高分子型電流變液對放電拋光特性之研究

Abstract

放電加工(EDM)是目前發展最成熟的模具加工方法之一，但加工後的表面因為放電火花衝擊造成有許多的放電坑與白層(再鑄層)，使得加工表面品質沒有達到理想的狀態，通常在放電後還要進行拋光後製程。本實驗室首創以電流變拋光液(Electro-rheological polishing fluid, ERP fluid)取代傳統放電加工液；利用電流變液施加電場時能形成纖維鏈狀的特性，並與磨粒結合形成拋光刷，即能進行放電拋光的複合加工製程，使工件在同一製程下達成鏡面效果的淨形加工。 由於高分子具質地較軟且拋光時不會對工件產生過切的性質，本研究利用高分子型電流變液進行放電拋光的加工實驗。實驗先以三種聚苯胺高分子及澱粉掺入矽油調製成電流變液進行放電加工，探討並比較其與在一般放電加工油的差異。發現雖然表面粗糙度及材料移除率雖不如放電加工油理想，但工件表面會增加矽元素而增加硬度及抗腐蝕性。接著將電流變液與氧化鋁磨粒調配成電流變拋光液，搭配不同的濃度進行放電拋光實驗。實驗結果發現，使用濃度100g/L的澱粉電流變拋光液，磨粒與電流變顆粒搭配濃度比例為1:1時並用負極性電極加工，可得最好的拋光效果，不只將原先使用傳統放電加工油所造成的約3μm白層完全去除，也使表面粗糙度從Ra 0.69μm大幅降低至Ra 0.10μm，改善程度高達86%。此外由電流變特性測試結果，顯示澱粉在電場下能承受的剪應力與抗擊穿電場強度皆最大，其證實了適用於放電拋光複合製程的高分子電流變液材料之選擇條件。本研究不只成功開發澱粉電流變拋光液實現放電拋光複合製程技術，並解明此新穎的澱粉電流變拋光液的材料特性。

Electrical discharge machining (EDM) is an important manufacturing process for mold and die. However, the work-piece surfaces after EDM process have the defects, such as micro-craters and white layer (recast layer). Therefore, the polishing is usually required after EDM process. This study presents a novel method of EDM that used Electro-rheological polishing fluid (ERP fluid) instead of conventional EDM oil. When applying high electric field between the electrode and work-piece, the dielectric particles are polarized and then the ER fluid will transform into fibrous chains. The fibrous chains seem to be working as polishing brushes dragging abrasive to carry out net shape by combining EDM and polishing in one process. In this study, the ER fluids were composed of polyaniline (PANI) or starch particles suspending in silicone oil. We first discussed EDM effects of ER fluids and compared it with conventional EDM oil. The surface roughness and material removal rate (MRR) in ER fluid are worse than EDM oil, but the workpiece surface is impinged by silicone, which can improve hardness and anti-corrosion of work-piece. In the experiments of ERP fluid mixing abrasive and ER fluid, using ER-abrasive ratio of 1:1 and negative electrode can obtain better surface roughness. We had the best machined surface without white layer and surface roughness of Ra 0.10μm when the starch concentration was 100g/L. There is 86% improvement of surface roughness from Ra 0.69μm for conventional EDM oil. Based on electro-rheological test, we found out starch is the most suitable particle for EDM-polishing because it has the largest shear stress and the highest breakdown electric field strength. This study successfully developed a novel ERP fluid using starch and alumina particles to carry out the combined process of EDM and polishing. Moreover, this study also indicates the key properties of ER fluid for EDM-polishing process.