Fabrication of Metallic Honeycomb Panels for Reusable TPS - Structures

Transcription

1 AT RM 90 B. Tabernig et al, 15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 Fabrication of Metallic Honeycomb Panels for Reusable TPS - Structures Bemhard Tabernig*, Wolfgang Thierfelder*, Hubert Alber*, Kees Sudmeijer + *Plansee AG, Reutte, Tyrol, Austria + Fokker Space, Leiden, The Netherlands Summary: The manufacturing technology with specific regard to high temperature brazing was developed to fabricate a honeycomb panel consisting of a thinsectioned PM 2000 core material sandwiched on both sides with PM 1000 face sheets. For brazing the PM 1000 / PM 2000 panel the braze alloy PdNi was selected due to the best oxidation behaviour while good mechanical properties and wetting behaviour compared with other tested filler alloys. To examine the concept of a hybrid PM 1000/2000 panel as a stiffened skin panel a number of engineering test samples of sub-scale and two full-size panels were fabricated at Plansee AG and supplied to Fokker Space for testing under representative in-service conditions. Engineering tests showed that the test samples were rather insensitive to temperature gradients even at temperature differences between the face sheets of 550 C. The engineering test samples exhibited no plastic deformation after testing at different heating rates ranging from 5 to 40 C/s and at temperature profiles representative for two flights. The requirement for the designed application regarding impact properties at low as well as high speed were met. Impact at low speed with an energy of 8J did not cause any cracks. Hail tests where ice bullets were fired with speeds to 208m/s at different angles from 25 to 90 C against the test piece showed no damage at 25 and caused slight indentation at 45 and cracks at 90, which demonstrated a good performance for the fly through a hail cloud without any problems. In tests to determine the response of a full-size panel to a number of simulated thermo-mechanical flight load cycles the panel passed 50 cycles successfully without damage.

2 B. Tabernig et al. RM " International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 Keywords: RLV, thermal protection system, TPS, PM 1000 / PM 2000 hybrid panel, honeycomb, brazing, ODS - superalloy 1. Introduction: Increased demand for delivering payloads to low earth orbit and reliable access to space will drive much of the civilian and military aerospace industry in the coming decade. Reusable launch vehicles (RLV) can be the most costand mission-effective approach under the condition of dramatic decreased launch costs combined with a high level of reliability for space transportation. Beside others one key factor for meeting these requirements of a future RLV system is a strong reduction in the maintenance costs. With regards of the existing Space Shuttle system the thermal protection system (TPS) which consists of ceramic tiles in the hot areas and insulating blankets in the cooler areas has only shown a limited reusability after re-entry in the past and was identified as one of the most significant contributors to maintenance and repair costs. Overall technology optimisation and revisions for the re-entry trajectories resulting in decreased temperature loading, however, lead to new, most promising design concepts which consider the introduction of high temperature metallic materials assemblies as replacement of the ceramics based TPS - structures (1). Due to the increased damage tolerant performance and service time of such metallic structures as well as improvement in terms of inspection and repairability the maintenance costs would be reduced decisively. The envisaged baseline design consists of load bearing metallic honeycomb panels fastened to a stand-off structure of the primary bulk of the spacecraft. In some areas at the outer surface these skin panels have to withstand operating temperatures up to 1050 C with emergency capabilities up to 1200 C in oxidising environment during the re-entry period and simultaneously carry compressive loads. Regarding this service range the most promising candidates for this application are oxide dispersion strengthened (ODS) - superalloys as the alloys PM 1000 and PM 2000 (2). The aim of this work was to develop the manufacturing technology of a hybrid PM 1000 / 2000 honeycomb panel and to examine this concept for a reusable stiffened skin panel. For this purpose a number of test samples of sub-scale size, and two full-size panels were fabricated at Plansee AG and supplied to Fokker Space for testing under expected service conditions.

