Low Leakage Fiber Metal Seals

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS A345 E. 41St. New York, N.Y GT-141 ^^ - The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of Its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published in an ASME Journal. Papers are available from ASME for fifteen months after the meeting. Printed in USA. Copyright 1992 by ASME Low Leakage Fiber Metal Seals G. P. JARRABET and L. LU Technetics Corporation DeLand, FL ABSTRACT To improve compressor efficiency, gas turbine engine manufacturers have focused on the need for impermeable low leakage rub strip seal materials. Rub strips are used for clearance control between rotating and stationary components in gas turbine engine compressors and turbines. Fiber metal materials are used in many clearance control seal applications and offer suitable rub characteristics, erosion resistance and temperature capability. The excellent abradability characteristics are related to the seal's high pore volume of many small but interconnected and open pores. A ceramic foam compositing process was developed for incorporating a closed cell foam into the fiber metal structure. This composite seal material achieves the low leakage needs of advanced engines. The properties of abradability, erosion resistance, oxidation resistance, and low weight inherent in fiber metal compressor seals are maintained. The seals are fabricated by brazing preformed rub strips to backing rings and final machining. Seal fabrication uses conventional processing. The low leakage composite rub strip is an innovative approach to improving compressor efficiency that offers a combination of desirable properties in a seal material. INTRODUCTION The efficiency of gas turbine engine compressors can be improved by holding blade tip clearances at or near zero. At ambient conditions the ideal material for blade tip seals is rubber, due to the fact that it provides a leak-free surface, is impermeable through the material and does not cause blade wear when contacted by the high speed rotors or blades. As air is compressed, the temperature due to compression rapidly exceeds the temperature capabilities of rubber and alternative materials are employed. For the last fifteen years, materials such as fiber metal seals and honeycomb have been used as alternatives to rubber in higher temperature sections of the compressor. These materials have been able to provide near zero tip clearance control and have had the necessary temperature capabilities. However, because these materials are porous (a necessary condition to allow the materials to abrade without blade damage), these materials have allowed leakage through their structure due to the necessary porosity. Leakage includes flow across the seal land as well as air bypassing the blade tip via compression into the porous rub strip. For today's advanced engines, a new abradable material is needed that is characterized by closed cell porosity. CONCEPT A recent improvement on the porous fiber metal seal has been the addition of a closed cell chromeoxide ceramic foam to the basic fiber metal skeleton, producing a leak-free composite material. A standard fiber metal blade tip seal is FM509D which is made from Hastelloy X 10 micron diameter fibers at 21% density (metal content by volume). The porous fiber metal structure is produced from randomly distributed fibers which are diffusion bonded to each other. This new material while utilizing the advantages of standard fiber metal seals (durability, erosion resistance, machinability, braze ability, etc.) is an improvement over the standard material in that the leakage through the material is zero and tip contact leakage (point leakage) is reduced %, depending upon surface finish. The material has the further advantage of being able to be ground to a surface finish of 150 micro inches Ra (0.4mm) to improve aerodynamic smoothness. The introduction of a filler material into standard fiber metal has been of interest to the company for a number of years. Because of the small pore size of fiber metal materials, insertion of material into the fiber metal matrix is not a trivial task. Through experimentation, a process was developed by which a closed cell chrome oxide ceramic foam was developed within the fiber metal seal which produces a light weight impermeable composite structure. Presented at the International Gas Turbine and Aeroengine Congress and Exposition Cologne, Germany June 1-4, 1992

2 The nature of the closed cell fiber metal seal is shown in Figures 1 and 2. Figure 1 shows the surface of the fiber/ceramic foam seal material. Figure 2 shows a cross section of the fiber/foam structure illustrating the closed cell bubble pores of the foam structure. The chrome-oxide fiber metal seal material meets the requirements for a rub strip material, namely: abradability, erosion resistance, leakage control and oxidation resistance. The product design has been chosen to provide a balance between rub tolerance and erosion resistance which can be adjusted for specific applications. FIBERMETAL/CERAMIC SEAL PROPERTIES Impermeability: Fiber metal and honeycomb materials are brazed onto substrate materials making permeability through the material unmeasureable after joining. Additionally, through permeability is not the property of concern with regard to blade tip seals. An alternative method is to supply a controlled flow to the surface of the material and measure the reduction in flow as air passes through the surface. An air supply regulated to 10 psi (.07 MPa)with a flow of 22 SCFH (.17 1ps) is delivered to a rubber or soft plastic tipped 3/8" (.95 cm) diameter steel tube. The rubber tip insures conformance with surface irregularities so that permeability of the material is measured and not the leakage due to surface imperfections. An air flow greater than zero indicates air passage through the seal and around the wall of the tube. Figure 3 shows the apparatus used for flow testing. Flow Meter Pressure Gauge Figure 1 - Surface morphology of the metal-fiber/ ceramic seal material. The bubbles or microspheroids are chromium oxide. The remaining areas are metal fibers. Air Flow Valve \ Ela tourer Tubing 304ss Tube Air Leak Seal Figure 3 - Schematic of Flow Test The following table summarizes the results using the flow test apparatus above: TABLE I Initial Measured Flow Flow % Reduction (SCFH) (SCFH) Figure 2 - The cross section of the metal-fiber/ ceramic seal material. The gray area (labeled as "f") is metal fiber. The arrows indicate chromium oxide bubbles. The dark regions are closed pores. No Test Std FM Chrome Oxide FM SCFH =.173 liters per sec. 1 SCFH = liters per sec.

