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Titanium Aluminide Applications in the High Speed Civil Transport

Paul A. Bartolotta and David L. Krause

National Aeronautics and Space Administration Lewis Research Center

Cleveland, Ohio

Abstract

It is projected that within the next two decades, overseas air travel will increase to over 600,000 passengers per day. The High Speed Civil Transport (HSCT) is a second-generation supersonic commercial aircraft proposed to meet this demand. The e>..'J)ected fleet of 500 to 1500 aircraft is required to meet EPA environmental goals; the HSCT propulsion system requires advanced tcclmologies to reduce exhaust and noise pollution. A part of the resultant strategy for noise attenuation is the use of an extremely large exhaust nozzle. In the nozzle, several critical components are fabricated from titanium alurninide : the divergent flap uses wrought gan1ma; the nozzle sidewall is a hybrid fabrication of both wrought gamma face sheet and cast gamma substructure. This paper describes the HSCT program and the use cf titanium aluminide for its components.

lnt roduction

In 1997, the National Aeronautics and Space Administration (NASA) developed an aeronautics and space transportation technology strategic roadmap called the "Three Pillars for Success". As the name suggests, this plan maps out NASA's future efforts and goals through the year 2020. Three categories (or Pillars) are described. The Pillar One focus is on Global Civil Aviation. Goals in Pillar One concentrate on increased civilian safety, reduced subsonic exhaust and noise emissions, and increased affordability. Pillar Two: Revolutionmy Technology Leaps is the location of the HSCT program. Also included in Pillar Two are programs to develop innovative design and manufacturing tools and technology. Finally , Pillar Three concentrates on the access to space. Included in this pillar are efforts to reduce costs of space flight by developing reusable launch vehicles (RL V) and advancing propulsion teclmologies. To achieve all of the goals listed in each pillar by the year 2020 requires strong partnerships between NASA. industry and academia.

Supersonic Technology CPillar Two goa))

To maintain the nation' s aeronautical leadership, NASA is working in concert with the aircraft industry to develop

· enabling teclmologies for a HSCT. The enabling technology goals to be reached within 20 years are: i) reduce overseas travel time by 50 percent, ii) reduce exhaust emissions to well below today's subsonic engines, iii) decrease noise levels slightly below present engines, and iv) achieve this witl1 at most a 15% increase in today 's subsonic fares. The focussed program chartered to tum these goals into reality is embodied in the High Speed Research (HSR) program. Present efforts are targeted for a 300 passenger aircraft that flies at supersonic speeds of Mach 2.4 and takes-off and lands at conventional airports (figure. 1). Many enabling technologies are required to meet this target configuration and the most critical are being addressed in tl1e HSR prog""ra::..;m;..:..;..... -----.

Figure 1: Artist rendering of the 300 passenger High Speed Civil Transport.

The HSR program is a partnership between NASA, Boeing, General Electric (GE), and Pratt & Whitney (PW). Due to the stringent environmental noise and emissions goals, most of this efJort is concentrated on the propulsion system. The HSCT engine size is larger than conventional subsonic engines such as the GE90 or Pratt & WhiLney PW4000. As shown in figure 2, the engine has two distinct sections. The front h.'llf of the engine consists of the turbomachinery and the combustor. The boxed region (figure 2) denotes the exhaust nozzle, which is primarily used for noise attenuation.

Fi~ure 2: Schell_latic of the HSCT propulsion system bemg developed m the HSR program. The boxed area is the exhaust nozzle portion of the engine.

To ~nect emissions goals, the operating temperature of the cngme core reaches supercmise temperatures ty pical cr military jet engines during supercruise, or conunercia1 engines during take-off conditions '. The key di.IIerence for the HSCT engine is that the major components also need to \~ithstand these temperature ex1:remes for longer periods of t1me (over 4 hours) and have design lives similar to today 's commercial engine components (-18,000 hours). This requirement developed the need for advanced materials t~t maintain their structural integrity during extrel?e high temperatures and for long exposure duraltons.

In addition to .their long-term temperature capabilities, the HSCT maten.aJs need to be lightweight. According to HSR calculatJons, for every unit of mass saved in the propulsion system, the Gross Take-Off Weight (GTOW) of the HSCT is reduced by ten times that amount. Weight plays an important role for several reasons. From an economics point-of-view, a low GTOW equates to m? re passeng~rs and longer cruise range (more passenger­miles) . . And m order to be an acceptable transportation ~ ltell_laltve, ~he HSCT should not require a drastic change m . a~rport mfrasuucture (i.e. , land and take-off using ex1~t111¥ . runways). This is also accomplished by mruntauung a low GTOW. Finally, from an airframe viewpoint, a low engine weight reduces the slJUctural requirements of the wings and fuselage.

The combination of long tem1 durability , high temperatures, and low weight goals make TiAI a viable candidate for several critical components in the HSCT propulsion system. TiAl is being considered in two product ~om1S (cast.subcomponents and wrought sheets). A~ong With ~ese different foffi1S, several compositions a T1Al are bemg studied. The HSR program is also addressing joining techniques, cast repair methods, and production fabrication processes. This paper shows the progress that the HSR team has achieved in U1e last s~veral . years and addresses future HSR requirements for T1Al 111 oU1er components of the HSCT propulsion system.

TiAl Applications in the HSCT Propulsion Svstem Perhaps the most extensive use of TiAl can be fmmd in the HSCT exhaust nozzle. This is where TiAI research was initiated in the HSR program. Originally, cast TiAl w~s selected for the divergent flap of the nozzle (figure 3). T1Al was chosen for its high specific stiffness (modulus­t~-wcight ratio) and its high temperature capability. The d1vergent flap is a relatively large component ( 1.8 m X 3.0 m) that is designed for very small deflections.

Figure 3: HSCT exhaust nozzle illustrating the use cf TiAJ for the major nozzle components.

The small deflection requirement is two-fold. First, the performance of the engine is dependent upon certain dimensions of the exhaust nozzle. One critical dimension is the exit area of U1e exhaust nozzle. Since U1e width cr the divergent flap is rela tively large, a smal l deflection can cause a significant change in that area. From a deflection limited structural viewpoint, the divergent flap is the back-suucture of U1e HSCT engine's acoustic treatment. The acoustic treatJnent consists of ceramic matrix composite (CMC) tiles and bulk acoustic absorber. The CMC tiles are used to protect the bulk absorber from U1e turbulent hot exhaust gases. Each tile is connected to the flap v ia a ceramic fastener. The survival of both U1e CMC tile and cerrunic fastener is directly dependent to the deflection of the divergent flap (i.e., large flap deflections will cause produce high bending stresses in U1e tiles and fasteners).

