EFFECT OF HEAT TREATMENT AND MULTIPLE PASSES THROUGH RESONANCE ON DAMPING AND VIBRATION FREQUENCIES OF ADDITIVELY MANUFACTURED STRUCTURE WITH INHERENT DAMPING

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 blade must be more often designed as the resonance-proof component under circumstances of the variable engine speed and varying thermal conditions. One of solutions to this challenge is damping optimization through advanced design of parts. Additive Manufacturing (AM) provides additional design space and capabilities for blading engineers. Introduction of inherent damping (energy dissipation between loose, non-melted metal powder particles) can increase damping by more than one order of magnitude comparing to 'solid' component. On the other hand AM parts often require additional Heat Treatment (HT) or Hot Isostatic Pressing (HIP) to reduce residual stresses and porosity. Additionally, components are affected by multiple passes through resonance and long-term exposure to vibrations during their service life. All these factors can cause changes in the powder: wear, compaction, fragmentation of compaction, micro-welding, sintering, sinter cracking and fragmentation. These changes are affecting damping and dynamic characteristics of structure. Several mock-up components have been studied. Three designs have been manufactured using Laser Powder Bed Fusion (LPBF) technology: a baseline 'solid' beam, beam with one large cavity with loose powder, beam with sixteen small pockets with pins surrounded by loose powder. All beams in as-built condition where then scanned using Computed Tomography (CT). Then, the damping ratio has been assessed for all components using electrodynamic shaker - the response was measured by laser vibrometer. The third beam (with sixteen small pockets) was subjected to Heat Treatment, CT-scanned and tested again. Several sweeps through resonance were executed and CT-scans were performed on this mock-up. The results of shaker tests show significant damping ratio changes: almost complete disappearance of inherent after first resonance passing and damping changes during further runs. Performed CT-scans show formation of the powder sinter after HT and progressive sinter fragmentation caused by resonance passing cycles.