Now showing 1 - 3 of 3
  • 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
    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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