摘要
放電披覆加工(EDC)是運用放電加工(EDM)的原理及調整放電參數在放電加工表面上進行材料披覆的一種加工技術。 一般EDC是在煤油中進行，煤油經過高溫裂解所產生的碳元素會與來自於電極材料的游離物質反應生成碳化物，所以EDC處理後的工件表面成份大都是碳化物，至於用EDC方法披覆非碳化物的研究相對較少。 本論文嘗試改變放電加工介質及自製不同電極材料進行EDC研究，主要是探討較特殊的碳化物及氮化物材料之適合參數條件及其產生的機制。 本研究選擇矽油為放電加工液，搭配石墨電極在高熔點難加工的鉬金屬表面披覆碳化矽材料是基於鉬金屬在微波管元件應用上的需求。 由實驗結果顯示選擇適當的放電加工參數不僅可以進行鉬金屬之加工成型，也可以對鉬金屬表面進行碳化矽吸波材料之披覆處理，因此可以簡化複雜的披覆程序與減少昂貴設備的投資，而具有應用在國防工業的潛力。 本研究製作鈦金屬及氮化鈦陶瓷兩類電極材料，分別在煤油(濕式EDC)與氮氣（乾式EDC）中對商用鋁合金表面進行披覆加工處理是著眼於改善鋁合金材料應用在燃料電池的雙極板時不耐酸蝕的缺點。 依據實驗的結果顯示：在濕式EDC時，高燒結密度（80%）的Ti金屬電極可以對鋁合金同時進行EDM及EDC處理，加工表面含有緻密的TiC層。 在煤油中添加氮化鈦粉末可以改善加工效率與表面品質，但採用高密度 (80%) 的TiN陶瓷電極，在鋁合金表面僅會形成多孔隙的TiC陶瓷層。 在乾式EDC方面，使用低密度 (60%) 的Ti燒結電極，在適當的參數條件下可以進行純TiN披覆處理，但高密度者需要更高的放電能量。 TiN陶瓷電極可以直接在鋁合金表面披覆純TiN層，然而低密度(60%)的TiN電極較不耐高放電能量，電極的耗損率會較高。 反觀採用高密度的情形，TiN披覆層的微觀形貌會受到峰值電流的影響，低峰值電流的TiN披覆層呈現緻密微細顆粒形貌，高峰值電流時則呈現多孔的網狀熔塊結構，表面粗糙度也比較高。 由於碳化鈦及氮化鈦均是屬於耐化學蝕腐之陶瓷材料，而放電加工技術也可被應用在燃料電池之金屬雙極板流道之成型製作，因此本研究結果具有作為一種複合加工技術應用在此領域之中的潛力。
並列摘要
Electrical discharge coating (EDC) is the application of electrical discharge machining (EDM) principle and modulating the discharge parameters to confer the coatings on machined workpiece. Normally, it is performed in kerosene, as a result, carbide layer may generate by reaction of pyrolysis carbon and metal ions. However, non-carbide formed by EDC process is rarely studied. This paper attempts to adopt different dielectric fluids and home-made electrodes to conduct EDC process to shed light on its optimal discharge parameters and different coating materials including special carbide and nitride. In this study, silicone oil selected to accompany with graphite electrode to conduct EDC process on refractory molybdenum (Mo) metal was in view of a requirement of SiC coating in microwave tube application. The experimental results show that by selecting the appropriate discharge parameters, it can not only proceed to machine process but also to coat SiC absorbing material on the surface of Mo. Evidently, EDC process can simplify the coating process and save the investment of expensive equipment, which demonstrates its potential application in the defense industry. In order to improve the poor corrosion resistance of Al alloy used in bipolar plate of PEMFC, titanium (Ti) metal and titanium nitride (TiN) ceramic were chosen as electrodes to carry out EDC in kerosene (wet) and nitrogen gas (dry), respectively. On one hand of wet EDC, high sintered density (80%) Ti electrode with appropriate discharge parameters can perform both EDM and EDC processes to produce dense TiC layer on Al alloy. Furthermore, it can improve the machining efficiency and quality of machined surface by adding the titanium nitride (TiN) powder in kerosene. But, using high density (80%) TiN ceramic electrodes with appropriate discharge parameters will result in a porous TiC layer on the surface of Al alloy. On the other hand of dry EDC, with appropriate discharging parameters, low density (60%) sintered Ti electrode can form pure TiN film on the surface of Al alloy. However, higher density Ti electrode needs using high discharge energy. In contrast, TiN electrode can directly deposit pure TiN layer on Al alloy. But, low density (60%) TiN electrode will result in high wear rate. By using high density TiN electrode can survive under high discharge energy. Whereas, the morphology of TiN layer is dependent of discharge current. With low discharge current currents, TiN layer exhibits a dense surface with fine grain, while high discharge current will result into porous network appearance and showing relatively high surface roughness. In view of TiC and TiN are good chemical corrosion resistance materials and EDM technology can be used to fabricate the fuel channel of metal bipolar plate in PEMFC, the results of this study may serve as a hybrid machining process in this field.
參考文獻
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