectively, activestate corrosion fatigue and fatigue under passive state conditions. The drilling of holes in mild steel and T130 showed a small stress concentration effect. No such effect was produced with holes in stainless steel. By contrast, however, the drilling of a hole in T318 resulted in a dramatic reduction in the fatigue limit: a reduction of the order of 50% was obtained in both distilled water and saline solutions. Unlike other materials S316 produced a wide scatter band of results in saline pH 6.5. This was attributed to a naturally fluctuating corrosion potential indicative of an irregular film breakdown and repair. This was attributed to a naturally fluctuating corrosion potential indicative of an irregular film breakdown and repair. The results are generally discussed and the evidence produced shows that the use of S316 for surgical implants cannot be recommended because of poor resistance to corrosion fatigue at low pH. T130 was superior in this respect. However, T318 showed the best resistance to corrosion fatigue even allowing for the drastic reduction in performance with a drilled hole.

A less well known form of transverse cracking in carbon manganese and low alloy steel weld metals, generally referred to as chevron cracking, has been found in a large number of industrial welds. These cracks are characterized by their macroscopic orientation, which is approximately transverse to the welding direction and at 45 with the plane of the plates (in a butt joint), and by the staircase morphology, which is easily recognized under microscopical examination. A special test was designed to reproduce these cracks under laboratory controlled conditions and a large number of welds was carried out. Chevron cracks were found in the tests with medium strength weld metals deposited by submerged arc with a basic agglomerated flux and by manual metal arc with basic and cellulosic electrodes. No chevron cracks were observed in the welds with rutile electrodes. High temperature baking of the submerged arc flux and the basic electrodes eliminated or reduced markedly the incidence of cracking in all cases indicating that hydrogen was the main factor controlling the cracking occurrence. Cont/d.