In addition to the low deflection criterion, the divergent flap needs to be lightweight and fabricated from high temperature resistant materials. It is est imated U1at certain regions of the divergent flap will ex-perience temperatures of over 750°C for long exposure times (over 4 hours). Several design concepts and material systems were initially considered (i.e., sheet metal, metal matrix composites, monolithic superalloys ru1d titanium alloys). After an exhaustive study, cast TiAl was chosen as the prime material for the divergent flap.

During the past several years, the HSR program made significant advances in casting technologies for TiAI, and tllis will be addressed in the next section. However, findings of several HSR studies proved that a cast TiAI flap did not have significant cost and weight savings as expected. These new studies did show that a divergent flap fabncated from wrought sheet TiAI would have U1ose savings. Fueled by recent successes of Plansee in producing wrought sheets of T iAl, it was decided to concentrate the flap efforts toward sheet TiAI fabrication as shown in figure 3. Here too, the HSR program has made

considerable contributions to the wrought TiAI arena in developing forming and joining techniques.

Cast TiAI is still being pursued for ot11er exhaust nozzle applications. These same studies showed that the nozzle sidewall would have significant savings by utiliz ing a cast TiAI subslructure and wrought TiAI face-sheet hybrid sidewall structure (figure 3). Many of the pioneering casting techniques and wrought sheet fabrication me tl1ods are being utilized to produce the sidewall subcomponents for the HSCT exhaust nozzle.

Cast TiAI Progress It has been shown tl1at variability in strength, ductility , and stiffness of TiAI is associated w itl1 variations in AI content

2 that can be related, among other factors, to the

TiAI microstmcture. There are three basic microslructures tl1at can be produced in TiAI depending on tJ1e Al content and material processing: equiaxed, duplex and lamellar. TiAI that is composed of either all equiaxed y grains or a ll lamellar colonies (y plus a.2 phase [D019 structure]) has mate ria l properties on opposite ends of the spectnun. The equia'\ed slructure provides for higher room temperature ductility, while the lamellar stmcture has better fracture toughness and creep properties. The duplex stmcture can be thought of as a compromise between the lamellar and equiaxed microslructures. Composed rf lamellar colonies tJ1at fom1 interspersed about equiaxed y grains, tJ1e duplex microslructure leads to higher strength and ductility, but lower frncture toughness. It a lso has been shown tJ1at AI content can influence the amount ci lamellar colonies that fom1 about equiaxed y grains2.

Table 1. Tensile Properties of cast Ti-4!t-2-2 and XD at...·25 ·c C1~ ~ ( .. )(, Property Yield Strength (MPa) Ultimate Tensi le Strength (MPa) Ducti lity (%) Modulus of Elasticity (GPa) Fracture Toughness (MPa..fm)

48-2-2 XD 275-380 360-500 1-3 160- 175 22

400-600 485-720 0.5- 1.5 160-175 17

Initially, tJ1ere were two types of TiAI being considered for the cast divergent flap: Ti-48Al-2Nb-2Cr (atomic %) and Ti-45Al-2Mn-2Nb (atomic %) + 0.8 T iB2 (volume %), respectively named Ti-48-2-2 and XD. The addition of TiB2 in the XD inoculates tl1e gamma alloy tJ1a t results in refired grain sizes3 ranging from 100 to 150 ~m. In contrnst, tJ1e Ti-48-2-2-cast mate rial lms grnins approximately 4 times larger tl1a11 tJ1e XD. As seen in table I, t11e refired grain size of the XD gives it a higher strength tlmn the T i-48-2-2. However, with the increased strength, XD has a lower ductility and fracture toughness. In general, botJ1 cast Ti-48-2-2 and XD TiAI a lloys have a duplex microstmcture, which consists of y grains, and a.2+y lamellar stmcture. XD lms a finer microslructu re and grain size, which primarily consists of lamellar grnins with TiB2 particles . T he Ti-48-2-2 a lloy has a somewhat larger grain stmcture and the amount of lamellar slructure

varies significantly within tl1e casting. In general, the Ti-48-2-2 shows more variation in the grain stmcture and exhibits textured material properties in thin components4

•

Botl1 alloys have attractive material cl1aracteristics; however, the program needed to proceed with only one cast TiAl com osition.

Figure 4: Proposed divergent flap prototype fabricated from cast TiAI (Ti-48-2-2).

To down-select to one TiAI alloy, divergent flap segments were cast from both Ti-48-2-2 and XD alloys (figure 4). The flap section shown in figure 4 is believed to represent the largest TiAI casting yet produced. The divergent flap segments proved to be a challenge in casting TiAI. The flap section shown in figure 4 incorporates all of the cast features of the product flap. Casting defects such as hot tears, porosity, no fill, and shrink were more prevalent in this component configuration than any ever attempted before with TiAI.

There were several factors (size, geometry, nmterial properties, and microstructure) contributing to tJ1ese problems. The flap segment, being a rib-stiffened component with severn! rib tJucknesses varying between 2 nm1 to 20 mm, has many features tJ1a t a re difficult to cast. Hot tears and inte rnal porosity occurred at many of tJ1e 90° intersections of the egg-crnte stmcture. The majority rf these issues were resolved fo r both TiAI a lloys during the HSR program. Afte r an exhaustive down-selection

process (based on castability, repairability, mechanical properties, machinability, and cost), it was shown that Ti-48-2-2 had a slight advantage over XD. Therefore, Ti-48-2-2 was chosen for all cast TiAl nozzle components.

As previously stated, the exhaust nozzle sidewall (figure 3) is a hybrid structure utilizing both a cast TiAI substructure and wrought TiAl facesheets. The sidewall substructure is comprised of tapered and curved cast TiAI I-bean1s. The average I-bean1 dimensions are 70 em in length, over 10 em in depth, and 2-10 mm in thickness. The !-beams will be electron beam welded together to form the sidewall. Subsequently, the TiAl facesheet is brazed on to the cast 1-beam substructure to finish the fabrication of the sidewall.

Figure 5: Cast Ti-48-2-2 I-bean1 and track illustrating salient features of the nozzle sidewall beanlS.

