As aerospace OEMs (original equipment manufacturers) and supply-chain companies face increasing pressure to produce more-fuel-efficient aircraft, wire EDM is emerging as a key machining technology — due in large part to a ‘body
of evidence’ generated by David Welling at the Laboratory for Machine Tools and Production Engineering (WZL) at RWTH Aachen University (Germany).

This shows that the process has the potential to replace broaching as the technology of choice for producing the fir-tree slots in high-pressure compressor and turbine disks (fir-tree slots are the spaces in turbine disks into which the blades’ fir-tree roots fit).

Mr Welling, who leads a research group, says his interest in aerospace machining technologies was triggered by an internship at one of the industry’s main OEMs.

After that, he finished his study of mechanical engineering for his diploma degree, and in 2010 produced the first of three ‘breakthrough’ studies on wire EDM in the manufacture of jet-engine components.

Mr Welling was part of the EU-funded ADMAP-GAS project, set up to investigate alternatives to broaching for fir-tree structures in gas-turbine blades and disks.

He says jet engines must become more-fuel-efficient, and that means developing advanced high-temperature materials that are often hard to machine.

The topic of fir-tree slot production using EDM was raised during discussions with the Research Group at WZL and machine tool and EDM machine manufacturer GF Machining Solutions (www.gfms.com).

“With the soaring demand for new aircraft by 2030, and with every passenger jet requiring more than 500 fir-tree slots, it was obvious that a more-efficient and secure method of manufacturing fir-tree slots was needed.

Broaching tools have obvious limitations, one of these being the high tool wear that results from the increased toughness of aerospace materials. Also, broaching machines require a lot of floor space, have a high initial investment cost and offer low levels of flexibility.

“It takes a relatively long time to change a broaching tool, and a special tool is needed for every fir-tree slot. Moreover, tools are costly to replace and can have a manufacturing lead time of more than six months.”

Wire EDM has long been considered a mature technology with little innovation potential, and until about 10 years ago, it was not considered a viable process for full fir-tree slot production; this was due to the ‘recast white layer’ left on components by earlier EDM generator technology.

Mr Welling’s research has shown that with the advent of advanced digital generators, the process now compares favourably with broaching in terms of surface integrity and part tolerances — both key requirements for fir-tree slots.

His research has also shown that wire EDM is comparable with broaching in terms of component fatigue life, and that it is capable of machining fir-tree slots with the required accuracy. Research also shows that the process has an
advantage when it comes to automation, since it offers the process-monitoring capabilities necessary to ensure machining quality.

“Whereas broaching is perceived as a difficult process to automate, with wire EDM you can monitor the control data, trace it back, and make the correlation to the surface to get a quick in-process quality assessment.

“Furthermore, there are not the tool-wear challenges inherent in broaching; the tool is the wire that is being unwound from the spool, so a ‘new tool’ is always in use.”

Mr Welling’s research has also shown that, while broaching uses brute force to produce forms, wire EDM is a virtually ‘force-free’ process capable of cutting the very fine radii that make up the geometry of fir-tree slots.