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estled estled betw een the 767 767 and the 747 in term ter m s of size, the Boeing Boeing 777 777 is the world ’s largest tw inengine airp lane. It was initially initially conceived ceived a s an enlarged version of the 767, 767, but it grew gr ew to 85% of the 747 in actual size, size, and sports a w ingspan of nearly 200 feet and a fuselage appr oximately oximately 11 feet feet in diam eter. Its Its passenger seating and ran ge combicombination put it in in a u nique niche niche that has allowed allowed developm ent of a gengeneration eration of stretch and range variants. T o e n a b l e su su c h a l a r g e t w i n engine airplane, Boeing had to achieve significant reductions in structural weight wh ile ile maintaining overall affordab affordab ility. ility. This This w as m ade possible by the development of breakthrough materials. The 777 Program enabled the matu ration of a large number of materials terials that were und er development in the mid - to late-1980 late-1980s. s. Materials that were transitioned transitioned into prod uction included included new advan ced ced 7000 7000 and 2000 series aluminum alloys, dam age-tolerant age-tolerant comp osites, osites, and advanced titanium alloys. These materials as well as non-structural materials advances enabled enabled a redu ction in weight of over 5800 5800 poun ds.
Th e Boein Boeing g 777 The development of the Boe Boeing 777 was was made made possi possib ble by the development of breakthrough materials tha th at allow allowed redu reductions ctions in struc stru ctura tu rall we w eight ight while maintaining affordability.
used extensively extensively in interface areas. In add ition, ition, titanium titanium replaced replaced m any steel components compon ents in the land ing gear and engine strut area in an effort effort to reduce weight and imp rove corrocorrosion resistance. Although stru ctural materials receive ceive the m ost attention, it is is imp ortant to n ote that the 777 also also paved the way for a wid e variety of nonstru ctural ad van ced mat erials. SigSignificant material applications includ clud ed th e introduction introduction of improved passenger windows, and dust covers covers m ore resistant resistant to the environment and more able to to withstand wear and tear. More-durable materials were also developed developed and implemented implemen ted for insulation blankets, interior paints, d ecorative ecorative inks, cargo floors, and cargo liners. liners. Furth ermore, many of these improved materials also generated significant weight savings.
Alloy developments During the waning d ays of the mid1980s, a frustrated Boeing and its aluminum aluminum suppliers suppliers shut down mas Bria Brian Smith Sm ith sive eff efforts orts to develop alum inum Boe Boeing Aircra Aircraft ft Co. Co. lithium alloys. As a result of this exSeatt Seattle le,, W ashington ashington perien ce, Boeing initiated a p rocess with these supp liers liers in in wh ich ich alloys alloys were first first stud stud ied on paper. Sup pliers were asked to proThe aluminum airplane pose various ‘what for m ajor ajor stru ctural app lications. lications. ‘what if’ alloys for From a stru ctural-weight ctural-weight stand point, the 777 is primarily These ‘what if’ alloys were evaluated for benefit benefit and a fan alum inum airplane. Seventy Seventy percent of the overall strucfordability. This un ique appr oach allowed prom ising ising alture is aluminu m, including including the wing box and fuselage. fuselage. Of loys to be identifie identified d early on an d, un like their aluminu mcourse, the alum inum alloys alloys are not th e garden-variety lithium lithium counterparts, these alloys alloys were robust to price and aerospace materials of the past. These are engineered alprop erty changes changes during loys offering offering imp roved development. strength, toughn ess, and The ‘what if’ pr ocess fofocorrosion resistance. cused on advanced alDespite the predomiloys for for wing an d fusenance of aluminu m, the lage ap plications. For 777 777 does contain sign ifiifithe wing, Boeing identicantly cantly m ore composite fied fied a general need for materials materials by w eight eight th an higher-strength alloys earlier Boeing Boeing aircraft. with good toughness The vertical fin, horiand improved corrosi corrosion on zontal stabilizers, and resistance – relatively passenger-floor passenger-floor beams standard targets. targets. H owutilize a Boeing/ Boeing/ supplier sup plier ever, in the case of the developed developed toughened, fuselage, Boeing Boeing h ad just dam age-resis age-resistant tant carbon completed a rigorous refiber epoxy resin system. view of fatigue fatigue an d corTitanium alloy imrosion issu es in its fleet fleet pr ovemen ts are critical critical of aging airplanes. This in combating the galeffort effort brou ght into focus vanic potential differThe Boeing 777-300ER’s new semi-levered landing gear system has performed ence between between alum inum flawle flawless ssly ly during during the flightflight-tes testt pro progra gram. The unique gea gear, which which is is manufactur manufactureed by by the need for advances and Carbon Fiber ReinRein- Goodrich Goodrich Corp., allows allows t he airplane airplane to t o rotate rotate early by shiftin g the t he cent center er of rotation rotation in toughness, fatigue esistance, forced Plastic (CFRP), from from the main main axle to aft aft axle axle of of the three-a three-axle xle la landing gear gear truck. As the airpla airplane ne crack growth r esistance, rotates, the t he nose is allowed to t o rise higher earlier. earlie r. and corrosion resistance. and titaniu m alloys are A D V A N C E D M A T ER ERI A LS LS & P RO C E S SE S E S / S EP EP T E M B ER ER 2 0 0 3
