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A honeycomb structure serves as an excellent gas flow seal and a sacrificial wear-surface to rotating turbine blades in high-temperature turbines. Achieving the ideal honeycomb construction requires careful attention to the amount and placement of brazing filler metal and brazing time and temperature.
Honeycomb structures, one of nature's unique designs, are widely used in such diverse applications as automotive, packaging, high-pressure containers, lightweight aerospace wing panels and engine nacelles, and high-temperature turbine seals for ground power and aircraft jet engines, taking advantage of honeycomb's high structural strength with minimum weight.
In the gas-turbine industry, honeycomb is used primarily in shaft-type labyrinth seals and rotating (rotor) blade shroud seals. This article focuses on the latter, and more specifically, on the use of open-face metallic honeycomb structures in high-temperature gas-turbine seal applications in aircraft jet engines and in industrial ground-power gas/steam applications. Figure 1 shows some typical honeycomb made of Hastelloy X (AMS 4436) used in high-temperature applications. Honeycomb cell size used in such applications typically varies from about 0.031 to 0.125 in. (0.8 to 3 mm) diameter. The depth of finish-machined honeycomb varies from less than 0.062 to 0.5 in. (1.5 to 13 mm) or greater. Figure 2 shows some typical spot-welded nodes between the honeycomb cells. Because they form capillary paths, these vertical nodes are brazed during the brazing cycle in addition to the joint that forms between the base of the honeycomb and the backing member (support ring, etc.) to which it is being joined.
Two primary purposes for using honeycomb for rotor-blade tip seals (rubbing seals, abradable seals, etc.) are: (1) to provide a gas-path seal to prevent hot engine gases from getting around the ends of the turbine rotor blades, and (2) to simultaneously provide a sacrificial surface against which the rapidly rotating blades will rub. When these purposes are realized, greater fuel efficiency should result, as well as longer service life for all rotating components.
Honeycomb seals are required in turbines because engine materials expand with increasing temperature and centrifugal force. By brazing a sacrificial layer of relatively "soft" honeycomb into a wide slot built into the turbine wall all along the blade path, the blades are allowed to grow, and their tips can cut a path into the honeycomb...