摘 要 放電加工表面因在加工液中受到急熱急冷作用容易產生微裂紋、氣孔、殘留應力等缺陷，這些缺陷不僅會造成加工表面精度不佳，更是造成機件使用壽命縮短的主因。因此如何維持加工效率的同時又能對放電加工表面進行改質強化，以提升加工表面之耐磨、耐蝕能力，是本文主要探討的主題。 經由本研究的實驗結果分析顯示，新開發的簡易複合電極以Cu-20wt%Cr的混合比例及在20MPa的壓力下所製作的電極具有最佳的材料去除率，而且實驗結果亦顯示在本實驗條件範圍內，此複合電極比傳統熔煉Cu電極具有更高的材料去除率，且其工件表面的微裂紋較少，再鑄層亦較薄，同時也發現複合電極之Cr成分會轉移到工件表面，增進表面耐蝕性能。而使用蒸餾水中添加尿素作為加工液對純鈦金屬之實驗，由於放電加工時產生高溫使尿素中解離出氮(N)元素並與工件材料(純鈦)產生化學作用，形成氮化鈦(TiN)的陶瓷表面硬化層；同時經由耐磨耗測試後發現，加工液中添加適量尿素可提升加工表面的耐磨耗能力、降低表面摩擦力，達到表面改質強化的效果。而在尿素加工液中添加鈦粉末進行放電加工時，由於添加粉末而造成極間間隙增大、促進放電分散的效應，使得加工屑容易排除，也因此促進放電穩定的進行，所以可提高材料的去除率、降低表面的粗糙度，同時尿素加工液中添加鈦粉末進行放電加工，會經由加工過程中放電柱的離子化作用解離出氮(N)元素並轉移至加工表面而形成表面改質層，將可提升加工表面的機械性質。 Abstract The Electrical discharge machining (EDM) surfaces have the defects of microcracks, pores and residual stresses formed by the strong temperature gradient during machining. These defects result not only in poor surface precision, but also in a shortened service life of machinery parts. Thus, the machined surface modification and machining efficiency need to be considered simultaneously for EDM applications, and they are also the interesting topics in this investigation. From the experimental results show that Cu-Cr composite electrodes with a Cu-20wt.%Cr mixing ratio and a 20 MPa sintering pressure obtained an excellent MRR. Moreover, this work also reveals that the composite electrodes obtained a higher MRR than Cu metal electrodes; the recast layer was thinner, and fewer cracks were present on the machined surface. Furthermore, the Cr contained in the composite electrode migrated to the work piece, resulting in good corrosion resistance of the machined surface after EDM. Moreover, workpiece surface modification by adding urea into the dielectric used in machining pure titanium metal. Experimental results indicate that the nitrogen element decomposed from the dielectric that contained urea, migrated to the work piece, forming a TiN hard layer, resulting in good wear resistance and exhibited improved friction of the machined surface after EDM. In addition, added conductive particle into urea solution could widen 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 channels. Therefore, the machined surface achieves the modification and reinforcement.
