Browsing by Author "Caballero Ramos, Armando"

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    Comparison of microstructure refinement in wire-arc additively manufactured Ti–6Al–2Sn–4Zr–2Mo–0.1Si and Ti–6Al–4V built with inter-pass deformation
    (Springer, 2022-09-22) Davis, Alec E.; Caballero Ramos, Armando; Biswal, Romali; Williams, Stewart; Prangnell, P. B.
    The titanium alloy Ti–6Al–2Sn–4Zr–2Mo–0.1Si (Ti6242) has been deposited for the first time by a directed energy deposition process using a wire and arc system—i.e., wire-arc additive manufacturing (WAAM)—with and without inter-pass machine hammer peening, and its microstructure investigated and compared to the more commonly used alloy Ti–6Al–4V (Ti64). The application of inter-pass machine hammer peening—where each added layer was deformed before deposition—successfully refined the strongly textured, coarse, columnar β-grain structure that is commonly seen in α + β titanium alloys, producing a finer equiaxed grain structure with a near-random α texture. The average grain diameter and texture strength decreased with the peening pitch. When Ti6242 was deposited under identical conditions to Ti64, by switching the alloy feed wire in-situ, the refined β-grain size decreased across the alloy-to-alloy transition reaching on average 25 pct less in Ti6242 than in Ti64. A similar 25 pct scale reduction was also found in the Ti6242 α-lath transformation microstructure. This comparatively greater microstructure refinement in Ti6242 was attributed to the dissimilar alloying elements present in the two materials; specifically, molybdenum, which has a lower diffusivity than vanadium and led to slower β-grain growth during reheating as well as a finer transformation microstructure.
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    Microstructure transition gradients in titanium dissimilar alloy (Ti-5Al-5V-5Mo-3Cr/Ti-6Al-4V) tailored wire-arc additively manufactured components
    (Elsevier, 2021-11-03) Kennedy, Jacob R.; Davis, Alec E.; Caballero Ramos, Armando; White, M.; Fellowes, Jonathan W.; Pickering, E. J.; Prangnell, P. B.
    The nature of the chemical mixing and microstructure gradients that occur across the interface transition, when manufacturing tailored components with the two high-performance dissimilar titanium alloys (Ti-6Al-4V (Ti-64) and Ti-5Al-5V-5Mo-3Cr (Ti-5553)) by the wire-arc additive manufacturing (WAAM) process, are reported. It has been shown that a relatively long-range chemical gradient occurs during the transition between layers produced with the two different titanium alloys, due to convective mixing in the melt pool between the substrate layers and new alloy wire. This resulted in a stepwise exponential decay composition profile normal to the layers, the width of which can be described by a simple dilution law, with steep local composition gradients seen within the boundary layers at the fusion boundary of each individual layer. The alloy-alloy composition gradients had little effect on the β-grain structure. However, they strongly influenced the transformation microstructure, due to their effect on the parent β-phase stability and the β → α transformation kinetics and reaction sequence. The microstructure gradient seen on transitioning from Ti-64 → Ti-5553 was significantly more abrupt, compared to when depositing the two alloys in the reverse order. Under WAAM thermal conditions, Ti-64 appears to be more sensitive to the effect of adding β-stabilising elements than when Ti-5553 is diluted by Ti-64, because at high cooling rates, stabilisation of the β phase readily suppresses α nucleation when cooling through the β transus, and the normal Ti-64 lamellar transformation microstructure is abruptly replaced by finer scale α laths generated by precipitation during subsequent reheating cycles.
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    The potential for grain refinement of wire-arc additive manufactured (WAAM) Ti-6Al-4V by ZrN and TiN inoculation
    (Elsevier, 2021-02-23) Kennedy, Jacob R.; Davis, Alec E.; Caballero Ramos, Armando; Williams, Stewart; Pickering, E. J.; Prangnell, P. B.
    Wire-Arc Additive Manufacturing (WAAM) of large near-net-shape titanium components has the potential to reduce costs and lead-time in many industrial sectors including aerospace. However, with titanium alloys, such as Ti-6Al-4V, standard WAAM processing conditions result in solidification microstructures comprising large cm-scale, <001> fibre textured, columnar β grains, which are detrimental to mechanical performance. In order to reduce the size of the solidified β-grains, as well as refine their columnar morphology and randomise their texture, two cubic nitride phases, TiN and ZrN were investigated as potential grain refining inoculants. To avoid the cost of manufacturing new wire, experimental trials were performed using powder adhered to the surface of the deposited tracks. With TiN particle additions, the β grain size was successfully reduced and modified from columnar to equiaxed grains, with an average size of 300 µm, while ZrN powder was shown to be ineffective at low addition levels studied. Clusters of TiN particles were found to be responsible for nucleating multiple β Ti grains. By utilizing the Burgers orientation relationship, EBSD investigation showed that a Kurdjumov-Sachs orientation relationship could be demonstrated between the refined primary β grains and TiN particles.