Now showing 1 - 9 of 9
  • Publication
    Assessment of Phase Evolution in Titanium-Niobium-Based Alloys During Rapid Solidification
    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.
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  • Publication
    Effects of Laser Power on Geometry, Microstructure and Mechanical Properties of Printed Ti-6Al-4V Parts
    This study investigated the effect of laser power on the properties of Ti-6Al-4V alloy parts produced by additive manufacturing. The printing study was carried out using the laser beam powder bed fusion (PBF-LB) technique (Renishaw RenAM 500M). The laser power was altered in the range of 100–400 W, in order to evaluate the effects of changing the input energy received by the powder particles on the as-built parts. The impact of changing laser power was investigated based on printed part dimensions, porosity, morphology, micro/nanostructure, wear, hardness and tensile properties. It was determined that laser power has a direct influence on part dimensional accuracy, with larger dimensions compared with CAD design under the processing conditions used, obtained at higher powers i.e. 2 % at 250 W, while 4 % at 400 W. The border thickness for rounded edges was found to be ∼0.2 ± 0.06 mm greater than that obtained for straight edges, printed on the same quarter circle samples. A more homogeneous morphology, along with an improved surface finish, was obtained for parts printed using the higher laser powers. The microstructure of the high power alloy, was characterised by wider prior β grains with longer and finer needles, along with superior as-built mechanical properties, when compared to parts produced using lower laser power (100 W). Additionally, shifts in the XRD peak position for parts printed at the lower and higher laser powers, indicate some reduction in the level of residual stress for parts produced at higher powers.
      212Scopus© Citations 42
  • Publication
    The effects of geometry and laser power on the porosity and melt pool formation in additively manufactured 316L stainless steel
    The present work investigates the effects of geometry and laser power on the porosity and melt pool formation for 316L stainless steel samples fabricated using the laser powder bed fusion (LPBF) technique. Both cylindrical and conical parts with the same heights were processed at a range of laser powers (60–70 W). An analytical model was used to select a suitable laser power, based on the established processing parameters, but also to predict the resultant melt pool dimensions. Based on the combination of experimental work and mathematical modelling, a novel geometrical factor is proposed, which was demonstrated to successfully improve the implemented model. A decrease in melt pool depths towards the building direction was determined in all the printed samples; this was however not predicted by the mathematical model. Furthermore, the variation in heat extraction exhibited by the conical and cylindrical parts allows the correlation between the melt pool dimensions and the geometrical factor. Finally, the influence of conical and cylindrical shapes on part hardness with increasing distance from the build plate was demonstrated; based on this comparison, it was determined that the cone geometries exhibit both a higher Vickers hardness and density.
      176Scopus© Citations 10
  • Publication
    The 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.
      48Scopus© Citations 1
  • Publication
    The Effect of Powder Size and Morphology on the Sinterability of Bioresorbable Mg-Sr/Ca Alloys
    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 %).
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  • Publication
    Mechanism of Stress Relaxation and Phase Transformation in Additively Manufactured Ti-6Al-4V via in situ High Temperature XRD and TEM Analyses
    Additive manufacturing is being increasingly used in the fabrication of Ti-6Al-4V parts to combine excellentmechanical properties and biocompatibility with high precision. Unfortunately, due to the build-up of ther-mal residual stresses and the formation of martensitic structure across a wide range of typical processingconditions, it is generally necessary to use a post-thermal treatment to achieve superior mechanical perfor-mance. This investigation aims to obtain a deeper understanding of the micro/nanostructural evolution(a0martensite phase decomposition), accounting for the kinetics of phase transformation during the heattreatment of 3D-printed Ti-6Al-4V alloy. As the mechanism of phase transformation and stress relaxation isstill ambiguous, in this study the changes in crystal lattice, phase, composition and lattice strain were investi-gated up to 1000°C using bothin situhigh temperature X-ray diffraction (XRD) and transmission electronmicroscopy (TEM). Based on the result a mechanism of phase transformation is proposed, via the accommo-dation/substitution of Al, V and Ti atoms in the crystal lattice. The proposed mechanism is supported basedon elemental concentration changes during heat treatment, in combination with changes in crystal structureobserved using the high temperature XRD and TEM measurements. This study provides a deeper under-standing on the mechanism of phase transformation through martensitic decomposition, as well as a deeperunderstanding of the influence of post-thermal treatment conditions on the alloy’s crystal structure.
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  • Publication
    In-situ XRD Study on the Effects of Stress Relaxation and Phase Transformation Heat Treatments on Mechanical and Microstructural Behaviour of Additively Manufactured Ti-6Al-4V
    Additively Manufactured (AM) titanium (Ti) components are routinely post-thermal heat treated (HT), to reduce internal stresses, as well as to obtain more desirable microstructural features, yielding improved mechanical performance. Currently, there is no consensus on the optimum HT method for AM Ti-6Al-4V, as the mechanism for the main phase transformation (α′ (martensite) → α + β (equilibrium)) is still ambiguous. In this study, stress relaxation and phase transformation in the alloy are investigated in detail, via isothermal heat treatments and in situ high temperature X-ray Diffraction (XRD). The latter was carried out at heating rates of 5 and 200 °C/min. The relationship between crystallographic evolution during isothermal treatments and mechanical behaviour was determined. Isothermal holding at 400 °C resulted in an increase in ultimate tensile strength (UTS) and yield strength (YS) by 3.4% and 2.1%, respectively, due to the relief of tensile microstrain. It was found that isothermal treatment conducted between 550 and 700 °C promotes martensitic decomposition, resulting in the formation of a transitional - αtr phase, which has an asymmetrical hexagonal crystal lattice. The formation of this αtr phase was determined to be the main factor contributing to a major decrease in ductility.
      94Scopus© Citations 19
  • Publication
    The Study on Microstructural Evolution During Post-processing of Additively Manufactured Ti64
    The effect of furnace heat treatments to 850 °C, on the evolution of microstructure in Ti–6Al–4V alloy produced via selective laser melting (SLM), was studied using optical microscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). Columnar prior-β grains in the build direction with lamellar α-martensite laths contained within the prior-β grains were determined. α-martensite laths present in the as-built microstructure had thicknesses around 236 nm while the heat-treated microstructure showed an α-lath thickness values of around 1.8 μm. Based on XRD analysis, upon heat treatment the formation of β-phase was determined with associated peaks around 41° and 58°, corresponding to (110) and (200) planes, respectively.
      142Scopus© Citations 1
  • Publication
    Development of Magnesium-Strontium/Calcium (Mg-Sr/Ca)-Based Alloys with Improved Sinterability for Next-Generation Biomedical Implants
    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).
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