aviation

Blisk devel­op­ment: How blade and disk became one

With the space-saving and lighter rotor disks in the com­pres­sor with integrated blades, engines consume less fuel. The blisk success story began in the Euro­fighter EJ200 engine.

05.2019 | Text: Denis Dilba

Text:
Denis Dilba holds a degree in mechatronics, is a graduate of the German School of Journalism, and founded the “Substanz” digital science magazine. He writes articles about a wide variety of technical and business themes.

Kopf

Blades and disks from a sin­gle piece—so-called blade in­te­grat­ed disks, or blisks for short—have long been a fix­ture in air­craft en­gines. The high-tech com­po­nents, which are used in com­pres­sors, not on­ly save space and weigh less than con­ven­tion­al ro­tors with in­di­vid­ual blades, but they al­so per­mit bet­ter blade aero­dy­nam­ics. Fur­ther­more, they re­duce as­sem­bly work and thus costs. As a re­sult, en­gines be­come more com­pact and lighter over­all and con­sume less fu­el, which in turn re­duces CO2 emis­sions and ben­e­fits the en­vi­ron­ment.

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In 1995, when Arthur Schäf­fler first un­veiled the new full-blisk low-pres­sure com­pres­sor to the four Eu­rofight­er cus­tomers for their EJ200 jet en­gine, the re­sponse was less than en­thu­si­as­tic. “Heat­ed dis­cus­sions broke out im­me­di­ate­ly,” re­calls the then Tech­ni­cal Di­rec­tor of the EJ200 con­sor­tium Eu­ro­jet, think­ing back to that meet­ing in Lon­don. The rep­re­sen­ta­tives from Spain, Ger­many, Italy and the Unit­ed King­dom had some grounds for their skep­ti­cism. Al­though a first blisk had al­ready been used in a he­li­copter en­gine back then, the blisks that MTU’s Schäf­fler was propos­ing had a much greater di­am­e­ter than the he­li­copter com­po­nent. “With the EJ200 blisks, we’d gone out to the very fron­tier of de­vel­op­ment tech­nol­o­gy,” says the en­gi­neer, who is now 81 years old. Dri­ven there by ne­ces­si­ty, the MTU en­gi­neers were forced to try the new tech­nol­o­gy in or­der to ful­fill the ser­vice life re­quire­ments for the EJ200. The high ro­ta­tion­al speeds of the ro­tors in the jet en­gine—and thus the cen­trifu­gal forces—were so great that fret­ting cor­ro­sion be­came a prob­lem for the con­ven­tion­al in­di­vid­ual blade tech­nol­o­gy. Fret­ting cor­ro­sion here refers to the for­ma­tion of lit­tle pits on the sur­faces of the blade root and ro­tor groove, which can lead to cracks and ul­ti­mate­ly to loss of the blade.

**Snapshot:** Good to see how this blisk is milled out of a single workpiece. Hover over the image for a bigger view

Snapshot: Good to see how this blisk is milled out of a single workpiece.

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Snapshot: Good to see how this blisk is milled out of a single workpiece.

**Blisk production on about 10,000 square meters:** In Munich, MTU operates one of the world’s biggest and most flexible manu­facturing facilities for pro­ducing blisks for high- and medium-pressure com­pres­sors. Hover over the image for a bigger view

Blisk production on about 10,000 square meters: In Munich, MTU operates one of the world’s biggest and most flexible manu­facturing facilities for pro­ducing blisks for high- and medium-pressure com­pres­sors.

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Blisk production on about 10,000 square meters: In Munich, MTU operates one of the world’s biggest and most flexible manu­facturing facilities for pro­ducing blisks for high- and medium-pressure com­pres­sors.

“The first 85 EJ200 en­gines that were de­liv­ered with­out blisks in the high-pres­sure com­pres­sor in stages 1 and 2 were thus lim­it­ed to 400 flight hours—where­as the de­sign had planned for 4,000 hours,” says Chris­t­ian Köh­ler, who joined MTU in 1990 and is to­day the Chief En­gi­neer of the EJ200 pro­gram. The blisks re­solved the prob­lem and ful­ly con­vinced the cus­tomers of the in­te­grat­ed disk-blade so­lu­tion. By the start of 2019, 558 jets had al­ready been de­liv­ered with the EJ200 en­gines—and more are on or­der. MTU’s blisk ex­pe­ri­ence was then fur­ther uti­lized in an ex­per­i­men­tal high-pres­sure com­pres­sor that was de­vel­oped as part of the HDV12 tech­nol­o­gy pro­gram and which fit more or less ex­act­ly in­to the PW6000 en­gine for the A318. “This al­lowed us to show Pratt & Whit­ney our tech­no­log­i­cal abil­i­ties,” Köh­ler says. With a few mod­i­fi­ca­tions, the high-pres­sure com­pres­sor is a stan­dard com­po­nent in the PW6000 to­day, and for blisk tech­nol­o­gy this rep­re­sent­ed the leap in­to com­mer­cial busi­ness.

