Mechanical & Materials Engineering Research Collection
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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.
34 - PublicationThe Effect of Powder Size and Morphology on the Sinterability of Bioresorbable Mg-Sr/Ca Alloys(2024-03-07)Possessing outstanding biocompatibility and bioresorbability, magnesium (Mg) alloys with strontium (Sr) and calcium (Ca) additions have shown potential to be used as temporary implants in orthopaedic applications. Having a low elastic modulus (45 GPa) close to the human bone lowers the stress shielding effects. Low temperature Additive Manufacturing (AM) techniques (e.g., Fused Deposition Modelling) have potential to be used for the fabrication of complex Mg components while avoiding safety concerns associated with high temperature AM. However, low sinterability of common Mg-alloys is the main limiting factor. The objective of this work is to investigate the effect of powder particle size / morphology on the sinterability of Mg-Ca/Sr based alloys produced via powder metallurgy. Laser Diffraction and Scanning Electron Microscopy (SEM) were used to characterize particle size and morphology. The study also focused on assessing the role of liquid phase sintering (LPS) mechanism by thermodynamic calculations and microstructural characterisation (SEM). Porosity measurements using density analysis and image processing were employed to determine the effects of powder size and morphology on sinterability of the alloys. It was found that the non-homogeneous particle size distribution with more spherical powder particles, facilitated the compaction and accordingly higher densification was obtained. This was achieved for powders milled at higher speeds (900 rpm), resulting in significantly lower porosity levels (~ 6-8 %) compared to the dry-milled state (~40-60 %).
106 - 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).
63 - 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.
25 - PublicationA numerical study on the sheltering effects of the central wind barriers on the Rose Fitzgerald Kennedy Bridge(CERAI, 2022-08-26)This study aims to examine the sheltering effects of central wind barriers installed near the pylons of the Rose Fitzgerald Kennedy Bridge. A full-scale Computational Fluid Dynamics (CFD) model is developed, which includes high-precision geometries of the bridge and the terrain. Simulations using this model are performed at realistic wind conditions as boundary conditions are mapped from mesoscale Weather Research and Forecasting (WRF) simulations. Wind velocities at multiple locations on the bridge predicted by the CFD simulations are compared with field measurement data where a good agreement is reached. The validated model is then applied with bridge geometries with and without the central wind barriers at high wind conditions. Comparisons between these two groups of simulations show that the wind barriers can effectively reduce wind velocities on traffic lanes near the pylon, which validates the current design of the barriers on the bridge.
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