3 748 RM 90 B. Tabernig et al. 15 ih International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol Material: PM 1000 is a cold-workable oxide dispersion strengthened (ODS) Nickelbased superalloy with high chromium content. Thanks to dispersion strengthening with Y 2 O 3 and a recrystallized, coarse-grained microstructure, PM 1000 features high tensile and creep rupture strengths at very high application temperatures, near its melting point. Due to the 20 % chromium content, the material is resistant to hot gas corrosion. The formed oxide layer exhibits good short-term cyclic oxidation resistance and a high emission coefficient at very high temperatures. PM 1000 additionally shows excellent low-cycle fatigue properties. These unique properties make PM 1000 most suitable as face sheet material. PM 2000 is an iron-based ODS alloy with the high aluminium content of 5.5wt%. The oxide dispersion strengthening confers a structural capability for operation temperatures of up to 1350 C. Its excellent high temperature oxidation resistance is achieved by the formation of a dense and tightly adherent alumina scale. The results of cyclic oxidation experiments demonstrate its superior behaviour compared to conventional superalloys such as Haynes 214 (3). PM 2000 is some 15 % lighter than PM 1000 and can be worked to very thin sections while still retaining the excellent high temperature strength and oxidation resistance (4). These features are crucial for the use as honeycomb core material. Consequently the optimized panel design consists of a PM 2000 honeycomb core joined onto PM 1000 face sheets, a set-up which is referred in the following as hybrid panel. According design of Fokker the thickness of the PM 1000 face sheet was 250um, the honeycomb core was manufactured from a 125um thick PM 2000 foil. This foil was corrugated into a half hexagon shape, the strips were then laser welded to form the full hexagon shape and the honeycomb structure built up layer by layer. 3. Development of Joining Technology: In order to join the honeycomb core to the face sheets a brazing technology had to be developed which ensures strength, ductility and oxidation resistance of the joint up to 1200 C comparable to the base material. For basic brazing experiments in a high vacuum furnace with a pressure p < 10~ 5 mbar five different commercial filler alloys (table 1) were selected regarding their

4 B. Tabernig et al. RM ;h International Plansee Seminar, Eds. G. Kneringer, P. Rfldhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 high melting temperature: the nickel based MBF 51, the cobalt based MBF 103 and the precious metals based braze alloys PdNi, PdCo and PdAu. PM 2000 honeycomb cores with a size of 50 x 50 x 8mm were brazed to face sheets by these different filler alloys. The brazing temperatures depended on the melting temperature of the brazing materials. Typical brazing cycles showed a total time of approximately 5 hours by a holding time between 5 to 10 min at brazing temperature. The processing temperature was monitored with a Pt/lr thermocouple. Material MBF51 MBF 103 PdNi PdCo PdAu Composition [wt%] Ni 15Cr1.4B7.2Si0.06C Co 15Ni 21Cr4.5W 1.6B 4.4Si 3Pd Pd 40Ni Pd 35Co Pd 92Au Table 1: Composition and melting temperature T M of filler alloys T M [ C] Wetting and Metallography: Cross sections of brazed samples were examined with an optical microscope to study the wetting and penetration behaviour of the tested filler alloy. After optimising of the brazing temperature and time all filler alloys showed excellent wetting and full penetration with the face sheet and the honeycomb core. Figure 1a: Joint by PdAu filler Figure 1b: Joint by PdNi filler

5 750 RM90 B. Tabernig et al. 15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 Only PdAu was too aggressive to the base material so that the honeycomb was totally dissolved (fig. 1a). Using short brazing times and low brazing temperatures, however, metallographically sound joints can be achieved by the PdNi filler (fig. 1b). 3.2 Oxidation Behaviour: The oxidation properties of the joints were evaluated by the mass change at 1200 C for 20 hours under stagnant air. Experimental results of all tested filler alloys are presented in comparison with the base material (PM 2000 core and PM 1000 face sheets) in fig. 2. The MBF filler alloys showed a high corrosion attack and spalling of the oxide layer. The best oxidation resistance which was comparable to the base material was achieved with the filler alloy PdNi. D flaked off oxide [%] g 20 D change of weight [%] O 15 change of we MBF 51 MBF103 PdNi PdCo PdAu base metal Figure 2: Change of weight after oxidation at 1200 C for 20 hours at air 3.3 Mechanical Properties: For the evaluation of mechanical properties tensile tests were carried out at RT, 700 C and 1100 C on test samples where face sheets PM 1000 (80 x 40mm 2 ) were brazed by the selected filler metals. The tests were done at a strain rate of 10" 3 s" 1 with 2 samples at each temperature. Excellent strength values and a sufficient elongation up to 1100 C could only be gained on

