Nestled between the 767 and the747 in terms of size, the Boeing

777 is the world’s largest twin-engine airplane. It was initially con-ceived as an enlarged version of the767, but it grew to 85% of the 747 inactual size, and sports a wingspanof nearly 200 feet and a fuselage ap-proximately 11 feet in diameter. Itspassenger seating and range combi-nation put it in a unique niche thathas allowed development of a gen-eration of stretch and range variants.

To enable such a large twin-engine airplane, Boeing had toachieve significant reductions instructural weight while maintainingoverall affordability. This was madepossible by the development ofbreakthrough materials.

The 777 Program enabled thematuration of a large number of ma-terials that were under developmentin the mid- to late-1980s. Materialsthat were transitioned into produc-tion included new advanced 7000and 2000 series aluminum alloys,damage-tolerant composites, andadvanced titanium alloys. These materials as well as non-structural materials advances enabled a reduc-tion in weight of over 5800 pounds.

The aluminum airplaneFrom a structural-weight standpoint, the 777 is primarily

an aluminum airplane. Seventy percent of the overall struc-ture is aluminum, including the wing box and fuselage. Ofcourse, the aluminum alloys are not the garden-varietyaerospace materials of the past. These are engineered al-loys offering improvedstrength, toughness, andcorrosion resistance.

Despite the predomi-nance of aluminum, the777 does contain signifi-cantly more compositematerials by weight thanearlier Boeing aircraft.The vertical fin, hori-zontal stabilizers, andpassenger-floor beamsutilize a Boeing/supplierdeveloped toughened,damage-resistant carbonfiber epoxy resin system.

Titanium alloy im-provements are criticalin combating the gal-vanic potential differ-ence between aluminumand Carbon Fiber Rein-forced Plastic (CFRP),and titanium alloys are

used extensively in interface areas.In addition, titanium replaced manysteel components in the landing gearand engine strut area in an effort toreduce weight and improve corro-sion resistance.

Although structural materials re-ceive the most attention, it is impor-tant to note that the 777 also pavedthe way for a wide variety of non-structural advanced materials. Sig-nificant material applications in-cluded the introduction of improvedpassenger windows, and dustcovers more resistant to the envi-ronment and more able to with-stand wear and tear. More-durablematerials were also developed andimplemented for insulation blankets,interior paints, decorative inks, cargofloors, and cargo liners. Further-more, many of these improved ma-terials also generated significantweight savings.

Alloy developmentsDuring the waning days of the mid-

1980s, a frustrated Boeing and itsaluminum suppliers shut down mas-sive efforts to develop aluminum-lithium alloys. As a result of this ex-perience, Boeing initiated a processwith these suppliers in which alloys

were first studied on paper. Suppliers were asked to pro-pose various ‘what if’ alloys for major structural applications.These ‘what if’ alloys were evaluated for benefit and af-fordability. This unique approach allowed promising al-loys to be identified early on and, unlike their aluminum-lithium counterparts, these alloys were robust to price and

property changes duringdevelopment.

The ‘what if’ process fo-cused on advanced al-loys for wing and fuse-lage applications. Forthe wing, Boeing identi-fied a general need forhigher-strength alloyswith good toughnessand improved corrosionresistance – relativelystandard targets. How-ever, in the case of thefuselage, Boeing had justcompleted a rigorous re-view of fatigue and cor-rosion issues in its fleetof aging airplanes. Thiseffort brought into focusthe need for advances in toughness, fatiguecrack growth resistance,and corrosion resistance.

The Boeing777

The development of the Boeing 777 was made possible

by the developmentof breakthrough materials that allowed reductions in structural weight while

maintaining affordability.

Brian SmithBoeing Aircraft Co.Seattle, Washington

ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2003 41

The Boeing 777-300ER’s new semi-levered landing gear system has performedflawlessly during the flight-test program. The unique gear, which is manufactured byGoodrich Corp., allows the airplane to rotate early by shifting the center of rotationfrom the main axle to aft axle of the three-axle landing gear truck. As the airplanerotates, the nose is allowed to rise higher earlier.

0903cent.qxd 8/10/03 8:12 PM Page 1

As a result of this innovativeprocess, Boeing and Alcoa wereable to generate and bring tomarket a number of break-through alloys and heat treat-ments. The advanced fuselagealloy 2524 yielded significant im-provements in the design prop-erties associated with fuselageskin durability. To further ad-dress fleet corrosion issues, 777designers worked diligently to

maintain the clad surface on the inte-rior of the airplane, particularly in themoisture-laden bilge area.

This material breakthrough was

married with advancements in 7000series alloy heat treatment (T77511 –retrogression re-age), which allowedhigher-strength 7150 materials forfuselage extruded stringers. The resultwas a structure that is tougher,stronger, and more corrosion-resistantthan earlier designs.

The same technological break-throughs that enabled application of7150 alloys on the fuselage, were alsoincorporated into wing alloy ‘what if’studies. These studies identified a can-didate alloy that had a particularlyunique combination of properties,pricing, and corrosion resistance: 7055-T7751. This alloy provides a nearly10% gain in strength, with highertoughness and significantly improvedcorrosion resistance.