In an effort to demonstrate and optimize the casting process prior to a final sidewall design, I-beanlS were cast in several configurations incorporating different salient features of the sidewaiJ. One of tlle configurations is shown in figure 5. The most prevalent defect during the initial casting trials was hot tears located at the intersection of the flange and web of the !-beam. Afier several iterations the gating was optimized to eliminate these hot tears.

Joining of cast TiAI components is not trivial. Although mechanical fastening of cast TiAI has been successful, special expensive machining operations are required and the required reinforcing features for the mechanical fasteners add additional weight to the component. A less cost ly attaclunent met11od is electron beam welding. Electron beam welds of cast Ti-48-2-2 up to 25 em in length and 15 nun thickness have been successfully made as shown in figure 6. This type of weld requires special handling and unique weld procedures. To electron beam weld cast TiAI, the parts are heated in a controlled

atmosphere to a prescribed temperature5. The parts are

then slowly removed from the furnace and welded. As the weld is placed, the parts are simultaneously returned to a second furnace at the same prescribed temperature as the first and receive a final heat treatment. Following this process crack free electron beam welds have been produced in Ti-48-2-2 on a regular basis without difficulty. When properly heat treated, t11e all-weld metal room temperature tensile properties are generally better t11an the base metal's, while the creep and fracture toughness are equivalent.

Figure 6: Defect free electron beam weld of cast Ti-48-2-2 .

Conventional large structural titanium castings typically exhibit numerous defects. Depending on their location in the part, many of these defects are repairable. It is more cost effective to repair a large casting than to scrap it out. Typically, the castings are repaired by grinding out the defect and filling in by Gas Tungsten Arc Welding (GT A W) deposition. A repair teclmigue was also developed for cast Ti-48-2-2 using GT A W . These repairs are done in an inert atmosphere while the part is held at a unifom1, elevated temperature. Simple castings of Ti-48-2-2 have been successfully repaired by GT A W. More complex geometry parts like rib-stiffened faceplates and sections from t11e prototype flap (figure 4) also have been repaired. Examples of relatively complex repair welds using GT A W of a 13-mm thick plate are shown in figure 7.

Figure 7: GTW A repair on cast Ti-48-2-2 slab.

Further HSCT applications of cast TiAI Present applications have been limited to the relatively large exhaust nozzle components where the weight benefits of cast TiAl are substantial. Since the conception of the HSR materials development program, cast TiAl technologies have matured, and this has prompted HSCT engine designers to look at other applications for cast TiAl. Even though maximum use temperatures for long term exposures are approximately 760°C, designers have targeted ancillary components in the engine's combustor region for cast TiAl. The back-structure of t11e combustor liner is a prime candidate for cast TiAl. The weight advantage of cast TiAl over conventional superalloys makes it Vel)' attractive for this application in spite cr TiAl ' s shortcomings.

Other applications are e>.1ensions of what is already demonstrated in present commercial aircraft engines. For example, GE is currently evaluating Ti-48-2-2 compressor cases (figure. 8). The T700 compressor case has a much smaller dian1eter than what will be required for the HSCT, but still similar casting technologies can be applied. Here again, ilie anticipated weight savings justify the development costs for a larger diameter casing.

I )

Cast Ti-48AI-2Cr-2Nb T700 Compressor Case

Figure 8: T700 Compressor case fabricated from cast Ti-48-2-2. The HSR program is considering Ti-48-2-2 for a similar application in the HSCT engine.

GE is also developing a Ti-48-2-2 low-pressure turbine (LPT) blade for tl1e GE90 engine (figure 9). These blades are similar in dimension to the LPT blades being designed for the HSCT. Casting techniques for tl1e GE90 blades can be directly applied to the HSCT blades. Here the weight savings potential is two-fold. Obviously, tl1e TiAl blades will lighter than the conventional superalloy blades. However, tl1e real weight saving will be in tl1e turbine disk. The lower blade weight decreases t11e centrifugal force exerted on the disk, tl1ereby decreasing

the associated stresses and consequently the mass of the superalloy disk.

Gl:90 Cnst -to ·Siza Gnmma LPT B lAde

Figure 9: Cast Ti-48-2-2 Low Pressure Turbine (LPT) blade prototype for ilie GE90 engine. Similar size and geomell)' of tl1e HSCT L TP blades.

As can be seen from t11e previous discussion, significant progress in the advancement of cast TiAl teclmologies has been made v ia ilie HSR program. With these advancements, cast TiAI is being considered in many critical applications of the HSCT propulsion system. The opportunities for further applications in commercial aircraft are limited by the lack of understanding by t11e engine designers of cast TiAl capabilities and by its high temperature capabilities. The prior can be resolved by successes as seen in ilie HSR program. The latter will require new compositions of TiAl to be developed and verified. These new compositions must have temperature capabilities approaching superalloys (over 850°C).

Wrought Sheet TiAI Progress Wrought sheet TiAl was down-selected over cast TiAl as the prime divergent flap material for ilie HSCT exhaust nozzle. The divergent flap (figure. 3) is comprised of two superalloy box bean1s supporting a series of sheet TiAI subelements (figure. 10). The fabrication of the sheet TiAl subelements required a significant international effort wiili contributions from indusll)' , academia, and government. Sheet TiAl fabrication processes were optimized, forming methods were developed, and joining techniques were evaluated.

Figure 10: Sheet TiAI subelement of the Divergent F lap concept for the HSCT with salient features of the f1tll-scale flap. [Materia l: Ti-46.5Al-4(Cr-Nb-Ta)-O.lB]

Prior to HSR involvement witJ1 wrought TiAl, Plansee cr Austria had developed ro lling techniques for TiAl as a subcontractor with a DoD program in the early 90 ' s6

.

The original DoD program focussed on diffusion bonding and superplastic forming (SPF) methods of fabricating components from sheet TiAI6

. Therefore, much cr Plansee 's efforts were concentrated on the ingot material (IM) process of TiAl sheet manufacturing. In the IM process, an ingot of TiAl is forged into a pancake prematerial. This pancake is then machined into a rectangular shape, canned, and hot rolled into thin sheets. The thin TiA1 sheets are trimmed and surface ground to the fmal thickness. In their IM efforts, Plansee had selected a composition for the TiAl ingot that optimized certain material properties for the SPF process. This composition was Ti-46.5Al-4(Cr-Nb-Ta)-O. lB and was used for the HSR program too. The IM process is very costly and has high. material rejection rates. This is due to the forging step of the IM process. However, the properties of the IM sheet TiAl are exceptional and have proven to be an excellent material for SPF.

