ME September 2015

techview Jonathan Chomicz

Raising Effi ciency Levels in the Machining of Blisks

A

ccording to the energy effi ciency report from the German Airline Association (the Bundesverband der Deutschen Luftverkehrswirtschaft), a modern

airplane uses just three liters of fuel per passenger per 100 km of fl ight route. The consumption level has been falling for years now, made possible by a multitude of measures includ- ing designers developing increasingly effi cient jet engines consisting of high-performance material components. This development poses a challenge to the mechanical engineer- ing industry closely connected with aircraft production as aerospace manufacturers are on the lookout for new manu- facturing solutions to ensure the effi cient machining of central engine components such as blisks and single blades. The number of blisks required by the aircraft industry is rising steadily. Larger engines now feature a number of blisk stages in their compressors. To ensure that these key components can withstand the extreme temperatures gener- ated, they are made of very hard, heat-resistant materials such as nickel-based alloys (Inconel) or titanium aluminides. Many well established conventional machining processes are pushed to their economic limit as the increasing hardness of the material reduces the life expectancy of expensive milling tools and thus raises production costs. There are other chal- lenges too. The conventional machining of superalloys pro- duces higher temperatures that have a negative infl uence on the material’s structure and endanger the process integrity. What alternative machining strategies are available that guarantee the effi cient processing of blisks? A system- atic, detailed answer is provided by a new study from the Fraunhofer-Institute for Production Technology and the ma- chine tool laboratory of the RWTH Aachen University. Their specialists compared different blisk machining strategies— from multiaxis milling combined with polishing to high-pres- sure waterjet cutting combined with precise electrochemical metal removal (PECM) fi nishing. It became clear that even with an annual production of just 800 nickel-based HPC- blisks the unit cost can be reduced by 50% compared to conventional machining methods as long as the right alterna- tive strategy is chosen. Noteable is that they concluded that

ECM/PECM Technology Specialist EMAG LLC

Farmington Hills, MI

the best methods cannot succeed without PECM fi nishing. Inclusion of the electrochemical process makes the material especially smooth and ensures that tool wear is minimal.

ECM makes material smooth and ensures minimal tool wear.

During the electrochemical process an electrolyte solution passes between the workpiece (the positive anode) and the tool (the negative cathode). This removes metal ions off the workpiece. The shape of the cathode—i.e., the tool with its active, conductive sectors—ensures that the removal of the material from the workpiece will achieve the desired compo- nent contour. This results in the best possible surface quality without burr formations, without changing the material struc- ture and with low roughness values. Subsequent refi nement processes on the blades, such as barreling, are noticeably shorter or may, in certain cases, even become unnecessary. Either result provides a vital reduction in unit costs. The PECM technology from EMAG represents a targeted development of the successful, basic electrochemical ma- chining (ECM) principle. In this case, the gap between work- piece and tool, through which the electrolyte solution passes, is particularly narrow. At the same time, the feed rate for the electrolyte is overlaid by a mechanical, oscillatory movement. In combination, these two factors provide for more effective and accurate material removal, which results in the economic advantages identifi ed by the machine tool laboratory of the RWTH Aachen University. For years the appropriate engineering technology has

been provided by EMAG ECM GmbH—the technology center for electrochemical machining and part of the EMAG Group. EMAG provides two sizes of PECM machines: the compact model PO 100 SF, for the complex machining of single turbine blades, and the larger model PO 900 SF for the machining of complete, and consequently much larger, disks of up to 900-mm diameter and a maximum weight of 500 kg. Aero components produced by a number of North American companies have already entered the qualifi cation phase.

45 — Aerospace & Defense Manufacturing 2015

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