Investigation of metallic Foams for Aero-Engine Sealing

Aero-engine sealing is a large concern using current material solutions for providing longer service life and better performance. For even optimum conditions current materials have inconsistencies in performance. A new novel set of materials is proposed to provide consistencies in performance and to accommodate for any spontaneous scenarios which occur such as harsh manoeuvres or environmental conditions.
Metallic foams were thought to have suitable mechanical and thermal properties for abradable linings. This report outlines the methodologies of how metallic foams were tested as well as an outline of how metallic foams behaved under certain loading conditions. Alantum foam is industrially available and comprised of a Nickel-Chromium (NiCr) alloy. The effects of filler material within the open cellular structure is also evaluated to understand the effects on the wear mechanisms and material behaviours. Additionally, as thermal limitations arise conventional thermal abradable material in combination with metallic foams is tested to investigate material compliance to understand whether sealing can be achieved.
These tests were achieved through use of the low-speed rig available at the University of Sheffield. This set-up includes front-on stroboscopic imaging of the blade and the abradable sample, the stroboscopic imaging for the blade helps to identify the material transfer, adhesion and damage during the test. The stroboscopic imaging allows for digital image correlation (DIC) post processing to identify areas of weakness, stress and strain accumulation and the fracture mechanics. Vacuum impregnation techniques are adopted to assess and analyse internal structure without breaking any of the metal foam in the sectioning process.
This project focuses on the interaction of an Inconel 718 blade (Rotatory - 100m/s) with a metallic foam abradable (stationary), a consistent incursion rate of 2 μm/pass is used for a suitable comparison of the different wear behaviours for the impact of filler and effects of pore size in open cellular structures. Furthermore, with consistent filler material different incursion rates of 2, 0.2 and 0.02 μm/pass are tested to identify the impact filler has on the wear mechanisms and behaviours during interaction with the blade. Thermal ageing of the samples is carried out to investigate the thermal capabilities. For capped metal foam substrates failure is evident at the lower incursion rates and so incursion rate is used as a cut off criteria where tests are not carried out at the lower incursion rates if they failed at the faster incursion events.
Filled foams rub tests established that filler density and thermally aged samples have a big impact on the rub tests where increases in filler density showed positive results. However, with increased thermal ageing temperatures of + 300 °C failures within the rub tests started to appear such as cracking and thermal smearing.
Finally for capped metallic foams factors such as cap hardness, pore size and incursion rates strongly determine the outcome of tests where softer caps and larger pore size combinations showed the best results at the fast incursion rate conditions. Additionally, patterned coatings have been tested and results have shown that cutting mechanisms are controlled and located along fault lines within the top surface pattern.