With the aid of the HSR program, Plansee developed a new powder metal (PM) processing met11od for wrought sheet TiAI. The PM process starts with TiAl powders that have a composition the same as the IM material The powder is then consolidated into a prematerial rectangle, canned, and hot rolled , similar to t11e IM process. Again after rolling, the sheets are de-canned, trimmed to fmal shape and surface ground to final thickness. According to HSR estimates, there is a significant cost saving with PM TiAl sheets because t11e forging step is eliminated. However, the PM TiAl sheet has a slight disadvantage over IM TiAI. Areas of micro-porosity are found in the PM sheets, and consequently this slightly limits the strength of the material. For the HSCT application, strength is not a primary design requirement, and tllerefore this is not an issue for the HSCT designers. All of the results described in th.is section are based on PM TiAl sheet material t11a t has been hot rolled in only one direction.

To fabricate the subelement shown in figure 10, several different joining techniques were considered and evaluated. Diffusion bonding was initially the joining method cf

choice. Preliminary diffusion bond trials7 are shown in figure 11. In figure lla, the first attempt of a diffusion bond between two PM TiAl sheets exhibits a visible bond line (arrow in figure lla) . After optimiz ing the bond temperature the bond line disappears (figure 11 b). Subsequent tests showed the bond strength to be greater than the parent sheet material, indicating the diffusion bonding was very successful. However, the bond preparation and required fi-xturing made it impractical for t11e large divergent flap application.

An alternative joining method for tl1e HSCT flap application is brazing. Brazing is not as strong as a diffusion bond but provides an economical option. Brazed joints using TiCuNi70 brazing film were successfully demonstrated in tl1e laboratory7 An example of the brazed joint is shown in figure 12. As implied by the micro­hardness indicators (dian1ond marks in figure 12), the braze and its two reaction zones are more brittle tllan the parent TiAl material. This could be a problem in low cycle fatigue (LCF) situations ; however, for this application, LCF does not limit t11e desig n life. Initia l strengtll tests indicate the brazed joint to be structurally sound and providing full coverage witltin ilie joint area. These results provide confidence in using ilie TiCuNi70 braze, the primary joining met110d for the divergent flap.

Figure 12: TiCuNi70 braze of Gamma TiAl sheet [Ti-46.5Al-4(Cr-Nb-Ta)-0.1B 7•

Mechanical fasteners will be required to attach the TiAl sheet subelements to the box beams of the divergent flap (figure 3). As a part of the effort to evaluate mechanical joining methods for sheet gamma, a series of room temperature and 700°C static tensile tests were con~ucted on riveted specimens (figure 13 a&b). The specunens were fabricated by joining two 13 mm X 64 mrn X lmm sheets with a 6 nun "Cherry max" stainless steel rivet. The holes for the rivets were drilled using water jet machining and subsequently honed to fmal dimensions. Initial test results showed smaller than expected failure loads7 and failures initiating within the sheet TiAl at tl1e rivet hole (figure 13a). No rivet failures occurred in any cf the specimens. It was hypothesized that the drilling still produced some residual stresses. Therefore, a stress relieving heat treatment was prescribed on subsequent test samples. Results from subsequent tests wiili the heat treatment showed a marked increased in failure loads, and the majority of the failures occurred wiili ilie stainless rivet.

To evaluate wrought gamma TiAl as a viable material candidate for the exhaust nozzle, a divergent flap subelement was fabricated using 1-nun thick sheets cf TiAl (figure 10). This subelement is the largest structure fabricated out of sheet TiAl. The subelement was approximately 85 em in length and has 10-cm corrugations. Incorporated into the subelement were features that might be used in the fabrication of a full-scale divergent flap. These features included t~e use. of i) s~ear clips to join together sectiOns of corrugallons, 11) muluple face sheets, iii) double corrugation sections and iv) b~ joints. Fabrication processes of double corrugat.Jon forming and face-sheet-to-corrugation brazes were extremely successful. Shear clip brazes were not as successful. Due to an incorrect process interpretation from laboratory to production unit, ilie braze coverage in t11e shear clip area only averaged between 70 to 85 percent. However, it was shown tl1at the braze coverage was not as

important as the stress concentration caused by the shear clip itse;::lf:.:· _____________ ---,

Figure 13: Mechanical fas tening of sh~et Garnm~ .TiA} by rivet (a) as received, and (b) stress relieved condtt.Jons .

It was decided to cut the subelement in half (lengtltwise) and test only one corrugation at a time. The su?element was tested at room temperature in a three-pomt bend using a uniform pressure instead of a point load (figure 14). The subelement had epo;..:y potted ends to e~ure tlmt the comtgations would not buckle due to the pomt load reactions at the roller supports.

Figure 14: Static test of subelement. Load at 3.35 Predicted failure load of 1.95 kN.

Periodically during ilie load-up, the subelement w~ exanuned for any external damage. Only a small crack 111

one of the brazed shear clips in the braze material was observed (note: this location was NOT tl1e failure location). The beam deflection was noticeable wiili tl1e naked eye at 3.35 kN (figure 14). Failure occurred shortly after reaching 3.75 kN, which ·was 90~ ~~her ~n the predicted failure load. The subelement mtttally faded. at the center shear clip edge within the stress concentrai.Jon area (figure 15).

Pretest finite element analysis (FEA) results accurately predicted measured corrugation strains/stresses. Corrugation stresses were within 4% of predicted stresses. Post-test FEA using the fai lure load of3.75 kN shows the stress at the failure location was 520 MPa. Since this is within 5% of the sheet garruna 's ultimate tensile strength (UTS) of 550 MPa, it can be stated that the fabrication process of hot forming and brazing did not significantly affect the materials structural capability. The final conclusion from this test is that sheet gamma TiAl has a tremendo.us potential for the HSCT ropulsion system. L........,.....----__ ~rr

Figure 15: Failure of subelement at 3.75 kN, which is 190% of predicted failure load. Failure initiated at t11e edge of the center shear clip towards the apex of the TiAI corrugation.

