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Thermal spray tooling is one of a number of technologies which have been developed to satisfy the need for low cost tooling, which is used when prototype components are required in the correct engineering material. In its current state, the technology has a number of fundamental shortcomings. The aim of this portfolio was to address these shortcomings, via a combination of experimental work and technology demonstrators - these are summarised as follows: • An experimental programme aimed to quantify many of the problems associated with thermal spray tooling. A wide variety of tests on thermal spray surfaces was carried out, in order to compare their performance with other 'low cost' tooling techniques. For the first time, tooling shells were produced using the High Velocity Oxy-Fuel (HVOF) technique. This system is capable of producing extremely high quality tool shells, and the technique developed involves the novel use of castable ceramic patterns - the first time a releasable pattern has been developed for this spray system. In addition, it was established that 'hybrid' tooling shells could be produced - these were formed using a combination of arc spray and HVOF layers. The work proved that these hybrid shells could provide substantial performance benefits in terms of wear performance and vacuum integrity, when compared to conventional arc sprayed shells - this benefit was also achieved without significant cost penalty. The programme also investigated the effect of thermal cycling on thermal spray samples - it was shown that repeated cycling at high temperature had an adverse effect on both arc sprayed and HVOF samples - the extent of this effect was very much dependent on the material. • The portfolio includes a technology demonstrator programme, which was carried out for Rover Group to show the potential of thermal spray tooling. The programme entailed the manufacture of a suite of 5 tools for compression moulding of Glass Mat Thermoplastic(GMT). The actual route used for the production of the tooling suite involved many unique features, which had not previously been utilised for thermal spray tooling production. One of the tools is the largest ever produced for compression moulding using thermal spraying, being approximately 4m2, and weighing in excess of 3 tonnes. Due to the compressive stresses involved in the moulding process, conventional resin backing systems were unsuitable for this tooling. It was therefore necessary to use a Chemically Bonded Ceramic (CBC) material, with an exceptionally high compressive strength. However, this material does not adhere to thermal spray surfaces, and it was therefore important to develop a novel fixing method at the interface of the materials. Further to this, in certain cases the use of thermal spraying was precluded by the component geometry - in these cases it was necessary to use the CBC material as the direct tool face. This was the first time that CBC tooling had been used for compression moulding GMT, and it was therefore necessary to develop new post-treatments for this inherently porous material. The moulding operation then entailed the development of specific techniques and conditions for this prototype tooling, which would not generally be used in production - normal moulding conditions for 'production' tooling were therefore inappropriate. Further work will entail materials development, the introduction of automation and development of design rules, specifically aimed at the production of large tooling for the aerospace and automotive sectors - this will be carried out via a successful project submission under the Innovative Manufacturing Initiative (IMI).

Zinc coated steels are widely used in the automotive industry, because of the improved protection against corrosion. Their use has consequences for the resistance welding process, which is the most widely used method of joining body panels. The zinc coating alloys with the copper electrode, resulting in increased electrode wear, and a reduction in electrode life. The welding current must be increased, because of the reduced contact resistance and thus heavier cables and power sources are required. A novel form of power source, the Medium Frequency Direct Current inverter, offers advantages over the traditional AC transformer. The higher operating frequency results in a lighter transformer, and a smaller welding current may be used, because the DC welding current generates heat at a constant rate, and is thus more effective than an AC power source. A potential advantage of this technology is that the increased frequency allows improved resolution in monitoring and control. Novel signal conditioning circuitry was developed, allowing significant improvement in the time resolution of the voltage and resistance signals. A series of welding trials was conducted, while monitoring the welding process. The correlation between weld quality and various process variables was assessed, and a control algorithm to compensate for electrode wear was proposed. This algorithm, based on a constant voltage principle, was implemented on a bespoke welding timer. A significant improvement in electrode life was obtained using this technique. The control algorithm was shown effective experimentally, but practical limitations do not permit testing under all possible conditions. A numerical model of the spot welding process, using Finite Difference technique, was developed. Following successful validation, the model was used to predict the performance of the control algorithm under various conditions of electrode wear. The results indicate that a constant voltage algorithm can compensate for an increase in electrode tip diameter, but that a change in contact resistance may result in unsatisfactory welds.

This thesis reports upon a second phase of research into the effects of free-stream turbulence quantities on heat transfer to impermeable gas turbine blading. It describes the development of a novel form of turbulence generator, to control separately the turbulence quantities intensity and frequency upstream of a cascade of blades at levels typical of the gas turbine. The turbulence generator was calibrated for these individual quantities, with a hotfilament anemometer system combined with on-line analog and digital signal processing. Blade heat transfer coefficients measured by two independent techniques are compared. A large quantity of data is presented, taken from a first stage high pressure rotor blade and a nozzle guide vane. These were subjected to steady flow and turbulent streams induced by both the novel turbulence generator and by more conventional turbulence grids. Surface pressure measurements have also been made, to predict the heat transfer rates by applying formulae derived from simple geometries. Much of the boundary layer over the two blades was apparently laminar. For the laminar regions, the simple formulae for heat transfer (flat plate for example) multiplied by a turbulence term, will provide as good a correlation as any. The intensity Tu is the most important turbulence quantity, but there is some evidence that the frequency of the perturbations can effect heat transfer. Other evidence presented would s'uggest that profile geometry is an overriding factor, which dictates the development of the turbulence, whatever its origin, as well as controlling its interaction with the boundary layer. None of the correlations available for the prediction of boundary layer transition are applicable. On both blade suction surfaces separation seems to have occurred, and the analysis indicates that transition on the pressure surfaces of modern blades will be inhibited by the high free-stream accelerations. Beyond transition, heat transfer is little affected by turbulence. It is now clear, that measurements of the turbulence as it develops through the cascade must be performed before a successful prediction procedure for all of the boundary layer regions can emerge.

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No description available.

No description available.

No description available.

No description available.