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As a result of this innova tive pr ocess, ocess, Boeing Boeing and Alcoa were able to generate and bring to market a number of breakthrou gh alloys and heat treatments. The advan ced ced fuselage fuselage alloy 2524 2524 yielded significant significant im provements in the design design prop erties associated associated w ith fuselage skin du rability. To furt furt her address fleet corrosion issues, 777 designers worked diligently to to maintain th e clad clad su rface rface on the interior of the airplane, particularly in the moisture-laden bilge area. This material breakthrough was
marr ied with ad vancements in 7000 7000 series alloy heat treatment (T77511 – retrogression retrogression re-age), re-age), w hich allowed higher-strength 7150 materials for fuselage extruded stringers. The result was a structure that is tougher, stronger, and more corrosion-resistant than earlier earlier d esigns. esigns. The same technological breakthroughs that enabled ap plicatio plication n of 7150 7150 alloys alloys on the fu selage, were also incorporated into w ing alloy alloy ‘what if’ stud ies. These stud ies identified identified a candid ate alloy alloy that had a particularly unique combination of properties, pricing, and corrosion resistance: 7055T7751 T7751.. This alloy pr ovides a nearly 10% gain in strength, with higher toughn ess and significantl significantly y imp roved corrosion resistance.
747 777
767
757 707/720 Regional and i ntercontinental ntercontinental market flexibi lity • State-of-the-art, service-ready features and technology • Industry-leading performance and econom ics • Range and capacity growth ens ure future family commonality
727
737 1950
1960
1970
1980
1990
2000
Boei Boeing ng airc aircra raft ft and and the years years they were were introduce introduced d into into ser service vice.. y d t i l t i e v h b o g d a r i e r p u m e v a D i W s Alloys: Ti 10-2-3 Al 2XXX-T3, -T42, -T36 Al 7055-T77 Al 7150-T77 Ti- 6-4 ELI Ti 15 -3 -3 3 Ti B21S Ti 6-2-4-2
9: Fin and stabili zer 3: Upper skin and s tringers tringers 4: Upper spar 2: Aft chord bulkhead 4: Seat tracks 9: Floor beams
5: Stabilize r attach fittings
Composites: Toughened CFRP Pitch core Perforated CFRP/Nomex 4: Crow n s tringe tringe rs
4: Keel beam 4: Belly stringers
Uncolored
2: Fuselage skin 1: Truck beam and braces
Glare bulk cargo floor 6013 Al alloy Lightweight sealants Al mesh AV-30 corrosion inhibiting compound Dense core potting CFRP comp. cascade Al-Li 8090 sound damping angles RTM CFRP chine
Break Breakout out of of advanc advanced ed materia materials ls on the 777. 42
8:Aft heat shi eld 8: Engi Engi ne mounts
6: ECS ducting
7: Tail cone outer slee ve 7: Tail cone pl ug 7 & 8: Aft core cowl 10 & 11: Thrust reverser cowl 11: Inlet cowl inner barrel
Toughened car carbon fiber epoxy Efforts to develop an improved carbon fiber fiber ep oxy resin resin system date back to the early- to mid-1980s. mid-1980s. These efforts efforts also o riginated with Boeing’s oeing’s in-service fleet fleet experience. Since Since the pr odu ction of the 757 757 and 767, 767, airline customers customers have had to contend contend w ith thin-gage composite structures in a wid e num ber of app lications. lications. Complaints abou t this material’ ma terial’ss sensitivity to impact d amage and the d iffi ifficulty culty of repair were man y. In response to these comp comp laints, laints, Boeing initiated an d led a signific significant ant effort to develop a toughened epoxy matrix that wou ld be more resistant resistant to dam age. Supp lier lier efforts efforts were repeatedly thwarted by the negative impact of toughening toughening agents on hot/ wet compression strength. Fortunately for Boeing, Boeing, Toray h ad been w orking d iligently iligently on a resin system that involved involved a tou ghening interlayer. The resulting system set a new standard for toughness and strength in composite material techtechnology. Imp act test results dem onstrated to th e airlines airlines that th is new system also suffered sign ific ificantly antly less dam age, and that such dam age could could be repaired in a man ner similar similar to repair of existing existing aluminu m structures. This breakthrou breakthrou gh in CFRP CFRP tough ness was optim ized to enable ContinContinuo us Tape Laying Machines (CTLM) (CTLM) to fabricate stru stru ctures, resulting in redu ced man ufacturing costs. costs. The new toug hened mat rix CFRP CFRP is is used for the main box cover panels and the main box spars. The main torque box cover panel consists of an integrally
A D V A N C E D M A T ER ERI A LS LS & PRO C E S SE S E S / S EP EP T E M B ER ER 2 0 0 3
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7% 11%
70%
200
, p p a K s s 2 e / n 1 . h n g i - 150 u i s o k t e r u t c a r F
Misc. Steel Titanium Composites Aluminum
Alu A lu m in um alloy s an d ot her adv anced materials by weight on the Boeing Boeing 777.
