Now showing 1 - 9 of 9
  • 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.
      91Scopus© Citations 19
  • Publication
    Impact of print bed build location on the dimensional accuracy and surface quality of parts printed by multi jet fusion
    Multi Jet Fusion (MJF) is one of the newly developed additive manufacturing techniques, based on the use of powder bed fusion technology. It provides the opportunity to build up 3D, complex polymer geometries, without the need for support structures. This study evaluates the effect of build location across a 380 × 284 mm2 build plate, on both the dimensional accuracy and surface quality of polyamide 12 (PA12) parts printed using the MJF technique. The cube test samples were printed at each of the four corners and the center of the build plate. Dimensional deviations were determined using optical metrology measurements, while surface deviations were measured using 2D tactile and 3D optical profilometers. The density and the degree of crystallinity of the samples were determined using the Archimedes method and Differential Scanning Calorimetry analysis, respectively. Moreover, the morphology of the internal polymer surfaces was evaluated using Scanning Electron Microscopy. It was concluded that while overall printed part dimensions and crystallinity homogeneity were relatively uniform across the build plate, some variations were observed. Parts printed in particular, closer to the front of the build plate exhibited higher porosity, higher surface roughness along with the highest level of geometry deviation, compared with the CAD drawing. This is likely to be associated with some non- uniformities in heating and cooling of the PA12 polymer parts across the build plate.
    Scopus© Citations 11  192
  • Publication
    Low‐pressure additive manufacturing of continuous fiber‐reinforced polymer composites
    Continuous fibre reinforced polymer composites have found a wide range of applications in the automotive and aerospace industry, due to their lightweight properties. Recently the use of additive manufacturing (AM) has been developed for the fabrication of these composites. This study investigates the use of both atmospheric and for the first time, low-pressure (1 Pa) processing conditions, for the AM of continuous carbon, glass and Kevlar fibre reinforced nylon composites. DSC was used to compare the thermal properties of the three types of fibre reinforced filament, prior to printing. It was found that the melting peak was dependent on filament type, which can be related to the polymer processing conditions used during their fabrication. Based on computed tomography measurements, it was found that the use of low-pressure printing conditions yielded a reduction in porosity for the carbon, glass and Kevlar printed composites of 5.7, 1.0 and 1.7 % respectively. The mechanical properties of the composites were compared, using a short beam shear test, which assisted in the measurement of interlaminar properties. An increase in interlaminar shear strength of 33, 22 and 12% was obtained for the carbon, glass and Kevlar fibre reinforced polymer composites respectively, when printed under low-pressure, compared with that obtained at atmospheric pressure.
    Scopus© Citations 58  568
  • 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.
    Scopus© Citations 10  171
  • Publication
    Application of Additive Manufacturing in Design & Manufacturing Engineering Education
    This paper details how education and training in both digital manufacturing and materials processing was implemented as part of an undergraduate engineering programme involving 90 students. The programme was provided through additive manufacturing (3D printing), which enabled the students to develop an understanding of part design, fabrication and performance. The 3D printing study was carried out using Fused Deposition Modelling (FDM) and involved the fabrication of a turbocharger turbine part. This was subsequently evaluated using a customized test rig to assess the printed turbine rotation speed, under a fixed air-flow. The dimensions and morphology of the fabricated parts were also evaluated. Students benchmarked the performance of their turbine parts, against the part which was found to exhibit the highest rotation speed. A pre- and post-course survey was conducted to track the learning experience and feedback from the students involved. The results demonstrated that incorporation of digital manufacturing, self-guided and peer learning improved the engagement and learning experience.
    Scopus© Citations 11  107
  • Publication
    Enhancing the bearing strength of woven carbon fibre thermoplastic composites through additive manufacturing
    (Elsevier, 2019-03-15) ;
    This paper examines a novel additive manufacturing (AM) technique for the fabrication of woven multilaminate composites. The printing studies were carried out using nylon coated carbon fibre Tow in the form of a filament. This pathing technique allows for a woven structure to be integrated with features (such as notches) previously only possible through destructive machining processes. In order to evaluate the performance of these printed composites, bearing response studies were carried out. 6 mm holes were routed into a multilaminate woven composite structure, the resulting part’s mechanical performance was then tested and compared with specimens which had been drilled post printing. Specimen were comprised of 9 woven laminates stacked to form a 3.1 mm thick standardised test coupon for single and double shear testing (ASTM D5961). Current industry standard machining techniques result in fibre discontinuity and damage, this results in suboptimal mechanical performance of composite components. These new ‘Tailor Woven’ specimens achieved single shear bearing strengths of up to 214 MPa and double shear bearing strengths of up to 276 MPa. These values represent an increase of 29% and 63% respectively compared with equivalent composites in which the hole had been drilled.
    Scopus© Citations 48  422
  • 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.
    Scopus© Citations 42  210
  • 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
    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.
    Scopus© Citations 1  132