Blisk devel­op­ment: How blade and disk became one

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.

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.

“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.

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.

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.”