stiffened stiffened skin w ith I-section I-section stiffeners stiffeners at a constan constan t spacing. The basic skin ply lay-ups are quite simple, with doublers inserted inserted as pr e-kitted e-kitted u nits. This app roach permits the pan els to be laid laid up by the CTLM, resulting in significant cost red uctions. To achieve accurate p art control, the stiffeners stiffeners are pre-cured pre-cured and co-bo co-bond nd ed to th e skin skin pan el dur ing the p anel cure cycle. cycle. Titanium Titanium alloys Titanium applications have increased creased with each major commercial airplan e introd uction. In the case of the 777, 777, the use of titanium wa s expand ed into p revious revious CFRP CFRP structure areas to minim ize the risk of galvanic corrosion that is present with aluminu m. For this ap plication, plication, betaann ealed Ti-6Al-4V Ti-6Al-4V ELI ELI (Extra (Extra Low Interstitial) Interstitial) was introd introd uced into the commercial fleet, fleet, and it provid es the maximum dam age toleranc tolerancee properties for for titaniu m alloys. Titanium was also selected for landing gear components. The single largest largest titanium app licati lication, on, and perhap s the biggest challenge, challenge, was applying Ti 1010-2-3 2-3 to the m ain land ing gear tru ck beam . This ap plication challenged Boeing’s oeing’s m etallurgists to develop tight process controls for welding the three pieces pieces that made u p this compo nent . (Not (Not e: As part of a subsequent cost reduction effort, Boeing ultimately converted the thr ee forgings to a sing le forging forging .) The resulting truck beam saved substan tial weight and also resulted resulted in a d esign esign without the typical corrosion and paint d amage risks associated associated w ith high-strength steel landing gear components. Titanium Titanium alloy alloy d evelopm evelopm ents in the early- to mid-1980 mid-1980ss were pu shed int o new p rodu ct forms forms and applicatio applications ns for the 777 777 as well. While earlier Boeing airplanes included titanium for landing-gear springs and high-
2XXX 17%improveme nt in toughness 2024
100
, 1000 N d / a d e t a r e h t l c w y c o / . 100 r n g i k m c a r c e u g i t a F
60%slow er fatigue crack growth 2024 2XXX
10 10 20 30 40 Stress intensity factor K max, ksi-in.1/2
40 50 60 70 Typical tensile yield strength, ksi
Toughened 2000 series aluminum alloy properties. This alloy is for the body skin.
7055-T77 7150-T77
Pitting
g n i t a r n o i t a i l o f x e , e c n a m r o f r e p n o i s o r r o C
A 7150T77
7050T76
7075T73
7055T77
B Goodness C 7075-T6
Traditional strength/ corrosion behavior
7150-T6
D
Severe 60
65
70
75 80 Compresssion yield strength, ksi
85
90
95
Advanc A dvancees in in 7000 serie seriess co corrosio rrosion-re n-resis sistant tant aluminu aluminu m allo alloys. ys.
250
2 / 1 .
Fuselage
Lower wi ng surface (increased (increased d urability) urability)
Upper wing surface (increased (increased strength)
200
n i / i s k , p p a150 K s s e n h g u o100 t e r u t c a r F
2324-T39 Type II
2024-T351
2324-T39
C h a l l e n g e
1 9 7 0 s - 1 9 9 0 s 2224-T3511
1 9 3 0 s - 1 9 6 0 s
7150-T651 7055-T7751 7075-T651
50 7178-T651
40
60
80 Typical yield strength, ksi
100
Advanc A dvanceed aluminum aluminum alloys alloys with highe higherr toughnes toughnesss and and improved improved corro corrosio sion n resis resistance tance..
