Effect of Kerosene Supply on the Performance of Micro-Electrical Discharge Machining of Aluminum Nitride

Abstract

As interest in aluminum nitride (AlN), one of the high-performance ceramics, has been rapidly increasing, many researchers have put a lot of efforts to machining it. Since AlN is classified as a difficult-to-cut material, the EDM process using an assisting electrode is emerging as an effective machining method. Kerosene, as a dielectric fluid, plays the important role in the formation of a conductive carbon layer called as the assisting electrode, which deposited on the surface of the workpiece and inducing discharge. Most of the previous methods used the hollow electrodes to stably feed the dielectric fluids through their center hole. However, in the case of micro-EDM, their extremely small electrode diameter makes it difficult to fabricate a through hole in the electrode and very narrow gap hinders the flow of dielectric fluid. In order to overcome feeding problems in micro-EDM of AlN, in this study, two kinds of kerosene feed enhancing methods are introduced: one is to use a D-shape electrode to obtain a wider asymmetric flow channel; and the other is to use graphite powder mixed kerosene (GPMK) and increase the gap between the tool and the workpiece. Flow simulation results show that both methods have the effect of discharge gap widening that promotes kerosene flow, and the experimental results show similar results as well. When using a D-shape electrode, the material removal rate (MRR) increased under the negative electrode condition but fast tool wear was observed. On the other hand, with GPMK, the MRR increased 64% and the electrode wear decreased 73% compared to the conventional ways under the positive electrode condition.

본 논문에서는 기존 질화 알루미늄 미세 방전가공에서 문제가 되었던 공구 마모와 가공면으로의 등유 공급을 극복하기 위한 공정을 제시하였다. 일반적으로 실린더형 공구를 사용한 보조전극법이 사용되나 마이크로 스케일에서는 가공면에 등유의 공급이 어렵기에, 다양한 복합 가공 연구가 시도되고 있다. 그 중에서 D-형 공구와 흑연 파우더 혼합 등유를 통해 가공면으로의 등유 공급을 촉진시켜, 탄소로 이루어진 전도층의 생성을 원활하게 함을 전도층 모니터링과 실험적으로 증명하였다. 이를 기반으로 질화 알루미늄에 관통 구멍을 가공했으며 기존 방식보다 64%의 Material removal rate (MRR) 증가와 73%의 relative wear ratio (RWR) 감소를 얻어냈다. 더욱이 전도층 모니터링을 통해 다른 종류의 비전도성 세라믹 미세 방전가공의 활용성을 제시하였다.