| 摘要: | 放電加工對高硬度、高強度的難加工材料及精密微細零件的加工製造，具有十分優越的加工特性。但放電加工表面因急熱急冷作用容易產生微裂紋、氣孔等缺陷，及材料去除時所形成的放電坑洞，都對加工表面的品質造成嚴重的影響。因此對放電加工表面進行精修及改質強化以提升加表面品質及強度的研究，對擴展放電加工的應用範圍具有舉足輕重的效益。 本研究針對放電加工表面品質不佳的缺點，提出表面精修及改質強化的方法，以實驗探討的方式研究加工液中添加粉末顆粒、整合放電與超音波加工機制及結合放電與球磨加工機制的複合加工法對放電加工表面產生精修及改質強化的效果。由於放電加工對材料的去除效率較傳統切削加工低，因此對於如何維持加工效率、加工精度同時獲得高品質加工表面的研究，本就是放電加工應用上必須同時面對的問題，因此也是本研究中所關注的主題。 經由本研究的實驗結果分析顯示，加工液中添加導電性的混合粉末顆粒，在加工時可擴大放電間隙促進排渣效果，同時具有放電能量分散的效益可降低放電表面的粗糙度，而且添加的粉末顆粒可經由放電柱的離子化作用，轉移、滲入加工表面達到表面改質強化的效果。而放電與超音波複合加工由於同時具有放電的蒸發、熔融及超音波振動趨使磨料的錘擊、衝擊作用，因此材料去除效率明顯高於傳統放電加工法；而且放電與超音波複合加工所添加的硬質磨料在加工的過程中，可成為表面改質強化的成分，由實驗的結果分析中發現可產生顆粒強化及固溶強化機構，獲得放電合金化的改質強化效果。而放電與球磨複合加工的實驗結果顯示，放電表面藉光滑的ZrO2圓球所產生的塑性變形，可消除放電表面的微裂紋、氣孔及放電坑洞，獲得平整細緻的加工表面，達到表面精修的效果，同時ZrO2圓球在加工的過程中，並不會對放電造成干擾而減低加工的效率。經由本研究一系列的探討可在高效率、高精度的加工條件下，同步對放電表面產生精修及改質強化的效果。 Electrical discharge machining (EDM) possesses excellent machining performances for difficult-to-machining materials and micro precision parts. The EDM surface reveals the defects of micro cracks and pores due to violent temperature gradient during machining, and the surface topography exhibits discharge craters owing to the material is removed from machined surface. Generally, the surface quality of EDM is poor. Therefore, the investigation of surface fine-finish and modification to improve the surface quality and strength is necessary for extending the EDM applications. This study proposes surface fine-finish and modification methods to ameliorate the surface defects on EDM. In this investigation, a series of experiments were carried out to explore the effects on surface fine-finish and modification including added conductive powder into dielectric, integrated the mechanisms of EDM with ultrasonic machining (USM), and combined EDM with ball burnish machining (BBM). Since the material removal rate (MRR) of EDM is lower than conventional machining, the objective of surface fine-finish and modification methods for EDM not only concern the effects of surface improvement to obtain high quality surface but also the machining efficiency and precision. Thus, the machined surface quality, machining efficiency, and precision need to be considered simultaneously for EDM applications and they are also the interesting topics in this investigation. Experimental results show that added conductive particle of mixture powder into dielectric could expand the discharge gap to facilitate the expelling of debris and divide the discharge energy to reduce the surface roughness. Moreover, the added powder could be transferred and penetrated to the machined surface by ionization of the discharge column. Therefore, the machined surface achieves the modification and reinforcement. In addition, the combined process of EDM with USM possesses not only the electrical discharge to vapor and melt the material but also the abrasive that driven by ultrasonic vibration to harm and impact the workpiece. Thus, the MRR of the combined process of EDM with USM is higher than conventional EDM. Furthermore, the abrasive that is added into dielectric becomes the agent of surface modification, experiment results show that the machined surface could obtain the effects of particle reinforcement and solid-solution strengthening. The machined surface received the discharge alloying via combined EDM with USM. In addition, experimental results show that the combined process of EDM with BBM deformed materials via the action of smooth ZrO2 balls on EDM surface to eliminate the cracks, pores and discharge craters. The combined process can achieve surface fine-finish and modification effects to gain flat and finish surface integrity. Moreover, the ZrO2 balls will not disturb the EDM proceeding to reduce the machining efficiency. From the series of empirical investigations, we can obtain the surface fine-finish and modification via our processes to reach the high quality surface with high machining efficiency and precision for modern industrial applications. |