6 B. Tabernig et al. RM " International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 specimens brazed by PdNi and PdCo (table 2). The low ductility of the Nibased braze alloys was referred to the formation of brittle intermetallics and a strong increase in hardness in the diffusion zone. Filler alloy Rm [MPa] PM 1000 MBF51 MBF 103 PdNi PdCo PdAu RT > C > C > RT > Table 2: Mechanical properties of the filler alloys f [%] 700 C > C > Due to the best oxidation behaviour while good mechanical properties and wetting behaviour compared with the other filler alloys PdNi was selected for brazing the PM 1000 / PM 2000 honeycomb structures. 4. Manufacturing of Full-Size Panel: 4.1 Design: After the evaluation of first experimental results the design of the full size pane! was fixed by Fokker. The panel will be composed of the following components (as indicated in fig. 3): PM 1000 upper face sheet (420 x 300mm): "2" PM 1000 lower face sheet (377 x 300mm): "3" Holes of 00.4mm were drilled in the lower face sheet to provide air circulation within the honeycomb structure and hence to ensure severe testing condition during oxidation tests. PM 2000 honeycomb core (400 x 300mm): "4" The edges were grinded in angle of 45 as a consequence of the selected sealing design. PM 1000 sealing members (bended sheets with a length of 300mm): "1"

7 752 RM 90 B. Tabernig et al. 15* International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 Figure 3: Schematic illustration of the full-size panel design 4.2 Development Route and Manufacturing Procedures Manufacture of test samples: For selection of braze alloy and first experimental testing a number of test samples up to dimensions of 50 x 50mm were manufactured. The core material had a cell size of 4.76mm and a height of 8mm. According design study the cell size was changed for following development steps to 8mm and the height to 10mm to further optimise the weight per area of the designed panel. Up-scaling to engineering test samples: The first step of up-scaling included the confirmation of developed brazing technology on test samples with new core design and dimensions of 50 x 50mm. A visual and metallographic investigation of specimens thereof showed comparable results with test samples in the basic development. In following steps successful up-scaling of test samples to dimensions of 120 x 120mm was performed. After control of joint quality a number of such engineering test samples was delivered to Fokker for thermo-mechanical investigations and impact tests.

8 B. Tabernig et al. RM " International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 Development of sub-scale panels: The next step in the development route was the integration of the PM1000 sealing members in test samples of 120 x 120mm. A panel assembly consisting of the face sheets, the honeycomb core and the sealing elements was brazed. Special tooling had to be developed to fix the assembly and to promote uniform heating of the panel. An example of a sub-scale panel with details of the sealing is presented in fig. 4. Figure 4: Sub-scale panel with integrated sealing members Visual and metallographic examinations have shown uniform brazing over the total area of the panel with no irregularities to previous development steps and an excellent brazing connection in the sloping edges. Tests to manufacture the holes in one side of brazed panels by mechanical drilling in dry conditions showed good results. Manufacturing of full-size panels: Based on gained experience in the previous development steps two full-size panels were fabricated. Manufacturing started with the preparation of the prematerial. The PM 1000 material was grinded to thickness of 250um. The face sheets and sealing elements were machined thereof, specific attention to the tolerances was paid when bending the sealing members. The PM 2000 honeycomb was grinded after corrugation, too, in order to ensure flatness less

