BRAZING & SOLDERING TODAY WELDING JOURNAL 45 Fig. 3 — Honeycomb-lined interstage seals and abrasive-coated seal teeth. called outer and inner air seals, and three inter-stage seals, all using honeycomb lands to minimize clearances and leakage flows thereby improving module and overall engine efficiency. Figure 3 shows the honeycomb lined interstage seals and knife edges coated with an abrasive, thermally sprayed alumina/ titania, to further reduce the wear of the rotor parts during rub incursions. To improve overall engine thermal efficiencies, there is an ongoing effort to push turbine entry temperatures to higher values. First and foremost, this affects the high-pressure turbine (HPT) hardware, but with increased temperatures there, the LPT stages and their air seals see steadily increasing temperatures as well. Typically, the first LPT stages of modern engine designs today see temperatures of up to 1100°C (2010 °F) on take-off conditions. Later stages will see lower temperature loading, typically down to 600°C (1110°F). The seals must withstand these temperatures for extended periods of time and under highly cyclic loading conditions. Therefore, apart from good abradability of the honeycomb seal lands, they must show adequate oxidation and hot gas corrosion resistance to survive the aggressive service environment. Honeycomb and Seal Component Manufacture Seal honeycomb is manufactured from thin metallic foil, typically 75 to 150 μm (0.003 to 0.006 in.) thick, which is corrugated and then laser spot welded in the nodes, to hold adjacent corrugated strip walls together, forming hexagonal cells. The welded honeycomb structure can be cut and shaped to fit backing members onto which the honeycomb is first tack welded and then joined by means of brazing. Welded honeycomb can also be formed into rings that are then brazed to turbine components to form integral ring seal faces such as interstage seals. To provide inner air seals, honeycomb segments are brazed to the shrouds of the nozzle guide vanes while outer air seals are often provided by brazing honeycomb lands to sheet metal segments that are then inserted into holder structures of the LPT casing. Such a brazed outer seal segment is shown in Fig. 4. Honeycomb Materials Seal honeycomb for application in engine compressors is frequently made from stainless steel foil. However, for the temperatures encountered in the turbine section, more heat- and oxidation-resistant materials have to be used. The materials are typically nickel-based alloys with high chromium alloying content. For enhanced oxidation resistance, Haynes® 214, which is a material often used, shows a high alloying content of aluminum. The most popular alloys for turbine seal honeycomb applications are Hastelloy® X and Haynes® 214. Nimonic 86™ is used in some specialty applications. Two iron-based materials, having a high concentration of aluminum and additions of rare earth metal (Y, Hf, and Zr) or oxide (Y2O3), are generally referred to as FeCrAlYs. Of these ironbased materials, MI2100 composition is already in commercial use in gas turbine engine sealing applications, the yttria dispersion strengthened FeCrAlY material, MI 2200, remains an experimental alloy. Braze Filler Materials To accommodate the high service temperatures, turbine side-seal honeycomb is typically brazed with nickel- or cobaltbased braze filler material (BFM) providing high initial as well as high remelt temperatures. Table 1 gives an overview of common braze filler compositions. Requirements for Brazing Seal Honeycomb The brazing of the honeycomb to a carrier structure does not only need to join these two elements together but also needs to reinforce the honeycomb structure itself. This is achieved by filling the gaps between adjacent honeycomb walls with braze filler material using capillary action and forming fillets at the base of the honeycomb. However, it is not desirable that the BFM travels up the entire honeycomb height. Typical specifications will limit the height of the fillet or how high the braze can climb up the faces of each cell. Filling of the nodes is typically also required to reach a certain minimum percentage of the honeycomb height, for Fig. 4 — Stepped outer air seal segment with brazed on seal honeycomb of 1.59- mm (1⁄16-in.) cell size. Table 1 — Chemical Composition and Liquidus Temperature of Braze Filler Metals Braze Ni Cr Si B Others Liquidus AMS 4777 Bal. 7 4.5 3.0 3 Fe 1000°C 1830°F AMS 4778 Bal. 4.5 3.2 1040°C 1900°F AMS 4779 Bal. 3.5 2.0 1065°C 1950°F
Welding Journal | February 2014
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