In­no­v­a­tive blisk man­u­fac­tur­ing

In the clas­sic method, the con­tours of the blades are milled out of a sol­id met­al disk. How­ev­er, the fur­ther for­ward you move in the en­gine—that is, where the disks get small­er and the blades big­ger—the less eco­nom­i­cal the ma­chin­ing process be­comes. “A lot of ex­pen­sive ma­te­r­i­al winds up in scrap, and the milling takes a very long time,” Köh­ler ex­plains. Mea­sur­ing up to 20 cen­time­ters in size, the blades of the first two EJ200 low-pres­sure com­pres­sor stages are there­fore forged in­di­vid­u­al­ly and on­ly then joined to the disk in a lin­ear fric­tion weld­ing process de­vel­oped es­pe­cial­ly for this pur­pose. The ex­perts at MTU sub­se­quent­ly de­vel­oped a fur­ther tech­nique for blisk man­u­fac­tur­ing: “With pre­cise elec­tro­chem­i­cal ma­chin­ing (PECM), we’re now able to man­u­fac­ture blisks even from nick­el al­loys, which are dif­fi­cult to weld and to ma­chine,” says Köh­ler. In the tech­nique, the blisk blank is dis­solved out of the al­loy us­ing a liq­uid elec­trolyte, an elec­tric cur­rent and a 3D mold­ing tool.

„Over the com­ing years, we’ll be man­u­­fac­tur­ing up to 6,000 blisks per year here. With­out all the ex­pe­ri­ence that we’ve built up over the years and our con­stant watch­­ful­ness and care, that would not be pos­si­ble.”

Dr. Stephan Bock, Director Engineering Advanced Programs MTU Aero Engines, Munich

One of the ad­van­tages of PECM over milling is that be­cause they make no con­tact with the com­po­nent dur­ing the process, the tools do not ex­pe­ri­ence wear and tear. More­over, due to the much high­er re­pro­duc­tion pre­ci­sion of the chem­i­cal-elec­tri­cal method, there is no need for fur­ther post-pro­cess­ing steps. Both these fac­tors re­duce costs. Us­ing the in­no­v­a­tive pre­ci­sion tech­nique, MTU man­u­fac­tures on be­half of Pratt & Whit­ney the fifth and sixth blisk stages of the Geared Tur­bo­fan™ (GTF) high-pres­sure com­pres­sor, which has eight stages in to­tal and is laid out ful­ly ac­cord­ing to blisk de­sign. “To man­u­fac­ture the ti­ta­ni­um blisks of the first four stages of the GTF high-pres­sure com­pres­sor, which were de­signed by MTU, we built a spe­cial­ly de­signed pro­duc­tion fa­cil­i­ty at the Mu­nich site,” says Dr. Stephan Bock, Se­nior Vice Pres­i­dent for Ad­vanced Com­mer­cial and Mil­i­tary Pro­grams. At this fa­cil­i­ty, large ti­ta­ni­um blisk blades are joined in­di­vid­u­al­ly to the disk by means of lin­ear fric­tion weld­ing and then ad­just­ed us­ing adap­tive milling; mean­while, small and medi­um-sized ti­ta­ni­um blisk blades are milled from a sin­gle piece.

Interaction: Automated blisk production

Automated blisk production

Blisks for the Pratt & Whitney GTF™ Engine Family are produced in a new and highly automated manufacturing hall. To the interaction ...

Up to 6,000 blisks per year

The cen­ter of ex­cel­lence for man­u­fac­tur­ing ti­ta­ni­um com­pres­sor disks is a prime ex­am­ple of the im­ple­men­ta­tion of In­dus­try 4.0 think­ing at MTU and the most state-of-the-art pro­duc­tion fa­cil­i­ty of its kind in the world. It con­tains a high de­gree of au­toma­tion, dig­i­tal­iza­tion, con­nec­tiv­i­ty, and self-con­trol­ling that is unique world­wide. The high­ly func­tion­al, en­er­gy-ef­fi­cient new fac­to­ry build­ing al­so meets the lat­est in mod­ern build­ing stan­dards. “Over the com­ing years, we’ll be man­u­fac­tur­ing up to 6,000 blisks per year here,” says MTU ex­pert Bock. “With­out all the ex­pe­ri­ence that we’ve built up over the years and our con­stant watch­ful­ness and care, that would not be pos­si­ble.” The process­es have to be ex­treme­ly sta­ble: one man­u­fac­tur­ing de­fect on a sin­gle blade can ren­der the en­tire com­po­nent un­us­able. How­ev­er, Bock be­lieves this is no time for MTU to rest on its lau­rels: “We’re al­ready work­ing on the blisks for the next GTF gen­er­a­tion.”

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AEROREPORT is an aviation magazine published by MTU Aero Engines, Germany's leading engine manufacturer. Neatly summed up, AEROREPORT offers an MTU perspective on the world of aviation. The word “REPORT” in the title stands for the high-tech and outstanding service “made by MTU”. “AERO” represents broader horizons and general aviation topics.

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