Advantages of Additive Manufacturing Technology in Damping Improvement of Turbine Blading

Abstract

Classical turbine blade design philosophy assumes so-called resonance-free dynamic solution (avoiding resonances for characteristic rotational speeds) achieved by eigenfrequency tunning. To meet current market demands, modern engines need: to operate with higher load, operate at higher firing temperatures, to startup and shutdown faster and more frequently. Therefore, the rotating blade must be more often designed as the resonance-proof component under circumstances of the variable rotational speed and varying thermal conditions. A century of turbine engine development has provided many solutions for improvement of High Cycle Fatigue lifetime of the blading. One of them is damping optimization through advanced design of parts. There are few main damping mechanisms occurring during blade vibrations: material damping, aerodynamical damping (usually below 0.3%) and frictional damping (depending on the design). Nowadays, the Additive Manufacturing (AM) and especially Laser Powder Bed Fusion (LPBF) allow to manufacture multifunctional and complex components with high structural integrity and extended lifetime. An example of uncooled turbine blade design of a jet engine has been studied. Two designs have been modelled and manufactured using LPBF technology: a baseline design (‘Solid Blade’) and a new design where the airfoil was filled with a matrix of pockets with pins and lattice bars surrounded by non-fused powder (‘Lattice Blade’). Then, the damping ratio has been assessed for both designs using electrodynamic shaker tests — the response was measured by laser vibrometer. Except material damping occurring in the baseline design, the new sophisticated design has additional damping mechanisms: the wave propagates through different media (changes of wave propagation speed, wave reflections), energy dissipates in the non-fused metal powder (friction between powder particles), solid pins in the pockets vibrate independently (act as dynamic dampers and improve energy dissipation in the powder), lattice bars in the pockets transfer the vibration wave to the powder (activate energy dissipation in the whole volume of the non-fused powder). The results of shaker tests show significant damping ratio increase for all investigated modes in this study — comparable to such damping features like friction under-platform dampers and damping bolts. Additionally, the LPBF approach has a multi-functional character — except significant improvement of damping ratio, the mass can be reduced (in this case decreased by about 6%), eigenfrequency can be tuned to avoid resonance, the stress concentration factors can be reduced (which is planned for next studies), etc. The proposed new design has not been optimized so far, giving wide margin for further improvements of the damping performance.