Fuh1re applications for wrought sheet TiAI With the success of the sheet TiAI subelement fabrication and test, sheet TiAI is gaining support as a potential replacement material for other components in tl1e HSCT. Some candidate components are hot ducts and chute doors. Mentioned in the cast TiAI section, sheet TiAl is the primary material for the sidewall facesheet of t11e HSCT exhaust nozzle. As confidence continues to build and more successes arise, sheet TiAI may be considered as a lightweight replacement for sheet superalloys in other areas within the HSCT engine.

Like cast TiAl, wrought sheet TiAI is hindered by its high temperature capabilities. Improvements in TiAl compositions to increase its use temperatures to above 850°C would enhance its likelihood to be used as a superalloy replacement. Likewise, an increase in ductility and fracture toughness would make it more attractive to design engineers. Even with the success of braze joining methods, more work is needed to improve high temperah!Ie durability of these joints. Joining methods such as transient liquid phase (TLP) bonding for sheet TiAl would be a great benefit to the aerospace community. With the HSR hot forming methods and a TLP bond, advanced concepts like TiAl honeycomb (figure 16) could be produced for the HSCT engine.

Figure 16: Example of advanced concept for sheet TiAI applications in HSCT engine.

Honeycomb structures have saved weight in conventional aircrafl and could save weight in the HSCT. f successfully developed, TiAl honeycomb panels could be used in hot ducts and doors within the HSCT engine. TiAl honeycomb panels could also replace the sheet TiAl being used in the divergent flap and sidewall of the HSCT nozzle. Presently, NASA 's reusable launch vehicle (RLV) program is bookkeeping TiAI honeycomb as the primary them1al protection system (TPS) for the leeward side cr the VentureSta/M (figure 17). The pioneering HSR fabrication and joining techniques have been transferred to the RL V program, representing a synergistic use cr technology between space and aeronautics applications.

Figure 17: TiAI Honeycomb panels are being considered in the Reusable Launch Vehicle (RL V) program.

Summarv Many of the presented advancements in casting, fabrication and joining technologies for TiAl are attributed to the HSR program. Much more work is required to achieve acceptance of this material system within the design community. However, the potential weight savings of TiAI over conventional superalloys have enticed design engineers to consider this material for high temperature applications where high stiffness is required. Each successful accomplishment in programs such as

RL V and HSR creates an optimistic future for TiAI in aerospace applications. To truly capitalize on the potential for this class of material, more research is required. Areas for improvement include low cost material production, robust joining methods, and increased materials property database.

Aknowledgemenfs The authors would like to acknowledge the dedicated reseaiChers of the HSR exhaust nozzle program for their contributions towards the advancement of TiAI material systems for aerospace applications. Without the contnbutions of the following people this worlc would not have advanced this far: Srivats Ram of Precision Castparts Corporation; Thomas Kelly and Russell Smashey cf General Electric Aircraft Engines; Gopal Das, Robert Warburton, and Steve McLeod of Pratt & Whitney; and Helmut Clemens of Plansee.

Re(erences 1 R.J. Shaw, L. Koops, and R. Hines, "Progress

Toward Meeting the Propulsion Teclmology Challenges for a 21st Centwy High-Speed Civil Transport", (NASA TM 113161/ ISABE 97-7045), 1997.

2 Nishiyama, Y., Miyashita, T., Isobe, S., and Noda, T., Proceedings of 1989 Symposium on High Tempemture Aluminides and Inteonetamcs, 1989, pp557-573.

3 Larsen, D.E., Wheeler, D.A., and London, BM "Processing and Manufacture of Gamma and XDT Gamma Titanium Aluminide Components by Investment Casting", Proceedings of Processing and Fabrication of Advanced Materials ill, TMS, 1994,pp631-641.

4 Bartolotta, P.A., Kantzos, P., and Krause, D.L., "In­plane Biaxial Yield Swface Study of Cast Titanium Aluminide (TiAI)", Proceedings of the Fifth International on Biaxial/Multiaxial Fatigue & Fracture, Cracow, Poland, 1997,pp389-402.

5 Unavailable infonnation due to Limited Exclusive Rights under Government contract NAS3-26385.

6 Shih, D.S., and Schwartz D:S., "Defonnation and Failure Behavior of Gamma TiAI Alloys", WL-TR-94-4088, June 1994.

7 Un-published data from Dr. Gopal Das, Pratt & Whitney, Florida. 1998.

• 2000 R&D 100 Award ENTRY FORM

Deadline: March 6, 2000

1. Submitting Organization

Organization:

Address: City: State: Zip:

NASA Glenn Research Center (NASAIGRC) 21000 Brookpark Road MS 51-1 Cleveland Ohio 44135

Submitter's Name: Paul A. Bartolotta Phone: Fax: Email:

(216) 433-3338 (216) 433-8011

Paui.A.Bartolotta@grc.nasa.gov

AFFIRMATION: I affirm that all information submitted as a part of, or supplemental to, this entry is a fair and accurate representation of this product.

Submitter's signature

2. Joint entry with ...

Organization: Address: City: State: Zip: Country:

Organization: Address: City: State: Zip: Country:

Organization:

Address: City: State: Zip: Country:

3. Product name:

Pratt & Whitney P.O. Box 109600 West Palm Beach Florida 33410-9600 USA

Plansee Aktiengesellschaft

Reutte

6600 Austria

BF Goodrich Aerospace Aerostructures Group 850 Lagoon Drive .Chula Vista California 91910-2098 USA

Submitter's Name: Phone: Fax: Email:

Robert E. Warburton (561) 796-2347 (561) 796-5866 warburt@pwfl.com

Submitter's Name: Dr. Michael Schwarzkopf Phone: +43 5672 600 2215 Fax: +43 5672 600 501 Email: michael.schwarzkopf@plansee.at

Submitter's Name: David Perryman Phone: (619) 691-2733 Fax: (619) 691-6771 Email: dperryman@aerostructures.bfg .com

Gamma-met Titanium Aluminide Sheet: Production and Component Fabrication

4. Briefly describe what the entry is (e.g. balance, camera, nuclear assay, etc.)

The entry is a commercially-available titanium aluminide {TiAO sheet material named Gamma-met and its economical fabrication methods for high temperature TiAI sheet components. This includes a new TiAI composition, innovative sheet rolling technique, novel component hot-forming method, and pioneering material joining approaches.

5. When was this product first marketed or available for order? Proof of actual sale or intent to sell must be submitted (i.e. invoice, marketing brochure, a one-page letter from a company using your product).

