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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 shroud seal, thereby preserving the integrity of the blades and the turbine wall. Ensuring proper surface wear characteristics at the blade/honeycomb interface during operation is of utmost importance.
For this to work properly, it is critical that the honeycomb's top surface be softer than the rotating blade tip so the honeycomb material will wear away preferentially, thereby preserving the shape, size and life of the blade.
Honeycomb seal problems
A primary concern in honeycomb brazing is the quantity of brazing filler metal (BFM) used, because it has a major affect on the rubbing/wear characteristics of the honeycomb and blade tips in service. Problems can occur in both aircraft and ground-based power turbines due to improper application of BFM to bond the honeycomb to its substrate.
As shown in Table 1, only nickel-base BFMs are appropriate for use in high-temperature turbine service. Nickel-base BFMs use boron, silicon or phosphorus in their chemical compositions as temperature depressants to help lower the melting point of the BFMs. However, in a brazed joint, these materials can form borides, silicides and phosphides, which can have hardnesses as high as HRC 60 or greater. This often is harder than the parent metals being joined. (High-hardness BFMs containing phosphorus are not recommended for use in aircraft seals.) If high-hardness BFMs come into direct contact with rotating blades, they can wear away the blade tips, or stress the blades to the point where they may crack. There must be minimal contact between the rotor blades and the hard BFM to prevent turbine problems. This requires absolute control of BFM quantity used in honeycomb brazing. Thousands of such seals have been successfully made and operated over the years when the correct (very small) amount of BFM is used.
Honeycomb and backing substrate materials must be thoroughly cleaned prior to brazing. Honeycomb traditionally is bonded to its substrate using nickel-base BFM in the form of powder, paste, transfer tape or amorphous foil. After resistance welding the honeycomb to the backing, powder can be added to the honeycomb by hand (known as "salt and pepper" technique) or using an automated applicator such as the dump chute , then held in place by lightly spraying acrylic cement over the powder. This fairly straightforward method is commonly done. Brazing paste often is added to the outside edges of honeycomb to create fillets along the edges of the honeycomb for inspection. This practice requires extreme care because excess BFM can easily erode the thin walls of the honeycomb.
Transfer tape (Fig. 3) is made by mixing BFM powder with a suitable binder, cast/rolled out into flat sheets and rolled up and slit to various widths. The brazing practice consists of placing the tape onto the honeycomb material and pressing or rolling into each cell. The honeycomb surface is scraped to reveal the bottom edges (see right side of Fig. 3) for subsequent resistance welding to its substrate prior to brazing.
Amorphous foil is a 0.0015 to 0.003 in. (0.04 to 0.08 mm) thick rapidly solidified, flexible, Ni-base BFM used to join honeycomb in one of two ways: (1) Foil is placed on a substrate and the honeycomb placed on top of it for brazing. Top pressure is applied to the honeycomb to ensure that it makes intimate contact with the substrate by settling through the BFM when the foil melts. This process is not widely used because special tooling is required to keep the honeycomb from "skating" over the molten BFM and becoming misaligned.
(2) Foil is added during the honeycomb manufacturing process so it becomes a part of the honeycomb itself (Fig. 4). The foil is added as either straight foil strips through the centers of each cell (bottom honeycomb in the photo) or as a corrugated foil conforming to the sides of each honeycomb cell (the panel above the pencil in the photo). The primary advantages of this method are no binders to outgas during the brazing cycle, use of only the precalculated exact amount of BFM needed for the joint and no separate BFM application step needed during turbine honeycomb seal assembly (as is required with powder, paste and tape).
The higher cost of amorphous-foil honeycomb often is offset by the savings from fewer manufacturing steps, lower brazing-reject rates compared with other BFM forms, simpler honeycomb replacement during repair cycles, and even a reduction in engine weight from less BFM being used. However, the substrate surfaces must be very flat and smooth for correct use of this type of honeycomb. Thus, it is ideal for new construction, or for use in repair operations in which the substrate surfaces can be machined smooth. During rework and repair, if extensive hand grinding/shaping is done, it might not produce the required surface smoothness to allow the use of this form of honeycomb.
The art of resistance welding honeycomb into place for brazing is not well developed enough in some brazing shops. In some cases, it is assumed that just enough spot welds should be used to lightly hold the honeycomb in position on its backing member until the BFM melts and bonds the honeycomb in place. This can result in honeycomb that partially lifts up during the brazing cycle, because the thin, lightweight honeycomb heats up and expands faster than the heavier backing material, lifting up off its backing support and not brazing properly.
Therefore, it is necessary to make sufficient spot welds (across the full width of the honeycomb and in ring seals along the honeycomb edges) to ensure that the honeycomb remains in intimate contact with the backing support during the entire brazing cycle. Spot welds should overlap to lock as much of the honeycomb as possible to the backing support. It usually is necessary to experiment to determine the proper degree of overlapping needed, based on the spot welding electrode used.
Figure 5 shows an excellent spot-welding electrode, made of copper strips and copper braided wire. This electrode, when pressed against the honeycomb, conforms to and covers a lot of area, as opposed to just one small spot made using a solid tapered electrode on a standard spot-welding machine. The electrode surface must be regularly cleaned for good performance. Special graphite resistance-welding heads are preferred by some shops because they do not pick up surface contamination like the copper heads do.
