Additive manu­facturing: Printing a compo­nent, layer by layer

The borescope boss is a small and seem­ing­ly in­signif­i­cant com­po­nent. Just about able to fit in­side a per­son’s fist, it has lat­er­al ex­ten­sions to the left and right, each with a hole. The mid­dle of the boss has an open­ing with a sil­ver-col­ored screw thread, and the rest of its sur­face has a gray­ish mat­te fin­ish. Once screwed on­to the tur­bine cen­ter frame, this ac­ces­so­ry al­lows tech­ni­cians to look in­side the low-pres­sure tur­bine with an in­spec­tion cam­era—the borescope—and thus to check the con­di­tion of the blades. Most peo­ple would sim­ply have no idea what they were look­ing at if some­one showed them the com­po­nent. How­ev­er, for Dr. Jür­gen Kraus, Head of Ad­di­tive Man­u­fac­tur­ing for MTU Aero En­gines in Mu­nich, the borescope boss holds a very spe­cial sig­nif­i­cance. “With it, we have made the leap to in­dus­tri­al-scale ad­di­tive pro­duc­tion,” says Kraus. While in the past these ac­ces­sories were milled from a sol­id block, to­day they are el­e­gant­ly print­ed through the use of se­lec­tive laser melt­ing.

3D mi­crow­eld­ing process­es

This process is gen­er­al­ly clas­si­fied un­der the broad­er field of 3D print­ing, but from a strict­ly tech­no­log­i­cal stand­point, it in­volves 3D mi­crow­eld­ing process­es. In this method, the 3D mod­el of the com­po­nent is “sliced” on a com­put­er in­to in­di­vid­ual lay­ers mea­sur­ing 20 to 40 mi­crom­e­ters thick. A pow­er­ful laser then melts ma­te­r­i­al in pow­der form with­in a con­struc­tion cham­ber ex­act­ly at the lo­ca­tions spec­i­fied by the com­put­er-gen­er­at­ed com­po­nent de­sign da­ta, join­ing it to the lay­er be­low. In this way, com­po­nents are built up, lay­er by lay­er, with new lay­ers con­tin­u­al­ly be­ing added. Ex­perts such as Kraus, there­fore, speak of ad­di­tive man­u­fac­tur­ing. To­day, the low-pres­sure tur­bines in the A320neo’s Geared Tur­bo­fan™ PW1100G-JM are be­ing out­fit­ted with the borescope boss­es. MTU is there­by one of the first com­pa­nies in the avi­a­tion in­dus­try to have re­ceived reg­u­la­to­ry ap­proval for the use of this in­no­v­a­tive tech­nol­o­gy in vol­ume pro­duc­tion of com­po­nents, and one of the first to have im­ple­ment­ed it.

Start­ing with tools and blanks

MTU took the first step on this path very ear­ly on, start­ing work on the ad­di­tive process as far back as the late 1990s. In the ear­ly years, the com­pa­ny fo­cused on the­o­ret­i­cal as­pects, but quick­ly shift­ed to prac­ti­cal ap­pli­ca­tions. “We be­gan with the pro­duc­tion of tools and mas­ter forms for pre­ci­sion cast­ing, along with sim­ple de­vel­op­ment parts,” Kraus re­calls. In the sec­ond phase, the com­pa­ny pro­duced fix­ture com­po­nents to re­place al­ready ex­ist­ing parts, such as spray noz­zles and grind­ing wheels used for man­u­fac­tur­ing com­po­nents. It was dur­ing this pe­ri­od that Geared Tur­bo­fan™ borescope boss­es were de­vel­oped. “These sim­ple com­po­nents, which are not crit­i­cal to the im­me­di­ate func­tion­ing of an en­gine, were ide­al for ex­plor­ing how ad­di­tive man­u­fac­tur­ing could work in vol­ume pro­duc­tion—thus paving the way for us­ing it to man­u­fac­ture com­po­nents that are more com­plex and more crit­i­cal,” ex­plains Dr. Karl-Heinz Dusel, Se­nior Man­ag­er Ad­di­tive Man­u­fac­tur­ing Tech­nol­o­gy at MTU. For in ad­di­tion to the man­u­fac­tur­ing tech­nol­o­gy in and of it­self, at that time the en­tire process chain al­so had to be re­built from scratch.

