Plasma‑assisted shaped tube electrochemical machining for ultra-large aspect ratio holes: Radical field shielding mechanisms and ultra-low electrode wear

High-temperature structural parts with ultra-large aspect ratio holes have important applications in aerospace. Both mechanical drilling and electrical discharge machining have tool wear problems. Besides, when electrochemical machining is used, the machining accuracy and efficiency are difficult to balance, and the insulation layer is easy to be damaged. Although laser-electrochemical hybrid processes demonstrate the capability for fabricating large aspect ratio holes, they introduce a complex process flow. The electrolytic plasma process, on the other hand, has significant advantages in improving the surface quality of machining, achieving high-precision machining, and enhancing cost-effectiveness. On this basis, this study proposes a plasma‑assisted shaped tube electrochemical machining (PA-STEM) method with tube electrode insulation and servo control. Process experiments were conducted to investigate the effects of sidewall insulation material, exposed electrode length, maximum speed, spray pressure, and voltage amplitude on machining performance. The results show that adjusting the feed speed by servo control while increasing the thickness of quartz glass can significantly enhance the durability of the tool electrode, thus ensuring machining accuracy. The exposed length of the electrode determines the shielding effect of the plasma against stray electric fields, which in turn has an impact on the machining quality of the hole sidewall. If the feed velocity is too low or too high, it will cause turbulence in the flow field and an increase in abnormal discharges, which will result in a decline in the surface quality of the sidewall. By adjusting the spray pressure and voltage amplitude, the discharge plasma can be stably induced during tube electrode feeding, thereby maintaining the electric field shielding of the tube electrode sidewall. Finally, by optimizing the process parameters, a typical hole with an ultra-large aspect ratio (> 50:1), high surface quality (Ra< 0.2 μm), and high accuracy (taper −0.08°) was achieved. This study significantly advances the applicability of PA-STEM for machining holes with an ultra-large aspect ratio.