3 Table I indicates that the new fiber metal/ceramic composite significantly reduces surface leakage due to open porosity. Equivalent leakage results are obtained on an as machined seal surface as well as a rubbed seal surface. Abradability/Rub Characteristics: The abradability of the new composite material has been evaluated extensively for its suitability as an abradable material with tipped Titanium blades. Much of the turbine industry is moving toward Ti blades tipped with some abrasive medium. Due to this fact, evaluations were performed using Al203 tipped Ti 6-4 blades. A series of rub tests were performed on conventional and chrome-oxide fiber metal seals to compare performance. The rub test rig has been described previously.' In the rub test, a rotor is spun by an air turbine to up to 800 fps (244 mps) tip speed. A flat seal sample is advanced at a controlled feed rate into the two rotating tips. Both blade tip and knife edge configurations are tested usually at room temperature. Computerized data acquisition records rotor speed, interaction depth, face load and torque applied to the seal. Videotaping records the rub interaction, typically about 5 seconds long. Subsequent analysis involves seal groove and tip measurement, visual and microscopic seal and tip examination and metallography. Rub testing is used as 1 Tolokan, R.P., Erickson, A.R., and Frank, R.T., "Rub Testing of Feltmetal Seals", ASME 80-GT-154, March, a ranking tool against a baseline of materials of known engine performance. Results of the test series are shown in Table II. Table II indicates that the standard material FM509D oxidized for 2 hrs. has an average torque value of in-lb. sec (171 w)and an average U.R.E. of 23,714 ft lb/in 3 (1961 J/cm 3 ) while the new composite material has an average torque of in-lb. ses (141 w) anc3 an average U.R.E. of 26, ft-lb/in (2205 J/cm ). This information indicates that the new material vs standard material has an equivalent or slightly better performance when abraded with Al203 tipped Ti blades. Erosion Resistance: The erosion resistance of the new composite fiber metal/ ceramic falls in the range of cc/150 gm of sprayed grit. This range is similar to the standard FM509D and varies as a function of the tensile strength of the material. It should be noted that the new composite material requires a higher starting density of fiber metal (Table II, % Density) to hold the erosion resistance to the standard FM509D material. Erosion resistance is determined on an in-house test rig. This rig delivers -240 mesh alumina grit at a 45 angle to the seal surface at a controlled and reproducible rate. Six readings per sample and five sample sets are used to establish each data point. Erosion resistance is reported as volume of material removed per 150 grams of grit fed. TABLE II ABRADABILITY TEST RESULTS TEST INCURSION PERCENT UTS RPM RPM RUB U.R.E. TIP NUMBER FM BLADE (IPS) DENSITY (PSI) (INITIAL) (FINAL) DEPTH (FT.- LBS./IN.CU ) WEAR TORQUE (INCHES) (IN- LB.SEC) x WTiALTIP x WTiALTIP x WTiALTIP CrO WT1ALTIP CrO WTiALTIP CrO WTiALTIP CrO WTiALTIP CrO WTiALTIP CrO WTiALTIP CrO WTiALTIP CrO WTiALTIP CrO WTiALTIP CrO WTiALTIP Legend: x = Standard FM509 Oxidized 2 Hrs. 509-CrO = FM509 Filled Processed with Chrome Oxide WTiALTIP =.060" Wide Titanium Blade Tipped with ALuminum Oxide U.R.E. = Unit Rub Energy Tip Wear = Positive Number Means Blade Pick Up/Negative Number Means Blade Wear SI Conversions: Incursion.005 1ps =.13 mm/s UTS 1000 psi = MPa Rub Depth & Tip Wear.001 in =.025 mm Unit Rub Energy (U.R.E.) 1000 ft-lb/in 3 = 82.7 J/cm 3 Torque 1000 in -lb/sec = 113 watts

