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- PublicationApplication of in situ process monitoring to optimise laser parameters during laser powder bed fusion printing of Ti-6Al-4V parts with overhang structures(Springer Science and Business Media LLC, 2023-12-15)Enhanced levels of alloy print defects such as porosity are associated with the printing of overhang structures by laser powder bed fusion (L-PBF). This study compared the microstructure and porosity of Ti-6Al-4V overhang structures, with that observed for the bulk alloy. It was observed in the region around the overhang structure that the microstructure exhibited larger grain sizes and was less homogenous, compared to the that obtained within the bulk alloy. An increased level of porosity of up to 0.08% was also observed in the overhang print alloy, compared with the corresponding < 0.02% in the alloy bulk. It is hypothesised that these microstructural changes are associated with the excess heat generated in the overhang region, due to the decreased thermal conductivity of the powder immediately below the print layers, compared with solid alloy. During L-PBF alloy printing, in situ process monitoring of the melt pool emissions was obtained in the near-infrared range and correlated with the properties of the printed parts. This in-process data was used to assist in selecting optimal laser processing conditions, in order to help prevent melt pool overheating at the overhang. By systematically controlling the laser energy during the printing of the first fifteen layers over the overhang structure, the level of porosity was reduced, to the < 0.02% level of the bulk alloy. There was also an associated reduction in the roughness (Ra) of the overhang itself, with its Ra decreasing from 62.4 ± 7.3 to 7.5 ± 1.9 µm.
- PublicationDevelopment of Magnesium-Strontium/Calcium (Mg-Sr/Ca)-Based Alloys with Improved Sinterability for Next-Generation Biomedical Implants(Springer, 2023-02-06)The use of biodegradable magnesium (Mg) alloys for bone fixation devices has potential to improve patients’ quality of life by avoiding the necessary secondary operations conducted regularly for the removal of implants fabricated from conventional non-resorbable alloys. Mg-alloys have excellent biocompatibility and biodegradability along with a low modulus of elasticity which will decrease bone-shielding effects. However, low corrosion resistance and relatively poor mechanical performance limit the use of Mg-based alloys for biomedical applications. This study focuses on the processing of Mg-Ca- and Mg-Sr-based alloys via powder metallurgical route. Thermodynamic calculations are used to predict the liquid phase fractions in order to optimise sinterability and porosity levels. Materials characterisation was conducted to validate the thermodynamic modeling results using optical and scanning electron microscopy (SEM/EDS) as well as X-ray Diffraction (XRD).
- PublicationAssessment of Phase Evolution in Titanium-Niobium-Based Alloys During Rapid Solidification(Springer, 2023-02-07)In this work, microstructural evolution in β-Ti alloys during solidification is studied as the cooling rate increases, approaching the cooling rates found in additive manufacturing processes. Using suction casting of thin rods, high cooling rates can be studied and compared, to find a trend in how these phases evolve under a broad range of solidification conditions. The effect of varying cooling rates is studied on the microstructural evolution of Titanium-Niobium (Ti-Nb)-based alloys with Tantalum (Ta) additions. A combined simulation and experimental approach is used to investigate the predictability of differences in microstructural evolution during rapid-solidification casting. Rods of binary Ti–25Nb and ternary Ti–20Nb–10Ta (wt% and hereafter) alloys were synthesized in diameters of 3, 5, and 10 mm using suction casting into copper moulds. Finite element (FE) and thermodynamic modelling was used to calculate the cooling rates and temperature gradients of the alloys. The microstructural and mechanical differences were determined via XRD, SEM/EDS, and mechanical testing.
- PublicationProceedings of the 38th International Manufacturing Conference (IMC38)(University College Dublin. School of Mechanical and Materials Engineering, 2022-08-30)The 38th International Manufacturing Conference, IMC38, showcases current research in the field of "manufacturing engineering" undertaken in Ireland by postgraduate students and experienced researchers. Indicative topics, in line with the contents of these proceedings, include; sustainable and energy efficient manufacturing, additive manufacturing, Industry 4.0 and digital manufacturing, machine tool, automation and manufacturing system design, surface engineering, forming and joining process research. The IMC community is also involved in research aimed at improving the learning experience of undergraduate and graduate engineers and developing high level skills for the manufacturing engineer of the future. The theme for this year’s conference is Sustainable Manufacturing, with a particular emphasis on a) Digitalisation of Manufacturing – its impact on sustainability and b) Addressing sustainability in Engineering Education, Industrial Training and CPD.
- PublicationThe role of microstructural evolution on the fatigue behavior of additively manufactured Ti–6Al–4V alloy(Elsevier, 2022-11-24)The fatigue behaviour of additively manufactured Ti–6Al–4V via Laser Powder Bed Fusion (L-PBF) was evaluated in three different conditions, as-built, heat-treated and hot isostatically pressed (HIP'ed). Fractography analysis interpreted together with the S–N curves indicates that fatigue failure in as-built and heat-treated conditions where <0.2% porosity was present, was mainly driven by early-stage crack growth. However, crack initiation was determined to be the main controlling factor for fatigue deformation of HIP'ed samples. Moreover, a strong correlation between the impact energy and fatigue limit was found. The findings were based on detailed microstructural and crystallographic characterization, as well as mechanical testing. The as-built and heat-treated conditions exhibited poor fatigue response in comparison to HIP'ed which is largely attributed to the lower levels of porosity identified. Even though similar levels of porosity are present in as-built and heat-treated samples, improvement in fatigue limit was determined in the heat-treated condition due to phase transformation and microstructural coarsening leading to reduction in micro-strain.
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