Browsing by Author "Cottam, R"

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    Diffraction line profile analysis of 3D wedge samples of Ti-6Al-4V fabricated using four different additive manufacturing processes
    (MDPI, 2019-01-09) Cottam, R; Palanisamy, S; Avdeev, M; Jarvis, T; Henry, C; Cuiuri, D; Balogh, L; Abdul Rahman Rashid, R
    Wedge-shaped samples were manufactured by four different Additive Manufacturing (AM) processes, namely selective laser melting (SLM), electron beam melting (EBM), direct metal deposition (DMD), and wire and arc additive manufacturing (WAAM), using Ti-6Al-4V as the feed material. A high-resolution powder diffractometer was used to measure the diffraction patterns of the samples whilst rotated about two axes to collect detected neutrons from all possible lattice planes. The diffraction pattern of a LaB6 standard powder sample was also measured to characterize the instrumental broadening and peak shapes necessary for the Diffraction Line Profile Analysis. The line profile analysis was conducted using the extended Convolution Multiple Whole Profile (eCMWP) procedure. Once analyzed, it was found that there was significant variation in the dislocation densities between the SLMed and the EBMed samples, although having a similar manufacturing technique. While the samples fabricated via WAAM and the DMD processes showed almost similar dislocation densities, they were, however, different in comparison to the other two AM processes, as expected. The hexagonal (HCP) crystal structure of the predominant α-Ti phase allowed a breakdown of the percentage of the Burgers’ vectors possible for this crystal structure. All four techniques exhibited different combinations of the three possible Burgers’ vectors, and these differences were attributed to the variation in the cooling rates experienced by the parts fabricated using these AM processes. © This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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    Effect of laser clad repair on the fatigue behaviour of ultra-high strength AISI 4340 steel
    (Elsevier, 2014-06-12) Sun, SD; Liu, Q; Brandt, M; Luzin, V; Cottam, R; Janardhana, M; Clark, GA
    The fatigue behaviour of an ultra-high strength steel (>1800 MPa) was evaluated to assess the potential of using laser cladding as a repair tool for such steels in aeronautical structural applications. AISI 4340 and AerMet 100 steel powder were used to clad over a grind-out region in an AISI 4340 steel substrate using a 2.5 kW ND:YAG laser. Post-clad heat treatment (PCHT) was also investigated. Results showed very poor tensile properties and significantly reduced fatigue life of the AISI 4340 as-clad with a very high hardness and brittle fracture in the clad and HAZ zone. Residual stress results showed a compressive residual stress in the clad region and tensile residual stress in the HAZ. Changing the alloy of the clad layer to AerMet 100 steel, as well as applying a PCHT process, showed promising results as the fatigue life was improved from that of the grind-out substrate. © 2014, Elsevier B.V.
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    Investigation into heat treatment and residual stress in laser clad AA7075 powder on AA7075 substrate
    (Springer US, 2013-06-04) Cottam, R; Luzin, V; Liu, Q; Wong, YC; Wang, J; Brandt, M
    The laser cladding of AA7075 powder onto a AA7075 substrate was conducted to evaluate the effect of heat treatment and to measure residual stress between the clad layer and substrate to better understand the effect of laser cladding. The microstructure formed in the clad region was characteristic of a high cooling rate, which is typical for laser cladding. The heat-affected zone (HAZ) showed coarser precipitates when compared with the substrate and was attributed to the heating from the laser. A solution heat treatment followed by aging was employed to restore the strength in the HAZ. Nanohardness traverses of the clad and substrate was performed and it was shown the hardness in the 7075 clad layer was lower than the substrate both pre- and post-heat treatment and was attributed to the vaporization of zinc and magnesium. Neutron diffraction was employed to measure the residual stress both before and after heat treatment. The residual stresses formed in the clad layer were tensile and about 50 MPa in magnitude; heat treatment increased the stress level to approximately 100 MPa. © 2013, Springer.
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    The role of metallurgical solid state phase transformations on the formation of residual stress in laser cladding and heating
    (Trans Tech Publications, 2014-02) Cottam, R; Luzin, V; Thorogood, KJ; Wong, YC; Brandt, M
    There are two major types of solid state phase transformations in metallic materials; the formation of second phase particles during heat treatments, and the transformation of the matrix from one crystalline packing arrangement to another during either heating or cooling. These transformations change the spacing between adjacent atoms and can thus influence the residual stress levels formed. The heating and cooling cycles of materials processing operations using lasers such as cladding and melting/heating, can induce phase transformations depending on the character of the material being processed. This paper compares the effects of the different phase transformations and also the influence of the type of laser processing on the final residual stress formed. The comparisons are made between laser clad AA7075, laser clad Ti-6Al-4V and laser melted nickel-aluminium bronze using neutron diffraction and the contour method of measuring residual stress. © 2014 Trans Tech Publications.
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    The role of microstructural characteristics in the cavitation erosion behaviour of laser melted and laser processed nickel–aluminium bronze
    (Elsevier, 2014-09-01) Cottam, R; Luzin, V; Moody, H; Edwards, DP; Majumdar, A; Wong, YC; Wang, J; Brandt, M
    In this study two types of laser surface treatment, laser surface melting and laser processing, were used to treat the surface of as cast Nickel–Aluminium Bronze. The two treatments were then subjected to cavitation erosion testing and were compared against as-cast Nickel–Aluminium Bronze. While the cavitation performance of the two types of laser surface treatment was equivalent, the morphology of the eroded surfaces was different. Several materials characterisation techniques including neutron diffraction for residual stress measurements and SEM were used to explain why the two eroded surfaces were different. It was found that the tensile residual stresses in the laser melted sample weakened the sample, which negated its superior strength when compared with the laser processed sample. It was also observed that the erosion and pitting in the laser melted sample were deeper and they were attributed to the tensile residual stresses accelerating the attack at grain boundaries. © 2014, Elsevier B.V.
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    The role of microstructure in the stress relaxation and tempering of laser clad Ti–6Al–4V
    (Elsevier, 2014-04-17) Cottam, R; Luzin, V; Liu, Q; Mayes, E; Wong, YC; Wang, J; Brandt, M
    In this work a heat treatment procedure was performed on laser clad Ti–6Al–4V, which resulted in a reduction of tensile residual stresses in the clad samples as well and a decrease in the hardness of the clad layer with an increasing heat treatment time. The reduction in the residual stress was attributed to inter-phase stress relaxation brought about by the growth of the β phase. The reduction in hardness was attributed to an increase in the volume fraction of the soft β phase. Both the reduction in residual stress and hardness due to this heat treatment makes it an attractive method to heat treat both laser clad and additively manufactured Ti–6Al–4V components for improved mechanical properties.© 2014, Elsevier B.V.