Currently, fir tree slots for blade assembly in jet engine disks are predominantly machined via broaching. Due to material developments in terms of thermal resistance – which enables for more efficient jet engines – broaching has reached its economical limit, especially where the machining of nickel-based alloys is concerned. Consequently, aero engine manufacturers have pronounced an increased need for economic alternatives for fir tree slot production.
Wire EDM has great potential as an alternative to broaching due to important process developments over the past decades. Nevertheless, there is a lack of comprehensive process knowledge regarding the machining of nickel-based alloys. The following two main challenges and tasks for the research in this thesis were identified. The first main task was to investigate the effect of wire EDM on the surface integrity and related component strength. The second task was to assess the process productivity to determine the economic competitiveness for wire EDM in order to become a viable alternative to broaching. The objective of this thesis was therefore to comprehensively research the wire EDM process for fir tree slot production in nickel-based alloys in terms of process signatures and resulting functional properties.
Initially, a methodology was developed to establish wire EDM parameters for fir tree slot production in nickel-based alloys. By using the methodology machining parameters were set up and the surface integrity as well as the component life of parts machined by wire EDM and broaching were compared and analyzed. In terms of component life evaluation, HCF and LCF tests indicated that wire EDM machined surfaces have component strengths of the same magnitude as broached surfaces. Subsequently, a fir tree slot production case study was designed in order to investigate the process capability of wire EDM to machine fir tree slots. It was determined that the process is capable of machining to the accuracy required by the jet engine manufacturers. A cost model was then compiled for an economic comparison of wire EDM and broaching. The outcome of the cost model evaluation demonstrated that, in economic terms, the wire EDM process is a serious alternative to broaching. Finally, the technology readiness level scale was taken to assess the overall capability of wire EDM to manufacture fir tree slots. The developments within the thesis and the final evaluation of the process capability on a realistic case study object led to a level of six. This showed the functional suitability of wire EDM to machine fir tree slots.