Toughened carbon fiber epoxy Efforts to develop an improved

carbon fiber epoxy resin system dateback to the early- to mid-1980s. Theseefforts also originated with Boeing’sin-service fleet experience. Since theproduction of the 757 and 767, airlinecustomers have had to contend withthin-gage composite structures in awide number of applications. Com-plaints about this material’s sensitivityto impact damage and the difficultyof repair were many.

In response to these complaints,Boeing initiated and led a significanteffort to develop a toughened epoxymatrix that would be more resistantto damage. Supplier efforts were re-peatedly thwarted by the negative im-pact of toughening agents on hot/wetcompression strength.

Fortunately for Boeing, Toray hadbeen working diligently on a resinsystem that involved a toughening in-terlayer. The resulting system set anew standard for toughness andstrength in composite material tech-nology. Impact test results demon-strated to the airlines that this newsystem also suffered significantly lessdamage, and that such damage couldbe repaired in a manner similar to re-pair of existing aluminum structures.

This breakthrough in CFRP tough-ness was optimized to enable Contin-uous Tape Laying Machines (CTLM)to fabricate structures, resulting in re-duced manufacturing costs. The newtoughened matrix CFRP is used forthe main box cover panels and themain box spars. The main torque boxcover panel consists of an integrally

42 ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2003

Breakout of advanced materials on the 777.

Boeing aircraft and the years they were introduced into service.

1950 1960 1970 1980 1990 2000

747

767

757

777

727

737

707/720 • Regional and intercontinental market flexibility• State-of-the-art, service-ready features and

technology• Industry-leading performance and economics• Range and capacity growth ensure future family

commonality

Dur

abil

ity

impr

oved

Wei

ght

save

d

Alloys:Ti 10-2-3Al 2XXX-T3, -T42, -T36Al 7055-T77Al 7150-T77Ti- 6-4 ELITi 15 -3 -3 3Ti B21STi 6-2-4-2

Composites:Toughened CFRPPitch corePerforated CFRP/Nomex

4: Crown stringers

UncoloredGlare bulk cargo floor6013 Al alloyLightweight sealantsAl meshAV-30 corrosion inhibiting compoundDense core pottingCFRP comp. cascadeAl-Li 8090 sound damping anglesRTM CFRP chine

2: Fuselage skin1: Truck beam

and braces

4: Keel beam4: Belly stringers

8: Aft heat shield8: Engine

mounts 6: ECS ducting

7: Tail cone outer sleeve7: Tail cone plug

7 & 8: Aft core cowl10 & 11: Thrust reverser cowl

11: Inlet cowl inner barrel

5: Stabilizer attach fittings

3: Upper skin and stringers

4: Upper spar chord

9: Floor beams

9: Fin and stabilizer

2: Aft bulkhead

4: Seat tracks

0903cent.qxd 8/10/03 8:12 PM Page 2

stiffened skin with I-section stiffenersat a constant spacing. The basic skinply lay-ups are quite simple, with dou-blers inserted as pre-kitted units. Thisapproach permits the panels to be laidup by the CTLM, resulting in signifi-cant cost reductions. To achieve ac-curate part control, the stiffeners arepre-cured and co-bonded to the skinpanel during the panel cure cycle.

Titanium alloysTitanium applications have in-

creased with each major commercialairplane introduction. In the case ofthe 777, the use of titanium was ex-panded into previous CFRP structureareas to minimize the risk of galvaniccorrosion that is present with alu-minum. For this application, beta-annealed Ti-6Al-4V ELI (Extra LowInterstitial) was introduced into thecommercial fleet, and it provides themaximum damage tolerance proper-ties for titanium alloys.

Titanium was also selected forlanding gear components. The singlelargest titanium application, and per-haps the biggest challenge, was ap-plying Ti 10-2-3 to the main landinggear truck beam. This applicationchallenged Boeing’s metallurgists todevelop tight process controls forwelding the three pieces that made upthis component. (Note: As part of asubsequent cost reduction effort,Boeing ultimately converted the threeforgings to a single forging.) The re-sulting truck beam saved substantialweight and also resulted in a designwithout the typical corrosion andpaint damage risks associated withhigh-strength steel landing gear com-ponents.

Titanium alloy developments in theearly- to mid-1980s were pushed intonew product forms and applicationsfor the 777 as well. While earlierBoeing airplanes included titaniumfor landing-gear springs and high-

ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2003 43

Aluminum alloys and other advancedmaterials by weight on the Boeing 777.

Toughened 2000 series aluminum alloy properties. This alloy is for the body skin.

Advances in 7000 series corrosion-resistant aluminum alloys.

Advanced aluminum alloys with higher toughness and improved corrosion resistance.