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temperature environmental control ducting, these alloys had several performance and inservice shortcomings. During the d esign of th e 777, 777, Boeing’s Boeing’s metallurgists worked closely with parts manu facturers facturers to up grade to Ti 15-3-3-3 for both clockclock-type type spr ings and du cting. cting. Another m ajor ajor step forward w as th e selection of Beta-21S Beta-21S titanium for the engine plug and nozzle hot structure, normally fabricated of n ickel-base alloys. Beta-21S, Beta-21S, develop ed for its high r esistance esistance to oxidation, resulted in significant significant w eight redu ction ction for this exhau exhau st comp comp onent. N on-structura on-structurall materials materials In the interest of creating a p referred airplane, Boeing’s materials engineers concentrated on every detail, and identified the potential for breakthrou ghs in some less obvious obvious areas, for example: By filling filling tr ad itional sealants with microballoons, over 300 pou nd s of weight was eliminated wh ile ile keeping the same basic prop erties. erties. Through d etailed etailed analyses and tests, Boeing Boeing confirmed firmed that an entire coat coat of paint could be eliminated from the lower portion of the fuselage interior. Amazingly, wh ile ile this change eliminated 3.6 square inches 0.5 square inch over 250 pounds of Comparativ Comparativ e composit composite e material damage damage resistance. Aft er a weight, the primary dri270 in.-lb impact, the conventional composite material on the ving force behind its inleft shows mu ch more damage damage than the t he new 777 advanced comcom- corporation was im posite posite materia materiall on on the right. right. proved paint adhesion Ti-6Al-4V, betaannealed:
100 2 / 1 .
n i i s k , C I K s s e n h g u o t e r u t c a r F
Goodness
1980s damage tolerant structure
90 80
VT-22STA: 2000s, damage tolerant forgings
70 60
Ti-6Al-4V, mill annealed:
50
1960s general structure
40 30
120
140
Ti-10V-2Fe-3Al STA: 1980s, 1980s, hi gh Ti-6Al-6Vstrength forgings 2Sn/Ti-6Al-4V VT-22 STA: STA: 1960s general 2000s, 2000s, hi gh structure strength
160 Ultimate tensile strength, ksi
180
200
Titanium alloy development has progressed in both strength and toughness.
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and better corrosion resistance. These chang chang es typify the innovative thinking that enabled the development of a preferred airplane in terms of cost, cost, weight, and affordab ility. ility. Boeing 777 to to 7E7 The 777 777 represented a breakthrou gh in materials app lications lications for comm comm ercial cial aircraft. aircraft. The introdu ction ction of this airplane was w ell-ti ell-timed med to drive a nu mber of critical critical adv adv ances in in m aterials terials technologies technologies to matu rity, with the end resu lt being being implementation. The rate of incorpor incorpor ation for these advances onto th e 777 777 is remarkable, and reflec reflects ts the h igh d egree of alignm alignm ent in research research w ork over th e five five years preceding the d esign effort. effort. This research was clearly focused on fleet concerns concerns raised by airlines, and th e deliberate development of enhanced performance materials that w ere costcosteffective. Just as th e 777 777 was a br eakthrou gh in terms of materials app lications, lications, the 7E7 7E7 prom ises ises to provide an even even greater opportu nity for innovation, innovation, both in technical technical advan ces ces and in the creation of the cooperative process needed to develop these technologies with our global partners. To comp comp ete against against prod ucts that are based based on many of the same material techn techn ologies foun foun d on th e 777, 777, the 7E7 engineers mu st consider consider further technology technology breakthroughs an d expand the app licati lication on of advan ced ced technologies beyond the current norm. Fortunately, Fortunately, m aterials aterials d evelopm evelopm ent in the last five years has been promising. Today, confidence has increased creased in comp osites osites as a prim ary structu re, based on 777 777 successes. successes. EnEncouraging couraging p rogress has been been m ade in aluminu m, steel steel and titanium technologies. Finally, understanding the need for environmentally responsible processes has also also grow n. Many technologies nologies are now matu ring in this area and offer an opportu nity to design design and p rodu ce an airplane that is not only cost cost and performance preferred, but m ore environmentally friendly friendly s than airp lanes of the past. For more info rmation: Brian Smith is the Chief Engineer of Commercial Airplane Boeing Materials Techn Techn ology Organ ization at the Boeing Airplane Co., Seattle, Wash ingt on; tel: 425/ 425/ 237-3516 237-3516;; e-mail: brian.w
[email protected].
[email protected].
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