Blisk machining in the fast lane

Blisk (bladed disk) production volumes for aircraft are increasing rapidly. Large engines use several of these high pressure compressors, which are manufactured from a single piece of high performance, often exotic alloy materials. Efficiently machining these materials is a focal point for project planning – a wide range of methods available, from mechanical cutting to waterjet cutting. A recent study by the Fraunhofer Institute for Production Technology (IPT) and the Laboratory of Machine Tools and Production Engineering (WZL) at RWTH Aachen, Germany, has now shown that precise electro-chemical machining (PECM) from EMAG drastically cuts costs for users, particularly when dressing the workpiece surface.

Aircraft engine components experience extreme stresses, high temperatures, and long service lives. Therefore, developers use very hard, heat-resistant materials such as nickel-based super alloys for the construction of blisks and disks with single blades. This development presents a range of problems to manufacturing engineers, since conventional cutting methods become uneconomical as the material hardness increases and the service life of expensive tools drops. A Fraunhofer IPT study compared seven different blisk machining strategies – from multi-axis milling combined with polishing to high pressure waterjet cutting united with PECM dressing. For an assumed production volume of 800 nickel-based HPC blisks, the unit costs can be reduced by more than 50% versus mechanical cutting with the correct machining strategy. PECM dressing can lead to radical improvements to the efficiency of the production process.

EMAG PECM technology is being closely studied by many OEMs and their suppliers. The fast electro-chemical process removes material without contact and causes minimal tool wear.

Electro-chemical machining creates a positive anode out of the workpiece as the tool acts as the negative cathode. An electrolyte solution flows between them, removing metal ions from the workpiece. The form of the cathode (tool) – with its active, conductive zones – results in material removal from the workpiece at the required component contours. Ring ducts, grooves, bell hollows, and other contours can be produced without contact with very high precision and excellent surface quality.

In developing the PECM process, EMAG engineers learned that the gap between the workpiece and tool, through which the electrolyte solution flows, is particularly narrow. Secondly, the supply of electrolyte solution is enhanced by a mechanical oscillation motion. Both factors ensure that that material removal is effective and precise.

EMAG ECM offers two machines for engine production: the small PO 100 SF model for machining single turbine blades; the larger PO 900 BF for machining the complete, larger, blisks. The systems differ in machining area size, footprints, generator capacity, and the number of machining axes.

“We launched the PO 900 BF in 2011 for blisk machining. The PO 100 SF development was the second stage in 2013 when an ever-increasing number of customers demanded an electro-chemical machine solution for single blades. We now have an ideal basis for the development of precise, cost-efficient production solutions for small and large engine components,” says Richard Keller, member of the board of directors at EMAG ECM GmbH.
 

Components certified for use

This method has already been implemented into production. In 2014, a well-known engine manufacturer certified single blades made on the PO 100 SF for aircraft use, unusually fast after the start of production at a supplier’s plant. A similar stage is currently pending for blisk production. Components from a whole host of engine companies are currently in the qualification phase for use in practice.

“The technology is currently in the validation phase in a number of test programs,” Keller states. “The long-term strategic focus on these components and application technology is proving successful and each additional certification of components manufactured with PECM will only boost our sales success.”

EMAG LLC
www.emag.com