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. 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. “A lot of expensive material winds up in scrap, and the milling takes a very long time,” Köhler explains. 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. In the classic method, the contours of the blades are milled out of a solid metal disk. 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. “This allowed us to show Pratt & Whitney our technological abilities,” Köhler says. 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. By the start of 2019, 558 jets had already been delivered with the EJ200 engines-and more are on order. The blisks resolved the problem and fully convinced the customers of the integrated disk-blade solution. “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. 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 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. 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. “With the EJ200 blisks, we’d gone out to the very frontier of development technology,” says the engineer, who is now 81 years old. 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. The representatives from Spain, Germany, Italy and the United Kingdom had some grounds for their skepticism.

“Heated discussions broke out immediately,” recalls the then Technical Director of the EJ200 consortium Eurojet, thinking back to that meeting in London. 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.

Blisk whiskers