Browsing by Author "Dirisu, Philip"

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    Analysis of fracture toughness properties of wire + arc additive manufactured high strength low alloy structural steel components
    (Elsevier, 2020-08-14) Dirisu, Philip; Ganguly, Supriyo; Mehmanparast, Ali; Martina, Filomeno; Williams, Stewart W.
    The uncertainty surrounding the fracture behaviour of CMT-WAAM deposited steel, in terms of crack tip condition (J and CTOD) needed to cause crack tip extension, has made this manufacturing technique unpopular to date. Fracture toughness parameters are crucial in the structural integrity assessment of components and structures in various industries for assessing the suitability of a manufacturing process and material. In the offshore wind industry, the EN-GJS-400-18-LT ductile cast grade for the mainframe and hub has lower fracture toughness resistance for its high strength grade. Its high weight level affects the Eigen frequency of the tower and imposes high installation cost incurred from heavy lifting equipment usage. Poor fracture toughness is currently a challenge for wind turbine manufacturers in the quest for a cleaner and cheaper energy in the form of offshore wind. In this study, CMT-WAAM is used in depositing steel components with an oscillatory and single pass deposition strategy. The effects of microstructural variation, as a result of layer by layer deposition and the layer band spacing, on the fracture resistance in the build and welding direction was shown here. The fracture mechanics and failure mode of the WAAM deposited parts were investigated. The microstructural variation, again as a result of the layer by layer deposition and the layer band spacing, are the key parameters that control the fracture toughness of WAAM steel. Anisotropic behaviour in the values was observed between both fracture orientations. The constructive transformation mechanism of the WAAM oscillatory process made way for intragranular nucleation of acicular ferrite on the Ti containing inclusion, thereby improving the toughness of the ER70S-6 deposit with a unique microstructure and Jq value of 640kJ/m2.
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    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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    Microstructure and mechanical properties of TOP-TIG-wire and arc additive manufactured super duplex stainless steel (ER2594)
    (Elsevier, 2019-07-02) Zhang, Xiaoyong; Wang, Kehong; Zhou, Qi; Ding, Jialuo; Ganguly, Supriyo; Marzio, Grasso; Yang, Dongqing; Xu, Xiangfang; Dirisu, Philip; Williams, Stewart W.
    As the excellent combination of mechanical properties and corrosion resistance for super duplex stainless steel, a prospective method – Wire and Arc Additive Manufacturing – for fabricating this material was proposed, and a wall component was deposited in this study. The microstructure of the as-deposited wall was carefully analyzed along with the variation of mechanical properties. The results revealed that, in the wall-body, the austenite/ferrite phase balance was broken by the overgrowing the austenite phase. During this process, the intergranular secondary austenite leading the increase of austenite phase together with some contributions made by the precipitation of intragranular secondary austenite. Propagation of the intermetallic phases, chi and sigma phase, was not the major reason for the low impact toughness in the last layer area and the root region. Instead, the presence of CrN and “inclusions” (Cr2N and impurities) took the main responsibility not only in the impact toughness but also the ductility. The anisotropic analysis revealed that the UTS and elongation appeared distinct difference in vertical and horizontal direction samples. The varieties in YS were eliminated by the nitrogen work hardening effect to a large extent.
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    Wire plus arc additive manufactured functional steel surfaces enhanced by rolling
    (Elsevier, 2019-08-20) Dirisu, Philip; Ganguly, Supriyo
    Surface waviness (SW) is one of the major problems confronting the economical use of as-deposited components made with the wire plus arc additive manufactured (WAAM) process. The SW acts as a stress raiser, thereby reducing the tensile properties and fatigue life of the component. In this study, the effect of compressive residual stress on the mechanical and fatigue behaviour of the as-deposited WAAM mild steel component was carried out using a process which combines deposition and rolling on the WAAM component surface. The fractured faces and microstructure were characterised by a scanning electron and optical microscope. The microstructural changes were characterised by X-ray diffraction techniques. The results revealed that an increase in the notch radius and compressive stress induced by rolling on the as-deposited condition reduced the SW from 0.18 to 0.08 mm with a reduction of stress concentration, consequently reducing crack initiation and propagation and improving fatigue life. This work shows that rolling has a dual effect on as-deposited WAAM mild steel components with a new functional surface.