On the design optimisation of direct energy deposited support structures to repair aero-engine turbine segments

Abstract

A novel approach for down-selection of a repaired support structure design produced using Laser Blown Powder – Direct Energy Deposition (LBP-DED) and filled with interstitial Ni-Al powder (∼0.75 area fraction) in a turbine segment was investigated. Simulation of flattening and un-flattening of the segment with implications to degradation of the support structure was quantified using a four-point bend test to identify the role of axial Young’s modulus in out-of-plane flexure. Two markedly different LBP additive structures; Diamond Lattice (DL) - nodal and Continuous Path (CP) – non-nodal, were produced and compared with the un-repaired condition. At room temperature, the forward and rear walls and internal nodes of the original equipment (OE) and DL support structures were found to contribute significantly to the Young’s modulus, with significantly reduced stiffness observed in the CP structures. Oxidation plays a key role in the development of internal compressive stresses within the abradable, with a two-fold increase in elastic modulus in the CP structure, but a smaller increase occurred in OE and DL support structures. A decrease in elastic modulus and concomitant increase in radius of curvature (flattening) occurred with an increasing number of flexural cycles. Cracking is most prominent in the nodal design within the front and rear walls and cracks propagate either to the surface or towards the base of the abradable lattice. No such degradation was observed for equivalent flexural cycles in the original and CP support structures, even up to a significant number of cycles. A criterion for catastrophic failure of the abradable was deduced from a steep decrease in flexural elastic modulus accompanied with a marked change in curvature. A non-nodal design support structure is optimum to counter in- service flattening/un-flattening.