9 754 RM 90 B. Tabernig et al. 15" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 than 50um and to avoid missing joints after brazing. The edges of the honeycomb core were cut in 45 angles on two sites. For joining a thin braze foil PdNi with a thickness of 50um was used to minimize its effect on the base material. The individual raw parts of the panel, the face sheets, the edge members, the core material and inserted braze foils, were assembled to an adequate fixture by laser spot welding and subsequently joined by a high temperature high vacuum brazing process at temperatures of about 1250 C C. A sample which was brazed in the same cycle was investigated with metallographic methods to certify the brazing step. Additionally a C-scan, a non destructive pulse echo ultrasonic technique which can detect different types of defects in brazed honeycomb structures (4) was applied to characterise the quality of brazing. The C-scan indicated good results with only a few minor imperfections in the joints over the total area. The final treatment was the mechanical drilling of holes in one side of the panel. One of the full-size panel which was delivered to Fokker for further testing is presented in fig. 5. Figure 5: Full-size panel with drilled holes on one face sheet

10 B. Tabernig et al. RM '" International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol Testing: 5.1 Engineering Tests Thermal cycling tests: 2 engineering test samples (120 x 120mm) were cycled thermically in order to identify at small scale the thermal response of a hybrid honeycomb panel to heating rate and thermal flight cycles. The test panels were heated at one side by infrared radiation. The temperature was controlled by two sets of opposing thermocouples at 5mm distance from the center line on the heated and unheated side of the panels. Before and after each test C-scan pictures were made to check the condition of the braze joints. The objective of the tests on the first panel was to study the impact of different heating rates. The test procedure included 4 heating up and cooling down cycles. The upper face sheet of the sample was heated up to 1050 C in all 4 cycles. The thermal load, however, was increased in the 4 cycles by increasing the heating rate. The values were 5 C/s, 10 C/s, 20 C/s and 40 C/s. After the heating up phase the sample was kept for 100s before cooling down with the same temperature gradient as used for heating. The thermal load can be illustrated by the temperature difference between upper and lower side of the panel. The maximum value was 550 C measured in the test at the heating rate of 40 C/s. The second panel was tested by a representative temperature profile of two thermal flight load cycles. In spite of the applied high temperature difference between both face sheets both samples showed no damage after all tests and were flat as before the tests. The C-scan pictures indicated that the braze joints were still good. This was further supported by a following metallographical investigation on the first test sample. The conclusion can be drawn that the panel is rather insensitive to thermal stress. Impact tests: Low and high speed impact tests were performed on 4 engineering test panels to determine the sensitivity to external impacts at different speed. The low-velocity impact tests were performed with an instrumented drop tower using spherical headed drop weights with a diameter of 7.5mm. The test panel surface was positioned perpendicularly to the drop path. Impact speed and energy were determined by different drop heights and weights. The impact energy was stepwise increased by 10J starting from 8.4J. A load cell in the head of the drop weight measured the contact force.

11 756 RM 90 B, Tabernig et al. 15* International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 The result of the impact test with an impact energy of 8.4J is presented in fig. 6a. The impact did not cause cracks as revealed by visual inspection and a C-scan, but only some plastic deformation. Hence the requirement for the panel to withstand impact energies of 8J without cracking was met. Only after additional 20.3 Joules cracks appeared (fig. 6b). Instead of highly deformed cell walls the braze joints showed still good quality (fig. 6c). Figure 6: Deformation of test sample tested at an impact energy of a) 8.4J and b) 8.4J J; c) braze joint after impact test of 20.3J The high-velocity impact tests were performed with an air gun and ice "bullets" of 25mm diameter. The impact speed was 208m/s which was measured by a sensor in the air gun. The test panel was positioned at different angles ranging from 25 to 90 to the bullet path. The hail tests showed no damage at 25 and cause slight indentation at 45 and cracks at 90, which demonstrated a good performance for the fly through a hail cloud without any problems. 5.2 Testing of Full-Size Panel The objective of this test was to determine the response of a full-size panel (300mm x 420mm) to a number of thermo-mechanical flight load cycles in order to simulate the in-service behaviour. A schematic drawing of the test equipment is given in fig. 7. The panel which was fixed on a support frame was loaded mechanically in bending by applying a load on the bearing and/or thermically by the heating of a quartz radiator on the upper side. The dimensions of the load application areas were 20mm x 5mm. The test machine was equipped with thermocouples and strain gages at defined positions to register temperature and record displacements.