Month: July 1999

6. Principal Developers:

Team Leader: Position: Organization: Address: City: State: Zip: Country: Phone: Fax:

PW Developer: Position: Organization: Address: City: State: Zip: Country: Phone: Fax:

Plansee Developer: Position: Organization: Address: City: State: Zip: Country: Phone: Fax:

BFG Developer: Position: Organization: Address: City: State: Zip: Country: Phone: Fax:

7. Product price

Paul A. Bartolotta Senior Materials Research Engineer NASA Glenn Research Center 21000 Brookpark Road MS 51-1 Cleveland Ohio 44135 USA (216) 433-3338 Email: Paui.A.Bartolotta@qrc.nasa.gov (216) 433-8011

Gopal Das Materials Technologist Pratt & Whitney P.O. Box 109600 West Palm Beach Florida 33410-9600 USA (561) 796-6744 Email: dasgopal@pwfl.com (561) 796-7454

Heinrich Kestler Head of gamma-TiAI development Plansee Aktiengesellschaftnse

Reutte

6600 Austria +43 5672 600 2772 Email: heinrich.kestler@plansee.at +43 5672 600 514

Robert Leholm I John Meaney Staff Engineer I Senior Staff Engineer BF Goodrich Aerospace, Aerostructures Group 850 Lagoon Drive Chula Vista California 91910-2098 USA (619) 691-37521 3682 (619) 691-3550

Email: rleholm@aerostructures.bfg.com jmeaney@aerostructures.bfg.com

Gamma-met TiAI sheet is presently available at $13001pound. Within 5 years, the price is projected to drop to -$750-5001pound (depending on demand). In full industrial production, the price is projected to drop to the level of advanced titanium-alloys (-$1501pound). Component fabrication costs are dependent on size, geometry, and quantity.

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8. Do you hold any patents or patents pending on this product?

Yes No

9. Describe your product's primary function as clearly as possible in one page.

The Titanium Aluminide (TiAI) sheet material, Gamma-met, and its associated fabrication methods are revolutionary advancements for lightweight high-temperature intermetallic materials. Gamma-met is the first and only TiAI sheet material that is economical to produce and fabricate into components. It is 111 oth the cost of other TiAI sheet processed using other methods and will soon have costs competing with titanium alloys (-$150/lbs.). The specific strength of Gamma-met is similar to superalloys while its specific stiffness is significantly higher. For the temperature range of 500-800°C, Gamma-met can replace superalloys thus reducing component weight by approximately 50% (15% weight savings for titanium alloys replacement).

What: The product is TiAI sheet called Gamma-met developed by a team of researchers from NASA, Pratt & Whitney, and Plansee. This TiAI sheet offers lower densities and higher specific stiffness and strength compared to its competition Q.e., conventional superalloys and Ti-alloys). TiAI in sheets/foils forms are unique in that they are beneficial for both propulsion and airframe components. The composition of Gamma-met was developed to provide better rolling characteristics and improved post-rolling mechanical properties. As seen in the provided sales literature and summarized in the comparison matrix in section 1 Ob, Gamma-met can provide the same specific strength (strength normalized with respect to density) and a significantly higher specific stiffness compared to its competition. Likewise, due to the revolutionary processing and rolling methods, Gamma-met has a significantly lower price than its TiAI competitor that is processed using conventional ingot metallurgy (IM) methods. To take full advantage of all of the benefits of Gamma-met, forming and joining techniques had to be developed. The fabrication techniques that were developed by a team of researchers from NASA, P&W, Plansee, and BF Goodrich do not require special operations and environment as typical with other intermetallic alloys. Thus Gamma-met component fabrication can be done in normal production settings using equipment common to conventional titanium alloy component fabrication. These production techniques make Gamma-met very economical to fabricate components.

How: The Air Force and its industrial partners have tried for over ten years to develop a cost effective high temperature sheet TiAI material system but they failed. Typical TiAI sheet production method was based on ingot metallurgy (IM). The IM processing route consists of the following steps:

I) an ingot of a certain TiAI composition was produced using a VAR (Vacuum Arc Remelt) method, II) the ingot was then hot isostatically forged to a -30" diameter pancake pre-material blank (this step of the

process had a 50% part rejection rate) Ill) if you were lucky and the pancake was in one piece, the pancake was cut into a rectangle and the top and

bottom was machined parallel IV) the squared-off blank was then encapsulated (or canned) and hot rolled down to the desired thickness V) the sheet was then decanned and finished ground to final thickness

This old IM method produced a limited quantity because of the forging rejection rate, was limited in the sheet size both in overall dimensions and sheet thickness (>0.060"), and had inconsistent properties between sheets. Ultimately the cost of the sheet was too high -$1 0,000/lb. The new Gamma-met sheet production is NOT based on the old IM TiAI technology, but a new revolutionary powder metallurgy technology. The PM processing route for Gamma-met is as follows:

I) powder TiAI (of the Gamma-met composition) is produced and consolidated into a rectangular pre-material blank (compared to the IM route this step has a near zero rejection rate)

II) the top & bottom are machined parallel, the blank is canned, rolled, and decanned Ill) final grinding to the finish thickness completes the process

This new revolutionary processing along with its new composition produces a TiAI sheet that is very consistent and economical. The sheet dimensions are not limited and thinner sheets can be produced to (<0.025").

Several production level fabrication techniques were developed for the Gamma-met sheet material. A new low­temperature hot-forming technique of Gamma-met sheets negates the need for hot presses with environmental

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chambers. This equates to increased production rates and ultimately lowers fabrication costs. Innovative joining methods for Gamma-met such as brazing, transient liquid phase bonding, and laser welding make it possible fabricate components as small as 1" long turbine blades to large 30' long hot ducts on rocket based propulsion systems. All these methods have been developed for economical fabrication of production components and are presently available.

1 OA. List vour product's competitors The Gamma-met TiAI sheet is competing against several material systems in the aerospace industry namely: Titanium-alloys and Superalloys (Nickel-based and Cobalt-based).

1 OB. Supply a matrix or table showing how the key features of your product compare to existing products or technologies.