4 Oxidation Resistance (Operating Life): Oxidation resistance of the new composite material has not been completed. However, the base fiber metal (FM509D) is the starting material for the composite and it is reasonable to conclude that the oxidation resistance would be no worse than the base starting material. Figure 4 displays the measures of operating life (oxidation resistance) of the base FM509D material. Additional testing is planned to determine long term cyclic oxidation resistance of the composite material. 10, micron Haynes 1 3, O x ai 4- -J 1, m a micron Hastelloy X Operating Temperature Limit, OF (Basis: 40% reduction in original tensile strength) Figure 5 - A segment of metal fiber preform which has been cut and rolled. The microstructure of the sintered fiber preform is porous as shown in Figure 6 which was photographed from the surface of the preform. The sintered fibers are randomly oriented with fiber length preferentially aligned parallel to the felt plane. The links between fibers are provided by the sintered bond, in most cases a neck between fibers can be seen. The degree of necking determines the strength of the sintered preform. Between fibers there are pores and these pores have an open path to the surface. In order to reduce the gas leakage, a ceramic is composited into the fiber preform to intercept these open paths. Figure 4: Oxidation Limits for Feltmetal Seals FABRICATION As indicated earlier, the low leakage seal is a metal-fiber/ceramic composite material. Since the ceramic constituent is inherently fragile and lacks deformability, the fabrication route is designed for the application geometry to conform to the fixed backing ring. The fabrication process can be generally divided into two steps, metal fiber preform fabrication and fiber/ceramic compositing process. The technique of manufacture of fiber metal materials has been used for more than 25 years in industry. The process can be discussed as metal fiber manufacture, stacking the fibers into a felt, and consolidating the stacked fiber felt by sintering. The density and strength of the sintered fiber metal are controlled by varying the fiber size, the sintering parameters and weight. For this particular application, the sintered fiber metal ha d a 23% of theoretical density (Hastelloy-X) with a tensile strength around 1900 psi (13.1 mpa). The sintered fiber preform was also ductile (an ultimate elongation of 5%), therefore it is able to be cut and cold rolled into the desired geometric configuration. A strip of cold formed preform used in the compressor seal is shown in Figure 5. Figure 6 - Typical structure of the sintered metal fiber preform, viewed from the surface. The integration of fiber preform with ceramic was achieved by an innovative process. In this process, a ceramic foam-like bubble structure is developed in the pores of the preformed seal. The properties of the ceramic, such as volume fraction, structural morphology and fiber/ceramic interface strength is largely controlled by controlling the parameters of the compositing process. With current methods, the process is able to control the ceramic constituent within a deviation of +/- 0.5% of the volume fraction.

