Development of Ti-Fe-based powders for laser additive manufacturing of ultrafine lamellar eutectics

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Years of academic research has gone into developing Ti-Fe-based ultrafine eutectic and near-eutectic alloys with remarkable mechanical properties. Cast ingots (few mm in dimensions) have demonstrated high compressive strengths (> 2 GPa) similar to bulk metallic glasses (BMGs), while retaining more than 15 % plasticity at room temperature [1–3]. However, conventional casting methods are incapable of providing uniform and high cooling rates necessary for growing such ultrafine microstructures over large dimensions without introducing significant heterogeneities. On the other hand, laser-based Additive Manufacturing (AM) techniques with inherently very high cooling rates like Selective Laser Melting (SLM) (ranging 106 K/s) or Laser Metal Deposition (LMD) (ranging 104 – 105 K/s) are appropriate for such microstructural growth and their track and layer-wise building approach maintains an almost constant cooling rate throughout bulk. This strongly motivates the development of high-quality powders for SLM and LMD trials. In this work, pre-alloyed powder of Fe-rich near-eutectic composition Fe82.4Ti17.6 (at %) was developed for LMD, while powders of two Ti-rich compositions: near-eutectic Ti66Fe27Nb3Sn4 (at %) and off-eutectic Ti73.5Fe23Nb1.5Sn2 (at %) were explored for SLM trials. Three gas atomisation methods, namely Crucible-based Gas atomisation (CGA), Crucible-Free atomisation (CFA) and Arc-melting Atomisation (AMA) were investigated for optimising powder production. In addition to conventional techniques, a novel methodology was proposed for one-step screening of powders’ key features based on advanced image analysis of X-Ray Computed Tomography (XCT) data. The methodology generated volume-weighted particle size distributions (which were validated against conventional laser diffraction), provided accurate estimations of internal porosity and quantitatively evaluated the 3D morphology of powders. In order to create a solidification knowledge dataset and further optimise the processing of powders under high cooling rates, in-depth microstructural studies were performed on these powders sieved into different particle size ranges (experiencing different solidification rates during atomisation). Results revealed that powder particle size is clearly related to, and can possibly predict, the solidification pathway followed during gas atomisation as well as its degree of completion. The ultrafine interlamellar spacing λ (< 190 μm) of lamellar eutectics observed in powders of near-eutectic compostitions increased almost linearly with particle size and revealed solidification rates similar to those encountered during SLM/LMD processing of the same or similar compositions. Therefore, this work highlights the potential of gas atomisation as a method to study rapid solidification and Laser-AM processing. Finally, two alloys were consolidated by AM using pre-alloyed powders and characterised mechanically, i.e. LMD-built Fe82.4Ti17.6 with lamellar eutectic microstructure and SLM-built Ti73.5Fe23Nb1.5Sn2 (off-eutectic) showing a unique “composite” microstructure of α-Ti and β-Ti grains strengthened by FeTi dispersoids that partially arranged themeselves as fine lamellas. Both alloys showed high compressive yield strengths (≈ 1.8 GPa and ≈ 1.9 GPa) at room temperature, with Ti73.5Fe23Nb1.5Sn2 showing high plasticity up to 20 %. The alloy showed higher tensile yield strength and elongation at intermediate temperatures (450 °C to 600 °C) than popular (α+β) aerospace alloys, like Ti-6Al-4V built by laser-AM [4–6]. LMD-built Fe82.4Ti17.6 largely remained brittle below 500 °C, but out-performed similar induction cast [7] and sintered alloys in compressive yield strength, thus proving an impressive candidate for compression-based applications (like tools) in the intermediate temperature range.
Eutectic alloys, Additive manufacturing, Gas atomisation, X-Ray Computed Tomography, Characterization and evaluation of materials
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