Precise electrochemical machining: A new development at MTU

In 2015, things got really serious for “Set 1” when the first two ECM/PECM series pro­duction systems at MTU Aero Engines’ Munich site had to show what they could do. The elec­tro­chemi­cal duo was tasked with manu­facturing the com­pli­ance hard­ware for the PurePower^® PW1100G-JM Geared Turbofan™, the en­gine for the A320neo: the fifth and the sixth high-pressure com­pres­sor stages. These are blisks with a di­am­eter of around 450 mil­li­me­ters and blades with ex­treme­ly com­pli­cat­ed ge­om­etries. The sample com­po­nents were rig­or­ously tested by client Pratt & Whitney with regard to strength and geometry—and they passed. Series pro­duction commenced in September 2015, and since then two further systems have come on stream; a third pair is under con­struc­tion and another two are in planning.

MTU has been working on the cutting-edge PECM tech­nol­ogy for some time now. This de­vel­op­ment was set in motion by the re­ali­za­tion that the blisk design was gaining more and more ac­cept­ance—in­clud­ing growing popu­lar­ity in the area of high-pres­sure com­pres­sors. Blisks have one major ad­van­tage over their coun­ter­parts with in­di­vid­ually in­sert­ed blades: with integral com­po­nents, the blade angle can be set such that the re­spec­tive stage works more ef­fi­cient­ly. As a result, the in­di­vid­ual stages can each compress air more ­ef­fec­tive­ly. Thanks to the integral design, the edge load on the rotor disks is reduced, which saves weight. In ad­di­tion, the elimi­na­tion of leaks en­hances the ef­fi­ciency in the compressor. “Both these things together reduce fuel con­sumption—and there­fore the engine’s CO₂ emis­sions as well,” explains MTU engineer Thomas Frank, who heads rotor pro­duction op­era­tions and is re­spon­sible for nickel blisks.

New method for new geometries and materials

From a technical point of view, however, the fuel con­sump­tion ad­van­tages and emis­si­ons re­duc­tions are not granted auto­mati­cally. Tem­pera­tures of around 650 de­grees Celsius prevail in stages 5 and 6 in the high-pres­sure com­pres­sor. Titanium is the material usually used to build com­pres­sors, but the light­weight metal ceases to have the requisite strength at these tem­pera­tures. For the PW1100G-JM, high-tem­pera­ture-re­sist­ant nickel alloys are used instead, but these alloys cannot be processed cost ef­fec­tive­ly using con­ven­tional milling tech­niques because of the high level of tool wear. A further dif­fi­culty is the ex­treme­ly complex 3D geometry of the blades, which stretches the ECM tech­nique pre­vi­ously used with suc­cess for larger blisks to its limits. An even more precise method had to be de­vel­oped—PECM.

As with the ECM method, PECM also involves using an elec­tro­lyte and elec­tri­cal current to care­fully and precisely dis­solve a me­tal­lic material. The material to be pro­cessed acts as an anode (positive pole) and the three-di­men­sional, me­tal­lic tool acts as a cathode (negative pole). The big ad­van­tage PECM has over con­ven­tional machining tech­niques is that the tool does not touch the com­po­nent, which means it does not incur process-related wear. An aqueous sodium nitrate so­lu­tion is used as an elec­tro­lyte, which flows be­tween the anode and cathode. This liquid has three functions: it establishes an electrically conductive connection; it carries away the removed material and the hydrogen created by the process; and it cools the process. Compared to the ECM tech­nique, the PECM method achiev­es higher ac­cu­ra­cies by em­ploying ex­treme­ly small working gaps in the mi­crom­eter range be­tween workpiece and electrodes.

In contrast to con­ven­tional, single-axis processes, the work­piece is pro­cessed sim­ul­ta­neously with two elec­trodes which travel toward each other. This was not easy to master. Moreover, the electro­lyte so­lu­tion also had to be improved. Con­se­quently, MTU decided to de­vel­op and build the mass pro­duction systems itself. “We’d already built up special know-how to the extent that we couldn’t find the same quality among external sup­pli­ers anymore,” explains Martin Bußmann, pro­ject manager for in­dus­tri­ali­za­tion of the new method. “And, natu­rally enough, we also wanted to keep our edge in the knowl­edge we had acquired.”

MTU also plans to exploit the advantages of the PECM process for other components or manufacturing steps in the future. After all, in principle, the technique is suitable for many applications, such as for edge rounding or for manufacturing individual rotor blades and guide vanes. “The blade geometries for high-pressure compressors are becoming even more complex, and the materials are becoming ever more heat-resistant. This is pushing conventional machining technology more and more to the limits of its technical possibilities and cost effectiveness,” says production manager Frank. “PECM offers a future-proof alternative.”