PhD, EngD and MSc by research theses (SATM)

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Browsing PhD, EngD and MSc by research theses (SATM) by Course name "PhD in Renewable Energy Marine Structures (REMS)"

Now showing 1 - 3 of 3
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    ItemOpen Access
    Development of wire and arc additive manufacture for large scale application for the energy industry.
    (2019-10) Dirisu, Philip; Ganguly, Supriyo; Martina, Filomeno
    Wire + arc additive manufacture (WAAM), as the most discussed tool in manufacturing, can complement the design of a fully optimised structure through multi-material deposition and incorporation of geometric design features not possible by traditional manufacturing technologies. Despite all the progress made in WAAM, detail studies on the implication of WAAM parameters on the structural integrity of steel-built components in both machined and as-built condition, and their fracture mechanics properties are largely unknown. To effectively and economically commercialise WAAM, the integrity and reliability of built components under specific loading conditions must be assured. In these studies, the properties of steel components built with WAAM were determined, and the possible methods to improve strength, fatigue performance, and fracture toughness, by tailored deposition of different wire grades, through the modification of the build surface from rolling, and through the addition of cold wire during deposition were determined. The results were presented through mechanical test data (tensile properties, hardness, Charpy and fracture toughness, and fatigue crack growth rate and fatigue life results, along with microstructural analysis (electron microscopy of fractured surfaces, EBSD, XRD, and optical microscopy), and the surface topography of the WAAM build, as characterised by the surface waviness. The studies showed that WAAM process parameters are responsible for the fatigue initiation of WAAM as-deposited (ASD) structures and that reduction of SW by process parameter optimisation and the use of side rolling will improve the fatigue life of ASD WAAM components. The possible cause of variability in fracture toughness in the WAAM steel deposits is the hardening and softening regions in the layer bands caused by the variation in peak temperature and cooling rates of the adjacent layer in the building direction. The addition of cold wire into the melt pool mop-up the excess heat, controlled the surface morphology and improved the mechanical properties and the deposition rate. In this study, a deposition rate of 6.27kg/hr was achieved. In conclusion, low carbon and high strength steel wires will be suitable for making bespoke components with WAAM application for large scale energy manufacturing, and WAAM mild steel components could be used in the as-deposited condition for engineering application when rolling is applied to reduce its SW.
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    ItemOpen Access
    Investigation of wire and arc additive manufacturing methods for high integrity, high productivity fabrication of large steel structures.
    (2021-06) Ehigiator, Osahon; Ganguly, Supriyo
    This thesis describes advances in aspects of deposition process control, mechanical performance optimisation and improved material property development, undertaken for high productivity and integrity wire and arc additive manufacture of large heavy-walled components. One key objective of this study was to undertake a fundamental study to understand the important factors at play during tandem-GMAW deposition of single bead and multi-layer wall structures, with a focus on achieving substantial improvement in deposition rate and surface quality of WAAM components. The study of the influence of key deposition parameters on the instantaneous arc characteristics and deposited bead geometry was conducted. The study found that while wire feed speed and travel speed significantly affected the arc and bead characteristic, the influence of contact tip to work distance was minimal. However, the later strongly influences the arc stability, due to its effect on the mode of metal transfer. The study also showed that consistent high quality single bead deposit is achievable when the lead wire is set about 1-2m/mins lower than the trail wire and increase in total wire feed speed of tandem process above 24m/mins, produced insignificant effect on bead width. Furthermore, tandem parallel wire configuration, produced the optimum surface quality and metal deposition efficiency; however, it was more susceptible to defect formation at higher travel speed, compared to tandem series wire set-up. The fundamental experience and knowledge gained through the aforementioned process study was instrumental in building a rectilinear wall structure, with excellent surface and geometrical quality and, subsequently a large skin and core part, with significant improvements in deposition rate and surface quality of the component. Thermal cycle generation during deposition of a thick-walled WAAM structure, built using 2.25Cr 1Mo type steel composition and response of the structure to heat treatment were studied. The main focus was to improve understanding on the effect of thermal cycle on the mechanical performance of the component and develop a suitable heat treatment regime to restore an acceptable material property, without requiring the more expensive and complex austenitisation heat treatment process. The study found that the generated thermal cycle produced microstructural heterogeneity, high hardness level, with large gradient in hardness of the multi-layered structure, in as-deposited condition. This resulted in poor impact toughness. However, the applied post deposition heat treatment parameters, was beneficial in homogenising the material, reducing the high hardness and variation in hardness, and restored the impact toughness. Tandem GMAW modification of alloy 2.25Cr 1Mo type wire composition, (using ER90S-G wire), with ER120S-G wire, having higher nickel content, was studied to exploit the potential for compositional modification, through this manufacturing route. The mixing of the two wires was carried out to determine whether higher nickel content can improve the impact toughness and eliminate the need for PDHT. The focus was to increase the as deposited charpy impact toughness, while maintaining excellent all-round static mechanical properties. The result showed that significant increase in charpy impact toughness, in both testing direction, at -30deg C, was achieved with the modified composition, containing equal proportions of ER90S-G and ER120S-G wire materials. Finally, laboratory sour service corrosion test was conducted to compare the corrosion performance of WAAM alloys, obtained under different processing conditions, to a similar but conventional wrought steel variant. The results showed that both the exposure temperature and time, accelerated the corrosion degradation of the materials. Also, while 2.25Cr 1Mo WAAM alloy and the wrought variant showed comparable corrosion performance, the modified WAAM alloy containing equal blend of ER90S-G and ER120S-G wire, exhibited superior corrosion performance compared to the wrought alloy, which was attributed to the higher nickel content of the former.
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    ItemOpen Access
    Mitigation of start/stop defects in circumferential laser and hybrid laser welding of tubular offshore structures.
    (2021-04) Lai, Wai Jun; Ganguly, Supriyo; Suder, Wojciech
    In recent years, the offshore wind industry's growth and increased demand for larger and stronger offshore wind turbines have led to significant research into the joining and manufacture of thick section tubular structural steels. Advances in joining processes, such as high productivity laser and hybrid laser welding, will reduce lead times and manufacturing costs of offshore wind turbine support structures compared to conventional arc welding processes. However, defect formation associated with laser keyhole initiation and termination at the weld intersection of circumferential welds and methods of mitigation is not fully understood nor documented. Therefore, to enable high productivity laser and hybrid laser welding for circumferential welding applications, further research into the mitigation of defects at the weld intersection of circumferential welds due to laser and hybrid laser weld start and stops are necessary. In this thesis, investigations into factors affecting the formation of laser keyhole initiation and termination related defects at the weld overlap start/stop region and methods of mitigation were carried out, by controlled experimentation, for both laser and hybrid laser-arc welding processes. Defect formation at the weld overlap start/stop region were monitored using weld cameras and characterised using optical microscopy. The underlying cause of defects at the weld overlap start/stop region in laser keyhole, and hybrid laser-arc welding is associated with keyhole collapse and instability during laser termination. These defects can be successfully mitigated using various weld termination regimes, such as laser power ramp-down and laser defocusing in autogenous laser keyhole welding, and a combination of laser power ramp-down and arc current sloping in hybrid laser-arc welding. Results show that the amount of deoxidising elements in the steel influences the weld penetration depth and defect formation in the steady-state welding condition and during the application of a laser termination regime in autogenous laser keyhole welding.