Factor Gamma-met TiAI sheet Ti-alloy Superalloy Competitive TiAI sheet IM Advantage

PM Density (g/cm} 3.8 .3.8 4.5 8.2 >50% lower density than

superalloys >15% lower than Ti-alloys

Specific St~~ngth 160 170 180 165 Similar specific strength (MPa/g*cm-RT Specific 40 40 25 25 Significantly higher specific Stiffness stiffness at all temperatures (GPa/g*cm-~ Ductility (%) 1.7/4.8 1.0/1.7 15/50 15/50 Lower than IM TiAI. RT I 760°C Not as high as Ti-alloys

Max. Use 800 800 600 900 Maximum use Temperature Temperature ~C) within superalloy levels

Cost ($/pound) 1300 10,000 150 20 Significantly lower than IM TiAI. Projected costs to be

same as Ti-alloy

1 OC. Describe how your product improves upon competitive products or technologies.

• 15% lighter than titanium alloys and >50% lighter than superalloys. • Replacing superalloys for aerospace propulsion system components saves vehicle weight by a factor of 20:1

O.e., 1 00 pound superalloy component is replaced by 50 pound Gamma-met component will equate to a vehicle weight reduction of over 1000 pounds).

• Higher temperature capabilities than Ti-alloys by over 200°C and can replace superalloys in the temperature range of 500-800°C.

• Approximately 2X higher specific stiffness compared to Ti-alloys and superalloys (based on modulus normalized with respect to density)

• Significantly better acoustic attenuation due to higher specific stiffness • 1/10 the cost of IM TiAI sheet with projected costs the same as Ti-alloys within 5 years (depending on demand) • _the component fabrication costs compared to other intermetallic alloys and approximately the same cost as

Ti-alloy and superalloy fabrication.

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1 OC. Describe how your product improves upon competitive products or technologies (Continuation).

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Figure 1:

Operates 200°C Hotter than conventional Titanium alloys

Higher Operating Temperatures

-50% greater specific stiffness than conventional alloys

Higher Specific Stiffness (GPa/g*cm·")

$1 0,000/lbs

$1 ,300/lbs.

_ the weight of superalloys for comparable operating temperatures

Lower Density

- 1 /1 0 the processing costs for thin TiAI sheet

Lower Costs

Primary benefits of new Gamma-met material as compared to the competition . (a) Higher operating temperatures. (b) Lower weight. (c) Higher specific stiffness. (d) Lower costs.

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11 A. Describe the principal applications of this product.

The principal application for this product is the fabrication of lightweight high-temperature aerospace components for both propulsion systems and airframe structures. The first applications will be found in space and military applications, however, there have been interests in the product from the commercial aviation and automobile industries. Due to its lightweight characteristics, Gamma-met significantly lowers vehicle weight, which improves fuel efficiencies and vehicle acceleration. The high temperature properties of Gamma-met allows it to replace superalloys in many key propulsion system applications where Ti-alloys would never survive. Due to its high specific stiffness and high temperature capabilities, Gamma-met is very desirable for leading edges and control surfaces for high speed (high MACH number) aircraft and other lightweight structures where deflections need to be kept to a minimum.

11 B. List all other applications for which your product can now be used.

Lightweight heat-resistant structures are required to meet NASA's goals to significantly reduce costs of putting payloads into space. NASA is developing a Reusable Launch Vehicle (RLV) as a future replacement for the Shuttle. Current next generation RLV design concepts have identified many structural areas where high service temperatures warrant the use of Gamma-met TiAI. Planned applications include the lightweight Thermal Protection System (TPS) and control surfaces structures such as vertical fin rudders, body flaps, and canted fin elevons. These structures are expected to endure temperatures up to 900°C (1650°F) for up to 100 hours, with moderate-to­high mechanical and acoustic loads, and no oxidation. Weight estimates by BFG, prime contractor of the RLV TPS system, show that the vehicle weight can be reduced by approximately 4,000 pounds by replacing the present lnconel617 TPS design with Gamma-met TiAI. Another study by Lockheed Martin Skunk Works has shown that weight savings of nearly 1 ,000 pounds can be achieved by using the same material on control surfaces.

Thermal Protection System (TPS) ,-----using Gamma-met saves 4,000 lbs

8ontrol surfaces using Gamma-met :;aves 1,000 lbs.

Figure 2:

in vehicle weight

RLV I VentureStar

Higher operating temperatures and high specific strength and stiffness properties allow new material (Gamma-met) to be used in new applications not possible for conventional Ti-alloys with significant weight savings over superalloys.

Current and future applications of Gamma-met TiAI sheets include military engine components, nozzle structures, and commercial and military space thermal protection systems (TPS). Fabrication of simple shaped components (trusscore beams) has been perfected by NASA, P&W, and BFG using Gamma-met sheets and these developed fabrication technologies. Honeycomb structures have shown to be a very efficient structure for applications requiring a high specific stiffness. BFG also has been successful in fabricating TiAI honeycomb structures for the RLV on a production scale. Gamma-met TiAI sheet technologies are being considered in various advanced airframe and propulsion concepts for both military and civilian aerospace applications. As the demand for low cost

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access to space increases, Gamma-met TiAI sheet technologies have again caught the interest of airframe manufacturers.

TPS for VentureStar

Figure 3:

Honeycomb construction

Trusscore beams for propulsion system and airframe components

Higher operating temperatures and high speci fic strength and stiffness properties allow new material (Gamma-met) to be used in new applications not possible for conventional Ti-a lloys with significant weight savings over supera lloys. New revolutionary techniques developed by NASA, P&W and BFG allow complex components to be economically fabricated from Gamma-met.

In aerospace propulsion systems, Gamma-met TiAI has been identified as having a significant impact on increasing thrust-to-weight ratios. The Joint Strike Fighter (JSF) program has identified exhaust nozzle tiles, hot ducts, and doors as Gamma-met TiAI sheet candidates. The Navy's Global First Strike program identified Gamma-met TiAI sheet combustor backstructures and compressor section components. TiAI hot ducts and airframe substructure are baselined in NASA's Rocket Based Combined Cycle and Turbine Based Combined Cycle programs. The Army's next generation Comanche helicopter will use Gamma-met TiAI in the exhaust nozzle components too. Future high speed aircraft such as the proposed High Speed Civil Transport, hypersonic (MACH 5) transport vehicles, and commercial jetliners have identified stator vanes, turbine/compressor blades, combustor backstructures, and nozzle components as TiAI sheet candidates for saving weight. Finally, the next generation Tomahawk missile program is considering the usage of Gamma-met TiAI sheet in the exhaust nozzle structure.

