Leading Spanish company Ona carried out a study to analyse a wire electro-erosion process (WEDM) as an alternative to conventional machining (broaching) of firtree profiles in discs for the aeronautical sector. The EDM company says that firtree slot WEDM could become a viable industrial option and replace broaching in future.
In its research, Ona, with its 65-year experience in EDM scrutinized the EDM process, namely, for broaching and milling. Focus parameters were surface integrity requirements, precision and processing time for the machining of firtrees. The test carried out was the cutting of a turbine disc made of Inconel 718. The outside diameter of the disc was 350mm and 82mm thick. It had to be fitted with 28 firtree slot profiles at 30° angles and given an allowance for profile tolerance of 0.01mm and, a firtree radial and angular positioning tolerance of 0 .08mm.
The aim was to cut the firtree teeth utilising the wire electro-erosion process (WEDM) for the test, while complying with tolerence requirements. The following were analysed: Utilising WEDM, the following were analysed:
- Roughing and finishing time of the entire profile.
- Total erosion time of an entire profile.
- Total roughing and total finishing time of the turbine disc, including preparation time.
- Total machining time including the time for configuration.
- Material removal rate MRR (mm2/min).
- Recasting the layer thickness.
The configuration and machine conditions
Standard Ona EDM technology was deployed (selecting the use for nickel alloy, as the test was with Inconel) on a 2-axis rotary table with two differing flushing conditions; a good flushing condition with closed nozzles and the other, poorer flushing condition with open nozzles. Also, a 0.25 coated wire was utilised.
In the study, the differing flushing conditions enabled Ona to carry out different tests to properly evaluate erosion time, this being greatly dependent on flushing conditions. The company explained that, often, closed nozzles cannot be applied because of the geometrical characteristics of the turbine discs.
According to Ona, the results using WEDM technology after one roughing pass and two finishing passes in a three-cut strategy show that a recast layer thickness in the required range of about 5µm and precision in the firtree profile within 0.01mm tolerance were possible. Furthermore, in the open-nozzle flushing condition, material removal rate in roughing was 110mm2/min, while a material removal rate of 60mm2/min was achieved when a 1-pass roughing and 2-pass finishing were deployed.
In closed-nozzle flushing, on the other hand, an improvement in the material removal rate was observed and in the roughing process alone, it was above 400mm2/min.
Most turbine discs have firtree profiles with angles between five and 30°. The taper-cutting function of Ona’s WEDM machine achieves good results when the geometrical precision for profiles is below 10°. For greater angles, a 2-axis rotary table is recommended. For the removal of material and to improve machining times, the dielectric must be cleaned in the reheating area. lt is essential that most of the nozzles are closed in the profile. For turbines discs with smaller diameters, in which the nozzles interfere with the fastening device, special nozzles are recommended.
Finally, the use of coated wires is recommended to reduce the processing of WEDM, especially in the roughing process.
Ona's conclusion based on the study of the results
Compared to conventional machining for broaching and milling, the company concludes that the WEDM process of turbine firtree profiles, as done in this study, is indeed an alternative to broaching as an industrial solution in the aeronautical sector. It allows for machining time optimisation and the reduction of white layer thickness, while achieving good geometrical precision. The 3-cut strategy deployed meets geometrical tolerances to achieve a white layer below 5µm.
While the certification of the complete machining process using EDM to standard aerospace standards maybe a long and costly process, according to Ona, it is a viable option is to combine WEDM for roughing as it is cheaper and quicker, together with broaching for final finishing.