Misc. Steel Titanium Composites Aluminum

1% 11%

7%

11%

70%

200

150

100

2XXX 17% improvement in toughness

2024

40 50 60 70Typical tensile yield strength, ksi

Frac

ture

toug

hnes

s K

app,

ks

i-in

.1/2

2XXX

1000

100

10

Fati

gue

crac

k gr

owth

rate

da/

dN,

��-i

n./c

ycle

10 20 30 40Stress intensity factor Kmax, ksi-in.1/2

60% slower fatigue crack growth

2024

2XXX

Cor

rosi

on p

erfo

rman

ce, e

xfol

iati

on ra

ting

Pitting

A

B

C

D

Severe

7055-T777150-T77

7075-T73

7075-T6 7150-T6

7050-T76

7150-T77 7055-

T77

Goodness

Traditionalstrength/corrosion behavior

60 65 70 75 80 85 90 95Compresssion yield strength, ksi

40 60 80 100Typical yield strength, ksi

250

200

150

100

50

Frac

ture

toug

hnes

s K

app,

ksi

/in.1/

2

Fuselage Lower wing surface

(increased durability)

Upper wing surface

(increased strength)

2324-T39 Type II

7055-T7751

7150-T651

2324-T392024-T351

2224-T3511

7075-T651

7178-T651

1930s - 1960s

1970s - 1990s

Challenge

0903cent.qxd 8/10/03 8:13 PM Page 3

temperature environmental con-trol ducting, these alloys hadseveral performance and in-service shortcomings. Duringthe design of the 777, Boeing’smetallurgists worked closelywith parts manufacturers to up-grade to Ti 15-3-3-3 for bothclock-type springs and ducting.

Another major step forwardwas the selection of Beta-21S ti-tanium for the engine plug and

nozzle hot structure, normally fabri-cated of nickel-base alloys. Beta-21S,developed for its high resistance to ox-idation, resulted in significant weightreduction for this exhaust component.

Non-structural materialsIn the interest of creating a preferred

airplane, Boeing’s materials engineersconcentrated on every detail, andidentified the potential for break-throughs in some less obvious areas,for example:

By filling traditionalsealants with micro-balloons, over 300 poundsof weight was eliminatedwhile keeping the samebasic properties.

Through detailed analy-ses and tests, Boeing con-firmed that an entire coatof paint could be elimi-nated from the lower por-tion of the fuselage inte-rior. Amazingly, whilethis change eliminatedover 250 pounds ofweight, the primary dri-ving force behind its in-corporation was im-proved paint adhesion

and better corrosion resistance. These changes typify the innovative

thinking that enabled the develop-ment of a preferred airplane in termsof cost, weight, and affordability.

Boeing 777 to 7E7 The 777 represented a breakthrough

in materials applications for commer-cial aircraft. The introduction of thisairplane was well-timed to drive anumber of critical advances in ma-terials technologies to maturity, withthe end result being implementation.The rate of incorporation for these ad-vances onto the 777 is remarkable, andreflects the high degree of alignmentin research work over the five yearspreceding the design effort. This re-search was clearly focused on fleetconcerns raised by airlines, and the de-liberate development of enhanced per-formance materials that were cost-effective.

Just as the 777 was a breakthroughin terms of materials applications, the7E7 promises to provide an evengreater opportunity for innovation,both in technical advances and in thecreation of the cooperative processneeded to develop these technologieswith our global partners.

To compete against products thatare based on many of the same ma-terial technologies found on the 777,the 7E7 engineers must consider fur-ther technology breakthroughs andexpand the application of advancedtechnologies beyond the currentnorm.

Fortunately, materials developmentin the last five years has beenpromising. Today, confidence has in-creased in composites as a primarystructure, based on 777 successes. En-couraging progress has been made inaluminum, steel and titanium tech-nologies. Finally, understanding theneed for environmentally responsibleprocesses has also grown. Many tech-nologies are now maturing in this areaand offer an opportunity to designand produce an airplane that is notonly cost and performance preferred,but more environmentally friendlythan airplanes of the past. ■■

For more information: Brian Smith is theChief Engineer of Commercial AirplaneBoeing Materials Technology Organiza-tion at the Boeing Airplane Co., Seattle,Washington; tel: 425/237-3516; e-mail:brian.w.smith@boeing.com.Titanium alloy development has progressed in both strength and toughness.

Comparative composite material damage resistance. After a270 in.-lb impact, the conventional composite material on theleft shows much more damage than the new 777 advanced com-posite material on the right.

3.6 square inches 0.5 square inch

Frac

ture

toug

hnes

s K

IC, k

si-i

n.1/

2

120 140 160 180 200Ultimate tensile strength, ksi

VT-22 STA:2000s, high

strength

Ti-6Al-4V, beta annealed:

1980s damage tolerant structure

Ti-6Al-4V, mill annealed:

1960s general

structure

VT-22STA:

2000s, damagetolerant forgings

Ti-6Al-6V-2Sn/Ti-6Al-4VSTA:

1960s generalstructure

Ti-10V-2Fe-3Al STA:

1980s, high strength forgings

Goodness100

90

80

70

60

50

40

30

44 ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2003

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