Design, Prototyping and Preliminary Testing of a Ti-Al Gas Turbine Blade

This work investigates the feasibility of manufacturing a turbine blade made of a Ti-Al intermetallic alloy by means of investment casting. The work is based on a multidisciplinary approach that combines a conventional CFD analysis of the flow field around the statoric and rotoric blades with the results of several metallurgical studies aimed at the optimization of the alloy composition by finding the best compromise among fracture toughness, oxidation resistance at high temperatures and mechanical properties. The combination of the two techniques lead to an iterative procedure (of which only the first two steps are reported in this paper): a conventional blade is first modeled and the corresponding investment cast is produced via a 3D printing technique; a first version of the blade is built; a modified blade shape is then obtained by a refined CFD study; as a last step the final version of the blade is cast. On the basis of standard operational specifications representative of modern gas turbines, a turbine blade was therefore designed, tested by CFD (ANSYS-FLUENT) to ensure proper fluid dynamic performance, and its levels of thermo-mechanical stress under working conditions were calculated via a commercial CAD software (ANSYS). The fully 3D version of the component was subsequently prototyped by means of fused deposition modeling. A full-scale set of blades (blade height approximately 7 cm, blade chord approximately 5 cm) was produced by means of investment casting in an induction furnace. The produced items showed acceptable characteristics in terms of shape and soundness. The blade alloy was analyzed by performing metallographic investigations and some preliminary mechanical tests. At the same time, the geometry was refined by a complete and more complex CFD study, and a slightly modified shape was obtained. Its final testing under operative conditions is left for a later study. The paper describes the spec-to-final product procedure and discusses some critical aspects of this manufacturing process such as the considerable reactivity between the molten metal and the mold material, the resistance of the ceramic shell to the molten metal impact at temperatures as high as 2073 K and the limit mold porosity that may compromise the component surface finish. Furthermore, a detailed account is provided for the CFD results that led to the modification of the original commercial shape: pressure, velocity and temperature fields in the statoric and rotoric channels are described in some detail, and a preliminary performance assessment of the turbine stage is presented and discussed.