No description available.

The aim of this research was to develop a low-cost, desktop Additive Layer Manufacturing (ALM) system. A review of commercial ALM systems, a number of which are also called 3D printers, has been undertaken with the intention of identifying a suitable technology to embody within a system demonstrating low-cost and desktop characteristics. The review resulted in a commercially unexploited powder deposition technology, electrophotography, being identified. The significant barriers to implementation of this technology were the limitation of build height in the Z-axis due to electric field depletion and the formation of part material fringing due to the non-uniform electric field present at the boundaries of printed artefacts. Initial trials were undertaken using a laser printing system to determine the printing characteristics of low-cost sacrificial and recyclable materials such as Mica, flour and sugar as well as an engineering polymer, Nylon 12. Mixed results were seen due to the large distribution of particle sizes and their tribocharging characteristics. The identified limiting phenomena were recreated and analysed in order to develop possible solutions, and further testing on the electrostatic behaviour and print acceptance of substrate transfer materials was undertaken with standard styrene co- polymer based toner. Consolidation techniques were investigated and powder layer transfer mechanisms were trialled, culminating in the development of the novel thermal transfer system, eliminating both the build height phenomena and artefact fringing issues. Development of a complete prototype system was undertaken, producing a compact desktop system with novel process architecture. The system functioned through the electrostatic deposition of a polymeric thermoplastic material onto the surface of a registered PTFE transfer substrate. The powder image present on the transfer substrate, was brought into close proximity to a build platform, where the powder layer was heated, consolidated and mechanically transferred using a single moving mechanism. Later concepts describe the novel continuous printing process, exhibiting high productivity while maintaining accuracy and resolution. This work demonstrates a significant step forwards in the apparatus for use in an electrophotographic ALM system. In doing so, solutions to fundamental electrostatic transfer problems, and a clear route for the further development of a commercial electrophotography ALM process have been demonstrated. The system conceptualised, designed and produced within this research holds much novel value and provides a basis and direction for further development.

The demand for ultra precision machined devices and components is growing at a rapid pace in various areas such as the aerospace, energy, optical, electronics and bio-medical industries. Because of their outstanding engineering properties such as high refractive index, wide energy bandgap and low mass density, there is a continuing requirement for developments in manufacturing methods for hard, brittle materials. Accordingly, an assessment of the nanometric cutting of the optical materials silicon and silicon carbide (SiC), which are ostensibly hard and brittle, has been undertaken. Using an approach of parallel molecular dynamics simulations with a three-body potential energy function combined with experimental characterization, this thesis provides a quantitative understanding of the ductile-regime machining of silicon and SiC (polytypes: 3C, 4H and 6H SiC), and the mechanism by which a diamond tool wears during the process. The distinctive MD algorithm developed in this work provides a comprehensive analysis of thermal effects, high pressure phase transformation, tool wear (both chemical and abrasive), influence of crystal anisotropy, cutting forces and machining stresses (hydrostatic and von Mises), hitherto not done so far. The calculated stress state in the cutting zone during nanometric cutting of single crystal silicon indicated Herzfeld–Mott transition (metallization) due to high pressure phase transformation (HPPT) of silicon under the influence of deviatoric stress conditions. Consequently, the transformation of pristine silicon to β-silicon (Si-II) was found to be the likely reason for the observed ductility of bulk silicon during its nanoscale cutting. Tribochemical formation of silicon carbide through a solid state single phase reaction between the diamond tool and silicon workpiece in tandem with sp3-sp2 disorder of carbon atoms from the diamond tool up to a cutting temperature of 959 K has been suggested as the most likely mechanism through which a diamond cutting tool wears while cutting silicon. The recently developed dislocation extraction algorithm (DXA) was employed to detect the nucleation of dislocations in the MD simulations of varying cutting orientation and cutting direction. Interestingly, despite of being a compound of silicon and carbon, silicon carbide (SiC) exhibited characteristics more like diamond, e.g. both SiC iii workpiece and diamond cutting tool were found to undergo sp3-sp2 transition during the nanometric cutting of single crystal SiC. Also, cleavage was found to be the dominant mechanism of material removal on the (111) crystal orientation. Based on the overall analysis, it was found that 3C-SiC offers ease of deformation on either (111) <-110>, (110) <001> or (100) <100> setups. The simulated orthogonal components of thrust force in 3C-SiC showed a variation of up to 45% while the resultant cutting forces showed a variation of 37% suggesting that 3C-SiC is anisotropic in its ease of deformation. The simulation results for three major polytypes of SiC and for silicon indicated that 4H-SiC would produce the best sub-surface integrity followed by 3C-SiC, silicon and 6H-SiC. While, silicon and SiC were found to undergo HPPT which governs the ductility in these hard, brittle materials, corresponding evidence of HPPT during the SPDT of polycrystalline reaction bonded SiC (RB-SiC) was not observed. It was found that, since the grain orientation changes from one crystal to another in polycrystalline SiC, the cutting tool experiences work material with different crystallographic orientations and directions of cutting. Thus, some of the grain boundaries cause the individual grains to slide along the easy cleavage direction. Consequently, the cutting chips in RB-SiC are not deformed by plastic mechanisms alone, but rather a combination of phase transformation at the grain boundaries and cleavage of the grains both proceed in tandem. Also, the specific-cutting energy required to machine polycrystalline SiC was found to be lower than that required to machine single crystal SiC. Correspondingly, a relatively inferior machined surface finish is expected with a polycrystalline SiC. Based on the simulation model developed, a novel method has been proposed for the quantitative assessment of tool wear from the MD simulations. This model can be utilized for the comparison of tool wear for various simulation studies concerning graphitization of diamond tools. Finally, based on the theoretical simulation results, a novel method of machining is proposed to suppress tool wear and to obtain a better quality of the machined surface during machining of difficult-to-machine materials.