Up to that point, MTU had on­ly bought blanks, and had not man­u­fac­tured them it­self; as a re­sult, Dusel and his col­leagues were not able to fall back on al­ready ex­ist­ing process­es, pro­ce­dures or struc­tures for the man­u­fac­tur­ing process or the se­cur­ing of reg­u­la­to­ry ap­proval. “Sim­ply work­ing out the nec­es­sary norm­ing sys­tem and cal­cu­lat­ing the ma­te­r­i­al da­ta took more than two years,” says Dusel. More­over, it was nec­es­sary to de­vel­op and im­ple­ment new meth­ods for com­po­nent test­ing and qual­i­ty as­sur­ance. With the process now firm­ly es­tab­lished, MTU is work­ing step by step to im­ple­ment it with more com­plex com­po­nents and oth­er en­gine types. Cur­rent pro­jects in­clude, for ex­am­ple, new bion­i­cal­ly de­signed and thus es­pe­cial­ly light­weight brack­ets for oil lines, and a stiffer, more cost-ef­fi­cient seal car­ri­er pro­duced with ad­di­tive man­u­fac­tur­ing. This in­ner ring with in­te­grat­ed hon­ey­combs will be in­stalled in the high-pres­sure com­pres­sor in the fu­ture. The brack­ets, which play an equal­ly crit­i­cal role in the en­gine’s func­tion, have a curved, fil­i­gree form.

Bion­i­cal­ly formed light­weight com­po­nents

The new de­sign has made it pos­si­ble to cut the weight of the com­po­nent by a third, with­out in­ter­fer­ing with its strength or damp­ing char­ac­ter­is­tics, ac­cord­ing to Dusel. Hence, ad­di­tive com­po­nents help re­duce en­gine weight, which re­duces fu­el con­sump­tion and there­fore emis­sions. It will be a while be­fore this point is reached, how­ev­er. “Com­po­nents such as these that are sub­ject to in­tense stress­es must be val­i­dat­ed in en­gine per­for­mance tests,” Kraus ex­plained. While this work is be­ing car­ried out, his team and ex­ter­nal ex­perts are al­ready con­duct­ing fea­si­bil­i­ty stud­ies on com­plete­ly new com­po­nents that could be uti­lized in the Next Eu­ro­pean Fight­er En­gine (NEFE), and in the com­ing gen­er­a­tion of Geared Tur­bo­fans™. “For the next gen­er­a­tion of en­gines, we can en­vi­sion us­ing ad­di­tive man­u­fac­tur­ing for up to 15 per­cent of the com­po­nents,” says Kraus. It is al­ready clear, he con­tin­ues, that in the fu­ture, en­gine man­u­fac­tur­ers will not be able to sur­vive if they don’t im­ple­ment the new process.

For that rea­son, start­ing at the be­gin­ning of last year, MTU has stepped up its com­mit­ment to ad­di­tive man­u­fac­tur­ing by es­tab­lish­ing a sep­a­rate de­part­ment. “By clus­ter­ing all ac­tiv­i­ties from de­sign and tech­nol­o­gy de­vel­op­ment to vol­ume pro­duc­tion in one or­ga­ni­za­tion­al unit, we aim to main­tain our lead and pull even fur­ther ahead,” says Lars Wag­n­er, the MTU COO. Kraus al­ready has an idea as to how all of that could work out: the next step is to de­vel­op and man­u­fac­ture new light­weight com­po­nents. It is nec­es­sary to come up with new de­signs, new com­po­nents—which could con­ceiv­ably in­clude bear­ing hous­ings, brack­ets and struts—and new ma­te­ri­als. The ad­van­tages of the new process can be seen par­tic­u­lar­ly in the man­u­fac­ture of com­plex com­po­nents. “The fu­ture lies with ad­di­tive man­u­fac­tur­ing—every op­ti­mized com­po­nent makes an en­gine a bit more ef­fi­cient,” Kraus says.