The force of the electrode against the material must be carefully controlled; too much force can damage the honeycomb structure. For most applications, 5 to 15 lbf usually is sufficient. Experimentation also is needed to determine the right power settings for the specific honeycomb involved; too much power will burn holes in the honeycomb.
Automated spot-welding machines (500 to 1000 W) offer repeatability from part to part. Smaller capacitive-discharge spot-welding equipment (automated or hand-held) often is preferred for tacking very light-gage (0.002 to 0.003 in., or 0.05 to 0.07 mm thick) material.
For consistent resistance-welded honeycomb structures, proper electrode amperage and force electrode settings, once developed, should be documented in written procedures, then controlled and monitored. This can help to prevent arcing, flashing, sparks, burn holes in the honeycomb and possible crushing of the honeycomb at the weld interface. It is not good practice to allow weld operators to modify weld-machine settings to satisfy their own preferences. (Improper tack welding of honeycomb segments has hurt a number of brazing shops.)
Brazing honeycomb into place
Most honeycomb material for turbine applications is furnace brazed, usually in vacuum. While the quantity of Ni-base BFM used is probably the most important factor for successful honeycomb brazing, brazing temperature and times also are very important, since the honeycomb material is very thin and can be easily eroded by the aggressive BFM.
In the brazing cycle, the honeycomb assembly should be heated to an intermediate holding temperature about 100 F (55 C) below the published BFM solidus temperature until the load temperature is uniform, then quickly raised to a brazing temperature about 100°F above the published liquidus temperature (unless experience shows that a slightly lower brazing temperature can be used). Since many shops tend to put on more BFM than is actually required to braze the honeycomb to its substrate, the load should be held at the brazing temperature for only a short time (1 to 5 min) to minimize BFM/honeycomb interaction or significant erosion of the honeycomb. Remember that the ideal amount of BFM required for brazing honeycomb is just enough to fill the volume of space in the nodes between each cell, as well as the space between the bottom of the honeycomb and the substrate to which it is being joined, which means only a tiny amount of BFM is actually needed. Any extra BFM applied to the honeycomb will either build up unneeded fillets in each cell, or could erode the honeycomb.
Some shops have success holding the honeycomb at temperature for up to 30 minutes to diffuse the BFM into the base metals and create a more ductile joint with a much higher remelt temperature. To do this, it is critical that only the correct minimal amount of BFM be applied to prevent erosion. However, holding for a long time increases the probability that any excess BFM will spread and coat all honeycomb surfaces.
Slowly cool parts to below the BFM solidus temperature before any rapid cooling occurs to the structures. The BFM must fully solidify before being disturbed by quenching gases, etc., to prevent porous, weakened and/or cracked joints.
Some shops use a back-filling gas in the vacuum furnace during brazing such as dry argon with a dewpoint of -50 F (-45 C) or drier (measured at the furnace, not its source). Back filling to a pressure of 50 to 100 microns helps to suppress outgassing from the parent metals and BFMs, and the presence of some oxygen in the gas (represented by its dewpoint) can help retard "flashing," or excess flow, of the BFM along the surfaces of the honeycomb material.
Honeycomb wear surface
Many companies that use honeycomb seals in their high-temperature turbine products specify where the BFM can and cannot be visible in the final brazed product. It is commonly known in the industry that hard, wear-resistant layers of nickel-base BFM should not coat the surface of the honeycomb that will touch the rotating blades.
Most honeycomb brazing operations include some form of machining of the honeycomb surfaces to final dimensions after brazing (such as mechanical cutting or grinding, electrical-chemical or spark-erosion methods). It would be wise, however, to develop the BFM application procedure as if the as-brazed surface of the honeycomb would be the rubbing surface. This would then require that appropriate procedures be in place to ensure that only a small amount of BFM is applied so that it does not coat the surfaces and the top of the honeycomb during brazing.
Two ways to keep BFM off the top wear-surfaces of non-machined honeycomb are: (1) building a capillary break into the top of the unmachined honeycomb via special tooling during manufacture to spread apart the two contacting surfaces of each node at the top of each cell so the BFM cannot wick up to the top of the node, and (2) placing a narrow film of braze inhibitor (stop-off) just inside the top of each node during the brazing cycle.
Because most honeycomb is machined, it is essential that the remaining height of the honeycomb after machining is free from excess BFM coating, as illustrated in Fig. 6. Properly brazed honeycomb (no excess BFM at the top) can then deform or wear away when rotor blades rub against the machined surface.
Some brazing shops operate on the incorrect premise that if a little bit of BFM is good, then more is better. Also, some customers demand to see significant fillets at the base of each honeycomb cell and along the outside edges of the honeycomb, leading to the over-application of BFM where too much powder BFM was used (Fig. 7). Contrast that with the minimal amount of BFM present in the foil-brazed honeycomb (Fig. 8).
Some aerospace manufacturers now require that the BFM not climb more than about 25% of the way up the faces of each cell. Figure 9 is an example of such a specification. This kind of requirement needs to become more widespread.
Are final-inspection changes needed? Figure 10 shows a foil-brazed honeycomb with no significant fillets at the base of the cells; the full shape of each cell at the bottom and the base metal substrate are clearly visible. Such a brazement might need revised visual inspection requirements, calling for greater dependance on pouring liquids into the honeycomb to check for cell leaks and drainage, because the "fish eyes" (caused by BFM fillets in each cell) that inspectors are used to seeing (Fig. 11) are no longer present. IH