No description available.

Sheet metal spinning is one of the metal forming processes, where a flat metal blank is rotated at a high speed and formed into an axisymmetric part by a roller which gradually forces the blank onto a mandrel, bearing the final shape of the spun part. Over the last few decades, sheet metal spinning has developed significantly and spun products have been widely used in various industries. Although the spinning process has already been known for centuries, the process design still highly relies on experienced spinners using trial-and-error. Challenges remain to achieve high product dimensional accuracy and prevent material failures. This PhD project aims to gain insight into the material deformation and wrinkling failure mechanics in the conventional spinning process by employing experimental and numerical methods. In this study, a tool compensation technique has been proposed and used to develop CNC multiple roller path (passes). 3-D elastic-plastic Finite Element (FE) models have been developed to analyse the material deformation and wrinkling failure of the spinning process. By combining these two techniques in the process design, the time and materials wasted by using the trial-and-error could be decreased significantly. In addition, it may provide a practical approach of standardised operation for the spinning industry and thus improve the product quality, process repeatability and production efficiency. Furthermore, effects of process parameters, e.g. roller path profiles, feed rate and spindle speed, on the variations of tool forces, stresses, strains, wall thickness and wrinkling failures have also been investigated. Using a concave roller path produces high tool forces, stresses and reduction of wall thickness. Conversely, low tool forces, stresses and wall thinning have been obtained in the FE model which uses the convex roller path. High feed ratios help to maintain original blank thickness but also lead to material failures and rough surface finish. Thus it is necessary to find a “trade off” feed ratio for a spinning process design.

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No description available.

Steel strip has been made from iron powder by the BISRA process. A water atomised powder and a reduced powder have been characterised. The water atomised powder has been sieved, elutriated and re-blended to form a series of approximately log-linear size distributions, and these have also been characterised. Particular attention has been paid to particle size distribution, apparent density, tap density, compressibility and weight specific surface. Mechanical and physical properties of the strip have been measured at various stages in its production. The properties have been found to depend on the processing conditions and on the powder characteristics. The processing conditions used did not produce satisfactory strip when very fine water atomised powder was used; an explanation for this has been proposed.

No description available.

No description available.