EDM as alternative to fir-tree profiling

ONA has carried out a study to analyse a wire electro-erosion process (WEDM) as an alternative to the conventional machining (broaching) of fir-tree profiles in discs for the aerospace sector.

In its research, ONA compared the EDM process with more conventional machining like broaching and milling. As key points in the research, ONA compared parameters such as the surface integrity requirements, precision and processing time of fir-tree machining. The test involved cutting a turbine disc with the following characteristics: Inconel 718 material; 350 mm outside diameter; 28 fir-tree slot profiles; slot profiles at 30°; 82 mm thick; profile tolerance of 0.01 mm; and fir-tree radial and angular positioning tolerance of 0.08 mm.
On completion of the tests, the results proved it possible to obtain a recast layer thickness in the necessary range (around 5 µm) and with sufficient precision in the fir-tree profile within a tolerance of 0.01 mm using WEDM technology. These results were obtained following one roughing pass and two finishing passes (three-cut strategy).
In worse flushing conditions (open nozzles), the material removal rate for roughing was 110 mm2/min. In fact, for the strategy used in the research (one pass for roughing and two passes for finishing), material removal rate was 60 mm2/min. On the other hand, in good flushing conditions (closed nozzles), a slight improvement in the material removal rate was observed; in the roughing process alone, it reached over 400 mm2/min.
Following the research, ONA concludes that WEDM machining of turbine disc fir-tree profiles is an extremely valid alternative to compete with broaching as an industrial solution in the aerospace sector. State-of-the-art and increasingly powerful generators, and more advanced numerical controls, allow for the optimisation of machining time and the reduction of white layer thickness.
For further information www.onaedm.com