5 Brazing of the Composite Abradable Seal With existing brazing technology, the composite abradable seal is capable of being brazed to various heat-resistant alloys, such as tempered martensite stainless steels or precipitation hardened super alloys. To date, the seal has been successfully brazed to Greek Ascoloy steel (tempered martensite type with a composition of Fe-12wt%Cr-2wt%Ni-3wt%W) and Incoloy 909 (precipitation hardened nickel based super-alloy with a composition of Ni - 42wt%Fe - 13wt%Co-4.7wt%Nb-1.5wt%Ti- 0.4wt%Si-0.03wt%A1). The brazing alloy selected for brazing onto these two backing materials was AMS 4777 (Ni-7wt%Cr-3.lwt%B-4.5wt%Si-3wt%Fe). A typical brazed joint strength for this type of brazing was over 800 psi (5.5 MPa) which coincided with the ultimate strength of the composite seal felt in the vertical direction (perpendicular to the felt plane). The strength test was performed in a tensile mode on an Instron universal tester, using a cross head speed of 0.1 in/min. The coupon was brazed in such a manner that the backing material was sandwiched by two seal materials. The fracture occurs consistently inside the seal material. The brazing cycle used was vacuum brazing at 1900 F (1038 C) for 10 minutes. It should be noted that, in most cases, the brazing process is also concurrent with the solution heat treatment of the backing material, therefore the brazing temperature and dwell time may vary depending on the particular application. The microstructure of the brazed joint is shown in Figure 7. From the micrograph, it is clear that the brazing alloy had good penetration about 8-9 microns into the porous seal material. It was this penetration that provided a good mechanical interlock between the metal fibers and brazing alloy and, consequently, supplied a stronger brazing strength than that of the seal materials. 100 Jim Figure 7 - An optical micrograph showing brazing interfacial/microstructure. The backing material (in the low section of the picture) was Greek Ascoloy. AMS 4777 braze alloy was used. After brazing, the seal to backing joint is inspected by methods used for fiber metal such as ultrasonic or laser holography NOT. The final seal land inside diameter is achieved by lathe turning similar to conventional fiber metal. Cutting is performed dry with sharp carbide inserts. Typical cutting rates are 150 SFPM (.5 cm/s) with a.003 in/rev (.08 mm/rev)feed. A finish cut of " ( mm) is recommended with.030" (.75 mm)rough cuts. The composite material is somewhat easier to machine than conventional fiber metal since the ceramic interrupts the tendency of the metal surface to glaze. Finish machining of the seal ring is done in water soluble oil (5% solution) with a post machining bake step to remove water. DISCUSSION An abradable seal in compressor applications should be sacrificial to minimize compressor blade tip wear, which generally requires the material be porous and low in mechanical strength. On the other hand, the seal also needs to be strong enough to have adequate erosion resistance for preventing wear by high velocity gas flow during the operation. These two contrary factors impose the difficulties in selection and processing of the seal materials. The past 25 years experience in research and development of abradable seals indicates that low density but high strength skeleton structure materials, such as metallic honeycomb and sintered metal fibers, balance good abradability with good erosion performance. However, the skeleton structure inherently lacks impermeability due to its opened pore structure. This is undesirable for the seal application. This disadvantage has been greatly improved by compositing a closed cell ceramic foam into the metal fiber skeleton, resulting in an impermeable seal material as clearly indicated by the leakage data in Table I. The good abradability of the metal-fiber/ceramic seal is largely due to its high porous volume faction (as much as 65 volume percentage). With adequate strength of the seal, the porosity reduces frictional force and frictional heating due to less contact area between the blade tip and the seal material during a rub. Among normal abradable seal materials, such as sintered metal or metal/ceramic powder abradable, thermally sprayed metal or metal/ceramic abradables and sintered fiber or fiber/ceramic abradables, fibrous seal materials are the most capable of being fabricated with a high volume porosity (up to 80 Vol%) while still maintaining reasonable strength. In contrast, the consolidated powder structure seal materials, either made of sintered metal-powder or thermally sprayed powder coatings, have to be kept at a high density (a low porosity) state (typically less than 30 volume percentage) to maintain adequate erosion resistance. In these cases, a high blade wear ratio is potentially a problem. In addition, the high porosity of the sintered fiber/ceramic abradable offers another advantage in weight saving due to its pore fraction. The specific gravity of the sintered fiber/ceramic abradables is 2.56 g/cc. In comparison, the typical thermally sprayed metal/ceramic abradable has a specific gravity about 5 g/cc. Therefore, a fifty percent weight saving is evident compared to thermally sprayed abradables.

6 Another property of the porous seal material is its insulating capability. A fiber metal rub strip has a typical thermal conductivity of.066 BTU-ft/HR-ft 2 - F (.0011 w/cm K) at 200 F (93 C). Thus, the rub strip will retard ring growth during engine start up and operation. The fiber metal/ceramic composite seal material overcomes the open pore structure of the fiber metal seal to produce and impermeable structure. Because the ceramic is in the form of a closed cell foam, properties of the seal such as rub characteristics, erosion resistance, temperature capability, weight and insulating properties are maintained. Fabrication of composite seal rings is conventional using curved fabricated segments, brazing and machining. The application of this new material is expected to reduce leakage and improve efficiency in gas turbine engine compressors.