12 B. Tabernig et al. RM * International Plansee Seminar, Eds. G. Kneringer, P. Rddhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 radiation T T T T T T t t t f t f t T T t T full size panel load Figure 7: Schematic illustration of equipment for testing the full-size panel The tested thermo-mechanical load profile which is representative for the TPS during ascent and re-entry of RLV is shown in fig. 8. The test on the fullsize panel included 50 cycles, e.g. simulation of 50 flights. In the first test cycle only the mechanical load profile, in the second only the thermical load was applied. In the following 47 cycles the panel was exposed to the combined thermo-mechanical load profile. ^""temperature ( C) load (N) time (sec) Figure 8: Representative thermo-mechanical load profile of a skin panel during ascent and re-entry of the RLV

13 758 RM90 B. Tabernig et al. 15~ International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 The panel passed successfully all 50 thermo-mechanical cycles without damage which was controlled by visual inspections and C-scans. Figure 9 showed no changes in the C-scan after the cycles compared to the C-scan taken before the cycling tests. isii i Figure 9: C-scan of the full-size panel a) before and b) after the 50 thermomechanical cycles A final test at 1100 C was carried out by increasing the mechanical load until failure occurred. The panel demonstrated once again good performance as the actual failure load was 1186N and hence about 10% higher than the predicted one. Failure occurred over the midline where the bending moment reached a maximum (fig. 10a). Apparently the failure mode was initiated by dimpling of the compressed facing (fig. 10b) causing a reduction of stiffness and subsequent failure of the panel.

14 B. Tabernig et al. RM " International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol. 1 Figure 10: a) failure of full-size panel and b) dimpling of compressed facing For a detailed failure analysis specimens were cut from the tested full-size panel and examined in the LM/SEM (fig. 11). The pictures clearly reveal the high quality of the braze joints, the uniform brazing with small diffusion zone and the deformation of the compressed face sheets with cracks initiated in tensile regions. Figure 11: a) specimen cut from tested panel and b) a metallographic section thereof

15 760 RM 90 B. Tabernig et al. 15 ; ' International Plansee Seminar, Eds. G. Kneringer, P. Rodhammer and H. Wildner, Plansee Holding AG, Reutte (2001), Vol Conclusions: The manufacturing technology of a hybrid PM 1000 / 2000 honeycomb panel was developed and its use for a reusable stiffened skin panel investigated. For this purpose a number of test samples of sub-scale size, and two full-size panels were fabricated and tested under expected service conditions. Engineering tests showed that the test samples were rather insensitive to temperature gradients even at temperature differences between the face sheets of 550 C. The engineering test samples exhibited no plastic deformation after testing at different heating rates ranging from 5 to 40 C/s and at temperature profiles representative for two flights. The requirement for the designed application regarding impact properties at low as well as high speed were met. Impact at low speed with an energy of 8J did not cause any cracks. Hail tests where ice bullets were fired with speeds to 208m/s at different angles from 25 to 90 C against the test piece showed no damage at 25 and caused slight indentation at 45 and cracks at 90, which demonstrated a good performance for the fly through a hail cloud without any problems. In tests to determine the response of a full-size panel to a number of simulated thermo-mechanical flight load cycles the panel passed 50 cycles successfully without damage. References: (1) M.L Blosser: "Reusable Metallic Thermal Protection System Developments" (3 rd European Wokshop on Thermal Protection Systems, Noordwijk, The Netherlands, 1998) (2) F.E.H. Muller, D. Sporer:,,ODS - Superlegierungen fur metallische Warmeschutzsysteme" (Werkstoffwoche 98, Munchen, 1998) (3) C. Brown, E. Verghese, D. Sporer, R. Sellors: Proc. Int. Gas Turbine & Aeroengine Congress & Exhibition, pp. 1-9 (ASME, Stockholm, 1998) (4) J.O. Taylor: in Review of Progress in QNDE, Vol. 16, eds. D.O. Thompson and D.E. Chimenti, pp (Plenum, New York, 1997)