Wire Electrochemical Trimming of Wire-EDMed Surface for the Manufacture of Turbine Slots

Abstract

Turbine slots are categorized as crucial structures in aero engine and have strict demands on surface integrity concerning fatigue strength. In the last decade, low speed wire electrical discharge machining (LS-Wire EDM) with multiple cuttings has been proposed as a method to replace the well-established broaching process, which needs expensive assets investment and lacks flexibility to new designs. However, recast layers and heat-affected zones still remain in doubt from industries. Wire electrochemical machining (Wire ECM) has similar kinematic flexibility with Wire EDM, but possess exclusive advantages like being free of tool wear and recast layers. Unfortunately the state of art of Wire ECM could not afford efficient precision machining of macro structures with a thickness over tens of millimeters.This work proposes an integrated process combining high speed wire electrical discharge machining (HS-Wire EDM) and wire electrochemical trimming (Wire ET) for the manufacture of turbine slots. Here, HS-Wire EDM takes on the low-cost efficient pre-slotting while a single cutting of Wire ET plays the roles of roughing, semi-finishing and finishing. Experimental results indicate that the electrode trimming rate have a significant influence on material removal rate. Under the process conditions as a pulsed voltage of 40% in duty ratio, 16 V in amplitude and 20 kHz in frequency, a trimming depth of 10 μm and an electrode trimming rate of 7.2 mm/min, the material removal depth is 30 μm and recast layers overlapped on WEDM surface could be entirely removed. Finally, a 20 mm-thick fir-tree slot in Inconel 718 free of recast layers was machined in the proposed integrated process. The average machining efficiency is 64 mm2/min and the profile allowance could reach -0.010 to +0.010 mm.