Additive Manufacturing for the Restoration and Enhancement of Titanium Compressor Blades in Jet Engines: Small-Scale Testing and Characterization
Authors:   , Johnson James 
Published: April 2024

The gas turbine engine has had continual efforts for improvement, in terms of safety, efficiency and reducing their emissions. To achieve the goal of improved efficiency, jet engines are designed to be lighter and therefore more economical and greener to run. One such component that has received attention is the engine compressor. A traditional design of a separate disc and blades has been developed into the BLISK, a bladed disc that incorporates the blades and disc into one single piece. This gives the advantages of a reduced weight and higher aerodynamic efficiency by eliminating leakage flow by reducing the clearance between the blade tip and casing, however it makes it difficult to repair or replace individual blades. Additive manufacturing has been adapted to partially or completely rebuild any damaged blades, with nickel based repairs already been shown to work, but has not been concluded for titanium based components as of yet. Directed energy deposition (DED) is a powder based additive manufacturing (AM) method, which can provide near net shape components. This method lends itself to the repair of damaged blades, however before this method is used in practice, the mechanical properties and microstructure of the build needs to be understood and qualified. Since in AM, the geometry of the build will affect the microstructure and therefore the mechanical properties, it is difficult to create a representative microstructure, however it does have value in the early TRLs (technology readiness level). Instead, testing of samples extracted from components would give a much more representative result. The main drawback for this is that the component may not be the right size or shape for a standard sample to be extracted from it. In this case the use of small scale testing needs to be implemented. For this study, Ti 6242 was fabricated using laser DED for material characterisation and assessment of the mechanical properties using small scale testing. Small scale testing such as small punch testing is a small specimen test technique, where a disc with a diameter of 3-10 mm and a thickness of 0.2-0.5 mm is tested using a hemispherical punch. Small punch tensile and creep tests were performed with the aim to correlate the results to more traditional uniaxial testing. Small punch tensile testing showed good promise when comparing to uniaxial samples for the DED Ti 6242 as well as a forged Ti 6246 alloy. Small punch creep of both of these materials showed very inconsistent results and is a topic of future research. The difficulty in small punch testing is the nature of stress state. Translating biaxial stresses seen in small punch testing to uniaxial stresses is a challenging process, for which a universal equation does not exist. Previous developments have focused on steel alloys, which when transferred to titanium alloys, do not exhibit the same correlation. Therefore, new methods for small punch tensile and creep have been developed for uniaxial correlations for the forged Ti 6246 and additive Ti 6242 respectively. Additionally, shear punch testing was also undertaken and assessed, as the stress state exhibited by the flat punch tip is less complex compared to the hemispherical punch tip used for small punch testing. For this small scale test technique, it was shown that a universal approach using a range of different materials is also sufficient when adding both titanium alloys.