Figure 3: Joint Strike Fighter exhaust nozzle tile made from Gamma-met will save over several hundred pounds per aircraft.

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The reduction in raw material prices that will be brought about by the initial volume generated by military and space applications will be the catalyst that inserts TiAI structures into commercial aircraft applications. This will in turn provide a long term commercial market. Commercial aerospace products such as hot nacelle and engine exhausts employ a range of high temperature advanced materials. Examples include the Ti-6-2-4-2 nozzle plugs for the PW4000, GE90 and V2500D5 engines, superalloy exhaust ducts on the JT8D and others, and many Ti-6-4 components. BFGoodrich currently fabricates almost 250,000 lb. ofTi-6-4 and Ti-6-2-4-2 components annually. Those same components fabricated from Gamma-met could weigh less than 210,000 lb. thus reducing aircraft fleet weight.

Commercial aircraft engine exhaust gases today can reach temperatures beyond the capability of existing Ti. Alloys. When this occurs, the structures are fabricated from superalloys with a 100% weight penalty over titanium alloy structures. As engine temperatures are driven higher in order to improve efficiency and reduce emissions, exhaust gas temperatures will also increase further. With the continuing drive for reduced weight on long range applications, Gamma-met TiAI has the potential to become the material of choice in many commercial engine applications. Industry projections for the world commercial airline fleet show a continued increase of about 750 planes per year for the next 20 years.

Vehicle Component Number of Vehicles Projected Gamma-met TiAI material usage

Global first strike/ Hot structures, compressor, nozzle 2000+ 30,000 lb.+ Tomahawk missiles

Comanche Nozzles 500+ 10,000 lb.+ helicopter F-22, JSF Nozzles, Exhaust structures, engine 700+ 100,000 lb. +

components Military Space TPS, Wingbox and Vertical Tail Hot 2+ 5,000 lb.+ Plane Structures RLVs TPS, Wingbox and Vertical Tail Hot 2-8 20,000 lb. - 80,000 lb.

Structures

12. Summarv: State in layman's terms why you feel your product should receive an R&D 1 00 Award. Why is it important to have this product? What benefits will it provide? (The value of the award for its promotional value is understood.)

The TiAI sheet material, Gamma-met, and its associated fabrication methods are revolutionary advancements for lightweight high-temperature intermetallic materials. Gamma-met is the first and only TiAI sheet material that is economical to produce and fabricate into components. It is 1/10ththe cost of other TiAI sheet processed using other methods and will soon have costs competing with titanium alloys (-$150/lbs.). The specific strength of Gamma­met is similar to superalloys while its specific stiffness is significantly higher. For the temperature range of 500-8000C, Gamma-met can replace superalloys thus reducing component weight by approximately 50% (15% weight savings for titanium alloys replacement).

The ever increasing demand associated with advanced aerospace and high-performance automobile design requirements in terms of efficiency, emissivity and economy can only be met by lightweight, high strength materials that can manage high thermomechanical stresses and adverse operating temperatures. Gamma-met titanium aluminide (TiAI) material system can cover the majority of these high temperature requirements and can ~ecome the most significant new material system for such vehicles.

Revolutionary advancements in forming, and joining methods achieved by Pratt & Whitney, Plansee, BF Goodrich, and NASA in NASA's High Speed Research (HSR) program are making it possible to significantly reduce vehicle weights. Hot forming process made it possible to fabricate component shapes at lower temperatures without the need for special non-oxidizing environments. Joining techniques such as brazing, diffusion bonding, and TLP bonding have been optimized for the production of advanced aerospace components (both propulsion & airframe).

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Gamma-met TiAI is considered to be an enabling technology for low cost Access to Space vehicles which will pave the way for the commercialization and industrialization of space thus becoming the same for the 21 51 century as the automobile industry for the 201

h century. Vehicle types that are planning to· use and will benefit from Gamma-met components include;

1) Future Space Shuttle replacement, Reusable Launch Vehicles (RLV), and VentureStar providing significant weight reduction and bigger payload capacity,

2) Next generation of hypersonic military vehicles guaranteeing fast access to targets over long distances, 3) Low cost and high performance military aircrafts achieving double the performance at half the cost, 4) Commercial subsonic (and future supersonic) and high capacity aircrafts maintaining the world trade with

less emission and guaranteeing the growing mobility of a growing population in a global economy, 5) High performance low emission cars which fullfil! the low emission standards while maintaining

performance.

Without Gamma-met's lower weight and higher temperature capabilities these vehicles would not meet their weight, performance, and efficiency requirements.

ORGANIZATION DATA

13. Chief Executive Officer

Name: Position: Organization: Address: City: State: Zip: Country: Phone: Fax:

Donald J. Campbell Center Director NASA Glenn Research Center 21 000 Brookpark Road Cleveland Ohio 44135 USA 216 433 2929 216 433 5266

Email: Donald.J.Campbell@grc.nasa.gov

14. Contact person to handle all arrangements on exhibits, banquet, and publicity.

Name: Position: Organization: Address: City: State: Zip: Country: Phone: Fax:

Laurie J. Stauber Office of Commercial Technology NASA Glenn Research Center 21 000 Brookpark Road MS 4-2 Cleveland Ohio 44135 USA 216 433 2820 216 433 2555

Email: Laurei.J.Stauber@grc.nasa.gov

15. To whom should reader inquiries about your product be directed?

Name: Position: Organization: Address: City: State: Zip: Country: Phone: Fax:

Paul A. Bartolotta Senior Materials Research Engineer NASA Glenn Research Center 21 000 Brookpark Road MS 51-1 Cleveland Ohio 44135 USA 216 433 3338 Email: Paui.A.Bartolotta@grc.nasa.gov 216 433 8011

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Name: Position: Organization: Address: City: State: Zip: Country: Phone: Fax:

Name: Position: Organization: Address: City: State: Zip: Country: Phone: Fax:

Klaus Rissbacher Manager business segment aerospace Plansee Aktiengesellschaft

Reutte

6600 Austria +43 5672 600 2778 +43 5672 600 514

Nigel Barker

Email: klaus.rissbacher@plansee.at

Manager- Business Development, CRAD BF Goodrich Aerospace, Aerostructures Group 850 Lagoon Drive Chula Vista California 91910-2098 USA 619 691 2568 Email: barker@aerostructures.bfg.com 619 691 6189

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